Two-phase microchannel system is a promising technology to achieve enhanced heat removal and more effective cooling of hotspots. The excellent thermodynamic properties of water make it a prime candidate as the working fluid in two-phase microchannel systems. While typical integrated circuit components require die temperature to remain below 95 °C, most of the earlier microchannel flow boiling studies were conducted at or above ambient pressure, where the saturation temperature of water is equal to or higher than 100 °C. In this paper, we tested flow boiling at sub-atmospheric pressure such that the saturation temperature of water can be significantly reduced below 95 °C. We study the pressure drop and heat transfer characteristics of our two-phase cold plate configuration, under uniform and hotspot (non-uniform) heating conditions at sub-atmospheric system pressures. A cold plate with 61 μm wide and 272 μm deep microchannels was tested at two systems pressures of 35 and 46 kPa and at two mass flow rates of 67 and 107 kg/m2-s. High-speed flow imaging was used for identifying flow patterns in the microchannels with the above test conditions. Pressure drop data were compared with the available semi-empirical correlations and the annular flow model. An explanation was proposed for the mismatch between the models under current microchannel configuration.

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