Flow boiling heat transfer of HFC-134a inside Helically coiled tubes

Abstract

The present study investigated the effect of coiling of a straight tube on heat transfer enhancement in an evaporator. For this purpose, straight and helically coiled concentric tube-in-tube heat exchangers with three different inclinations (horizontal, vertical and with 45º angle) were employed. The outer diameter of the coil was 305 mm and its pitch was 45 mm. Refrigerant HFC-134a flowing inside the inner tube was heated by the water flowing in the annulus. The ranges of refrigerant mass fluxes was between 112 and 152 kg/m2s and inlet vapor quality ranges varied from 10% to 80%. Analysis of the collected data indicated that, the heat transfer coefficient elevates by increasing the mass velocity and vapor quality in coiled tubes like the straight tubes. The flow boiling heat transfer coefficient increases when the coiled tube is used instead of a straight tube. The horizontal helically coiled tube was more effective than vertical and 45º inclined ones in enhancing the heat transfer coefficients. Also two different correlations were developed to predict the average boiling heat transfer coefficients of R-134a inside vertical and horizontal helically coiled tubes, respectively.