Flamelet Generated Manifold Model Research Articles

A comparative study of combustion models for simulating partially premixed swirling natural gas flames

Combustion models are important for simulating chemical reaction properties and the turbulence-chemistry interaction (TCI) in combustion. However, little attention has been paid to the temperature/species distributions/velocity flow fields predicted by different combustion models and their requirements for mesh resolution. In the present work, a partially premixed swirling natural gas flame was separately simulated by two commonly-used combustion models in Reynolds-Averaged-Numerical-Simulation (RANS) simulations: the flamelet generated manifold (FGM) model and the eddy dissipation concept (EDC) model. It is found that the EDC model requires a higher quality of mesh than the FGM model from the temperature and species distributions perspective. It is possible for the EDC model to overpredict temperature, however, such a problem can be solved by improving the mesh quality. The combustion models do not differ obviously for the mesh refinement for flow fields. The further analysis find that the EDC model brings forward the reaction location (11 mm upstream than the FGM model). The weakened turbulence mixing and early chemical reactions make the EDC model derive discrepancies with measured values, resulting from the variations between its modelling principles and actual applications in the software ANSYS Fluent. The low turbulence viscosity makes the EDC model flame thicker. Overall, the FGM model showed better suitability and accuracy for such a partially-premixed swirling natural gas flame. This work connects the two common combustion models and partially premixed swirling flame characteristics in the RANS simulations, and highlights the effects on the temperature/species concentration/turbulence flow fields, in concerns of mesh resolution and reaction properties rather than sole applications, which can provide foundation for subsequent researchers.

Numerical investigation of supercritical combustion dynamics in a multi-element LOx–methane combustor using flamelet-generated manifold approach

The article investigates liquid oxygen (LOx)–methane supercritical combustion dynamics in a multi-element rocket-scale combustor using large eddy simulation (LES). A complex framework of real gas thermodynamics and flamelet-generated manifold (FGM) combustion model is invoked to simulate transcritical oxygen injection and supercritical methane combustion. A benchmark Mascotte chamber, rocket combustion Modeling (RCM) test case, i.e., RCM-3 (V04)/G2 test case, is used to validate the real gas FGM model in the LES framework. The validation study accurately reproduces experimental flame structure and OH concentration, demonstrating the FGM model's importance in incorporating finite rate kinetics in LOx–methane combustion. Subsequently, the numerical framework investigates a specially designed multi-element combustion chamber featuring seven bidirectional swirl coaxial injectors. The analyses capture the complex hydrodynamics and combustion dynamics associated with multiple swirl injectors operating at supercritical pressure, effectively demonstrating the initiation of transverse acoustic waves and examining the effect of local sound speed on the evolution of acoustic modes in the combustor. The dominant frequency modes shed light on understanding the role of injectors in enhanced combustor dynamics. Spectral analysis reveals the interplay of the upstream injector and chamber acoustics due to possible frequency coupling. The results also highlight the effect of fuel injection temperature on the stability of the combustor, revealing a violent dynamic activity for lower fuel injection temperature associated with the longitudinal acoustic mode of the combustor. The investigation appropriately reproduces self-sustained limit-cycle oscillations at lower fuel injection temperatures and corroborates the conventional understanding of combustor instability.

Numerical prediction of fire dynamics and the safety zone in large‐scale multiple pool fire in a dike using flamelet model

AbstractThis research aims to develop a computational fluid dynamics (CFD) methodology for estimating the safety zone around a dike with multiple pool fire (MPF). This study predicts the safety zone and burning characteristics of heptane MPF inside a square dike in calm wind and worst‐case crosswind situations using unsteady simulations. The heptane MPF is modelled using flamelet approach with large eddy simulation (LES) turbulence model incorporating the soot generation. Discrete ordinates and Moss–Brookes model estimate the effect of participating medium radiation on prediction of safety distance and soot production. The discretization is incorporated using an isotropic trimmed cell mesher with local refinement to resolve the flame characteristics in Simcenter STAR CCM+. An extensive grid‐independence study has been executed to find the optimal mesh. The flame temperature and O2 and CO2 mass fraction predictions in calm wind conditions are in good agreement with the experimental findings of Koseki and Yumoto and the maximum flame temperature predictions are within 2.8% error. The safety zone is predicted using the estimated radiative heat flux. The effect of different ordinate sets (angular discretization S4, S6, and S12) approach on safety distance prediction is investigated. The validated grid independent CFD model combined with flamelet generated manifold model and LES turbulence model is proposed to predict the safety zone for industrial MPF in crosswind scenarios, thereby preventing human fatalities and property loss.

