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

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.

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.

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.

A dynamic similarity subgrid-scale (SGS) unmixedness model is presented for large eddy simulation (LES) of turbulent reacting flows. The model is assessed both a priori and a posteriori via data obtained by direct numerical simulations (DNS) of homogeneous compressible turbulent flows involving a single step Arrhenius reaction. The results of a priori analysis indicate that the local values of the SGS unmixedness are accurately predicted by the model. A posteriori results also indicate that the statistics of the resolved temperature and scalars as obtained by LES compare favorably with DNS values.

The coupling of turbulence and chemistry in a premixed bluff-body flame as studied by LES

: The goals of this project were the development of new sub-filter models for large eddy simulation of turbulent combustion and of chemical mechanisms for jet fuel surrogates. The sub-filter modeling work focuses on the development of a framework for describing multiple combustion regimes using the flamelet approach, on describing the scalar dissipation rate in turbulent non-premixed combustion, and on modeling strain effects in turbulent premixed combustion. The chemistry work proposes a method for defining jet fuel surrogates, describes how different sub-mechanisms can be incorporated into the definition, and finally creates and validates a mechanism that serves as an accurate surrogate for jet fuel behavior.

Tabulated chemical kinetics for efficient and detailed simulations of diesel engine combustion

Recent progress at CTR in the control of jet mixing using numerical simulations and experiments is summarized. These control experiments use a basic form of evolution strategies and have been successful in enhancing turbulent mixing. Progress in large eddy simulation of turbulent combustion is also discussed. Results from two new flamelet based techniques for simulation of diffusion flames are presented.

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.

Coupled large eddy simulations of turbulent combustion and radiative heat transfer

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.

Large eddy simulation of turbulent stratified combustion using dynamic thickened flame coupled with tabulated detailed chemistry

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

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.