Flamelet Generated Manifold Research Articles

Effect of progress variable definition on the mass burning rate of premixed laminar flames predicted by the Flamelet Generated Manifold method

This paper investigates how the choice of progress variable in tabulated chemistry affects the mass burning rates of premixed laminar flames. Simulations are carried out using finite rate, detailed chemistry (DC) and Flamelet Generated Manifolds (FGM). Through comparison of detailed chemistry and FGM (using different progress variable definitions), it is found that for FGM the mass burning rate depends on the choice of progress variable and thus results in a different mass burning rate than detailed chemistry. Since the mass burning rate is influenced by stretch and transport phenomena, the effects of these on mass burning rates are analysed. While FGM qualitatively predicts the effect of stretch on the mass burning rate compared to detailed chemistry, there are quantitative differences. It is shown that this is mainly caused by a lack of projection in usual FGM applications. When the projection of the source term and the diffusion term are included in the table, FGM becomes independent of the choice of progress variable and the effects of stretch are better represented by FGM similar to detailed chemistry.

Numerical investigation of multiphase reactive processes using flamelet generated manifold approach and extended coherent flame combustion model

For the calculation of multiphase reactive processes in computational fluid dynamics (CFD), detailed chemical kinetics and simplified combustion models are commonly applied. An appropriate modelling approach to overcome the high computational demand of chemical kinetics is the flamelet generated manifold (FGM), which prescribe the calculation of chemical kinetics in preprocessor for the generation of the look-up databases that are used during CFD simulations with interpolation procedure. For the calculation of the chemistry kinetics in processor, combustion models are commonly applied, such as Three-zones extended coherent flame model (ECFM-3Z) that features calculation of flame speed in turbulent conditions. The primary goal of the research is to investigate and validate FGM and ECFM-3Z models on the multiphase reactive process inside a compression ignition engine for single and multiple injection strategies. Additionally, an overview of the modelling methodology and capability of FGM and ECFM-3Z models is presented, where the impact of their features is analysed on the results inside a compression ignition engine. For the numerical simulations, CFD code AVL FIRE™ was used, where the calculated results such as in-cylinder pressure, temperature, rate of heat release, and nitric oxide emissions are computed. The FGM modelling approach showed higher ignition delay compared to the ECFM-3Z model for single-injection strategy, which can be attributed to the pretabulated autoignition conditions in three zones of the ECFM-3Z model. For the multi-injection strategy, such an ignition delay difference between FGM and ECFM-3Z is not observed since the small amount of injected fuel in pilot injections tends to have quicker ignition, which then creates better conditions for combustion of the more significant amount of injected fuel in the main injection. The experimental nitric oxide emission trend is achieved with both combustion modelling approaches, where the CFD calculation time for cases with FGM is reduced approximately by half. In comparison against the experimental values, both FGM and ECFM-3Z combustion modelling approaches showed the capability of predicting the influence of fuel injection strategy on the combustion process in passenger car compression ignition engines.

Experimental and numerical study of water sprayed turbulent combustion: Proposal of a neural network modeling for five-dimensional flamelet approach

Owing to the increasing worldwide demand for natural gas, the development of a large submerged combustion vaporizer is required. Its burner is equipped with a water spray nozzle to reduce nitrogen oxides, and a practical simulation method is required for the optimal design. The non-adiabatic flamelet approach can predict the combustion emissions and is useful for reducing simulation costs. However, as the number of control variables increases, the database requires larger memory and cannot be dealt with by general computers. In this study, an artificial neural network (ANN) model based on a five-dimensional flamelet database, which includes the effects of heat loss and vapor concentration by sprayed water evaporation, is developed. Furthermore, large eddy simulations (LESs) for turbulent combustion fields with and without water spray are conducted employing flamelet generated manifold (FGM) approach with this ANN model, and the validity is investigated. For comparison, a lab-scale burner equipped with a water spray nozzle is manufactured, and combustion experiments with and without water spray are conducted. The results show that CO, NO, temperature, and reaction rate of progress variable predicted by the present ANN model are in good agreement with those of a five-dimensional flamelet database. In the condition without water spray, the flame behavior predicted by the LES employing the FGM/ANN approach is in good agreement with that employing the conventional FGM approach, while indicating much lower memory, although there appeared some quantitative discrepancies in the temperature against the experiment probably partially because of the insufficiency of the FGM approach for the present complex flame structure. In the condition with water spray, the LES employing the FGM/ANN approach is able to capture the effect of the water spray on the flame behavior in the experiment, such that the water spray decreases the temperature, which causes the decrease in NO but increase in CO.

