A numerical study on the chemical kinetics process during auto-ignition of n-heptane in a direct injection compression ignition engine

This paper presents a numerical analysis of the ignition process of an n-heptane spray in a compression ignition engine using a post-processing tool developed by West Virginia University. Such a tool is able to process the CFD simulation data for the examination of chemical reaction process without revising the CFD source code. The main functions of the post-processing tool include: (1) The calculation of the instantaneous rate of production (ROP) using CHEMKIN with the temperature, pressure and species concentration in each cell simulated using CFD; (2) the development of the representative destruction reaction (RDR) and destruction pathway of the key species involved in a specific area; and (3) the visualization of the analysis results. Such a tool was applied to examine the chemical reaction process during ignition delay of the n-heptane spray in a direct injection compression ignition engine. The H abstraction of n-heptane by O2, OH, HO2, and H radical during ignition period was further examined. The destruction pathway of key species in RDROH/RDRHO2 region and their development leading to the autoignition of n-heptane were studied. It is found that both the n-heptane/air mixture and bulk gas movement play an important role in the formation of RDRHO2 region. The RDRHO2 region featured with medium temperature (around 1000 K) produces more H2O2/HO2 radical before the auto-ignition of n-heptane than low-temperature combustion. The rapid dissociation of H2O2 provides a large amount of OH radical that enhances the chain branching reaction as well as heat release process which then initiates the autoignition of n-heptane. The reaction rates of nC7H16 + OH = C7H15 + H2O and nC7H16 + HO2 = C7H15 + H2O2 were examined to reveal their competition in destructing n-heptane. The H atom is also found to promote the chain branching during auto-ignition. Such a tool provides the convenience for commercial CFD research community to conveniently elaborate the CFD simulation results for better understanding of the fundamental aspects of the combustion and emissions phenomenon observed using CFD code.