Heat Loss Analysis of Flow Boiling Experiments Onboard International Space Station with Unclear Thermal Environmental Conditions (1st Report: Subcooled Liquid Flow Conditions at Test Section Inlet)

Two series of flow boiling experiments have been conducted onboard the International Space Station (ISS) as a part of the TPF (Two-Phase Flow) experiment promoted by JAXA during July 2017–March 2018, February 2019–July 2019. Microgravity data on two-phase flow and heat transfer in flow boiling of n-Perfluorohexane (FC-72) have been obtained by a copper heated test tube and a transparent glass heated tube in a wide experimental range of mass velocity, liquid subcooling, vapor quality and heat flux. Furthermore, detailed two-phase flow behaviors have been observed by using high frame rate camera in the unheated observation section. In order to elucidate the accurate influence of gravity on flow boiling, it is essential to compare the heat transfer data and two-phase flow behaviors obtained under normal (terrestrial) gravity and microgravity environments at the same flow and heating conditions. However, both experiments cannot be performed by using the same experimental apparatus and under the same thermal environmental conditions. In addition, the heat loss cannot be negligible due to the forced avionics air flow inside the experimental apparatus in ISS. Therefore, exact evaluation of the fluid conditions at the inlet of the heated test tube requires the heat loss model with high-accuracy. In the present paper, the heat loss models for evaluating the degree of liquid subcooling at the inlet of the heated test tube and net heat flux from the heated tube to test fluid has been developed by using the results of preliminary heat loss experiments conducted onboard ISS. The correction of the degree of liquid subcooling by the proposed heat loss models is not negligible for the accurate analysis of gravity effects. The accuracy of the heat loss models has been verified through the evaluation of heat transfer coefficients for single-phase turbulent forced convection obtained from the heating experiments onboard ISS.