IMPACT OF CHANNEL GEOMETRY ON 2-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 n umber of variables control the two-phase flow heat transfer coefficien t. The pressure, the surface heat flux, and the mass flux significantl y affect the thermal transport. Experiments were conducted on a setup that was built for testing microchannel heat exchangers. The parameter s considered in the study are power input: 20 to 300 W, volume flow ra te. 35 to 300 ml/min, quality: 0 to 0.5, inlet subcooling: 5 to 15 deg rees C. The results indicate that the heat transfer coefficient and pr essure drop are functions of the flow quality, the mass flux, and, of course, the heat flux and the related surface superheat. The hear tran sfer 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 v apor quality is increased from 0.01 to 0.65.