Flow boiling heat transfer of R407C in a microchannels based heat spreader

Abstract New flow boiling experimental results for R407C in a microchannel based heat spreader are presented. Boiling curves were obtained for heat fluxes up to 310 kW/m 2 (based on the footprint area), mass velocities from 400 to 1500 kg/m 2 s, liquid subcoolings at the test section inlet of 5, 10 and 15 °C and saturation temperatures referred to the pressure at the heat sink inlet of approximately 25 °C. Based on these results, heat sink averaged heat transfer coefficients during convective boiling were estimated. The heat sink evaluated in the present study is comprised of fifty parallel rectangular channels with cross-section dimensions of 100 × 500 μm, and total length of 15 mm. Average heat transfer coefficients up to 30 kW/m 2  °C were obtained. It was also found that the boiling curve moves to the left hand side with decreasing the mass velocity and liquid subcooling at the heat-sink inlet. Moreover, for a fixed heat-sink averaged vapor quality, the average heat transfer coefficient increases with increasing mass velocity. Under similar experimental conditions, the refrigerant R134a provided higher heat transfer coefficients than R407C. Additionally, during flow boiling of R407C, pressure oscillations with lower amplitude and frequency were observed compared to R134a. No one of the heat transfer predictive methods evaluated in the present study was accurate enough to predict the present R407C database.

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