Nusselt Number Behavior on Deep Dimpled Surfaces Within a Channel

Abstract

Experimental results, measured on a dimpled test surface placed on one wall of a channel, are given for a ratio of air inlet stagnation temperature to surface temperature of approximately 0.94, and Reynolds numbers R eH from 12,000 to 70,000. These data include friction factors, local Nusselt numbers, spatially-resolved local Nusselt numbers, and globally-averaged Nusselt numbers. The ratio of dimple depth to dimple print diameterand the ratio of channel height to dimple print diameter is 1.00. These results are compared to measurements from other investigations with different ratios of dimple depth to dimple print diameter ␦/D to provide information on the influences of dimple depth. At all Reynolds numbers considered, local and spatially-resolved Nusselt number augmentations increase as dimple depth increases (and all other experimental and geometric parameters are held approximately constant). These are attributed to: (i) increases in the strengths and intensity of vortices and associated secondary flows ejected from the dimples, as well as (ii) increases in the magnitudes of three-dimensional turbulence production and turbulence transport. The effects of these phenomena are especially apparent in local Nusselt number ratio distributions measured just inside of the dimples, and just downstream of the downstream edges of the dimples.

Reviews on drag reducing polymers

“…When Reynolds number increased from 12000 to 70000, the friction factor ratio also increased from 1.6 to 2.6. Silva et al [80] and Burgess et al [81] opined that the use of dimples increased pressure drop and these results are contradictory to that reported by Kim et al [79].…”

“…Hence, the uses of dimples are suggested only for laminar flow application such as in microelectronic cooling. In addition, Burgess et al [81] found experimentally that dimples enhanced heat transfer performance; however, friction factor ratios increased with the increasing Reynolds number. When Reynolds number increased from 12000 to 70000, the friction factor ratio also increased from 1.6 to 2.6.…”

Optimum design of a channel roughened by dimples to improve cooling performance

“…This is also supported by Refs. [6,7,11]. Moon et al [11] have concluded that the normalized Nusselt number is not a function of Reynolds number, and the friction factor is relatively independent of Reynolds number.…”

“…Mahmood et al [5] have presented heat transfer characteristics when dimples and protrusions are placed on opposite walls. Experimental study of staggered arrays of dimples on a single surface has been reported by Burgess and Ligrani [6] and Burgess et al [7]. Seven different dimple depressions on the surface have been studied by Park and Ligrani [8].…”

Flow characteristics along and above dimpled surfaces with three different dimple depths within a channel

“…Mahmood et al [9] describe the mechanisms responsible for local and spatially-averaged heat transfer augmentations on flat channel surfaces with an array of dimples on one wall for one channel height equal to 50 percent of the dimple print diameter. Other recent investigations consider flow and heat transfer in single spherical cavities [10], effects of dimples and protrusions on opposite channel walls [11,12], the effects of dimple depth on vortex structure and surface heat transfer [13,14], the effects of deep dimples on local surface Nusselt number distributions [15], the combined influences of aspect ratio, temperature ratio, Reynolds number, and flow structure [16], and the flow structure due to dimple depressions on a channel surface [17].…”