Experimental investigation of heat transfer and pressure drop characteristics of internal finned tubes

This study investigates experimentally laminar flow in a water-to-water double pipe heat exchanger for counter flow arrangement. The hot water flows in inner pipe and cold water in the outer annulus. The test section, which has a length of 1400 mm, is a horizontal annular passage formed by aluminum alloy tube with an inner diameter 20.9 mm and outer diameter 35 mm. Three different finned tubes with four, six, and eight longitudinal trapezoidal fins, which have length 1400 mm, height 5.23 mm, and thickness 3.63 mm at base and 1 mm at the tip, attached to the wall surface of the inner tube. For comparison, the heat transfer and pressure drop characteristics are also studied for a smooth inner tube. The results show that the differences in Nu are insignificant among the internal finned tubes with four, six, and eight fins at a smaller Re , whereas the inner tube with more fins has a larger Nu at a larger Re . At a fixed Reynolds number, the friction factor of tube with eight fins is the largest, followed by tube with six fins, tube with four fins and smooth tube for Re < 10 0 0; however, the differences in friction factors of the four heat exchangers are not pronounced for Re > 1500. In addition, the heat transfer benefit tends to decrease as the pressure drop increases for internal finned tubes. Finally, empirical correlations of Nusselt number and the friction factor with the Reynolds number are proposed. (c) 2021 Elsevier Ltd. All rights reserved.