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 1250 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 that 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.

Forced Convection Heat Transfer Analysis through Dimpled Surfaces with Different Arrangements

“…Case [8].It can also be seen that the variation in the Nusselt number is gradual with Reynolds number as expected [14,15]. It can be seen that 'Nu' increases as 'Re' increases in all the three cases.…”

“…They observed a considerable increase in the distribution of local heat transfer coefficient everywhere on these dimple surfaces as compared to flat surface. Mahmood et al [8], experimentally investigated the effect of dimples on heat transfer augmentation .They used the flow visualization techniques and concluded that the periodic nature of shedding off of vortices is the main cause of enhancement of heat transfer and is much more pronounced at the downstream rims of the dimples. Xie et al [9] numerically investigated the heat transfer and fluid flow characteristics of teardrop dimple along with teardrop protrusion having different eccentricities.…”

An Experimental Study on Heat Transfer Enhancement of Flat Plates Using Dimples

“…The different arrangements of dimples used are [1,11]. The formation of vortex pairs, periodically shedding off from the dimples at low angles, a large up wash regions with some fluids coming out from the central regions of the dimples, are the main causes of enhancement of Nusselt number [6].This happens mostly at low angle orientation of dimples (30 0 ), as at low angles the dimples are very close to each other so a pronounce effect of dimples is felt. It can also be seen that the variation in the Nusselt number is almost gradual with orientation as expected [12,13].…”

“…They observed a considerable increment in the heat transfer rate for the surfaces having dimples (About 2.5 times then their smooth surfaces). Mahmood et al [6] conducted an experiment to investigate the effect of dimples on heat transfer using the flow visualization techniques and concluded that the periodic nature of shedding off of vortices is the main cause of enhancement of heat transfer (much more pronounced at the downstream rims of the dimples). Mahmood et al [7] conducted an experiment to study the effect of Reynolds number, aspect ratio, and temperature ratio in a channel with dimples.…”

Review on Recent Progresses in Heat Transfer Enhancement Through the Use of Passive Vortex Generators and Its Applications in Solar Parabolic Trough Systems

“…Afanasyev [35] used spherical cavities as the vortex generator and got heat transfer enhancement of 30-40% with air as the working fluid. Mahmood [36] also tried to use dimpled channel to enhance the heat transfer but obtained a very less increase in heat transfer and pressure drop as no form drag was produced by the protruding objects. …”