Numerical study of a high-hydrogen micromix model burner using flamelet-generated manifold

The flamelet-generated manifolds (FGM) method was adopted in this study to consider the preferential diffusion in a high-hydrogen micro-mixing model burner. That is, when solving the FGM flamelet, accurate diffusion rate was obtained from two methods: multicomponent formulation and constant detailed Lewis numbers assumption. Then a new method of filling the thermochemical state and the source term in the mixture fraction and the process variable space also was proposed, namely the linear triangular dissection interpolation method, to predict the position of the hydrogen-rich micro-mixing flame front. Compared with the Fluent approach to establish the diffusion FGM flamelet, the results showed that the two FGMs have similar flame predictions in high hydrogen content fuels, and both can accurately capture the location of the internal and external shear layer boundaries of the micro-mixing multi-jet flame in the steady state, while the Fluent approach based on the uniform Lewis number assumption predicts results that deviated significantly from the experimental results. However, for the internal shear layer, both methods have large predicted OH gradients compared to the experimental results due to the lack of effective Lewis number correction for the control variable transport equation. The results using linear triangular dissection interpolation maybe superior to the method with linear interpolation of the process variable quenching boundary toward zero, which leads to flashback due to overestimation of the process variable source term in the region below the diffusion FGM quenching boundary.

An assessment of the sectional soot model and FGM tabulated chemistry coupling in laminar flame simulations

Discrete sectional method (DSM)-based soot models are computationally demanding since several transport equations are required to be solved for sectional soot variables using large chemical kinetic mechanisms. In this paper, the sectional soot model is coupled with Flamelet Generated Manifold (FGM) tabulated chemistry to achieve computationally efficient chemistry reduction for combustion simulations. Different approaches are explored for incorporating soot-gas phase coupling with FGM chemistry, and a comprehensive assessment of these FGM-DSM approaches is conducted for their predictive accuracy and computational performance in simulations of laminar ethylene counterflow flames. The accuracy of soot prediction with FGM chemistry is found to be sensitive to the tabulated concentrations of PAH species. An unaccounted consumption of PAH originating from PAH-based processes leads to significant overprediction of soot. In this context, the performance of two strategies (i) solving the transport equation of PAH species (ii) including a contribution of soot nucleation during the manifold generation stage is compared. The latter approach showed relatively better accuracy and computational efficiency than the former. Numerical results further reveal that the strategy of including the complete soot model during the manifold generation stage reproduces the detailed chemistry solutions most accurately. The influence of unsteadiness on predictive capabilities of FGM-DSM approaches is also investigated by imposing time-dependent strain rates in unsteady simulations. The computational performance analysis indicates that by adopting FGM chemistry, up to two orders of magnitude reduction in CPU time can be achieved, based on the choice of section number and simulation approach. Promising results obtained for FGM-DSM strategies in one-dimensional configuration, provide a good outset for extending the application of FGM for soot estimation in turbulent flames.

Development of a Multiphase Flamelet Generated Manifold for Spray Combustion Simulations

Abstract In this study, a model flame of quasi-one-dimensional (1D) counterflow spray flame has been developed. The two-dimensional (2D) multiphase convection-diffusion-reaction equations have been simplified to one dimension using similarity reduction under the Eulerian framework. This model flame is able to directly account for nonadiabatic heat loss, preferential evaporation, as well as multiple combustion regimes present in realistic spray combustion processes. A spray flamelet library was generated based on the model flame. To retrieve data from the spray flamelet library, the enthalpy was used as an additional controlling variable to represent the interphase heat transfer, while the mixing and chemical reaction processes were mapped to the mixture fraction and the progress variable. The spray flamelet generated manifolds (SFGM) approach was validated against the results from the direct integration of finite rate chemistry as a benchmark. The SFGM approach was found to give a better performance in terms of predictions of temperature and species mass fractions.

