Eddy Simulation Of Turbulent Combustion Research Articles

Adaptive mesh refinement towards optimized mesh generation for large eddy simulation of turbulent combustion in a typical micro gas turbine combustor

Over the last years, several advanced micro Gas Turbine (mGT) cycle developments have been proposed, aiming at making the mGT more fuel and operational flexible. However, accurate data on real industrial combustors, assessing the performances and emissions of the combustion under unconventional diluted conditions or fuels involved in these novel cycles, are still missing. In this framework, Large Eddy Simulations, who allow to accurately assess the unsteady effects coupled to turbulent-chemistry interaction of reacting flows, offer an opportunity to better assess the combustion behaviour under these specific conditions. However, the computational cost remains much higher compared to RANS simulations. The mesh generation process might be complex, especially when the region of interest is not intuitively known. Therefore, Adaptive Mesh Refinement (AMR) methods allow the mesh to be refined only in the region where finer cells are required to capture important phenomena, i.e. in the flame front. By dynamically refining the mesh all along the simulation, the mesh is optimized in terms of cell quantity and distribution for more accurate results at potentially lower computational costs. In this work, an adaptive mesh refinement method based on a predefined criterion is successfully applied to the LES of a typical industrial mGT combustor, the Turbec T100. The results show that the adaptation strategy allows to automatically generate a dynamic mesh that is able to capture correctly the flame over time for an increased computational cost of 15%. Therefore, the human meshing effort is reduced while the automatic meshing leads to an optimized mesh.

Adaptive detached eddy simulation of turbulent combustion with the subgrid dissipation concept

Detached eddy simulation has become a widely used method in eddy simulations due to its balance between cost and accuracy. The recently developed subgrid dissipation concept (SDC) combustion model [Liu et al., “On the subgrid dissipation concept for large eddy simulation of turbulent combustion,” Combust. Flame 258, 113099 (2023)] is found to be more reasonable and accurate than the conventional eddy dissipation concept model in large eddy simulation (LES). In this paper, the SDC model is adapted to the ℓ2-ω adaptive detached eddy simulation framework, named DES-SDC. The required key quantities, including the fine structure mass fraction and dissipation rate, are appropriately blended across Reynolds-averaged Navier–Stokes and LES regions. The DES-SDC approach is validated using premixed bluff body stabilized flame, non-premixed swirl flame, and premixed swirl flame with complex geometry. It is much more tolerant to coarse mesh resolution than pure LES, yet it preserves the capability of resolving the key unsteady feature critical for the combustion process, as it is designed to be. The DES-SDC approach is relatively insensitive to the grid resolution. The present research provides a promising approach for accurately simulating practical unsteady turbulent combustion problems at an affordable computational cost.

Quantitative studies of NO emissions from various reaction pathways in swirling combustion of hydrogen-enriched methane

Hydrogen-enriched methane is a promising alternative fuel for carbon dioxide reduction. Large eddy simulations of turbulent combustion and NO emission are carried out using a new numerical method, which incorporates an improved NO prediction model and a recently-developed NO reaction pathway separation approach. Quantitative analyses of NO formations from all reaction pathways are conducted in an open swirl burner (combustion in the open environment) with hydrogen-enriched methane at a constant thermal power of 10.85 kW at the atmospheric pressure. Results indicate that different NO reaction pathways response very differently with hydrogen addition. In the open burner with hydrogen enrichment up to 60 %, the prompt NO makes the largest (around 50 %) contribution to the total NO emission, but its relative contribution decreases with hydrogen addition. The thermal NO strongly increases with hydrogen addition, but it accounts for only around 25 % of the total NO emission in hydrogen-enriched cases, because of the reduced high-temperature zone by co-flow air dilution. NO formation from the NNH pathway largely increases and contributes around 20 % to the total NO emission with hydrogen enrichment. Results would provide quantitative understanding of NO emission in the bluff-body stabilized swirling combustion of hydrogen-enriched methane in an open burner.

On the subgrid dissipation concept for large eddy simulation of turbulent combustion

A new subgrid turbulent combustion model for LES is derived based on the eddy dissipation concept (EDC). To be consistent with the LES framework, the relationship between the subgrid quantities and the fine structure ones is built from the unresolved part of the energy cascade, hence the subgrid dissipation concept (SDC). The fine structure quantities are directly computed using the subgrid scale ones. Resolving local laminar flame structures is allowed if the grid resolution permits. The model is examined using premixed piloted jet flames and non-premixed swirl flame. Satisfactory predictions from the SDC model are obtained in the test cases, showing improvements with respect to the standard EDC model. The new SDC model is relatively insensitive to the grid resolution.

