Evaporation and combustion characteristics of hydrocarbon fuel droplet in sub- and super-critical environments

An experimental study on evaporation and combustion phenomena of single suspended hydrocarbon droplets was conducted in sub-and super-critical pressure environments under normal gravity. Droplet temperature and photographs were obtained by utilizing embedded thermocouple and high-speed video camera respectively. Experimental results show equilibrium vaporization stage exists during droplet combustion in sub-critical pressure environments, which accords with quasi-steady assumption. However, the equilibrium vaporization stage disappears during droplet combustion in super-critical pressure environments, quasi-steady assumption is not applicable. Droplet combustion time decreases rapidly with the increase of ambient pressure at sub-critical conditions and phase equilibrium controls the droplet burning rate. In super-critical conditions, the interface between droplet and ambient gas becomes ambiguous, droplet combustion time does not decrease and approaches a stable value; phase change disappears and diffusion coefficient starts to affect the droplet burning rate. The trend change of combustion time variation at critical pressure indicates that the critical pressure point is an important basis for judging whether the droplet enters the super-critical combustion or not. The ratio of droplet evaporation time to droplet combustion time has little change in sub-critical pressure environments, but it decreases rapidly in super-critical pressure environments, this indicates droplet evaporation process completes earlier in super-critical pressure environments.