1999 Fiscal Year Final Research Report Summary
On the Bubble Behavior and Heat Transfer Mechanism in Microbubble Emission Boiling
| Project/Area Number |
09650219
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Thermal engineering
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| Research Institution | TOHOKU UNIVERSITY |
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Principal Investigator |
KUMAGAI Satoshi Department of Machine Intelligence and Systems Engineering, Tohoku University, Associate Professor, 大学院・工学研究科, 助教授 (30134026)
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| Project Period (FY) |
1997 – 1999
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| Keywords | Boiling Heat Transfer / Microbubble Emission Phenomena / Liquid Subcooling / Bubble Motion / Pressure Fluctuation / Temperature Fluctuation / Power Spectrum |
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| Research Abstract |
High heat flux far exceeding CHF can be achieved in microbubble emission boiling (MEB) without a marked increase in the wall superheat from CHF under high liquid subcooling condition, accompanying a lot of microbubbles emitted from the heating surface. Those high heat flux should be considered a result of violent behavior of coalescent bubbles in that regime. A rapid collapse of a bubble which shrinks toward the heating surface induces a strong flow of subcooled liquid like a micro-jet to make steady solid-liquid contact even in the high surface temperature region. This bubble behavior generates a moment of high pressure in the vicinity of the surface. In this study, a measurement of the pressure and the temperature fluctuation in the liquid near the surface was performed synchronously with a recording of the bubble motion by a high speed video camera to look into the mechanism of the water supply to the surface and therefore the heat transfer.
The experimental results reveals that there exists a close relation among the pressure and the temperature fluctuations and the bubble motion especially the collapsing process of it. The waveform of a pressure fluctuation is a repetition of a group of peaks which consists of a first large peak and following smaller peaks. The pressure steeply increases when a bubble begins to collapse. A sharp positive large peak in the pressure fluctuation corresponds to the collapse of a bubble, and after a short delay negative peak in the temperature fluctuation is observed, indicating a liquid flow onto the surface. The peak frequencies of the power spectrums of the temperature and the pressure fluctuation as well as the bubble collapse frequency are coincident with each other and are expressed by a simple relation against heat flux, showing a higher frequency at a higher heat flux.
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