Effect of heat-transfer surface structure on critical heat flux

This study deals with the critical heat flux for saturated and subcooled flow boiling in rectangular narrow channels with boiling heat-transfer enhancement surfaces manufactured by a thermal spray coating. The coatings were fabricated on sandblasted copper plates by vacuum plasma spraying using fine copper particle. HCFC123 and FC72 were used as the working fluid for saturated and subcooled flow boiling, respectively. The one side of the wall at the center of the narrow channel was replaced with the heating surface. The channel height was varied 1, 2, and 4 mm, and the channel width was 20 mm. In the saturated flow boiling experiments, the channel was placed in various inclined angles, and the effect of flow and heating direction to gravity on the critical heat flux had beenmeasured. For subcooled boiling, the flows were horizontal flows with bottom heating, and the effect of inlet subcooling degree on the critical heat flux had been measured. These experimental results were compared with those for a smooth surface. As for the results, for saturated flow boiling, the critical heat flux was very sensitive to the change in the channel inclined angle. The effect of the surface structure on the critical heat flux was minimal. On the other hand, for subcooled flow boiling, the critical heat flux of the coating increased by about 20% in the condition with a large degree of subcooling of 40 K. The reason might be that vapor bubbles generated on the coating were immediately condensed in bulk subcooled liquid due to its smaller diameter. For both surfaces, large pressure fluctuation was observed just before burnout in the flow condition with a large degree of subcooling of 40 K. © 2012 by Begell House, Inc.