Effects of turbulence and flamelet combustion modelling on the CFD simulation of a dual inlet ramjet combustor

Abstract This manuscript presents Computational Fluid Dynamics simulation of a ramjet combustor using Reynolds Averaged Navier–Stokes approach for turbulence with flamelet combustion modelling. In the approach a one-dimensional premixed flame simulation is performed to cover a wide range of mixture fraction space. After a mesh study, effects of turbulence, Realizable k-ε and SST k-ω, and combustion modelling, Steady Laminar Flamelet and Flamelet Generated Manifold, are investigated in detail. The Flamelet Generated Manifold model that offers a reaction delay by employing an additional progress variable, shows there are no reactions taking place in the vicinity of jet inlets, hence the unrealistic temperature rise that is seen in the case of using Steady Laminar Flamelet is not observed. A study on the choice of progress variable based on the combustion products is carried out. The present study readily demonstrates good predictability of Flamelet Generated Manifold combustion modelling in such a dual inlet ramjet combustion chamber.

Combining flamelet-generated manifold and machine learning models in simulation of a non-premixed diffusion flame

Flamelet Generated Manifold (FGM) is an example of a chemistry tabulation or a flamelet method that is under attention because of its accuracy and speed in predicting combustion characteristics. However, the main problem in applying the model is a large amount of memory required. One way to solve this problem is to apply machine learning (ML) to replace the stored tabulated data. Four different machine learning methods, including two Artificial Neural Networks (ANNs), a Random Forest (RF), and a Gradient Boosted Trees (GBT), are trained, validated, and compared in terms of various performance measures. The progress variable source term and transport properties are replaced with the ML models. Particular attention was paid to the progress variable source term due to its high gradient and wide range of its value in the control variables space. Data preprocessing is shown to play an essential role in improving the performance of the models. Two ensemble models, namely RF and GBT, exhibit high training efficiency and acceptable accuracy. On the other hand, the ANN models have lower training errors and take longer to train. The four models are then combined with a one-dimensional combustion code to simulate a counterflow non-premixed diffusion flame in engine-relevant conditions. The predictions of the ML-FGM models are compared with detailed chemical simulations and the original FGM model for key combustion properties and representative species profiles.

Generalizing progress variable definition in CFD simulation of combustion systems using tabulated chemistry models

In the Computational Fluid Dynamics (CFD) simulation of advanced combustion systems, the chemical kinetics must be examined in detail to predict the emissions and performance characteristics accurately. Nevertheless, the combustion simulation with detailed chemical kinetics is complicated because of the number of equations and a broad timescale spectrum. The Flamelet-Generated Manifold (FGM) is one of the examples of tabulation methods that has received much attention in recent years due to its fast and accurate prediction of combustion characteristics. The Progress Variable (PV) definition in FGM and other PV-based tabulated approaches is often selected randomly or depending on the user's experience. When complicated combustion systems are involved, such choices can become extremely difficult. In the current work, a generic approach for formulating a global PV is developed and tested in various operating conditions relevant to combustion engines. The method is based on a genetic algorithm optimization to maximize the monotonicity of PV, ensuring that for each value of PV, the dependent thermophysical properties have unique values. The FGM model's ability to reproduce the detailed kinetics evolution of the essential combustion and emission parameters of a non-premixed diffusion flame in Spray A configuration is evaluated in both one-dimensional counterflow and CFD simulation. It is concluded that with the use of the current approach, important combustion characteristics can be predicted much better compared to non-optimized PV while eliminating the manual selection of PV definition by the user. Since the algorithm needs to be executed before the chemistry tabulation in the pre-processing step, it does not increase the runtime of the FGM simulation. The algorithm only needs a few minutes to be finished on a standard desktop. The improvement in the results and the distribution of the values of important species in the computational domain is examined.

Ammonia-methane combustion in a swirl burner: Experimental analysis and numerical modeling with Flamelet Generated Manifold model

Ammonia/methane flames gained significant attention since this is a probable step toward a sustainable, carbon-free economy in the near future. Therefore, three such flames were numerically and experimentally investigated in a swirl burner, focusing on robust modeling methods to facilitate the spreading of ammonia combustion in the industry. Chemistry was considered by the mechanism of Okafor and employed through the Flamelet Generated Manifold model. Both Particle Image Velocimetry and OH* measurements confirmed the appropriateness of the numerical model in all cases. It was demonstrated that the modeling approaches are applicable at both near stoichiometric conditions and close to the flammability limit of ammonia. The comparison of steady-state and the mean unsteady results implied that the steady calculations are appropriate only for the chemical conversion and fall behind in flow field modeling. The Root Mean Square velocity was identical up to the reaction zone for all cases, while its value decreased with the increase of ammonia concentration. The CO emission matched underpredicted experiments by a magnitude; however, the concentration was very low. Regarding the NOx emission, the CFD underpredicted it by a factor of two.

