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Simultaneous Rayleigh temperature, OH- and CH2O-LIF imaging of methane jets in a vitiated coflow
Abstract
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)
... Fundamental studies of these flames have not only provided significant insight into autoignition processes, but have been used to generate extensive datasets in simplified configurations for validating turbulence-chemistry interaction models for numerical modeling of combustion systems. Fundamental studies of laminar and turbulent jet flames issuing into high temperature, low oxygen environments have been undertaken in jet in hot coflow (JHC) burners (Dally et al., 2002;Medwell et al., 2007Medwell et al., , 2008Oldenhof et al., 2010Oldenhof et al., , 2011Oldenhof et al., 2012;Medwell and Dally, 2012a;Sepman et al., 2013;Ye et al., 2016Ye et al., , 2017Ye et al., , 2018Evans et al., 2017bEvans et al., , 2019aKruse et al., 2019), hot cross-flow burners (Sidey and Mastorakos, 2017), vitiated coflow burners (VCBs) (Cabra et al., 2002(Cabra et al., , 2005Gordon et al., 2008Gordon et al., , 2009Macfarlane et al., 2018Macfarlane et al., , 2019Ramachandran et al., 2019), and partially premixed jet burners (PPJBs) (Dunn et al., 2007a;Dunn et al., 2009), as have spray flames in hot coflow burners (Correia Rodrigues et al., 2015a,b;Wang et al., 2019b). In each case, fresh fuel issues from a jet into a stream of hot gas generated by lean premixed flames, resulting in 15% O 2 (by vol.). ...
... Such improved understanding these transitions-and the structure of the upstream flames leading to their formation-may be achieved through targeted laser-diagnostics studies and subsequently validated, complementary numerical modeling. The high fidelity data that laser diagnostics can provide allows for the detailed study of reactive structures across the broad range of spatial and temporal scales in different optically-accessible JHC burners and reactors (Plessing et al., 1998;Cabra et al., 2002Cabra et al., , 2005Dally et al., 2002;Medwell et al., 2007Medwell et al., , 2008Gordon et al., 2008Gordon et al., , 2009Oldenhof et al., 2010Oldenhof et al., , 2011Oldenhof et al., 2012;Medwell and Dally, 2012a;Sepman et al., 2013;Sorrentino et al., 2015Sorrentino et al., , 2016Ye et al., 2016Ye et al., , 2017Ye et al., , 2018Evans et al., 2017bEvans et al., , 2019aSidey and Mastorakos, 2017;Macfarlane et al., 2018Macfarlane et al., , 2019Kruse et al., 2019;Ramachandran et al., 2019). Laser diagnostics a provide means to investigate the small-scale and distributed reaction zones in macroscopically-near-homogeneous MILD combustion conditions. ...
... An important consideration beyond laser wavelength and pulse energy is the intensity profile emanating from the laser. These profiles may be approximated as Gaussian, triangular, or "top-hats" (Gordon et al., 2008;, but are often significantly more complex due to imperfections in mirrors and sheet-forming-optics, and vary between individual laser pulses (i.e., shot-to-shot). Variations in beam profile may also be caused by variations in refractive index, inherent in flames, which result in refraction of the beam termed "beamsteering" (Kruse et al., 2018). ...
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.
... Therefore, in the last few decades, numerous experimental and numerical tools have been developed to measure the flow properties in reacting and non-reacting flows. Several non-intrusive laser diagnostic techniques have been developed for the multi-dimensional measurement of temperature [1][2][3][4][5][6][7], species distribution [7,8], mixture fraction [2][3][4]6,7], and velocity [9,10]. In particular, for temperature and major species concentration measurement, Rayleigh/Raman scattering [2][3][4]7] proved to be an effective and reliable tool. ...
... The relative quenching rate (q 0, perturber /q 0, N 2 ) is computed using the Kr PLIF signal (S Kr ), used krypton mole fraction (χ Kr ), and overlap integral (G) according to Eq. (8). The Kr PLIF spectral profile (Voigt fits) for various perturber species, namely, N 2 , air, CH 4 , CO 2 , and C 2 H 4 corrected for the used Kr mole fraction (χ Kr ), is shown in Fig. 5. ...
Quenching rate is an important parameter to include in fluorescence measurements that are aimed at quantifying the thermochemical field of a reacting flow. Traditionally, the quenching measurements were obtained at low pressures using the direct measurements of quenching times followed by a linear scaling to the desired pressure. This approach, however, cannot account for the possible deviation from the linear pressure scaling at elevated pressures due to three and multi-body collisions. Furthermore, the best accuracy on the quenching rate is obtained with ultra-short pulse lasers that are typically not readily available. This study offsets these limitations by demonstrating a new approach for making direct quenching measurements at atmospheric conditions and using nanosecond lasers. The quenching measurements are demonstrated in a krypton-perturber system, and the $5p{\Big[\frac{3}{2}\Big]}_2 \leftarrow \leftarrow 4{p^6}{\,^1}{S_0}$ 5 p [ 3 2 ] 2 ←← 4 p 6 1 S 0 two-photon electronic transition is accessed. A theoretical construct is presented that relates the absorption spectral parameters and the integrated fluorescence signal to the quenching rate, referenced to a given species and conditions. Using this formulation, the relative quenching rates for different perturber species, namely, air, ${{\rm CH}_4}$ C H 4 , ${{\rm C}_2}{{\rm H}_4}$ C 2 H 4 , and ${{\rm CO}_2}$ C O 2 , are reported as measured at 1 atm and 300 K. As such, the present technique is limited to the measurement of the relative quenching rate, unlike the previous studies where absolute quenching rates are measured. Nonetheless, when the reference quenching rate is independently measured, the relative quenching rates can be converted to absolute values.
... In general, autoignition and flame propagation are two different mechanisms that can stabilize flames in high-temperature flows. Lifted flames have been largely investigated for various configurations experimentally [28][29][30][31][32] and numerically [11,28,[33][34][35][36][37][38][39] . The main anchoring mechanism is sometimes attributed to autoignition (e.g. ...
... The main anchoring mechanism is sometimes attributed to autoignition (e.g. [28][29][30][32][33][34][35]39] ), in other situations to flame propagation (e.g. [36,37] ), to a mix between the two depending on the flame branch (e.g. ...
This numerical study investigates the combustion modes in the second stage of a sequential combustor at atmospheric and high pressure. The sequential burner (SB) features a mixing section with fuel injection into a hot vitiated crossflow. Depending on the dominant combustion mode, a recirculation zone assists flame anchoring in the combustion chamber. The flame is located sufficiently downstream of the injector resulting in partially-premixed conditions. First, combustion regime maps are obtained from 0-D and 1-D simulations showing the coexistence of three combustion modes: autoignition, flame propagation and flame propagation assisted by autoignition. These regime maps can be used to understand the combustion modes at play in turbulent sequential combustors, as shown with 3-D large eddy simulations (LES) with semi-detailed chemistry. In addition to the simulation of steady-state combustion at three different operating conditions, transient simulations are performed: (i) ignition of the combustor with autoignition as the dominant mode, (ii) ignition that is initiated by autoignition and that is followed by a transition to a propagation stabilized flame, and (iii) a transient change of the inlet temperature (decrease by 150 K) resulting into a change of the combustion regime. These results show the importance of the recirculation zone for the ignition and the anchoring of a propagating type flame. On the contrary, the autoignition flame stabilizes due to continuous self-ignition of the mixture and the recirculation zone does not play an important role for the flame anchoring. These findings are important for the design and operation of practical sequential combustion systems.
... The lifted turbulent jet flames in a vitiated coflow have been analyzed both experimentally and numerically by many researchers. Gordon et al. [58] gathered an experimental repository of data including temperature, OH and CH 2 O mass fractions of this flame. The turbulence structure [59], autoignition [60], flame stabilization [61], flame structure [62], swirls dynamics, visualization [63], the micro-scales structures and the turbulence intensity distribution in the flame [64], sensitivity of preflame zone [65], ignition/extinction [66] of lifted H 2 / N 2 jet flame issuing into vitiated coflow have also been extensively studied during the last decade. ...
... The rate of reactions is also analyzed at the ignition kernel. This analysis provides accurate results in terms of sensitivity and comprehensiveness, as the transported PDF Scalar has the advantage of calculating nonlinear chemical elementary reactions without any approximation, the fact that was previously authenticated in capturing the strong sensitivity of flame lift-off height to combustion parameters [58,70,74,89]. Considering the statistical nature of turbulence, the concentrations of species and as a result instantaneous reaction rates includes strong fluctuations, the least square smoothing filter was employed to smooth out the fluctuations and noisy results of instantaneous reaction rates [90]. ...
This paper gives an in-depth insight into NO X (NO, NO 2 , and N 2 O) formation of H 2 /N 2 turbulent Cabra jet flame issuing into a hot vitiated coflow. The joint composition probability density function (PDF) was employed to model the combustion and to specify the characteristics of the flame (i.e., scalar variables, concentration of species etc.). The turbulent transport term was modelled by Reynold-Average-Naiver-Stokes (RANS) SSG and molecular mixing was modelled by modified curl model. A combustion mechanism including 13 species and 34 reactions was employed to define the thermochemical state of the flame. The chemical reaction terms were resolved and accelerated by In Situ Adaptive Tabulation (ISAT). The simulation was performed at different equivalence ratios (ER), fuel jet nitrogen content (Y N 2 ,C ), coflow (T C ) and jet temperatures (T J ), coflow oxygen (Y O 2 ,C ) and water contents (Y H 2 O,C ). Results reveal NO X is composed of 30% NO 2 and 70% NO in the burner. Reaction rate analysis at different operating points in the ignition kernel demonstrates that N+OH⇌NO+H and NO 2 +H⇌NO+OH are dominant reactions in NO formation, while NO+HO 2 ⇌NO 2 +OH is the main reaction in NO 2 formation.
... ture mapping [8,9] , planar laser-induced fluorescence (PLIF) imaging can give species concentration [10] , laser-induced incandescence yields the soot volume fraction [11] , velocity fields can be deduced using Particle Imaging Velocimetry [12] , to name a few examples. Laser sheet imaging can also be used to study interactions between key combustion species by simultaneously recording the spatial distribution of different species, which is especially important for correlation studies of species that overlap in space [13][14][15][16][17][18][19][20][21][22] . However, experimental approaches for "simultaneous" detection of several combustion species often rely on sequential detection using several time-gated cameras, often in combination with spectral filters/mirrors that separate the emitted signal onto the different cameras (or regions on the sensor) [13][14][15][16][17][18][19][20][21][22] . ...
