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Research Papers: Nuclear Power

Photographic Study on Bubble Motion in Subcooled Pool Boiling

[+] Author and Article Information
Tomio Okawa, Takahiro Harada, Yuta Kotsusa

Department of Mechanical Engineering, Osaka University, 2-1, Yamadaoka, Suita-shi, Osaka 565-0871, Japan

J. Eng. Gas Turbines Power 132(10), 102922 (Jul 14, 2010) (6 pages) doi:10.1115/1.4001064 History: Received August 24, 2009; Revised August 31, 2009; Published July 14, 2010; Online July 14, 2010

Using a static contact angle of a vertical heated wall as a main experimental parameter, a photographic study was carried out to elucidate the mechanisms to determine the vapor bubble dynamics during subcooled pool boiling. The test fluid was distilled water and the experiments were performed under the atmospheric pressure; liquid subcooling was set to around 5 K. To enable clear observation of bubble behavior with a high speed camera, the experiments were conducted in an isolated bubble regime near the onset of nucleate boiling. Distinctly different bubble behaviors were observed on hydrophobic and hydrophilic surfaces: the bubbles were adhered to the surface for a long period of time when the contact angle was large while lifted-off the surface within a short period of time after the nucleation when the contact angle was small. Since buoyancy does not remove the bubble from the vertical surface, the mechanisms of bubble lift-off were investigated. It was indicated that the change in bubble shape induced by the surface tension force, unsteady growth force, and local liquid flow induced by heterogeneous condensation around the bubble are considered to promote the bubble lift-off while the surface tension force acting on the three-phase common line prevented the lift-off. Effects of the surface wettability on the lift-off bubble diameter, the elapsed time from the nucleation at the lift-off, and the condensation rate after the lift-off were also investigated.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic diagram of experimental apparatus

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Figure 2

Time-elapsed images of a bubble attached to the heated surface (W=2.6 mm and Δt=0.625 ms)

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Figure 3

Time-elapsed images of bubble lift-off (W=3.7 mm and Δt=0.625 ms)

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Figure 4

Time-elapsed images of bubble rebound (W=3.9 mm and Δt=1.25 ms)

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Figure 5

Classification of bubble behaviors on θ−A0 map

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Figure 6

Effects of the initial bubble growth rate on the bubble aspect ratios in the initial stage and at lift-off

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Figure 7

Bubble growth curves prior to the lift-off obtained when θ=0 deg: (a) raw data and (b) scaled data

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Figure 8

Dimensionless growth force estimated using Eq. 5

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Figure 9

Dimensionless bubble dimensions for hydrophilic heated surface (θ=0 deg)

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Figure 10

Effects of static contact angle on lift-off characteristics: (a) time periods required for bubble size to be maximized and for bubble lift-off and (b) maximum bubble size and the bubble size at lift-off

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Figure 11

Dependence of condensation time on surface wettability

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