Nucleate Pool Boiling eXperiment (NPBX) in microgravity: International Space Station

Abstract

A series of nucleate pool boiling experiments were conducted in the Boiling Experimental Facility (BXF) located in the Microgravity Science Glovebox (MSG) on board the International Space Station (ISS) during the period March–May, 2011. Nucleate Pool Boiling eXperiment (NPBX) was one of the two experiments housed in the BXF. Results of experiments on natural convection, nucleate pool boiling heat transfer and critical heat flux are described. Perfluoro-n-hexane was used as the test liquid. The test liquid contained dissolved gas. The test surface was a polished aluminum disc (89.5mmdia.) heated from below with strain gage heaters. Five cylindrical cavities were formed on the surface with four cavities located at the corners of a square and one in the middle, to study bubble dynamics and initiate nucleate boiling. During experiments the magnitude of mean gravity level normal to the heater surface varied from 1.7×10−7ge to 6×10−7ge.The results of the experiments show that at low superheats, bubbles generated on the heater surface slide and merge to yield a large bubble located in the middle of the heater. At high superheats, the large bubble may lift off from the heater but continue to hover near the surface. In both these scenarios, the large bubble serves as a vapor sink. Natural convection heat transfer in microgravity was found to be consistent with that predicted by available correlations. Steady state nucleate boiling and maximum heat fluxes are found to be lower than those obtained under earth normal gravity conditions. The heat transfer coefficients for nucleate pool boiling are found to be weakly dependent on the level of gravity (h/hge∝(g/ge)1/8). Maximum heat flux also shows a weaker dependence on gravity than that given by the hydrodynamic theory of boiling. The data are useful for calibration of results of numerical simulations. Any correlations that are developed for nucleate boiling heat transfer under microgravity condition must account for the existence of vapor escape path (large vapor bubble acting as a sink) from the heater, relative size of the large bubble and heater, and the size and geometry of the chamber used.