... Laser sheet imaging can also be used to study interactions between key combustion species by simultaneously recording the spatial distribution of different species, which is especially important for correlation studies of species that overlap in space [13][14][15][16][17][18][19][20][21][22] . However, experimental approaches for "simultaneous" detection of several combustion species often rely on sequential detection using several time-gated cameras, often in combination with spectral filters/mirrors that separate the emitted signal onto the different cameras (or regions on the sensor) [13][14][15][16][17][18][19][20][21][22] . While the use of several cameras can be advantageous in some situations e.g. for detection of significantly different wavelengths where a combination of cameras having different characteristics/sensitivities facilitates the experiment, the methodology has certain drawbacks; (1) systems based on several intensified cameras are expensive and, sometimes, impractical, (2) different species that have spectrally overlapping emissions can be challenging to probe due to the risk of signal cross-talk and (3) procedures to compensate for differences in the collection optics and camera efficiencies as well as post-processing means to achieve an accurate pixel-to-pixel correspondence are often required. ...
Imaging the interaction between different combustion species under turbulent flame conditions requires methods that both are extremely fast and provide means to spectrally separate different signals. Current experimental solutions to achieve this often rely on using several cameras that are time-gated and/or equipped with different spectral filters. In this work we explore a technique called Frequency Recognition Algorithm for Multiple Exposures (FRAME) as an alternative solution for instantaneous multispectral imaging of flame species. The method is based on exciting different species with different spatial “codes” and to separate each signal component using a spatial frequency-sensitive lock-in algorithm. This methodology permits the signal from several different species to be recorded at the exact same time with a single camera. Furthermore, since the signals are recognized based on the superimposed spatial codes, there is no need for spectral separation prior to detection. The entire fluorescence envelope from each species can thus, in principle, be detected. In the current work, we present simultaneous planar laser-induced fluorescence imaging of OH and CH2O in a turbulent dimethyl ether (DME)/air flame.
... For the study of flame stabilization and auto-ignition in systems where (cold) fuel is injected into a hot, oxygen-containing environment, Jet-in-Hot-Coflow (JHC) burners provide an excellent configuration. The flame stabilization [7][8][9][10][11][12][13][14][15][16][17][18][19], and to some extent also the influence of boundary conditions on the flame stabilization in JHC burners [14][15][16][17][18][19], have been described in the literature. Cabra et al. [14] reported an increasing lift-off height with increasing coflow-and jet velocity, while the lift-off height decreased with increasing coflow temperature. ...
... The transient formation of auto-ignition kernels in Jet-in-Hot-Coflow flames has also been studied in the literature [11,12,17,[21][22][23][24][25][26][27]. For example, Gordon et al. [10] used planar imaging of temperature, CH 2 O, and OH to examine the structure of individual ignition kernels upstream of stably burning flames resulting from natural gas issuing into a vitiated coflow. The imaging results indicated isolated ignition kernels, inferred from temperature and CH 2 O increases, which were not always accompanied by OH and which occurred in regions of low-temperature gradients, indicating auto-ignition (and not flame propagation) was a primary mechanism governing the lifted flame stabilization. ...
Transient auto-ignition is a key factor for flame stabilization and flame initialization in several technical combustion systems such as internal combustion engines or gas turbine combustors. Reliable numerical simulations of auto-ignition stabilized flames are important for the development of new combustor systems. For detailed model validation, knowledge of the sensitivity of different system response quantities (SRQs) of interest to the boundary conditions in combination with the accuracy of boundary conditions is essential, especially with respect to uncertainty quantification of numerical simulations. In the current study, the flame stabilization and auto-ignition in the DLR Jet-in-Hot-Coflow burner was examined experimentally using high-speed OH* chemiluminescence. Here, methane was either injected continuously to study the flame stabilization mechanism of steady state lifted jet flames, or in a pulsed manner to study the formation of auto-ignition kernels, into the hot exhaust gas of a lean, premixed hydrogen/air flame. The flame stabilization height, and the location and time of initial auto-ignition kernels for a case with transient auto-ignition were evaluated with respect to several boundary conditions, such as coflow temperature as well as coflow- and jet-velocity. A relative sensitivity of the measured SRQs on the boundary conditions was introduced in order to quantitatively compare steady state flame to transient auto-ignition characteristics and to assess the quantitative influence of different boundary conditions. Comparison of the auto-ignition dynamics in the steady state and during transient fuel injection allowed assessing the role of auto-ignition in the flame stabilization mechanism for different boundary conditions; accompanying chemical kinetic calculations were used to quantify the influence of strain on auto-ignition and flame propagation for the current conditions, allowing further insight into the flame stabilization mechanism in Jet-in-Hot-Coflow flames.
... Accordingly, Gordon et al. (2008) used the PLIF technique to examine the stability of methane turbulent flames in a high-temperature vitiated co-flow system. Also, Frank et al. (2005) utilized PLIF diagnostics to acquire two-dimensional measurements of mixture fraction, temperature, scalar dissipation rate, and forward reaction rate in turbulent partially premixed flames with argon dilution. ...
... Specifically, when the fuel is injected into sufficient coflow air at higher X f , the most reactive mixture micro clusters can be formed faster to start autoignition (Kerkemeier et al. 2013). In the meantime, higher X f can enhance chemical reactions and accelerate the buildup of a precursor radical pool, which was pointed to be the key stage of initiating autoignition spots (Gordon, Masri, and Mastorakos 2008;Macfarlane et al. 2018). It is essential to clarify the mechanism dominating intermittency to eliminate low-frequency pressure pulsation and understand the unsteady autoignited flame. ...
... [6] and references therein. Specific examples of multiple species PLIF studies are the combined imaging of the hydroxyl (OH) radical and formaldehyde (CH 2 O) for heat-release imaging presented by Paul and Naim [8], simultaneous visualization of CH and OH radicals by Kiefer et al. [9] and simultaneous imaging of distributions of temperature, OH and CH 2 O by Gordon et al. [10]. Multi-species PLIF imaging in highly turbulent premixed flames operating in the distributed reaction zone regime has been realized by Zhou et al. [11]. ...
A method based on femtosecond two-photon excitation has been developed for simultaneous visualization of interference-free fluorescence of H and O atoms in turbulent flames. This work shows pioneering results on single-shot simultaneous imaging of these radicals under non-stationary flame conditions. The fluorescence signal, showing the distribution of H and O radicals in premixed CH4/O2 flames was investigated for equivalence ratios ranging from ϕ = 0.8 to ϕ = 1.3. The images have been quantified through calibration measurements and indicate single-shot detection limits on the order of a few percent. Experimental profiles have also been compared with profiles from flame simulations, showing similar trends.
... Imaging multiple scalars simultaneously in turbulent flames is common practice to examine turbulence-chemistry interactions (e.g., [1][2][3][4][5] ). Minor species, i.e., combustion radicals or pollutants, are often desired and are typically imaged using planar laserinduced fluorescence (PLIF) (e.g., [1-3 , 5-10] ). ...
This study describes a technique that utilizes a single, tunable, pulsed dye laser and two intensified CCD cameras to image NH and NO simultaneously in turbulent ammonia-hydrogen-nitrogen jet flames. The NO radical is excited at 236.214 nm in its (0,1) band, while NH is excited in its A3Π-X3Σ–(1,0) band using the residual energy of the beam at 303.545 nm, necessary to yield 236.214 nm via mixing with the fundamental of the pump laser at 1064 nm. Data show that it is possible to image the instantaneous structure of these turbulent flames, specifically, NH delineates the reaction zone while NO also marks the location of burnt products.
... In contrast to the aformentioned studies at elevated pressures, there have been several studies performed in open-flame burners at atmospheric pressure which facilitate the decoupling of chemistry, mixing and flow-field effects. Many of these experiments have been carried out using so-called jet-in-hot-coflow (JHC) burners [22,23], or similar designs such as the vitiated coflow burner [24,25]. In these burners, mild combustion conditions are achieved using an additional burner upstream of the main combustion zone, with the primary fuel jet issuing into the hot and low-O 2 coflow of combustion products. ...
Combustion in hot and low oxygen environments, such as those encountered in practical devices including inter-turbine burners and sequential gas turbines, is not yet fully understood at a fundamental level, particularly in terms of the effects of pressure. To meet this gap in understanding, a confined-and-pressurised jet-in-hot-coflow (CP-JHC) combustor has been developed to facilitate optical diagnostics of turbulent flames in hot and vitiated coflows for the studies of flame stabilisation, structure and soot formation at elevated pressures. The CP-JHC burner has been designed for steady operation at 10 bar with internal temperatures of up to 1975 K, with a water-cooled central jet issuing into a hot oxidant stream of combustion products from a non-premixed natural gas/H2 burner. This work describes the key features and operational capabilities of the CP-JHC burner and presents a selection of experimental results showing characteristics not previously available. Specifically, temperature measurements of the hot coflow are used to estimate the enthalpy deficit of the stream, revealing an increase in thermal efficiency with increasing heat input, and a decrease with increasing pressure. Chemiluminescence imaging of OH* and CH* is performed for turbulent jet flames to study the flame structure under various operating conditions, and true-colour imaging results are also included to highlight the change in soot formation under elevated pressures. The mean images indicate a change in stabilisation behaviour with changes in pressure and jet Reynolds number (Rejet), which is further investigated by a statistical analysis of the short-exposure CH* images. This analysis reveals that an increase in Rejet from 10,000 to 15,000 leads to an increase in the mean lift-off height (from the jet exit plane) from approximately 1.5 to 6 jet diameters at atmospheric pressure, while the flames at elevated pressures show significantly less variation and tend to stabilise at the jet exit for P > 3.5 bar(a). The experimental findings are complemented by numerical simulations of laminar opposed flow flames, providing additional insights into the fundamental chemical kinetics effects which influence these flames. In particular, a monotonic reduction in both the maximum and integrated OH* and CH* mass fractions is observed with increasing pressure. This reduction is particularly pronounced at lower pressures, with a reduction to 10% of the atmospheric-pressure value at 3 bar(a) for the integrated OH* mass fraction. Additionally, this behaviour is shown to be related to the combined effects of a shift in the formation pathways and the increased impact of collisional quenching.
... The uncertainty for OH number density is the principle source of uncertainty for the Rayleigh scattering and can be estimated to be smaller than 2 % (Sutton, Williams, Fleming 2008). The typical uncertainty in the temperature data in the coflow and reaction zone varies from 5% to 10 % (Gordon, Masri, Mastorakos 2008;Medwell 2007;Ye et al. 2018). For the 3% case, the mean temperature computed in this study using the comprehensive and 36-species mechanisms agree well with measurements and the results of Li et al. (2021a). ...
