Microgravity Heat Transfer in Flow Boiling

To investigate flow boiling in microgravity, test sections of transparent heated tube and transparent heating surface were developed, and heat transfer characteristics were directly related to the liquid-vapor behaviors observed. The experiments were performed on board aircraft where the boiling system was exposed in series to normal, hyper and reduced gravity fields along a parabolic trajectory. In the experiments using a round tube and an analytical model, an important gravity effect on two-phase forced convective heat transfer where heat transfer is deteriorated in microgravity at low mass velocity was clarified. As regards the dryout phenomenon, measurement of critical heat flux was attempted in a short microgravity duration and the process of dryout was investigated for a moderate quality region based on the measured temperature fluctuation and corresponding liquid-vapor behaviors. Flow boiling in narrow channels was also investigated as one of the systems to be applied to space heat exchangers, and a few important characteristics were clarified concerning the gravity effect. Because of the limited opportunity for experiments and the short microgravity duration created by aircraft, the results obtained here could not cover all aspects of the phenomenon for the gravity effects on flow boiling for different systems and parameters, but the results are intended to become a powerful aid for further investigation in the present discipline utilizing longer microgravity periods in a new space platform to be realized soon.