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Large eddy simulation of two isothermal and reacting turbulent separated oxy-fuel jets
Abstract
In this work, a Large eddy simulation (LES) method and a tabulated chemistry approach according to the Flamelet Generated Manifold (FGM) strategy are coupled to numerically study the interactions of turbulent isothermal and reacting flows stemming from two aligned jets providing alternately fuel (natural gas) and oxidant (pure oxygen gas).The jets feature different geometries and deliver unequal momentums at the boundaries. The effect of oxygen in comparison to air environment on the FGM tabulation and results is pointed out. In addition, the impact of combustion on the flow and mixing field evolvement is analyzed. The LES relies on a dynamic Smagorinsky subgrid scale (SGS) model and a linear eddy diffusivity ansatz to close the SGS stresses and the SGS scalar fluxes for describing the turbulent flow field and the turbulent scalar field, respectively. For model assessment, available laser-based experimental data are used for model validation. In particular the numerical results are compared with available experimental data for the flow field. The latter are gained experimentally by the Particle Image Velocimetry (PIV) and laser tomography, respectively. In the first part of this paper, the jets interaction process is studied for the isothermal case while the oxy-fuel combustion in the reacting case is analyzed in the second part. The analysis is achieved in terms of statistical quantities for the flow velocity, mixture fraction, chemical species and temperature. An overall satisfactory agreement is reported.
... However, Mayer et al. in [21] discussed the usability and limits of the SFM approach in oxy-fuel combustions. Hidouri et al. in [22] applied in the LES framework an alternative approach based on the joint PDF method coupled to FGM technique based on the mixture fraction and the reaction progress variable to investigate the behavior of two reacting separated oxy-fuel jets. Thereby the sub-grid PDF shape is described by the presumed beta-PDF assumption. ...
... One of the early turbulent simulations incorporating the FPV approach in the LES framework has been reported in [17], where it was shown that the LES-FPV approach possesses in turbulent regimes potential promise in addressing the issue of differential diffusion in such a flame. Similar to FPV, the FGM method has been used in [22] with Le = 1 to tackle oxy-fuel combustion process in twin-jets. ...
... It is usually modeled from the FPV table as function of the reaction progress variable along with the presumed PDF. According to previous LES studies in oxy-flames [17,22], the application of a presumed shape of PDF with Beta-function experienced shortcomings regarding the general prediction capability of the temperature distribution and minor species evolution. As pointed out in [17], the results obtained within the LES-FPV framework are affected by the pre-calculated flame structures through the tabulated transport properties and source terms from the manifolds, and all data are additionally PDF-integrated. ...
Abstract: The oxidation of methane under oxy-fuel combustion conditions with carbon capture is attractive and deserves huge interest towards reducing CO2 and NOx emissions. The current paper
reports on the predictions and analysis of combustion characteristics of a turbulent oxy-methane non-premixed flame operating under highly diluted conditions of CO2 and H2 in oxidizer and fuel
streams, respectively. These are achieved by applying a novel, well-designed numerical combustion model. The latter consists of a large eddy simulation (LES) extension of a recently suggested hybrid
model in Reynolds averaging-based numerical simulation (RANS) context by the authors. It combines a transported joint scalar probability density function (T-PDF) following the Eulerian
Stochastic Field methodology (ESF) on the one hand, and a flamelet progress variable (FPV) turbulent combustion model under consideration of detailed chemical reaction mechanism on the
other hand. This novel hybrid ESF/FPV approach removes the weaknesses of the presumed-probability density function (P-PDF)-based FPV modeling, along with the solving of associated
additional modeled transport equations while rendering the T-PDF computationally less affordable. First, the prediction capability of the LES hybrid ESF/FPV was appraised on the well-known air-piloted methane jet flame (Sandia Flame D). Then, it was assessed in analysing the combustion properties of a non-premixed oxy-flame and in capturing the CO2 dilution effect on the oxy-fuel flame behavior. To this end, the so-called oxy-flame B3, already numerically investigated in a RANS context, was analysed. Comparisons with experimental data in terms of temperature, scalar distributions, and scatter plots agree satisfactorily. Finally, the impact of generating the FPV chemistry table under condition of unity Lewis number, even with CO2 dilution, was investigated on the general prediction of the oxy-fuel flame structure, stability and emissions. In particular, it turns out that 68% molar percentage of CO2 leads to 0.39% of CO formation near the burner fuel nozzle and 0.62% at 10 d-fuel above the nozzle.
... Burner geometry and ways of fuel injection were shown to have strong impact on flame stability and consequently on the efficiency of the combustion system [26]. Hidouri et al. and Yahia et al. [27][28][29] studied numerically the mixing process between isothermal and reacting jets. Their results showed that mixing was enhanced by the use of separated jet burners compared to a simple jet burner. ...
... Their results showed that mixing was enhanced by the use of separated jet burners compared to a simple jet burner. This was confirmed experimentally by Boushaki et al. [30] using the same burner with separated jets as those of Hidouri et al. [27,29]. Recently, Yang et al. [31] investigated numerically the combustion and emission characteristics of a new elliptical jet burner. ...
This work investigates the combustion characteristics of CH4-air/O2 turbulent swirling flames in a co-axial burner with radial fuel injection. The burner configuration consists of two coaxial tubes with a swirler positioned within the annular space for the oxidizer flow. The central tube delivers the fuel through eight holes placed that are uniformly distributed along the circumference of the inner tube nearby the burner exit. The experiments were carried out in a 25-kW combustion chamber of dimensions 50x50x120 cm³. Stereo Particle Image Velocimetry (SPIV) was used to characterize the different velocity fields, whereas the OH* Chemiluminescence is employed to examine the flame structure and stability. The exhaust gas compositions were measured using multi-gas analyzers. The structure and stability of flames, temperature evolutions, CO2, and pollutant emissions (NOx, CO) were examined under different parameters, such as: oxygen enrichment (21-50%), global equivalence ratio (0.4 -1), swirl number (0.5 and 1.4), injection type of fuel (axial or radial). Experiments were conducted at a constant power of 9.41 kW and variable powers in the range from 9.41 to 22.42 kW. The present results show that the oxygen enrichment decreases the lift-off height and enhances the flame stability using constant flame power. Increasing the swirl number improves the flame stability and produces a better attached flame. At constant flame power and variable oxidizer flow rate, a high oxygen concentration yields lower NOx emissions. For lean combustion with excessive oxygen enrichment over 35%, retaining the same flame power leads to a significant decrease in NOx emissions. The radial injection of CH4 does not affects the NOx emissions, however, the axial injection is shown to be advantageous in terms of CO emissions in the present studied configurations.
... Dealing with oxy-combustion LES modeling, Hidouri et al. in [68] employed in the LES framework an alternative method based on the joint PDF method coupled to the FGM technique based on the mixture fraction and the reaction progress variable in order to investigate the behavior of two reacting separated oxy-fuel jets. Thereby the sub-grid PDF shape is described by the presumed beta-PDF assumption. ...
... One of the early turbulent simulations incorporating the FPV approach in the LES framework has been reported in [58], where it was shown that the LES-FPV approach possesses in turbulent regimes a potential promise in addressing the issue of differential diffusion. Similar to FPV, the FGM method has been used in [68] with Le = 1 to tackle oxy-fuel combustion Similarly to the previous LES case, the inlet turbulent flow field velocity of the current case was generated employing an in house turbulence inflow generator which was developed according to Klein et al in [90]. ...
