Lagrangian filtered density function modeling of a turbulent stratified flame combined with flamelet approach

Abstract

To simulate turbulent flames with high accuracy at low computational cost, Rieth et al. [“A hybrid flamelet finite-rate chemistry approach for efficient LES with a transported FDF,” Combust. Flame 199, 183–193 (2019)] have developed a hybrid method combining a combustion sub-grid model with assumed filtered density function (FDF) with a transported FDF approach. The present paper extends the hybrid approach to a stratified flame from the Cambridge stratified flame series. In contrast to the conventional Lagrangian FDF transport approach, the hybrid model applies Lagrangian particles to solve FDF transport only in selected regions, while an assumed FDF is applied in the remaining domain. With the hybrid model, the overall number of particles is strongly reduced compared to the conventional Lagrangian FDF transport model, promising great savings in computational cost. To provide a basis for the comparisons, simulations with assumed FDF or transported FDF only have also been performed. The present work aims to show the advantage of the Lagrangian transported FDF and the hybrid approach for a highly stratified flame, one of the most challenging members of the well-known Cambridge stratified flame series. Different criteria are tested for triggering the switch-over between the methods to maximize the efficiency of the hybrid approach, where basic flame quantities such as mixture fraction were predicted well with the assumed FDF model, and the temperature and mass fraction of carbon monoxide were predicted better by the hybrid method, featuring the transported FDF technique.