Heat transfer coefficient and pressure drop of water flow boiling in porous open microchannels heat sink

Abstract

• Porous open microchannels (POM) heat sink was proposed and fabricated. • Heat transfer coefficient was greatly improved by porous wall at high heat fluxes. • Periodic slug-stratified flow was newly observed in POM heat sink. • No local dryout was noted due to the enhanced capillary actions in porous wall. • Two-phase flow could maintain stable under all operating conditions tested. The open microchannel is attracting more attention due to the reduced pressure drop and the improved flow stability, but the local dryout easily occurs at the downstream of the microchannels and subsequently deteriorates the flow boiling heat transfer performance. In this study, two open microchannels heat sinks were fabricated utilizing solid copper substrate and the porous substrate sintered by copper powder particles, respectively. Flow boiling experiments were conducted using deionized water as the working fluid with the inlet temperate of 87 °C at different mass fluxes ranging from 151.5 to 551.1 kg/m 2 s over a heat flux range of 29.6-1871.1 kW/m 2 in the porous open microchannels (POM) and the solid copper open microchannels (SOM). Experimental results showed that the heat transfer coefficients (HTC) were greatly improved in POM compared with that in SOM at moderate (300 kW/m 2 < q eff < 600 kW/m 2 ) and high ( q eff > 600 kW/m 2 ) heat fluxes. No local dryout was observed even at the highest heat fluxes in POM heat sink. Therefore, the flow boiling HTC in POM no longer deteriorated with increment in heat flux at high heat flux region, unlike in SOM. A new periodic flow pattern named slug-stratified flow was found in POM heat sink during which the slug flow and the stratified flow alternated temporally and spatially. Besides, although the two-phase pressure drop of POM was larger than that of SOM due to the increased friction and acceleration loss, the flow could still maintain stable under all operating conditions tested here.