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Research Papers

Enhancement of Heat Transfer Behind Sliding Bubbles

[+] Author and Article Information
D. Keith Hollingsworth, Larry C. Witte, Marcelino Figueroa

Department of Mechanical Engineering, University of Houston, Houston, TX 77204-4006

J. Heat Transfer 131(12), 121005 (Oct 15, 2009) (9 pages) doi:10.1115/1.3216039 History: Received February 12, 2008; Revised March 24, 2009; Published October 15, 2009

The time-dependent temperature distribution on an inclined, thin-foil uniform-heat-generation heater was used to infer the heat transfer enhancement caused by the passage of an FC-87 bubble sliding beneath the lower surface of the heater. A two-camera system was used: One camera recorded color images of a liquid crystal layer applied to the upper (dry) side of the heater while a second camera simultaneously recorded the position, size, and shape of the bubble from below. The temperature response of the heater could then be correlated directly to the bubble characteristics at any given time during its passage. The data along the line bisecting the bubble wake from the nine bubbles comprising 54 bubble images were analyzed. The heat transfer in the wake behind the sliding cap-shaped bubbles is very effective compared with the natural convection that occurs before the passage of the bubble. The maximum values of heat transfer coefficient in the range of 2500W/m2K were produced in very sharply peaked curves. The point of maximum cooling measured as a fraction of the local driving temperature difference before the bubble passage was identified and correlated with some success to the streamwise length of the bubble. The location of the maximum heat transfer coefficient was reasonably correlated with bubble width. The level of the maximum heat transfer coefficient when cast as a Nusselt number based on bubble width grew to a saturation value as the bubble moved across the plate. A constant value of Nusselt number requires that the heat transfer coefficient falls as the bubble grows past some critical bubble size. This behavior was observed for the larger cap-shaped bubbles.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic drawing of the test apparatus

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Figure 2

Detailed sketch of test surface

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Figure 3

Timing of lower and upper image sequences: The positions of the bubbles drawn in dashed lines are interpolated

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Figure 4

Sequence of liquid crystal images for a typical sliding-bubble: The position of the bubble is shown with an ellipse in each image—the images are 140 ms apart and the bubble motion is to the right

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Figure 5

Sketch of sliding bubbles, showing how length (L), width (W), and height (H) are defined: N refers to the bubble nose

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Figure 6

The liquid crystal image (top), the temperature distribution (middle), and the local Nusselt number (bottom) for run 6—T∞=26.8°C and wall flux of 1026 W/m2: The smooth line represents Eq. 1

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Figure 7

Temperature distributions at various times for run 4: Times in the legend are in seconds and the shaded bars represent the location of the bubble at the time indicated in the legend

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Figure 8

Nondimensional temperature distribution for run 4

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Figure 9

Nondimensional temperature distribution for run 7

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Figure 10

Distance to minimum fractional surface temperature scaled on bubble length

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Figure 11

Heat transfer coefficients for run 4 (corresponds to Fig. 7)

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Figure 12

Heat transfer coefficients for run 7 (corresponds to Fig. 8)

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Figure 13

Distance to location of maximum heat transfer coefficient scaled on bubble width

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Figure 14

Maximum Nusselt number versus width-based Reynolds number

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