Investigation of Flow Boiling in Micro-Channels: Heat Transfer, Pressure Drop and Evaluation of Existing Correlations

In this study, the flow boiling heat transfer and pressure drop characteristics of refrigerant R134a in micro-channels were experimentally investigated. The tests were performed in circular horizontal micro-channels with inner diameters of 0.5 mm and 1 mm and a heating length of 300 mm. The mass velocities varied from 500 kg/m2s to 2500 kg/m2s, and the heat fluxes varied from 15 kW/m2 to 147 kW/m2. The heat transfer coefficient (HTC) and frictional pressure drop (FPD) were measured and discussed in detail. According to the results, HTC was significantly affected by heat flux, whereas it was independent of mass velocity. Nucleate boiling was the dominant heat transfer mechanism for R134a flow boiling in the micro-channels. In comparison to the 1 mm channel, the 0.5 mm channel shows better performance in heat transfer, with a maximum increase of approximately 22 %. In addition, FPD increased with increasing mass velocity and decreasing channel diameter. Finally, several existing correlations for HTC and FPD were evaluated by comparing them with the experimental values. Tran’s correlation (1996) presented a better agreement in terms of the average HTC, while for the FPD, the model of Kim and Mudawar (2013b) showed good prediction accuracy.