Effects of stratification on premixed cool flame propagation and modeling

Low-temperature cool flames play a critical role for the performance of stratified-charge internal combustion engines. Accurate modeling of stratified combustion in engines requires a clear understanding of cool flame propagation in a compositionally stratified mixture. There are many studies on hot flame propagation in fuel-stratified mixtures. However, comparatively little effort has been made to explore stratification effects on cool flame propagation and modeling. To this end, a series of one-dimensional numerical simulations is performed in this work to investigate unsteady stratified dimethyl ether (DME)/air cool flames propagating in six stratification configurations (i.e., lean-to-stoichiometric (L2S), stoichiometric-to-lean (S2L), stoichiometric-to-rich (S2R), rich-to-stoichiometric (R2S), lean-to-rich (L2R), and rich-to-lean (R2L) stratified mixtures). To better understand stratification effects, the above stratified cool flames are compared with compositionally homogeneous cool flames. Furthermore, a well-established premixed flame modeling approach – Artificially Thickened Flame (ATF)/Flamelet-Generated Manifolds (FGM) – is evaluated with respect to its suitability for modeling stratified cool flames. The comparisons between stratified cool flames and homogeneous cool flames demonstrate that the stratification with an increase (e.g., Cases L2S, S2R and L2R) or decrease (e.g., Cases S2L, R2S and R2L) in equivalence ratio can promote or suppress cool flame propagation. Both the positive and negative effects would be augmented when decreasing stratification layer thickness. Moreover, sensitivity analysis and reaction path flux analysis of OH consumption are conducted to identify a cause and effect chain that elucidates how stratification affects cool flame propagation. It is found that stratification can significantly change the competition between reactions CH2O+OH⇌HCO+H2O and CH3OCH3+OH⇌CH3OCH2+H2O and hence affect the fuel consumption rate. Furthermore, a scaling law is proposed based on a stratification Damköhler number, in order to comprehensively characterize stratification effects on cool flame propagation. In addition, the evaluation of ATF/FGM in the laminar context suggests that the FGM method is a good candidate for describing low-temperature chemistry, while the ATF approach may need further consideration for accurately capturing cool flame propagation speeds.

The effect of vertical and horizontal air curtain on smoke and heat control in the multi-storey building

Air curtains are used as an industrial tool to control heating and cooling, over time they have been found to be useful for controlling fire and dust. In this paper, the effect of air curtain on fire was investigated in two different positions (above the door in a horizontal mode and next to the door in a vertical mode). Examination of fire requires careful simulation of toxic species and heat transfer; therefore, the combustion model of flamelet generated manifold (FGM) is used to investigate all toxic species. However, in this study, toxic samples of CO and CO2 are studied. After verifying the combustion model, the vertical air curtain was studied in different heat release rate (HRR) of 62.9, 500, and 5000 kW and in different positions of the fuel bed. Having examined the vertical air curtain at different velocities and ventilation angles, it was observed that the vertical air curtain functioned poorly in under-ventilated case; for example, almost 13 out of 24 rooms in the building had highly dangerous conditions, although in the over-ventilated case, it controlled the fire. But, the horizontal air curtain played the role of a fire controller and kept the mass fraction of toxic species (CO and CO2) below 5 and 170 ppm in the whole building, respectively.

Simulation of Pool and Compartment Fire Using Flamelet Generated Manifold With/Without Radiation Coupling

One of the critical issues in fire dynamic simulation is the selection of suitable combustion model. Typically, combustion models based on infinite fast chemistry are used in fires. In this paper, the flamelet generated manifold (FGM) has been used as a combustion model and the fireFoam solver of OpenFOAM has been used. Two distinct FGMs, with and without radiation coupling, have been investigated to illustrate the role of radiation in simulations. The energy equation with discrete ordinates radiation model is used for coupling of FGM with radiation. These models will be examined in two scenarios of the pool and compartment fire. The FGM with the heat equation is in a good agreement with the experiments. In addition, the basic FGM has a deviation from the experimental results, in a pool fire scenario. The mean of vertical velocity of basic FGM and FGM with energy equation has 40 and 15 percent relative error, respectively, in the pool fire scenario. In the compartment fire, FGM model, with and without radiation coupling shows the same behavior and the velocity results have 9% relative error in the doorway. In general, the radiation coupling in pool fire is more important than compartment scenario.

