Geometrical effects on heat transfer mechanisms during pool boiling in Dual Tapered Microgap with HFE7000

Abstract

Dual tapered microgap configuration provides a simple external device that can be retrofitted on a boiling surface. It has been shown to be very effective for enhancing pool boiling heat transfer coefficient (HTC) and Critical Heat Flux (CHF) with water. Recent testing showed that the same geometry is not effective with refrigerants and dielectric fluids. A systematic experimental and theoretical study is conducted to bring out the underlying mechanisms and identify the effect of geometrical parameters mainly on HTC, as higher values are desired in electronics cooling application. Five different taper angles of 5°, 10°, 15°, 20°, and 25° and two inlet gap heights of 0.8 mm, and 1.27 mm were experimentally evaluated with dielectric liquid HFE7000. Theoretical analysis has identified two mechanisms from bubble squeezing and pressure recovery for inducing flow in the microgap. These mechanisms are influenced by the fluid properties and the geometrical parameters. The best geometrical configuration was able to achieve a 2X enhancement in HTC over a plain copper test surface. Further enhancements are expected with introduction of additional surface features such as microchannels on the heater surface in the microgap region. The theoretical model was able to estimate the pressure drop, mass flux, and the resulting HTC with high accuracy for different geometric configurations.