In the prevailing situation of unsustainable fossil fuel resources and the elevated levels of air pollutant emissions, the state-of-the-art of combustion investigations confronts primarily two challenges. These are on the one hand the optimization of the fossil fuel combustion efficiency and on the other hand the development and the application of robust strategies to reduce the amount of the released pollutant gases with respect to the new emission standards in accordance with the global energy policies.Within this context, the carbon dioxide capture and storage (CCS) technologies play an important role as an accepted strategy towards the mitigation of CO 2 emissions. One of the important aspects of the CCS techniques is the oxidation of natural gas under oxy-fuel combustion conditions. However, very few scientific contributions have been devoted to the research of these systems, so that there is a lack of understanding of the oxy-combustion processes.The present work aims at the development and the application of an advanced numerical approach for the simulation of oxy-fuel combustion in which the TCI is adequatelyaccounted for within non-premixed combustion regimes using the OpenFOAM platform.The suggested model which is designed for both RANS and LES applications consists of a combination of a transported probability density function approach following the Eulerian Stochastic field methodology and the flamelet progress variable (FPV) chemistry reduction mechanism. In the LES framework, the proposed method accurately represents the effect of the sub-grid fluctuations on the flame structure and on combustion characteristics along with the interaction between turbulence and chemistry.The implemented developed combustion model is first verified, and then validated and applied to different turbulent non-premixed combustion configurations featuring an increasing order of complexity. In particular, Sandia flame D which consists of a turbulent piloted methane-air jet flame is first employed for model validation in both RANS and LES contexts. The next flames are more challenging cases, namely the non-premixed Sandia oxy-flame series (A & B), which are operated under different Re numbers and characterized by various CO 2 and H 2 enrichments in the oxidizer and fuel streams, respectively. All investigated cases are well documented with available experimentalmeasurements.The comparison of the obtained results with experimental data in terms of temperature, scalar distributions, PDFs and scatter plots agree satisfactorily, essentially in the LES context.This work finally reveals that the hybrid ESF/FPV approach removes the weaknesses of the presumed probability density function based FPV modeling (β-PDF).
... Murthy et al. has developed a solar generated steam injection for a diesel engine and they showed that NOx emissions and exhaust temperature decrease however the soot emissions, thermal efficiency, power and SFC increase at full load conditions [10]. Several studies have been interested to the pollutant emissions reduction and to the combustion stability by the using of new burners [11,12] or by the substitution of air with pure oxygen as an oxidizer [13,14]. ...
... In this expression, the heat transfer correlation coefficients denoted by a, b and c are given by the (13) where B is the cylinder diameter, h is the heat transfer coefficient, c p is the specific heat at constant pressure, λ is the thermal conductivity, μ is dynamic viscosity, ρ is the density and w is the average cylinder gas speed using Woschni Correlation [21]. To obtain the average cylinder gas velocity, Woschni proposed a correlation that relates the gas velocity to the mean piston speed and to the pressure rise due to combustion: ...
Combined effects of compression ratios and steam injection on performance, combustion and emission characteristics of a HCCI engine are numerically investigated. The pollutant emission is controlled by the dilution of the reactant by steam injection. Combustion is performed by using the Internal Combustion Engine (ICE) model. For model assessment, computed results are compared to the published data available in the literature obtained with same boundary conditions. An overall satisfactory agreement is reported. Three values of steam injection ratio are tested. Results show that the performance of the HCCI engine is very low if the steam injection exceed 20%.
... In addition, NO emissions are reduced by 50% compared to the circular burner. Hidouri et al. (2016Hidouri et al. ( , 2017 investigated numerically the mixing process between the isotherm and reactive jets. They showed that mixing was improved using separated-jet burners compared to a singlejet. ...
This work investigates numerically a non-premixed swirling flame in a coaxial burner with a radial fuel injection under lean and rich conditions of mixtures. The swirler is placed in the annular part of the burner. The fuel is injected radially through eight holes symmetrically distributed on the periphery of the central tube. All simulations are carried out using the ANSYS-Fluent CFD code. The turbulence is captured using the Reynolds Averaged Navier-Stokes approach. Chemistry/turbulence interaction is resolved using the Eddy Dissipation Model. Simulations are performed with a global equivalence ratio ranging from 0.5 to 1.3. Model validation is achieved by comparing computed results to our experimental data of Stereoscopic Particle Image Velocimetry obtained in the case of the stoichiometric regime. Good agreement between numerical results and experimental measurements is assigned. The central recirculation zone and the swirling jet region due to the presence of the swirl are well predicted by the simulations. The effect of the global equivalence ratio on the profile of axial velocity, temperature distributions and pollutant emissions (CO and NOx) is numerically studied. From dynamic point of view, the equivalence ratio modifies the mean axial velocity of the swirling diffusion flame. The increase of equivalence ratio destabilizes the flame by increasing the liftoff height. NO production decreases by the increasing of the equivalence ratio.
... They showed that the use of a non-equal scale model improved the obtained numerical results. The mixing process between the isotherm and reacting jets was investigated numerically by Hidouri et al. and Yahya et al. [24][25][26]. The study showed that the mixing was enhanced using separated jet burners compared to a simple jet burner. ...
This paper reports an experimental and numerical investigation of a methane-air diffusion flame stabilized over a swirler coaxial burner. The burner configuration consists of two tubes with a swirler placed in the annular part. The passage of the oxidant is ensured by the annular tube; however, the fuel is injected by the central jet through eight holes across the oxidizer flow. The experiments were conducted in a combustion chamber of 25 kW power and 48 × 48 × 100 cm3 dimensions. Numerical flow fields were compared with stereoscopic particle image velocimetry (stereo-PIV) fields for non-reacting and reacting cases. The turbulence was captured using the Reynolds averaged Navier-Stokes (RANS) approach, associated with the eddy dissipation combustion model (EDM) to resolve the turbulence/chemistry interaction. The simulations were performed using the Fluent CFD (Computational Fluid Dynamic) code. Comparison of the computed results and the experimental data showed that the RANS results were capable of predicting the swirling flow. The effect of the inlet velocity ratio on dynamic flow behavior, temperature distribution, species mass fraction and the pollutant emission were numerically studied. The results showed that the radial injection of fuel induces a partial premixing between reactants, which affects the flame behavior, in particular the flame stabilization. The increase in the velocity ratio (Rv) improves the turbulence and subsequently ameliorates the mixing. CO emissions caused by the temperature variation are also decreased due to the improvement of the inlet velocity ratio.
... A growing concern about air pollution in urban environments results in stringent legislation for devices certification and energy utilization [1]. Several studies have been interested in the pollutant emissions reduction and the optimization of combustion systems by the use of new burners [2,3] or by capturing target species [4]. To assess the problem in detail, accurate prediction of the transport and dispersion of airborne contaminants in urban environments is needed. ...
Hazardous gas dispersion within a complex urban environment in 1:1 scaled geometry of German cities, Hanover and Frankfurt, is predicted using an advanced turbulence model. The investigation involves a large group of real buildings with a high level of details. For this purpose, Computer Aided Design (CAD) of two configurations are cleaned, then fine grids meshed in. Weather conditions are introduced using power law velocity profiles at inlets boundary. The investigation focused on the effects of release locations and material properties of the contaminants (e.g., densities) on the convection/diffusion of pollutants within complex urban area. Two geometries demonstrating different topologies and boundaries conditions are investigated. Pollutants are introduced into the computational domain through chimney and/or pipe leakages in various locations. Simulations are carried out using Large Eddy Simulation (LES) turbulence model and species transport for the pollutants. The weather conditions are accounted for using a logarithmic velocity profile at inlets. CH4 and CO2 distributions, as well as turbulence quantities and velocity profiles, show important influences on the dispersion behavior of the hazardous gas.
... In order to speed up the integration of the chemistry equations and reduce the overall computational cost, chemical mechanism reduction and chemistry tabulation/storage/retrieval approach, different studies have been published to be common representatives of such practices. See as examples the work in references [3,4], where the Flamelet Generated Manifolds (FGM) tabulation technique was utilized as one of the chemical reduction techniques. ...