Presumed Joint-PDF Modelling for Turbulent Stratified Flames

Turbulence–chemistry interaction models such as the conditional moment closure and various flamelet models require a presumed Probability Density Function (PDF) model of the conditioning variables. In turbulent stratified flames, a joint-PDF model for a reaction progress variable and the mixture fraction is required. The joint-PDF is often modelled by two beta functions using the first and second moments of the two conditioning variables. In this work, the performance of a joint-PDF model based on the flamelet PDF approach is compared to the double beta-PDF model. The conditional PDF of the reaction progress variable conditioned on mixture fraction is modelled with the flamelet PDF. Unlike the beta PDF, the flamelet functional form for the conditional PDF varies with the local mixture fraction. The two PDF models are coupled with a two-dimensional premixed flamelet-generated manifold chemistry model. Two stratified flames of the Cambridge–Sandia bluff-body burner are simulated using a Reynolds-Averaged Navier–Stokes (RANS) approach. The results indicate that the flamelet PDF model has a superior, albeit marginal, predictions of the temperature, major and minor species mass fractions.

Large eddy simulation of the Cambridge/Sandia stratified flame with flamelet-generated manifolds: Effects of non-unity Lewis numbers and stretch

The Cambridge/Sandia turbulent stratified flame (SwB5) is simulated with the LES and Flamelet-Generated Manifolds (FGM) combustion model. Three 3D FGM manifolds are adopted. With the purpose to examine the influence of transport properties, unity and non-unity Lewis numbers (Le) are included in the first two manifolds, respectively. The combined effects of non-unity Le and stretch are investigated in the third manifold. Heat loss to the wall is also modeled. Good agreement is found between the simulation and experiment. The equivalence ratio, temperature and mass fractions of CO and H2 are all well reproduced in contrast with previous simulations. It is found that using non-unity Le can even deteriorate the near-wall temperature modeling. Non-unity Le is proposed to be crucial for the CO prediction as well, besides H2. The equivalence ratio modeling is observed to be very important, which accounts for several non-unity Le effects. Flame stretch shows almost no impact on the velocity fields, whereas its effects on the species, equivalence ratio and temperature are identified, although to a limited extent for the Cambridge/Sandia flame.

Numerical simulations of gaseous flames in combustion chamber applications

Recent developments and assessments of combustion models, numerical schemes and high-power computing allow simulations to be applied to real industrial thermal oxidizers and burners. In this paper, two type concepts in a complex geometry of a burner and combustion chamber is reviewed by means of measurements data from on-site during operations compared with the numerical simulation’s analysis. The combustion models as Flamelet, Flamelet Generated Manifolds (FGM) and Hybrid BML/Flamelet are performed to assess modeling and fundamental flow aspects of combustion instabilities in a swirl concept in the context of the Reynolds-averaged Navier Stokes (RANS) equations for gaseous flames. Simulations in real thermal oxidizers illustrate the prospective of the approach but the combustion modeling and chemistry sub-grid models are limited cases in terms of validations due to the lack of available advanced set of measurements. Specific issues associated to real thermal oxidizer are presented: on-site measurements during operations, multi-perforation of heat in the combustor walls and flame instabilities. The examples are assigned as mean flow predictions (velocity, temperature and species) and transient phenomena (ignition and flame instabilities). Finally, the conceptual differences of the potential perspectives are discussed in detail from a theoretical and practical point of view.

