Heat Loss Analysis of Flow Boiling Experiments Onboard International Space Station with Unclear Thermal Environmental Conditions (2nd Report: Liquid-vapor Two-phase Flow Conditions at Test Section Inlet)

This is the second report on the development of heat loss model for flow boiling experiments conducted onboard International Space Station (ISS). In the first paper (Inoue et al. 2021), a heat loss model was developed for subcooled flow boiling experiments, a single-phase liquid at the inlet of heated test tube was discussed. In the present report, a heat loss model is developed for saturated flow boiling experiments, where two-phase liquid and vapor mixture flows to the inlet of heated test tube. The accurate evaluation of vapor quality is essential for the exact investigation of gravity effects on flow boiling. Since long straight test tube could not be introduced due to the strict dimensional restriction of experimental setup in the ISS experiments, a high-power serpentine-type preheater is employed for adjusting vapor quality at the inlet of heated test tube. Therefore, in addition to the heat loss model for subcooled flow boiling developed in the first report, where the temperature of subcooled liquid is measured directly behind the preheater, an additional heat loss model for the sections including the preheater and the downstream piping is needed. A new heat loss model is developed based on the heat transfer data obtained in the preliminary heat loss experiments with liquid single-phase flow and in the liquid-vapor two-phase flow where power is supplied only to the preheater. For the ISS experiments, the amount of heat loss is quite large due to the existence of the avionics air flowing inside the experimental setup. Decrease of vapor quality due to the heat loss increases as the mass velocity decreases and it reaches a difference of predicting quality as much as 0.53 in an extreme example for low mass velocity of 50 kg/(m²·s). Developed heat loss model can estimate vapor quality at the inlet of heated test tube with sufficient accuracy for investigating detailed flow boiling characteristics under microgravity conditions.