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The filtered tabulated chemistry (FTACLES) approach utilizes data from pre-tabulated explicitly filtered 1D flame profiles for closure of the LES-filtered transport terms. Different methodologies are discussed to obtain a suitable progress variable c from detailed chemistry calculations of a methane/air flame. In this context, special focus is placed on the analytical modeling of the reaction source term using series of parameterized Gaussians. For increasing effective filter sizes in LES (i.e. including the flame thickening) the precise shape of the reaction rate profile becomes less and less relevant. In particular, it is shown that for one-step chemistry, a single Gaussian is sufficient to derive an explicitly expressible 1D flame profile with a prescribed laminar flame speed and thermal flame thickness. The resulting artificial flame profile is shown to have similarities with profiles based on carbon chemistry and detailed reaction mechanisms. Next, the behavior of the filtered c-transport equation is analyzed and several possible closure methods are compared for a wide range of filter widths. It is shown that the unclosed contribution of the filtered diffusion term can be combined with the subgrid convection term, thus simplifying the FTACLES formulation. The model is implemented in OpenFOAM and validated in 1D for a variety of LES filter sizes in combination with artificial flame thickening. A power-law-based wrinkling model is modified for use with artificial flame thickening and combined with the FTACLES model to enable 3D simulations of a premixed turbulent Bunsen burner. The comparison of 3D Large Eddy Bunsen flame simulations at increasing levels of turbulence intensity shows a good match to experimental results for most investigated cases. In addition, the results are mostly insensitive to the variation of the mesh size. [ABSTRACT FROM AUTHOR]

This paper provides a numerical study on n-dodecane flames using Large-Eddy Simulations (LES) along with the Flamelet Generated Manifold (FGM) method for combustion modeling. The computational setup follows the Engine Combustion Network Spray A operating condition, which consists of a single-hole spray injection into a constant volume vessel. Herein we propose a novel approach for the coupling of the energy equation with the FGM database for spray combustion simulations. Namely, the energy equation is solved in terms of the sensible enthalpy, while the heat of combustion is calculated from the FGM database. This approach decreases the computational cost of the simulation because it does not require a precise computation of the entire composition of the mixture. The flamelet database is generated by simulating a series of counterflow diffusion flames with two popular chemical kinetics mechanisms for n-dodecane. Further, the secondary breakup of the droplet is taken into account by a recently developed modified version of the Taylor Analogy Breakup model. The numerical results show that the proposed methodology captures accurately the main characteristics of the reacting spray, such as mixture formation, ignition delay time, and flame lift-off. Additionally, it captures the "cool flame" between the flame lift-off and the injection nozzle. Overall, the simulations show differences between the two kinetics mechanisms regarding the ignition characteristics, while similar flame structures are observed once the flame is stabilised at the lift-off distance. [ABSTRACT FROM AUTHOR]

This paper assesses the accuracy of partially-premixed turbulent combustion models based on the tabulation of chemical kinetics, under multi-injection diesel engine-relevant conditions. For this purpose, 2-D direct numerical simulation (DNS) is carried out. Pockets of gaseous n-heptane are randomly distributed in a turbulent field of a partially burnt n-heptane/air mixture. The burnt gases composition and enthalpy correspond to the partial oxidation of a pilot injection that precedes the main injection, represented here by the fresh fuel pockets. The DNS domain is enclosed in a larger volume, permitting quasi-constant pressure autoignition. Chemical kinetics is modeled by a 29-species skeletal reaction mechanism for n-heptane/air mixture autoignition and flame propagation. A homogeneous isotropic turbulence spectrum is used to initialize the velocity field in the domain. A DNS database is generated varying the progress of the pilot injection combustion c 0 and the velocity fluctuation level u ′ of the turbulence spectrum. Three different modeling approaches are tested a priori against the DNS data: (1) the tabulated homogeneous reactor, which is a direct exploitation of the chemistry tabulation ignoring any local mixture heterogeneity; (2) the presumed conditional moment model, which includes a separate statistical description for the mixture and the combustion progress; (3) the approximated diffusion flame model, which considers the heterogeneous turbulent reactor as a diffusion flame. Since the same chemical kinetics mechanism is used for the generation of the chemistry tabulation, the study is entirely focused on the evaluation of the different modeling assumptions. Results show that accounting for initial progress variable of the mixture ( c 0 ) is mandatory for such models. They also indicate the omission of mixture fraction Z and progress variable c heterogeneities and the assumption of the statistical independence of Z and c as the main responsible for model discrepancies under the studied conditions. [ABSTRACT FROM AUTHOR]

Direct numerical simulation (DNS) of turbulent combustion is a research area becoming ever more important and has been established as a powerful tool in combustion science to complement theory and experiment. The rapid advancement of supercomputing has lately enabled a series of interesting DNS studies with some practical relevance. This advent along with the severe lack of experimental data sets with a high degree of data completeness motivates the use of DNS for combustion modeling. Simultaneously, enormous progress is made in the utilization of DNS data as a valuable resource to develop and validate combustion models from DNS data. In this paper, several promising and useful analysis techniques are identified and reviewed. Their usefulness is demonstrated on the basis of selected modeling studies, which comprise examples of various commonly employed modeling frameworks, and which integrate DNS in a systematic way into the process of model development and validation. It is expected that the modeling routes outlined here can be used to address currently open modeling questions and to advance the fidelity of existing models. [ABSTRACT FROM AUTHOR]

The present paper reports on the assessment of different turbulence-chemistry interaction closures for modeling turbulent combustion in the Moderate and Intense Low oxygen Dilution (MILD) combustion regime. 2D RANS simulations have been carried out to model flames issuing from two burners i.e. Delft-Jet-in-Hot-Coflow (DJHC) burner and Adelaide JHC burner which imitate the MILD combustion. In order to model these flames, two different approaches of turbulence-chemistry interaction models, i.e. Eddy Dissipation Concept (EDC) and PDF based models, are considered; while in the PDF based modeling, two different variants are invoked to understand the applicability of the PDF based models in the MILD regime: one is based on presumed shape PDF (i.e. steady flamelet (SF) model) approach and the other one is transported PDF approach. For transported PDF method, two different solution approaches namely, Lagrangian solution method (LPDF) and Multi-Environment Eulerian PDF (MEPDF) model are considered. A comprehensive study has been carried out by comparing the results obtained from these different models. For the DJHC burner, the computations are carried out for a jet speed corresponding to Reynolds numbers of Re = 4100, whereas the Adelaide JHC burner computations are performed for a jet speed corresponding to Reynolds number of Re = 10000. The effects of molecular diffusion on the flame characteristics are also studied by using different micro-mixing models. In the case of DJHC burner, it has been observed that the mean axial velocity and the turbulent kinetic energy profiles are in good agreement with the measurements. However, the temperature profiles are over-predicted in the downstream region by both EDC and the PDF based models. In the context of Adelaide JHC burner, the profiles of the temperature and the mass fraction of major species (CH, H, O, HO, CO) obtained using LPDF approach are in better agreement with the measurements compared to those obtained using EDC model; although, both the solution approaches fail to capture CO and OH radical profiles. [ABSTRACT FROM AUTHOR]