Moderate, intense or low-oxygen dilution (MILD) Combustion has many attributes such as low emissions, noiseless combustion with high thermal efficiency which are attractive for greener combustion systems. Past studies investigated flow and chemical kinetics effects in establishing this combustion mode for methane-air mixtures. Many of the practical fuels are large hydrocarbons and hence this study aims to develop a skeletal mechanism suitable for turbulent MILD combustion simulations of n-heptane/air. Computer Assisted Reduction Mechanism approach is employed to develop a mechanism involving 36 species and 205 reactions, which is validated for wide range of conditions using measurements or results obtained from a comprehensive mechanism. This mechanism is found to be excellent for predicting both ignition delay times and flame speeds for MILD conditions. The OH* and CH* obtained using quasi-steady state assumptions agree well with those obtained using the comprehensive mechanism with these chemiluminescent species. The performance of the skeletal mechanism for turbulent MILD combustion simulation is tested using Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations with a finite-rate chemistry model. The computed statistics of temperature and OH number density agree quite well with measurements for highly diluted combustion cases.
... The hydroxyl radical (OH) is widely used as a reaction zone and post-oxidation zone marker in combustion research [4]. Therefore, the measurement and analysis of combustion species such as soot, PAH, OH, CH 2 O, etc. [5][6][7] augment the fundamental understanding of soot formation and validation of predictive models. In particular, two-dimensional (2D) and/or three-dimensional (3D) optical measurements will help to better "track" and access the local and global distribution of the species in a flame [7]. ...
We report the three-dimensional (3D) mapping of polycyclic aromatic hydrocarbons (PAHs), soot, and hydroxyl radicals (OH) in ethylene/air diffusion flames. A structured illumination-based frequency recognition algorithm for multiple exposures (FRAME) approach is combined with sample translation to intersect the flame in several two-dimensional planes. The FRAME technique has been used for recording a snapshot of multiple species on a single camera. It relies on extracting the amplitude of spatial modulation of two or more probed species encoded on a single sub-image. Here, the FRAME technique is first applied for simultaneous imaging of PAH by laser-induced fluorescence (PAH-LIF) and soot by laser-induced incandescence (LII). Sequentially, it is employed for simultaneous mapping of OH-LIF and soot-LII. The LII signal is converted to absolute soot volume fraction (fv) maps using a line-of-sight light extinction measurement. Finally, we have demonstrated the approach for layer-wise 2D imaging of soot volume fraction and averaged 3D mapping of multiple species.
... Common optical temperature measurement methods mainly include laser induced fluorescence (LIF) and tunable diode laser absorption spectroscopy (TDLAS). LIF [5] provides high resolution in time and space, which not only meets the demand of flame point temperature measurement but also measures the surface temperature distribution of the flame, as well as the high-frequency pulsating flame temperature. However, in this method, it is necessary to select appropriate groups as seed gases. ...
The tomography schlieren method is useful for calculating the non-axisymmetric temperature field, which is relevant to many industrial applications. However, the traditional tomography schlieren method requires bulky imaging devices, which limits its application. Thus, this paper proposes a non-contact three-dimensional temperature field measurement method based on the rotating tomography schlieren method. Projections from different directions of the temperature field are obtained at different angles using a transmission schlieren system, which is controlled by a designed rotating tomographic mechanism. The calibration schlieren method and the projection reconstruction algorithm are used to calculate the 3D temperature field. Verification experiments of single flame and double flame showed that the relative error of the reconstructed temperature was approximately 3.7% compared with a precision thermocouple.
... Since their study, others [108][109][110] also showed good agreement between overlap-layers and the region of heat release in flames. Based on these findings, numerous investigations [13,19,[111][112][113][114][115][116][117]124] have utilized the overlap-method to visualize reaction layers in turbulent flames. While concerns regarding the fidelity of the overlap-method have been raised [17,19,21,121], recent DNS studies by Aspden et al. [30] and Wang et al. [45] showed a strong correlation between heat release rate and overlap-layers in turbulent premixed dodecane-and methane-air flames, respectively. ...
Developing next-generation propulsion and energy production devices that are efficient, cost-effective, and generate little to no harmful emissions will require highly-accurate, robust, yet computationally tractable turbulent combustion models. Models that accurately simulate turbulent premixed combustion problems are particularly important due to the fact that burning in a premixed mode can reduce exhaust emissions. A common tool employed to identify when a particular model might be more appropriate than others is the theoretical Borghi Diagram, which possesses boundaries that are meant to separate various regimes of combustion (i.e. where a particular model is superior to others). However, the derivations of these boundaries are merely based upon intuition and dimensional reasoning, rather than experimental evidence. This thesis aims to provide such evidence; furthermore, it proposes novel approaches to delineating regimes of combustion that are consistent with experimental results. To this end, high-fidelity flame structure measurements were applied to premixed methane-air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with turbulence levels (u'/S_{L}) as high as 246, longitudinal integral length scales (L_{x}) as large as 43 mm, and turbulent Karlovitz (Ka_{T}) and Reynolds (Re_{T}) numbers up to 533 and 99,000, respectively. Two techniques were employed to measure the preheat and reaction layer thicknesses of these flames. One consisted of planar laser-induced fluorescence (PLIF) imaging of CH radicals, while the other involved taking the product of simultaneously acquired PLIF images of formaldehyde (CH_{2}O) and hydroxyl (OH) to produce ``overlap-layers." Average preheat layer thicknesses are found to increase with increasing u'/S_{L} and with axial distance from the burner (x/D). In contrast, average reaction layer thicknesses did not vary appreciably with either u'/S_{L} or x/D. The reaction layers are also observed to remain continuous; that is, local extinction events are rarely observed. The results of this study, as well as those from prior investigations, display inconsistencies with predictions made by the theoretical Borghi Diagram. Therefore, a new Measured Regime Diagram is proposed wherein the Klimov-Williams criterion is replaced by a metric that relates the turbulent diffusivity (D_{T}) to the molecular diffusivity within the preheat layer (D*). Specifically, the line defined by D_{T}/D* ~ 180 does a substantially better job of separating thin flamelets from those with broadened preheat yet thin reaction layers (i.e. BP-TR flames). Additionally, the results suggest that the BP-TR regime extends well beyond what was previously theorized since neither broken nor broadened reaction layers were observed under conditions with Karlovitz numbers as high as 533. Overall, these efforts provide tremendous insights into the fundamental properties of extremely turbulent premixed flames. Ultimately, these insights will assist with the development and proper selection of accurate and robust numerical models.
... These theories can be categorized based on the degree of fuel-air premixing upstream of the flame base [104][105][106], or on the local turbulence effect on the flame base [107,108]. For lifted flames in a heated coflow [37,109], autoignition was considered as an additional contribution to the flame stabilization phenomenon. Using DNS, Yoo et al. [96] concluded that autoignition is indeed the key mechanism for stabilization of lifted jet flames in heated coflow. ...
Current projection of energy consumption trends has shown that combustion of fossil fuel will continue to play an important role in industrial thermal processes, power generation, and transportation for a substantial period. In order for these sectors to sustain under the finite fossil fuel reserves, improvements in existing devices and development of novel concepts that emphasize on energy efficiency are necessary. Numerical simulations can be used to address this need, in particular by complementing experiments with extensive and quick parametric studies. However, this is only viable if numerical predictions of the combustion processes are accurate, which requires adequate modeling of the multi-physics phenomena in turbulent reacting flows. In this work, the flamelet-type combustion model, one of the most widely used approaches for turbulent reacting flow simulations, is thoroughly analyzed in terms of the validity of its underlying assumptions and limitations in its description of different combustion regimes. Diagnostic tools that account for the flamelet formulation are developed and applied to two different direct numerical simulation (DNS) results. These analyses show that the omission of the higher-order and unsteady flamelet effects by most conventional flamelet models is not valid in realistic configurations that are characterized by complex vortical structures, flame extinction and reignition, and turbulence-chemistry interactions. Following these findings, a higher-order flamelet model that describes the conventionally omitted flamelet effects is developed for large-eddy simulations (LES) applications. This model is based on the physical interpretation of flamelets as quasi one-dimensional structures in the turbulent flow, and the consideration of the effects that the spatial-filtering in LES methodology has on these structures. The model is applied in LES of a turbulent counterflow diffusion flame configuration, demonstrating improved agreement with the reference DNS solutions of the same case than the steady flamelet/progress variable (FPV) and laminar approximation models.
... In many practical applications, however, the ambient oxidizer around a fuel jet is hot enough to initiate autoignition of the fuel/air mixture upstream of the lifted flamebase. As such, autoignition has been considered as one of the important stabilization mechanisms of turbulent lifted flames in vitiated coflows [3,[6][7][8][11][12][13][14]. For instance, previous 3-D direct numerical simulations (DNSs) of hydrogen [7] and ethylene [8] jet flames in heated coflows revealed that turbulent lifted jet flames are stabilized primarily by the autoignition of fuel-lean mixtures supported by the hot coflow where the temperature is high. ...
Three-dimensional direct numerical simulations of turbulent lifted hydrogen jet flames in heated coflows are performed with a detailed H2/air chemical mechanism to understand their ignition dynamics and stabilization mechanisms. Turbulent lifted jet flames with four different coflow temperatures, Tc, between 750 K and 1100 K are investigated by examining the instantaneous/time-averaged values and conditional means of heat release rate and species critical to ignition, and by performing a displacement speed analysis and a local combustion mode analysis with an indicator, α. Although Tc at 950 K is higher than the autoignition limit, the flame is primarily stabilized by flame propagation rather than autoignition, while at 1100 K, flame stabilization is found to be highly affected by autoignition. The local combustion mode analysis further reveals that at 950 K, even if a local ignition mode with |α|<1 first appears in the near field of the jet, it develops into a local extinction mode with α<−1 as local temperature decreases due to the excessive mixing of heated coflow and cold H2 within vortical structures, which inhibits the ignition kernel development upstream of the flamebase. At 1100 K, however, a local ignition mode prevails upstream of the flamebase. To further identify the effect of a vortex on the early development of an ignition kernel in a mixing layer between the heated coflow and cold H2, a series of two-dimensional DNSs are performed, varying several vortex parameters and air temperature, as a reference for the more complicated corresponding 3-D turbulent DNS cases. The results substantiate that the development of a vortex in the mixing layer tends to retard the autoignition within the vortex, especially when its temperature is slightly above the autoignition limit.
... The ribbon-like structures of OH and CH 2 O are captured in these images, which are the typical structures in non-premixed flames, particularly in turbulent jet flames. 47 However, the ribbon-like structures in these bluff-body flames are quite different from those in jet flames. As referred to the methane jet flame, the ribbon-like structures of OH and CH 2 O were observed, respectively. ...