Co-optimized machine-learned manifold models for large eddy simulation of turbulent combustion

Many modeling approaches in large eddy simulation (LES) of turbulent combustion employ a projection of the thermochemical state onto a low-dimensional manifold within state space to reduce the number of transported variables and hence computational cost. Flamelet-generated manifolds (FGM) is an example of a well-established, physics-based approach, but increasingly, principal component analysis (PCA) is being used as a data-driven method for generating manifold models. For both approaches, the nonlinear relationship between the location on the predefined manifold and the outputs of interest, such as reaction rates, can be tabulated or encoded in a neural network. This work proposes a new approach for manifold modeling that extends these existing approaches. A modified neural network structure simultaneously encodes the definition of the manifold variables, the nonlinear mapping, and the subfilter closure for LES. This allows all three of these aspects of the model to be co-optimized, generating a model from any source of combustion thermochemical state data. The manifold parameterizing variables are constrained to be linear combinations of species, as in FGM and PCA-based models, to aid in interpretability and implementation. For LES, subfilter variances of the manifold variables are also included as inputs. Two types of a priori analysis are performed to evaluate the new approach. In the first, the model is trained on data from one-dimensional premixed flames. In this case, the approach recovers the behavior of flamelet-based manifold approaches, and in fact slightly improves performance by identifying an optimized progress variable. The approach is also applied to data from direct numerical simulations of spherical ignition kernels in isotropic turbulence. For any specified manifold dimensionality, the new approach provides substantially lower prediction errors than a PCA-based model developed from the same data set. Additionally, the LES formulation of the new approach can provide accurate predictions for filtered reaction rates across a variety of filter widths.

Adaptive Mesh Refinement Towards Optimized Mesh Generation for Large Eddy Simulation of Turbulent Combustion in a Typical Micro Gas Turbine Combustor

Adaptive Mesh Refinement Towards Optimized Mesh Generation for Large Eddy Simulation of Turbulent Combustion in a Typical Micro Gas Turbine Combustor

Investigation of scalar–scalar-gradient filtered joint density function for large eddy simulation of turbulent combustion

The scalar–scalar-gradient filtered joint density function (FJDF) and its transport equation for large eddy simulation of turbulent combustion are studied experimentally. Measurements are performed in the fully developed region of an axisymmetric turbulent jet (with jet Reynolds number UjDj/ν=40 000) using an array consisting of three X-wires and three resistance-wire temperature probes. Filtering in the cross-stream and streamwise directions is realized by using the array and by invoking Taylor's hypothesis, respectively. The FJDF and the terms in the transport equation are analyzed using their means conditional on the filtered scalar and the subgrid-scale (SGS) scalar variance. The FJDF is unimodal when the SGS scalar variance is small compared to its mean value. The scalar gradient depends weakly on the SGS scalar. For large SGS variance, the FJDF is bimodal and the gradient depends strongly on the SGS scalar; therefore, the often-invoked independence assumption is not valid. The SGS scalar under such a condition contains a diffusion layer structure and the SGS mixing is similar to the early stages of binary mixing. The isoscalar surface in the diffusion layer has a lower surface-to-volume ratio than that in a well-mixed scalar. The conditionally filtered diffusion of the scalar gradient has an S-shaped dependence on the scalar gradient, which is expected to be qualitatively different from that of a reactive scalar under fast chemistry conditions. However, because modeling is performed at a higher level and because the scalar–scalar-gradient FJDF contains the information about the scalar dissipation and the surface-to-volume ratio, the FJDF approach is expected to be more accurate than scalar filtered density function approaches and has the potential to model turbulent combustion over a wide range of Damköhler numbers.

A-priori and a-posteriori studies of a direct moment closure approach for turbulent combustion using DNS data of a premixed flame

The direct moment closure (DMC) approach for modeling the filtered chemical source terms in large eddy simulation of turbulent combustion is comprehensively assessed by using direct numerical simulation data of a turbulent premixed flame with a skeletal chemistry. In this approach, closure is achieved by linearizing the chemical source terms using the Taylor series expansions. Different from traditional method, the exponential term is treated as a single variable in the expansion, and the filtered chemical reaction rate can be readily modeled with the first-order and second-order moments, plus higher-order correlations. The a-priori investigations demonstrate that the DMC model is able to reasonably represent the chemical reaction rates, and the second-order correlation of reactive scalars can be well modeled with the algebraic gradient-type model. Compared to the first-order moment closure, the second-order one makes a significant improvement on the predictions. These improvements are also observed in the a-posteriori study in which the premixed flame can be better resolved by the second-order moment closure model. Consequently, the second-order moment closure model provides an alternative for turbulent combustion modeling and is recommended to be used as an improvement to the laminar chemistry closure model. The sensitivity of the DMC approach to the grid size needs further investigation.

