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Refrigerant Flow Boiling in a Microchannel Cold Plate Evaporator
Keywords: microchannel, heat transfer, refrigerant
The local two-phase heat transfer coefficient is the key parameter needed for accurate prediction of the heat transfer rate in microchannel cold plate evaporators used in electronics cooling systems. Many of the recent studies of the heat transfer coefficients in microchannel heat exchangers for electronics cooling were developed for water as the heat transfer fluid and in the subcooled or low vapor-quality region. In contrast, the present study focuses on the investigation of the local flow boiling heat transfer coefficient at different vapor qualities of the refrigerant HFC-134a in a microchannel copper cold plate evaporator. An experimental setup was designed and built to measure the local heat transfer coefficient and to obtain a better understanding of the underlying physics. The heat transfer coefficient is measured locally for several vapor qualities starting from subcooled liquid to superheated vapor. In this way the local heat transfer coefficient can be mapped as a function of the average refrigerant quality. In addition measurements were carried out for subcooled boiling. The current test piece contains 17 parallel, rectangular microchannels with hydraulic diameters of 1.09 mm and aspect ratios of 2.5. The design of the test setup is supported by a robust energy balance and comparison of single phase heat transfer coefficients with the literature. Results are presented for four different refrigerant mass flow rates of 0.5, 1.0, 1.5, and 2.0 g/s which correspond to mass velocities from 20.3 to 81 kg/m2s and at three different pressures 4.0, 5.5 and 7.5 bar corresponding to saturation temperatures of 8.9, 18.7, and 29C. The primary result is a map of the quality-based local heat transfer coefficient for saturated flow boiling. In addition, results for subcooled boiling at a pressure of 7.5 bar are presented. In this case, the wall heat flux varied from 0 to 20 W/cm2. Future work will include a study of microchannels with different geometries, a comparison to current literature, and predictive model development.
Stefan S. Bertsch, Graduate Student
Purdue University
West Lafayette, IN
USA


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