The present paper gives the results of the investigation of transition boiling and film boiling crisis. The investigation of transition boiling and film boiling crisis is now of great importance for many fields of science and engineering. These processes are realized in the cryogenic systems. The knowledge of transition boiling law is the basis for the calculation of superconducting device stability. However, the transition boiling remains till now one of the least explozed section of the heat transfer theory. P. Berenson, V. G. Pronko, K. Nisikawa, V. G. Grigoryev and a number of other Soviet and foreign scientists, up to now, have made a contribution to the study of boiling. A certain experimental material accumulated must be generalized. We have carried out a transition boiling complex study combining the development and analysis of theoretical models and the experimental investigation of physical mechanism of heat transfer process and correlations.
In order to study effects of irradiation at low temperature on insulating materials of superconducting magnets, flexural and impact tests are carried out at 4.2K without warmup after low temperature irradiation for several fiber reinforced plastics. The used materials are glass fiber reinforced epoxies and polyimide, and carbon fiber reinforced epoxies. After irradiation of 1.1×109rad, the reduction in flexural strength of G-10 CR is about 70% and that of G-11 CR about 25%. No change are observed in strength of glass fiber reinforced polyimide by low temperature irradiation. Other kinds of glass fiber reinforced epoxies show a reduction in strength but the flexural strength of carbon fiber reinforced epoxies increases a small by irradiation. Irradiation effect of these materials on impact value is similar to that on flexural strength.
A single layer race track magnet with a large Nb3Sn conductor has been constructed and tested in order to obtain engineering information on fabrication of a 10T multishell dipole magnet. A bronze processed Nb3Sn monolithic wire is used in the wind and react method. The cross section of the conductor is 2.3×6.0mm2 and the critical current is 4.60kA at 10T and 4.2K. The coil has an inner width of 25mm and an outer width of 181mm with a thickness of 6mm. The length of the straight section is 800mm in the total length of 956mm. The number of turns is 30. Mica glass tapes are sandwiched as insulators between the turns, and mica boards bonded with inorganic adhesives are used between the coil and two shielding iron plates. The coil has not been impregnated with epoxy resin after the heat treatment to keep good thermal contact with a liquid helium. After the construction of this magnet the following things became clear. A large current monolithic conductor can be wound in a small diameter as 25mm. The coil keeps its shape without an impregnation of epoxy resin. Though the conductor expanded longitudinally about 0.5% after the heat treatment, no local wire looseness and no stress concentration in the windings were induced. A mica glass tape withstands the heat treatment, which is surely useful as an insulator for a Nb3Sn magnet in the wind and react method. In the excitation tests, the magnet experienced two quenches at the currents of 5.32kA and 6.82kA, and reached 7.00kA which was the upper limit of our power supply. The central field is 4.65T at 7.00kA. The estimated critical current of the Nb3Sn conductor is about 11.83kA at 4.65T and 4.2K. Voltage taps having each interval of 2cm are attached to the wire in every 5 turns of the windings. Propagations of normal zone, temperatures of the conductor after quenches and voltage oscillations were measured using the voltage taps. Such voltage oscillations have been found in excitations of other superconducting magnets and always observed just before quenches during training. Therefore the measurement of the voltage oscillation can be used as a detector of a mechanical disturbance in a magnet. The starting points of both of two quenches deduced from the normal zone propagations were at the boundaries between the straight section and the end part of the windings, and then the voltage oscillations were surely observed just before the quenches. The cause of these quenches and training is considered that the wires at the end part are not fixed sufficiently and moved by the Lorentz force. The minimum quench energy of the magnet and the propagation velocities of normal zone were measured using a heater placed on the windings. From the result of these experiments and thermal analyses, it is estimated that there is no degradation of the critical current caused by the winding and heat treatment, and the thermal margin of the magnet is sufficiently large for the local heat disturbances.
The ac losses in multifilamentary superconducting composites with different superconducting filament bundle positions have been measured using the magnetization method in order to reveal the relation between filament bundle position and coupling losses. Loss components depending on dB/dt in a mixed matrix superconducting composite, whose filament bundle is located in a central region surrounded by an outer stabilizing copper sheath, has been compared with another superconducting composite whose stabilizing copper is located in a central region surrounded by an outer filament bundle. In both conductors, key parameters, such as filament twistpitch, wire diameter and amount of copper stabilizer, were almost the same. Applied magnetic field is 2Tesla with 0.05-2Tesla/sec field change rate. Experimental results indicate that coupling losses between filaments in the composite with the filament bundle located in the central region is smaller than the composite with the filament bundle located in the outer region. A similar conclusion was reached theoretically by B. Truck. Coupling loss values obtained by the experiment show good agreement with calculated values with the equations proposed by B. Turck. It is also pointed out that a copper stabilizer, divided by the CuNi barrier into small regions, like a honeycomb, causes anomalous increasing in the copper resistivity due to Ni diffusion during heat treatment.