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High resolution imaging of flameless and distributed turbulent combustion
Abstract
Planar laser-induced fluorescence (PLIF) and Rayleigh scattering measurements were used for the study of turbulence/combustion interactions in distributed reaction regimes including flameless or MILD combustion. A novel laboratory scale burner (Distributed and Flameless Combustion Burner – DFCB) was used to reach uniquely high Karlovitz numbers, presently reported up to 14,400. It consists of a highly turbulent piloted high speed jet burner with a vitiated coflow. Six cases are reported whereas two of them (leaner cases) led to an invisible reacting zone, though still emitting light in the UV and near infrared range. Simultaneous OH/CH2O PLIF image with 50μm spatial resolution were achieved to capture the variation of intermediate species in the reaction layer. When complemented with temperature images obtained by Rayleigh scattering measurement, it provided insights of the reaction front structures as well as measures of the flame brush thicknesses. In particular, variations in the jet velocity highlighted the influence of turbulent mixing (hence turbulence/chemistry interaction) on the flame structures as depicted by the formation of relatively large pools of CH2O. Further, variations in the jet stoichiometry impacted on the reaction zone visibility but only marginally on the intensity and moderately on the overall shape of the OH and CH2O signals.
... To date, several experimental (Cavaliere and de Joannon 2004;Tsuji et al. 2003;Duwig et al. 2008Duwig et al. , 2012Suzukawa et al. 1997;Dally et al. 2002Dally et al. , 2004Plessing et al. 1998;Özdemir and Peters 2001;Christo and Dally 2005;Medwell et al. 2007;Oldenhof et al. 2011) and numerical (Coelho and Peters 2001;Aminian et al. 2011; van Oijen 2013;Minamoto et al. 2013Minamoto et al. , 2014Swaminathan 2014a, b, 2015;Doan et al. 2018;Swaminathan 2019;Awad et al. 2021; Abo-Amsha and Chakraborty 2023) investigations focussed on the physics of MILD combustion, and the advancements in high-performance computing has enabled Direct Numerical Simulations (DNS) of MILD combustion (van Oijen 2013;Minamoto et al. 2013Minamoto et al. , 2014Swaminathan 2014a, b, 2015;Doan et al. 2018;Swaminathan 2019;Awad et al. 2021;Abo-Amsha and Chakraborty 2023). These analyses provide valuable insights into the physics of the MILD combustion process and help to explain the differences between MILD combustion and the conventional combustion processes in terms of reaction zone thickness and its morphology (Minamoto et al. 2013(Minamoto et al. , 2014Swaminathan 2014a, b, 2015;Doan et al. 2018;Swaminathan 2019;Awad et al. 2021;Abo-Amsha and Chakraborty 2023). ...
... These analyses provide valuable insights into the physics of the MILD combustion process and help to explain the differences between MILD combustion and the conventional combustion processes in terms of reaction zone thickness and its morphology (Minamoto et al. 2013(Minamoto et al. , 2014Swaminathan 2014a, b, 2015;Doan et al. 2018;Swaminathan 2019;Awad et al. 2021;Abo-Amsha and Chakraborty 2023). However, most of the analyses of MILD combustion were conducted for simple fuels such as H 2 ( van Oijen 2013), CH 4 (Duwig et al. 2008(Duwig et al. , 2012Suzukawa et al. 1997;Dally et al. 2002Dally et al. , 2004Plessing et al. 1998;Özdemir and Peters 2001;Christo and Dally 2005;Medwell et al. 2007;Minamoto et al. 2013Minamoto et al. , 2014Swaminathan 2014a, b, 2015;Doan et al. 2018;Swaminathan 2019;Awad et al. 2021), C 2 H 4 (Dally et al. 2004) and C 3 H 8 (Dally et al. 2004) while limited attention is given to the MILD combustion of heavier hydrocarbons (e.g. C 7 H 16 ) (Ye et al. 2017;Abo-Amsha and Chakraborty 2023), which are often used in industrial furnaces and gas turbines. ...
... 1c and d for methane and n-heptane cases, respectively. A significant amount of self-interaction of flame elements can be seen in both cases from Figs. 1c and d, which is consistent with previous experimental (Plessing et al. 1998;Özdemir and Peters 2001;Dally et al. 2004;Christo and Dally 2005;Medwell et al. 2007;Duwig et al. 2012;Oldenhof et al. 2011) and computational (Minamoto et al. 2013(Minamoto et al. , 2014Swaminathan 2014a, 2015) observations. These interactions lead to the appearance of the thickened (or distributed) reaction zones, which are qualitatively similar for both cases considered here. ...
Three-dimensional Direct Numerical Simulations of Exhaust Gas Recirculation (EGR)-type Moderate or Intense Low Oxygen Dilution (MILD) combustion of homogeneous mixtures of methane- and n-heptane–air have been conducted with skeletal chemical mechanisms. The suitability of different choices of reaction progress variable (which is supposed to increase monotonically from zero in the unburned gas to one in fully burned products) based on the mass fractions of different major species and non-dimensional temperature have been analysed in detail. It has been found that reaction progress variable definitions based on oxygen mass fraction, and linear combination of CO, CO2, H2 and H2O mass fractions (i.e. cO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${c}_{O2}$$\end{document} and cc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${c}_{c}$$\end{document}) capture all the extreme values of the major species in the range between zero and one under MILD conditions. A reaction progress variable based on fuel mass fraction is found to be unsuitable for heavy hydrocarbons, such as n-heptane, since the fuel breaks down to smaller molecules before the major reactants (products) are completely consumed (formed). Moreover, it has been found that the reaction rates of cO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${c}_{O2}$$\end{document} and cc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${c}_{c}$$\end{document} exhibit approximate linear behaviours with the heat release rate in both methane and n-heptane MILD combustion. The interdependence of different mass fractions in the EGR-type homogeneous mixture combustion is considerably different from the corresponding 1D unstretched premixed flames. The current findings indicate that the tabulated chemistry approach based on premixed laminar flames may need to be modified to account for EGR-type MILD combustion. Furthermore, both the reaction rate and scalar dissipation rate of cO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${c}_{O2}$$\end{document} and cc\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${c}_{c}$$\end{document} are found to be non-linearly related in both methane and n-heptane MILD combustion cases but the qualitative nature of this correlation for n-heptane is different from that in methane. This suggests that the range of validity of SDR-based turbulent combustion models can be different for homogeneous MILD combustion of different fuels.
... This definition of MILD combustion (alternatively known as flameless combustion) follows from the well-stirred reactor (WSR) theory, which can be readily applied to turbulent premixed combustion. MILD combustion has been analysed extensively [4][5][6][7][8][9][10][11][12][13][14][15] by experimental means to understand the underlying combustion process. Plessing et al. [6] compared MILD combustion with conventional combustion by investigating both planar laser-induced fluorescence images of OH radicals (OH-PLIF) and Rayleigh thermometry. ...
... Minamoto, Swaminathan and co-workers [18][19][20][21][22] analysed the distribution of species, temperature, and reaction-rate fields [18,19], morphological and topological structures [20] and scalar gradient statistics in terms of temperature and equivalent OH-PLIF signal [21] constructed from DNS data based on three-dimensional simulations of the combustion an exhaust gas recirculation (EGR)-type homogeneous-mixture under MILD conditions. They concluded that the appearance of a distributed combustion is due to the interaction of thin reaction zones [19][20][21][22][23], which were also reported based on OH-PLIF visualisations [4,10,14,15]. These analyses have been further extended by Doan, Swaminathan and co-workers for stratified-mixture MILD combustion [24][25][26] and provided important insights into the flame structure [24,25], reaction-zone topology [24,25] and markers of combustion mode [26]. ...
... It can be seen from Figure 4 that the OH-PLIF signal reveals a much thicker reaction zone in MILD combustion cases than in the corresponding conventional premixed flames, but there is still a clearly defined interface separating reactants and products for turbulent MILD combustion. The observations made from temperature and the equivalent OH-PLIF signal from the DNS data are consistent with previous experimental [6][7][8][9][10][11][12][13][14][15] and numerical [19][20][21][22][23] findings. ...
Moderate or intense low-oxygen dilution (MILD) combustion is a novel combustion technique that can simultaneously improve thermal efficiency and reduce emissions. This paper focuses on the differences in statistical behaviours of the surface density function (SDF = magnitude of the reaction progress variable gradient) between conventional premixed flames and exhaust gas recirculation (EGR) type homogeneous-mixture combustion under MILD conditions using direct numerical simulations (DNS) data. The mean values of the SDF in the MILD combustion cases were found to be significantly smaller than those in the corresponding premixed flame cases. Moreover, the mean behaviour of the SDF in response to the variations of turbulence intensity were compared between MILD and premixed flame cases, and the differences are explained in terms of the strain rates induced by fluid motion and the ones arising from flame displacement speed. It was found that the effects of dilatation rate were much weaker in the MILD combustion cases than in the premixed flame cases, and the reactive scalar gradient in MILD combustion cases preferentially aligns with the most compressive principal strain-rate eigendirection. By contrast, the reactive scalar gradient preferentially aligned with the most extensive principal strain-rate eigendirection within the flame in the premixed flame cases considered here, but the extent of this alignment weakened with increasing turbulence intensity. This gave rise to a predominantly positive mean value of normal strain rate in the premixed flames, whereas the mean normal strain rate remained negative, and its magnitude increased with increasing turbulence intensity in the MILD combustion cases. The mean value of the reaction component of displacement speed assumed non-negligible values in the MILD combustion cases for a broader range of reaction progress variable, compared with the conventional premixed flames. Moreover, the mean displacement speed increased from the unburned gas side to the burned gas side in the conventional premixed flames, whereas the mean displacement speed in MILD combustion cases decreased from the unburned gas side to the middle of the flame before increasing mildly towards the burned gas side. These differences in the mean displacement speed gave rise to significant differences in the mean behaviour of the normal strain rate induced by the flame propagation and effective strain rate, which explains the differences in the SDF evolution and its response to the variation of turbulence intensity between the conventional premixed flames and MILD combustion cases. The tangential fluid-dynamic strain rate assumed positive mean values, but it was overcome by negative mean values of curvature stretch rate to yield negative mean values of stretch rate for both the premixed flames and MILD combustion cases. This behaviour is explained in terms of the curvature dependence of displacement speed. These findings suggest that the curvature dependence of displacement speed and the scalar gradient alignment with local principal strain rate eigendirections need to be addressed for modelling EGR-type homogeneous-mixture MILD combustion.
... Besides the above-noted experimental work on the nonpremixed JHC flame, 22,25,26,63−74 Duwig et al. 75 conducted a set of experiments on the premixed JHC flame of the methane/air mixture, aiming to examine the turbulence−combustion interaction in a laboratory-scale pilot-jet burner operating at very high Karlovitz numbers (∼14000). These investigators found both visible and invisible reaction layers accompanied by radiations indicating the presence of the radical OH. ...
... Both layers were viewed to be thickened or thinned by primary large-scale coherent structures (rings and helices) arising in the reactant-jet shear layer. Duwig et al. 75 showed a complex and intermittent interaction between turbulence and chemistry that involves the motion of coherent structures. The result of the interaction consists of thick brushes of OH and CH 2 O with alternating thicknesses. ...
... Although having distinct visibilities, the two lean cases were found to exhibit very similar characteristics of the turbulence/chemistry interaction. Duwig et al. 75 raised an interesting question to further address in future for such a complex autoignition process; i.e., what is the exact role played by coherent structures in autoignition of the JHC flame? Energy & Fuels pubs.acs.org/EF ...
... Many efforts have been done to reduce the emission of pollutions caused by the utilization of fossil fuel, such as NO x , SO x , and soot. In NO x reduction, one of those advanced approaches is diluting the combustion mixtures [1][2][3][4] especially with high temperature diluents to create a distributed reaction zone and mild combustion [2,5,6] which can lower the temperature of the reaction zone and reduce the formation of NO x . The dilution can be achieved with exhaust gas recirculation (EGR), steam, or low calorific value (LCV) fuels utilization, which normally contain a massive amount of incombustible gas, such as N 2 , CO 2 , and H 2 O. ...
