Convection and Flow Boiling in Microgaps and Porous Foam Coolers

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An open and foam-filled microgap cooler, providing direct liquid cooling for a simulated electronic/photonic component and which eliminates the problematic thermal resistance of the commonly-used thermal interface material (TIM), is examined. The single phase heat transfer and pressure drop results of water are used to validate a detailed numerical model and, together with the convective FC-72 data, establish a baseline for microgap cooler performance. The two-phase heat transfer characteristics of FC-72 are examined at various microgap dimensions, heat fluxes, and mass fluxes and the results are projected onto a flow regime map. Infrared (IR) thermography is used to explore the two-phase characteristic of FC-72 inside the channel instantaneously. Also the single and two-phase heat transfer and pressure drop of porous metal foam which can enhance the cooling capability of low conductive fluid are studied and compared with the performance of the open channel microgap cooler in terms of volumetric heat transfer rate and required pumping power.

The single-phase experimental results were in good agreement (within 10% error) with classical correlation of single-phase heat transfer coefficient and pressure drop in micro single gap channel with heat transfer coefficients as high as 23 kW/m2-K at 260 µm gap with water and 5 kW/m2-K at 110 µm gap with FC-72. Annular flow was found to dominate the two-phase behavior in the open channel yielding FC-72 heat transfer coefficients as high as 10 kW/m2-K at 110 µm gap channel.

The single-phase pressure drop and heat transfer coefficient experimental results are compared with existing correlations and achieved 10 kW/m2-K of heat transfer coefficient at 95% porosity and 20PPI with water and 2.85 kW/m2-K with FC-72 at the same configuration. For the two-phase flow boiling, it is found that large pore size provides better cooling capability.