Phase change in microchannel heat sink under forced convection boiling

A microchannel heat sink system, consisting of parallel microchannels, distributed temperature micro-sensors and a local heater, has been fabricated and characterized. V-grooves with hydraulic diameter of either 40 /spl mu/m or 80 /spl mu/m were formed by bulk silicon etching. The heater and temperature microsensor array were fabricated using surface micromachining. Microchannels were realized by bonding a glass wafer to the silicon substrate, resulting in a transparent cover for flow visualization. Phase change during the boiling process was studied under forced convection conditions, where DI water was used as the working fluid. No boiling plateau, associated with latent heat, has been observed in the boiling curves of microchannel heat sinks. Flow visualization was carried out to understand the boiling mechanism in such a system. Three phase-change modes were observed depending on the input power level. Local nucleation boiling within the microchannels occurred at low power level. At moderate levels, large bubbles developed at the inlet/outlet regions, and the upstream bubbles were forced through the channels and out of the system. At higher input power levels, a stable annular flow mode was observed, where a thin liquid film coated each channel wall until critical heat flux conditions developed with a dryout of the system.