Heat release multiplicity and subcritical-supercritical droplet transition in autoignition of n-dodecane sprays under detonation-relevant conditions

•Evaporation alters autoignition by modifying the gas temperature and composition.•Droplet evaporation induces additional low-temperature ignitions.•Variations in diameter affect autoignition by changing the evaporation rate.•Droplets that fail to evaporate before gas ignition may enter the supercritical state. This paper investigates the autoignition characteristics of two-phase n-dodecane spray and air mixtures through detailed simulations, utilizing a skeletal mechanism with optimized low-temperature chemistry (LTC). The study examines the effects of fuel equivalence ratio (ER), initial gas temperature and droplet size on the autoignition process. The results demonstrate that the mass and heat transfer resulting from droplet evaporation have a dual effect on chemical kinetics with different initial conditions. Both the chemistry and evaporation time scales are altered as a result of this competition. The LTC introduces additional complexity to the autoignition process, causing the two-phase mixture system to exhibit not only common single-stage high-temperature ignition (HTI) and two-stage ignition (first low temperature, then high temperature), but also a new low-temperature ignition (LTI). Furthermore, autoignition is delayed with higher liquid ER at low or high gas temperatures, while it accelerates in the intermediate temperature range. Moreover, variations in droplet diameter significantly affect the autoignition process by changing the evaporation rate. Additionally, the study identifies the cases where the fuel droplets fail to fully evaporate before the HTI thermal runaway in the gas phase, leading to the possibility of entering the supercritical state. The findings of this study unveil the peculiar phenomena that may occur under elevated temperature and pressure conditions, including additional LTI and the possibility of fuel droplets reaching a supercritical state. These insights enhance the fundamental understanding about the interactions between evaporating droplets and igniting gas under isochoric conditions, providing valuable implications for future research and practical applications of detonation engines.