The flame stabilization is a complex problem, especially when reducing the fuel supply, as it involves complicated interactions of turbulence, mixing, and chemistry. In this study, the flamelet progress variable combined with large eddy simulation has been used to simulate a bluff-body non-premixed flame to reveal the mechanisms of flow, mixing, and flame stabilization during the central fuel jet velocity reduction. The two flow patterns of jet dominant and coflow dominant are first analyzed by the concept of persistence of decay similarity of jet. The results show that this concept is informative to interpret whether and where the flow is jet dominant and understand the competition between two flows in detail, not only for flow but also for mixing. The results further show that the jet-coflow interaction, which has a pronounced impact on flame topology, has a minimal impact on flame stabilization for the bluff-body stabilized non-premixed flames over a wide range of fuel jet velocities, because of approximately close ignition delay time and flow convection velocities. In addition, it is observed that a ribbon-like structure of formaldehyde forms upstream of the hydroxyl. This phenomenon is caused by autoignition which is favored by high temperature in the recirculation zone and takes place far upstream of the flame. That would particularly facilitate flame stabilization in bluff-body burners. Published under an exclusive license by AIP Publishing. https://doi.
... Also, these radicals have much more homogeneous distribution throughout the reaction region. Many scholars (Gordon et al. 2008;Medwell et al. 2012;Najm et al. 1998Ye et al. 2018;Zhou et al. 2017) have concluded that HCO radicals have an excellent spatial and temporal correlation with (major) heat release zones in hydrocarbons' combustion. Since most of the fuel carbon is flowing to products (CO and CO 2 ) through reaction paths involving HCO, it can be used as a suitable monitor for fuel consumption speed and local heat release rate. ...
The combustion of kerosene spray under hot-diluted conditions and conventional conditions was experimentally investigated. By examining flame photographs, chemiluminescence images, and in-field temperature measurements, the separate effect of different variables including oxygen concentration, temperature and velocity of the co-flowing air, fuel flow rate and injection pressure, and eventually the type of spray nozzle on multiple parameters such as flame stability, structure, luminosity, temperature field, and qualitative CH radical distribution, as well as HCO and NO2 with lower precision, in the reaction region, have been studied. It was observed that an increment in injection pressure and co-flow temperature enhances the spray flame stability, while dilution exacerbates it. Also, a solid cone spray pattern with a lower spray angle has better stability than hollow cone ones with a higher spray angle. Moreover, it was noted that liquid fuels, compared to gaseous fuels, require higher preheating temperatures, for the same dilution level, to engender a stable flame. For combustion of spray in conventional conditions, a double-flame structure was observed consisting of a bluish section at the leading edge emerging into a yellowish sooting trail. An increase in co-flow velocity, as well as injection pressure, strengthens the inner flame front, whereas raising the co-flow temperature or diluting the oxidant, deteriorates the inner flame front. In the case of highly preheated air (without dilution), the flame liftoff height is reduced to as close to the atomizer as a few millimeters, forming a single flame structure similar to gaseous flames. In this case, the peak temperature was considerably higher than the conventional combustion, yet the gain was much lower than the preheating level. Combined effects of preheating and dilution alter the spray flame structure in a way that the flame volume is reduced, the temperature field has become more homogeneous, the peak temperature is limited to less than 1500 K, and temperature fluctuations have significantly decreased, seemingly approaching MILD combustion regime conditions.
... Laser Rayleigh scattering was utilized by Sutton [22] and Arndt et al [23]. to determine density, Feikema et al. [24], Barlow et al. [25], and Balla et al. [26] to determine mixture fraction, along with Gordon et al. [27] and Barat et al. [28] to determine temperature. It is possible to perform this diagnostic in both reacting and non-reacting flows and it has many useful properties. ...
An experimental methodology for studying the mixing and heat transfer characteristics of effusion cooling jets was developed alongside an experimental test section. Preliminary investigations into these phenomena were performed in an environment relevant to gas turbine combustors. An array of effusion jets were injected into a hot vitiated crossflow containing combustion products at an average temperature of 1500K. Planar images of gas temperature via Rayleigh scattering were obtained parallel to the injection plane at various heights for two density ratios (4.65 and 3.05) and three blowing ratios (5, 7, and 10). The main goal of this data collection was for validation of the methodology and improving the fundamental understanding of the fluid mechanics involved. The instantaneous gas temperature distributions reveal that there are significant fluctuations in the motion of the effusion jets which can be attributed to 3 different sources: 1.) Flow interactions between the individual jets. 2.) Entrainment of the crossflow. 3.) Geometry of the effusion jet hole creating turbulence prior to injection. It was also observed that for a set blowing ratio, as density ratio decreased, fluctuations in the jet motion increased. Additionally, jet penetration depth increased with decreasing density ratio.
... Formaldehyde (CH 2 O) is formed in the preheat layer of hydrocarbon flames from the initial fuel decomposition reactions and is consumed in reaction layer [59]. It is an important precursor radical prior to autoignition [60]. The points in Fig. 7 are colored by the Signed Flame Index (SFI), which is defined as [ ...
Two analysis methods for time scale and energy balance relevant to flame ignition and stabilization in cavity-stabilized flames are developed. The interaction time of hot product in the recirculation zone of the cavity with the surrounding unburned mixture and the reaction induction time of the mixture are estimated in the time scale method. The energy release from chemical reactions and the energy loss due to species exchange in the recirculation zone are included in the energy balance method. The autoignition and propagation of supersonic ethylene flames in a model supersonic combustor with a cavity is investigated first using highly resolved large eddy simulation. The evolutions of the two time scales are then calculated in the ignition process of the supersonic ethylene flames. It is found that the time scale theory is well valid in the flame propagation and stabilization stages. The rates of energy generation and loss are then analyzed in the cavity. It is found that initially the local energy generation rate is relatively small, resulting in slow net energy accumulation in the cavity. Then the energy generation increases due to the intermittent flame propagation in the cavity, whereas the energy loss oscillates consistently since the burned gas leaves the cavity. Also, energy generation and loss are generally balanced in the cavity and all tend to zero after the flame is globally stabilized. The two methods present the characteristic time scales and energy balancing during the transient ignition process for the first time.
... The distributions of OH and CH2O were also measured. Auto-ignition is considered to be the stabilization mechanism of the flame in high-temperature coflow [8] . Wang et al. [9] investigated the stabilization mechanism of a high Ka number CH 4 /air premixed flame by using DNS. ...
The stability limit of a supersonic ethylene jet flame in a fuel-rich hot coflow was examined by investigating the influence of the injection pressure, which was varied from 2.0 atm to 4.5 atm, and of the equivalence ratio of the coflow, which was varied from 1.2 to 1.6. The flames were investigated with time-resolved chemiluminescence and schlieren images, as well as a large-eddy simulation of combustion. The results show that, with increasing injection pressure, the flame state changes from stable to unstable and blow-off, and the flame brush thickness, heat release, and height of coflow decrease. The flame stability limits decrease as the equivalence ratio of coflow increases. Lastly, a large-eddy simulation was performed to investigate the mechanism of flame stabilization, and the numerical simulation results are in good agreement with the experimental results. It was found that the stability of a supersonic flame is affected by the chemical time scale and flow time scale.
... MILD combustion is characterized as fuel autoignition, which is physically indicated by an intense buildup of precursor (CH 2 O) and subsequent initiation of reaction (represented by OH radical) as well as stable flame [40]. Fig. 9 shows the spatial distributions of OH and CH 2 O inside the MCF for the three atmospheres. ...
To deepen the understanding of combining moderate or intense low-oxygen dilution (MILD) combustion with oxy-fuel combustion for enhancing flame stability while realizing carbon capturing and storage, this paper presents a numerical study of methane MILD combustion in three atmospheres, i.e.: O 2 /N 2 , O 2 /CO 2 and O 2 /H 2 O, with both computational fluid dynamics (CFD) and kinetic calculation approaches. Firstly, CFD predictions for the three conditions were performed following a systematic validation of the numerical method against experimental measurement from methane/air MILD combustion in a laboratory-scale closed furnace. Subsequently, kinetic calculations with a well-stirred reactor model was used to quantitatively identify the operating ranges of MILD combustion in the three atmospheres for methane. Moreover, the kinetic calculation provided additional insight into the fuel oxidation pathway. The results reveal that replacing N 2 with either CO 2 or H 2 O would help to establish MILD combustion mode from the viewpoint of lower temperature increase, due to both physical and chemical property discrepancies among the diluents. Specifically, the chemical effect and physical effect are responsible to the lower temperature rise for CO 2 -diluted case and H 2 O-diluted case, respectively. Inside the MILD combustion furnace, the negative heat release region disappears in regardless of atmospheres, indicating the eliminated fuel pyrolysis process under MILD combustion mode. Detailed analysis of the flame structure suggests that the combustion regimes inside the furnace in the three atmospheres are all in well-stirred combustion regime, and CO 2 -diluted case has the most extended reaction zone. Kinetic calculation indicates that CO 2 or H 2 O dilution would result in a wider MILD combustion operating range compared to N 2 dilution, while it is more pronounced for CO 2 . These observations all imply that MILD combustion will be more easily established with CO 2 dilution than N 2 or H 2 O dilution. However, higher CO formation is obtained in O 2 /CO 2 , forcing more attention to be paid on CO emission under CO 2 -diluted MILD combustion. Furthermore, the hydrocarbon recombination route is negligible under MILD combustion in spite of the atmospheres, implying lower sooting tendency as compared with conventional combustion.
... Based on the mean liftoff height positions seen in Fig. 7, it was decided to use a shorter confinement collar (152 mm height) such that the mean positions of the flame base would be slightly downstream of the collar. Given the relative low coflow velocity (hence Froude number) of our jet flame in vitiated coflow burner as compared to the burners of Cabra et al. [23] and Gordon et al. [48], the two-stream condition was not expected to be preserved downstream of the confinement collar due to the entrainment of room temperature air. Thus, the short confinement collar enabled a direct investigation of the impact of downstream ambient air entrainment on the resulting flame base motions. ...
Turbulent combustion of non-premixed jets issuing into a vitiated coflow is studied at coflow temperatures that do not significantly exceed the fuel auto-ignition temperatures, with the objective of observing the global features of lifted flames in this operating temperature regime and the role played by auto-ignition in flame stabilization. Three distinct modes of flame base motions are identified, which include a fluctuating lifted flame base (mode A), avalanche downstream motion of the flame base (mode B), and the formation and propagation of auto-ignition kernels (mode C). Reducing the confinement length of the hot coflow serves to highlight the role of auto-ignition in flame stabilization when the flame is subjected to destabilization by ambient air entrainment. The influence of autoignition is further assessed by computing ignition delay times for homogeneous CH4=air mixtures using chemical kinetic simulations and comparing them against the flow transit time corresponding to mean flame liftoff height of the bulk flame base. It is inferred from these studies that while auto-ignition is an active flame stabilization mechanism in this regime, the effect of turbulence may be crucial in determining the importance of auto-ignition toward stabilizing the flame at the conditions studied. An experimental investigation of auto-ignition characteristics at various jet Reynolds numbers reveals that turbulence appears to have a suppressing effect on the active role of auto-ignition in flame stabilization.
