| Budget Amount *help |
¥3,400,000 (Direct Cost: ¥3,400,000)
Fiscal Year 2004: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 2003: ¥1,100,000 (Direct Cost: ¥1,100,000)
Fiscal Year 2002: ¥1,800,000 (Direct Cost: ¥1,800,000)
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| Research Abstract |
In highly subcooled boiling, many microbubbles are emitted from coalesced bubbles on the heating surface and the heat flux increases higher than the ordinary critical heat flux in transition boiling. The boiling regime has been called Microbubble Emission Boiling, shortened MEB. MEB occurred remarkably in subcooled flow boiling and the maximum heat flux obtained was 10MW/m^2 for distilled water in the horizontal rectangular channel of 17mm height and 14mm width with square heating surface of 10mm×10mm placed on the bottom surface of the channel. According to the bubble behaviors and the pressure fluctuations, MEB was categorized into two type, they were violent MEB and silent MEB. In the violent MEB, the pressure fluctuations rose high and the heat flux increased steeply with the temperature rise of heating surface. A periodic type of MEB was observed in the violent MEB.
In the periodic type of MEB, a series of bubble collapse, liquid supply, bubble generation and bubble growth was cond
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ucted periodically and the periodic pressure waves were observed in the channel. The heat flux increased proportionally to the frequency of pressure fluctuations. The pressure frequency is considered to be the frequency of liquid supply into the heating surface.
After MEB reached the maximum heat flux point, the heating surface was covered with a thin vapor film and it brightened like a miller surface, then the surface temperature rose rapidly and the heat flux decreased. This is a terminal stage of MEB. The surface temperature at terminal stage was about 200℃ for water and it was very high compared with the case of non MEB. Then the boiling turned rapidly to film boiling.
MEB was investigated for horizontal circular channels of 2.5mm, 5mm, 10mm and 16mm in diameter. The channel was manufactured in the center of circular heating block made of copper and straight circular tubes were connected with the heating block. The heating surface was a part of the channel and the length was 10mm for the channels. Thirty cartridge heaters were assembled parallel to the channel in the heating block. MEB occurred in transition boiling and the heat flux was higher than the ordinary critical heat flux. The heat fluxes in MEB increased with increasing liquid flow velocity. For example, the maximum heat fluxes in MEB were 5MW/m^2 at 0.5m/s, 6MW/m^2 at 1.0m/s, 9MW/m^2 at 1.5m/s and 10MW/m^2 at 2.5m/s at 30K of liquid subcooling in the channel of 10mm diameter. The liquid velocity is one of the strong factors in MEB as same as liquid subcooling. For the various channels with different diameters, the heat flux in MEB increased for the channel with the larger diameter 0.25m/s of low liquid velocity, however, no differences of heat fluxes between the channels were observed at 1.0m/s of liquid velocity. A periodic MEB also occurred in the circular channels and the heat flux increases with the pressure frequency regardless of liquid subcooling, liquid velocity and channel diameter.
The experimental results obtained in the present study will be developed to an ultra-high heat flux cooling technology for high powered electronic devices. Less
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