In the present paper, the behaviour of an oxy-fuel non-premixed jet flame is numerically investigated by using a novel approach which combines a transported joint scalar probability density function (T-PDF) following the Eulerian Stochastic Field methodology (ESF) and a Flamelet Progress Variable (FPV) turbulent combustion model under consideration of detailed chemical reaction mechanism. This hybrid ESF/FPV approach overcomes the limitations of the presumed- probability density function (P-PDF) based FPV modelling along with the solving of associated additional modelled transport equations while rendering the T-PDF computationally less demanding. In Reynolds Averaged Navier-Stokes (RANS) context, the suggested approach is first validated by assessing its general prediction capability in reproducing the flame and flow properties of a simple piloted jet flame configuration known as Sandia Flame D. Second, its feasibility in capturing CO2addition effect on the flame behaviour is demonstrated while studying a non-premixed oxy-flame configuration. This consists of an oxy-methane flame characterized by a high CO2 amount in the oxidizer and a significant content of H2 in the fuel stream, making it challenging for combustion modelling. Comparisons of numerical results with experimental data show that the complete model reproduces the major properties of the flame cases investigated and allows achieving the best agreement for the temperature and different species mass fractions once compared to the classical presumed PDF approach.
This paper reports results on the effect of oxygen enrichment on flow and turbulent flames in a coaxial swirl burner. The investigation was performed by S-PIV in CH4-air-O2 non-reacting and reacting flows. This work is the continuation of studies carried out previously by the authors on the same burner, with different new operating conditions. The burner consists of 2 concentric tubes, with a swirler located in the annular part through which the oxidant (air-O2) is supplied. The CH4 gas is delivered into the central tube and injected radially into the combustion chamber through small holes. The burner is placed in a parallelepiped combustion chamber with a volume of 120×50×50 cm³. The paper reports detailed results on mean velocity fields, profiles of the three velocity components (U,V,W) and velocity fluctuations (U’,V’,W’) in the case of a 150 Nl/min oxidant flow rate, a swirl number Sn = 1.4 and an equivalence ratio Φ = 1. Three cases of O2 addition were investigated, 21%, 25%, and 30% (in vol.) corresponding to 9.4 kW, 11.7 kW, and 14.5 kW of combustion power, respectively. In-depth analysis of the field velocity differences between reactive and non-reactive cases was carried out and showed a significant difference in flow velocities between reacting and non-reacting flows. The profiles of the mean velocities, fluctuations and velocity decays differed, and higher values were observed in the reacting case. The enrichment of air with oxygen affected the velocity fields and their fluctuations in both distribution and values. The addition of O2 increased the flame temperature, leading to a greater radial expansion and a reduction in the size of the recirculation zone. The maximum axial velocity increased significantly as a function of the O2 rate, whereas the swirling velocity varied only slightly. The velocity fluctuation results also revealed a difference in profiles and values depending on O2 enrichments and locations along the flow.
We report the results of laboratory investigations of the behaviour during explosion mitigation by channels typical of those used in commercial explosion arrester devices. Experimental results are presented first of shock and detonation wave interaction with a miniaturised matrix of narrow channels aligned parallel to the incoming flows. The results with a larger scale device using the same channel dimensions tested at more extreme conditions representative of overdriven detonation certification test conditions are consistent with the reduced scale laboratory measurements. To evaluate the applicability of laboratory scale evaluation of arrester elements for other types of explosion arrester, a further preliminary study is also reported with deflagration arrester elements. Finally, the present observations are interpreted in the light of published theories potentially relevant to explaining successful explosion mitigation, or otherwise, by commercial arrester devices. Using this approach, laboratory scale measurements are shown to be as a useful tool for investigating the dominant explosion mitigation mechanisms relevant to practical end user application safety devices and provide an opportunity for assisting with the design and evaluation of practical explosion, especially designs that could be supported by more comprehensive detailed CFD simulations where the supporting laboratory test configurations could also facilitate the use of more sophisticated diagnostic methods
De nos jours, l’énergie délivrée par la combustion dépasse 80% de l‟énergie totale dans le monde, et ce pourcentage restera probablement élevé le long des 100 prochaines années. La plupart des systèmes réactifs qui génèrent la combustion turbulente sont utilisés dans la fabrication, le transport et l‟industrie pour la génération des puissances. Comme résultat, l‟émission des polluants est parmi les problèmes majeurs qui sont devenus des facteurs critiques dans notre société. Dans ce cadre, une étude détaillée des systèmes réactifs est alors nécessaire pour la conception de systèmes de haute performance qui s‟adaptent aux technologies modernes. L'optimisation des performances de ces systèmes énergétiques permet d‟une part d‟économiser l'énergie et d‟autre part de réduire la pollution. Les jets turbulents sont impliqués dans l'efficacité de ces divers systèmes. Dans le cas isotherme, la complexité des écoulements turbulents résulte principalement de la coexistence des structures de tailles très différentes et de l‟interaction non linéaire entre ces structures. Les plus grandes structures dépendent fortement de la géométrie du domaine considéré, elles sont donc anisotropes. De plus, elles ont une grande durée de vie et elles sont responsables du transport de la quasi-totalité de l'énergie. Les plus petites structures, quant à elles, ont souvent un caractère beaucoup plus "universel" (dû à leur comportement relativement isotrope) et sont à l'origine du processus de dissipation visqueuse. Prédire numériquement la dispersion et le mélange d‟un scalaire non réactif dans un écoulement turbulent est considéré comme un problème primordial et reste toujours actuel. Plusieurs recherches sont attachés à ce sujet afin d‟approfondir de plus à la connaissance de différents phénomènes pour pouvoir les mieux prédire. La prédiction numérique du mélange turbulent existant dans plusieurs applications industrielles et environnementales, a un important intérêt en génie chimique. Il est nécessaire donc de bien comprendre la majorité de propriétés du mélange et de l‟écoulement. En combustion, la complication du comportement des jets résulte de l‟interaction entre le dégagement de la chaleur, les processus de mélange, l'entraînement et la recirculation des gaz. Pour bien comprendre la complexité de ce phénomène, il est nécessaire de connaître parfaitement l'évolution dynamique et scalaire des jets turbulents isothermes en présence d'importantes différences de densité, comme elles peuvent lors de la combustion. Cette optimisation passe par la compréhension de l'effet de la variation des conditions d'entrée sur les processus de mélange dans le cas non réactif et sur la stabilité et la nature de la flamme dans le cas réactif. Ainsi, des études théoriques, expérimentales et numériques, doivent être menées en parallèle pour mieux identifier les effets d'une telle intervention. Bien des questions demeurent ouvertes dans le but de mieux caractériser les différents écoulements turbulents réactifs. Les objectifs des études menées dans ce domaine sont la réduction des émissions de polluants et l‟amélioration du rendement de combustion. Une compréhension du mélange et leur interaction avec les différents processus chimiques traduit donc un enjeu majeur. Elle est considéré alors comme un facteur déterminant la qualité des variétés des procèdes. Ce travail de thèse se base sur les jets coaxiaux qui constituent un cas particulier de jet axisymétrique. Ils sont communément rencontrés dans des différents brûleurs industriels qui assurent le contact entre le comburant et le carburant sous une forme de jets coaxiaux. Cette technique est le siège d‟une amélioration du mélange et de la stabilité des flammes.