... For example, Cabra et al. [12] performed simultaneous Raman/Rayleigh/laser-induced fluorescence (LIF) measurements and used joint statistics of temperature versus mixture fraction and OH mole fraction versus mixture fraction to assess the thermo-chemical state of the flame. Gordon et al. [22] performed quantitative OH planar laser-induced fluorescence (PLIF) in combination with CH 2 O PLIF and planar Rayleigh scattering to study the stabilization mechanism of methane jets in a hot coflow. They found isolated ignition kernels upstream of the stably burning flame. ...
For the detailed understanding of transient combustion processes, in particular, of auto-ignition, quantitative measurements with high spatio-temporal resolution are desirable. These can, for instance, serve as validation data for time-resolved numerical simulations and in particular for the combustion models used in those simulations. In the current study, a jet-in-hot-coflow (JHC) burner, developed at the German Aerospace Center (DLR), the DLR JHC, was used to inject a turbulent methane jet into the hot exhaust gas of a lean hydrogen/air flame, and a steady state jet flame was established. In addition, fuel could be injected in a transient manner. Here, an auto-igniting jet was observed. The flame stabilization of the steady state jet flame and the auto-ignition during transient fuel injection were studied using high-speed laser-based and optical measurements. A strategy for quantifying high-speed OH planar laser-induced fluorescence is presented, and the measurement uncertainties are evaluated. The flame stabilization mechanism in steady state jet flames was assessed using probability density functions of the OH concentration at different axial and radial locations. The formation of auto-ignition kernels during transient fuel injection is evaluated based on time series of the OH concentration. It is shown how the OH concentration levels and PDF shapes can be used to characterize the chemical state of the reacting flow and to distinguish between auto-ignition and flame propagation.
... In general, autoignition and flame propagation are two different mechanisms that can stabilize flames in high-temperature flows. Lifted flames have been largely investigated for various configurations experimentally [26][27][28][29][30] and numerically [11,26,[31][32][33][34][35][36][37]. The main anchoring mechanism is sometimes attributed to autoignition (e.g. ...
This numerical study investigates the combustion modes in the second stage of a sequential combustor at atmospheric and high pressure. The sequential burner (SB) features a mixing section with fuel injection into a hot vitiated crossflow. Depending on the dominant combustion mode, a recirculation zone assists flame anchoring in the combustion chamber. The flame is located sufficiently downstream of the injector resulting in partially premixed conditions. First, combustion regime maps are obtained from 0-D and 1-D simulations showing the co-existence of three combustion modes: autoignition, flame propagation and flame propagation assisted by autoignition. These regime maps can be used to understand the combustion modes at play in turbulent sequential combustors, as shown with 3-D large eddy simulations (LES) with semi-detailed chemistry. In addition to the simulation of steady-state combustion at three different operating conditions, transient simulations are performed: (i) ignition of the combustor with autoignition as the dominant mode, (ii) ignition that is initiated by autoignition and that is followed by a transition to a propagation stabilized flame, and (iii) a transient change of the inlet temperature (decrease by 150 K) resulting into a change of the combustion regime. These results show the importance of the recirculation zone for the ignition and the anchoring of a propagating type flame. On the contrary, the autoignition flame stabilizes due to continuous self-ignition of the mixture and the recirculation zone does not play an important role for the flame anchoring.
The current study focuses on characterizing the auto-ignition process from turbulent fuel injection into a high-temperature, vitiated environment using the jet-in-hot-coflow (JHC) burner configuration. High-speed (10-kHz acquisition rate) optical and laser-based diagnostics are used to identify ignition kernel formation and to determine the most probable mixture fraction and temperature conditions that directly facilitate ignition. Four operating cases are studied that present variations in coflow temperature, jet velocity, and fuel type. High-speed OH* chemiluminescence (CL) imaging is used to obtain data on the spatial position and delay time (following fuel injection) of the formation of the first auto-ignition kernels. Over the limited conditions tested, the ignition delay times and heights show a strong sensitivity to temperature and fuel type, but only a mild sensitivity to jet velocity. High-resolution, kHz-rate laser Rayleigh scattering (LRS) is used to provide simultaneous mixture fraction and temperature measurements prior to and at the onset of ignition. High signal-to-noise ratios (SNR >200) enable reliable measurements of mixture fraction at ultra-lean conditions that seemingly promote auto-ignition. A statistical characterization of the most probable mixture fraction values leading to ignition (ξig) is presented and compared with calculated values of the most reactive mixture fraction, ξMR, which has been identified previously from theory and simulations as the parameter governing auto-ignition. Results show that very lean conditions near ξMR are preferentially encountered, but the probability density function (PDF) of ξig is described by a near-exponential distribution, spanning values across the flammability limits. Some limitations to the current methodologies for determining ξig are discussed, which are due primarily to the fact that the discretely sampled mixture fraction data is inherently asynchronous with the ignition event itself.
New understanding of turbulent ethylene (C2H4) jet flames, issuing into a range of preheated coflowing oxidisers with reduced oxygen (O2) concentrations are reported. These conditions emulate moderate or intense low-oxygen dilution (MILD) combustion. To resolve previously reported non-monotonic trends, comparisons are made for coflow O2 concentrations of 3 %, 4 %, 5 %, 6 %, 9 % and 11 % and coflow temperatures of 1250 K, 1315 K and 1385 K. Instantaneous and simultaneously planar imaging measurements of temperature, hydroxyl radicals (OH) and formaldehyde (CH2O) were taken at eight downstream locations ranging from 9 mm to 75 mm. The new data reveal non-monotonic lift-off trends in the OH and CH2O formation heights at all three temperatures. Compared with extant measurements, the higher resolution and greater control of coflow composition provide a detailed exploration of the transitional behaviour that occurs when MILD combustion conditions are achieved. The results experimentally demonstrate non-monotonic variation in lift-off height, and show that the same behaviour is present in both the OH and CH2O formation heights, and occurs at all three temperatures, which is due to the movement of the location of the stoichiometric and most reactive mixture fractions to the coflow side of the jet shear layer. The results also show that increasing the oxidiser temperature does not have a significant effect on the OH number density for a given temperature but does result in a significant decrease in lift-off height and CH2O concentration.
This chapter sheds light on the historical development of the flameless combustion based on various numerical and experimental studies. The first section presents state of the art on discovering flameless combustion and preliminary investigations in industrial furnaces and burners. It is discussed that how a nonconventional combustion regime, achieved through specific flow and temperature conditions, helped lower the NOx emissions while maintaining the overall system efficiency. Modeling approaches for the flameless regime are discussed in detail using computational fluid dynamics. It is observed that both turbulence and chemistry equally drive the flameless regime, and hence, models accounting for better turbulence-chemistry interaction capture the flameless regime well. It is concluded based on the state-of-the-art survey that a standard/modified κ-ɛ model for turbulence works satisfactorily. For combustion modeling, the eddy dissipation concept with detailed chemical kinetics is required to capture the flameless combustion characteristics. This chapter also highlights the importance of the selection of combustor geometry and how it generates increased recirculation levels required to sustain flameless combustion conditions. Among the presented investigations, geometries based on a cyclonic flow field provide promising results regarding pollutant emissions. In the later sections of this chapter, flameless combustion's potential to burn low graded dirty fuels is discussed. It is presented how creating a heated and diluted environment can lead to clean burning of low calorific value fuels such as syngas, biogas, ammonia, coke oven gas, etc. It is observed that fuel flexibility is an inherent talent of this combustion regime, which is needed to be explored further in future studies. In the final section of this chapter, studies based on the applicability of flameless combustion in gas turbine engines are presented. The issues observed are high-pressure loss, narrow operational range, etc. Solutions in terms of fuel/air staging two-stage combustors are discussed.
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.
This paper aims to understand the effects of coflow oxygen content on the lift-off height of autoignited jet flame and develop a lift-off height prediction model. A jet-in-coflow burner was used to performed autoignition experiments in hot air coflow with different oxygen contents. At a fixed oxygen content, as the fuel jet velocity increases, two types of autoignited flames were observed: (1) nozzle-stabilized flame (NS); (2) lifted flame (LF). As the oxygen content increases, it takes a larger fuel jet velocity for the transition from NS to LF and the transition from LF to blow-out. For a fixed fuel jet velocity, the lift-off height decreases significantly with increasing oxygen content and is more sensitive to the oxygen content at oxygen-lean conditions. Based on the modified large-scale mixing model, a lift-off height model was developed that considers the combined effect of small-scale strain and large-scale mixing.
Flame dynamics and combustion oscillation are complex problems in propulsion systems. In this study, the combustion oscillation characteristics of a supersonic ethylene jet flame in a hot coflow were investigated utilizing a 5 kHz high-speed hydroxyl planar laser-induced fluorescence (OH-PLIF) technique and an advanced postprocessing method, namely, dynamic mode decomposition (DMD). A PLIF system equipped with a large laser sheet was used to collect the dynamic development process of the jet flame. An ethylene jet flow was burned at a hot coflow temperature of 1900 K at different Mach numbers (Mach 0.55–1.6). The dynamic evolution of flame microstructures was clearly obtained. The local extinction events and flame area distribution in the flame were statistically analyzed to characterize the instability of the jet flame. The results indicated that flame instability was enhanced with increasing Mach number, but the jet velocity did not affect the global flame oscillation frequency. Based on DMD analysis, the spatiotemporal three-dimensional oscillation characteristics of the jet flame were quantified. The DMD results indicated that during different time periods, the jet flame is dominated by coherent structures with different frequencies. For supersonic flow, increased jet velocity might lead the dominant mode frequency shift to a higher level.
In this study, we conduct three-dimensional nonlinear large-eddy simulation to investigate the interaction between turbulence and reaction during the initial ignition process of a turbulent methane/hydrogen jet-in-hot-coflow flame under moderate or intense low-oxygen dilution (MILD) condition. Special focus has been placed on the spatial development of the flame and the temporal evolution of representative ignition spots that characterize the range of ignition behaviors observed in the case. Results show that the ignition process of the flame consists of four consecutive phases. Ignition occurs initially with relatively lean mixtures, and compared to the corresponding homogeneous stagnant adiabatic combustion, the loss of radical species associated with flow transportation causes a delay in ignition. The initial ignition spots formed during the autoignition phase provide sufficient conditions for the stabilization of the flame, including the provision of a variety of key radicals. Results also show that the flow convection accompanying the hot coflow dominated the slow flame propagation, and the turbulent mixing is of great importance for rapid flame propagation. These findings will broaden our knowledge of MILD combustion and provide useful insights into advanced ignition control.
