Study on Cooling Technology for Superconducting Magnets Employed in Space-Borne Applications

利用带光学窗口的低温恒温器和高速摄像机对超流氦的沸腾现象进行了可视化研究，测量了超流氦在较宽的压力和温度范围内的沸腾模式和传热系数的变化，揭示了超流氦用于超导磁体高效冷却的热特性。实验结果表明，在空间微重力环境下，即使加热器上方的超流氦量与重力环境下的超流氦量基本相同，也不会出现饱和超流氦特有的沸腾现象&噪声膜沸腾。由于汽相的快速增长，出现了“粘性指进”现象，混合相的运动可以提高系统的导热系数，因此，环路热管具有重量轻、可靠性高、传热量大等优点，被推荐用于冷却系统的排热。在微重力条件下，研究了它们在顶部加热和底部加热的情况下的启动和性能特性。发现当相当大的热量施加到热管时，与热管水平放置时的情况相比，传热能力没有差异，并且一旦启动就观察到稳定的性能。当热负荷较小时，有时会观察到不稳定的性能。这意味着当启动时，蒸汽的流动路径可能在蒸发器的出口处被液体阻塞。因此，在微重力条件下，液体的初始分布对热管的启动有很大的影响。无论液体的初始分布如何，储液器的温度控制似乎都能够确保热管的启动。

Boiling phenomena of superfluid helium have been visualized by employing a cryostat with optical windows on it and a high-speed camera, and variations of film boiling modes and heat transfer coefficients have been measured in wide ranges of pressure and temperature to unveil the thermal characteristics of superfluid helium for efficient cooling of superconducting magnets. It is concluded from the experimental results that the noisy film boiling, which is the unique boiling phenomenon of saturated superfluid helium, does not appear under the microgravity environment of space, even if almost the same amount of superfluid helium is located over the heater as that under the gravity. Because of rapid growing of vapor phase, "viscous fingering" phenomenon was observed, and higher thermal conduction may be obtained by the effect of mixed phases motion.Loop heat pipes are suggested for the heat exhaust from cooling systems, since the loop heat pipes are light in weight, highly reliable, and can transfer large amount of heat. Their start-up and performance characteristics under microgravity are studied in cases of top-heating and bottom-heating. It is found out that when rather large heat was applied to the heat pipes, there was no difference in heat transfer capacity from the case when heat pipes are placed horizontally, and stable performance was observed once they had started up. Unstable performance had been sometimes observed when the heat load was small. This means the flow path of the vapor may be choked with the liquid at the outlet of the evaporator when start-up. Hence the initial distribution of the liquid may affect largely the start-up of heat pipes under the microgravity condition. The temperature control of the reservoir seems to be able to ensure the start-up of the heat pipes regardless of the initial distribution of the liquid.