NUMERICAL SIMULATION ON COMBUSTION AND EMISSION PROCESSES OF PREMIXED/DIRECT-INJECTED FUEL STRATIFICATION COMBUSTION

摘要

A fully coupled multi-dimensional computational fluid mechanics and reduced chemical kinetics model is adopted to investigate the combustion and emission mechanisms of premixed/direct-injected fuel stratification combustion, stratification of which was achieved by most uniform fuel and other small partial direct-injected fuel. The results show that the low temperature reaction of the premixed / direct-injected fuel combustion first occurs in the regions with homogeneous fuel and the high temperature reaction begins from high fuel concentration regions involved in the spray process. At the end of the low temperature reaction, the concentrations of H_2O_2 and CH_2O in the rich mixture regions are richer than other regions. There are two kinds of stratifications in this study: the premixed/direct-injected fuel stratification combustion injecting fuel near the start of the low temperature reaction and the premixed/direct-injected fuel stratification combustion with appropriate early fuel injection in the case of a small direct injection (Dl)ratio. In the first case, with the increase of the ratio of the direct-injected fuel to the total fuel (DI ratio), the onset of the high temperature reaction advances, the maximum pressure rise rate decreases, high unburned hydrocarbon (UHC) emissions decrease, CO emissions increase, and NOx emissions rapidly increase. In the second case, with the advance in the injection timing, the UHC emissions increase, CO emissions decrease, and NOx emissions reduce. Such stratification combustion has the potential for reducing UHC emissions and CO emissions simultaneously, while NOx emissions are very low. The characteristics of the premixed / direct-injected fuel stratification combustion are consistent with the imposed stratification combustion in our previous work.