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Abstract: Three-dimensional direct numerical simulation (DNS) of exhaust gas recirculation (EGR)-type turbulent combustion operated in moderate and intense low-oxygen dilution (MILD) condition has been carried out to study the flame structure and flame interaction. In order to achieve adequate EGR-type initial/inlet mixture fields, partially premixed mixture fields which are correlated with the turbulence are carefully preprocessed. The chemical kinetics is modelled using a skeletal mechanism for methane–air combustion. The results suggest that the flame fronts have thin flame structure and the direct link between the mean reaction rate and scalar dissipation rate remains valid in the EGR-type combustion with MILD condition. However, the commonly used canonical flamelet is not fully representative for MILD combustion. During the flame–flame interactions, the heat release rate increases higher than the maximum laminar flame value, while the gradient of progress variable becomes smaller than laminar value. It is also proposed that the reaction rate and the scalar gradient can be used as a marker for the flame interaction. [Copyright &y& Elsevier]

Abstract: The behaviour of the turbulent scalar flux in premixed flames has been studied using Direct Numerical Simulation (DNS) with emphasis on the effects of Lewis number in the context of Reynolds-averaged closure modelling. A database was obtained from DNS of three-dimensional freely propagating statistically planar turbulent premixed flames with simplified chemistry and a range of global Lewis numbers from 0.34 to 1.2. Under the same initial conditions of turbulence, flames with low Lewis numbers are found to exhibit counter-gradient transport, whereas flames with higher Lewis numbers tend to exhibit gradient transport. The Reynolds-averaged transport equation for the turbulent scalar flux is analysed in detail and the performance of existing models for the unclosed terms is assessed with respect to corresponding quantities extracted from DNS data. Based on this assessment, existing models which are able to address the effects of non-unity Lewis number on turbulent scalar flux transport are identified, and new or modified models are suggested wherever necessary. In this way, a complete set of closure models for the scalar flux transport equation is prescribed for use in Reynolds-Averaged Navier–Stokes simulations. [Copyright &y& Elsevier]

Abstract: Turbulent reaction rate closure modelling using the concept of Flame Surface Density (FSD) is now well-established in the context of Reynolds Averaged Navier Stokes (RANS) simulations. In the present study, three-dimensional DNS data is explicitly filtered in order to evaluate different terms of the FSD transport equation in the context of Large Eddy Simulation (LES). Existing sub-models for these unclosed terms are assessed with respect to the filtered DNS data, resulting in a recommended modelled form for each term. Previously assessed modelling of the propagation and curvature terms is combined with new model formulations for the turbulent transport and strain rate terms to produce a complete modelled FSD transport equation for use in LES. Displacement speed is shown to play a pivotal role in the modelling of several terms and its curvature dependence is explicitly accounted for in order to ensure that the FSD model will be valid in both the corrugated flamelets and the thin reaction zones regimes. [Copyright &y& Elsevier]

Abstract: The effects of tangential strain rate on the displacement speed of turbulent premixed flames in the thin reaction zones regime are studied for three different Lewis numbers (Le =0.8, 1.0 and 1.2) using three-dimensional compressible direct numerical simulation (DNS) of statistically planar flames. For non-unity Lewis numbers it is shown that the variations of temperature and scalar gradient in response to tangential strain rate on a given reaction progress variable isosurface have a profound influence on displacement speed behaviour. In the case of Le =0.8, temperature and tangential strain rate are found to be positively correlated at locations of zero curvature whereas the opposite behaviour is apparent for the case of Le =1.2. It is demonstrated that the effects of the temperature-curvature and tangential strain rate-curvature correlations are implicitly present in the response of the temperature to local strain rate. The temperature-curvature correlation and strain rate-curvature correlation are found to be in agreement with previous experimental results. Displacement speed and strain rate are found to be weakly correlated in general, but their conditional joint pdf at zero curvature locations shows a negative correlation. [Copyright &y& Elsevier]

Abstract: The effects of tangential strain rate and curvature on the surface density function (SDF) and on source terms within the SDF transport equation are studied for lean methane–air and hydrogen–air flames using two-dimensional direct numerical simulations with detailed chemistry. A positive correlation is observed between the SDF and the tangential strain rate, and this is explained in terms of the interaction between the local tangential strain rate and the dilatation rate due to heat release. Curvature is also seen to affect the SDF through the curvature response of both tangential strain rate and dilatation rate on a given flame isosurface. Strain rate and curvature are found to have an appreciable effect on several terms of the SDF transport equation. The SDF straining term in both methane and hydrogen flames is correlated positively with tangential strain rate, as expected, and is also correlated negatively with curvature. For methane flames, the SDF propagation term is found to correlate negatively with flame curvature toward the reactant side of the flame and positively toward the product side. By contrast, for hydrogen flames the SDF propagation term is negatively correlated with curvature throughout the flame brush. The variation of the SDF curvature term with local flame curvature for both methane and hydrogen flames is found to be nonlinear due to the additional stretch induced by the tangential diffusion component of the displacement speed. Physical explanations are provided for all of these effects, and the modeling implications are considered in detail. [Copyright &y& Elsevier]