TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)

Slush nitrogen is a mixture of liquid and solid nitrogen. Fine solid particles are dispersed in liquid phase. It enables us to utilize latent heat and increase the quantity of refrigerant supplied. Therefore, it can be expected to be useful as a high-performance refrigerant for high-temperature superconductive devices. This study discusses the heat transfer characteristics of slush nitrogen when flowing in pipes. The relationship between the heat transfer coefficient and velocity is investigated experimentally. The flow velocity, temperature (sub-cooled liquid nitrogen) and mass solid fraction are controlled parameters. The results revealed that, under the condition when the Reynolds number is less than 100,000, the heat transfer coefficient of slush nitrogen is larger than that of sub-cooled liquid nitrogen. Furthermore, the heat transfer characteristic of sub-cooled liquid nitrogen is almost equal to the prediction by Colburn's equation (i.e., empirical equation on heat transfer in pipes during turbulent flow).

View full abstract

Show abstractHide abstract

The internal stress and residual strain of Chevrel-phase PbMo₆S₈ wires were analyzed using an elastic-perfect plastic model. Comparing these results with those previously obtained from our experimental results, it was found that the order of size of the residual pre-strains qualitatively agrees with the experimental results except for part of the wires although the analyzed values were 0.2-0.4% larger than the experimental values. Concerning critical current (I_c) degradation in a small 3-layer PbMo₆S₈ coil using a mandrel made of alumina, the difference between the experimental result and estimated value was argued. It was concluded that it is insufficient to explain the difference even when considering the effects of hoop's strain due to electromagnetic force and the variation in the cross-sectional area of the compound layer. Moreover, the internal stress and residual strain were analyzed for six kinds of assumed MgB₂ wire in which different sheaths were used. As a result, the wire with a Fe sheath had better mechanical characteristic than one with a Cu sheath in the case of using a single sheath. As for adopting composite sheaths, the wire with a Fe-barrier/Cu/Cu-Ni composite sheath had better strain characteristic as compared to the wire using a Nb-barrier/Cu/Cu-Ni sheath.

View full abstract

Download PDF (941K)

Register with J-STAGE for free!