Local Heat Transfer and Flow Structure on and Above a Dimpled Surface in a Channel

Abstract

Experimental results, measured on and above a dimpled test surface placed on one wall of a channel, are given for Reynolds numbers from 1,250 to 61,500 and ratios of air inlet stagnation temperature to surface temperature ranging from 0.68 to 0.94. These include flow visualizations, surveys of time-averaged total pressure and streamwise velocity, and spatially-resolved local Nusselt numbers, which are measured using infrared thermography, used in conjunction with energy balances, thermocouples, and in situ calibration procedures. The ratio of channel height to dimple print diameter is 0.5. Flow visualizations show vortical fluid and vortex pairs shed from the dimples, including a large upwash region and packets of fluid emanating from the central regions of each dimple, as well as vortex pairs and vortical fluid which form near dimple diagonals. These vortex structures augment local Nusselt numbers near the downstream rims of each dimple, both slightly within each depression, and especially on the flat surface just downstream of each dimple. Such augmentations are spread over larger surface areas and become more pronounced as the ratio of inlet stagnation temperature to local surface temperature decreases. As a result, local and spatially-averaged heat transfer augmentations become larger as this temperature ratio decreases. This is due to the actions of vortical fluid in advecting cool fluid from the central parts of the channel to regions close to the hotter dimpled surface.

Computational modelling of the flow and heat transfer in dimpled channels

“…13. This is consistent with experimental findings reported by Kwon et al (2) (for the constant temperature BC case) and Mahmood et al (12) (for the constant heat flux case).…”

“…It is worth noting that for a constant temperature wall boundary condition, Figure 12, the contour lines of Nusselt number extend further in the lateral direction when compared to those of the constant heat flux case, Figure 13. This is consistent with experimental findings reported by Kwon et al (2) (for the constant temperature BC case) and Mahmood et al (12) (for the constant heat flux case).…”

“…Mahmood et al (12) conducted an experiment to investigate the heat transfer characteristics over such surfaces for a range of Reynolds numbers. They found that the overall heat transfer enhancement level was fairly independent of Reynolds number.…”

Effect of outline curvature degree on heat transfer and flow characteristics inside dimpled tubes and spirally grooved tubes

“…The heat transfer can be enhanced by roughening the surface to increase the effectiveness of heat exchangers, including rib, 2 dimple, 3,4 groove, 5 and so on. The rough surface disturbs the straight‐line flow and destroys the boundary layer, which is conducive to heat transfer but increases the pressure loss 6,7 . Generally, the heat transfer and flow performances are taken into consideration together for heat exchangers.…”

Numerical study on heat transfer and nanofluid flow in pipes fitted with different dimpled spiral center plate

“…So, the ongoing attempt has been made on the progression of dimple combined cooler methods. Plenty of empirical researches including the dimples arrays in channels with rectangular shape have been done [29,30]. Flow structure characteristics on one of the dimpled walls of the channel were experimentally investigated by Ligrani et al [31], on which both with and without protrusions were located on the other wall.…”