Experimental study on evaporative heat transfer and pressure drop of R-134a in a horizontal dimpled tube

Abstract

The heat transfer and pressure drop characteristics of R-134a during evaporation in smooth and dimpled tubes are experimentally examined. All test sections used in this study are double-tube heat exchangers, where the hot water in an annulus transfers its heat to R-134a in the inner tube. The experiments are conducted in a mass flux range of 300–500 kg/m2 s, heat flux range of 20–30 kW/m2, and evaporating temperature range of 7–13 °C. Smooth and dimpled tubes with a length of 1500 mm and an inner diameter of 8.1 mm are tested. The results indicate that the tube with protrusions enhance the heat transfer up to 70% along with a pressure drop increase up to 587% over the smooth tube. The effects of relevant parameters on heat transfer and pressure drop are explored. In this experiment, the heat transfer enhancement factor falls in the range of 1.26–1.7, while the pressure drop penalty factor fluctuates between 3.78 and 6.87. In addition, the efficiency index is applied to estimate the performance of dimpled tube. It is found that all efficiency index values are in the range of 0.18–0.42.