Spray cooling characteristics of water and FC-72 under reduced and elevated gravity for space application

For the application to space thermal management systems, spray cooling characteristics under both reduced and elevated gravity conditions have been experimentally studied with the aid of the parabolic flights of an aircraft, MU-300, during each of its parabolic flight (10^-2g for 20 seconds) and also during the entry and recovery periods of each parabolic trajectory (1.5-2.0g for 15-20 seconds). Water and FC-72 (perfluorocarbon) were employed as the test liquids to be sprayed and the spray parameters for each liquid were controlled by alternately using three different spray nozzles and adjusting the injection pressure at a prescribed level. The experiments were carried out in two alternative modes. In the first mode, each liquid was sprayed onto a copper block heater surface, causing its transient cooling from a high superheating exceeding the MHF (minimum heat flux) point to a lower superheating below the CHF (critical heat flux) point. In contrast to this, in the second mode the liquid was sprayed onto a transparent glass plate heater which was Joule-heated such that a relatively low heat flux below the CHF was maintained steadily at the heater surface, a detailed observation from the rear side of the transparent heater was schemed so that the behavior of the impinging droplets on the heater surface could be recognized. A remarkable difference was found in the gravity dependency of spray cooling characteristics between water and FC-72. The observation through the transparent heater indicated that the difference in the low heat flux region is mainly ascribable to a difference between the two liquids in the behavior of liquid layer formed on the heater surface