... These gases make the chemical reaction rate drop significantly and combustion process out of the normal flame-let regime more easily [4][5][6][7][8][9][10][11]. Recently, many studies about the effect of dilution on the characteristics of combustion have been performed, such as turbulent burning velocity, flame stability, flame structure, and pollution emission [2,[12][13][14][15][16]. 2 of 11 Among these researches, laser diagnostic techniques were adopted widely, and much essential information of flames has been obtained, e.g., the structure of turbulent flames [1][2][3]17,18], OH radical laser measurement [19][20][21][22], and the intermediate product CH 2 O [20,[23][24][25][26][27][28][29][30][31][32][33]. Dally et al. [34] and Medwell et al. [2] used planar laser induced fluorescence (PLIF) and Rayleigh scattering (RS) measured the flame structure of turbulent non-premixed jet flames. ...
... The greater the Ka, the faster the turbulent mixing rate compared to the chemical reaction rate of the flame. The flame with a high rate of dilution was also compared with the flame with extreme low equivalence ratio (ϕ = 0.4), which can obtain some fundamental understanding about the interaction between turbulent transfer and chemical reaction in distributed reaction regimes including mild combustion [1,5,6]. In addition, the spatial distribution of OH and CH 2 O of several jet flames were imaged by PLIF to have a look at the detail structure of the reaction zone from the downstream position 5d (d: Diameter of jet tube) to 37d. ...
Diluting the combustion mixtures is one of the advanced approaches to reduce the NOx emission of methane/air premixed turbulent flame, especially with high diluents to create a distributed reaction zone and mild combustion, which can lower the temperature of reaction zone and reduce the formation of NOx. The effect of N2/CO2 dilution on the combustion characteristics of methane/air premixed turbulent flame with different dilution ratio and different exit Reynolds number was conducted by OH-PLIF and CH2O-PLIF. Results show that the increase of dilution ratio can sharply reduce the concentration of OH and CH2O, and postpone the burning of fuel. Compared with the ultra-lean combustion, the dilution weakens the combustion more obviously. For different dilution gases, the concentration of OH in the combustion zone varies greatly, while the concentration of CH2O in the unburned zone is less affected by different dilution gas. The CO2 dilution has a more significant effect on OH concentration than N2 with the given dilution ratio, but a similar effect on the concentration of CH2O in the preheat zone of flame. However, dilution does not have much influence on the flame structure with the given turbulent intensity.
... However, actual combustion devices are often designed to operate in the lean combustion zone (e.g., < 0.8) to reduce NO x emissions and to improve thermal efficiency. Additionally, in a flameless combustion environment, there is a lack of markers for the flame reaction zone, since the CH concentration is even lower [6]. Therefore, we need to find a substitution of CH to visualize the flame surface structure in these situations. ...
... We did not see a significant difference in the intensity of the CH 3 signal at different heights; the turbulence did not cause an apparent broadening of the CH 3 -labeled region. Hence, CH 3 is a potential substitution for CH as a direct marker to visualize the reaction zone of hydrocarbon flames under extreme conditions, where the low CH concentration or strong turbulence prevails (e.g., flameless combustion [6]). ...
Visualization of the reaction zone of flames using CH radicals as markers is restricted by the low concentration of CH in fuel-lean conditions. To address this, methyl radicals ( ${{\rm{CH}}_3}$ C H 3 ) are employed as a substitution of CH in premixed methane/air flames. A pump-probe method was adopted with the pump laser photolyzing ${{\rm{CH}}_3}$ C H 3 and the probe laser detecting the photolyzed CH ( ${{\rm{X}}^2}\Pi$ X 2 Π ) fragments. Laser excitation scans were performed to ensure that the fluorescence detected was from CH only. Visualization of the reaction zone of flames was accomplished by a ${{\rm{CH}}_3}$ C H 3 photofragmentation laser-induced fluorescence technique in fuel-lean conditions (the equivalence ratio of 0.4), where CH planar laser-induced fluorescence did not work in both laminar and turbulent jet flames. The proposed pump-probe method of ${{\rm{CH}}_3}$ C H 3 can be used to visualize the reaction zone of hydrocarbon combustion under both fuel-lean and fuel-rich conditions with a superior signal-to-noise ratio.
... Simultaneous planar LIF (PLIF) imaging provides increased spatial information about species distributions and may allow for direct, two-dimensional comparison of the spatial distributions and, in some cases, concentrations of intermediary and radical species (as well as the flow-and temperature-fields). Simultaneous planar imaging of OH and CH 2 O has been undertaken as part of numerous experimental campaigns in the JHC configurations (Medwell et al., 2007(Medwell et al., , 2008Gordon et al., 2008Gordon et al., , 2009Duwig et al., 2012;Macfarlane et al., 2017Macfarlane et al., , 2018Ye et al., 2018). One such experimental configuration is represented in Figure 4, providing an overview of the optical layout required for simultaneous imaging of OH, CH 2 O and Rayleigh scattering for temperature (discussed in more detail in section 5), presented in Figure 5. ...
... Imaging of CH 2 O is most commonly performed using the Nd:YAG third-harmonic wavelength of 355 nm (Gordon et al., 2007(Gordon et al., , 2008Duwig et al., 2012;Macfarlane et al., 2017Macfarlane et al., , 2018Ye et al., 2018) or-less often-near 341 nm using a frequencydoubled tunable-dye laser (Medwell et al., 2007(Medwell et al., , 2008. The latter approach targets a specific energy transition, which allows for amelioration of the effect of Boltzmann fraction on the CH 2 O signal (Medwell et al., 2007). ...
There is a wealth of existing experimental data of flames collected using laser diagnostics. The primary objective of this review is to provide context and guidance in interpreting these laser diagnostic data. This educational piece is intended to benefit those new to laser diagnostics or with specialization in other facets of combustion science, such as computational modeling. This review focuses on laser-diagnostics in the context of the commonly used canonical jet-in-hot-coflow (JHC) burner, although the content is applicable to a wide variety of configurations including, but not restricted to, simple jet, bluff body, swirling and stratified flames. The JHC burner configuration has been used for fundamental studies of moderate or intense low oxygen dilution (MILD) combustion, autoignition and flame stabilization in hot environments. These environments emulate sequential combustion or exhaust gas recirculation. The JHC configuration has been applied in several burners for parametric studies of MILD combustion, flame reaction zone structure, behavior of fuels covering a significant range of chemical complexity, and the collection of data for numerical model validation. Studies of unconfined JHC burners using gaseous fuels have employed point-based Rayleigh-Raman or two-dimensional Rayleigh scattering measurements for the temperature field. While the former also provides simultaneous measurements of major species concentrations, the latter has often been used in conjunction with planar laser-induced fluorescence (PLIF) to simultaneously provide quantitative or qualitative measurements of radical and intermediary species. These established scattering-based thermography techniques are not, however, effective in droplet or particle laden flows, or in confined burners with significant background scattering. Techniques including coherent anti-Stokes Raman scattering (CARS) and non-linear excitation regime two-line atomic fluorescence (NTLAF) have, however, been successfully demonstrated in both sooting and spray flames. This review gives an overview of diagnostics techniques undertaken in canonical burners, with the intention of providing an introduction to laser-based measurements in combustion. The efficacy, applicability and accuracy of the experimental techniques are also discussed, with examples from studies of flames in JHC burners. Finally, current and future directions for studies of flames using the JHC configuration including spray flames and studies and elevated pressures are summarized.
... Combustion radicals can also be measured by fs-TPLIF. Hydroxyl (OH) is one of the essential combustion radicals, and OH-PLIF is widely used to visualize the reaction zone and product zone of a flame [60][61][62][63]. Different from CO, H, and O, OH can be excited with single-photon strategy. ...
... A typical single-photon OH-LIF uses an ns laser at~283 nm to excite OH through a transition line in A 2 Σ + ← X 2 Π (0, 1) band, which allows observation of the fluorescence emission from the (1, 1) and (0, 0) bands at~310 nm [64]. There are many researches of OH-LIF, including ns-laser excitation [62,63] and ps-laser excitation [65]. Stauffer et al. [31,32] performed the fs-TPLIF of OH measurements with two-photon excitation at 620 nm. ...
The applications of femtosecond lasers to the diagnostics of combustion and flow field have recently attracted increasing interest. Many novel spectroscopic methods have been developed in obtaining non-intrusive measurements of temperature, velocity, and species concentrations with unprecedented possibilities. In this paper, several applications of femtosecond-laser-based incoherent techniques in the field of combustion diagnostics were reviewed, including two-photon femtosecond laser-induced fluorescence (fs-TPLIF), femtosecond laser-induced breakdown spectroscopy (fs-LIBS), filament-induced nonlinear spectroscopy (FINS), femtosecond laser-induced plasma spectroscopy (FLIPS), femtosecond laser electronic excitation tagging velocimetry (FLEET), femtosecond laser-induced cyano chemiluminescence (FLICC), and filamentary anemometry using femtosecond laser-extended electric discharge (FALED). Furthermore, prospects of the femtosecond-laser-based combustion diagnostic techniques in the future were analyzed and discussed to provide a reference for the relevant researchers.
... Duwig et al. [62] presented a coflow burner with a premixed central jet surrounded by a McKenna burner to generate the coflow (Fig. 10). The burner was named Distributed and Flameless Combustion Burner (DFCB). ...
... The burner employed by Duwig et al.[62]. The central jet plug was placed in the centre of a McKenna burner. ...
Since its discovery, the Flameless Combustion (FC) regime has been seen as a promising alternative combustion technique to reduce pollutant emissions of gas turbine engines. This combustion mode is often characterized by well-distributed reaction zones, which can potentially decrease temperature gradients, acoustic oscillations and, consequently NOx emission. However, the application of FC to gas turbines is still not a reality due to the inherent difficulties faced in attaining the regime while meeting all the engine requirements. Over the past years, investigations related to FC have been focused on understanding the fundamentals of this combustion regime, the regime boundaries, its computational modelling, and combustor design attempts. This article reviews the progress achieved so far, discusses the various definitions of the FC regime, and attempts to point the directions for future research. The review suggests that modelling of the FC regime is still not capable of predicting intermediate species and pollutant emissions. Comprehensive experimental databases with conditions relevant to gas turbine combustors are not available, and moreover, many of the current experiments do not necessarily represent the FC regime. By analysing the latest developments in computational modelling, the review points to the most promising approaches for the prediction of reaction zones and pollutant emissions in FC. The lessons learned from previous design attempts provide valuable insights into the design of a successful gas turbine engine operating under the FC regime. The review concludes with some examples where the gas turbine architecture has been exploited to advance the possibilities of FC in gas turbines.
... Therefore, in comparison with flames in the 6% and 9% O 2 coflows, a more spatially distributed temperature and heat release can be deduced from the OH * profile in the 3% O 2 case. Based on the distributed nature of MILD combustion [1,50,51] , flames in the 3% O 2 coflow agree better with MILD combustion conceptually than flames in the coflow with a higher oxygen level. ...
... This change is most prominent at Z st , where the gradient of the temperature profile is considerably sharper in the 9% O 2 case than the 3% O 2 case. Thus, flames in the 3% O 2 case agree better with the definition of MILD flame being distributed with a low temperature increase [1,50,51] . ...
This study compares the flame structure of ethanol and dimethyl ether (DME) in a hot and diluted ox- idiser experimentally and computationally. Experiments were conducted on a Jet in Hot Coflow (JHC) burner, with the fuel jet issuing into a 1250-K coflow at three oxygen levels. Planar measurements using OH-LIF, CH2O-LIF, and Rayleigh scattering images reveal that the overall spatial distribution and evolution of OH, CH2O, and temperature were quite similar for the two fuels. For both the ethanol and the DME flames, a transitional flame structure occurred as the coflow oxygen level increased from 3% to 9%. This indicates that the flames shift away from the MILD combustion regime. Reaction flux analyses of ethanol and DME were performed with the OPPDIF code, and ethane (C2H6 ) was also included in the analyses for comparison. These analyses reveal that the H2/O2 pathways are very important for both ethanol and DME in the 3% O2 cases. In contrast, the importance of fuel-specific reactions overtakes that of H2/O2 reactions when fuels are burnt in the cold air or in the vitiated oxidant stream with 9% O2 . Unsteady laminar flamelet analyses were also performed to investigate the ignition processes and help interpret experimental results. Flamelet equations were solved in time and mixture fraction field, which was pro- vided by non-reactive Large-Eddy Simulation (LES).
