Material and geometry effects of wire surface coating on heat transfer in He II bath

Porous coating could be employed to improve heat transfer from a wire to superfluid helium. Peak and recovery heat fluxes four times the values associated with bare wire were achieved for wires cooled by saturated superfluid helium (He IIs) in our previous work. However, different materials and geometries of coatings may have different influence on heat release performance of samples. Some porous materials, different coating styles and geometries are adopted in this paper to find out their effects on heat transfer. Capillary filter, gypsum/alumina mixture and pure gypsum are taken as the alternative coating materials for copper wire (d=40μm) in direct and/or indirect contact with He IT. Straight and coiled copper wires with mixture coatings are constructed and measured. The measurement shows that peak heat fluxes in excess of 2.5 times those associated with bare wire are obtained for different coatings. Copper coil with mixture coating is obviously improved by a great increase in its heat transfer performance, which is extremely significant for improving the cryostability of superconducting magnet in practical usage. It is concluded that the micro-structure geometry of porous coating rather than material and macro geometry is the main factor affecting its heat release performance.