Evaluation of different turbulent combustion models based on tabulated chemistry using DNS of heterogeneous mixtures

This study used direct numerical simulations (DNSs) of combustion processes in turbulent heterogeneous mixtures for self-igniting partially-premixed configurations to assess the accuracy of partially-premixed turbulent combustion models that are based on the tabulation of chemistry progress in homogeneous reactors (HRs). DNS coupled with n-heptane/air detailed chemistry solving was considered as a reference result. Because the same detailed chemistry was used to generate the chemistry databases, the study was focused entirely on validating the modelling assumptions. Various HR-based tabulation models were tested: (1) the tabulated homogeneous reactor (THR) model, which is a direct exploitation of HR tabulation lacking any statistical information concerning mixture heterogeneity; (2) the presumed conditional moment (PCM) model, which includes a limited statistical description of the mixture and/or of the combustion advancement; (3) approximated diffusion flame (ADF) models, which consider the heterogeneous turbulent reactor as either a unique diffusion flame (simple ADF model formulation) or as a collection of flamelets with different strain rates (ADFχ model). The a priori response of the above mentioned models was compared with detailed chemistry DNS results. The main findings are as follows: (i) a direct use of HR tabulation (THR model) led to overly inaccurate results; (ii) an assumed independence between mixture fraction and progress variable was responsible for most PCM modelling failures in the context of turbulent heterogeneous self-ignited combustion; (iii) the presumed β-function of the progress variable distribution is likely to fail because of the complexity of autoignition kinetics; (iv) the best results were obtained with the ADF models; (v) a simple ADF formulation is preferable to ADFχ, which showed limitations in terms of accuracy concerning the distribution of the progress variable; (vi) all tested models provided an acceptable prediction of the autoignition delays, but only the ADF and ADFχ models are able to represent the temporal evolution of the progress variable.