LES Analysis of Fuel/Air Mixing and Heat Release Processes in a Diesel Spray

Numerical calculations were performed to investigate the mixture formation, ignition, and combustion processes in a diesel spray. The spray was formed by injecting n-heptane into a constant volume vessel under high-temperature and high-pressure conditions. The fuel droplets were described by a discrete droplet model (DDM). Numerical calculations for the flow and turbulent diffusion processes were performed on the basis of large eddy simulation (LES) to describe the processes of local non-homogeneous mixture formation and heat release. The oxidation processes in the mixture were calculated by Schreiber's five-step mechanism for n-heptane. Calculations were performed for sprays formed by single-stage injection and pilot/main two-stage injection. The flame structure in a diesel spray and its temporal change were discussed using a flame index proposed by Yamashita et al. The results show that combustion of a premixture release heat around the boundary between the unburnt mixture in the upstream region and burnt mixture in the downstream region, and diffusion flame is formed in the surrounding region of the burnt mixture. The different flame structures are formed according to the surrounding gas conditions. In addition, in the pilot/main two-stage injections, the ignition delay of the main spray becomes very short and the region of premixture combustion locates upstream, which could result in larger soot emissions.