Flow boiling in plain and porous coated microchannels

Abstract

Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 kg/m2s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 160.7 kW/m2. HFE-7200 was used as the working fluid. Flow visualisation results and SEM surface analyses are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide, while shallow but larger cavities up to 6 µm were found on the plain copper channels. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern evolution occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux. Microchannel flow boiling heat transfer is enhanced by up to 43.5 % at low heat fluxes where the nucleate boiling mechanism is dominant. Heat transfer enhancement diminishes with further increase in heat flux to 13.2 %, potentially due to nucleate boiling suppression with flow regime transition.