Laminar triple flames are investigated numerically using detailed and reduced chemical reaction mechanisms. Triple flames are believed to play an essential role in flame propagation in partially premixed systems. In order to reduce the computational cost of the simulations, the flamelet-generated manifold (FGM) method is used to simplify the chemistry model. The FGM method is based on a library of premixed laminar flamelets. Mixture fraction variations in partially premixed flames are taken into account by using the mixture fraction as an extra degree of freedom. A comparison of the results computed with FGM and with detailed chemistry shows that FGM predicts the structure of triple flames very accurately. The gradient of the mixture fraction in the unburnt mixture is varied, and its influence on the structure and propagation of triple flames is studied. For decreasing gradients, the curvature of the premixed flame branch decreases and the propagation velocity increases. Due to the diffusive nature of the flow and the use of symmetry boundary conditions in the lateral direction, high mixture fraction gradients could not be realized at the position of the flame. As a result, the trailing diffusion flame branch is only weakly present. The heat release in the premixed flame branches is two orders of magnitude higher than in the diffusion flame branch. The premixed and diffusion flame branches are studied using a flamelet analysis. Flame stretch and curvature appear to be significant in the premixed flame branch of a triple flame. These effects cause a decrease in the local mass burning rate of almost 15%. The structure of the diffusion flame branch appears to be similar to the structure of a counterflow diffusion flame with the same scalar dissipation rate.
The use of oxy-combustion in separated-jet burners opens interesting possibilities in pollutant and flame control, and has very important impacts on flame stability. An experimental study of the combustion characteristics and flame structures has been conducted on a simplified version of a separated-jet burner that includes a central natural gas jet surrounded by two pure-oxygen jets. The flame stability and its structural properties are analyzed by the chemiluminescence of OH. Liftoff positions, flame front, flame length, and the concentrations of CO2, CO, O2 and NOx have been measured. The effects of jet exit velocities, jet diameters and separation distance between jets on flame characteristics have been examined. The results quantify the increase of central jet velocity and the distance between the jets makes possible to control and reduce appreciably the NOx emission. The liftoff height and the flame length depend strongly on the spacing between the jets and the jet exit velocities. The flame front spreads noticeably with the distance between the jets; however, the effect of jet exit velocities is lower, especially if the distance between the jets is high.
The influence of co-flow on a turbulent binary gas mixing round jet is numerically studied using a first and a second order turbulence closure models. The objective of the study is to obtain a better understanding of the flow structure and mixing process in turbulent variable-density jets. Comparisons between recently published experimental results and mean mixture fraction, the scalar turbulent fluctuation, and the jet spreading rate, feature reasonably good agreements. It is mainly shown that the co-flow reduces the jet spreading rate, but on the otherhand increases the mixing efficiency.
A study of turbulence/combustion interactions in a relatively large turbulent diffusion flame of an axisymmetric methane
jet into air is presented. A first order k–ɛ turbulence closure model is used along with two different models (equal scales and non-equal scales) for the submodel describing
the scalar dissipation rate. The flamelet concept is used to model the turbulent combustion along with a joint mixture fraction/strain
rate probability density function (PDF) for the prediction of the average parameters of the turbulent diffusion flame. The
numerical approach is that of Patankar and Spalding, while the flamelet simulations are obtained from the RUN-1DL code of
Rogg and co-workers based on a 17 species detailed reaction mechanism. The chosen configuration is that of the experimentally
studied turbulent diffusion flame of Streb [1]. A comparison between these experimental results and the obtained numerical
ones is thus presented. Relatively good agreements are obtained which show the usefulness of the two-scale model compared
to the classical one-scale model for predicting turbulent diffusion flames. Nonetheless some discrepancies are obtained in
the outer and downstream regions of the jet, especially in comparison with the experimental data. These are attributed to
short coming of the considered turbulence model and soot radiation which is not accounted for.
A new approach to chemistry modeling for large eddy simulation of turbulent reacting flows is developed. Instead of solving transport equations for all of the numerous species in a typical chemical mechanism and modeling the unclosed chemical source terms, the present study adopts an indirect mapping approach, whereby all of the detailed chemical processes are mapped to a reduced system of tracking scalars. Presently, only two such scalars are considered: a mixture fraction variable, which tracks the mixing of fuel and oxidizer, and a progress variable, which tracks the global extent-of-reaction of the local mixture. The mapping functions, which describe all of the detailed chemical processes with respect to the tracking variables, are determined by solving quasi-steady diffusion-reaction equations with complex chemical kinetics and multicomponent mass diffusion. The performance of the new model is compared to fast chemistry and steady flamelet models for predicting velocity, species concentration, and temperature fields in a methane-fueled coaxial jet combustor for which experimental data are available. The progress-variable approach is able to capture the unsteady, lifted flame dynamics observed in the experiment, and to obtain good agreement with the experimental data and significantly outperform the fast chemistry and steady flamelet models, which both predict an attached flame.
The continuity of evolving pollutant emission regulations requires the design and the development of new burners. The separated-jets burner concept is an example which implies a careful understanding of the turbulent oxy-fuel flames properties developing in a furnace. Modeling, such as turbulent combustion, is a challenge it combines complex flow and transport phenomena with combustion event including a vast amount of species and reactions. The Flamelet Generated Manifold (FGM) method is a promising technique to model reacting flows using tabulated chemistry approach. It is considered as a function of the mixture fraction (Z) and a reaction progress variable (Y). In this work, the interaction of three aligned separated jets is numerically investigated. The central jet provides the natural gas and the others sides jets provide the pure oxygen. Turbulence is modeled using the Large Eddy Simulation (LES) model in which the dynamic Germano approach is applied to determinate the sub grid scale stress. In reacting case, the turbulence chemistry interaction is achieved by coupling the Flamelet Generated Manifold (FGM) tabulated chemistry within the Computational Fluid Dynamic (CFD) code. The numerical results show a good agreement with experimental data notably in the isothermal case. In reacting case, statistical flow field quantities for velocity, mixture fraction and temperature are analyzed in details.
Multiple turbulent jet flames are widely employed in industrial furnaces. The flame pattern and emissions depend on the jet interaction and mixing. This paper presents an experimental and numerical investigation of the mixing characteristics and the resulting concentration fields in unconfined, non-reacting multiple turbulent jets. Experimentally, Planar Laser Induced Fluorescence (PLIF) was employed to study the effects of the Reynolds number, separation distance, and momentum ratio on jet interaction and mixing in three round collinear jets. The three jet configuration was chosen to represent a center fuel jet surrounded by two air jets. The experimental results are compared with predictions using the Fire Dynamics Simulator (FDS) developed by National Institute of Standards and Technology (NIST). The numerical results were found to be in good agreement with the experimental data. FDS is then used to test different jet configurations and will be used for simulating confined and reacting jets in furnaces. The results of this study show that greater dilution in the mean centerline concentration of the center jet occurred at lower separation distance due to enhanced jet interaction. Increasing the momentum of the side jets showed more dilution and better mixing as a result of increased entrainment of the ambient fluid. Extensive numerical studies in a representative five turbulent jet configuration with a center fuel jet surrounded by four oxidizer jets are presented. These provide fundamental guidelines for optimization of the control parameters, jet spacing, momentum ratio of fuel to oxidizer jet, for industrial furnaces with multiple turbulent jets.
This paper investigates two furnaces which work under oxy-fuel condition with natural gas. One is a 0.8 MW furnace where detailed inflame measurements are available. The other furnace is an 11.5 kW lab-scale furnace with temperature measurements. The furnaces were investigated by CFD (Computational fluid dynamics) analysis. The main focus was on using combustion models that are not computationally demanding. Therefore the SFM (steady flamelet) approach was used with two detailed mechanisms. The advantage of the SFM is that the calculation time can be reduced from 4 weeks to 4 days on 8 CPU-cores. The applicability of two detailed mechanisms under oxy-fuel condition is pointed out in this paper. The investigation showed that the skeletal25 mechanism and the SFM are in very good accordance with measurements. If the strain rate between CH4 and O2 stream is too low, the SFM fails to predict the flame shape correctly. The influence of three different turbulence models was also investigated. Furthermore simulations with the eddy dissipation model and numerically expensive eddy dissipation concept model were conducted. Different WSGGM (weighted sum of grey gases model) were applied. The comparison of the WSGGMs showed that the difference between them is insignificant for small furnaces.
