Experimental Measurements of R134a Flow Boiling Inside a 3.4-mm ID Microfin Tube

The minimization of the refrigerant charge in refrigerating and air conditioning equipment is now an important issue for the new environmental challenges. This paper presents R134a flow boiling heat transfer and pressure drop measurements inside a mini microfin tube with internal diameter of 3.4 mm. This study was carried out in a new experimental facility built at the Dipartimento di Ingegneria Industriale of the University of Padova, especially designed to study both single- and two-phase heat transfer processes in microstructured surfaces. The microfin tube was brazed inside a copper plate and electrically heated from the bottom by means of a wire resistance. Several T-type thermocouples were inserted in the wall to measure the temperature distribution during the phase-change process. In particular, the experimental measurements were carried out at constant saturation temperature of 30 °C, by varying the refrigerant mass velocity between 190 kg m-2 s-1 and 940 kg m-2 s-1, and the vapor quality from 0.2 to 0.99 at three different heat fluxes: 10, 25, and 50 kW m-2. The experimental results are presented in terms of two-phase heat transfer coefficient, vapor quality at the onset of dryout, and frictional pressure drop