Condensation characteristics of liquid ammonia direct injection under diesel engine-like conditions

Ammonia has gained widespread attention in internal combustion engines as a promising hydrogen carrier. However, its large latent heat of evaporation leading to a severe cooling effect brings a challenge for direct utilization of its liquid injection. The present study aims to unravel the condensation features of liquid ammonia injection. First, an efficient Euler-Lagrange simulation framework coupled with a condensation model was established, and the simulation results were verified against experimental data. Then, extensive simulations of multi-hole liquid ammonia injections under various diesel engine-like conditions were conducted, and the results indicate that the proportion of condensation mass relative to its injection mass gradually increases with hole numbers. The condensation in spray transient period gathers at the nozzle exit primarily due to the large instantaneous evaporation rate. Therefore, smaller nozzles, hotter fuels, lower injection pressures and higher ambient temperatures tend to produce more condensation in the transient period. As the spray enters the quasi-steady period, the condensation region moves away from the nozzle exit and becomes widen, primarily depending on the long-term cooling effect, which is jointly affected by evaporation mass, heat transfer rate, and phase envelope. Accordingly, larger nozzles, higher injection pressures, and lower ambient temperatures can facilitate condensation. For the supercritical injection of ammonia, due to its fast phase transition, substantial condensation occurs at the nozzle exit, and its distribution is totally different from the normal evaporating spray. Finally, a characteristic isotherm, only increasing with ambient pressure, was proposed to qualitatively indicate the condensation penetration length for large-scale condensation.