Numerical Simulations of Pre-Chamber Combustion in an Optically Accessible RCEM

In this work, numerical simulations of an automotive-sized scavenged pre-chamber mounted in an optically-accessible rapid compression-expansion machine (RCEM) have been carried out using two different turbulence models: Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES). The RANS approach is combined with the G-equation combustion model, whereas the LES approach is coupled with the flamelet generated manifold (FGM) model for partially-premixed combustion. Simulation results are compared with experimental data in terms of OH* chemiluminescence in the main chamber. Both RANS and LES results were found to qualitatively reproduce the main features observed experimentally in terms of spatial flame development. Simulation results are further analysed by means of early flame propagation within the pre-chamber (related to the fuel and turbulence intensity distributions) and the ignition process in the main chamber. During the turbulence jet ignition (TJI) process, the analysis of the LES progress variable variance reveals that during the intensive jet mixing the mixture in the main chamber is predominantly ignited by autoignition followed by a progressive transition to a deflagrative premixed flame propagation mode. For the lean fuel-air mixture considered (λ=2) the mixing of the additional fuel (previously injected into the pre-chamber) within the main chamber was found to play a major role on the ignition process.