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International Symposium on Heat and Mass Transfer in Refrigeration and Cryogenics
September, 1-5, 1986, Dubrovnik, Yugoslavia

DOI: 10.1615/ICHMT.1986.IntSympHMTinRefCryo


ISBN Print: 978-3-54017-957-3

Transient Heat Transfer to a Forced Flow of Supercritical Helium at Very Low Temperatures

pages 599-618
DOI: 10.1615/ICHMT.1986.IntSympHMTinRefCryo.420
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ABSTRACT

When certain metals are cooled to very low temperatures they become perfect conductors of electricity. Because this phenomenon of superconductivity occurs only below the critical temperature of these metals, e.g. Tc of NbTi is 9.5 K and T of Nb3Sn is 18.5 K, it is required to cool the superconducting devices to low temperatures. The suitable cooling fluid in this temperature region is helium. In practice, the usual operating temperature for superconducting systems is 4.2 K, the boiling temperature of liquid helium under atmospheric pressure.
During the last few years large superconducting systems are cooled by a forced flow of supercritical helium. This cooling technique has advantages above pool boiling, e.g. i) no spacers between windings are required, ii) the heat transfer conditions along the conductor are uniform, and iii) for thermal insulation only a vacuum vessel is required. The vessel has got not to resist high pressures because in most designs the cooling fluid flows in tubes along or inside the conductors of the superconducting system.
To get a stable performance of a superconducting system it is required that the cooling absorbs the stationary heat-inleak. Secondly, the cooling has to be designed in such a manner that during the initiation of a quench (vanishing of the superconducting properties, the conductor becomes "normal") no harm occurs to the system before the safety systems are activated. Usually, the origin of a quench is a small energy release in the conductor caused by magnetic and mechanical instabilities [1]. This happens within a fraction of a second, so a better knowledge of transient heat transfer to supercritical helium is desirable. Transient heat transfer data to a forced flow of helium in a small test sample have been obtained by Giarratano and Steward [2].

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