An Improved NO Prediction Model for Large Eddy Simulation of Turbulent Combustion

Accurate prediction of nitrogen oxides (NOX) emissions is of great importance in combustion simulations. This work proposes an improved model to predict NO distributions in turbulent flames, based on the large eddy simulation approach and flamelet-progress-variable turbulence combustion model. A separate table for NO reaction rate is constructed to account for both unsteady and nonadiabatic effects on NO production. Different from the existing models, the nonadiabatic effect is considered by specifying different enthalpy defects by a scaling factor in the flamelet calculations. Additionally, it is observed that the transient variation of NO reaction rate with NO concentration exhibits two linear stages. As a result, only three flamelet databases are included to describe the NO reaction rate in a transient process, thereby simplifying the flamelet table while maintaining model accuracy. The proposed NO prediction model is validated in large eddy simulations of turbulent combustion, including the classic Sandia D and the Sydney swirling SM1 flames. The calculated NO variations show good agreement with experimental data, demonstrating the applicability and accuracy of the proposed model.

Large eddy simulations of turbulent combustion of kerosene-air in a dual swirl gas turbine model combustor at high pressures

In advanced gas turbine engines, high-pressure combustion generally occurs with combustion chamber pressure exceeding the critical pressure of the engine fuel. In this paper, large eddy simulations are conducted in a dual swirl gas turbine model combustor to analyze the effects of different fuel-air equivalence ratios and chamber pressures on swirling injection and turbulent combustion. Numerical results reveal that intricate interactions exist between swirling flows and high-pressure turbulent flame. Strong swirling injection of air could cause reverse flows in the inner injector, which lead to a V shaped partially premixed turbulent flame at a chamber pressure of 3 MPa. As the operating pressure increases to 5 MPa, decreased injection velocity would prevent reverse flows in the inner injector, leading to the turbulent diffusive flame. On the other hand, combustion causes flow acceleration, gaseous expansion, and strong thermophysical property variations, which would influence the reverse flows, increase the expansion angle in swirling injection, and dictate the size of central recirculation zone and the oscillation frequency of precessing vortex core. Results obtained herein would help gain fundamental understanding on high-pressure swirling flows and flame dynamics in advanced gas turbine engines.

A Reduced Order Model based on Large Eddy Simulation of Turbulent Combustion in the Hybrid Rocket Engine

A combined method of large eddy simulations for non-premixed combustion in a turbulent boundary layer coupled with proper orthogonal decomposition of instantaneous velocity, pressure and temperature fields is developed in order to obtain a reduced order model. First, we investigate a channel turbulent reacting flow using Large Eddy Simulations (LES) technique. Polypropylene/O2 has been considered as fuel/oxidant pair. The turbulence-combustion interaction is based on a combination of finite rate/eddy dissipation model applied to a reduced chemical mechanism with four reactions. The LES numerical results are analyzed with respect to RANS simulations and with other reference data. The second part of the paper refers to the derivation of a Reduced Order Model (ROM) based on proper orthogonal decomposition (POD) technique for the unsteady flow field. In order to achieve that, the eigenmodes of the flow are computed from several snapshots of the instantaneous fields uniformly spaced and the most energetic ones are used to set up the Reduced Order Model. Constant regression rate of the fuel grain is considered. The flow and thermal fields obtained with ROMs are compared with the ones obtained from the full simulation and an analysis on the number of modes required to achieve the desired accuracy is presented.

