Numerical studies for diesel spray assisted methane ignition at low temperature conditions

This dissertation belongs to the research field of computational physics and numerical combustion modeling. Computational fluid dynamics (CFD) and finite-rate chemistry are employed to numerically investigate reacting spray combustion at internal combustion engine (ICE) conditions. The main focus of the study is the ignition characteristics of dual-fuel (DF) sprays, where a high-reactivity liquid fuel spray (here n-dodecane) is injected into an hot ambient environment with premixed low-reactivity fuel (here methane) and oxidizer. In particular, operating conditions similar to reactivity controlled compression ignition (RCCI) combustion technology are adopted. Such an approach may provide better ignition timing control with lower emissions and higher thermal efficiency. Although DF combustion and RCCI are promising engine combustion concepts, there are still operational issues related to RCCI such as ignition delay time (IDT) and heat release rate (HRR) control, as well as finding optimal operating conditions. Numerical simulations and high-performance computing enable detailed investigations on the 3D physics and chemistry of such reacting flow problems. The dissertation consists of three journal publications. Large-eddy simulation (LES) coupled with finite-rate chemistry approach is utilized using OpenFOAM, a C++ library for CFD simulations. In Publication I, we investigate the effect of ambient temperature on DF spray ignition characteristics, and the inhibiting effect of methane on n-dodecane oxidation chemistry at lower temperatures. In Publications II and III, the effect of spray injection timing using single and double injection strategies within RCCI context is investigated by introducing a compression heating model to account for dynamic thermophysical conditions due to piston compression. The ignition characteristics of DF sprays are investigated and the effect of injection timing is analyzed. The main findings of this dissertation can be summarized as follows: 1) In stationary conditions without dynamic compression, DF spray simulations at varying engine-relevant ambient temperatures indicate that methane has an inhibiting effect on n-dodecane spray IDT, especially at lower temperatures. This behavior was also shown to be qualitatively insensitive to the selection of chemical mechanism. 2) Under dynamic compression, in RCCI relevant conditions, it is observed that advancing injection timing first advances, then retards the IDT, due to over-leaning of the injected spray at very advanced injection timings. The finding is noted to be consistent with experimental observations in a laboratory engine. 3) It was found that the source of reactivity stratification in DF spray combustion is mostly mixture stratification, rather than thermal stratification. 4) With advanced injection timing, the contribution of lean mixture conditions and low-temperature chemistry modes to HRR increases. 5) In a double-injection scenario, the injection timings and the dwell time between the two injections can be adjusted to control the local reactivity, IDT, and HRR characteristics.