Investigation and Comparative Analysis of Onset of Nucleate Boiling (ONB) in a Narrow Rectangular Channel in Microgravity and Earth Gravity Environments

Understanding the onset of nucleate boiling (ONB) in different gravitational environments is crucial for a wide variety of industrial and space applications of flow boiling systems for thermal management. This thesis provides a comparative analysis of ONB in both microgravity (µg) and Earth gravity (1g) environments, with the aim of identifying the key differences. NASA’s experimental facility called Flow Boiling and Condensation Experiment (FBCE) equipped with Flow Boiling Module (FBM) was used for both the experiment onboard the International Space Station (ISS) and on Earth. The experiment was conducted in a rectangular channel of 5mm (depth) ×2.5 mm (width) ×114.6 mm (length) where N-perfluorohexane (nPFH) was used as coolant with subcooled inlet condition in a single-side-heated condition. Key operating parameters of investigations are mass flux of (200 - 3000 kg/m2s)) with subcooling degrees (6 - 24℃), where a uniform heat flux is applied to the heating surface in a stepwise manner. Seven thermocouples were installed into the heated surface to measure the wall temperature distribution and sequential images acquired by a high-speed camera. ONB was detected from the transient wall temperature graphs, and it was found that in microgravity multiple ONB like phenomenon occurs whereas in Earth’s gravity most of the time it occurs only once. Visualization images was compared, and it showed that after ONB bubbles are seen throughout the whole channel in µg, while bubbles appeared only on the last half of the channel in 1g. Location wise temperature drop at ONB was investigated and it was found that the graph is flatter in µg than 1g. Transient behavior of bulk fluid temperature, inlet and outlet pressure, mass flux during ONB was analyzed for mass flux of 200 kg/m2s in four different subcooling. Parametric effects of subcooling at different mass flux on ONB heat flux and ONB wall superheat was examined. Two correlations for µg and 1g were developed using non dimensional parameters for predicting ONB heat flux and superheat and compared with existing correlations. Spatial and temporal patterns of ONB occurrence was determined and discussed. It was found that the time span from the first ONB to last ONB is higher in µg than 1g for most cases. Lastly, correlations were developed to predict the first ONB location in microgravity and Earth gravity with a very good agreement with the experimental data.