| Budget Amount *help |
¥2,100,000 (Direct Cost: ¥2,100,000)
Fiscal Year 2004: ¥400,000 (Direct Cost: ¥400,000)
Fiscal Year 2003: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 2002: ¥1,200,000 (Direct Cost: ¥1,200,000)
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| Research Abstract |
The heat transfer characteristics from a flat copper surface to liquid ^3He were studied previously from the non-boiling state to the film boiling one between 0.5 and 1 K under saturated vapor pressure by our group. The temperature difference ΔT across the thin boundary layer existing between a copper surface and bulk liquid ^3He was measured as a function of heat flux q^^・ under steady-state conditions. In the non-boiling state and the film boiling one, the observed q^^・-ΔT relation curves were similar to other liquids. However, in the nucleate boiling state, the data were not explained by Kutateladze's correlation, that is, the heat transfer rate was more than one order large than that expected from the correlation and the observed q^^・-ΔT curve was subdivided into three regions by its slope. Liquid ^3He has the lowest boiling point and the smallest surface tension among elemental substances. These features mean that there is no pre-existing bubble nucleus to stimulate the bubble for
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mation. Thus liquid ^3He should exhibit a unique bubble formation process. In order to clarify the nucleate boiling state, we have observed the bubble by a visualization method. The bubble formation process was taken by a high-speed camera (500 frames per second).
We have constructed two types of cryostats and sample cells. The time for a bubble to grow and depart from the surface was 14 ms at q^^・ = 8×10^<-5>W/cm^2 and a bulk liquid ^3He temperature of 0.7K. The shape of bubble on the surface was spheroid-like, which was explained by the low surface tension of liquid ^3He. On the other hand, a nitrogen bubble observed by Bland et al.was spherical with a short neck on the surface, and the time for a bubble to grow and depart was about 20 times longer than ^3He. This means that ^3He has a high frequency of bubble formation per time. The average bubble size measured in the vertical direction increased as the heat flux increased. The relation between heat flux and bubble number per time was obtained The curve had an inflection point near the change from region I to region II.
The characteristic feature of nucleate boiling in liquid ^3He are summarized as follows : (1)The shape of bubble is spheroid-like ; (2)The bubble growth rate is fast compared with one of nitrogen ; (3)In the region of a large quantity of heat flux, the large spheroidal bubble covers the surface and the increasing rate of bubble number becomes dully. So, the agitation of liquid near surface by bubble becomes less and some parts of surface are covered with film, which results in the gentle slope of q^^・-ΔT curve at region II. Less
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