Extreme microgap based hotspot thermal management with convective boiling of refrigerant
Nasr, Mohamed Haitham Helmy
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Performance of the next generation microprocessors is rapidly reaching its limits due to inability to remove heat, especially at high power density from so-called local “hotspots”. Convective boiling heat transfer in microgap heat sinks has the potential to dissipate ultra-high heat fluxes. This thesis presents an experimental investigation of heat transfer performance of three dedicated microgap coolers for hotspot thermal management. In this study, a rectangular microgap, batch micromachined in silicon and instrumented with thin-film resistive thermometry, is employed to assess its capability of dissipating extreme heat fluxes of multiple kW/cm2 while keeping the wall temperature within the limits dictated by electronics reliability. Convective boiling in microgap with heights of 5 μm and 10 μm was tested with and without pin fins in the microgap. The test section was heated from the bottom using resistive heaters and capped with glass to enable visual observation of two-phase flow regimes. Microgap pressure drop and wall temperature measurements, mapped into flow regimes, were obtained with R134a as the coolant, for heat fluxes up to 5 kW/cm2, mass fluxes up to 7,000 kg/m2s, at maximum pressures up to 1.5 MPa and outlet vapor qualities approaching unity. These experimental parameters constitute extreme values in terms of microgap height, mass fluxes, and heat fluxes. New flow regimes, including vapor plumes, liquid slugs, and ultra-thin wavy liquid film, were observed as a function of increasing heat flux and microgap geometry. Dominant mechanism(s) of two-phase heat transfer responsible for each regime have been postulated based on flow visualization correlated with pressure drop and thermal resistance measurements. A 3D numerical model was used to extract two-phase heat transfer coefficient and quality from experimental data and the results were compared to correlations found in literature.