A numerical experimental investigation is presented for a steady methane lifted-flame and a non-reaction jet flow in a co-flow of hot combustion products from lean premixed air/hydrogen combustion. A pressurized vitiated co-flow burner has been employed to study the methane lifted flame and non-reaction jet flow under different background pressures. The lift-off height has been measured with a high-speed camera, and the central jet flow velocity has been measured by means of a Schlieren imaging system. The experimental results show that the lift-off height decreases for an increment in the background pressure and in the co-flow temperature. As far as the experimental tests on the non-reaction jet flow is concerned, the jet velocity becomes extinct faster as the background pressure rises. The evolution of the jet velocity has been proved to be another important factor that affects the lift-off height under different background pressures, in addition to the fuel autoignition delay. The simulation data led with a RANS/PDF model show that an increment in the background pressure makes the temperatures increase and induces a brighter yellow part of lifted flame, which leads to more soot production. This proves that the flame is not completely premixed. On the other hand, the Schlieren images of a non-reaction jet flow highlight that the flame is partially premixed, since the edge of the jet is not well defined, as the jet penetration increases with time.
Turbulent combustion will remain central to the next generation of combustion devices that are likely to employ blends of renewable and fossil fuels, transitioning eventually to electrofuels (also referred to as e-fuels, powerfuels, power-to-x, or synthetics). This paper starts by projecting that the decarbonization process is likely to be very slow as guided by history and by the sheer extent of the current network for fossil fuels, and the cost of its replacement. This transition to renewables will be moderated by the advent of cleaner engines that operate on increasingly cleaner fuel blends. A brief outline of recent developments in combustion modes, such as gasoline compression ignition for reciprocating engines and sequential combustion for gas turbines, is presented. The next two sections of the paper identify two essential areas of development for advancing knowledge of turbulent combustion, namely multi-mode or mixed-mode combustion and soot formation. Multi-mode combustion is common in practical devices and spans the entire range of processes from transient ignition to stable combustion and the formation of pollutants. A range of burners developed to study highly turbulent premixed flames and mixed-mode flames, is presented along with samples of data and an outline of outstanding research issues. Soot formation relevant to electrofuels, such as blends of diesel-oxymethylene ethers, hydrogen-methane or ethylene-ammonia, is also discussed. Mechanisms of soot formation, while significantly improved, remain lacking particularly for heavy fuels and their blends. Other important areas of research, such as spray atomization, turbulent dense spray flames, turbulent fires, and the effects of high pressure, are briefly mentioned. The paper concludes by highlighting the continued need for research in these areas of turbulent combustion to bring predictive capabilities to a level of comprehensive fidelity that enables them to become standard reliable tools for the design and monitoring of future combustors.
Premixed staged combustion in gas-turbine combustors, where a premixed fuel–air mixture is injected into a hot vitiated environment, has promise for better emission control. In these hot environments that occur at high engine pressure, the flames may stabilize by a combination of autoignition and premixed flame propagation. In this work, a laminar and steady premixed jet in vitiated coflow was studied numerically to examine the stability behavior of such flames that are impacted by both autoignition and flame propagation. Simulations with detailed chemistry were used with boundary conditions matching previous experiments to understand the stability mechanisms. At the conditions studied, the laminar flame was lifted, with the region of highest heat release rates occurring downstream of the jet exit. However, non-zero heat release rates an order-of-magnitude below the maximum value were observed continuously to the jet exit. An energy budget analysis was conducted along streamlines through different portions of the flame. Streamlines flowing through relatively low heat release rate regions near the burner exit were only influenced by mixing between the jet and vitiated coflow. Streamlines which flowed through higher heat release rates in the lifted flame were influenced by heat transfer from both the coflow and the existing flame, leading to heat-transfer assisted propagating premixed-flame behavior. Species budgets analyzed in the low- and high-heat release regions of the flames were also used to characterize autoignition locations. The species budget through autoignition regions showed clear markers such as high CH2O concentrations present ahead of high heat release rates. The streamlines through premixed propagation regions of the flame also had high CH2O concentrations upstream of the high heat release region, but the species budget shows this is due to mixing from autoignition regions as opposed to reactions occurring on the streamline ahead of the flame. Thus, caution must be taken when using CH2O as a marker for autoignition in such flames with mixed stabilization modes.
A turbulent n-heptane jet flame in a jet-in-hot-coflow burner is numerically and experimentally investigated, revealing distinct features of this fuel in a jet-in-hot-coflow burner. The RANS k-ε turbulence model is adopted in combination with a dynamic partially-stirred reactor (PaSR) combustion model. The simulation results are used to support newly-obtained experimental measurements of mean temperature, OH number density and normalised CH 2 O-PLIF signal values at several axial locations. The simulations capture the transitional phenomenon observed experimentally for the low coflow oxygen concentration case, which is determined to be due to the two chemical pathways which exist for the n-heptane fuel. The predicted flame weak-to-strong transition heights based on the streamwise (axial) gradient of OH number density show non-monotonic behaviour. Furthermore, an investigation on negative heat release rate region shows that the absolute value of negative heat release rate increases with reduced coflow oxygen content, in contrast to the suppression phenomenon seen in laminar opposed-flow flames. * zl443@cam.ac.uk
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.
This paper extends the technique of laser-induced fluorescence from OH & CH2O (LIFOH-
CH2O) to provide a measure of heat release zones in turbulent, moderately dense spray
flames of ethanol and biodiesel fuels. A custom filter is used to remove interference from
stimulated Raman scattering arising from the liquid fragments. Mie scattering is measured jointly on a separate camera to co-locate the fluid fragments with respect to the reaction zones. It is found that while the overall spray flame structure is similar to that of a turbulent gaseous diffusion flame, additional complexities arise due to the continuous release of fresh fuel vapor from the evaporating fragments. Structures referred to as burning rings of different sizes are observed, and these are ignited by interfacing with a hot edge before they grow, propagate, and burnout. Statistics on the occurrence of these rings, with and without clouds of liquid fragments within them, are presented. These findings are common to both ethanol and biodiesel, although measurements in the latter were affected by strong fluorescence interference from the excited components in the biodiesel blend.
In this work, a new 3 kWe flameless combustor for hydrogen fuel is designed and analyzed using CFD simulation. The strategy of the design is to provide a large volumetric combustion for hydrogen fuel without significant rise of the temperature. The combustor initial dimensions and specification were obtained from practical design procedures, and then optimized using CFD simulations. A three-dimensional model for the designed combustor is constructed to further analysis of flameless hydrogen combustion and consideration that leads to disappearance of flame-front and flameless combustion. The key design parameters including aerodynamic, temperature at walls and flame, NO X , pressure drop, combustion efficiency for the hydrogen flame is analyzed in the designed combustor. To well demonstrate the combustor, the NO X and entropy destruction and finally energy conversion efficiency, and overall operability in the microturbine cycle of hydrogen flameless combustor is compared with a 3 kWe design counterpart for natural gas. The findings demonstrate that hydrogen flameless combustion is superior to derive the microturbines with significantly lower NO X , and improvements in energy efficiency, and cycle overall efficiency with low wall temperatures guaranteeing the long-term operation of combustor and microturbine parts.
The scenario of fuel injected into hot surrounds is found in a range of practical combustion applications. These flame conditions have been emulated using a jet-in-hot-coflow-burner using prevaporised n-heptane and mixtures of n-heptane and toluene, relevant to gasoline and diesel fuel surrogates. This paper reports measurements of six lifted, turbulent flames, with a constant jet flow of a prevaporised fuel/N2 mixture at 380 K into various hot and vitiated coflow conditions. Five of these flames issued into coflows generated by the combustion of different mixtures of ethylene/air and one had a coflow from a natural gas/air flame. Two n-heptane/toluene fuel blends were also measured to study the effect of soot propensity. Gas sampling, non-linear excitation regime two-line atomic fluorescence (NTLAF) and laser-induced incandescence (LII) were used to characterise the flames, investigate the mixing between the hot coflow and the surrounding air, and measure the flame temperature for the different coflow configurations. A comparison of results of the flames issuing into hot coflows is presented, indicating that the hottest flame is not associated with the coflow containing the highest concentration of O2, but with the minimum soot loading and, consequently, the minimum radiative heat loss. Subsequent numerical simulations of canonical opposed-flow flames demonstrate that the soot loading in the downstream region of the flames is strongly dependent on PAH formation in the hot coflow region and further analyses reveal the chemical pathways which are most impacted by small variations in hot coflow composition.
The stabilization mechanism of non-premixed jet flames of methane diluted with helium has been investigated experimentally. Effects of fuel mole fraction, XF,O and nozzle diameter, D on the lifted flame characteristics of diluted methane jets were studied. Such methane jet flames could be lifted despite the Schmidt number was less than unity. Regimes of lifted flames were evaluated according to Richardson number and liftoff height compared with the length of developing zone. Such flames obtained using D = 9.4 mm nozzle were stabilized due to buoyancy induced convection in buoyancy dominated regime whereas for D = 0.95 mm nozzle methane jet flames could be lifted even at nozzle exit velocities much higher than stoichiometric laminar flame speed in jet momentum dominated regime. The chemiluminescence intensities of OH* radical (good indicators of heat release rate) were measured using monochromatic system for these lifted flames. It was confirmed that, in jet-momentum dominated regime an increase in radius of curvature in addition to OH* concentration stabilizes such lifted flames. Heat release rate near the triple point inferred by the OH* chemiluminescence intensity was inversely proportional to XF,O and had maximum at blowout conditions. © 2018 International Information and Engineering Technology Association.
This paper explores turbulent autoignition and flame stabilisation for a range of fuels, utilising a jet in a hot coflow burner. Jet fuels including: hydrogen, dimethyl ether and hydrocarbons ranging from CH4 to C4 H8 are investigated with the influence of partial premixing, dilution and hot coflow temperature. Simultaneous acoustic emission measurements and high-speed chemiluminescence imaging at 10 kHz are performed; investigating the flame lift-off dynamics and to study the initiation and evolution of autoignition kernels. For all fuels studied, a common trend is found for increasing coflow temperatures; where a transition from high lift-off flames exhibiting an autoignition kernel dominated flame stabilisation mechanism, to lower lift-off flames exhibiting a premixed flame propagation stabilisation mechanism. Three key findings are reported: (i) common to all fuels studied for the high lift-off flames, the lift-off height vs. time follows a sawtooth-like trend. The leading edge of the main flame body (flame base) drifts downstream with near constant velocity, whilst upstream of the flame base autoignition kernels form and grow rapidly merging with the flame base; thereby lowering the tip of the flame base. (ii) High amplitude acoustic emission events correlate well with auto-ignition kernel flame base merging events for high lift-off flames, for the fuels studied. The ethylene flames produced the highest sound levels for a given mean lift-off height. (iii) In the high lift-off height regime, the lift-off height for all fuels scales well with corresponding simple 0-D auto-ignition delay calculations. The good correlation of the lift-off height scaling with the computed autionition delay implies that chemical kinetics, rather than turbulent mixing controls the processes at the base of these flames for higher lift-off height flames, indicating that autoignition is the dominant stabilising mode.
