Spray Cooling of High Aspect Ratio Open Microchannels

Direct spraying of dielectric liquids has been shown to be an effective method of cooling high power electronics. Recent studies have illustrated that even higher heat transfer can be obtained by adding extended structures, particularly straight fins, to the heated surface. In the current work, spray cooling of high aspect ratio open microchannels was explored, which substantially increases the total surface area allowing more residence time for the incoming liquid to be heated by the wall. Five such heat sinks were EDM wire machined and their thermal performance was investigated. These 1.41times1.41 cm² heat sinks featured a channel width of 360 mum; a fin width of 500 mum; and fin lengths of 0.25 mm, 0.50 mm, 1.0 mm, 3.0 mm, and 5.0 mm. The five enhanced surfaces and a flat surface with the same projected area were sprayed with a full cone nozzle using PF-5060 at 30degC and nozzle pressure differences from 1.36-4.08 atm (20-60 psig). In all cases, the enhanced surfaces improved thermal performance compared to the flat surface. Longer fins were found to outperform shorter ones in the single-phase regime. Adding fins also resulted in two-phase effects (and higher heat transfer) at lower wall temperatures than the flat surface. The two-phase regime appeared to be marked by a balance between added area, changing flow flux, channeling, and added conduction resistance. Spray efficiency calculations indicated that a much larger percentage of the liquid sprayed onto the enhanced surface evaporated than with the flat surface. Fin lengths between 1 and 3 mm appeared to be optimum for heat fluxes as high as 124 W/cm and the range of conditions studied