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Phase-Change Heat Transfer Characterization In Microscale Pin Fin Heat Sinks
Erfan Rasouli and Vinod Narayanan

Subcooled and saturated flow boiling of Performance Fluid PF5060 through four micro pin fin heat sinks (μPFHSs) with varied pitch and aspect ratios is investigated in this study. The geometrical variations include pin sizes of 400 μm and 200 μm, corresponding to aspect ratios of 3.1 and 4.2, and transverse pitch ratios between 2.4 and 3.8 respectively. Flow boiling performance is characterized for mass fluxes ranging from 30–100 kg/m2.s at a fixed inlet subcooling of 12.5°C. Saturated inlet flow condition is also tested for the lowest mass flux of 30 kg/m2.s to evaluate the effect of inlet subcooling on two-phase heat transfer coefficient trends. Qualitative visualization using high speed imaging is provided to augment global measurements trends. For both saturated and subcooled conditions and the range of mass fluxes, nucleate boiling heat transfer is the dominant mechanism for xout  <0.5 whereas semi-annular film evaporation becomes dominant at higher qualities. The pitch and aspect ratio effects on two-phase heat transfer rate and hydrodynamic performance of pin fin arrays are compared under the same flow conditions. The phenomenon of vortex shedding observed in single phase flows [1–2] for the higher pitch ratio heat sinks is seen to result in increased two-phase heat transfer coefficients in the nucleate boiling dominant regime. Additionally, the unsteady vortex shedding eliminates the wall temperature overshoot in the range of experiments performed. The experimentally determined heat transfer coefficients are compared with predicted values by available correlations in the literature. Predictions by correlations in literature are not in good agreement with the experimental values and show a systematic deviation depending on the μPFHS dimensions.

Keywords: Phase-change heat transfer, subcooled boiling, micro pin fin heat sink, nucleate boiling mechanism, flow visualization, vortex shedding.

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