Over the years numerical modelling and simulation techniques have constantly been improved with the increase in their use. While keeping the computational resources in mind, numerical simulations are usually adapted to the required degree of accuracy and quality of results. The conventional Reynolds Average Navier Stokes (RANS) is a robust, cheap but less accurate approach. Large Eddy Simulation (LES) provides very detailed and accurate results to the some of the most complex turbulence cases but at higher computational cost. On the other hand, Direct Numerical Simulation (DNS) is although the most accurate of the three approaches but at the same time it is computationally very expensive which makes it very difficult to be applied to the most of the complex industrial problems. The current work is aimed to develop a deeper understanding of multi-hole Gasoline Direct Injection (GDI) sprays which pose many complexities such as; air entrainment in the multi-hole spray cone, Jet-to-Jet interactions, and changes in the spray dynamics due to the internal flow of the injector. RANS approach is used to study multi-hole injector under cold, hot and superheated conditions. Whereas, LES is utilized to investigate the changes in the dynamics of the single spray plume due to the internal flow of the GDI injector. To reduce computational cost of the simulations, dynamic mesh refinement has been incorporated for both LES and RANS simulation. A thorough investigation of air entrainment in three and six hole GDI injectors has been carried out using RANS approach under non superheated and superheated conditions. The inter plume interactions caused by the air entrainment effects have been analysed and compared to the experimental results. Moreover, the tendencies of semi collapse and full collapse of multi-hole sprays under non superheated and superheated conditions have been investigated in detail as well. A methodology of LES has been established using different injection strategies along with various subgrid scale models for a single spray plume. In GDI multi-hole sprays, the internal flow of the injector plays a very crucial role in the outcome the spray plume. A separate already available internal flow LES simulation of the injector has been coupled with the external spray simulation in order to include the effect of nozzle geometry and the cavitation phenomenon which completely change the dynamics of the spray.
This paper presents a numerical and analytical study of structure and flow development in planar (2-dimensional) turbulent jets. The numerical results are used to develop a phenomenological solution, here referred as Bending Model, for twin turbulent plane jets. The effects of nozzle-nozzle spacing and injection angle are emphasized in numerical study. This study includes a wide range of nozzle-nozzle spacing, 4.25, 9 and 18.25 and injection angle between -20 to 20. In the farfield, the k–ε model predicts the velocity of the jet higher than the experimental results; however, the overall performance of k–ε model is acceptable in the prediction of velocity field. Mean velocity and static pressure fields are presented. The Bending Model can predict the attachment of two plane jets for a wide range of nozzle-nozzle spacing and injection angle. Based on the model and modified Reichardt's hypothesis, the flow field in the domain is predicted. The results are compared with the numerical simulation of the k-є model and previous experimental results. The results show encouraging agreement with the numerical and experimental results. © Copyright 2012 Authors -This is an Open Access article published under the Creative Commons Attribution License terms (http://creativecommons.org/licenses/by/2.0). Unrestricted use, distribution, and reproduction in any medium are permitted, provided the original work is properly cited. Nomenclature cp Combined point d0 Nozzle diameter I Turbulence intensity Ka Axial velocity decay rate constant Kr spread rate constant Lx Domain length in x-direction Ly Domain width in y-direction M Total momentum M0 Total momentum at nozzle exit Mf Momentum of combined jet at farfield M(x) Total momentum at location x mp Merging point (also called confluence point) P Static pressure S Nozzle-nozzle spacing u Velocity magnitude U0 Velocity magnitude at the nozzle exit uc Velocity at symmetry line (y=0) um Maximum velocity in each cross section us1, us2 Velocity profile single jets 1 and 2. x,y Coordinate system xmp Length of converging region xcp Length of converging region plus merging region y0 Position of zero velocity in recirculation zone; representative of shear layer boundary between jets yc Centerline distance from coordinate in y direction Greek α Constant equal to 0.693 θ Angle relative to x-axis θ 0 Angle between two jets at the nozzle exit θ(x) Angle relative to x-axis in
A finite volume Conditional Moment Closure (CMC) formulation has been developed as an LES sub-grid combustion model. This allows unstructured meshes to be used for both LES and CMC grids making the method more applicable to complex geometry. The method has been applied to an oxy-fuel jet flame. This flame offers new challenges to combustion modelling due to a high CO2CO2 content in the oxidiser stream and significant H2H2 content in the fuel stream. The density ratio of the two streams is of the order 5 and the viscosity of the two streams will also differ. All the flames simulated showed localised extinction in the region around 3–5 jet diameters downstream of the nozzle, which is in very good agreement with the experiment. Trends for conditional and unconditional statistics with changing levels of H2H2 in the fuel are correctly captured by the LES–CMC method, although different levels of agreement are observed for different species and temperature and possible reasons for this are discussed. The degree of extinction is also correctly predicted to increase as the H2H2 content of the jet is reduced, showing the ability of the CMC method to predict complex turbulence-chemistry interaction phenomenon in the presence of changing fuel composition.
A semi-analytical model for the interaction of twin circular jets is developed, requiring the solution of a second-order differential equation for the momentum balance. This model (‘Bending Model’) can predict the trajectory and the attachment of two jets. Using Reichardt’s hypothesis, the three-dimensional velocity field of dual-hole circular jets can be predicted. The model is used to predict the interaction of converging, diverging, parallel and non-equal jets. The results show encouraging agreement with k–ε numerical simulations and experimental results from the literature. The position of the maximum velocity is predicted to within 5%, and the model can predict the velocity field as well as the k–ε model. The current model can be useful in phenomenological engine simulation of multi-hole injectors.
Large Eddy Simulations of dilute spray-air reacting two phase flow are performed following an Eulerian–Lagrangian approach. The method includes a full two-way coupling in which phase properties and spray source terms are interchanging between the two phases within each coupling time step. To achieve sub-grid scale closures in the filtered equations used for LES of the carrier phase, the Smagorinsky model with dynamic procedure is applied for the flow field while an eddy diffusivity approach is used for the scalar fluxes. The phase transition of droplets is captured using a non-equilibrium evaporation model. The evaporating droplets are tracked using a Langragian procedure. They are injected in a polydisperse manner and generated in time dependent boundary conditions. The combustion is described by a reaction progress variable and a mixture fraction as well as their variances in the line of the Flamelet Generated Manifold method. The ultimate objective of this work is to appraise the ability of this LES-based spray module developed here to retrieve the spray flow and combustion properties of the configuration under study. The configuration consists of a spray jet issued into a pilot flame and a co-flowing atmospheric air in which droplets traverse a pre-evaporation distance and release part of their mass before reaching the combustion zone. Thereby the spray pre-evaporation turns the combustion regime from diffusion to partially premixed. The fuel used is liquid acetone, which is modeled by a detailed reaction mechanism including 84 species and 409 elementary reactions. A series of spray established at different operating conditions are investigated, analyzed and compared. Analysis allows to gain a deep insight into the process ongoing. Comparisons include exhaust gas temperature, droplet velocities and corresponding fluctuations, droplet mean diameters and spray volume flux at different distances from the exit planes. An overall good agreement is reported.
