A coupled level-set and volume-of-fluid (CLSVOF) method for prediction of microgravity flow boiling with low inlet subcooling on the international space station

The present study is part of a series of NASA-supported investigations of flow boiling of n-perfluorohexane (nPFH, C6F14) under microgravity on the International Space Station (ISS). The fluid physics and heat transfer characteristics of flow boiling with low inlet subcooling are explored using the Flow Boiling and Condensation Experiment (FBCE) (which is actual name of the test facility), the largest endeavor in microgravity two-phase research conducted to date. Experimental data are collected from the FBCE's Flow Boiling Module (FBM), which features a rectangular channel with 5.0-mm x 2.5-mm cross-section and 114.6-mm heated length. Results for three flow rates are used to explore the effects of mass velocity on flow structure and heat transfer characteristics in the absence of the body force. A Computational Fluid Dynamics (CFD) model is constructed to predict the measured FBM results. The CFD model employs the Coupled Level-Set and Volume-of-Fluid (CLSVOF) method, which is modified with improved interface capture and additional force terms in the momentum equation to determine the bubble dynamics more accurately. Validity of the CFD model is assessed in two ways, by comparing predictions against video-captured interfacial behavior and heat transfer data. The CFD model shows good ability to capture detailed evolution of the interfacial structure both across and along the flow channel, including bubble nucleation, growth, departure, and coalescence, as well as axial development of dominant flow patterns. Details of the flow structure are examined via axial development of both flow velocity and void fraction profiles. Similarly, heat transfer results are presented in terms of streamwise variations of both wall temperature and fluid temperature, which are also predicted accurately with the CFD model. Overall, this study proves the CFD model is an effective method for both design and performance assessment of flow boiling subsystems in space vehicles.