Enhanced flow boiling in silicon nanowire-coated manifold microchannels

An experimental study was carried out to investigate the heat transfer, pressure drop and flow instability characteristics associated with the flow pattern of deionized water during two-phase boiling in a silicon-based manifold microchannel heat sink coated with silicon nanowires (SiNWs) compared to a plain-wall device. The manifold microchannel device featured parallel transverse microchannels etched on a silicon substrate and longitudinal microchannels etched on a glass cover plate. Silicon nanowires were generated on the bottom and the sidewalls of the silicon microchannels. A closed-loop experimental system was constructed to demonstrate thermal and hydraulic performance. Experimental results were presented with mass fluxes ranging from 250 to 1250 kg/m² s and subcooled inlet temperatures from 15 K to 65 K. Results for the SiNWs device showed an approximate 20% improvement in heat flux rejection compared to the plain-wall device under the same wall superheat conditions. A subcooled inlet temperature of 65 K associated with a mass flux of 1250 kg/m² s is shown to be capable of dissipating an effective heat flux of 431.3 W/cm² with a wall superheat of about 85 K. Overall, the SiNW coatings proved positive effects on enhancing the flow boiling heat transfer with slower pressure drop increase, meanwhile the three-dimensional manifold microchannel design is revealed to effectively mitigate flow instability during the entire single and two-phase flow regions. This indicates great potential in utilizing three-dimensional flows by integrating SiNWs surface structures in high heat flux cooling applications.