A study of turbulent mixing of two confined jets in a side-jump combustor was carried out with complementary numerical and experimental efforts. The side-inlet angle was varied from 15° to 135°, air-to-fuel ration from 1.8 to 6.4, and combustor Reynolds number from 1.1 × 104 to 5.9 × 104, respectively, to investigate their effects on the flow and mixing patterns. The fuel concentration distribution calculated from the temperature measurements using the Mach-Zehnder interferometry and thermocouple probing is in good agreement with that predicted with an algebraic Reynolds stress turbulence model. The side-inlet angle is found to affect largely the dominant role played by the jet impingement or entrainment. The existence of the critical side-inlet angle and Reynolds number and the increase of uniformity of mixing with increasing air-to-fuel ratio are believed to be useful for the design of side-dump combustors.
The divertor concept for DEMO fusion reactor is based on modular design cooled by multiple impinging jets. Such divertor should be able to withstand a surface heat flux of at least 10MW/m2 at an acceptable pumping power. To reduce the thermal loads the plasma-facing side of the divertor is build up of numerous small cooling fingers. Each cooling finger is cooled by an array of jets blowing through the holes on the steel cartridge.The size, number and arrangement of jets on the cartridge influences the heat transfer and pressure drop characteristics of the divertor. Five different cartridge designs are analyzed in the paper. The most critical parameters, such as structure temperature, heat removal ability, pressure drop, cooling efficiency and thermal stress loadings in the cooling finger are predicted for each cartridge design. A combined computational fluid dynamics and structural model was used to perform the necessary numerical analyses. The results have shown that the cartridge design with the best heat transfer and pressure drop characteristics is not also the most favorable choice from the point of view of minimum stress peaks.
An experimental study of combustion characteristics and structure of interacting 2, 3, and 5 propane jets in a cross flow is presented. The separation distance between the burners was set at 8, 12, and 16 exit diameters. The ratio of jet momentum flux to cross flow momentum flux was varied over a range of 44-390. Flame length, blowout stability, temperature, radiation emission, and concentrations of CO, NO, and O2 are measured. The effects of number of jets, their separation distance, and their geometrical arrangement relative to cross flow, on flame length, radiation and stability characteristics are examined. The changes in the radial profiles of temperature, and concentrations of CO, O2, and NO are studied as functions of separation distance and ratio of jet momentum flux to cross flow momentum flux of 3-jet flames. The range between the upper and lower stability limits of the jet velocity of lifted flames in cross flow is higher for multiple-jet flames than for single-jet flames. A third limit, which is newly identified, however is independent of the number of jets or their separation distance. Carbon monoxide concentration and flame radiation are maximum at moderate values of separation distance and NO concentration decreases monotonically with it.
This paper reviews recent and ongoing work on numerical models for turbulent combustion systems based on a classical LES approach. The work is confined to single-phase reacting flows. First, important physico-chemical features of combustion-LES are discussed along with several aspects of overall LES models. Subsequently, some numerical issues, in particular questions associated with the reliability of LES results, are outlined. The details of chemistry, its reduction, and tabulation are not addressed here. Second, two illustrative applications dealing with non-premixed and premixed flame configurations are presented. The results show that combustion-LES is able to provide predictions very close to measured data for configurations where the flow is governed by large turbulent structures. To meet the future demands, new key challenges in specific modelling areas are suggested, and opportunities for advancements in combustion-LES techniques are highlighted. From a predictive point of view, the main target must be to provide a reliable method to aid combustion safety studies and the design of combustion systems of practical importance.
The mass transport properties of a round turbulent jet of water discharging into a low velocity co-flowing water stream, confined in a square channel, is investigated experimentally. The measurement region is the self-similar range from x/d=70 to x/d=140. Combined laser-induced fluorescence and 2D laser Doppler velocimetry are used in order to measure simultaneously, instantaneously and in the same probe volume, the molecular concentration of a passive scalar and two components of the velocity. This technique allows the determination of moments involving correlations of both velocity and concentration fields, which are necessary to validate the second-order modelling schemes. Both transport equations of Reynolds shear stress uv and turbulent mass flux vc have been considered. In both cases, advection, production and diffusion terms have been determined experimentally. The pressure-strain correlation and the pressure scrambling term are inferred with the help of the budget of Reynolds shear stress and mass turbulent transport equations. Second order closure models are evaluated in the light of the experimental data.The turbulent Schmidt number is found to be almost constant and equal to 0.62 in the center region and decreases strongly to zero in the mixing layer of the jet. The effects of the co-flow on the turbulent mixing process are also highlighted.
The active control of oxy-fuel flames from burners with separated jets is investigated. The control system consists of four small jet actuators, placed tangential to the exit of the main jets to generate a swirling flow. These actuators are able to modify the flow structure and to act on mixing between the reactants and consequently on the flame behavior. The burner (25 kW) is composed of separated jets, one jet of natural gas and one or two jets of pure oxygen. Experiments are conducted with three burner configurations, according to the number of jets, the jet exit velocities, and the separation distance between the jets. OH chemiluminescence measurements, particle image velocimetry, and measurements of NO
x
emissions are used to characterize the flow and the flame structure. Results show that the small jet actuators have a significant influence on the behavior of jets and the flame characteristics, particularly in the stabilization zone. It is shown that the control leads to a decrease in lift-off heights and to better stability of the flame. The use of jet actuators induces high jet spreading and an increase in turbulence intensity, which improves the mixing between the reactants and the surrounding fluid. Pollutant measurements show important results in terms of NO
x
reductions (up to 60%), in particular for low swirl intensity. The burner parameters, such as the number of jets and the spacing between the jets, also impact the flame behavior and NO
x
formation.
In this paper, flameless combustion was promoted to suppress thermal-NOx formation in the hydrogen-high-containing fuel combustion. The PSRN model was used to model the flameless combustion in the air for four fuels: H2/CH4 60/40% (by volume), H2/CH4 40/60%, H2/CH4 20/80% and pure hydrogen. The results show that the NOx emissions below 30ppmv while CO emissions are under 50ppmv, which are coincident with the experimental data in the “clean flameless combustion” regime for all the four fuels. The simulation also reveals that CO decreases from 48ppmv to nearly zero when the hydrogen composition varies from 40% to 100%, but the NOx emission is not sensitive to the hydrogen composition. In the highly diluted case, the NOx and CO emissions do not depend on the entrainment ratio.
The interaction between turbulent acetylene flames was investigated experimentally by the analysis of their visible lengths. The study involved 1, 2, 3 and 5 turbulent flames formed by parallel vertical jets whose relative distance was varied. The tube internal diameters were 1, 2, 3 and 4.4 mm. The results quantify the increase of flame length of multiple jets of equal injection tube radii and equal initial velocities as the flame separation distance decreases and as the number of jets increases. An empirical formula relating the length of multiple jet flames to that of the individual flame of same burner diameter was derived. Results of experiments performed with parallel burners inclined of 45° in relation to the vertical direction are also presented.
The understanding of physical and chemical processes occuring in many applications in sciences and engineering is important to ensure stability and efficiency of their performance. Examples are the combustion process in direct-injection engines,,as turbine combustors. and liquid rocket propulsion systems. The objective of this paper is the numerical investigation of laminar methane/air and methane/oxygen flames where different Mixtures of nitrogen and oxygen in the oxidizer stream are studied. Moreover, liquid oxygen (LOX) spray flames with carrier gas methane directed against a methane stream are investigated in the counterflow Configuration. These structures may be used in (spray) flamelet library or flamelet generated manifold computations of turbulent combustion. The mathematical model is based oil two-dimensional equations which are transferred into one-dimensional equations Using a similarity transformation. The numerical simulation concerns the axisymmetric Configuration with all adaptive numerical grid for the gas phase. Detailed models of all relevant processes are employed: in particular, it detailed chemical reaction mechanism is used which comprises 35 species including C(2) involving 294 elementary reactions. The thermodynamic data for CH(4) and O(2) below 300K are implemented for normal and elevated pressures. For the CH(4)/air and all oxygenated flame, the present results are compared with results from literature. The CH(4)/O(2) flame is studied for elevated pressures up to 2MPa. Extinction conditions are evaluated for use ill turbulent flamelet computations. It is shown that oxygen dilution, pressure. and strain rate have it pronounced effect on flame structure. The use of liquid compared to gaseous oxygen strongly affects flame structure.
