Heat Transfer On Superconductor Wires With Porous Coatings In Superfluid Helium

The application of superconductor wires strongly depends on its cryostability affected by the external heat transfer performance between conductor and coolant. In most practical cases, the heat generated in a superconductor is released from its surface to the surrounding helium, which can be gaseous, liquid or superfluid. The best coolant is pressurized superfluid helium that has outstanding cooling properties. The largest values of peak heat flux as well as recovery heat flux are obtained in pressurized superfluid helium, but usual electrical insulation coatings on conductor surface make this advantage to nothing. Therefore investigations and improvements of the coatings of superconductor are of scientific and technical interest, especially in respect the hydrodynamic and heat transfer properties.Porous coating instead of varnish layer of superconductor is raised to improve the heat exchange between superconductor and superfluid. The distinguished advantage of the method is to obtain pressurized superfluid helium in saturated superfluid helium bath due to the present of porous coating on the surface of conductor, which results in a large scale of heat transfer improvement by the outstanding cooling properties of HeIIp. The phenomenon is theoretically explained in the paper by introducing the thermo-mechanical effect of superfluid inside porous coatings.It is emphatically pointed out that the pressure of liquid helium is the important parameter affecting heat transfer in superfluid. An important role in heat transfer of porous coated wires is played by the pressure increase inside the porous coatings. A conclusion is theoretically reached that surface heat flux has a logarithmic relation with the pressure increase inside porous coating q∝ln ?P, which strongly depends on the pore size and porosity of porous materials with the relation ?P m ax∝d?1ε?0.5. A measurement of pressure in porous coating is first performed at low temperature during a heat transfer measurement to verify the relations. The comparison shows a good agreement.On the basis of analysis above, a pressure parameter is also introduced into the analysis to take account of hydrostatic pressure, so that a mathematical model unifying the calculation of peak heat flux in both pressurized and saturated superfluid helium is recommended in the paper. The calculated results of the model according to the experimental condition well agree with those of measurement.Measurements of both normal conductor (RhFe, Cu) and superconductor (NbTi/Cu) with and without porous coatings are performed in order to determine the effect of porous surface coatings on heat transfer in superfluid. The samples with different porous coatings at HeIIs have same characteristics of heat transfer as that of bare wire at HeIIp, i.e. both peak heat flux qp and recovery heat flux qr are increasing monotonously with decreasing bath temperature. The results show that qp and qr of coated wires at HeIIs bath of 1.65K are respectively as 3 and 4 times large as those of bare wires, and as 5 and 4 times large as those of lacquered wires. Coated superconductor wires also show a recovery current Ir with a 40% increase than bare wire, quench current It and recovery current Ir with 20% and 40% increase than lacquered. The increase of recovery current is of more interest and significance in practical cases, because usually superconductor works below recovery current.A serious measurement fault is first mentioned here that the power circuit resistance affects the recovery heat flux if a sample is heated by its ohmic heat and heat flux cannot maintain constant during the jump from film boiling. The fault exists inevitably in such kind of measurement but no attention was paid by other researchers. The reason and pattern of affecting recovery heat flux are found out by means of theoretical analysis and mathematical derivation. A modifying measure to correctly get recovery heat flux is put forward in the paper, which is eventually tested and verified in experiments.