Pool boiling heat transfer and bubble dynamics over V-shaped microchannels and micropyramids: Does high aspect ratio always benefit boiling?

Abstract

Metallic microchannels have been widely applied in two-phase heat transfer devices to satisfy the high heat flux cooling requirements of compact devices, but the pool boiling heat transfer and bubble dynamics over metallic microchannels have been rarely reported. Herein, we investigate the pool boiling heat transfer and bubble dynamics over V-shaped open microchannels prepared on copper by ultrafast laser micromachining. The open microchannels are of various channel widths, 100 μm and 50 μm, and of depth-to-width ratios varying from 0.5 to 2. In addition to the conventional methods used for studying boiling, we utilized the captive bubble method and the capillary tube method to analyze the bubble dynamics and the surface wickability, respectively. We find that for the microchannels with a channel width of 100 μm, the critical heat flux (CHF) and the heat transfer coefficient (HTC) increase with the increase of depth-to-width ratio; but the opposite trend is observed for the microchannels with a channel width of 50 μm. We propose that the surface wickability affects the boiling heat transfer more significantly with the increase of heat flux. For narrow and deep V-shaped microchannels, there is a mismatch between the bubble removal rate and the liquid supplying rate and thus leading to the deteriorated boiling heat transfer. We also demonstrate that the wickability of pyramidal microstructures is much higher than V-shaped microchannels. The CHF and HTC of pyramidal microstructures increase by 5–40% and 35–157%, respectively, compared to the V-shaped microchannels with identical structural spacing and depth.