Flow Boiling in Silicon Microchannel Heat Sinks

The local flow boiling heat transfer and pressure drop in microchannel heat sinks are investigated with a dielectric fluid, Fluorinert FC-77. The effect of channel size on flow boiling is studied for mass fluxes ranging from 250 to 1600 kg/m²s for seven different test pieces consisting of parallel microchannels with nominal widths ranging from 100 to 5850 mum, all with a depth of 400 mum. High-speed visualizations are performed simultaneously with the local measurements of the temperature and pressure drop to investigate the flow boiling patterns and the conditions for transition between different regimes. The results of this study show that for microchannels of width 400 mum and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size, and have a weak dependence on channel width for smaller microchannels. This is consistent with the visualizations which show that flow regimes in microchannels of width 400 mum and larger are similar, while those in the 100 mum wide microchannels are distinctly different. Also, unlike the 100 mum wide microchannels, in which bubble nucleation at the walls is suppressed at a relatively low heat flux, nucleate boiling is dominant over a wide range of heat fluxes for microchannels of width 400 mum and larger.