In the current study, the auto-ignition dynamics of cold fuel jets issuing into a high-temperature, vitiated environments is investigated. Due to the short time scale of these events, high-speed measurements are used to resolve the coupled spatio-temporal behavior. The present study uses high-speed (20-kHz) OH* chemiluminescence imaging to identify the location and timing of the formation of the initial ignition kernels, providing visualization of the ignition dynamics and a detailed statistical evaluation of ignition heights and ignition delay times across a broad parameter space which includes variations in fuel type, dilution levels, coflow temperature, and coflow oxidizer content. The auto-ignition location and ignition delay times show a strong sensitivity to coflow temperature with increased sensitivities at lower coflow temperatures. Comparisons between kernel formation location for the transient jet and the fluctuating flame base of the subsequent, steady-state flame is presented, highlighting the role of flame propagation on flame stabilization. Results indicate that at lower temperatures the flame stabilization mechanism is dominated by auto-ignition, but at higher coflow temperatures, flame propagation plays a key role. The effects of variations in the hot, coflow oxidizer content on ignition properties were found to be noticeable, but still significantly less than variations in the temperature.
The temperature dependent corrections of the formaldehyde laser induced fluorescence raw signal are discussed for the 355nm excitation, which is widely available as the third harmonic of Nd-YAG lasers. The temperature dependence of the HCHO partition function is calculated explicitly and the effect of quenching corrections is discussed in view of the absence of experimental data on collision cross-sections. Particular reference is made to the case of HCHO layers in hydrocarbon diffusion flames. It is shown that the thickness of such layers is not affected drastically by the calculated corrections, which has implications for the estimate of the scalar dissipation rate in diffusion flames.
The autoignition of a laminar non-premixed flame placed in a field of homogeneous isotropic turbulence has been studied previously using single-step chemistry and/or simplified models for diffusion processes. The existence of a specific value of the mixture fraction, called “most-reactive,” and the importance of the scalar dissipation rate to predict the ignition location were demonstrated. The effect of the turbulence intensity on the ignition time was found to be non-monotonic. In this work, we wish to assess the influence of more realistic chemistry and transport models on ignition location and time. To do so, direct simulations are carried out using a detailed reaction scheme, multicomponent diffusion velocities and accurate thermodynamic properties. We observe that the turbulent non-premixed flame ignites always faster than the laminar one, even for the highest Reynolds numbers investigated. The scalar dissipation rate can still be used to predict the ignition site, as was observed in simple chemistry simulations. But the most-reactive conditions must of course be determined using the detailed modeling, and cannot any more be analytically predicted. The interest of repeating the direct simulations to get rid of the influence of random initial conditions is also demonstrated.
A model for the temperature-dependent collisional quenching of OH A(sup 2) Sigma(sup +) is presented. The model for quenching is based on a classical electron transfer mechanism. Predictions of the model are shown to adequately reproduce many of the experimentally observed trends: variation with collision partner, temperature, and vibrational level in OH A(sup 2) Sigma(sup +), and the disposition of the quenching products. A negligibly small electronic quenching cross-section is predicted for collision partners having positive ions that cannot be produced through a thermal collision with OH A(sup 2) Sigma(sup +). Results of the model are compared to experimentally measured cross-sections for a number of species of interest in combustion and aerothermodynamic applications. A general function for the temperature dependence of the cross-section for collisional quenching of OH A(sup 2) Sigma(sup +) is derived. Curve-fitting coefficients for a number of collision partners are tabulated.
Direct numerical simulation (DNS) results of autoignition in a non-premixed medium under an isotropic, homogeneous, and decaying turbulence are presented. The initial mixture consists of segregated fuel parcels randomly distributed within warm air, and the entire medium is subjected to a three-dimensional turbulence. Chemical kinetics is modeled by a four-step reduced reaction mechanism for autoignition of n-heptane/air mixture. Thus, this work overcomes the principal limitations of a previous contribution of the authors on two-dimensional DNS of autoignition with a one-step reaction model. Specific attention is focused on the differences in the effects of two- and three-dimensional turbulence on autoignition characteristics. The three-dimensional results show that ignition spots are most likely to originate at locations jointly corresponding to the most reactive mixture fraction and low scalar dissipation rate. Further, these ignition spots are found to originate at locations corresponding to the core of local vortical structures, and after ignition, the burning gases move toward the vortex periphery. Such a movement is explained as caused by the cyclostrophic imbalance developed when the local gas density is variable. These results lead to the conclusion that the local ignition-zone structure does not conform to the classical stretched flamelet description. Parametric studies show that the ignition delay time decreases with an increase in turbulence intensity. Hence, these three-dimensional simulation results resolve the discrepancy between trends in experimental data and predictions from DNSs of two-dimensional turbulence. This qualitative difference between DNS results from three- and two-dimensional simulations is discussed and attributed to the effect of vortex stretching that is present in the former, but not in the latter.
Flamelet models for turbulent combustion typically employ the assumption of unity Lewis number, i.e., equal thermal and species diffusivities. These models have been employed to predict ignition delay times and ignition location in combusting sprays. However, there is the interesting question: what would be the effects of including multicomponent species diffusion on the ignition predictions? In this work, a one-dimensional n-heptane–air diffusion flame is chosen to study the effects of multicomponent diffusion on predicted ignition characteristics. The ambient conditions selected include typical in-cylinder conditions of a medium-duty diesel engine: pressure 10–40 bar and air temperature 850–1000 K. The ignition and oxidation of n-heptane are predicted using a reaction mechanism consisting of 34 species and 56 steps. The mixture fraction is computed separately as a passive species, the diffusion coefficient, of which is equal to the local thermal diffusion coefficient. From these computations, the transient structure of the flamelet, including ignition, is obtained. The results are compared with those obtained with the unity Lewis number assumption. The implications of the unity Lewis number assumption on the predicted ignition characteristics are discussed.
Experimental methods are detailed for comprehensive further investigation of the turbulent premixed flames of natural-gas/air mixtures stabilized on a Bunsen-type burner already studied by joint imaging of OH and velocity (Franker et.al, 1999). Simultaneous two-dimensional measurements of reaction progress variable and OH mole fraction are made from planar imaging of Rayleigh scattering and laser-induced fluorescence of OH. Image in-plane and out-of-plane spatial resolution of the order of 100 μm has been achieved. Care is taken to ensure that the instantaneous flame-front structure can be adequately resolved, and the measured scalar structure in laminar flames agrees well with flame calculations with a C-3 mechanism. Mean velocity and turbulence intensity profiles are presented for the non-reacting flows together with measurements of longitudinal and transverse correlation functions, their associated length scales, and the dissipation rate of the turbulence kinetic energy. In this paper, we report the mean structure of the turbulent flame brush in terms of Reynolds- and Favre-averaged profiles of the reaction progress variable and its standard deviation, and selected probability density functions. The results are compared with those derived from the OH images using the thin flamelet assumption. The mean and standard deviation of OH mole fraction are also presented. Preliminary conclusions are drawn about the relationship of the mean flame brush structure to the turbulence and the admixture of co-flowing air. The validity of the thin-flamelet assumption appears to be questionable for the lean flames investigated. Results for the structure of the instantaneous flame fronts are reported in a companion paper (Chen and Bilger, 2001).
Rate coefficients are reported for vibrational energy transfer and electronic quenching of OH A(2) Sigma(+)(v' = 1) by N-2, O-2, CO, CO2, NO, Ar, Kr, and Xe. Rate coefficients for electronic quenching of OH A(2) Sigma(+)(v' = 0) by the same set of collision partners are also reported. The measurements were performed at high temperatures (1900 and 2300 K) behind reproducible shock waves. The cross sections for quenching in v' = 1 were observed to be quite similar to the values found for quenching in v' = 0. For all of the species studied, the cross sections were found to be independent of temperature from 1900 to 2300 K. However, all of the high-temperature cross sections were found to be smaller than the previously reported values for quenching and vibrational energy transfer at 300 K. The decrease in the cross sections with temperature was observed to be more pronounced for vibrational energy transfer than for electronic quenching.
Planar imaging of flow scalars is widely used in fluid mechanics, but the effects of imaging system blur on the measured scalar and its gradients are often inadequately quantified. Here, we present a 1-D analytical study that uses simplified models of the scalar profiles and imaging system blur to estimate the measurement errors caused by finite resolution. One objective of this paper is to give the experimentalist a methodology for quantitatively assessing the impact of imaging system blur on the accuracy of scalar measurements. The scalar profiles are modeled as either error or Gaussian functions, and the imaging system resolution is cast in terms of the line-spread function (LSF), which is modeled as Gaussian. The analysis gives the errors induced in the scalar structure thickness, gradient, and dissipation, for varying degrees of blur, the latter of which is quantified by , the standard deviation of the Gaussian LSF. The results show that, to keep errors in the peak scalar gradients and dissipation to less than 10%, the 20%-width of the scalar structures should be at least 7.5. Typical flow imaging experiments require fast (i.e., low f/#) optics that may suffer from significant blur and, therefore, this requirement may be difficult to meet in many applications. It is also shown that the resolution requirements for measuring the dissipation are more restrictive than for structure thicknesses. Further simulations were made to assess the effects of having clustered, or closely spaced, dissipation structures. Compared to the single structure results, there is a less severe resolution requirement to obtain scalar structure length scales, but a more severe requirement on the scalar gradient and dissipation.
Using laser-induced fluorescence (LIF), spatially resolved concentration profiles of formaldehyde (H2CO) were obtained in the preheating zone of atmospheric-pressure premixed CH4/air flames stabilized on the central slot of a multiple-slot burner similar in construction to domestic boilers. The isolated
pQ1(6) rotational line (339.23nm) in the 21
041
0 vibronic combination transition in the Ã1A2-
1A1 electronic band system around 339nm was excited in the linear LIF intensity regime. For a quantification of quenching effects
on the measured LIF signal intensities, relative fluorescence quantum yields were determined from direct fluorescence lifetime
as a function of height above the slot exit. Absolute H2CO number densities in the flames were evaluated from a calibration of measured LIF signal intensities versus those obtained
in a low-pressure sample with a known H2CO vapor pressure. Peak concentrations in the slightly lean and rich flames reached (994±298) and (174±52) ppm, respectively.
