Flow Boiling Heat Transfer in Microchannel Heat Sinks of Different Flow Orientations

A study of FC-72 flow boiling in a microchannel heat sink with different orientations is presented. The heat sink consisted of 21 microchannels in parallel, with the channel dimensions of 0.2mm (W) × 2mm (H) × 15mm (L). The heat sink was attached onto a flip chip BGA package with a thermal grease at the interface to minimize the contact resistance. The experiments were conducted based on a subcooling around 31 °C and inlet thermodynamic equilibrium quality x = −0.4 with varying flowrates and power inputs for both the vertical upflow (VU) and the vertical downflow (VD) orientations. Boiling with bubbles was observed at the heat sink outlet even if the exit quality x<0. It was found that the flow boiling regime can be represented by isolated bubble flow, slug flow in the present microchannels. Both the junction to inlet thermal resistances and pressure drops are presented as functions of the power input. The experimentally determined thermal resistances were also compared with an analytical analysis in the presence of both single phase convection and two phase saturated boiling heat transfer mechanisms. Good agreement is achieved at large power inputs.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.