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Research Papers: Heat Transfer Enhancement

The Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence

[+] Author and Article Information
Andrew R. Gifford, Thomas E. Diller, Pavlos P. Vlachos

Department of Mechanical Engineering, AEThER Laboratory, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

J. Heat Transfer 133(2), 021901 (Nov 02, 2010) (11 pages) doi:10.1115/1.4002595 History: Received September 05, 2008; Revised July 23, 2010; Published November 02, 2010; Online November 02, 2010

Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-resolved digital particle image velocimetry (TRDPIV) and a new variety of thin-film heat flux sensor called the heat flux array (HFA) are used simultaneously to measure the spatiotemporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation surface. Laminar flow and heat transfer at low levels of freestream turbulence (Tux¯=0.51.0%) are examined to provide baseline flow characteristics and heat transfer coefficients. Similar experiments using a turbulence grid are performed to examine the effects of turbulence with mean streamwise turbulence intensity of Tux¯=5.0% and an integral length scale of Λx¯=3.25cm. At a Reynolds number of ReD¯=U¯D/υ=21,000, an average increase in the mean heat transfer coefficient of 64% above the laminar level was observed. Experimental studies confirm that coherent structures play a dominant role in the augmentation of heat transfer in the stagnation region. Calculation and examination of the transient physical properties for coherent structures (i.e., circulation, area averaged vorticity, integral length scale, and proximity to the surface) shows that freestream turbulence is stretched and vorticity is amplified as it is convected toward the stagnation surface. The resulting stagnation flow is dominated by dynamic, counter-rotating vortex pairs. Heat transfer augmentation occurs when the rotational motion of coherent structures sweeps cooler freestream fluid into the laminar momentum and thermal boundary layers into close proximity of the heated stagnation surface. Evidence in support of this mechanism is provided through validation of a new mechanistic model, which incorporates the transient physical properties of tracked coherent structures. The model performs well in capturing the essential dynamics of the interaction and in the prediction of the experimentally measured transient and time-averaged turbulent heat transfer coefficients.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

A hypothetical mechanism for heat transfer augmentation at a stagnation surface

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Figure 2

Normalized grid turbulence power spectral density versus von Kàrmàn spectrum

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Figure 3

Hiemenz flow model and instrumentation

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Figure 4

Water Tunnel Facility and experimental setup

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Figure 5

Laminar flow and boundary layer

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Figure 6

The effects of freestream turbulence on time-averaged heat transfer

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Figure 7

Stream-wise variation in time-averaged flow properties and predicted heat transfer augmentation

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Figure 8

Peak coherence and associated frequencies between fluctuating velocity and surface heat transfer

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Figure 9

Comparison of fluctuating velocity and heat transfer signals in a region of high coherence

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Figure 10

A snapshot of TRDPV flow field animations showing coherent structures in the stagnation region

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Figure 11

Histogram of coherent structures identified in the stagnation region and corresponding physical properties

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Figure 12

Mechanistic model validations for an example streamwise coherent structure interaction

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