Impact of channel geometry on two-phase flow heat transfer characteristics of refrigerants in microchannel heat exchangers

Microchannel surfaces, often machined to 20 to 1000 mu m in width and depth, are employed in high-heat-flux applications. However, a large number of variables control the two-phase flow heat transfer coefficient. The pressure, the surface heat flux, and the mass flux significantly affect the thermal transport. Experiments were conducted on a setup that was built for testing microchannel heat exchangers. The parameters considered in the study are power input: 20 to 300 W, volume flow rate. 35 to 300 ml/min, quality: 0 to 0.5, inlet subcooling: 5 to 15 degrees C. The results indicate that the heat transfer coefficient and pressure drop are functions of the flow quality, the mass flux, and, of course, the heat flux and the related surface superheat. The hear transfer coefficient decreases from a value of 12,000 W/m(2)-K to 9000 W/m(2)-K as 80 degrees C, when the wall superheat is increased from 10 to 80 degrees C. The coefficient decreases by 30 percent when the exit vapor quality is increased from 0.01 to 0.65.