Transcritical vaporization of liquid fuels and propellants

An overview is provided of some challenges associated with predicting spray combustion processes in propulsion systems operating at pressures and temp eratures that are above the critical values of the pure fuel or propellant injected in the liquid phase (e,g,, diesel engines and cryogenic liquid roc ket engines), The issues determining high-pressure phase equilibria are out lined first. Then, the case of the gasification of a liquid fuel (propellan t) droplet in a quiescent environment is considered. The reviewed literatur e shows that the more advanced models now provide consistent predictions re garding, for instance, the variation of droplet lifetime with pressure. The droplet gasification process at these conditions is essentially unsteady. Recent studies using molecular dynamics simulations to investigate transcri tical droplet vaporization are briefly discussed. Next, the effects of conv ection, secondary atomization, and the proximity of neighbors on supercriti cal droplet combustion are! considered. Published results indicate that the latter tends to preclude droplets in clouds from reaching the critical mix ing state. Forced convective effects on the behavior of a droplet at superc ritical conditions are considerable because they couple with a significantl y reduced surface tension coefficient to produce secondary atomization and a one order of magnitude reduction in the droplet lifetime. Finally, a spec ific example is given of how supercriticality influences the overall perfor mance of propulsion systems.