Research Papers

Experiments on the Physical Mechanism of Heat Transfer Augmentation by Freestream Turbulence at a Cylinder Stagnation Point

[+] Author and Article Information
A. C. Nix

Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106andrew.nix@mail.wvu.edu

T. E. Diller

Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061tdiller@vt.edu

J. Turbomach 131(2), 021015 (Jan 29, 2009) (7 pages) doi:10.1115/1.2950079 History: Received July 30, 2007; Revised August 30, 2007; Published January 29, 2009

Detailed time records of velocity and heat flux were measured near the stagnation point of a cylinder in low-speed airflow. The freestream turbulence was controlled using five different grids positioned to match the characteristics from previous heat flux experiments at NASA Glenn using the same wind tunnel. A hot wire was used to measure the cross-flow velocity at a range of positions in front of the stagnation point. This gave the average velocity and fluctuating component including the turbulence intensity and integral length scale. The heat flux was measured with a heat flux microsensor located on the stagnation line underneath the hot-wire probe. This gave the average heat flux and the fluctuating component simultaneous with the velocity signal, including the heat flux turbulence intensity and the coherence with the velocity. The coherence between the signals allowed identification of the crucial positions for measurement of the integral length scale and turbulence intensity for prediction of the time-averaged surface heat flux. The frequencies corresponded to the most energetic frequencies of the turbulence, indicating the importance of the penetration of the turbulent eddies from the freestream through the boundary layer to the surface. The distance from the surface was slightly less than the local value of length scale, indicating the crucial role of the turbulence in augmenting the heat flux. The resulting predictions of the analytical model matched well with the measured heat transfer augmentation.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Wind tunnel and turbulence grid

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Figure 2

Instrumented cylinder model

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Figure 3

Velocity profile (no grid)

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Figure 4

Turbulence intensity (Grid 1)

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Figure 5

rms of streamwise velocity (Grid 1)

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Figure 6

Turbulent length scale (Grid 1)

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Figure 7

Velocity power spectral density (Grid 1)

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Figure 8

Heat flux power spectral density (Grid 1)

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Figure 9

Heat flux and velocity coherence (Grid 1)

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Figure 10

Hot-wire probe effect on heat transfer (no grid)

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Figure 11

Calculated change in heat transfer coefficients with turbulence (Grid 1)



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