Time- and space-resolved mixture fraction measurements have been made throughout a turbulent nonreacting propane jet issuing into coflowing air using laser Rayleigh scattering, The objective of the measurements has been to obtain a better understanding of the Row structure and mixing process in turbulent variable-density jets where turbulent mixing has been decoupled from the effects of chemical heat release found in highly exothermic reacting jets. The measurements yield probability density distributions of the mixture fraction, from which the means, higher moments, and intermittency are calculated, Time histories of the Rayleigh signal are analyzed to obtain the power spectra and autocorrelations, Comparisons are made with results for other constant- and variable-density turbulent jets, and the observed differences are discussed.
Direct numerical simulation (DNS) is a very powerful tool to evaluate the validity of new models and theories for turbulent combustion, but the application of detailed chemistry is limited. In this paper, a dimension-reduction technique called the flamelet-generated manifold (FGM) method is considered. In this method a manifold is created by solving a set of one-dimensional flamelet equations. The use of low-dimensional FGM’s in DNS of premixed turbulent flames in the thin reaction zones regime is investigated. A three-dimensional (3D) DNS is performed of a spherically expanding, premixed, turbulent, methane–air flame. 1D and 2D FGM’s are created and used in simulations of flamelets which are subjected to stretch and curvature effects derived from the 3D DNS results. The results are compared with results from flamelet simulations with detailed chemistry. The results show that deviations from the 1D FGM due to stretch and curvature effects are significant, but they appear to be embedded in a 2D manifold. This 2D manifold corresponds well with 2D FGM’s that are created in different ways, but it shows large differences with a 2D manifold based on chemical kinetics alone. This indicates that an attracting low-dimensional manifold exists which is not solely determined by chemical kinetics. As a consequence, the results of the flamelet simulations using 2D FGM’s are more accurate than when a 1D FGM is applied: the mean error in the burning velocity is almost an order of magnitude smaller.
This paper reports an investigation of Computational Fluid Dynamics (CFD) on the influence of injection momentum rate of premixed air and fuel on the flameless Moderate or Intense Low oxygen Dilution (MILD) combustion in a recuperative furnace. Details of the furnace flow velocity, temperature, O2, CO2 and NOx concentrations are provided. Results obtained suggest that the flue gas recirculation plays a vital role in establishing the premixed MILD combustion. It is also revealed that there is a critical momentum rate of the fuel-air mixture below which MILD combustion does not occur. Moreover, the momentum rate appears to have less significant influence on conventional global combustion than on MILD combustion.
In this paper, Numerical simulations of mean velocity and turbulent kinetic energy fields are presented for three-dimensional lateral jet in crossflow, at the injection angles of −60° and −30°. The RNG k-ɛ turbulence model, with the two-layer wall function method, is adopted to simulate the characteristics of this flow at the jet-to-crossflow velocity ratios, 1, 2 and 4. The results show that the injection angle and jet-to-crossflow velocity ratio can change the flow fields, and the range upstream affected by jet injected laterally increase and the curvature of jet trajectories varies along the flow direction. Furthermore, the separation events in the lee of the jet exit and behind the jet bending-segment have been found, and the mechanisms of two vortex systems are analyzed.
A new approach to chemistry modelling for large-eddy simulation of turbulent reacting flows is developed. Instead of solving transport equations for all of the numerous species in a typical chemical mechanism and modelling the unclosed chemical source terms, the present study adopts an indirect mapping approach, whereby all of the detailed chemical processes are mapped to a reduced system of tracking scalars. Here, only two such scalars are considered: a mixture fraction variable, which tracks the mixing of fuel and oxidizer, and a progress variable, which tracks the global extent of reaction of the local mixture. The mapping functions, which describe all of the detailed chemical processes with respect to the tracking variables, are determined by solving quasi-steady diffusion-reaction equations with complex chemical kinetics and multicomponent mass diffusion. The performance of the new model is compared to fast-chemistry and steady-flamelet models for predicting velocity, species concentration, and temperature fields in a methane-fuelled coaxial jet combustor for which experimental data are available. The progress-variable approach is able to capture the unsteady, lifted flame dynamics observed in the experiment, and to obtain good agreement with the experimental data, while the fast-chemistry and steady-flamelet models both predict an attached flame.
The reactants are generally injected into the industrial furnaces by jets. An effective method to act on combustion in such systems is to control the way injection jets. The present study concerns the control of turbulent flames by the jets deflection in a natural gas–oxygen burner with separated jets. The burner of 25 kW power is constituted with three aligned jets, one central natural gas jet surrounded by two oxygen jets. The principal idea is to confine the fuel jet by oxygen jets to favour the mixing in order to improve the flame stability and consequently to reduce the pollutant emissions like NOx. The flame stability and its structural properties are analyzed by the OH chemiluminescence. The Particle Image Velocimetry technique has been used to characterize the dynamic field. Results show that the control by inclined jets has a considerable effect on the dynamic behaviour and flame topology. Indeed, the control by incline of oxygen jets towards fuel jet showed a double interest: a better stabilization of flame and a significant reduction of nitrogen oxides. Measurements showed that the deflection favours the mixing and accelerates the fusion of jets allowing the flame stabilization.
Oxy-fuel combustion in separated-jet burners has been proven to increase thermal efficiency and to have a potential for NOx emission reduction. This paper presents an investigation into confined, turbulent, oxy-flames generated by a burner consisting of a central natural gas jet surrounded by two oxygen jets. The study is focused on the identifying the influence of burner parameters on the flame characteristics and topology, namely stability, lift-off height and flame length. The effects of the natural gas and oxygen jet exit velocities, the distance separating the jets and the deflection of oxygen jets towards the natural gas jet are examined. The OH chemiluminescence. Results show that the lift-off heights increase when jet exit velocities and the distance separating the jets are increased. The deflection of oxygen jets decreases the lift-off height and increases the volume of flame in the transversal plane. The flame length increases principally with the oxygen exit velocity and the separation distance, and decreases considerably when the angle of oxygen jets is increased.
The interaction of two parallel plane jets of different velocities is studied by flow visualization and PIV measurement to
examine the influence of velocity ratio on the development ofjets in the initial region. It is found that the parallel plane
jets develop toward the high velocity side and the jet width is reduced with a decrease in the jet velocity ratio. Corresponding
to the variation of mean velocity field to the velocity ratio, the magnitudes of turbulence intensities, Reynolds stress and
static pressure are weakened in the merging region of the jets and their peak locations of the properties are shifted to the
high velocity side. These results indicate that the interaction of two parallel jets is weakened with a decrease in the velocity
ratio of the jets.
KeywordsParallel jets-Flow measurement-Turbulent flow-PlY-Flow visualization
Various strategies have been proposed to tabulate complex chemistry for subsequent introduction into fluid mechanics computations. Some of them are grounded on laminar flame calculations, which are useful to seek out key relations linking a few control parameters with relevant species responses. The objective of this paper is to estimate whether approaches based on premixed flamelets (FPI or FGM) can be extended to partially premixed and diffusion flames. Prototypes of nonpremixed laminar and strained counterflow flames are simulated using fully detailed chemistry. The configuration studied is a jet of methane/air mixture opposed to an air stream. A set of reference flames is then obtained, to which FPI results are compared. By varying the equivalence ratio of the free stream of methane/air mixture, from stoichiometry up to pure methane, premixed, partially premixed, and diffusion flames are analyzed. When the fresh fuel/oxidizer mixture equivalence ratio takes values within the flammability limits, excellent results are obtained with FPI. When this equivalence ratio is outside the flammability limits, diffusive fluxes across isomixture fraction surfaces lead to a departure between the FPI tabulation and the reference detailed chemistry flames. This is associated mainly with the appearance of a double-flame structure, progressively evolving into a single diffusion flame when the fuel side equivalence ratio is further increased. Using an improved flame index to distinguish between premixed and diffusion flame burning, hybrid partially premixed combustion is reproduced from a combination of FPI and diffusion flamelets.
