Abstract
In this work, a comparative numerical study of Reactivity Controlled Compression Ignition (RCCI) combustion was conducted using both the Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) approaches. In order to clarify the associated chemical kinetic processes, a data processing method that is able to calculate average reaction pathways, and identify representative reactions for the species creation, consumption, and heat release was adopted. The LES case yielded a wrinkled spray-flame structure, in contrast to the smoother and concentrated structure predicted by the RANS case. However, despite the different distribution features, both cases demonstrated similar reaction pathways and heat release characteristics. Modeling results showed that the low-temperature heat releases in RCCI combustion were controlled by the n-heptane spray-combustion process and dominated by the reactions R8 (C7H15O2 - 2 = C7H15 - 2 + O-2), R95 (CH3COCH2 + O-2 = CH3COCH2O2), and R285 (HCO + O-2 = CO + HO2). In addition, low-temperature consumption pathways of n-heptane led to the substantial formation of a hydroxyl radical, which enhanced the consumption of iso-octane. As the combustion process progressed, high-temperature ignition pockets were found near the piston rim edge and squish regions, where heat release was dominated by the reaction R285. Intense high-temperature heat release was observed instantly from the ignition pockets, resulting in a rapid propagation of the reactive surfaces.