RESEARCH PAPERS: Bubbles, Particles and Droplets

The Thickness of the Liquid Microlayer Between a Cap-Shaped Sliding Bubble and a Heated Wall: Experimental Measurements

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

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

J. Heat Transfer 128(9), 934-944 (Feb 03, 2006) (11 pages) doi:10.1115/1.2241858 History: Received September 13, 2005; Revised February 03, 2006

A laser-based method has been developed to measure the thickness of the liquid microlayer between a cap-shaped sliding bubble and an inclined heated wall. 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 on the evolution of the microlayer that forms between the bubble and the surface. Past experiments have used heat transfer measurements on uniform-heat-generation surfaces to infer the microlayer thickness through an energy balance. These studies have produced measurements of 20–100 μm for refrigerants and for water, but they have yet to be confirmed by a direct measurement that does not depend on a first-law closure. The results presented here are direct measurements of the microlayer thickness made from a reflectance-based fiber-optic laser probe. Details of the construction and calibration of the probe are presented. Data for saturated FC-87 and a uniform-temperature surface inclined at 2 deg to 15 deg from the horizontal are reported. Millimeter-sized spherical bubbles of FC-87 vapor were injected near the lower end of a uniformly heated aluminum plate. The laser probe yielded microlayer thicknesses of 22–55 μm for cap-shaped bubbles. Bubble Reynolds numbers range from 600 to 4800, Froude numbers from 0.9 to 1.7, and Weber numbers from 2.6 to 47. The microlayer thickness above cap-shaped bubbles was correlated to a function of inclination angle and a bubble shape factor. The successful correlation suggests that this data set can be used to validate the results of detailed models of the microlayer dynamics.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Image collage from Bayazit (16,17)

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Figure 2

Contour plot of the change in Tw caused by the bubble: Tw(t=280ms)−Tw(t=0ms). Bubble is moving from left to right. From Bayazit (16,17).

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Figure 3

Sketch of the test chamber

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Figure 4

A collage of bubble images showing plan and side views for a 15deg inclination angle

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Figure 5

Schematic of the two-fiber probe system

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Figure 6

End of the fiber optic probe

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Figure 7

Schematic of the calibration system

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Figure 8

Calibration curve for the fiber optic probe

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Figure 9

Time history of bubble passage for a 2deg inclination (Runs 1 to 5)

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Figure 10

Time history of bubble passage for a 2deg inclination (Runs 6 to 11)

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Figure 11

Time history of bubble passage for a 2deg inclination (Runs 24 to 29)

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Figure 12

Time history of bubble passage for a 2deg inclination (Runs 49 to 55)

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Figure 13

Example traces of microlayer thickness: Run 3 at 2deg, Run 47 at 15deg

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Figure 14

Microlayer thickness versus bubble length

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Figure 15

Microlayer thickness versus bubble width

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Figure 16

Microlayer thickness versus bubble volume

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Figure 17

Definition of bubble width (W), length (L), height (H), and nose position (NP)

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Figure 18

Microlayer thickness versus Reynolds number

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Figure 19

Microlayer thickness versus Weber number

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Figure 20

Microlayer thickness versus Froude number

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Figure 21

Microlayer Reynolds number as given by Eq. 10

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Figure 22

Microlayer Reynolds number as given by Eq. 11

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Figure 23

twave∕ttotal versus μDeqububble∕(σδ)



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