... Il tutto senza tener conto che ogni specifica forma di "Colorless Distributed Combustion" è caratterizzata da un corrispondente meccanismo di interazione tra turbolenza e combustione, per il cui ottenimento e controllo (subordinazione agli scopi prefissi) si richiede una strategia progettuale "dedicata": <<… It also implies that flameless combustion for gas turbine application (lean flameless) would certainly differ in term of turbulence/chemistry interaction (hence burner design) compared to a flameless furnace (MILD combustion).>> [30]. Con questi intendimenti si è dato avvio alla sintesi di una nuova architettura fluidodinamica, atta ad allocare stabilmente fiamme distribuite nei tubi di fiamma dei turbogas. ...
... However, like all other fossil fuels, syngas emits pollutants such as NOx as it burns. Diluting the combustion mixture is a general method to control NOx emissions [1][2][3], especially with high temperature diluents to create a distributed reaction zone and mild combustion [2]. Gas diluents, which normally denote incombustible gas, including N2, CO2, H2O, and exhaust gas re-circulation (EGR) [4], can significantly reduce the chemical reaction rate and temperature of the reaction zone, and thus decrease the formation of NOx. ...
Syngas produced by gasification, which contains a high hydrogen content, has significant potential. The variation in the hydrogen content and dilution combustion are effective means to improve the steady combustion of syngas and reduce NOx emissions. OH planar laser-induced fluorescence technology (OH-PLIF) was applied in the present investigation of the turbulence of a premixed flame of syngas with varied compositions of H2/CO. The flame front structure and turbulent flame velocities of syngas with varied compositions and turbulent intensities were analyzed and calculated. Results showed that the trend in the turbulent flame speed with different hydrogen proportions and dilutions was similar to that of the laminar flame speed of the corresponding syngas. A higher hydrogen proportion induced a higher turbulent flame speed, higher OH concentration, and a smaller flame. Dilution had the opposite effect. Increasing the Reynolds number also increased the turbulent flame speed and OH concentration. In addition, the effect of the turbulence on the combustion of syngas was independent of the composition of syngas after the analysis of the ratio between the turbulent flame speed and the corresponding laminar flame speed, for the turbulent flames under low turbulent intensity. These research results provide a theoretical basis for the practical application of syngas with a complex composition in gas turbine power generation.
... For instance, in applications where light sheets are used (such as planar laser-induced fluorescence, particle imaging velocimetry, and laser Rayleigh scattering), the light sheet is usually a few hundred nanometers thick and the spatial resolution in the plane is 10 × 10 μm 2 or larger. [3][4][5][6] In pointwise or 1D-methods (like Raman or nonlinear optical spectroscopy), the reported measurement volumes are typically on the order of 0.5 mm and larger. [7,8] In the following, we focus on spontaneous and nonlinear Raman scattering techniques in the gas phase. ...
Thermometry utilizing the vibrational spectrum of nitrogen is a very common tool in combustion diagnostics via spontaneous and coherent Raman scattering spectroscopy. In the presence of strong temperature gradients, however, it is likely that hot and cold gas portions contribute to the signal. This is known as spatial averaging, and it can result in severe measurement errors. The present work proposes and demonstrates two straightforward approaches that allow the identification of spatial averaging as well as estimating the resulting error. For this purpose, simulated Raman spectra are considered in order to avoid any impact from experimental artifacts. The first approach utilizes difference spectra, whereas the second one is based upon principal component analysis (PCA). Two scenarios were tested: (1) spectra of 50:50 mixtures exhibiting a common mean temperature and (2) spectra of mixtures with systematically varied fractions of hot (2000 K) and cold (300 K) gas. It is shown that the resulting measurement error can be as high as 500 K. Spatial averging affects the temperature derived from Raman spectra of nitrogen. The paper presents simple methods to identify and quantify the errors.
... Similar uniform distribution has been reported based on visual examination of advanced laser measurement results of temperature and CH 2 O fluorescence [5][6][7][8] , which has been the basis of employing perfectly stirred reactor (PSR)-type approaches for theoretical analyses [2,9,10] and combustion modelling [11][12][13] . Contrary to these observations with uniformly-distributed reaction zones, reaction zones with reasonably well determined flame fronts with relatively large scalar gradients are also reported for MILD combustion based on OH planar laser induced fluorescence (PLIF) images [5,6,8,14] . These observations may justify the use of flamelet-type approaches [11,15,16] , with an appropriate treatment for the addition of radicals and intermediate species in the reactant and oxidizer streams due to EGR or FGR. ...
Direct numerical simulation (DNS) data of moderate or intense low-oxygen dilution (MILD) combustion and a planar flame are analysed to identify quantities influencing the unique features of co-existing combustion modes and develop a model to identify them in MILD combustion. The results show the existence of direct relationship between the scalar dissipation and reaction rates in MILD combustion, whereas the correlation is weaker than the planar flame. Also, rotational turbulent motions identified by the enstrophy-strain balance also show a substantial influence on the MILD combustion field, via the principal component analysis, suggesting the mechanism by which the non-flamelet part of reaction zones is controlled. A neural network (NN)-based model was proposed to identify the filtered local combustion mode for MILD combustion fields in LES context. The NN is trained based on DNS data of MILD combustion at two different conditions for different filter sizes. The model assessment was performed using the third MILD condition having a higher Ka and dilution level than those of the two training data sets. Several filter sizes ranging from 0.5 to 2.0 times of a corresponding laminar flame thickness were considered, and also prediction performance of a “zeroth-order model” is compared with that of the NN-based prediction. The assessment shows very high correlation between the NN-based prediction and the mode directly obtained from DNS. The predicted local combustion mode could be used to exploit advantages of both flamelet and non-flamelet-type combustion models to predict co-existing MILD reaction zones.
... The turbulence and Taylor microscale Reynolds numbers are Re t ≈ 96 and Re λ ≈ 35, respectively. The turbulence Reynolds number is similar to those in previous experimental studies of MILD combustion [21,33,34] . Case AZ1 has 3.5% by volume as maximum oxygen concentration. ...
Moderate or Intense Low-oxygen Dilution (MILD) combustion has drawn increasing attention as it allows to avoid the thermo-chemical conditions prone to the formation of pollutant species while ensuring high energy efficiency and fuel flexibility. MILD combustion is characterized by a strong competition between turbulent mixing and chemical kinetics so that turbulence-chemistry interactions are naturally strengthened and unsteady phenomena such as local extinction and re-ignition may occur. The underlying physical mechanisms are not fully understood yet and the validation of combustion models featuring enhanced predictive capabilities is required. Within this context, high-fidelity data from Direct Numerical Simulation (DNS) represent a great opportunity for the assessment and the validation of combustion closure formulations. In this study, the performance of the Partially Stirred Reactor (PaSR) combustion model in MILD conditions is a priori assessed on Direct Numerical Simulations (DNS) of turbulent combustion of MILD mixtures in a cubical domain. Modeled quantities of interest, such as heat release rate and reaction rates of major and minor species, are compared to the corresponding filtered quantities extracted from the DNS. Different submodels for the key model parameters, i.e., the chemical time scale τc and the mixing time scale τmix, are considered and their influence on the results is evaluated. The results show that the mixing time scale is the leading scale in the investigated cases. The best agreement with the DNS data regarding the prediction of heat release rate and chemical source terms is achieved by the PaSR model that employs a local dynamic approach for the estimation of the mixing time scale. An overestimation of the OH species source terms occurs in limited zones of the computational domain, characterized by low heat release rates.
... This way, the diluted nature of MILD combustion is imitated, without the need for real exhaust gas recirculation. The JHC configuration was utilized in both laminar (Sepman et al., 2013a,b) and turbulent conditions (Dally et al., 2002;Oldenhof et al., 2011;Duwig et al., 2012;Ma and Roekaerts, 2016) to achieve MILD combustion. ...
The energy demand in the world is ever increasing, and for some applications combustion is still the only reliable source, and will remain as such in the foreseeable future. To be able to mitigate the environmental effects of combustion, we need to move to cleaner technologies. Moderate or intense low oxygen dilution (MILD) combustion is one of these technologies, which offer less harmful emissions, especially nitric oxide and nitrogen dioxide (NOx). It is achieved by the recirculation of the flue gases into the fresh reactants, reducing the oxygen content, and thereby causing the oxidation reactions to occur at a milder pace, as the acronym suggests. This results in a flameless combustion process and reduces the harmful emissions to negligible amounts. To assist in the design and development of combustors that work in the MILD regime, reliable and efficient models are required. In this study, modeling of the effects of temperature variation in the oxidizer of a MILD combustion case is tackled. The turbulent scales are fully resolved by performing direct numerical simulations (DNS), and chemistry is modeled using multistage flamelet generated manifolds (MuSt-FGM). In order to model the temperature variations, a passive scalar which is created by normalizing the initial temperature in the oxidizer is defined as a new control variable. During flamelet creation, it was observed that not all the compositions are autoigniting. Several approaches are proposed to solve this issue. The results from these cases are compared against the ones performed using detailed chemistry. With the best performing approach, the ignition delay is predicted fairly well, but the average heat release rate is over-predicted. Some possible causes of this mismatch are also given in the discussion.
... Different studies have been conducted to understand the physics and chemistry of distributed combustion. Duwig et al. [17] employed laser based diagnostics to image the reaction zone of flameless and distributed turbulent combustion at high resolution. The change of reaction layer structure in relation to distributed (some call it MILD) combustion was investigated by Dally et al. [18]. ...
Colorless distributed combustion (CDC) combustion technology offers significant advantages of ultra-low pollutants emission, stable operation and improved pattern factor for high intensity stationary gas turbines applications. Detailed knowledge on distributed combustion behavior is required to further deploy this technology. This paper reports the evolution of swirl flame shape, flame expansion and pollutants emission characteristics using propane, methane, and 20% and 40% hydrogen enriched methane fuels. The entrainment of hot reactive gases was simulated by diluting the inlet air stream with inert nitrogen or carbon dioxide. OH* chemiluminescence signatures, captured at different dilution levels, manifested gradual reduction of flame luminosity when CDC was approached. Flame boundaries derived using image threshold technique helped to visualize the broadened reaction zone under CDC conditions, which had much reduced chemiluminescence signal intensity. The rms to mean OH* signal variation at different dilution levels were analyzed to detect the initiation of CDC. The higher flame lift-off observed with CO 2 dilution was due to higher heat capacity of CO2 , resulting in greater flame speed reduction. Flame expansion, evaluated from the area encompassed within the flame boundary at different dilution levels showed a power-law behavior with both the diluents. Expansion of 4 to 5 times of initial flame volumes was observed under CDC using CO 2 and N 2 dilution, respectively. Significant reduction of NO and CO emission achieved under CDC was due to reduction of overall flame temperature, hotspot mitigation, and widened reaction zone occupying large flame volume, which makes it favorable for many practical combustor applications.
... Considering the preheated reactant mixture being more reactive, the characteristics of transport-chemistry interaction in turbulent flames at MILD conditions are distinct from those at normal conditions. Experimental measurements suggest a uniform and relatively distributed combustion under MILD conditions, while the OH images present a contrasting view of well-defined thin reaction zones, making it difficult to categorize in the standard "turbulent combustion regime" context [9][10][11][12][13][14] . Three-dimensional direct numerical simulation (3D DNS) of turbulent methane flame at MILD conditions has been conducted [15] , reporting regions with strong chemical activity along with the interactions between different rehttps://doi. ...
Dominant physical processes that characterize the combustion of a lean methane/air mixture, diluted with exhaust gas recirculation (EGR), under turbulent MILD premixed conditions are identified using the combined approach of Computational Singular Perturbation (CSP) and Tangential Stretching Rate (TSR). TSR is a measure to combine the time scale and amplitude of all active modes and serves as a rational metric for the true dynamical characteristics of the system, especially in turbulent reacting flows in which reaction and turbulent transport processes compete. Applied to the MILD conditions where the flame structures exhibit nearly distributed combustion modes, the TSR metric was found to be an excellent diagnostic tool to depict the regions of important activities. In particular, the analysis of turbulent DNS data revealed that the system’s dynamics is mostly dissipative in nature, as the chemically explosive modes are largely suppressed by the dissipative action of transport. On the other hand, the convective transport associated with turbulent eddies play a key role in bringing the explosive nature into the system. In the turbulent MILD conditions under study, the flame structure appears nearly in the distributed combustion regime, such that the conventional statistics conditioned over the progress variable becomes inappropriate, but TSR serves as an automated and systematic way to depict the topology of such complex flames. In addition, further analysis of the CSP modes revealed a strong competition between explosive and dissipative modes, the former favored by hydrogen-related reactions and the convection of CH4, and the latter by carbon-related processes. This competition results in a much smaller region of explosive dynamics in contrast to the widespread existence of explosive modes.
