Direct Numerical Simulations of Low Temperature Combustion in IC Engine Related Conditions

Abstract: This thesis deals with direct numerical simulation (DNS) of low temperature combustion in internal combustion engine related conditions. In particular, HCCI (homogeneous charge compression ignition), SACI (spark assisted ho-mogeneous charge compression ignition) and PPC (partially premixed charge combustion) were investigated to gain deeper understanding on the details of the combustion process. In the study of HCCI combustion relevant to direct injection HCCI engines, it was found that auto-ignition is not first started at the stoichiometric mixture, but rather in lean mixtures where the local temperature is higher and scalar dissipation rate is low. The first ignition spots with the shortest ignition delay time were found to depend on the turbulence intensity, initial pressure and temperature. With higher turbulence intensity the ignition delay time is longer, and the ignition duration is longer. With higher combustor pressure, the ignition delay time is also longer but the ignition zone is significantly thinner. Compared with the perfectly homogeneous mixture, the ignition delay time in the turbulence case is shorter, owing to differential diffusion and stratifications in composition and temperature. In the study of SACI combustion, the interaction of the premixed flame and auto-ignition under different fuel and operation conditions were studied in 3D DNS and one-dimensional detailed numerical simulations. Several fuels including hydrogen, syngas and methane are studied. The results revealed that due to preferential diffusion of hydrogen, lean hydrogen/air flames tend to develop to cellular shapes, owing to the well known thermal diffusive instability. Furthermore, the imbalance of heat and mass transfer with respect to hydrogen molecular in the preheat zone was shown to inhibit the ignition process leading to a lower temperature and thereby a slower propagation of the reaction front. In the studies of PPC combustion, the effect of several parameters, e.g. the fuel mass ratio between the second and the first fuel injection, initial temperature, and turbulence intensity, was investigated using 2D and 3D DNS for syngas and PRF70. A tradeoff between NO emission and CO emission was identified when varying the fuel mass ratio. Spatial reaction structures showed that during combustion process, auto-ignition was formed first followed by premixed combustion and at last a diffusion controlled partially premixed combustion. In the homogeneous lean charge region, the fuel is consumed almost completely due to relatively fast combustion and abundant of oxygen. In contrast, in the stoichiometric charge region and fuel-rich region CO is hardly oxidized to CO2 even though the temperature is high.