A Thickened Stochastic Fields Approach for Turbulent Combustion Simulation

The Stochastic Fields approach is an effective way to implement transported Probability Density Function modelling into Large Eddy Simulation of turbulent combustion. In premixed turbulent combustion however, thin flame-like structures arise in the solution of the Stochastic Fields equations that require grid spacing much finer than the filter scale used for the Large Eddy Simulation. The conventional approach of using grid spacing equal to the filter scale yields substantial numerical error, whereas using grid spacing much finer than the filter length scale is computationally-unaffordable for most industrially-relevant combustion systems. A Thickened Stochastic Fields approach is developed in this study in order to provide physically-accurate and numerically-converged solutions of the Stochastic Fields equations with reduced compute time. The Thickened Stochastic Fields formulation bridges between the conventional Stochastic Fields and conventional Thickened-Flame approaches depending on the numerical grid spacing utilised. One-dimensional Stochastic Fields simulations of freely-propagating turbulent premixed flames are used in order to obtain criteria for the thickening factor required, as a function of relevant physical and numerical parameters, and to obtain a model for an efficiency function that accounts for the loss of resolved flame surface area caused by applying the thickening transformation to the Stochastic Fields equations. The Thickened Stochastic Fields formulation is tested by performing LES of a laboratory premixed Bunsen flame. The results demonstrate that the Thickened Stochastic Fields method produces accurate predictions even when using a grid spacing equal to the filter scale. The present development therefore facilitates the accurate application of the Stochastic Fields approach to industrially-relevant combustion systems.

Resolution Requirements in Stochastic Field Simulation of Turbulent Premixed Flames

The spatial resolution requirements of the Stochastic Fields probability density function approach are investigated in the context of turbulent premixed combustion simulation. The Stochastic Fields approach is an attractive way to implement a transported Probability Density Function modelling framework into Large Eddy Simulations of turbulent combustion. In premixed combustion LES, the numerical grid should resolve flame-like structures that arise from solution of the Stochastic Fields equation. Through analysis of Stochastic Fields simulations of a freely-propagating planar turbulent premixed flame, it is shown that the flame-like structures in the Stochastic Fields simulations can be orders of magnitude narrower than the LES filter length scale. The under-resolution is worst for low Karlovitz number combustion, where the thickness of the Stochastic Fields flame structures is on the order of the laminar flame thickness. The effect of resolution on LES predictions is then assessed by performing LES of a laboratory Bunsen flame and comparing the effect of refining the grid spacing and filter length scale independently. The usual practice of setting the LES filter length scale equal to grid spacing leads to severe under-resolution and numerical thickening of the flame, and to substantial error in the turbulent flame speed. The numerical resolution required for accurate solution of the Stochastic Fields equations is prohibitive for many practical applications involving high-pressure premixed combustion. This motivates development of a Thickened Stochastic Fields approach (Picciani et al. Flow Turbul. Combust. X, YYY (2018) in order to ensure the numerical accuracy of Stochastic Fields simulations.

Large eddy simulation of turbulent combustion by a dynamic second-order moment closure model

A dynamic second-order moment closure model is developed for turbulent combustion in the form of large eddy simulation. The filtered reaction rate is directly closed in the form of Arrhenius law, and the whole temperature exponential function is treated as a single variable to avoid the traditional Taylor series expansion. The sub-grid unresolved reaction rate is modeled with a second-order moment closure model. All the coefficients in the sub-grid models are evaluated by the dynamic procedures. To validate and evaluate this model, a priori validation using a DNS database and posteriori validation by LES of the Sandia piloted jet flame (Flame D) and the Sydney bluff-body swirling flame (SM1) are performed. The results demonstrate that the dynamic second-order moment closure model coupled with LES is able to reasonably predict turbulent combustion even with simple chemistry, and has potential to predict more complex combustion with detailed reaction mechanism and acceptable computational cost.

Large Eddy Simulation of Turbulent Combustion in a Spark-Assisted Homogenous Charge Compression Ignition Engine

A large eddy simuation (LES) model is presented for simulation of spark-assisted homogeneous charge compression ignition combustion. The model is based on tabulated chemical kinetic rate for ignition and flame surface density for flame propagation, taking into account interaction between flame propagation introduced by the spark and auto-ignition due to charge compression. The model is used to simulate the combustion process in an experimental HCCI engine, with operation conditions ranging from spark-ignition controlled flame propagation to auto-ignition controlled HCCI combustion. The model is shown to be able to predict the combustion behavior observed in previous engine experiments. With low initial temperature, the SI flame mode prevails; with high initial temperature, the HCCI mode prevails. With moderate initial temperature, the SI flame and HCCI ignition interact more closely, which results in higher sensitivity to the initial temperature and turbulence conditions. This may be the reason of the observed high cyclic variation in the experiments.

