R134a Flow Boiling Heat Transfer and Pressure Drop inside a 3.4 mm ID Microfin Tube

Abstract

In the last decades, traditional microfin tubes have been widely used in air and water heat exchangers for heat pump and refrigerating applications during condensation or evaporation. The possible downsizing of microfin tubes can lead to more efficient and compact heat exchangers and thus to a reduction of the refrigerant charge of the systems. This paper presents the R134a flow boiling heat transfer and pressure drop measurements inside a small microfin tube with internal diameter at the fin tip of 3.4mm. This study is 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. The microfin tube is brazed inside a copper plate and electrically heated from the bottom. Several T-type thermocouples are 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 190kg m-2 s-1 and 755kg m-2 s-1, the vapour quality from 0.2 to 0.99 at three different heat fluxes 10, 25, and 50kW m-2. The experimental results are presented in terms of two-phase heat transfer coefficient, vapour quality at the onset of dryout, frictional pressure drop as a function of the operative test conditions.