Pumped-loop liquid-cooled system involving phase-change (two-phase) is a promising technology for removing excess heat from high-density electronics. However, no consensus has been reached so far regarding the hydraulic and heat transfer characteristics of such two-phase (TP) micro flow passages with hydraulic diameters (Dh) on the order of several hundred micrometers or smaller. In a previous paper [Tong et al., ASME IMECE2007-42027], we reported flow boiling experiments for several microchannel (MC) cold plate devices with channel widths ranging from 61 to 340 μm (hydraulic diameters from 100 to 337 μm) and a micro-pin-fin array (μPFA) device. Two working fluids, deionized water and HFE-7100, were tested respectively. In this paper, we focus on the highspeed visualization study of the phase-change phenomena inside the MC cold plates. Features of the major TP flow patterns are discussed for each MC unit with varying sizes and with the two different working fluids. From direct visualization, we report the liquid thin film evaporation rate for the pulsating annular flow regions in MCs. The observed liquid film evaporation rate is much higher than previous predictions/assumptions from a steady-state annular flow picture as assumed by most theoretical investigations so far. The local transient heat flux is thus much higher than the average heat flux provided, especially for water and small hydraulic diameter MCs.

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