Nusselt Numbers and Flow Structure on and Above a Shallow Dimpled Surface Within a Channel Including Effects of Inlet Turbulence Intensity Level

Abstract

Experimental results from a channel with shallow dimples placed on one wall are given for Reynolds numbers based on channel height from 3,700 to 20,000, levels of longitudinal turbulence intensity from 3% to 11% (at the entrance of the channel test section), and a ratio of air inlet stagnation temperature to surface temperature of approximately 0.94. The ratio of dimple depth to dimple print diameter δ∕D is 0.1, and the ratio of channel height to dimple print diameter H∕D is 1.00. The data presented include friction factors, local Nusselt numbers, spatially averaged Nusselt numbers, a number of time-averaged flow structural characteristics, flow visualization results, and spectra of longitudinal velocity fluctuations which, at a Reynolds number of 20,000, show a primary vortex shedding frequency of 8.0Hz and a dimple edge vortex pair oscillation frequency of approximately 6.5Hz. The local flow structure shows some qualitative similarity to characteristics measured with deeper dimples (δ∕D of 0.2 and 0.3), with smaller quantitative changes from the dimples as δ∕D decreases. A similar conclusion is reached regarding qualitative and quantitative variations of local Nusselt number ratio data, which show that the highest local values are present within the downstream portions of dimples, as well as near dimple spanwise and downstream edges. Local and spatially averaged Nusselt number ratios sometimes change by small amounts as the channel inlet turbulence intensity level is altered, whereas friction factor ratios increase somewhat at the channel inlet turbulence intensity level increases. These changes to local Nusselt number data (with changing turbulence intensity level) are present at the same locations where the vortex pairs appear to originate, where they have the greatest influences on local flow and heat transfer behavior.

Measurement of local heat/mass transfer coefficients on a dimple using naphthalene sublimation

“…They conducted a detailed analysis of a tornado-like mechanism for enhancing the heat transfer in a spherical dimple with varying depth under conditions of turbulent flow. Ligrani et al [4][5][6] presented a detailed explanation of the flow structure and heat transfer on a channel with dimple arrays. They provided flow visualization of an instantaneous three-dimensional flow structure induced within a dimple.…”

Mechanics of drag reduction by shallow dimples in channel flow

“…Numerous empirical relations have been proposed relating practically useful parameters such as Nusselt numbers and friction factors with the dimple depth to diameter ratio, Reynolds number, inlet turbulence intensity, channel height, and even the channel aspect ratio. 2,[9][10][11] Dimple depth to diameter ratios used in these heat transfer studies generally range from 10% to as deep as 50%.…”

A Numerical Study of Small-Scale Longitudinal Heat Conduction in Plate Heat Exchangers

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