Experimental study on flow boiling in microchannel evaporators with top gap structures

Abstract

The efficient cooling of high-power electronic equipment requires developing a simple and effective method for enhancing boiling heat transfer. This study utilized R245fa as a refrigerant in a closed pump-driven two-phase system to enhance boiling heat transfer performance. The effects of the boiling curves, overall heat transfer performance, radial local heat transfer coefficients, and along-length local heat transfer coefficients were analyzed under five different mass fluxes and four top gap height structures. The results indicate that a top gap height of 0.9 mm leads to a higher critical heat flux compared to a structure without a top gap. For a given mass flux, the overall heat transfer coefficient peaks at a top gap height of 0.9 mm. The difference in radial local heat transfer coefficient (Δ$h_r$) consistently exceeds zero across different vapor qualities and mass flux conditions. As the vapor quality increased, the overall trend of the along-length heat transfer coefficient showed a significant increase at the $L_{nd}$= 0 position. Among the top gap structures, the 0.9-mm top gap height exhibits the highest and most uniform along-length heat transfer coefficient. The average local heat transfer coefficients for the 0.9-mm top gap were 1.17-, 1.20-, 1.21-, 1.11-, and 1.09 times higher than those of a gapless structure as the mass flux increased from 89 to 535 kg/m²/s. This study provides deeper insight into how the top gap height enhances the fluid redistribution and rewetting mechanisms in flow boiling within microchannels.