Stress-Blended Eddy Simulation/Flamelet Generated Manifold Simulation of Film-Cooled Surface Heat Transfer and Near-Wall Reaction

Abstract Accurate numerical prediction of surface heat transfer in the presence of film cooling within aero-engine sub-components, such as blade effusion holes and combustor liners, has long been a goal of the aero-engine industry. It requires accurate simulation of the turbulent mixing and reaction processes between freestream and the cooling flow. In this study, the stress-blended eddy simulation (SBES) turbulence model is used together with the flamelet generated manifold (FGM) combustion model to calculate the surface heat flux upstream and downstream of an effusion cooling hole. The SBES model employs a blending function to automatically switch between Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) based on the local flow features, and thus significantly reduces the computational cost compared to a full LES simulation. All simulations are run using ansys fluent®, a commercial finite-volume computational fluid dynamics (CFD) solver. The test case corresponds to an experimental rig run at Massachusetts Institute of Technology (MIT), which is essentially a flat plate brushed by a uniform freestream of argon with ethylene seeded inside, and is cooled by either a reacting air or a non-reacting nitrogen jet inclined at 35 deg to the freestream. Calculations are performed for both reacting and non-reacting jet cooling cases across a range of jet-to-stream blowing ratios and compared with the experimental data. The effects of mesh resolution are also investigated. Calculations are also performed across a range of Damköhler number (i.e., flow to chemical time ratio) from zero to 30, with unity blowing ratio, and the differences in the maximum surface heat flux magnitude in the reacting and non-reacting cases at a specific location downstream of the hole are investigated. Results from these analyses show good correlation with the experimental heat flux data upstream and downstream of the cooling hole, including the heat flux augmentation due to local reaction. Results from the Damköhler number sweep also show a good match with the experimental data across the range investigated.

Numerical study of a turbulent co-axial non-premixed flame for methanol hydrothermal combustion: Comparison of the EDC and FGM models

Eddy dissipation concept (EDC) model and flamelet generated manifolds (FGM) model are developed separately to study the temperature profiles and extinction limits of non-premixed hydrothermal flames. Predictions by the two models are evaluated comparatively by experimental data in literatures. FGM model shows relatively better prediction of temperature than EDC model in the near nozzle field. Extinction temperatures can be predicted by EDC model with deviations of 10–33 K. The extinction flow rates predicted by the FGM model are higher than those by the EDC model. Flow fields and reaction source terms are analysed to identify the inherent mechanism leading different results by the two models. It is illustrated that the positive effect of turbulence on reaction rate near the nozzle by the FGM model is the essential reason causing different flame characteristics from the EDC model by which the turbulence only has negative effect on reaction rate.

Numerical investigation of swirl-stabilized pulverized coal flames in air and oxy-fuel atmospheres by means of large eddy simulation coupled with tabulated chemistry

This paper investigates a self-sustained swirl-stabilized pulverized coal combustion chamber, specially designed to validate numerical models under atmospheric and oxy-fuel conditions. For this purpose, a comprehensive model, which accounts for both atmospheric and oxy-fuel conditions, is developed based on Large Eddy Simulation. To describe the turbulent gas-phase chemistry, a 4D Flamelet Generated Manifold based tabulated chemistry model is coupled with the artificially thickened flame approach and implemented in an Euler–Lagrange framework. The chemistry model is able to account for finite rate chemistry, heat losses, mixing devolatilization gases and char off-gases. Radiative heat transfer is included by solving the radiative transfer equation, applying the discrete ordinate method for angular discretization and a weighted sum of gray gas model for the spectral resolution, while particles are treated as gray. The simulation results from the combustion chamber are extensively validated against the available measurement data. Operating conditions with different atmospheres are investigated to validate the models’ prediction accuracy for air and oxy-fuel atmospheres. The validation process is carried out using velocity fields, particle temperature measurements and the gas-species concentrations. Overall, the comparisons between simulations and measurements reveal a favorable agreement with only small differences near the burner opening. These results demonstrate the suitability of large-eddy simulation combined with tabulated chemistry for predicting the combustion of pulverized coal in conventional air and oxy-fuel atmospheres. Further analyses of the self-sustained coal flames are carried out; in particular, the influence of different atmospheres on the interaction of coal particles with the gas phase is studied.