... Also, since the calculated flame scattering cross section is approximately 10% higher than that of air, the air entrainment beyond the flame front causes the reduction of scattering cross section. It inevitably results in a small temperature overestimation, though it is reported that the effective Rayleigh cross section of burned gas and unburned gas is almost the same for the case of the equivalent ratio less than the unit [20]. ...
This paper explores Rayleigh scattering thermometry via a wavelength of 355 nm through a unique measurement scheme. In this context, the $p$ p -polarization and $s$ s -polarization Rayleigh scattering of flame and air (as the temperature calibration reference) are measured. Subtraction of $p$ p -polarization Rayleigh scattering intensity from that of $s$ s -polarization is proposed to eliminate the background noise and fluorescence interference influence to reduce the temperature measurement uncertainties. To validate this method, the temperature field of $ {{\rm{CH}}_4}/{{\rm{N}}_2}/{{\rm{O}}_2} $ C H 4 / N 2 / O 2 premixed flame at $ \phi = 0.78 $ ϕ = 0.78 on a McKenna burner is detected by this Rayleigh scattering thermometry, and the axial temperature profile is validated with the literature data. Within the region of interest domain ( $ - 5\,\,{\rm{mm}}$ − 5 m m to 5 mm in the radial direction), an overall temperature measurement system precision of $ \pm 46.5\,\,{\rm{K}} $ ± 46.5 K is reported. The influence of both $ p $ p -polarization Rayleigh scattering and laser sheet inhomogeneity on the temperature measurement is further quantitatively studied. The measurement uncertainties relevant to laser energy variation and flame Rayleigh scattering cross-section variation due to temperature increase are specified as 1.4% and 2%–8%, respectively. Eventually, temperature measurements of single-shot images are attempted, and the large signal dynamic range (100–1000 [a.u.]) indicates a promising potential for temperature field interpretation of turbulence combustion.
... Also, views arising from OH-PLIF imaging of MILD combustion differ and seem to suggest that OH may not be a reliable marker for HRR. For example, OH-PLIF imaging of MILD combustion in a jet-in-hot-coflow (JHC) or a furnace showed thin regions of OH with a clear peak and strong gradients (Dally et al., 2004;Duwig et al., 2012;Medwell et al., 2007;Ozdemir and Peters, 2001;Plessing et al., 1998). On the other hand, Medwell et al. (2009) observed that there is a decrease in OH concentration with an increase in CH 2 O in MILD reaction zones compared to the conventional combustion. ...
Various commonly used markers for heat release are assessed using direct numerical simulation (DNS) data for Moderate or Intense Low-oxygen Dilution (MILD) combustion to find their suitability for non-premixed MILD combustion. The laser-induced fluorescence (LIF) signals of various markers are synthesized from the DNS data to construct their planar (PLIF) images which are compared to the heat release rate images obtained directly from the DNS data. The local OH values in heat releasing regions are observed to be very small compared to the background level coming from unreacted mixture diluted with exhaust gases. Furthermore, these values are very much smaller compared to those in burnt regions. This observation rises questions on the use of OH-PLIF for MILD combustion. However, the chemiluminescent image obtained using OH∗ is shown to correlate well with the heat release. Two scalar-based PLIF markers, OH×CH2O and H×CH2O, correlate well with the heat release. Flame index (FI) and chemical explosive mode analyses (CEMA) are used to identify premixed and non-premixed regions in MILD combustion. Although there is some agreement between the CEMA and FI results, large discrepancies are still observed. The schlieren images deduced from the DNS data showed that this technique can be used for a quick and qualitative identification of MILD combustion before applying expensive laser diagnostics.
... Indeed, this simple configuration has allowed for precise laser measurements 4,8 of various flow and thermochemical quantities, providing physical insights into several key features of this combustion mode, such as the importance of autoignition 10 and the role of turbulence in promoting thicker reaction zones. 17 However, other representative characteristics of MILD combustion commonly existing in practical burners with internal exhaust gas recirculation (EGR) are absent in the JHC configuration. ...
A cyclonic burner operating under moderate or intense low-oxygen dilution (MILD) conditions is simulated using a Perfectly Stirred Reactor (PSR) incorporated within a tabulated chemistry approach. A presumed joint probability density function (PDF) method is utilised with appropriate sub-models for the turbulence-chemistry interaction. Non-adiabatic effects are included in the PSR calculation to take into account the effects of non-negligible wall heat loss in the burner. {\color{black}Five different operating conditions are investigated and the computed mean temperatures agree well with measurements. A substantial improvement is observed when the non-adiabatic PSR is used highlighting the importance of heat transfer effects for burner configurations involving internal exhaust gas recirculation (EGR). Furthermore, enhanced reaction homogeneity is observed in this cyclonic configuration for the globally lean case, leading to a more spatially uniform temperature variation with MILD combustion.
... Axis unit: m. CH 2 O is a key precursor in the initiation process of reaction for fuel molecules with carbon atoms, especially in MILD combustion.[42][43][44][45] Therefore, it is adopted here in combination with temperature to identify the reactive region in the flame. ...
The current article focuses on the numerical simulation of the Delft Jet-in-Hot-Coflow (DJHC) burner, fed with natural gas and biogas, using the Eddy Dissipation Concept (EDC) model with dynamic chemistry reduction and tabulation, i.e. Tabulated Dynamic Adaptive Chemistry (TDAC). The CPU time saving provided by TDAC is evaluated for various EDC model constants and chemical mechanisms of increasing complexity, using a number of chemistry reduction approaches. Results show that the TDAC method provides speed-up factors of 1.4-2.0 and more than 10, when using a skeletal (DRM19) and a comprehensive kinetic mechanism (POLIMIC1C3HT), respectively. The Directed Relation Graph with Error Propagation (DRGEP), Dynamic Adaptive Chemistry (DAC) and Elementary Flux Analysis (EFA) reduction models show superior performances when compared to other approaches as Directed Relation Graph (DRG) and Path Flux Analysis (PFA). All the reduction models have been adapted for run-time reduction. Furthermore, the contribution of tabulation is more important with small mechanisms, while reduction plays a major role with large mechanisms.
... structure of the reactive region [3,4], the ignition and oxidation chemistry [5,6]. Previous studies employing direct photographs [7,8] and laser thermometry [9,10] suggest a uniform and distributed combustion under highly preheated and diluted conditions, often referred to as flameless combustion. On the contrary, the Probability Density Function (PDF) of temperature reported in [11] suggests the existence of thin reaction zones. ...
Distributed combustion regime occurs in several combustion technologies were efficient and environmentally cleaner energy conversion are primary tasks. For such technologies (MILD, LTC, etc…), working temperatures are enough low to boost the formation of several classes of pollutants, such as NOx and soot. To access this temperature range, a significant dilution as well as preheating of reactants is required. Such conditions are usually achieved by a strong recirculation of exhaust gases that simultaneously dilute and pre-heat the fresh reactants. However, the intersection of low combustion temperatures and highly diluted mixtures with intense pre-heating alters the evolution of the combustion process with respect to traditional flames, leading to significant features such as uniformity and distributed ignition. The present study numerically characterized the turbulence-chemistry and combustion regimes of propane/oxygen mixtures, highly diluted in nitrogen, at atmospheric pressure, in a cyclonic combustor under MILD Combustion operating conditions. The velocity and mixing fields were obtained using CFD with focus on mean and fluctuating quantities. The flow-field information helped differentiate between the impact of turbulence levels and dilution ones. The integral length scale along with the fluctuating velocity is critical to determine Damköhler and Karlovitz numbers. Together these numbers identify the combustion regime at which the combustor is operating. This information clearly distinguishes between conventional flames and distributed combustion. The results revealed that major controllers of the reaction regime are dilution and mixing levels; both are significantly impacted by lowering oxygen concentration through entrainment of hot reactive species from within the combustor, which is important in distributed combustion. Understanding the controlling factors of distributed regime is critical for the development and deployment of these novel combustion technologies for near zero emissions from high intensity combustors and energy savings using fossil and biofuels for sustainable energy conversion.
... This is a combustion regime characterized by fuel oxidation in an environment with relatively high dilution and preheating levels. Such operating conditions feature a process with a distributed reaction zone, relatively uniform temperatures within the combustion chamber, no visible flame, low noise, negligible soot formation and very low NO x and CO emissions [5,6]. In MILD combustion, the inlet temperature of the reactants is higher than the auto-ignition temperature of the mixture and, simultaneously, the maximum temperature increase due to oxidation reactions remains lower than the mixture auto-ignition temperature [7,8] because of high dilution levels. ...
The implementation of MILD combustion systems is limited by a lack of fundamental insight into such combustion regime and therefore novel tools are indispensable compared to traditional combustion systems. In this context CFD simulations for the prediction of the burner behaviour and for design and optimization appears essential for a successful introduction of such concept in some industries. Detailed chemistry has to be included in fluid-dynamics simulations in order to account for the strong turbulence-chemistry interaction in the MILD regime. An effective strategy to overcome this aspect is represented by tabulated chemistry techniques. In particular the implementation of Flamelet Generated Manifold with IML tabulation seems to be a promising tools for MILD systems and therefore high fidelity and comprehensive experimental data are needed for the assessment of such model. The present study is framed in this context and it investigates the characteristics of MILD Combustion in a Cyclonic lab-scale burner that operates with high level of internal recirculation degrees induced by a cyclonic fluid-dynamic pattern obtained by the geometrical configuration of the reactor and of the feeding system. Experimental tests were realized varying the mixture composition. Detailed measurements of local mean temperatures and concentrations of gas species at the stack for several operating conditions were used to validate the FGM model under such unconventional operating conditions. Results suggest that FGM with IML is a promising tool for modeling the complex flame structures of cyclonic MILD burner, with many aspects that need to be further investigated.
... A common approach to perform Rayleigh measurements is to use a 90 angle collection configuration. 71 In case of limited optical access, a picosecond lidar (light detection and ranging) concept can be adapted where the back-scattered Rayleigh signal 72-74 is collected. The short pulse duration is employed in picosecond lidar to spatially resolve the temperature distribution. ...
Gaining information of species, temperature, and velocity distributions in turbulent combustion and high-speed reactive flows is challenging, particularly for conducting measurements without influencing the experimental object itself. The use of optical and spectroscopic techniques, and in particular laser-based diagnostics, has shown outstanding abilities for performing non-intrusive in situ diagnostics. The development of instrumentation, such as robust lasers with high pulse energy, ultra-short pulse duration, and high repetition rate along with digitized cameras exhibiting high sensitivity, large dynamic range, and frame rates on the order of MHz, has opened up for temporally and spatially resolved volumetric measurements of extreme dynamics and complexities. The aim of this article is to present selected important laser-based techniques for gas-phase diagnostics focusing on their applications in combustion and aerospace engineering. Applicable laser-based techniques for investigations of turbulent flows and combustion such as planar laser-induced fluorescence, Raman and Rayleigh scattering, coherent anti-Stokes Raman scattering, laser-induced grating scattering, particle image velocimetry, laser Doppler anemometry, and tomographic imaging are reviewed and described with some background physics. In addition, demands on instrumentation are further discussed to give insight in the possibilities that are offered by laser flow diagnostics.
The current study investigates the influence of inlet air momentum and temperature for flameless characteristics. Three-dimensional computations were implemented by ANSYS FLUENT. A small-scale experimental combustor geometry was used for validation study. Turbulence modelling was performed by RNG k- ε model and the GRI-Eddy Dissipation Concept (GRI-EDC) model was used for the combustion modelling. The influence of inlet air velocity was investigated through using different air jet diameters at constant flow rate. Five different air jet diameters (4,6,8,10 and 12 mm) were studied. It was found that decreasing air jet diameter leads to an increase in the exhaust gas recirculation which reserve in mixture dilution (low O 2 content mixture) subsequently, the reaction rate decreases which support the flameless mode creation. Also, this recirculated gas participates in heating up of fresh reactants which make the mixture tends to complete combustion. The influence of combustion air preheating temperature on NO and CO emissions were studied. CO and NO concentration were decreased with reducing the temperature of inlet preheated combustion air.
