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TECHNICAL PAPERS: Bubbles, Particles, and Droplets

Dynamics of Bubble Motion and Bubble Top Jet Flows From Moving Vapor Bubbles on Microwires

[+] Author and Article Information
David M. Christopher

Thermal Engineering Department, Tsinghua University, Beijing, China 100084dmc@tsinghua.edu.cn

Hao Wang, Xiaofeng Peng

Thermal Engineering Department, Tsinghua University, Beijing, China 100084

J. Heat Transfer 127(11), 1260-1268 (Jun 15, 2005) (9 pages) doi:10.1115/1.2039109 History: Received December 01, 2004; Revised June 15, 2005

Rapid bubble sweeping along heated wires was observed during subcooled nucleate boiling experiments on very fine wires with jet flows emanating from the tops of the vapor bubbles for a variety of conditions. This paper presents experimental results with a numerical analysis of the physical mechanisms causing the experimentally observed bubble motion and jet flows. The results show that the moving bubble creates a nonuniform temperature distribution in the wire by cooling the wire as it moves along the wire with significant heat transfer in the wake behind the bubble. The results verify that the bubble motion is driven by the temperature difference from the front to the back of the bubble, which causes Marangoni flow. The Marangoni flow then thrusts the bubble forward along the wire with the calculated bubble velocities agreeing well with experimental measurements. In addition, the temperature difference from the bottom to the top of the bubble creates a vertical component to the Marangoni flow that results in the jet flows from the bubble tops. Comparisons with experimental observations suggest that the condensation heat transfer at the bubble interface is restricted by noncondensable gases that increase the surface temperature gradient and the resulting Marangoni flow. The numerical results also show that the heat transfer from the wire due to the Marangoni flow is significantly larger than the heat transfer due to the evaporation under the bubble.

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Copyright © 2005 by American Society of Mechanical Engineers
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Figures

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Figure 1

Experimental system

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Figure 2

Asymmetrical temperature distribution model (9)

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Figure 3

Typical moving bubble with trailing jets

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Figure 4

Bubble-wire geometry and coordinate system

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Figure 5

(a) Larger bubble moving to the right at about 40mm∕s in water for a heat flux of 7.29×105W∕m2, a bulk liquid temperature of 40°C, and an average wire temperature of 106°C. (b) Same bubble moving to the left at the same speed 0.1s later.

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Figure 6

(a) Large bubble overtaking a smaller bubble at t=0.670s in water with the same conditions as for Fig. 5. (b) Larger bubble has bounced over and absorbed the smaller bubble at t=0.674s. (c) Larger bubble has been thrust back onto the wire and continues traveling to the right at t=0.676s.

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Figure 7

(a) Small bubble about 0.15mm in diameter moving at about 25mm∕s to the right. (b) Smaller bubble returning to the left at the same speed.

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Figure 8

PIV velocity measurements for 0.35mm bubble moving in water with a bulk temperature of 40°C. The wire temperature was 108°C, the heat flux was 840kW∕m2 and the bubble velocity was about 11mm∕s. The arrow represents the size scale of the velocity vectors.

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Figure 9

Temperature contours in centerplane and exit plane of flow region showing asymmetric jet for bubble diameter of 0.4mm and velocity of 47.1mm∕s

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Figure 10

Temperature contours on the bubble surface for a bubble moving to the left for the conditions in Fig. 9.

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Figure 11

Heat flux variation along the wire near the bubble. The light region behind the bubble experiences the greatest heat flux.

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Figure 12

Effect of evaporation/condensation heat transfer coefficient ratio on the bubble velocity for a 0.2mm diameter bubble in ethanol and a subcooling of 50°C

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Figure 13

Effect of subcooling on the heat generation rate and bubble velocity for 0.2mm and 0.4mm diameter bubbles in ethanol with an evaporation/condensation heat transfer coefficient ratio of 100

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