Flow Boiling Critical Heat Flux and Condensation in Microgravity

Results from the Flow Boiling and Condensation Experiment (FBCE), which collected the first flow boiling and condensation data in long-duration, steady microgravity through experiments performed onboard the International Space Station (ISS), are presented. Prior to the ISS experiments, a new correlation for flow boiling critical heat flux (CHF) is developed from data obtained in Earth gravity at different orientations and short durations of microgravity onboard parabolic flight. The new correlation accounts for the influence of gravity in the direction of the flow, impacting vapor removal from the channel, and perpendicular to the heated walls, affecting bubble detachment from the walls, on flow boiling CHF. Novel flow boiling experiments in long-duration microgravity were performed with one or two opposite walls heated using the Flow Boiling Module (FBM), which simultaneously captures heat transfer data and high speed images of flow patterns. The unique microgravity CHF results are presented, and parametric trends are correlated to variations in flow patterns. The results are divided into subcooled and saturated inlet conditions and applicable correlations are assessed. The newly proposed correlation outperforms is the best preforming for the entire database, validating its use in microgravity. Visual observations leading up to CHF justify use of the Interfacial Lift-off model, which predicts CHF with good accuracy for all operating conditions. The data obtained onboard the ISS is consolidated with the prelaunch database to develop highly accurate artificial neural networks (ANNs) for flow boiling heat transfer and CHF in microgravity. The ANNs are developed using a systematic approach that enables the prediction of physical trends. Instabilities observed during subcooled flow boiling are further investigated in dedicated experiments performed at an elevated data capture rate of 30 Hz and extended image capture period up to 28 s. Criteria was proposed to demarcate the stable and unstable operating conditions, and a new correlation to predict the onset of flow instability is proposed. Lastly, microgravity flow condensation heat transfer experiments were conducted onboard the ISS, yielding the first flow condensation data in stable microgravity. Trends in the data are discussed and the two-phase mixture Reynolds number is found to be strongly correlated to local heat transfer coefficient. A separated flow model for annular flow is found to accurately predict trends in average heat transfer coefficient, but underpredicts the microgravity database.