Direct numerical simulation of ammonia spray combustion ignited by n-heptane flame under high-pressure conditions

Ammonia is considered as a promising clean and sustainable alternative fuel in internal combustion engines, and ammonia/diesel direct dual fuel stratification (DDFS) engines have attracted extensive research interests. However, the ignition process, subsequent flame development, and interactions of diesel/ammonia flames under DDFS conditions are not well understood. In the present study, ammonia spray ignited by n-heptane flame was investigated using three-dimensional direct numerical simulations (DNS). The thermochemical conditions in the DNS correspond to those in ammonia/diesel DDFS engines. The Eulerian and Lagrangian framework was employed for solving the gas and liquid phases, respectively. Two cases with different ammonia energy ratios (AER) were considered. Two-stage combustion of n-heptane was observed, and it was found that the first-stage ignition occurs in the fuel-lean mixture with minimal heat release, gradually transitioning into the fuel-rich mixture. In contrast, the second-stage ignition is initiated at the tip of the jet in the fuel-rich region, and non-premixed flames near the stoichiometric mixture fraction are identified afterward. The combustion mode analysis based on key radical species illustrates the complex multi-stage and multi-mode nature of the n-heptane flame. Ammonia and n-heptane spray flame interactions were investigated. In the case with low AER, the n-heptane flame is broken by the ammonia spray, and the combustion region is wide. In the case with high AER, the significant entrainment of cold ammonia/ambient gas mixtures into the n-heptane flame decreases the local fluid temperature. The combustion regime of ammonia/n-heptane DDFS combustion was also examined. It was shown that the contribution of premixed combustion decreases with increasing AER, and premixed combustion prevails in the ammonia spray flame in both cases.