ASSESSMENT OF STATISTICAL AND KINETICS-BASED MODELS FOR LOx-METHANE GREEN PROPELLANT COMBUSTION

In this article, we present a comprehensive review and numerical analysis of liquid oxygen (LOx)-methane (CH<sub>4</sub>) combustion at supercritical pressures. A detailed review of numerical and experimental investigations on LOx-CH<sub>4</sub> combustion was conducted to understand the transcritical injection and supercritical combustion process occurring in a typical high-pressure rocket engine. In this work, we performed comprehensive numerical tests using statistical models, such as steady laminar flamelet (SLF) and flamelet/progress variable [flamelet-generated manifold (FGM)], and the kinetic approach of eddy dissipation concept (EDC) combustion closure to develop an accurate but computationally efficient supercritical combustion modeling methodology. A benchmark ONERA Mascotte chamber [G2 RCM-3 (V04)] test case was utilized to simulate LOx-CH<sub>4</sub> combustion. A systematic study of three different combustion models with different step chemical kinetic mechanisms was performed to identify the role of the detailed and reduced chemical mechanisms. The results of the study showed the efficacy of the FGM framework, which incorporates finite-rate kinetics in the LOx-CH<sub>4</sub> turbulent diffusion flame. The simulated flame shape and peak temperature location closely matched the G2 test observation. The kinetic model study revealed that the reduced mechanism can also describe the flame structure accurately in the FGM framework. The FGM model reproduced the experimental flame structure and hydroxyl concentration accurately compared to the SLF and EDC models. This study highlights the importance of finite-rate effects in LOx-CH<sub>4</sub> combustion and reveals that the statistical turbulence-chemistry approach of the FGM model is accurate and computationally less expensive for sub-scale or full-scale LOx-CH<sub>4</sub> rocket engine simulations.

An extended FGM model with transported PDF for LES of spray combustion

An enhanced flamelet generated manifold (FGM) model for large eddy simulation (LES) of turbulent spray combustion is presented. In the enhanced FGM model, a transported probability density function (TPDF) description of the FGM variables is employed. The TPDF is represented using the Eulerian stochastic fields (ESF) approach, and the method is applied to LES of spray combustion under conditions relevant to internal combustion engines. The new ESF/FGM method achieves an improved accuracy of predictions due to the ESF modelling of the subgrid-scale turbulence-chemistry interaction. It also achieves high computational efficiency due to the FGM tabulation of the chemical kinetic mechanism. The performance of the new ESF/FGM model is assessed by simulation of the Spray-A flames from Engine Combustion Network (ECN) and comparison of the results, firstly, with experimental measurements, and secondly, with conventional FGM model simulation results. It is shown that the ESF/FGM method is capable of predicting both global and local combustion characteristics, i.e., pressure rise, ignition delay time, flame lift-off length and the thermo-chemical structure of the spray flames with improved accuracy compared to the conventional FGM model that is based on the presumed PDF description of FGM variables. The sensitivity of the predictions using ESF/FGM to the number of stochastic fields is examined by varying the number of these fields in the range of 4–128. Furthermore, the influence of different FGM reaction progress variables on the simulations is investigated, and a new reaction progress variable based on the local consumption of oxygen is proposed. The results show that the new progress variable improves predictions of spray combustion, including the prediction of the start of injection, the quasi-steady state liftoff length, the post-injection oxidation, and the pressure evolution.

Numerical Simulations of a Lifted Hydrogen Jet Flame Using Flamelet Generated Manifold Approach