A numerical study of soot formation in the near-field of a strongly radiating, nonpremixed, acetylene–air planar jet flame is conducted using Large Eddy Simulation in two dimensions to examine coupled turbulence, soot chemistry, and radiation effects. The two-dimensional, Favre-filtered, compressible Navier-Stokes, total sensible energy and mixture fraction equations are closed using the Smagorinsky subgrid-scale (SGS) turbulence model. Major species of gas-phase combustion are obtained using a laminar flamelet model by employing experimentally obtained laminar flame state relationships for the major species mass fractions as a function of gas-phase mixture fraction. A combination of a presumed Beta filtered density function and a scale-similarity model are used to account for SGS mixture fraction and scalar dissipation fluctuations on the filtered composition and heat release rate. A soot transport and finite-rate kinetics model accounting for soot nucleation, surface growth, agglomeration, and oxidation is used. Radiation is modeled by integrating the filtered radiative transfer equation using the discrete ordinates method. Both instantaneous and time-averaged results are presented in order to highlight physical and numerical modeling issues and to examine turbulence, soot chemistry, and radiation interactions. Qualitative comparisons are made to previous numerical results and experimental data.
Swirl flows play an important role in many engineering applications such as modern gas turbines, aero propulsion systems etc. While the enhanced mixing and stabilisation of the flame caused by the swirl are desirable features, such flows often exhibit hydrodynamic instabilities called precessing vortex core. For design purposes it is very important to predict such instabilities. Computational fluid dynamics (CFD) using Reynolds-averaged Navier–Stokes (RANS) type turbulence models are state of the art for the prediction of flow properties in engineering practice. The objective of this paper is therefore to evaluate the performance of the unsteady RANS (U-RANS) method in predicting the precessing vortex core phenomenon. To this end, an unconfined swirling flow with precessing vortex core at swirl number 0.75 and Reynolds number ranging from 10 000 to 42 000, investigated by means of both experiments and large eddy simulation, is utilised. The results show that U-RANS is able to capture the precessing vortex core both qualitatively and in parts quantitatively.
The stagnation point offsets of turbulent opposed jets at various exit velocity ratios and nozzle separations were experimentally studied by a hot-wire anemometer, smoke-wire technique and numerically simulated by Reynolds stress model (RSM). Results show that for 2D ≤ L ≤ 4D (where L is nozzle separation and D is nozzle diameter), the position of the impingement plane is unstable and oscillates within a region between two relative stable positions when the exit velocities are equal. However, the impingement plane deviates from the midpoint obviously and the flow field becomes stable relatively when there is very small difference of the exit velocities for opposed jets of 2D ≤ L ≤ 8D. At L < 2D or L > 8D, the position of stagnation point becomes insensitive to the variety of exit velocity ratio.
Three-dimensional turbulent jets in crossflow at low to medium jet-to-crossflow velocity ratios are computed with a finite-volume numerical procedure which utilizes a second-moment closure model to approximate the Reynolds stresses. A multigrid method is used to accelerate the convergence rate of the procedure. Comparison of the computations to measured data shows good qualitative agreement. All trends are correctly predicted, though there is some uncertainty on the height of penetration of the jet. The evolution of the vorticity field is used to explore the jet-crossflow interaction.
Premixed and nonpremixed flamelet-generated manifolds have been constructed and applied to large-eddy simulation of the piloted partially premixed turbulent flames Sandia Flame D and F. In both manifolds the chemistry is parameterized as a function of the mixture fraction and a progress variable. Compared to standard nonpremixed flamelets, premixed flamelets cover a much larger part of the reaction domain. Comparison of the results for the two manifolds with experimental data of flame D show that both manifolds yield predictions of comparable accuracy for the mean temperature, mixture fraction, and a number of chemical species, such as CO2. However, the nonpremixed manifold outperforms the premixed manifold for other chemical species, the most notable being CO and H2. If the mixture is rich, CO and H2 in a premixed flamelet are larger than in a nonpremixed flamelet, for a given value of the progress variable. Simulations have been performed for two different grids to address the effect of the large-eddy filter width. The inclusion of modeled subgrid variances of mixture fraction and progress variable as additional entries to the manifold have only small effects on the simulation of either flame. An exception is the prediction of NO, which (through an extra transport equation) was found to be much closer to experimental results when modeled subgrid variances were included. The results obtained for flame D are satisfactory, but despite the unsteadiness of the LES, the extinction measured in flame F is not properly captured. The latter finding suggests that the extinction in flame F mainly occurs on scales smaller than those resolved by the simulation. With the presumed β-pdf approach, significant extinction does not occur, unless the scalar subgrid variances are overestimated. A thickened flame model, which maps unresolved small-scale dynamics upon resolved scales, is able to predict the experimentally observed extinction to some extent.
A numerical method has been developed that is capable of describing turbulent, compressible, subsonic separated jet flow fields. The method uses a modification of the Brailovskaya two-step implicit artificial viscosity algorithm which includes an explicit artificial mass dissipation added to the continuity equation. Solutions are obtained by applying the numerical scheme to the four governing equations which describe the time-dependent, two-dimensional, turbulent, compressible flow of an ideal gas. Turbulent Reynolds stresses were described by means of the Prandtl-Görtler approximation. Starting with arbitrary initial conditions, a solution is marched forward in time until a condition satisfactorily close to steady state is achieved. Computations were performed for confined jet air flows at a March number of 0.45, wall offsets of two and four nozzle widths, and zero wall angle. The calculations were compared with experimental data and acceptable agreement was found.
A four-step mechanism for the combustion of methane in air in nonpremixed flames is obtained by making steady-state and partial equilibrium approximations for minor species. The model gives good predictions of laminar flame structure, including steady-state minor species, for a wide range of flame stretch and across the whole breadth of the reaction zone, including rich mixtures. A good prediction of extinction is obtained. Further reduction to a three-step or two-step mechanism is discussed.
Etude expérimentale du comportement de brûleurs à jets séparés: Application à la combustion gaz naturel-oxygène pur
- L Salentey
Salentey L. Etude expérimentale du comportement de brûleurs à jets séparés:
Application à la combustion gaz naturel-oxygène pur. Thèse de
doctorat: Université de Rouen; 2002.
Du contrôle passif au contrôle actif: Application à l'oxycombustion dans des brûleurs à jets séparés PhD thesis
- T Boushaki
Boushaki T. Du contrôle passif au contrôle actif: Application à l'oxycombustion dans des brûleurs à jets séparés PhD thesis. Université de
Rouen; 2007.
A dynamic subgridscale eddyviscosity model
- M Germano
- U Piomelli
- P Moin
- W H Cabot
Germano M, Piomelli U, Moin P, Cabot WH. A dynamic subgridscale eddyviscosity model. Phys Fluids A 1991;3(7):1760-5.
A Large-Eddy Simulation Technique for the Prediction of Flow, Mixing and Combustion in Gas Turbine Combustors PhD Thesis
- B Wegner
Wegner B. A Large-Eddy Simulation Technique for the Prediction of Flow,
Mixing and Combustion in Gas Turbine Combustors PhD Thesis. Technische
Universität Darmstadt; 2007.