Microfin tube, Flow boiling, R134a, heat transfer, pressure drop
The energy and environmental performance of refrigeration and air conditioning machines are commonly described by their Total Equivalent Warming Impact, so called TEWI, which is defined as the sum of the indirect and direct emissions. The direct emissions are related to charge inventory of the system and to the type of refrigerant used, while the indirect emissions basically depend on the system energy performance. Even if there is a strong interest in the new low-GWP refrigerants, the traditional HFC fluids, with huge GWPs, are still widely used in the refrigeration and air conditioning equipment. For this reason, there is a still strong demand of innovative solutions which can be implemented with the current fluids and then applied to the new ones, when there will be the final phase-out of the HFCs. From this standpoint, looking at the TEWI index, the charge minimization and the system performance optimization represent the main targets of the innovation to cope with the environmental challenges. Since the early 1970s, traditional microfin tubes have been widely used in air and water heat exchangers for heat pump and refrigerating applications because they have been demonstrated to significantly improve the heat transfer performance during both in-tube condensation and boiling. The possible downsizing of microfin tubes could lead to more efficient and compact heat exchangers and thus to a reduction of the refrigerant charge of the systems and to an overall improving of their performance. Nowadays, large manufacturers are exploring the possible use of mini microfin tubes and there is a strong interest in understanding the heat transfer and pressure drop behaviours of this enhanced tube. This paper presents the R134a flow boiling heat transfer and pressure drop measurements inside a mini microfin tube with internal diameter at the fin tip of 4.3 mm. This study is carried out in a new experimental facility built at the Dept. of Management and Engineering of the University of Padova. The microfin tube was brazed inside a copper plate and electrically heated from the bottom. Sixteen T-type thermocouples are located in the copper plate to monitor the temperature distribution during the heat transfer process. In particular, the experimental measurements were carried out at constant mean saturation temperature of 30 Â°C, by varying the refrigerant mass velocity between 200 kg m-2 s-1 and 800 kg m-2 s-1, the vapour quality from 0.1 to 0.95, at four different heat fluxes: 15, 30, 60, and 90 kW m-2. The experimental results are presented in terms of two-phase heat transfer coefficient, onset dryout vapour quality, and frictional pressure drop as a function of the operative test conditions.