R.G. Scurlock

THE INFLUENCE OF GEOMETRY ON SELF-FIELD AC LOSSES OF AG SHEATHED PBBI2223 TAPES

We measured axial fields along an Ag sheltered PbBi2223 superconducting tape 3 mm wide and 0.03 mm thick carrying 47 Hz alternating currents. Tte critical current of the tape at 77 K was 1 5 A corresponding to a critical current density of 16 000 A/cm2. The axial electric field was measured by pairs of voltage leads brought out in a plane perpendicular to the tape axis, turned axially to meet and so led out to a voltmeter. The spatial variation and the current dependence of the electric field, obtained from several pairs of taps closed at different radial positions, were consistent with the prediction for a uniform superconducting thin rectangle with a critical current Ic rather than a superconductor of elliptical cross-section. Our results also showed that the hysteretic losses at currents below Ic can be measured with an accuracy of about 5% using a pair of taps closed at a radial distance about three times the tape half-width.

Enhanced boiling heat transfer surfaces

Abstract The concept of enhanced boiling and the advantages to be gained in cryogenic plant are discussed. Experimental results are reported in liquid nitrogen, argon, oxygen and R12. The saturated boiling heat transfer chracteristics of flat surfaces are significantly enhanced by plasma spraying of porous pure aluminium and aluminium/silicon coatings. Such coatings possess heat transfer coefficients up to 10 times those for smooth untreated surfaces. The experimental results demonstrate there is an optimum thickness of plasma sprayed coating for each liquid and selected heat flux, in order to achieve maximum heat transfer coefficient. The effect of fouling by impurities has been studied, and the results suggest that smooth surfaces may show greater degradation in heat transfer performance than porous surfaces. Application of experimental results from small surfaces to the large reboiler/condensers used in cryogenic plant require consideration of the major differences between local boiling heat transfer at constant heat flux (laboratory condition) and constant wall temperature (cryogenic plant condition).

Miniature silicon diode thermometers for cryogenics

Abstract Silicon diode thermometers have become established as accurate and reproducible thermometers for the temperature range 1–400 K. However the diodes need to be carefully selected for reliable performance; they are consequently expensive, and they also have a relatively large thermal mass. This paper describes the development of fast response (∼1 μs at 4 K), low cost, light weight (5 mg) miniature silicon diode thermometers which may be directly mounted using soft solder.

Local phase evolution in (Pb/Bi)2Sr2Ca2Cu3Ox tapes

Abstract The phase evolution has been measured locally from the core-sheath interface to the core-centre of (Pb/Bi)2223 tapes sintered for up to 350 h. The rate of the phase conversion is found to be much faster at the interface than at the centre. For example, in a tape sintered at 832°C in air for 30 h, the percentage of 2223 phase is ∼90% at the interface compared with ∼27% at the centre. Consequently, there is a 2223 phase gradient from the interface to the centre of the core, even after a long sintering time. There appears to be an associated gradient in the grain size and Vickers microhardness across the thickness of the core. The influence of the silver sheath on the phase conversion in half-peeled tapes has also been studied. The half-peeled tapes were made by removing one side of the silver sheath of the normal (Pb/Bi)2223 tape before sintering.

AC V-I characteristics of Ag sheathed PbBi2223 tapes up to 10 kHz: phenomena and interpretations

The self-field AC V-I characteristics of PB2223 silver sheathed superconducting tapes were measured at 9 different positions across the tape at 77 K and 65 K between 47 Hz and 10 kHz. The significant difference found between the centre and the edge was interpreted as the result of the complex geometry of tapes. The implication of this difference to the measurement of the total losses is discussed. Considerable contributions from eddy current losses in the silver sheath were also found for frequencies above 200 Hz.<<ETX>>

A flying superconducting magnet and cryostat for magnetic suspension of wind-tunnel models

Abstract The quality of aerodynamic tests on aircraft or missile models in wind tunnels is improved by magnetically suspending these models, thus avoiding the intrusion of mechanical supports. The size of the electromagnet array required for this task may be prohibitive at large scales but can be reduced by increasing the magnetic moment of the models internal magnetic core. This article describes a new approach, in which a superconducting solenoid is carried within the model in place of the conventional ferromagnetic core. The potential advantages, the philosophy and details of the design of a prototype, and some experiences in its use are outlined.

Numerical modelling and flow visualization of mixing of stratified layers and rollover in LNG

Abstract A numerical model has been developed to study the mixing of two initially stratified layers which are subjected to a uniform lateral heat flux. An important distinction is made between the free surface and the liquid/liquid interface with regard to the different flow characteristics of the two layers. In the upper layer where warm liquid is cooled at the evaporating surface, the convective circulation is featured by a strong downward core flow; in contrast, the fluid flow in the lower layer is mainly confined to the wall boundary and is much weaker. Flow visualization experiments show that mixing of two stratified layers generally involves two stages in sequence: migration of the interface and rapid mixing between the remaining liquids. The interface movement is due to entrainment mixing at the interface. When the two layers approach density equalization, the interface becomes increasingly unstable and the core flow in the upper layer is able to break into the lower layer. The base to side heat flux ratio appears to be a major factor in determining the mode and intensity of the subsequent mixing at a rollover incident.

AC susceptibility of high-Tc superconductors: Hysteresis and intergranular critical state

Abstract AC susceptibility of granular YBCO samples with different grain sizes has been measured, both as a function of temperature and as a function of the applied DC field. Two types of systematic hysteresis were observed: (1) the significantly enhanced AC field screening ability in the field cooled (FC) virgin samples compared to the zero field cooled (ZFC) virgin sample; (2) enhanced AC field exclusion in both the ZFC and the FC samples compared to their virgin states, as the applied DC field was reduced. In addition, the grain size was found to have a strong influence on both the virgin and the hysteretic AC susceptibilities. The observed hysteresis is then explained by carefully analysing the intergranular local magnetic field (or the intergranular critical state), which is strongly modulated by the intragranular critical state, upon (1) cooling a sample with or without a DC field; (2) increasing or reducing the DC field applied to a virgin sample. Using the additional information from the grain size effect, a more detailed picture of the dynamic intergranular critical state is obtained.

Transport critical currents and flux pinning in melt-grown YBa2Cu3O7−δ

Abstract The critical current and flux pinning properties of melt grown YBa 2 Cu 3 O 7−δ bulk material were investigated by transport measurements and V-I characteristics. A J c of 16 000 A cm −2 in zero field and 3000 A cm −2 in 1 T at 77 K was obtained using a criterion of 3.3 μV cm −1 . No features of granularity could be detected as J c showed no hysteresis with increasing or decreasing field. By considering a distribution of activation energy U 0 together with the correlation volume of flux bundles V c and the pinning range r p , the measured V-I characteristics in the flux creep regime were found to be consistent with that of thermally activated flux flow (TAFF). The temperature and field dependence of the derived V c r p points to a collective pinning mechanism by point defects. The depinning current J c , one to tow orders of magnitude higher than the transport J c , was also obtained by using the U 0 and V c r p deduced from the V-I characteristics.

Cryogenic engineering of high temperature superconductors below 77 K

Abstract The j - H characteristics of presently available high temperature superconductors show a significant improvement with decreasing temperature. While the most attractive refrigerant would be liquid nitrogen at 77 K under 1 bar, with the superconductor operating at a somewhat higher temperature, say 80 K, for heat transfer purposes, this paper examines the possible and practical use of temperatures below 77 K for power engineering applications. The types of refrigeration to be employed, whether using wet cooling, gas cooling or dry cooling via thermal contact, are also examined.