Parametric experimental trends, interfacial behavior, correlation assessment, and interfacial lift-off model predictions of critical heat flux for microgravity flow boiling with subcooled inlet conditions – Experiments onboard the International Space Station

Abstract

This study examines the critical heat flux data obtained during the “Flow Boiling and Condensation Experiment (FBCE)” which was launched to the International Space Station (ISS) in August 2021. The overarching goals of FBCE are to obtain flow boiling and flow condensation data in high quality, long-duration microgravity, and investigate gravitational effects on two-phase flow physics. The first stage of FBCE completed features the Flow Boiling Module (FBM), which collected flow boiling heat transfer data and flow visualization images in a highly accurate and steady microgravity environment from February 2022 until July 2022. Flow boiling experiments were performed with subcooled inlet of n-Perfluorohexane in a rectangular channel of dimensions 114.6-mm heated length, 2.5-mm heated width, and 5.0-mm height with either one or two, opposite, heated walls. The long-duration microgravity database encompasses a broad range of operating conditions: mass velocity of 199 – 3200 kg/m2s, inlet subcooling of 2.6 – 45.6°C, and inlet pressure of 124.8 – 176.7 kPa. Image sequences leading up to and at CHF are presented to both illustrate the physical mechanism triggering CHF and explain the experimental trends observed in microgravity. Experimental results show CHF is strongly dependent on mass velocity and higher degrees of inlet subcooling, but less dependent on inlet pressure in the tested ranges. Examination of the relationships and parametric trends between dimensionless groups governing CHF reinforces conventional trends and reveals, for subcooled CHF, a dependence of Boiling number at CHF on outlet thermodynamic equilibrium quality. Comparison of the new microgravity CHF data with Earth-gravity CHF data reveals, for single-sided heating, CHF in microgravity is degraded up to ∼38% at low mass velocity, with diminishing differences as mass velocity is increased, and for double-sided heating, less significant differences in CHF between the two gravitational environments. Experimental data are compared to predictions of various flow boiling CHF correlations which previously demonstrated their merit, and the most suitable one for the entire database is recommended. Flow visualization reveals a wavy vapor layer with wetting fronts described in the Interfacial Lift-off Model, which is used to predict CHF values with good accuracy.