Effects of Exterior Surface Dimples on Heat Transfer and Friction Factors for a Cross-Flow Heat Exchanger

Experimental results which show the effects of adding spherical-indentation dimples to the exterior surfaces of tubes in a cross-flow heat exchanger are presented. This exchanger contains four streamwise rows of tubes which are staggered with respect to each other. Either shallow or deep dimples are located on the exterior tube surfaces. The tubes are surrounded by air and hot water is used inside the tubes. Air-side Reynolds numbers range from 10,500 to 12,800, and water-side Reynolds numbers range from 3460 to 6300. The deep dimples produce significant heat transfer augmentations (compared to tubes with smooth exterior surfaces) for tubes located in the tube row, which is farthest upstream. The shallow dimples produce significant heat transfer augmentations on the tubes, which are located in the first, third, and fourth rows of the tube bundle. This is due to thermal augmentations produced by the vortices which are shed from individual dimples. In some cases, these augmentations are enhanced by the relatively high advection speeds and absence of wake-mixing for the tube row which is located farthest upstream.