Thermal characteristics of two-phase flow of a dielectric fluid in surface-augmented microchannels

Chip-level heat flux levels have risen beyond 250 W/cm² due to the presence of circuit architecture-driven hot-spots. After intense scrutiny over the past two decades, microchannel heat sinks are closer to commercial implementation as evidenced by the advent of the single-chip silicon cooler announced recently by IBM. Though there are a multitude of studies and data related to multichannel configurations, a number of problems still exist which require further understanding. Contradictory results have been reported in the past about the flow behavior and its characteristics such as critical heal flux (CHF), flow transition and flow instabilities. Two-phase flow instabilities in particular, have been a disturbing feature of flow boiling in microchannels and the area of focus in the recent past. A major portion of the current study is directed towards the analysis and suppression of these instabilities. This paper reports an experimental investigation of flow boiling of dielectric fluid FC72 in a heal sink consisting of 19 microchannels of height 346 mum and width 200 mum (hydraulic diameter = 253 mum). The base of the microchannel is augmented with two 20 mum reentrant cavities to trigger nucleation activity and sustain nucleate boiling. The top cover of the channel is made of Pyrex glass, which allows high speed imaging of the How behavior in the microchannel. The flow was driven by a gear pump and conducted at mass fluxes of 500-2200 kg/m²-s and inlet subcooling up to 20degC. The results delineate the effects of mass flux, heat flux and inlet subcooling on the Onset of Nucleate Boiling (ONB), temperature overshoot and the frequency of flow instabilities calculated using a Fourier transform of the average pressure drop. The frequency of instabilities was found to be a stronger function of mixture quality than of the wall heat flux.