Large eddy simulation of an ignition sequence and the resulting steady combustion in a swirl-stabilized combustor using FGM-based tabulated chemistry

PurposeThis study aims to deal with the large eddy simulation (LES) of an ignition sequence and the resulting steady combustion in a swirl-stabilized liquid-fueled combustor. Particular attention is paid to the ease of handling the numerical tool, the accuracy of the results and the reasonable computational cost involved. The primary aim of the study is to appraise the ability of the newly developed computational fluid dynamics (CFD) methodology to retrieve the spark-based flame kernel initiation, its propagation until the full ignition of the combustion chamber, the flame stabilization and the combustion processes governing the steady combustion regime.Design/methodology/approachThe CFD model consists of an LES-based spray module coupled to a subgrid-scale ignition model to capture the flame kernel initiation and the early stage of the flame kernel growth, and a combustion model based on the mixture fraction-progress variable formulation in the line of the flamelet generated manifold (FGM) method to retrieve the subsequent flame propagation and combustion properties. The LES-spray module is based on an Eulerian-Lagrangian approach and includes a fully two-way coupling at each time step to account for the interactions between the liquid and the gaseous phases. The Wall-Adapting Local Eddy-viscosity (WALE) model is used for the flow field while the eddy diffusivity model is used for the scalar fluxes. The fuel is liquid kerosene, injected in the form of a polydisperse spray of droplets. The spray dynamics are tracked using the Lagrangian procedure, and the phase transition of droplets is calculated using a non-equilibrium evaporation model. The oxidation mechanism of the Jet A-1 surrogate is described through a reduced reaction mechanism derived from a detailed mechanism using a species sensitivity method.FindingsBy comparing the numerical results with a set of published data for a swirl-stabilized spray flame, the proposed CFD methodology is found capable of capturing the whole spark-based ignition sequence in a liquid-fueled combustion chamber and the main flame characteristics in the steady combustion regime with reasonable computing costs.Research limitations/implicationsThe proposed CFD methodology simulates the whole ignition sequence, namely, the flame kernel initiation, its propagation to fully ignite the combustion chamber, and the global flame stabilization. Due to the lack of experimental ignition data on this liquid-fueled configuration, the ability of the proposed CFD methodology to accurately predict ignition timing was not quantitatively assessed. It would, therefore, be interesting to apply this CFD methodology to other configurations that have experimental ignition data, to quantitatively assess its ability to predict the ignition timing and the flame characteristics during the ignition sequence. Such further investigations will not only provide further validation of the proposed methodology but also will potentially identify its shortfalls for better improvement.Practical implicationsThis CFD methodology is developed by customizing a commercial CFD code widely used in the industry. It is, therefore, directly applicable to practical configurations, and provides not only a relatively straightforward approach to predict an ignition sequence in liquid-fueled combustion chambers but also a robust way to predict the flame characteristics in the steady combustion regime as significant improvements are noticed on the prediction of slow species.Originality/valueThe incorporation of the subgrid ignition model paired with a combustion model based on tabulated chemistry allows reducing computational costs involved in the simulation of the ignition phase. The incorporation of the FGM-based tabulated chemistry provides a drastic reduction of computing resources with reasonable accuracy. The CFD methodology is developed using the platform of a commercial CFD code widely used in the industry for relatively straightforward applicability.

Air curtain to control smoke and fire spread in a ventilated multi-floor building

Compartment fire is a hazardous phenomenon because of the generated poisonous and high-temperature gases. Air curtain systems can be used in order to control the fire spread. This paper examines the numerical simulation method with a focus on the combustion model which can model the poisonous gases and investigates the effect of air curtain on fire. The results of the flamelet generated manifold (FGM) combustion model, which considers all of the poisonous species with detail, in compartment fire with single and multi-floor building have been verified. A three-floor compartment fire scenario with methane as a fuel of 62.9, 500, and 5000 kW heat release rate, which represents over to under-ventilation, was selected as a test case. The impact of air curtains with various supply angles, velocity, and ventilation on pollutants and temperature distribution was examined. The mass fraction of CO and CO2 species reached near zero at angles of 12 and 24, at the over-ventilated case when using air curtain. Therefore, there is no hazard for the upper floors in case of using air curtain. However, in the under-ventilated case, it is necessary to improve the efficiency of air curtain by using ventilation in the fire room and increasing the exhaust velocity of the air curtain.

