Flow Boiling of R245fa in a Single Circular Microchannel

Abstract

This paper describes an experimental setup for the investigation of two-phase heat transfer inside microchannels and reports local heat transfer coefficients measured during flow boiling of HFC-245fa in a 0.96-mm-diameter single circular channel. The test runs have been performed during vaporization at around 1.85 bar, corresponding to 31°C saturation temperature. As a peculiar characteristic of the present technique, the heat transfer coefficient is not measured by imposing the heat flux; instead, the boiling process is governed by controlling the inlet temperature of the heating secondary fluid. In the data, mass velocity ranges between 200 and 400 kg m⁻² s⁻¹, with heat flux varying from 5 to 85 kW m⁻² and vapor quality from 0.05 up to 0.8. Since these data are not measured at uniform heat flux conditions, a proper analysis is performed to enlighten the influence of the different parameters and to compare the present data to those obtained when the heat flux is imposed. Besides, the test runs have been carried out in a double mode: by increasing the water-to-refrigerant temperature difference and by decreasing it. Finally, the experimental data are compared to models available in the literature for predicting the heat transfer coefficients inside microchannels.

Acknowledgments

Stefano Bortolin graduated in mechanical engineering at the University of Padova, Italy. He is now a post doc researcher at the Department of Fisica Tecnica, University of Padova. At present, his research is focused on two-phase heat transfer, with particular regard to condensation and vaporization of refrigerants inside microchannels.

Davide Del Col took his Ph.D. at the University of Padova, Italy, and was visiting scholar at Pennsylvania State University, USA. At present, he is assistant professor at the Faculty of Engineering of the University of Padova, where he teaches fundamentals of thermodynamics and heat transfer. He is a member of the Commission B1 of IIR and a member of the ASME K-13 Committee. His research activity deals with heat transfer during condensation and vaporization of new refrigerants, design of condensers and evaporators, and microscale heat transfer. He is also active in the field of heat pumps and solar energy conversion. He is author of one book and more than 120 papers, most of them published in international journals or presented at international conferences.

Luisa Rossetto is a professor of heat transfer. She teaches advanced heat transfer and applied thermodynamics to the engineering students at the University of Padova. Her research activity concerns the field of enhanced heat transfer, particularly during single-phase flow or with condensation and vaporization of refrigerants (on integral fin tubes, in microfin tubes, and in mini- and microchannels). She is a member of ASHRAE (American Society of Heating, Refrigerating, Air-Conditioning Engineers), IIR (International Institute of Refrigeration), and UIT (Italian Society of Thermo-Fluid Dynamics). She is author or co-author of more than 150 papers, most published in the international scientific press.