FLOW BOILING OF R134a IN A LARGE SURFACE AREA MICROCHANNEL ARRAY FOR HIGH-FLUX LASER DIODE COOLING

Packaging high average power laser diode arrays that generate heat at an area average flux in excess of 1 kW.cm(-2) is a significant engineering challenge. While liquid microchannel coolers have demonstrated up to 11.9 kW.cm(-2), two-phase microchannel array coolers have not achieved 1 kW cm(-2) due to critical heat flux and flow instabilities. In the current study, flow boiling heat transfer was characterized by a 1 x 10 mm heated zone centered over a 5 x 10 mm array of 125 very small channels (45 x 200 mu m) with R134a as the phase change fluid. The high aspect ratio channels (4.4:1) were manufactured using MEMS fabrication techniques, which yielded a large heat transfer surface area to volume ratio. A test facility was used to characterize the heat transfer performance of boiling R134a over a range of saturation temperatures (15 degrees C to 25 degrees C), mass fluxes (735-2230 kg.m(-2).s(-1)), and heat duties (< 110.3 W). During the tests, the calculated outlet vapor quality exceeded 61%, and the base heat flux at the heater reached a maximum of 1.1 kW.cm(-2). The resulting average experimental flow boiling heat transfer coefficients are found to be as large a 13.4 kW.m(-2).K-1 over the approximately 3 mm two-phase region, with an average uncertainty of +/- 2.72%. A substantial amount of heat was spread downstream via the low thermal resistance silicon floor. Specifically, between 29.5% and 55.1% of the heat dissipated in the two-phase region was dissipated over the heater. The remaining heat dissipated in the two-phase region was dissipated in the 2 mm of channel downstream of the heater. This suggests that heat spreading from the hotspot played a vital role in dissipating the heat load.