Triple-Point Pressure Evaporation of Impacted Droplets

The spray evaporator is a heat rejection device used for thermal control of spacecraft. An expendable evaporant is used to cool a transport fluid by means of evaporative cooling. The evaporant is sprayed at vacuum conditions onto a warm, highly conductive surface and is evaporated. The vapor is then vented to space. An important aspect of the evaporator is the way in which the droplets impact and evaporate on the warm evaporator surface in a near-vacuum environment. A high-speed photographic study was made to observe the impact and evaporation of water droplets at pressures below the triple-point pressure of water. The droplets ranged in size from 50 micron to 600 micron in diameter. Surface temperatures ranged from 140 F to 35 F. It was determined from observations of the high-speed films that the degree of solidification before impact determines the impact mechanism to a large degree, and that the surface temperature determines the evaporation and wetting mechanism, while the other parameters have only a secondary effect on the system.