Performance of smooth and micro-fin tubes in high mass flux region of R-134a during evaporation

Abstract

The two-phase heat transfer coefficient and pressure drop during evaporation of pure R-134a in smooth and micro-fin tubes are experimentally investigated. Different from most previous studies, the present experiments have been performed at high mass flux conditions. The test section is a 2.5 m long counterflow horizontal double tube heat exchanger with refrigerant flow inside the tube while hot water flows in the annulas. The inner tubes are made from smooth or micro-fin horizontal copper tubing of 9.52 mm outer diameter. The test runs are done at average saturated evaporating temperatures ranging between 10 and 20 °C. The mass fluxes are between 400 and 800 kg/m2s. The experimental results of both smooth and micro-fin tubes show that the average heat transfer coefficient tends to increase with an increase of average quality, mass flux, and evaporating temperature. The pressure drop increases with an increase of average quality and mass flux, but tends to slightly decrease with a rise of evaporating temperature. The average heat transfer coefficient of the 9.52 mm OD micro-fin tube is 50% to 100% higher than that of the 9.52 mm OD smooth tube while the pressure drop is 10% to 60% higher. New correlations for the evaporation heat transfer coefficient and pressure drop in high mass flux region of R-134a are proposed for practical use.