Effect of outlet plenum design on flow boiling heat transfer in microchannel heat sinks

Abstract

Instabilities during flow boiling in microchannel lead to fluctuations in temperature, pressure, mass flux. It is well known that the instabilities can be suppressed by effective extraction of the accumulated vapor away from the outlet plenum. Here we investigate the effect of outlet plenum volume on instabilities during flow boiling in a heat sink comprising of fourteen parallel microchannels within a footprint area of 1 × 1 cm2. Plain and nanostructured microchannel surfaces are investigated with two outlet plenum designs including the one with same volume (baseline case) as the inlet plenum and another one with eight times the volume of the inlet plenum. An eight-fold increase in the outlet plenum volume facilitates a significant reduction in amplitude of fluctuations of surface temperature from 6.6 to 2.8 °C at heat flux of 137 W/cm2 and mass flux of 500 kg/m2s. Efficient vapor removal due to the increase in the manifold volume also lowers the pressure drop, thereby allowing maximum reduction in surface temperature of 4.8 °C and 3 °C for mass flux of 250 and 500 kg/m2s, respectively, compared to that of the baseline case. The enhancement in heat transfer by implementing nanostructured surface with larger outlet plenum volume is marginal with a temperature reduction of about 0.5 °C in comparison to that with plain microchannel with similar outlet plenum volume. Overall, it is shown that the performance improvement obtained with simple outlet plenum design modifications can be comparable or better in comparison to the enhancement due to complex surface modification strategies.