Enhancement of Cryogenic Pool Boiling Through Surface Modifications

Abstract

Cryogenic liquids are critical in many space systems, such as for propulsion, power generation, and for providing breathing air. Experiments were conducted to examine the pool boiling behavior of liquid nitrogen as a liquid-oxygen simulant. Cylindrical test specimens with different geometries and surface configurations (grooves) were heated by embedded electrical cartridge heaters. Multiple embedded thermocouple sensors were used, combined with corrections for variations in the thermal conductivity with temperature, corrections for surface area increase and fin effectiveness, to determine the rate of heat transfer in the nucleate boiling regime. The overall rates of heat transfer and boiling were within the comprehensive ranges of previously reported experiments. The rate of heat transfer for the grooved cylinders was seen to be higher by approximately a factor of ten for horizontal cylinders with smooth surfaces, and a factor of two for vertical cylinders. The heat flux increase was greater at the lower portion of the vertical cylinder compared with the middle section. Correcting the rates of heat flux for differences in surface area due to the addition of groves suggests a substantial decrease in the estimated amount of heat flux gain. Using a fin effectiveness correction for the grooved surfaces however suggests a 12-15\% increase in the overall rate of heat transfer coefficient over the initial measured values.