A variable volume approach of tabulated detailed chemistry and its applications to multidimensional engine simulations

Abstract

In the context of engine development for low fuel consumption and low emissions, combustion modeling is a challenging subject as the requirements for lower pollutant emissions mean that accurate temporal and spatial predictions of the heat release and species concentrations are needed. Among the various approaches developed recently to account for these processes in realistic configurations, tabulated techniques appear to be a promising approach. A good compromise can be obtained between the accuracy brought by detailed chemistry at limited computational cost. Tabulation approaches were first developed and successfully applied to stationary combustors at constant pressure. However, these approaches are severely limited for internal combustion engine applications: indeed, they are not capable of accounting for the enthalpy or energy loss due to the pressure work in the expansion stroke. To overcome this limitation a new Variable Volume Tabulated Homogeneous Chemistry (VVTHC) approach is proposed in this study. It provides the evolution of major species and radicals from the onset of auto-ignition up to the end of the expansion stroke for compression ignited and spark ignited engine applications. It is first validated for homogeneous engine cases where it compares very well to complex chemistry simulations. Implemented in a piston engine combustion model, it also succeeds in matching the burned gases volume variation behind a propagating flame at constant pressure and in reproducing the subsequent composition evolution. In these conditions, the new approach gives results similar to those obtained with classical constant pressure tabulations. Finally, a feasibility calculation of a realistic stratified Diesel engine configuration is presented. On these cases, a constant volume tabulation approach is shown to compare well with VVTHC and the experiment on the evolution of pressure but not on that of species mass fractions. This comparison demonstrates the interest of the VVTHC approach for piston engine applications.