MILD (moderate and intense low-oxygen dilution) combustion is a highly promising technology to deliver clean and efficient thermal energy. However, because of its unconventional reaction nature, the optimization of the MILD combustion in various industrial burners is still challenging, for which the design tool based on accurate and cost-effective numerical simulation is most desirable. To this end, the tabulated chemistry approach (TCA) is thoroughly assessed for the modeling of MILD combustion by simulations of the Adelaide Jet in Hot Coflow (JHC) burner. The sensitivities to the submodel accounting for the scalar micromixing and the canonical flame configurations (i.e., flamelet and PSR-based reactors), being relevant for MILD regime characterization in TCA, are studied. It is found that the scalar mixing enhanced through the dynamic adjustment of model parameter C s leads to an improved prediction, and the optimal value of C s = 8 is identified for the current flames. The proper parametrization of the detailed chemical structures in TCA is found to affect the accurate prediction of the MILD flame profiles, especially the mass fraction of minor species (e.g., OH, CO). Furthermore, the endothermic reaction path of O + C2H2 ⇒ CO + CH2 is indicated as the main contributing step to the disparities in the CO predictions. This implies that the multiple reaction regimes in complex MILD burning should be accounted for by the use of either flamelet or PSR structures, depending on the local microscale diffusion/chemistry competitions. Overall, the results highlight the influential role of multiscale mixing and its intercoupling with the finite-rate chemistry, the accurate determination of which is important for MILD combustion modeling.
In this chapter, the research dedicated to moderate or intense low-oxygen dilution (MILD) combustion (also called flameless combustion) that relied on direct numerical simulations (DNS) is summarized. In particular, the various DNS carried out are detailed and three different configurations are considered: the autoigniting mixing layer between fuel and hot and diluted oxidizer, the premixed MILD combustion resulting from internal exhaust gas recirculation, and the nonpremixed MILD combustion with internal exhaust gas recirculation. Focus is placed here on different aspects of MILD combustion. First, works that relate to the onset of MILD combustion and the apparition of the initial ignition kernels are discussed, in particular, a summary is provided on the findings that show the particular physics of MILD combustion, where the initial ignition kernels are mostly related to the distribution of mixture fraction and recirculating radicals. Subsequently, the identified physical mechanisms involved in the development of those ignition kernels are summarized. In particular, focus is placed on the balance between ignition and deflagrative mechanisms. Using different analysis methods, the works summarized here show that, while there is a coexistence between ignition and deflagration, ignition is the main contributor to the overall heat release. Finally, the implications of these findings on the modeling of MILD combustion are discussed through various studies that assessed a priori different modeling frameworks for MILD combustion. In those, models that capture this essential and dominant ignition behavior of MILD combustion were shown to be more accurate.
Burners are mechanical elements that warrant heat production from combustion by ensuring a mixture between a fuel (gaseous, liquid, or solid) and oxidizer (generally air, naturally containing oxygen) or injecting a premixed fuel-oxidizer mixture. Generally, burners consist of one or multiple injectors resistant to high temperatures so that the mixture is ignited as soon as it leaves injectors. The mixing process requires the best regulation such that combustion efficiency is maximum with low unburnt and pollutants. The ignition process can be operated directly (stove burner, water heater, boiler, oven, etc.), or indirectly, for example, to produce mechanical work in a heat engine. Numerous parameters can be used to classify burners, among them fluid flow regime, injection direction, reactants mixture, and combustion process. According to the injection process, burners can be classified into two categories: premixed and nonpremixed injection. In premixed burners, fuel and oxidizer are well mixed before injection and ignition, whereas, in the second category, fuel and oxidizer are injected separately and then mixed in the burner or combustion chamber before ignition. According to flow dynamics, burners can be operated in a laminar or turbulent regime; however, laminar burners are almost limited to research purposes. Nearly all combustion applications use turbulent burners. When the fuel and oxidizer (or their mixture) are injected in the same direction, the burner is said “coflow burner,” whereas when they are injected in opposite directions, the burner is classified as a “counterflow burner.” Premixed burners have the best efficiencies and less unburnt, and they also permit accurate temperature and emissions control. Despite all these advantages, premixed burners are not safe, as flashback can easily occur in these burners. On the other hand, nonpremixed burners are fully safe since reactants are separated before ignition. Here, we will deal with coflow and counterflow burners in the flameless combustion (FC) process. Coflow burners received more attention since they have simple geometries and can be operated easily with varying different parameters of interest. On the other hand, counterflow burners are generally laminar and are used in fundamental researches. In the following, several burners operated under FC and especially moderate or intense low-oxygen dilution (MILD) combustion will be described; furthermore, results obtained using these burners are summarized. Two main sections are dedicated to the coflow and counterflow burners. Every section presents different types of burners used by researchers to investigate characteristics of combustion occurring in this kind of burners.
A preheated premixed CH4/air jet flame was operated in an optically accessible rectangular combustion chamber in order to study the flame stabilization and flame expansion mechanisms. Due to an off-center position of the jet, a pronounced lateral recirculation zone had formed in the combustion chamber which stabilized the lifted flame by mixing burned and unburned gases in the shear layer. Simultaneous single shot measurements of the flow field by particle image velocimetry (PIV), of OH by planar laser induced fluorescence (PLIF) and of heat release by OH chemiluminescence imaging from two directions were performed at a frame rate of 5 kHz. The image sequences provided a detailed insight into the interaction between the flow field and the flame, particularly into the effects of the vortices that developed in the shear layer between the jet and the recirculation zone. Special interest was devoted to the role of autoignition and turbulent flame propagation near the flame base.
Towards achieving sustainable and decarbonized power, biomass to electricity is an attractive pathway. To that end, the humidified gas turbine cycle is a promising technology. Recirculated steam which contains low-grade heat can be used to replace part of the air flow. This immediately reduces power loss in the compressor and increases specific power output, benefiting higher electrical efficiency compared to dry cycles. With high steam content, wet combustion leads to the so-called flameless combustion (FC) or colorless distributed combustion (CDC) which is presently investigated using large eddy simulation and a detailed finite rate chemistry method. Further insight regarding the coherent structures is obtained. Proper orthogonal decomposition method is applied on both the velocity and the heat release field aiming to explore the in-depth dynamic of flow-flame interaction in a swirl burner. Our results are the first reporting two distinct sets of helical coherent structures. A higher frequency mode or structure at Strouhal number St ~ 0.7 is caused by the vortex shedding, and a lower frequency mode at St ~ 0.1 corresponds to the off-central motion of an intermittently occurring precessing vortex core (PVC). With high steam content (hence very distributed reaction regime), more frequent occurrence of the marginally stable PVC is observed. The wet flame local extinction is evidenced to be an important driver towards the promotion and suppression of the PVC structure. Compared to the very energetic flow-flame interaction in conventional flames, FC or CDC flames undergo less fluctuations.
Towards developing humidified gas turbines (HGT) capable of running at high electrical efficiencies and low emissions, wet/steam-diluted combustion in a premixed swirl burner is investigated using large eddy simulation and a partially stirred reactor method. Chemical explosive mode and extended combustion mode analyses are performed to promote the understanding of wet flame structures. The former identifies the key features of the wet methane oxidation processes, and the latter extends the flame regime classification method to describing the combustion status of fluid parcels using local properties. Three combustion regimes are extensively discussed: the swirl stabilized (SS), colorless distributed (CDC) and non-combustible. Using the combined analyses of the two approaches, it is found that compared to dry flames, wet flames present more fluid parcels defined in the practical CDC regime where local heat release is low and Damköhler number is smaller than unity. The wet fluid parcels are capable of self-igniting via radical explosion, while dry fluid parcels self-ignite via thermal runaway. The species CH2O and temperature are the first and second highest contributors towards the explosivity of dry flames, while temperature is insignificant to that of wet flames. The species C2H6 is found an important source to the self-ignitability of wet fluid parcels in the practical CDC regime due to the activation of the three-body ethane formation reaction R148: 2CH3+M=C2H6+M in the low O2% wet combustion environment. Proper use of proposed methods to quantify wet flame behavior guides stable and low emission operation of practical HGT.
Direct Numerical Simulations (DNS) data of Moderate or Intense Low-oxygen Dilution (MILD) combustion are analysed to identify the contributions of the autoignition and flame modes. This is performed using an extended Chemical Explosive Mode Analysis (CEMA) which accounts for diffusion effects allowing it to discriminate between deflagration and autoignition. This analysis indicates that in premixed MILD combustion conditions, the main combustion mode is ignition for all dilution and turbulence levels and for the two reactant temperature conditions considered. In non-premixed conditions, the preponderance of the ignition mode was observed to depend on the axial location and mixture fraction stratification. With a large mixture fraction lengthscale, ignition is more preponderant in the early part of the domain while the deflagrative mode increases further downstream. On the other hand, when the mixture fraction lengthscale is small, sequential autoignition is observed. Finally, the various combustion modes are observed to correlate strongly with mixture fraction where lean mixtures are more likely to autoignite while stoichiometric and rich mixtures are more likely to react as deflagrative structures.
Distributed combustion is a recent technology that has a feasibility to apply on the industrial scale. It provides relatively low combustion temperature with rather uniform distribution and very low emission (NOx). The two-stage distributed combustion system is established to reduce the complexity of operating in the industrial application. The operation range of this combustion system is governed by the equivalent ratio of the first and second-stages of combustion. This research is attempted to study the effects of equivalent ratio and to identify the operation range of the combustion system. There are two stages in this experiment: 1. To set up using a diesel combustor (first-stage) 2. To use an LPG distributed combustor (second-stage). Part of the aim of this project is to improve the efficiency of the single-stage diesel combustor. The air to fuel ratio of the first-stage combustor is controlled to provide the 5% excess oxygen for the second-stage combustor. The equivalent ratio of the second-stage is varied as 1, 0.9, 0.8, 0.7 and 0.6. The results have shown that stoichiometric distributed combustion yields the highest outlet temperature and first and second law efficiency. The temperature is decreased when reducing the equivalent ratio. Finally, this research also provides the formula that can be used to determine the appropriate operating range in the term of A/F for the two-stage distributed combustion system conveniently applied to the single-stage available in the industrial.
Flameless combustion, also called MILD combustion (Moderate or Intense Low Oxygen Dilution), is a technology that reduces NOx emissions and improves combustion efficiency. Appropriate turbulence-chemistry interaction models are needed to address this combustion regime via computational modelling. Following a similar analysis to that used in the Extended EDC model (E-EDC), the purpose of the present work is to develop and test a Novel Extended Eddy Dissipation Concept model (NE-EDC) to be better able to predict flameless combustion. In the E-EDC and NE-EDC models, in order to consider the influence of the dilution on the reaction rate and temperature, the coefficients are considered to be space dependent as a function of the local Reynolds and Damköhler numbers. A comparative study of four models is carried out: the E-EDC and NE-EDC models, the EDC model with specific, fixed values of the model coefficients optimized for the current application, and the Flamelet Generated Manifold (FGM) model with pure fuel and air as boundary conditions for flamelet generation. The models are validated using experimental data of the Delft Lab Scale furnace (9 kW) burning Natural Gas (T = 446 K) and preheated air (T = 886 K) injected via separate jets, at an overall equivalence ratio of 0.8. among the considered models, the NE-EDC results show the best agreement with experimental data, with a slight improvement over the E-EDC model and a significant improvement over the EDC model with tuned constant coefficients and the FGM model.
The present work quantifies the impact of fuel chemistry on burning modes using premixed dimethyl ether (DME), ethanol (EtOH) and methane flames in a back-to-burnt opposed jet configuration. The study considers equivalence ratios 0 ≤ Φ ≤ 1, resulting in a Damköhler (Da) number range 0.06 ≤ Da ≤ 5.1. Multi-scale turbulence (Re ≃ 19,550 and Ret ≃ 360) is generated by means of a cross fractal grid and kept constant along with the enthalpy of the hot combustion products (THCP = 1700 K) of the counterflow stream. The mean turbulent rate of strain exceeds the laminar extinction rate for all flames. Simultaneous Mie scattering, OH-PLIF and PIV are used to identify reactants, mixing, weakly reacting, strongly reacting and product fluids. The relative balance between conventional flame propagation and auto-ignition based combustion is highlighted using suitably defined Da numbers and a more rapid transition towards self-sustained (e.g. flamelet type) combustion is observed for DME. The strain rate distribution on the reactant fluid surface for methane remains similar to the (non-reactive) mixing layer (Φ=0), while DME and EtOH flames gradually detach from the stagnation plane with increasing Φ leading to stabilisation in regions with lower compressive rates of strain. The study further provides information on the conditions leading to burning mode transitions via (i) multi-fluid probabilities, (ii) structural flow field information and turbulence-flame interactions delineated by means of conditional (iii) velocity statistics and (iv) the rate of strain along fluid iso-contours.
