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Research Papers: Evaporation, Boiling, and Condensation

The Thickness of the Liquid Microlayer Between a Sliding Bubble and a Heated Wall: Comparison of Models to Experimental Data

[+] Author and Article Information
D. Keith Hollingsworth, Xin Li, Larry C. Witte

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

J. Heat Transfer 130(11), 111501 (Sep 03, 2008) (9 pages) doi:10.1115/1.2957485 History: Received August 21, 2007; Revised February 12, 2008; Published September 03, 2008

Sliding vapor bubbles are known to create high heat transfer coefficients along the surfaces against which they slide. The details of this process remain unclear and depend, in part, on the evolution of the liquid microlayer that forms between the bubble and the surface, as the bubble grows by evaporation. A mechanistic model of the microlayer thickness verified by direct observation of the microlayer thickness is needed. This paper describes a comparison of measurements from a recent set of experiments to the results of microlayer models from literature and to the predictions of a new model presented here for the first time. The measurements were produced by a laser-based method developed to measure the thickness of the liquid microlayer between a cap-shaped sliding bubble and an inclined heated wall. Microlayer thicknesses of 2255μm were obtained for saturated FC-87 and a uniform-temperature surface inclined at 2–15 deg from the horizontal. The basis of each model, input requirements, limitations, and performance relative to this data set is discussed. A correlation is developed based on the structure of the lubrication theory. It collects the measured microlayer thickness presented as a microlayer Reynolds number to within ±10%. This correlation depends only on bubble volume, inclination, and a bubble shape factor, all of which can be determined experimentally to within reasonable accuracy.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 6

Microlayer thickness calculations for sliding bubbles at inclinations of (top) 5 deg and (bottom) 10 deg using Addlesee and Kew’s model

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Figure 7

Microlayer thickness calculations for sliding bubbles at inclinations of (top) 5 deg and (bottom) 10 deg using the modified Addlesee and Kew model

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Figure 8

Assumed microlayer curvature

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Figure 9

Pressure developed under bubbles of various inclinations

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Figure 10

The dependence of Reδ on Fr22 for various inclination angles

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Figure 1

(a) Sketch of the bubble geometry. The velocity distribution in the microlayer is shown in the reference frame of the bubble. (b) Definitions of bubble width (W), length (L), and height (H).

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Figure 2

A collage of images of a single bubble moving from left to right under a heated wall inclined at 2 deg. The black spot is the fiber-optic film thickness probe. Side views of the bubbles are shown at the top of the figure. The scale at the bottom shows time measured from the first image on the left.

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Figure 3

Side view of a sliding bubble undergoing transition from a small to a large cap-shaped bubble at 10 deg inclination. Bubble motion is left to right. Picture is inverted: The heated surface is at the bottom of the frames.

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Figure 4

Example traces of microlayer thickness for bubbles at inclinations of 2 deg and 15 deg

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Figure 5

Microlayer thickness versus bubble length from Li (2-3)

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Figure 11

A curve fit for Reδ in terms of Froude number and inclination angle

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Figure 12

The dependence of Reδ∗ on a shape factor and inclination angle

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