Diesel engine has been widely used in the vast majority of combustion applications, and its popularity has steadily increased due to inherent benefits in terms of its thermal efficiency, fuel economy and durability. However, the resulting exhaust from the diesel fuel combustion includes PM (particulate matters) and gases, that represent thousands of different chemical substances known to be hazardous air pollutants. Recently, as the interest of environmental pollution is increased all over the world, emission standards are also getting gradually stricter. Therefore, better understanding of various processes inside the cylinder of diesel engine is critical to achieve a strategy for reducing pollutant formation. Diesel engine combustion can be characterized by spray dynamics and its combustion in terms of pollutant formation processes. Due to complex interactions among spray, ignition, combustion and pollutant formation processes, it is very hard to investigate experimentally one process independently from the others. On the other hand, it’s a good engineering practice to study numerically the characteristics of diesel spray and its effects on emission as the first step for understanding the diesel spray combustion.
In the present study, the characteristics of fuel spray and the influence of the operating parameters on the resulting exhaust emission are investigated to correlate the spray characteristics with exhaust emissions such as soot and NOx. For this purpose, numerical studies have been made in the spray jet injected into a pressurized, high temperature constant volume vessel using KIVA code. KIVA, which is a multi-dimensional CFD code for analyzing a chemically reacting flow with fuel spray, is modified to implement the reduced and detailed chemical reaction mechanisms as an engineering tool for studying of soot and NOx formation in diesel spray combustion and validated by comparing with a reliable experimental data. The simulations employ an n-heptane chemical reaction mechanism including 66 species and 274 reactions for investigating diesel ignition and emissions formation such as soot and NOx under various ambient gas temperatures. This detailed chemical reaction mechanism of n-heptane as a representative diesel fuel is coupled directly with the CFD calculations through a partially stirred reactor (PaSR) model to consider the interaction of turbulence and chemistry.
Using the updated KIVA code, the comparison with the experimental data gave a good reliability in terms of spray penetration length, total maximum soot mass concentration, auto-ignition point and soot formation. The numerical prediction of product emissions gave an insight of the relationship between the spray characteristics and exhaust emissions. However, there was a sensitive grid dependency on the computational results. This severe grid dependency is originated from the O’Rourke’s collision model used in this study. It has a built-in drawback that a computational mesh with small cell size will yield the increase of collision frequency and the decrease of domain meeting two parcels. Compared to the good consistency with the experimental data of temporal distribution of soot formation, soot mass concentration is lower than the measured one. It is because the soot formation process which is represented by particle inception, surface growth and oxidation and coagulation is not modeled phenomenologically. These drawbacks should be updated for the improvements of current numerical methods as future works.