A comparative study of experimental flow boiling heat transfer and pressure characteristics in straight- and oblique-finned microchannels

Flow boiling experiments are conducted in straight-finned and oblique-finned microchannels with similar channel dimensions and operating conditions using the FC-72 dielectric fluid. Both heat sink geometries consist of 40 parallel microchannels, and are fabricated on a copper block with a footprint area of 25 mm × 25 mm. The oblique fins are produced by introducing diagonal cuts with angle of 27° and width which is half of that of the parallel microchannels. The comparative study with straight fins shows significant augmentation in heat transfer and the delay in the onset of critical heat flux for the oblique-finned microchannels. This is due to enhancement in the flow boiling stability offered by the oblique fins in terms of reduced wall temperature gradients and pressure fluctuations. Flow visualisations performed on both microchannel geometries show increased bubbles generation in the nucleate boiling region and a continuously developing thin liquid-film in the convective boiling region for the oblique fins, which is believed to be the primary factor in heat transfer enhancement. However, the improved heat transfer performance incurs a higher pressure drop penalty compared to its straight-finned counterpart. This drawback could possibly be overcome by careful modifications in the oblique-finned geometry, so as to control the amount of secondary flow.