Experimental study and numerical analysis of heat transfer enhancement and turbulent flow over shallowly dimpled channel surfaces

Abstract

Experiments and numerical simulations have been done to investigate the heat transfer, pressure loss and turbulent flow characteristics over the surfaces with arrays of shallow dimples of different dimple depths within channels. Three different dimple depth to diameter ratios of 0.067, 0.1 and 0.2 are investigated with the Reynolds number (Re) ranging from 10,000 to 60,000. Heat transfer measurements were done by using the transient liquid crystal thermography technique to obtain the local heat transfer enhancement characteristics. Additional steady-state experiments were also done to obtain the overall heat transfer enhancement and the pressure loss of the turbulent flow over the dimpled surfaces. The experimental results indicated that the dimple depth has appreciable effects on heat transfer enhancement and pressure loss. The Reynolds number also shows appreciable effects on the local heat transfer enhancement distribution on the surface with shallower dimples. The overall heat transfer enhancement of all the dimpled surfaces approximately keep constant with the Reynolds numbers. The experiments still indicate that the shallow dimples with the depth to diameter ratio of 0.067 obtained almost constant overall heat transfer enhancement up to 1.4 with negligible pressure loss penalty, which indicates a high and constant Reynolds analogy factor of about 1.33 over the studied Reynolds number range. However, as the dimple depth ratio increases from 0.067 to 0.2, the pressure loss increases rapidly and surpasses the heat transfer enhancement, which leads to lower Reynolds analogy factor than 1.0. Furthermore, the numerical simulations show detailed turbulent flow characteristics over the dimpled surfaces. Different from the dimples with relatively deep depth ratios of 0.1 and 0.2, the shallower dimples with the depth ratio of 0.067 generate an obvious low-speed swirling horseshoe vortex zone near the upstream rim in the dimples at the relatively high Reynolds number of 50,500. The horseshoe vortex shed from spanwise edges of the shallow dimples produces lower but more uniform turbulent mixing intensity in the near-wall region, which is responsible for the higher thermal performance.