Improvement on Superconducting Magnet Performance using Fountain Effect of Superfluid Helium

在实验和数值分析的基础上，研究了超流氦(He)在不同类型的多孔隔板分段通道中的换热特性。考察了以下几种垫片的传热性能：(A)无孔FRP垫片，(B)直径为2 mm或4 mm的多孔FRP垫片，(C)由烧结Al_2O_3细粉制成的多孔垫片，(D)由高密度聚乙烯制成的比Al_2O_3更具弹性的多孔隔膜。多孔隔膜的平均直径约为1μm。应用多孔隔膜时，由于喷泉效应，可能会在通道中诱导氦流动。当使用直径为2 mm和4 mm的隔膜时，λ转变热流几乎相同。这一值大约是使用无孔垫片的情况下的1.5倍。λ转变的热流密度取决于隔板的孔隙率。当气孔率仅为5%左右时，可实现较高的冷却性能。这些结果表明，在He II通道中引入孔洞是抑制He II通道局部温升的一种非常有效的方法。由于喷泉效应在加热通道中诱导了亨恩流动，所以当使用多孔隔膜而不是直径为2 mm或4 mm的玻璃钢隔膜时，λ转变热流显著增加。还发现，在地狱通道之间使用多孔隔离层，不仅在通道中完全充满He II时，而且在通道中产生正常氦(He I)时，都能有效地抑制加热通道中的陡峭温升。

Heat transport characteristics of superfluid helium (He n) in channel segmented by different types of spacer having several holes have been investigated on the basis of both experiment and numerical analysis. The heat transport performance has been examined for the following spacers : (a) FRP spacer without any hole, (b) FRP spacer with several holes of 2 or 4 mm diameter, (c) porous spacer made of fine powder of sintered Al_2O_3, (d) porous spacer made of high density polyethylene (HDPE) which is more flexible than Al_2O_3. The average diameter of the porous is about 1μm. It is expected that helium flow is induced in the channel because of a fountain effect when the porous spacer is applied.The λ-transition heat flux when we use the spacers having holes of diameter 2 mm and 4 mm is almost the same. This value is about 1.5 times larger than that in the case of using spacer without any hole. The λ-transition heat flux depends on the porosity of the spacer. A relatively high cooling performance can be realized when the porosity is only about 5%. These results lead to the understanding that the introduction of some holes in the spacer is a very effective way to suppress local temperature rise in the He II channel.The λ-transition heat flux increases remarkably when the porous spacer is used instead of the FRP spacer with holes of 2 or 4 mm in diameter, because Hen flow is induced in the heated channel by a fountain effect. It is also found that the use of porous spacers between Hell channels is effective to suppress a steep temperature rise in the heated channel not only when the channel is filled with He II entirely but also when normal helium (He I ) is generated in the channel.