Abstract A turbulent lifted H2/N2 jet flame in a vitiating coflow environment is numerically investigated, using the Flamelet generated manifold (FGM) combustion model with large eddy simulations (LES). Due to the hot vitiated H2/air coflow, the primary stabilization mechanism is the auto-ignition followed by a premixed flame. In addition to using H2 as a fuel, this flame poses two other modeling challenges: (i) the auto-ignition, which is a transient chemistry-driven phenomenon; (ii) the existence of multiple combustion regimes, e.g., diffusion at auto-ignition location but premixed in the postflame. A series of LES/FGM simulations are completed in this work by reducing the coflow temperature from 1045 K to 1000 K. The FGM model can predict the characteristics of the flame by showing a lifted flame. It also accurately predicts the trend in the flame lift-off distance with a change in the coflow temperature. The current results are compared for mixture fraction, temperature, and OH mass fraction at multiple locations, which have also been correctly captured. It is noted that for a high coflow temperature (and hence a low lift-off distance), the flame's lift-off is highly sensitive to the inlet boundary conditions and the mesh resolution near the jet entry. A relatively coarse mesh is used for all the simulations, which is generated using a careful strategy that not only resolves the jet instabilities near the fuel inlet but also keeps the overall mesh count low and allows for a large computational time step. A systematic sensitivity analysis of the computational speed is also performed. This work provides some useful guidelines for simulating the H2 diluted flames using the FGM model, which may be valuable to the gas turbine industry.

Numerical analysis of dilute methanol spray flames in vitiated coflow using extended flamelet generated manifold model

This work focuses on the large eddy simulation and the study of turbulent dilute methanol spray flames in vitiated coflow using the secondary-oxidizer Flamelet Generated Model (FGM). The modified FGM model uses an additional secondary oxidizer parameter in addition to the three other parameters previously used for spray flames—progress variable, mixture fraction, and enthalpy. The results for gas phase and droplet properties are validated against the dilute methanol spray flame database for varying fuel injection amounts. The droplet statistics and the liftoff flame heights are accurately captured for all the cases. A proper orthogonal decomposition (POD) of the scalar fluctuating hydroxyl radical (OH) field and the velocity–temperature field captures the flame structures in the downstream region of ignition kernels. The detailed POD analysis reveals that the base frequency of the dominant OH field equals that of the dominant vortical structure of 67.3 Hz. The flame propagation happens around these dominant vortical structures because of the less-strained fluid mixing.

Verification of temperature and toxic species in methane compartment fire using flamelet generated manifold with radiation effect

This paper examined the effect of combustion model on compartment fire simulation. The results of the flamelet generated manifold (FGM) combustion model were compared with conventional combustion models in compartment fire simulation. Methane fuel was selected and the detailed kinetic mechanism was chosen for the FGM model. In addition, the radiation effect was investigated on the FGM combustion model. It was observed that it was necessary to apply the radiation effect in the FGM combustion model for the high heat release rate. For example, the temperature of FGM-base (in the absence of radiation effect) over-predicted near the ceiling of the room at heat release rate of 265 kW. However, if radiation effect was applied, the mean temperature reached 1250 K and the relative error was reduced to less than 10%. The important point is that the computational cost of the FGM was equal to other conventional combustion models in fire simulation. In addition, the conventional combustion models could model only one or two toxic species. However, the FGM was able to model all toxic species involved in the fire.

Large Eddy Simulations of a Low-Swirl Gaseous Partially Premixed Lifted Flame in Presence of Wall Heat Losses

The use of lifted flames presents some very promising advantages in terms of pollutant emissions and flame stability. The focus here is on a specific low-swirl injection system operated with methane and derived from an air-blast atomizer for aero-engine applications, which is responsible for flame lift-off. The key feature of this concept is the interaction between the swirling jet and the confinement walls, leading to a strong outer recirculation zone and thus to an upstream transport of combustion products from the main reaction region to the flame base. Here, the representation of the physics involved is challenging, since finite-rate effects govern the lift-off occurrence, and only a few numerical studies have been carried out on this test case so far. The aim of the present work is therefore to understand the limits of some state-of-the-art combustion models within the context of LES. Considering this context, two different strategies are adopted: the Flamelet-Generated Manifold (FGM) approach and the Thickened Flame (TF) model. A modified version of the FGM model including stretch and heat loss effects is also applied as an improvement of the standard model. Numerical results are compared with the available experimental data in terms of temperature and chemical species concentration maps, showing that the TF model can better reproduce the lift-off than the FGM approach.