Distributed combustion is a promising method to obtain more uniform thermal field inside a combustor and reduced ultra-low pollutant emission levels at a combustor outlet. In this way, oxygen concentration in the oxidizer is reduced, resulting in a lower reaction rate. This study aims at investigating thermal field distributions and pollutant levels of various biogas flames under distributed combustion conditions. Combustion characteristics of biogas flames have numerically been investigated by a commercial code on distributed combustion conditions in terms of fuel flexibility, diluent temperature, and diluent composition. k-Ɛ standard turbulence model, PDF/The Mixture Fraction combustion model and P-1 radiation model have been used during the predictions. The oxygen concentration in the oxidizer has been reduced to the oxygen concentration of 15% to investigate various types of biogas, different mixture temperature, and diluent composition. The results resulting from conventional conditions show that the predictions have been in good agreement with the existing measured temperature profiles in terms of values and trends. According to the further modellings obtained from distributed combustion conditions, it can be said that more uniform thermal field has been emerged inside the combustor as the oxygen concentration has been reduced. Therefore, it can be concluded that distributed combustion conditions have been achieved. It can also be determined that pollutant emission levels have been decreased to ultra-low levels as the oxygen concentration has been reduced in the oxidizer.
Direct Numerical Simulation (DNS) data of Moderate or Intense Low-oxygen Dilution (MILD) combustion are analysed to gather insights on autoignition and flame propagation in MILD combustion. Unlike in conventional combustion, the chemical reactions occur over a large portion of the computational domain. The presence of ignition and flame propagation and their coexistence are studied through spatial and statistical analyses of the convective, diffusive and chemical effects in the species transport equations. Autoignition is observed in regions with lean mixtures because of their low ignition delay times and these events propagate into richer mixtures either as a flame or ignition. This is found to be highly dependent on the mixture fraction length scale, ℓZ, and autoignition is favoured when ℓZ is small.
Nowadays, power demand is growing globally, and access to reliable, affordable energy is a critical issue. The International Energy Agency (IEA) reported that, by 2020, the global economy will grow by about 3.5% annually and the total population will rise by about one billion.
Femtosecond (fs)-laser electronic-excitation tagging velocimetry (FLEET) in a nitrogen flow field using a 267 nm laser was performed under the condition of fs-laser filamentation. The filamentous properties and their effects on velocity measurements were investigated and were compared with those of an 800 nm fs-laser. The results show that the required energy of the 267 nm laser pulse is as low as hundreds of μJ, and this is beneficial for reducing the potential perturbations to the flow flied. The filaments induced by the 267 nm laser are longer and thinner than are those induced by the 800 nm laser, which enlarges the velocity measurements region, and a precision of 1.3% was achieved.
This paper presents three-dimensional direct numerical simulations of lean premixed H2/air flames with equivalence ratios 0.4, 0.5 and 0.6, respectively. The initial Karlovitz number is around 2335 and the pressure is 20 atm, which is relevant to gas turbine conditions. The heat release in reaction zones under different equivalence ratios is examined statistically with the aim to extend our understanding of lean combustion under high-pressure conditions. With increasing equivalence ratio, the relative thickness of reaction zone (δf/δL) is increasing for both laminar and turbulent flames, but the extent of increase is reduced under high equivalence ratio. By examining the local structures of flame fronts, it is found that trenches and plateaus of local equivalence ratio are located on separate sides of the reaction zone edge. Due to the decreased Lewis number under high equivalence ratio, the trench ‘depth’ and plateau ‘height’ are reduced. For the flame under ultra-lean conditions, there are some spots with temperatures above adiabatic temperature. This is attributed to the high-fraction of radicals in these regions, which will promote heat release. Furthermore, the heat release rates of elementary reactions are investigated with the analysis of radical fractions and rate constants. When the mixture equivalence ratio varies, the local heat release is changed in different temperature windows due to the combined effects of radical fractions and reaction rate constants.
Innovative solutions in terms of energy efficiency and pollutant emissions abatement require the development of new combustion technologies. In particular, several solutions imply a process based on mixture dilution and preheating that can lead to very peculiar combustion regimes. New combustion concepts, such as MILD combustion, rely on local self-ignition mechanism due to the obtainment of burned gases/fresh reactants mixtures that lead to a process mainly stabilized by means of a distributed autoignition. Despite the very interesting features related to such concept, several modeling issues have to be properly investigated, in order to permit the development of MILD Combustion concepts through a computationally-driven design. The evaluation of characteristic times, on both the micro/macro scales, is strongly influenced by the emerging characteristics related to a strong coupling between mixing and kinetics times due to the high dilution levels of such technologies. In order to include detailed chemistry in CFD simulations, Flamelet Generated Manifold (FGM) seems to be a promising choice. Specifically, the aim of this work is to prove the reliability of the tabulated chemistry method with respect to a cyclonic burner that was used as a test-case for validation purposes. Reynolds averaged Navier-Stokes simulations were realized, and a chemistry tabulation approach was used to take into account detailed chemistry effects. Finally, an assessment of heat transfer mode was carried out by including both convection and radiation heat exchange in the modeling and by comparing experimental and numerical results with temperature and gas concentration measurements obtained in several locations of the experimental apparatus. The computational tool was able to catch in a satisfactory manner the main features of the combustion regime in the cyclonic burner and the validation was strongly improved when radiative heat transfer is included in the numerical model.
Large Eddy Simulation (LES) of a CH4/H2 diffusion Jet flame in Hot Coflow (JHC) is undertaken to find distinct behaviors of Moderate or Intense Low oxygen Dilution (MILD) Combustion (MILDC), High Temperature Combustion (HTC) and Traditional Combustion (TC). These three JHC flames are realized by using different coflow temperatures and oxygen mass fractions: (1) 1300K and 9% for MILDC; (2) 1300K and 30% for HTC; (3) 600K and 30% for TC. The modeling of LES combining the eddy dissipation concept (EDC) with a global four-step reaction mechanism is validated by the JHC measurements of Dally et al. (Proc. Combust. Inst. 2002, 29, 1147-1154). The instantaneous and time-averaged velocities, temperatures and species concentrations such as carbon monoxide (CO) are presented and compared for the three cases. It is demonstrated that the JHC flames of MILDC and HTC both develop from auto-ignition nearly immediately downstream of the nozzle exit, while the lift-off JHC flame of TC evolves from an induced-ignition with a significant delay. Manifestly, combustion reactions proceed gently in the MILDC case and highly aggressively in the TC case. In both MILDC and HTC cases, stable combustion ensues in the very near field. While most heat releases around the stoichiometric location and transfers away slowly, combustion species, e.g. CO, diffuse more rapidly across the jet flow. The JHC flame for TC behaves completely differently. With a wobbling flame base, large-scale flame oscillations enhance crosswise turbulent mixing and heat transfer. Consequently, high temperatures and high CO concentrations concurrently emerge across the central region in the mid field. Besides, local extinction and re-ignition appear to occur frequently in the TC and do not happen in the HTC and MILDC.
Flameless combustion is a recently developed combustion system, which provides zero emission product. This phenomenon requires auto-ignition by supplying high-temperature air with low oxygen concentration. The flame is vanished and colorless. Temperature of the flameless combustion is less than that of a conventional case, where NOx reactions can be well suppressed. To design a flameless combustor, the computational fluid dynamics (CFD) is employed. The designed air-and-fuel injection method can be applied with the turbulent and non-premixed models. Due to the fact that nature of turbulent non-premixed combustion is based on molecular randomness, inappropriate mesh type can lead to significant numerical errors. Therefore, this research aims to numerically investigate the effects of mesh type on flameless combustion characteristics, which is a primary step of design process. Different meshes, i.e. tetrahedral, hexagonal are selected. Boundary conditions are 5% of oxygen and 900 K of air-inlet temperature for the flameless combustion, and 21% of oxygen and 300 K of air-inlet temperature for the conventional case. The results are finally presented and discussed in terms of velocity streamlines, and contours of turbulent kinetic energy and viscosity, temperature, and combustion products.
The tremendous increase in energy demand due to increased population and rapid economics results in increased level of atmospheric pollutants and global warming. Th global shift to the use of renewable clean energies still has some restrictions in term of the availability of the advanced reliable technologies and the cost of application compared to conventional fossil fuels. Until we can have this full conversion to renewables, the development of novel techniques for clean combustion of fossil fuels is appreciated. Forced by the simultaneous increased pressure of strict emissions regulations and the target of limiting the global warming to 2 oC, gas turbine manufacturers developed novel combustion techniques for clean power production in gas turbines as per the present review study. These novel techniques depend either on the modification in the existing combustion system or developing novel burners for clean power production. In this review, different clean combustion techniques are presented including; flame type variability, burner design, and fuel and oxidizer flexibility. The combustion and emission characteristics of different flame types including; non-premixed/premixed, moderate or intense low-oxygen dilution (MILD) flameless combustion, colorless distributed combustion (CDC), and low-swirl injector (LSI) combustion flames are presented with their limitations for applications. Novel burner designs for clean burning in gas turbines are investigated in detail including; swirl stabilized, dry low NOx (DLN) and dry low emission (DLE), catalytic combustion, perforated plate, environmental vortex (EV), sequential environmental vortex (SEV), advanced environmental vortex (AEV), and lean direct injection (LDI) micromixer burners. As an effective technique to control combustion instabilities within the gas turbine combustor, fuel flexibility approach is studied considering mainly hydrogen-enrich combustion and the associated concerns about fuel variability technique are investigated. Oxidizer flexibility approach in gas turbines is also studied under premixed combustion mode considering lean premixed (LPM) air combustion and oxy-fuel combustion and both techniques are compared in terms of performance and emissions. Finally, the feasibility of the different clean combustion techniques is discussed along with the available market products utilizing such novel technologies.
The current study quantifies the probability of encountering up to five fluid states (reactants, combustion products, mixing fluid, fluids with low and high reactivity) in premixed turbulent DME flames as a function of the Damköhler number. The flames were aerodynamically stabilised in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Re≃ 18,400 and Ret > 370). The chemical timescale was varied via the mixture stoichiometry resulting in a wide range of Damköhler numbers (0.08 ≤ Da ≤ 5.6). The mean turbulent strain (≥ 3200 s ) exceeded the extinction strain rate of the corresponding laminar flames for all mixtures. Simultaneous Mie scattering, OH-PLIF and PIV were used to identify the fluid states and supporting computations show that the thermochemical state (e.g. OH and CH concentrations) at the twin flame extinction point correlates well with flames in the back-to-burnt geometry at the corresponding rate of heat release. For mixtures where the bulk strain (≃ 750 s ) was similar to (or less than) the extinction strain rate, fluids with low and high reactivity could accordingly be segregated by a threshold based on the OH concentration at the extinction point. A sensitivity analysis of the distribution between the fluid states was performed. The flow conditions were further analysed in terms of Damköhler and Karlovitz numbers. The study provides (i) the evolution of multi-fluid probability statistics as a function of the Damköhler number, including (ii) the flow direction across fluid interfaces and OH gradients, (iii) mean flow field statistics, (iv) conditional velocity statistics and (v) a tentative combustion regime classification.
Turbulent flames in the moderate or intense low oxygen dilution (MILD) combustion regime have previously exhibited less susceptibility to lift-off than conventional autoignitive flames in a jet-in-hot-coflow (JHC) burner. This has been demonstrated through laser-based diagnostics and examination of CH* chemiluminescence. New experimental observations are presented of turbulent flames of natural gas, ethylene and blends of the two fuels, in coflows with temperatures from 1250-1385 K and oxygen concentrations from 3-11% (vol./vol.). Zero- and one-dimensional simulations, as well as turbulent flame modelling, are used to explain the trends seen experimentally with different coflows and fuels. Numerical simulations using simplified batch reactors and opposed-flow flames demonstrate that blending of methane and ethylene fuels is most significant near 1100 K. Near this temperature, pure ethylene exhibits a transition between high and low temperature ignition pathways. Further analyses show that a 1:1 methane/ethylene blend behaves more like ethylene near MILD combustion conditions, and more like methane in conventional autoignition conditions. Two-dimensional modelling results of the turbulent flames are then discussed and explained in the context of the simplified reactor results. The flames confined by the lean flammability-limit in the coflow and high strain-rates in jet shear layer, in agreement with previous work using a semi-empirical jet model. The two-dimensional modelling is additionally able to qualitatively replicate the trends in lift-off height, with normalised heat release rate profiles reproducing, and serving to explain, the effects seen in experimental campaigns.
