V.E. Keilin

Nucleate and film pool boiling heat transfer to saturated liquid helium

Abstract Pool boiling heat transfer from a square copper heating surface to liquid helium has been investigated experimentally. The study includes both nucleate and film boiling regimes, the measurements of maximum and minimum heat fluxes and superheats of the heating wall in the pressure region from 0.445 up to 0.98 P/P c . The results are compared with the available experimental data and correlations.

Forced convection heat transfer to liquid helium I in the nucleate boiling region

Abstract Heat transfer and critical heat fluxes to helium boiling in a 2 mm id copper tube (100 mm long) were measured in the pressure range 1.1–1.5 atm and at mass velocities 18–96 kg m −2 s −1 . Corresponding Reynolds numbers are (1.2–6.2) × 10 4 . Experimentally obtained heat transfer coefficients show satisfactory agreement with those calculated according to the Kutateladze equation but with less pronounced pressure dependence. It was found that in the boiling region developed quality did not influence the heat transfer coefficient. An expression was obtained, which describes with ±10% error, the dependence of critical heat flux on mass flow rate in the pressure range 1.1–1.5 atm and mass quality 0.33–0.6.

Force-cooled superconducting systems

Abstract Force-cooled superconducting systems (f c s s) are compared with traditional ‘pool’ systems. A model is proposed to determine the stability criteria for current in f c s s and some results of the analysis of this model are presented. A force-cooled superconducting coil and the apparatus to test the coil, built in IV Kurchatov Atomic Energy Institute during 1968, are described. The results of the tests on the coil are also presented.

Device for measuring pressure drop and heat transfer in two-phase helium flow☆

Abstract A device for studying hydrodynamics and heat transfer in helium flow (helium being in two-phase liquid, and supercritical states) is described. Preliminary data about pressure drop and heat transfer in two-phase helium flow are presented. The tube used was about 0.45 mm i d and about 1 m long. The mass flow in the tests ranged from 1·5 × 10−2 gls, heat flux from 0·02 to 0·5 W/cm2 and quality x (mass vapour fraction) ranged from 0 to 100%. The dependence of Δp/[xΔpv + (1 - x)Δp1] on x is found to be a convenient (in the opinion of the authors) form of presentation of experimental two-phase pressure drop data.

Stability of compound superconductors under localized heat pulses

Abstract Superconducting state stability of commercial Nb-Ti superconductors under heat pulses, generated at different lengths of superconductor, was investigated. The case of a one-dimensional superconductor without heat transfer to liquid helium was examined. It was found that due to dissipation of heat along the superconductor, the energy of irreversible quench may exceed considerably the energy needed for its adiabatic heating to critical temperature Tc (at given magnetic field and current). Such overheating, as it follows from the simplified analytical model and from experiments, depends on current density, magnetic field, the length of the heat generation and on the properties of matrix, but it is in broad limits independent of the form and duration of the pulse. Possible reasons of quantitative disagreement between calculations and experimental results are discussed.

CRYOSTATS OF A NEW TYPE FOR SUPERCONDUCTING COILS.

The design of wide-necked cryostats which are to contain fairly large pieces of apparatus and yet have a reasonable evaporation rate has received attention in a programme for testing superconducting coils carried out in the U.S.S.R. Details are given of several improved versions with inner diameters of up to 300 mm.

The influence of Sc/Cu ratio and filament distribution on the stability of superconductors with respect to local heat pulses

Abstract Experimental and analytical data are presented which show that the stability of an intrinsically stable composite superconductor with respect to local heat pulses, depends on the Sc/Cu ratio and the filament distribution across the sc composite cross section. A variation in the Sc/Cu ratio corresponds to a change in the thermal and resistive properties of the composite. These two properties are opposed in their influence on the stability of the composite. Therefore, for a given current and cross section of the composite, the optimum Sc/Cu ratio which corresponds to the maximum stability of the composite can be found. The maximum stability depends on the current and occurs in the range of high Sc/Cu ratios. The value of minimum quenching energy can differ by an order of magnitude in composites of different filament distributions.

Stable optimized superconducting skit coils

Abstract A superconducting system with a field strength of about 50 kOe in a 20 cm bore is described. The system, with a total weight of 260 kg, consists of two coils divided into radial and axial sections; the axial gap between coils can be changed. The winding is made of stabilized copper strip into which wires of Nb-50%Zr alloy are inserted. In order to minimize the total weight as well as the weight of superconductor the winding is self consistent in regard to critical current and to stability. The number of wires ranges from 20 to 4 and the cross section of the strip from 6.5 to 4 mm 2 respectively. In preliminary tests a field of 47 kOe was achieved at a stable current of 575 A (without superconducting-to-normal transition).

The development and investigation of superconducting magnetic systems for physical experiments

Abstract Superconducting solenoids designed for different physical experiments are described. The solenoids are made of multifilament niobium titanium wires and are connected in series. The magnetic induction achieved at 4.2 K is 9.5 T and with the use of an iron yoke with magnetic flux concentrators an additional 1.5–2 T could be added to the operating volume of about 50 cm 3 . Various modified cryostats which allow operation for several days without liquid helium refilling and which permit a wide range of physical experiments are described.

Cryogenic problems of forced cooled superconducting systems

Abstract Methods of producing cold compressed helium, various ways of refrigerant distribution and ways of reducing cool-down time being applied to forced cooled superconducting systems (FCSS) are discussed. The conditions obtained are for preventing normal zone progapation from current leads inwards into the winding. The analysis shows that main cryogenic problems of FCSS can be relatively simply solved.