Two-dimensional direct numerical simulations have been performed of the autoignition of (i) laminar and turbulent shearless mixing layers between fuel and hotter air, (ii) thin slabs of fuel exposed to air from both sides, and (iii) homogeneous stagnant adiabatic mixtures. It has been found that the time for the first appearance of an ignition site is almost independent of the turbulence time scale, varies little in individual realisations of the same flow, decreases with partial premixing, is shorter in turbulent than in laminar flows, and decreases with decreasing width of the fuel stream. The autoignition time in the turbulent flows in longer than the ignition delay time of stagnant homogeneous mixtures and this implies that the heat losses due to mixture fraction gradients associated with mixture inhomogeneities increase the autoignition time. It has also been found that ignition always occurs at a well-defined mixture fraction fMR, which is accurately predicted by previous laminar flow analyses to depend only on the fuel and oxidant temperatures and the activation energy. As a measure of the heat losses of the heat-producing regions that eventually autoignite, the time evolution of the scalar dissipation rate, conditional on the most reactive mixture fraction, is examined and used to explain successfully all the observed trends of autoignition time with turbulent time scale, flow length scale, and partial premixing. The implications of these findings for modelling and for the interpretation of experimental data are discussed.
The autoignition behaviour of hydrogen in a turbulent co-flow of heated air at atmospheric pressures was examined experimentally. Turbulent flows of air, with temperatures up to 1015 K and velocities up to 35 m/s, were set up in an optically accessible tube of circular cross-section. The fuel, pure or diluted with nitrogen, was continuously injected along the centreline of the tube, with velocities equal to or larger than those of the air, and temperatures that were lower. The fuel mixing patterns hence obtained were akin to diffusion from a point source or to an axisymmetric jet within a co-flow. For a relatively wide range of temperatures and velocities, a statistically steady condition of randomly occurring autoignition kernels was observed, whose axial location was measured by hydroxyl radical chemiluminescence. The probability density function of autoignition location was sharp enough to allow the accurate determination of a minimum autoignition length and smooth enough to allow the mean and variance to be calculated. It was found that both autoignition lengths increased with the air velocity and decreased with the air temperature, as expected. An estimate of the residence time up to autoignition showed that the autoignition delay times increased with the air velocity for the same temperature, suggesting a delaying effect of the turbulence on autoignition. The connection between these findings and previous experimental and direct numerical simulation studies is discussed.
The present vitiated coflow flame consists of a lifted jet flame formed by a fuel jet issuing from a central nozzle into a large coaxial flow of hot combustion products from a lean premixed H2/air flame. The fuel stream consists of CH4 mixed with air. Detailed multiscalar point measurements from combined Raman–Rayleigh–LIF experiments are obtained for a single base-case condition. The experimental data are presented and then compared to numerical results from probability density function (PDF) calculations incorporating various mixing models. The experimental results reveal broadened bimodal distributions of reactive scalars when the probe volume is in the flame stabilization region. The bimodal distribution is attributed to fluctuation of the instantaneous lifted flame position relative to the probe volume. The PDF calculation using the modified Curl mixing model predicts well several but not all features of the instantaneous temperature and composition distributions, time-averaged scalar profiles, and conditional statistics from the multiscalar experiments. A complementary series of parametric experiments is used to determine the sensitivity of flame liftoff height to jet velocity, coflow velocity, and coflow temperature. The liftoff height is found to be approximately linearly related to each parameter within the ranges tested, and it is most sensitive to coflow temperature. The PDF model predictions for the corresponding conditions show that the sensitivity of flame liftoff height to jet velocity and coflow temperature is reasonably captured, while the sensitivity to coflow velocity is underpredicted.
Excited state lifetimes have been measured for the A-states of CH, OH, and NO in a number of low-pressure, premixed, laminar flow methane flames. From these lifetimes, collisional quenching rates were determined as a function of height above the burner and thus as a function of flame temperature and composition. The results were compared with values calculated using a model of the flame chemistry to predict collider mole fractions, together with parameterizations of quenching rate coefficients for each collider. Measured OH and NO quenching rates agree well with those calculated from these quenching rate coefficients and modeled flame composition data. This indicates that collisional quenching corrections for laser-induced fluorescence measurements can be calculated from knowledge of major species mole fractions and gas temperature. Predicted quenching rates for CH range from agreement with measured values to 27% higher than measured values. This discrepancies suggest insufficient knowledge of high temperature quenching by H2O and N2.
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 autoignition of spatially non-homogeneous hydrogen-air mixtures in 2-D random turbulence and mixture fraction fields is studied using the Direct Numerical Simulation (DNS) approach coupled with detailed kinetics. The coupling between chemistry and the unsteady scalar dissipation rate field is investigated over a wide range of different autoignition scenarios. The simulations show that autoignition is initiated at discrete spatially localized sites, referred to as kernels, by radical build-up in high-temperature, fuel-lean mixtures, and at relatively low dissipation rates. Detailed analysis of the dominant chemistry and the relative roles of reaction and diffusion is implemented by tracking the evolution of four representative kernels that characterize the range of ignition behaviors observed in the simulation. This evolution yields different autoignition delay scenarios as well as extinction at the different sites based on the local dissipation rates and their temporal histories. Where significant autoignition delay and extinction are observed, a shift in the relative roles of dominant reactions that contribute to radical production and consumption during this induction phase is observed. This shift is particularly characterized by an increased role of termination reactions during the intermediate stages of the induction period, which results in extinction in approximately two thirds of the ignition kernels in the computational domain. The fate of the different kernels is associated with: (1) the dissipation of heat that contributes to a slowdown in chemical reactions and a shift in the balance between chain-branching and chain-termination reactions; (2) the dissipation of mass that keeps the radical pool growth in check, and that is promoted by slower reaction rates; and (3) counter to the effects of dissipation of heat and intermediate species, the preferential diffusion of H2 relative to both heat and its diluent, N2, that promotes ignition. Ultimately, the balance between radical production and dissipation determines the success or failure of a given kernel to ignite. A new criterion for unsteady ignition is presented based on the instantaneous balance between radical production and dissipation. A Damköhler number, so defined, must remain above a critical value of unity at all times during the induction period if the kernel is to eventually ignite. Inherent in a multi-step kinetic description of ignition phenomena is the disparate time scales associated with different elementary reactions that, coupled with the characteristic scales of heat and mass dissipation, may yield different dominant chemistries at different stages of the induction process for a given kernel. To capture the strong history effects associated with radical build-up, new ignition progress variables based on key radical species are investigated.
Local heat release rate represents one of the most interesting experimental observables in the study of unsteady reacting flows. The direct measure of burning or heat release rate as a field variable is not possible. Numerous experimental investigations have relied on inferring this type of information as well as flame-front topology from indirect measures that are presumed to be correlated. A recent study has brought into question many of the commonly used flame-front marker and burning-rate diagnostics. This same study found that the concentration of formyl radical offers the best possibility for measuring flame burning rate. However, primarily due to low concentrations, the fluorescence signal level from formyl is too weak to employ this diagnostic for single-pulse measurements of turbulent-reacting flows.In this paper, we describe and demonstrate a new fluorescence-based reaction-front imaging diagnostic suitable for single-shot applications. The measurement is based on taking the pixel-by-pixel product of OH and CH2O planar laser-induced fluorescence (PLIF) images to yield an image closely related to a reaction rate. The spectroscopic and collisional processes affecting the measured signals are discussed, and the foundation of the diagnostic, as based on laminar and unsteady flame calculations, is presented. We report the results of applying this diagnostic to the study of a laminar premixed flame subject to an interaction with an isolated line-vortex pair.
Laser-induced fluorescence has been observed form the formaldehyde Ã1A2-X̃1A1 electronic transition in a well characterized, laminar methane/air diffusion flame burning at atmospheric pressure. This represents the first optical measurement in flames of naturally occurring formaldehyde, an important intermediate in the oxidation of hydrocarbons. Both 355 nm and tunable dye laser excitation of fluorescence are demonstrated. The observed fluorescence signals are corrected for partition function effects and for estimated collisional quenching rates to obtain relative concentration profiles.
Autoignition of hydrocarbon fuels is an outstanding research problem of significant practical relevance in engines and gas turbine applications. This paper presents a numerical study of the autoignition of methane, the simplest in the hydrocarbon family. The model burner used here produces a simple, yet representative lifted jet flame issuing in a vitiated surrounding. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD package, FLUENT. The in situ adaptive tabulation (ISAT) method is used to implement detailed chemical kinetics. An analysis of species concentrations and transport budgets of convection, turbulent diffusion, and chemical reaction terms is performed with respect to selected species at the base of the lifted turbulent flames. This analysis provides a clearer understanding of the mechanism and the dominant species that control autoignition. Calculations are also performed for test cases that clearly distinguish autoignition from premixed flame propagation, as these are the two most plausible mechanisms for flame stabilization for the turbulent lifted flames under investigation. It is revealed that a radical pool of precursors containing minor species such as CH3, CH2O, C2H2, C2H4, C2H6, HO2, and H2O2 builds up prior to autoignition. The transport budgets show a clear convective–reactive balance when autoignition occurs. This is in contrast to the reactive–diffusive balance that occurs in the reaction zone of premixed flames. The buildup of a pool of radical species and the convective–reactive balance of their transport budgets are deemed to be good indicators of the occurrence of autoignition.
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.
The balanced cross-rate model is proposed to analyze laser-induced molecular fluorescence signals when the laser pulse length is of the order of nanoseconds. Nanosecond pulse length lasers, specifically Q-switched Nd:YAG-pumped dye lasers, are attractive for saturated molecular fluorescence spectroscopy because of their high peak power and because their short pulse length minimizes the risk of laser-induced chemistry. In the balanced cross-rate model, single upper and lower rotational levels are assumed to be directly coupled by the laser radiation. Because the laser-induced processes which couple these levels are so fast at saturation intensities, a steady state is established between the two levels within picoseconds. Provided that the total population of the two laser-coupled rotational levels is constant during the laser pulse, the total molecular population can be calculated from the observed upper rotational level population using a two-level saturation model and Boltzmann statistics. Numerical simulation of the laser excitation dynamics of OH in an atmospheric pressure H(2)/O(2)/N(2) flame indicates that the balanced cross-rate model will give accurate results provided that the rotational relaxation rates in the upper and lower sets of rotational levels are approximately equal.