R134a Flow Boiling inside a 4.3 mm ID Microfin Tube

The minimization of the refrigerant charge in refrigerating and air conditioning equipment is an important issue due to the new environmental challenges. Recently, relatively small smooth copper tubes with outer diameter around 5 mm have been implemented in many air conditioning and refrigeration equipment. Since their introduction in late 1970s, microfin tubes have also been largely used to enhance both single and two-phase heat transfer. This paper presents R134a flow boiling heat transfer and pressure drop measurements inside a mini microfin tube with internal diameter of 4.3 mm. 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 inlet saturation temperature of 30 °C, by varying the refrigerant mass velocity between 100 kg m-2 s-1 and 800 kg m-2 s-1, and the vapor 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, vapor quality at the onset of dryout, and frictional pressure drop.