Flameless combustion is an attractive solution to address existing problems of emissions and stability when operating gas turbine combustors. Theoretical, numerical and experimental approaches were used to study the flameless gas turbine combustor. The emissions and combustion stability were measured and the limits of the flameless regime are discussed. Using experimental techniques and Large Eddy Simulation (LES), detailed knowledge of the flow field and the oxidation dynamics was obtained. In particular the relation between the turbulent coherent structures dynamics and the flameless oxidation was highlighted. A model for flameless combustion simulations including detailed chemistry was derived. The theoretical analysis of the flameless combustion provides 2 non-dimensional numbers that define the range of the flameless mode. It was determined that the mixture that is ignited and burnt is composed of 50% of fresh gases and 50% vitiated gases.
A constrained optimization type of numerical algorithm for removing noise from images is presented. The total variation of the image is minimized subject to constraints involving the statistics of the noise. The constraints are imposed using Lanrange multipliers. The solution is obtained using the gradient-projection method. This amounts to solving a time dependent partial differential equation on a manifold determined by the constraints. As t → ∞ the solution converges to a steady state which is the denoised image. The numerical algorithm is simple and relatively fast. The results appear to be state-of-the-art for very noisy images. The method is noninvasive, yielding sharp edges in the image. The technique could be interpreted as a first step of moving each level set of the image normal to itself with velocity equal to the curvature of the level set divided by the magnitude of the gradient of the image, and a second step which projects the image back onto the constraint set.
Rayleigh scattering cross sections are measured for nine combustion species (Ar, N2, O2, CO2, CO, H2, H2O, CH4, and C3H8) at wavelengths of 266, 355, and 532 nm and at temperatures ranging from 295 to 1525 K. Experimental results show that, as laser wavelengths become shorter, polarization effects become important and the depolarization ratio of the combustion species must be accounted for in the calculation of the Rayleigh scattering cross section. Temperature effects on the scattering cross section are also measured. Only a small temperature dependence is measured for cross sections at 355 nm, resulting in a 2-8% increase in cross section at temperatures of 1500 K. This temperature dependence increases slightly for measurements at 266 nm, resulting in a 5-11% increase in cross sections at temperatures of 1450 K.
Recent developments in heat recovery systems allow for preheating of combustion air up to temperatures of 1300°C and, thus, fuel savings up to 60% are achievable. In conventional burner/furnace designs, the higher the combustion air temperature the higher the NOx emissions. However, the most recent developments allow for low NOx combustion using high temperature combustion air. The objective of this paper is to establish conditions under which industrial furnaces should be operated in order to maximize the efficiency and minimize the pollutant emissions including carbon dioxide. To this end, semi-industrial scale experiments have been carried out using natural gas and vitiated air at 1300°C. A Nippon Furnace Kogyo burner that features a central air jet and two fuel gas injectors was used. Comprehensive in-furnace measurements of velocities, temperature, gas composition (O2, CO2, CO, H2, NO, CH4) and radiation have been carried out. The furnace was operated under conditions resembling a well-stirred reactor; the temperature and chemistry fields were uniform all over the furnace. Almost the whole furnace volume was filled with combustion products containing 2-3% oxygen at temperatures in the range 1350-1450°C, despite the high temperature (1300°C) of the vitiated air. The natural gas jets entrained many of the combustion products before they mixed with combustion air. This mode of combustion resulted in high and uniform heat fluxes and low NOx and CO emissions. It was concluded that industrial furnaces of tomorrow are likely to be designed as well-stirred reactors equipped with high efficiency heat regenerators. Conventional burners will be either replaced with individual fuel and air injectors or substantially redesigned to facilitate uniformity of combustion conditions within the furnace.
This paper details a quantitative joint temperature, OH, and CH{sub 2}O imaging experiment designed to investigate the stabilization of lifted turbulent methane flames issuing into a high temperature vitiated coflow. Temperature is determined through Rayleigh imaging, and the data are used to quantify OH-LIF excited at 283.011 nm, and to enable to semi-quantification of CH{sub 2}O-LIF excited at 355 nm. A fuel with Rayleigh cross-section equal to that of the vitiated coflow was used to improve accuracy in the processing of the Rayleigh temperature. Results of the experiment have been presented, and compared to simulations of laminar transient autoignition flamelets. The images were classified in three main categories: (i) CH{sub 2}O only, (ii) ignition kernels, and (iii) liftoff flames. Images of type (i) and (ii) were dominant in the early part of the jet, while images of type (iii) were dominant after the mean stabilization height. By examining OH and CH{sub 2}O conditional on the size of the kernel, it was found that the sequence of conditional data was analogous to the evolution of autoignition, following the key stages of (1) build-up of a precursor pool, (2) initiation of reaction, and (3) formation of a steady flame. Viewed in such a sequence, CH{sub 2}O peaks prior to the autoignition and then decays after ignition, and OH is found to peak at ignition and these peaks are maintained into the established steady flames. This is in qualitative agreement with the laminar transient flamelet calculations. The data are consistent with the view that autoignition is the main stabilization mechanism in this lifted flame. (author)
A nonpremixed lifted jet flame is studied dynamically in the hysteresis zone. High-speed laser tomography images show clearly that, in the case of an organized jet, the flame is located on streamwise counter-rotating vortex filaments generated to secondary instabilities and ejected towards ambient air. Particle image velocimetry is used to evaluate the amplitude of the translational and rotational velocity of these filaments. The use of an acoustic field to force jet instabilities shows that the flame, following large filament ejections, moves back upstream very close to the nozzle without anchoring at it. The role of streamwise vortices in the stabilization mechanism of the lifted flame is confirmed by measurements obtained with a disordered jet, from a straight tube burner. From these results, it is proposed that secondary vortices at the flame base are sufficiently strong to create a premixed zone and to oppose flame propagation.
Results are presented on the thermal and chemical characteristics or flames using high-temperature combustion air and liquified petroleum gas (LPG) as the fuel. The stability limits of these flames are extremely wide as compared to any other method of flame stabilization. This study is part of the Japan national project directed to develop advanced industrial furnace designs that provide approximately 30% energy savings and hence CO2 reduction, 30% reduction in the furnace size, and 25% reduction of pollutants including NOx as compared to current designs. The objective here is to establish conditions that permit significant reduction in energy consumption, high efficiency, and low pollution from a range of furnaces. Data have been obtained on mean flame temperature and temperature fluctuations, flame emission spectra, emission intensity Of C-2 and CH species from within the flames, and overall pollutant emission from the flames. The uniformity of temperature in the furnace was found to be far greater with low oxygen concentration combustion air preheated to 1000degreesC as compared to that obtained with room-temperature air or that found in conventional flames. Emission of NOx and CO was much lower with combustion air preheated to high temperatures with low oxygen concentration. The chemiluminescence intensity of CH and C2 radicals is significantly affected by the preheat temperature of the combustion air and oxygen concentration in the oxidant. The flame signatures revealed important flame characteristics under high-temperature air combustion conditions. The advantages of utilizing highly preheated combustion air (in excess of 1000degreesC) in various types of furnaces are given. The new and advanced furnace design utilizes high-efficiency regenerators and behaves essentially as a well-stirred reactor with uniform thermal and chemical characteristics. Because each furnace design requires unique features, it is imperative that each furnace must be optimized to satisfy the functional requirements of the furnace. In this paper a relatively simple diagnostic methodology is presented, which assists in a rational furnace design and optimization process.
The controlling processes in the operation of a non-premixed, stagnation point reverse flow (SPRF) combustor are investigated. The combustor consists of a central injector at the single open end of a cylindrical chamber. The injector inlet area is much less than the area through which exhaust products leave. The SPRF combustor operates stably at low equivalence ratios without external preheating or swirl, and produces low NOx emissions in both premixed and non-premixed modes of operation. Non-intrusive imaging diagnostics are used to understand the combustor operation. Simultaneous Planar Laser-Induced Fluorescence (PLIF) imaging of OH radicals and chemiluminescence imaging are used to characterize the reaction and heat release zone and the flame products. Separate measurements with Particle Image Velocimetry (PIV) provide the reacting velocity field, and elastic laser sheet scattering from liquid droplets seeded into the fuel characterize its mixing. The velocity and chemiluminescence data indicate the flame is stabilized in a region of low mean and high rms velocity. Together with some product entrainment, this enables stable operation of the combustor at very lean overall equivalence ratios. The non-premixed mode of operation is found to be similar to the premixed case in many ways. Though, in non-premixed operation, the flame is lifted well away from the injector, so that significant air and fuel premixing occurs before combustion. Similar NOx emission for both operating modes is attributed to efficient mixing of nearly all the fuel and air before burning. This is confirmed through a combination of oil droplet results, OH PLIF comparisons and laminar flame modeling. The latter indicates entrainment of products is not directly responsible for low NOx emissions at a given overall fuel–air ratio, but rather is likely to contribute to a slight increase in NOx levels compared to a system with no product entrainment.
An experimental and numerical investigation is presented of a lifted turbulent H2/N2 jet flame in acollow of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the complexity of recirculating flow. Scalar measurements are reported for a lifted turbulent jet flame of H2/N2 (Re=23,600, H/d=10) in a coflow of hot combustion products from a lean H2/Air flame (=0.25, T=1045 K). The combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow throughout the entire test region. Two combustion models (joint scalar probability density function and eddy dissipation concept) are used in conjunction with various turbulence models to predict the liftoff height (HPDF/d=7, HEDC/d=8.5). Kalghatgi's classic phenomenological theory, which is based on scaling arguments, yields a reasonbly accurate prediction (HK/d=11.4) of the liftoff height for the present flame. The vitiated coflow admits the possibility of autoignition of mixed fluid, and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility of turbulent premixed flame propagation by small-scale (on the order of the flame thickness) recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.
The design, theory and characteristics of combustors in which the reactants (or the combustion air alone) are preheated using heat recycled from beyond the flame zone, without mixing the two streams, are reviewed. There is a great variety of such systems, based on a combustor in between the two limbs of a heat exchanger, ranging from the so called “self-recuperative” burners which save a substantial proportion of fuel when used to replace conventional burners in heating up furnaces to a given temperature, over beds of particulates and “filtration” combustion, to systems able to burn mixtures normally considered incombustible, which are currently used mostly for incineration. A review of their theory shows that such devices have the potential for very high efficiencies. Their recent application to radiant burners, I.C. engines, and to the pollution-free combustion of lean hydrogen/air mixtures is surveyed.
Combustion characteristics of two different gaseous fuels (a low calorific value fuel and methane fuel) have been examined using high temperature and low oxygen concentration combustion air. The momentum flux ratio between the fuel jet and the combustion airflow was kept constant to provide similarity in mixing between the different experimental cases to understand the role of fuel jet property on combustion. Direct flame photography, 2-D Particle image velocimetry (PIV), Light Emission Spectroscopy and chemiluminescent NOX analyzer was used as the diagnostics. These diagnostics allowed information on global flame features, mean and rms components of axial and radial velocity, axial strain rates and vorticity, the spatial distribution of combustion intermediate species, such as, OH and CH, and overall NOX emission levels. The results indicate a slower mixing during high temperature air combustion with low calorific value fuel as compared to methane fuel. The results showed higher turbulence levels and higher axial strain rates for low calorific fuel jets as compared to methane fuel jet during the high temperature air combustion condition. This results in less intense (or mild) combustion conditions with the result of increased flame length and volume and lower NOX emissions. Even for the normal methane fuel high temperature and oxygen deficient combustion conditions provided lower NOX emission. Furthermore, the high temperatures obtained for methane combustion provided lower vorticity and axial strain rates than the low calorific value fuel due to the suppression of vortical structure formation from the stronger heat release. In the case of low calorific value fuel, higher fuel jet velocity into low-density high temperature air leads to longer jet length. This jet causes a local stagnation to the upstream cross-flow to create local higher value of turbulence levels immediately upstream of the jet. The spatial distribution of the flame generated radicals (OH and CH) revealed significant ignition delay of the LCV fuel jet and a far more uniform distribution of the intermediate species. The methane fuel jet showed a prolonged reaction zone and faster ignition at high temperature and oxygen deficient conditions when compared to normal temperature air combustion of methane.