A priori analysis of subgrid mass diffusion vectors in high pressure turbulent hydrogen/oxygen reacting shear layer flames

Direct numerical simulations of high pressure (100 atm), turbulent H2/O2 non-premixed temporally developing shear layer flames are conducted at initial Reynolds numbers ranging from 850 ⩾ Reδ0 ⩾ 4500, with up to ∼3/4 × 109 grid points. A real gas equation of state, real property models, detailed chemistry, and generalized diffusion models are included. The results of the simulations focus on the mass diffusion vectors and their subgrid contributions relevant to large eddy simulation of turbulent combustion. Comparisons of the actual filtered mass flux vectors with their corresponding forms evaluated with filtered primitive variables are shown through correlation coefficients, probability density functions (PDFs) of the ratios between the vector magnitudes, and PDFs of the angles between the vectors. The results show a reasonable correlation (⩾0.75) for all species and simulations when evaluated globally. However, the correlations weaken substantially in regions of large temperature subgrid scale (SGS) variance and filtered SGS scalar dissipation. Within these regions, the correlations are also shown to vary inversely with Reynolds number. Vector analysis indicates that evaluating the mass flux vector using only the filtered primitive variables accurately predicts the direction of the actual filtered flux; however, the magnitude is poorly predicted in the aforementioned regions. Comparisons of the subgrid mass fluxes to the subgrid scalar fluxes are also represented as PDFs of the ratios between the vector magnitudes. These results show the subgrid mass flux to be significantly smaller (≲5%) than the subgrid scalar flux, and this ratio is shown to diminish with increasing Reynolds number. A final comparison of the divergence of the subgrid mass fluxes to the subgrid scalar fluxes are also represented as PDFs of the ratios between the magnitudes. At lower Reynolds numbers, these ratios suggest comparable values between the subgrid diffusion and turbulent stirring terms in regions of large temperature SGS variance and filtered SGS scalar dissipation. However, this ratio is shown to diminish, but remain significant, with increasing Reynolds number.

An analysis of premixed flamelet models for large eddy simulation of turbulent combustion

When premixed flamelet models are applied in the context of large eddy simulation, a number of assumptions are implicity made. The validity of these assumptions depends on, for example, the simulated flame’s location within the premixed regime diagram, the accuracy of the presumed subfilter flamelet coordinate distributions, and the extent to which the asymptotic flamelets capture the turbulence-perturbed chemistry. Here, the errors that arise due to these assumptions are considered, analyzed, and compared using a direct numerical simulation of a premixed turbulent flame propagating in the thin reaction zones regime. Flamelet representations of the progress variable source term are formed in an a priori fashion. Level set flamelet methods in particular are considered because, although they offer a number of advantages, they make some of the most stringent flame structure assumptions. Errors due to the level set model are evaluated relative to other flamelet error sources, such as the shape of the presumed probability density function and the influence of the variance model. The results provide guidance on the importance of the individual modeling assumptions, and are used to propose a new modeling strategy in an effort to improve the level set framework.

Coupled large eddy simulations of turbulent combustion and radiative heat transfer

Radiative heat transfer is known to play an important role in combustion processes but is often neglected in simulations because of its complexity and the related numerical costs. An original approach is proposed here to perform large eddy simulations of turbulent combustion including radiation: unsteady combustion and radiative heat transfer are computed by two independent codes that exchange data when needed through a specialized language, CORBA, working on an internal computer network or over the Internet. The radiation code gets temperature and mass-fraction fields from the combustion code and returns radiative energy source terms. This coupling technique is easy to implement, portable, flexible, and versatile. Each code keeps its own structure and may be developed and optimized independently, especially when running on massively parallel machines. Preliminary results show that taking radiative heat transfer into account strongly modifies the flame dynamics, probably because the burnt gas temperature decreases, making the flame stabilization weaker and increasing the flame sensitivity to turbulent motions. Comparisons with experimental data are very encouraging.

Coupled large eddy simulations of turbulent combustion and radiative heat transfer

Coupled large eddy simulations of turbulent combustion and radiative heat transfer

Developments in Formulation and Application of the Filtered Density Function

An overview is presented of the state of progress in large eddy simulation of turbulent combustion via the filtered density function (FDF). This includes the formulations based on both the joint velocity-scalar FDF, and the marginal scalar FDF. In the former, the most up-to-date and comprehensive form of the model is presented along with its applications for LES of some relatively simple flows. In the latter, results are presented of the most recent LES of a complex turbulent flame. Both of the models are described in the context of a variable density flow via consideration of the filtered mass density function (FMDF).