Large eddy simulation of spray combustion using flamelet generated manifolds combined with artificial neural networks

In the present work, artificial neural networks (ANN) technique combined with flamelet generated manifolds (FGM) is proposed to mitigate the memory issue of FGM models. A set of ANN models is firstly trained using a 68-species mass fractions in mixture fraction-progress variable space. The ANN prediction accuracy is examined in large eddy simulation (LES) and Reynolds averaged Navier-Stokes (RANS) simulations of spray combustion. It is shown that the present ANN models can properly replicate the FGM table for most of the species mass fractions. The network models with relative error less than 5% are considered in RANS and LES to simulate the Engine Combustion Network (ECN) Spray H flames. Validation of the method is firstly conducted in the framework of RANS. Both non-reacting and reacting cases show the present method predicts very well the trend of spray and combustion process under different ambient temperatures. The results show that FGM-ANN can replicate the ignition delay time (IDT) and lift-off length (LOL) precisely as the conventional FGM method, and the results agree very well with the experiments. With the help of ANN, it is possible to achieve high efficiency and accuracy, with a significantly reduced memory requirement of the FGM models. LES with FGM-ANN is then applied to explore the detailed spray combustion process. Chemical explosive mode analysis (CEMA) approach is used to identify the local combustion modes. It is found that before the spray flame is developed to the steady-state, the high CH2O zone is always associated with ignition mode. However, high CH2O zone together with high OH zone is dominated by the burned mode after the steady-state. The lift-off position is dominated mainly by the diffusion mode.

Numerical Investigation of Combustion Instabilities in a Single-Element Lean Direct Inject Combustor Using Flamelet Based Approaches

Abstract In this paper, high-fidelity large eddy simulations (LES) along with flamelet-based combustion models are assessed to predict combustion dynamics in low-emission gas turbine combustor. A model configuration of a single-element lean direct injection (LDI) combustor from Purdue University (Huang et al., 2014, “Combustion Dynamics Behavior in a Single-Element Lean Direct Injection (LDI) Gas Turbine Combustor,” 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, OH, July 28–30.) is used for the validation of simulation results. Two combustion models based on the flamelet concept, i.e., steady diffusion flamelet (SDF) model and flamelet generated manifold (FGM) model are employed to predict combustion instabilities. Simulations are carried out for two equivalence ratios of φ = 0.6, and 0.4. The results in the form of mode shapes, peak to peak pressure amplitude and power spectrum density (PSD) are compared with the experimental data of Huang et al. (2014, “Combustion Dynamics Behavior in a Single-Element Lean Direct Injection (LDI) Gas Turbine Combustor,” 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, OH, July 28–30.). The effect of variation in the time-step size hence acoustic courant number is studied. Further, two numerical solver options, i.e., pressure-based segregated solver and pressure-based coupled solver, are used to understand their effect on the solution convergence regarding the number of time-steps required to reach the limit cycle of the pressure oscillations. A truncated (half) domain simulation is performed by applying an appropriate acoustic impedance boundary condition at the truncated location. Overall, the simulation results compare well with the experimental data and trends are captured accurately in all simulations. It builds confidence in flamelet-based combustion models for the use in combustion instability modeling which is traditionally done using finite rate chemistry models based on reduced kinetics.

Detached Eddy Simulation of syngas combustion in a reverse-flow configuration

The combustion of low calorific value syngas in a 3.3-kW reverse-flow chamber under moderate or intense low-oxygen dilution (MILD) (global equivalence ratio Φglobal = 0.89) and ultra-lean (Φglobal = 0.32) conditions have been numerically investigated with detailed chemistry. Reynolds averaged Navier Stokes (RANS)/Detached eddy simulation (DES) models for flow and Eddy dissipation concept (EDC)/Flamelet generated manifold (FGM) models for combustion have been used. Quantitative measurements of temperature, OH radical, and CO emissions performed in the same combustor are used to validate and compare the models. The prediction of the minor species OH is a special focus of the present study. The fields of velocity, mixture fraction, and kinetic parameters have been investigated in detail. The DES-EDC model performs the best in predicting the scalar distributions, although the computational cost is highest. The RANS model predicts insufficient mixing and higher velocities leading to a delayed reaction zone. The FGM model consistently predicts early ignition that has been attributed to the lack of a suitable progress variable and the use of the diffusion flamelet to represent the thermochemistry. However, the FGM model is promising with suitable modifications due to the low computational cost. The present study also provides a comprehensive dataset in a reverse-flow chamber operating under MILD and ultra-lean conditions for validation of numerical models.