In order to simulate an insulated superconductor in practical use, a 20-μm-thick stainless steel strip, both surfaces of which were coated with a PVF film and a carbon-paste film, was used as a test piece in the present experiment. Rapid transient (up to approx. 5, 000K/s) and steady-state boiling heat transfer from this vertical test strip, which was heated stepwise in saturated liquid helium-4 at atmospheric pressure, has been studied. In steady-state boiling, thermal resistance of the coating shifts the nucleate boiling curve to a higher wall-superheat region and suppresses burnout and a quick transition; consequently, heat transfer data were well correlated with a single boiling curve with smooth and continuous transition from nucleate boiling to film boiling. In the case of stepwise heating with a heat flux below the steady-state CHF, the wall superheat suddenly increases to a value corresponding to the Kapitza resistance just as the thermal wave reaches the test piece surface; steady-state nucleate boiling is attained within a very short time after rapid increase in the wall temperature. In the case of a heat flux above the steady-state CHF, just after the transition from transient nucleate boiling to film boiling, the wall heat flux sharply dips once; then steady-state film boiling is attained within a very short time. A rapid transient boiling curve has also been obtained.
A possibility of the new type Peltier refrigerator using a high-Tc superconductor as passive thermo-element has been investigated near the liquid nitrogen temperature. In a normal type Peltier refrigerator, a pair of the n-type semiconductor and the p-type one are usually used. However, at low temperatures no effective p-type material has been found out, though the Bi-Sb alloy is well known to be the promising candidate for the n-type material. Therefore, in this paper we have tried to use the high-Tc superconductor instead of the p-type material. With the Peltier refrigerator composed of the n-type Bi88Sb12 semiconductor and the Bi-2223 high-Tc superconductor, a cooling has been demonstrated and a maximum temperature difference of 6.6K has been obtained when the temperature of the hot junction is 78K. Besides the improvement in the performance of the Peltier refrigerator has been achieved by reducing the cross section of the superconductor.
A variable nozzle area radial turbine for 10-kW class helium refrigerator was developed. The turbine incorporates a mixed-flow turbine impeller (outer diam. φ55mm), self-acting gas journal bearings, spiral-grooved thrust bearings, and variable flow nozzles. This turbine can deliver high overall efficiency under partial refrigeration because it has a larger adiabatic head than a conventional turbine. To achieve high adiabatic efficiency, a numerical analysis of three-dimensional compressible turbulent flow is adopted in the impeller design. Test results show that adiabatic efficiency is above 83% at design point (mass flow rate of 0.174kg/s and rotational speed of 1, 348 1/s). To verify bearing reliability, the turbine shaft was driven at up to 1, 545 1/s. The characteristics of the variable flow nozzles were also examined.
Lubricant for a helium compressor requires not only lubrication but also few evolving rate of impurities, because helium refrigerators should be operated stably and continuously for a long time. In this investigation, evolving gases of different types of lubricant were measured to pay attention to heatproof and evolving gases at present operating compressors. It was found that specially refined mineral oil was suitable at the point of heatproof and economy as lubricant for a helium compressor.