Ammonia-Methane Combustion in a Swirl Burner: Experimental Analysis and Numerical Modeling with Flamelet Generated Manifold Model

Ammonia-Methane Combustion in a Swirl Burner: Experimental Analysis and Numerical Modeling with Flamelet Generated Manifold Model

Modelling of MILD combustion in a lab-scale furnace with an extended FGM model including turbulence–radiation interaction

The flamelet generated manifold (FGM) model is suitable for moderate or intense low oxygen dilution (MILD) combustion provided the flamelets underlying the manifold include the effects of strong dilution by products of the fuel/oxidizer mixture. Here we propose such an extended model based on the use of non-premixed flamelets diluted at the airside and develop its application to non-adiabatic combustion in a lab-scale furnace. The extended model is referred to as diluted air FGM (DA-FGM) model. In the DA-FGM model in addition to mixture fraction, progress variable and scaled enthalpy loss, one additional controlling parameter named air dilution level, is introduced leading to a four-dimensional lookup table for laminar flames. For turbulent flames also variances of mixture fraction and progress variable are taken into account as independent variables leading to a six-dimensional table. Using a RANS approach implemented in OpenFOAM-2.3.1, the DA-FGM model has been applied to MILD combustion of Dutch natural gas in a lab-scale furnace operated at a thermal power 9 kW and at equivalence ratio 0.8. Radiation is described using a weighted-sum-of-gray-gases (WSGG) model. The validation study is mainly done using a grey WSGG model with TRI taken into account. The relative importance of including turbulence radiation interaction (TRI) and spectral treatment of radiative transfer is also studied. The predicted velocity and temperature statistics are in good agreement with the experimental LDA and CARS data provided not only the mixture fraction fluctuations but also the progress variable fluctuations are taken into account.

Large-eddy simulation of a reacting swirling flow in a model combustion chamber

We perform Large-eddy simulations of a non-premixed swirling flame in a model of a combustion chamber with a swirling air bulk flow at Re = 15000 and a central pilot low-velocity jet with methane using the Flamelet-generated manifold model. The unsteady behaviour of this regime is well reproduced based on the flame dynamics. The distribution of turbulent kinetic energy suggests the presence of intensive vortical structures typical of high-swirl flows similar to the precessing vortex core.

The effects of swirl number on the combustion characteristics in an internally-staged combustor

The three-dimensional turbulent swirling flame in an internally-staged combustor is numerically investigated. Four cases over a range of swirl intensities are explored by the Flamelet Generated Manifold model in this paper. Special attention is paid to analyzing the variation of the flow field, temperature, major species concentrations and emissions. The results clearly show the effects of swirl number on the size of the center recirculation zone, fuel distribution and combustion characteristics. When the third premixed stage swirl number increases from 0.6 to 1.2, the axial length of the center recirculation zone decreases by 3.7%, while the radial length increases by 6.9%. The characteristics of the flow field play an important role in the spatial distribution of the fuel, which further affects the temperature distribution in the combustor. The backflow effect is enhanced, resulting in a greater concentration of fuel at the outlet of the swirler. After the maximum temperature is reached at the exit position of the pilot stage, the temperature decreases compared to the peak temperature downstream as the proportion of premixed combustion mode increases. This creates a high concentration region of OH at the outlet of the pilot stage injector and the heat release region is squeezed upstream. At the same time, the volume of the high-temperature region downstream of the pilot stage is reduced. In addition, as the swirl number of the third premixed stage increases from 0.6 to 1.2, the emissions of NO and CO decrease by 28.7% and 75%, respectively.

Large eddy simulation of premixed turbulent combustion using a non-adiabatic, strain-sensitive flamelet approach

In this study, a novel strained, non-adiabatic flamelet generated manifold (FGM) model is developed based on a counterflow premixed flame. The different levels of strain and heat loss are introduced into the flamelets by varying the stagnation point strain rate and burnt temperature of the counterflow configuration. The strain rate response of these flamelets is found to exhibit different extinction patterns for different ranges of burnt temperature. A priori studies are performed to compare the phase-space flame structures of the strained and unstrained flamelets. It is found that the strained flamelet degenerates towards the unstrained flamelet with burnt temperature decreasing. A bifurcation burnt temperature is further defined that separates the two types of flamelet quenching that are driven by strain and heat loss. An appropriate set of tabulation parameters is chosen that preserves the uniqueness of mapping between the manifold and the populated flamelets. The external boundaries of the tabulations are defined, where the proposed model degenerates into the conventional unstrained/non-adiabatic FGM manifold. Internal tabulation boundaries are defined to distinguish different combustion modes among stable combustion, strain-induced quenching, and heat-loss-induced quenching. The model is validated using large eddy simulation (LES) of a confined turbulent premixed jet flame. The proposed FGM model vastly increases accuracy of predictions when both strain and heat loss effects are included. Without accounting for these effects, there are discrepancies in the prediction of flame height and temperature as compared with experimental data. An analysis of the dominant physical process (strain, heat loss) that contributes to the evolution of the turbulent flame is performed.

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.

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.