FLOW BOILING HEAT TRANSFER OF R134a IN A MICROCHANNEL HEAT SINK

In the present study, the thermal performance of a micro heat spreader based on flow boiling mechanism was evaluated. The heat spreader under study has a foot print area of 15 x 15 mm(2) and is composed by 50 channels with width of 100 mu m and depth of 500 mu m. Experiments were performed for R134a, heat fluxes up to 310 kW/m(2) (based on the foot print area), mass velocities from 400 to 1500 kg/m(2)s, saturation temperatures of approximately 25 degrees C and liquid subcoolings of 5 and 15 degrees C. Heat-sink averaged heat transfer coefficients based on the effective heat transfer area up to 9 kW/m(2)K were obtained. From the analyses of the experimental results, for a fixed heat-spreader averaged wall superheating, it was found that the dissipated heat flux increases with decreasing the mass velocity and liquid subcooling. Moreover, the trend with varying mass velocity of the difference between the heat spreader averaged superheating for the onset of nucleate boiling (ONB) and the respective value under boiling conditions is not clear, however it seems that its value increases with increasing the liquid subcooling. The wall superheating excess necessary for the onset of nucleate boiling becomes negligible for mass velocities higher than 1000 kg/m(2)s.