SINGLE-PHASE PRESSURE DROP AND HEAT TRANSFER MEASUREMENTS OF TURBULENT FLOW INSIDE HELICALLY DIMPLED TUBES

Three-dimensional roughness is known to yield higher heat transfer performance and better fouling characteristics compared with two-dimensional roughness. In the present study, seven three-dimensional dimpled tubes having a range of geometric parameters (0.020 <= e/D <= 0.030, 5.0 <= p/e <= 10.0, and 6.0 <= z/e <= 14.0) were specially made from 22.2 mm outer diameter (19.9 mm inner diameter, I.D.) tubes with electric steam condenser application in mind. The effects of the roughness parameters on heat transfer and friction characteristics were systematically investigated, and an optimum configuration was deduced. The optimum configuration, which maximizes the thermal efficiency, was dimple height of 0.5 mm, axial dimple pitch of 3.0 mm, and circumferential dimple pitch of 5.0 mm (p/e = 6.0 and z/e = 10.0). This configuration is in close agreement with the one deduced from 15.0 mm I.D. tube. The present dimpled tube yielded a higher thermal efficiency than the tube with two-dimensional spiral rib or the tube with three-dimensional diamond-shaped roughness. Correlations of the friction and the heat transfer roughness functions are developed from the present data.