The study of mild combustion mode for both non-premixed and premixed mixture preparation was presented. The aerodynamic, chemical and thermal aspects of the mild combustion process were also studied in a combustor. The mixing rates, flue gas recirculation and strong shear produced by reactants from discrete jets were emphasized for the study. The results revealed that for the initiation and progress of the reaction the entertainment of flue gases in the fresh mixture was very important. It was also shown that the combustion region shifted away from the burner and extended further downstream with increase in equivalence ratio of nonpremixed mixture.
Detailed scalar structure measurements of highly sheared turbulent premixed flames stabilized on the piloted premixed jet
burner (PPJB) are reported together with corresponding numerical calculations using a particle based probability density function
(PDF) method. The PPJB is capable of stabilizing highly turbulent premixed jet flames through the use of a small stoichiometric
pilot that ensures initial ignition of the jet and a large shielding coflow of hot combustion products. Four lean premixed
methane-air flames with a constant jet equivalence ratio are studied over a wide range of jet velocities. The scalar structure
of the flames are examined through high resolution imaging of temperature and OH mole fraction, whilst the reaction rate structure
is examined using simultaneous imaging of temperature and mole fractions of OH and CH2O. Measurements of temperature and mole fractions of CO and OH using the Raman–Rayleigh–LIF-crossed plane OH technique are
used to examine the flame thickening and flame reaction rates. It is found that as the shear rates increase, finite-rate chemistry
effects manifest through a gradual decrease in reactedness, rather than the abrupt localized extinction observed in non-premixed
flames when approaching blow-off. This gradual decrease in reactedness is accompanied by a broadening in the reaction zone
which is consistent with the view that turbulence structures become embedded within the instantaneous flame front. Numerical
predictions using a particle-based PDF model are shown to be able to predict the measured flames with significant finite-rate
chemistry effects, albeit with the use of a modified mixing frequency.
KeywordsTurbulent premixed flames-Finite-rate chemistry-Distributed reaction regime-Flame front thickening-Reaction rate measurements
The spatial distributions of the hydroxyl radical (OH), formaldehyde (H2CO), and temperature imaged by laser diagnostic techniques are presented using a Jet in Hot Coflow (JHC) burner. The measurements are of turbulent nonpremixed ethylene jet flames, either undiluted or diluted with hydrogen (H2), air or nitrogen (N2). The fuel jet issues into a hot and highly diluted coflow at two O2 levels and a fixed temperature of 1100 K. These conditions emulate those of moderate or intense low oxygen dilution (MILD) combustion. Ethylene is an important species in the oxidation of higher-order hydrocarbon fuels and in the formation of soot. Under the influence of the hot and diluted coflow, soot is seen to be suppressed. At downstream locations, surrounding air is entrained which results in increases in reaction rates and a spatial mismatch between the OH and H2CO surfaces. In a very low O2 coflow, a faint outline of the reaction zone is seen to extend to the jet exit plane, whereas at a higher coflow O2 level, the flames visually appear lifted. In the flames that appear lifted, a continuous OH surface is identified that extends to the jet exit. At the “lift-off” height a transition from weak to strong OH is observed, analogous to a lifted flame. H2CO is also seen upstream of the transition point, providing further evidence of the occurrence of preignition reactions in the apparent lifted region of these flames. The unique characteristics of these particular cases has led to the term transitional flame.
The stabilisation region of turbulent non-premixed flames of natural gas mixtures burning in a hot and diluted coflow is studied by recording the flame luminescence with an intensified high-speed camera. The flame base is found to behave fundamentally differently from that of a conventional lifted jet flame in a cold air coflow. Whereas the latter flame has a sharp interface that moves up and down, ignition kernels are continuously being formed in the jet-in-hot-coflow flames, growing in size while being convected downstream. To study the lift-off height effectively given these highly variable flame structures, a new definition of lift-off height is introduced. An important parameter determining lift-off height is the mean ignition frequency density in the flame stabilisation region. An increase in coflow temperature and the addition of small quantities of higher alkanes both increase ignition frequencies, and decrease the distance between the jet exit and the location where the first ignition kernels appear. Both mechanisms lower the lift-off height. An increase in jet Reynolds number initially leads to a significant decrease of the location where ignition first occurs. Higher jet Reynolds numbers (above 5000) do not strongly alter the location of first ignition but hamper the growth of flame pockets and reduce ignition frequencies in flames with lower coflow temperatures, leading to larger lift-off heights.
Moderate and intense low oxygen dilution combustion is a newly implemented and developed concept to achieve high thermal efficiency and fuel savings while maintaining emission of pollutants at very low levels. It utilizes the concept of heat and exhaust gas recirculation to achieve combustion at a reduced temperature, a flat thermal field, and low turbulence fluctuations. An experimental burner is used in this study to simulate the heat and exhaust gas recirculation applied to a simple jet in a hot coflow. Temporally and spatially resolved measurements of reactive scalars are conducted on three different turbulent nonpremixed flames of a H2/CH4 fuel mixture at a fixed-jet Reynolds number, and different oxygen levels in the hot oxidant stream. The data were collected using the single-point Raman-Rayleigh-laser-induced fluorescence technique. The results show substantial variation in the flame structure and appearance with the decrease of the oxygen level. By reducing the oxygen level in the hot coflow, the flame becomes less luminous, the temperature increase in the reaction zone can get as low as 100 K, and the levels of CO and OH are substantially lowered. The levels of NO also decrease with decreasing the oxygen levels and at 3% by mass, it is less that 5 ppm. For this case, a widely distributed NO profile is found which is not consistent with profiles for other oxygen levels.
This study describes the performance and stability characteristics of a parallel jet MILD (Moderate or Intense Low-oxygen Dilution) combustion burner system in a laboratory-scale furnace, in which the reactants and exhaust ports are all mounted on the same wall. Thermal field measurements are presented for cases with and without combustion air preheat, in addition to global temperature and emission measurements for a range of equivalence ratio, heat extraction, air preheat and fuel dilution levels. The present furnace/burner configuration proved to operate without the need for external air preheating, and achieved a high degree of temperature uniformity. Based on an analysis of the temperature distribution and emissions, PSR model predictions, and equilibrium calculations, the CO formation was found to be related to the mixing patterns and furnace temperature rather than reaction quenching by the heat exchanger. The critical equivalence ratio, or excess air level, which maintains low CO emissions is reported for different heat exchanger positions, and an optimum operating condition is identified. Results of CO and NOx emissions, together with visual observations and a simplified two-dimensional analysis of the furnace aerodynamics, demonstrate that fuel jet momentum controls the stability of this multiple jet system. A stability diagram showing the threshold for stable operation is reported, which is not explained by previous stability criteria.
This paper describes the initial characterization of a piloted premixed jet burner (PPJB) designed to investigate finite-rate chemistry effects in highly turbulent lean premixed combustion. The PPJB consists of a high-velocity lean premixed central jet, piloted by a low-velocity stoichiometric premixed pilot, surrounded by a large-diameter coflow of lean premixed hydrogen–air combustion products. The configuration of a lean central jet supported by a stoichiometric pilot is similar to that of a lean premixed gas turbine combustor, but without additional complications such as swirl, recirculation, and complex boundary conditions. A significant feature of the PPJB is that under certain conditions the central jet combustion process appears to undergo an extinction–reignition process. It is considered likely that intense turbulent mixing after the nozzle drives an initial extinction process that reduces flame luminosity, with reignition occurring downstream where turbulent mixing has decreased, causing an increase in flame luminosity. Four flames are selected for further study, each with an equivalence ratio of 0.5 and with central jet velocities of 50, 100, 150, and 200 m/s. Simultaneous two-dimensional (2D) Rayleigh–OH planar laser-induced fluorescence (PLIF) imaging results are presented for the selected flames, showing that in the “extinction” region OH concentrations occur at reduced levels in isolated patchy regions, supporting the idea that extinction is predominantly occurring. Laser Doppler velocimetry (LDV) data are also reported for the flow field turbulence statistics, with the most significant result being that for the reacting cases the pilot delays the occurrence of peak turbulence intensity downstream to near the observed “extinction” region.
The Mild Combustion is characterized by both an elevated temperature of reactants and low temperature increase in the combustion process. These features are the results of several technological demands coming from different application fields. This review paper aims to collect information which could be useful in understanding the fundamentals and applications of Mild Combustion. The information in this field are still sparse, because of the recent identification of the process, so that many speculative considerations have been presented in order to make the whole framework more consistent and rich with potential new applications.
The development of the basic conceptual viewpoints, or paradigms, for turbulent combustion in gases over the last 50 years is reviewed. Significant progress has been made. Recent successes in the prediction of pollutant species and extinction/re-ignition phenomena in non-premixed flames are seen as the result of close interaction between experimentalists, theoreticians, and modellers. Premixed turbulent flames seem to be dependent on a much wider range of factors, and predictive capabilities are not so advanced. Implications for large eddy simulation (LES) and partially premixed combustion are outlined.
Recent advances in heat-recirculating combustion in industrial furnaces, particularly of the alternating flow type, are reviewed. A large amount of waste heat can be recovered by this type of system. Highly preheated combustion air, typically above 1300 K, is easily obtained due to advanced design and metarials employed. Although preheated air combustion generally produces high nitric oxide emissions, it has been used to generate high-temperature flames for some special applications. The energy saving achieved simultaneously by heat recirculation has become more attractive, from an ecological point of view. However, to enjoy the energy saving brought by a high rate of heat recirculation by applying highly preheated air combustion to generic industrial furnances, a reduction of nitric oxide emission is required. The possibility of low nitric oxide emission from highly preheated air combustion is intensively discussed. Dilution of the air with burned gases and combustion occurring in air with low oxygen concentration are shown to be indispensable factors in realizing low nitric oxide emissions. This has led to advanced furnance technology.
The common misconception that hydrogen flames are not visible is examined. Examples are presented of clearly visible emissions from typical hydrogen flames. It is shown that while visible emissions from these flames are considerably weaker than those from comparable hydrocarbon flames, they are indeed visible, albeit at reduced light levels in most cases. Detailed flame spectra are presented to characterize flame emission bands in the ultraviolet, visible and infrared regions of the spectrum that result in a visible hydrogen flame. The visible blue emission is emphasized, and recorded spectra indicate that fine spectral structure is superimposed on a broadband continuum extending from the ultraviolet into the visible region. Tests were performed to show that this emission does not arise from carbon or nitrogen chemistry resulting from carbon-containing impurities (hydrocarbons) in the hydrogen fuel or from CO2 or N2 entrainment from the surrounding air. The spectral structure, however, is also observed in methane flames. The magnitude of the broadband emission increases with flame temperature in a highly nonlinear manner while the finer spectral structure is insensitive to temperature. A comparison of diffusion and premixed H2 flames shows that the fine scale structure is comparable in both flames.
This report will present a special form of combustion, called flameless oxidation. In contrast to the combustion within stabilized flames, temperature peaks can be avoided at flameless oxidation. For that reason, the thermal NO-formation is largely suppressed, even at very high air preheat temperatures. A brief summary of the present NOx-reducing techniques will be given. The illustration of flameless oxidation will cover the explanation of the basic principle, the presentation of calculated and measured data and the introduction of some application examples. The results are encouraging the assumption that NO-emissions from a wide range of combustion sources could be largely eliminated in the future. Use of burners, operated in flameless oxidation mode in continuous industrial furnaces have proven to be reliable and well accepted for the very uniform product quality by furnace people.
All turbulent cases data sampled at y/d = 12. Top: joint probability density function (PDF) of OH and CH 2 O PLIF signal peaks' width (h) Bottom: PDF of the OH signal at the maximums of RMS as seen in Fig
- Fig
Fig. 11. All turbulent cases data sampled at y/d = 12. Top: joint probability density function (PDF) of OH and CH 2 O PLIF signal peaks' width (h). Bottom: PDF of the OH signal at
the maximums of RMS as seen in Fig. 6.
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