The types of the oscillations of very low temperature circulation systems are reviewed. Despite the considerable success of the techniques which were developed in '60s to analyse the dynamical and transient behaviors of cryogenic systems, the incompetence thereof to predict exactly the modes of oscillations and instabilities of the He systems operated around the critical point now becomes gradually aparent. The chief aim of this review is to clarify where they fail and suggest how we can save them. The evidences which show the abnormally swift diffusion of enthalpy along supercritical He channels are presented. The relationship between oscillation modes and inhomogenuities of temperature, density and velocity of He coolant is stressed.
This paper continues from part-1 to study on the cooling of superconducting magnets by the use of He II. In this part, the fact that the cooling efficiency in He II increases drastically by pressurizing the liquid is illustrated by examples. We also give examples in which the performances of magnet quenching currents and minimum propagating currents were improved by He II. On the other hand, we envisage several problems of instability on the lowered temperature region, such as the Kapitza resistance between the stabilizing material and the superconductor, and the a.c. loss caused by increased magnetization. The demerits, however, are probably overcome by the excellent cooling abilities of He II, especially of the pressurized He II. Finally, basic ideas of conceptual designs of superconducting troidal field coils in Torus II Supra (France) and NUMAK (U.S.A.) projects are introduced focusing on the cooling manner by means of the subcooled He II at 1.8K.
Two small venturi flow meters of the JIS standard size have been made to test their applicability for helium gas at low temperatures over a wide renge of the Reynold's number. The experimental results are consistent with the turburent flow theory for the Reynold's number larger than 8×104.
Steady and transient heat transfer from a horizontal surface to liquid helium was measured from convection to film boiling region at bulk liquid temperatures between 2.5 and 5.1K. The test copper cylinder was insulated with vacuum, except the flat heat transfer surface of 20mm in diameter. The influence of four different surface treatment, three kinds of finish and coating over the same copper sample on maximum heat flux was also investigated in the pressure range of 0.046<P/Pc<0.92 corresponding to the temperature from 2.5 to 5.1K. Experimental results obtained in the developed nucleate boiling region shifted to larger ΔT value than the reported data compared at the same heat flux and nucleate boiling data were correlated with a modified Rohsenow equation. The film boiling data were correlated well with Rayleigh number and Nusselt number based upon Laplace reference length. Three kinds of dynamic temperature variation were noted corresponding to three boiling regions for the step change of heat input. The peak of the maximum heat flux curves shifted toward 0.5Pc for the two more rough sufaces. Maximum heat flux was found to decrease with surface coating for the polished surface.