Interfacing small closed-cycle refrigerators to liquid helium cryostats

Several different means of connecting small closed cycle refrigerators to liquid helium cryostats for the purpose of reliquefying the boiloff or reducing the boiloff rate are described. Interface principles which enhance the reliability and serviceability are presented. Most of the interfaces which are described permit the refrigerator to be serviced while maintaining liquid helium in the cryostat.     

The stabilization of temperature in miniature continuously filled He cryostats

This paper describes a continuously filled cryostat which consumes a negligible amount of liquid helium. The cryostat can measure temperatures below 4.2K to room temperature. The method of controlling the pressure of the exchange gas in the vacuum can will be described.     

Analysis of the cooling of continuous flow helium cryostats

A mathematical model of the cooling of a continuous-flow cryostat which takes into account real values of the specific and latent heat of the cryogenic fluid and of the specific heat of the cryostat material is presented. The amount of liquid in the cooling fluid and four parasitic heat flows, caused by radiation and heat conduction in the construction materials and in the rest gas in the vacuum insulation, are also taken into account. The influence of different model parameters on performance, particularly in the non-stationary regime, is demonstrated by means of numerical solutions of the modelling equations. A quantitative criterion which assesses the properties of the planned cryostat, is formulated. The theoretical conclusions are compared with measurements performed on a continuous flow helium cryostat.     

A helium transfer gas-liquid separator

A cryostat incorporating a dilution refrigerator and high field superconducting solenoid has been constructed which requires automatic helium transfer. A gas-liquid separator for this has been designed and is described.     

An analytical study of two-phase helium flow within a radial rotating duct

A study was made of the behaviour of separated two-phase helium flow within a radial rotating duct. The liquid film thickness, phase velocities, heat-exchange coefficients, pressure and temperature differences were evaluated. The plots obtained for helium flow in thermodynamic equilibrium are used to calculate the radius dependence of pressure and quality at a given rotational frequency and inlet duct conditions. The equilibrium pressure model is presented which describes pressures not too close to the critical values.     

Calculation of the thermodynamic parameters of helium in a centrifugal force field

The design of cooling systems for a cryoalternator's superconducting field windings must be based on the knowledge of the distribution of the thermodynamic parameters (primarily temperature) of the helium in both its liquid and gaseous phases in centrifugal force fields. This paper proposes a set of ordinary differential equations describing the distributions. By numerically solving the set, the effects of the angular velocity of rotation, filling factor and temperature (pressure) at the axis of rotation upon the pressure, temperature and density distributions in helium in rotating vessels have been studied. Formulee for the approximation of the distributions have been obtained, which are applicable to rotation axis temperatures from 3 to 4.6 K (corresponding to saturation pressures 0.24 × 10⁵ to 1.4 × 10⁵ Pa), angular velocities of rotation from 50 to 300 rad s^?1 and filling factors from 0.1 to 0.9.     

过冷态超流氦恒温器的热力学分析

对过冷态超流氦恒温器的热力学过程进行分析,考察了不同的设计温度和J-T换热器热效率下恒温器的制冷性能,得到了理论制冷量的变化曲线.结果表明,理论制冷量与设计温度和J-T换热器的热效率密切相关.计算发现,160 L液氦在2W制冷量下从4.2K降至1.8K大约需要24 h.     

An inexpensive liquid helium level sensing unit

Two units to measure the level of liquid helium in a superconducting magnet and in a storage dewar are described.     

Optimization of the designs of small-size helium cryostats

Using a computer program that simulates thermophysical processes occurring in small helium cryostats, the surface emissivities of actual cryostats are verified. The efficiency of the heat transfer from the walls of the reservoir suspension tube to the helium vapors is determined. The program makes it possible to optimize the designs of cryostats according to the specific conditions of their use.Notation Stefan-Boltzmann, constant - _bg, _ho, _sh, _res, _det emissivities of the background surrounding the cryostat, housing surface of the cryostat, radiation shield, helium reservoir, and radiation detector - T _bg,T _ho,T _sh,T _res temperatures of the background, cryostat housing, shield, and reservoir - _re1, _re2, _re3 reduced emissivities of the shield, housing, and reservoir - F _sh,F _ho,F _res surface areas of shield, housing, and reservoir - F ₁,F ₂,F ₃ areas of the entrance window in the cryostat housing, of the filter on the radiation shield, and of the inlet hole of the matching optics - ₁, ₂ emissivities of the filter and entrance window - ₁ solid angle at which the background radiation is absorbed by optical filter - ₂ solid angle at which radiation is transferred by matching optics to the radiation detector - ₁, ₂, ₃ transmission of radiation by way of the entrance window, filter, and matching optics - S cross-sectional area of the helium reservoir suspension tube - (T) temperature dependence of the thermal conductivity of the tube material - n ₁,n ₂,n ₃ number of mechanical couplings between the housing and the shield, electric wires, and mechanical couplings between the shield and reservoir - S ₁,S ₂,S ₃ cross-sectional areas of the mechanical coupling between the housing and shield, of the electric wire, and of the mechanical coupling between the shield and reservoir - l ₁,l ₂,l ₃,l ₄ lengths of mechanical couplings and electric wires over the body-shield and the shield-reservoir portions - ₁(T), ₂(T), ₃(T) temperature dependences of the thermal conductivities of the materials of mechanical couplings between the housing and shield, of electric wires, and mechanical couplings between the shield and reservoir - specific resistance of the material of electric wires - I working electric current - d inner diameter of the helium reservoir suspension tube - L ₁,L ₂,L ₃ lengths of the portions of the tube in the first, second, and third zones participating in heat exchange with helium vapors - U ₁,U ₂,U ₃ coefficients of heat conduction from the tube walls to helium vapors in the first, second, and third zones - T ₁, T ₂, T ₃ temperature differences in the corresponding zones of the tube - G ₁ weighted quantity of nitrogen evaporated per unit time - r ₁ latent heat of nitrogen vapor generation - G weighted quantity of helium evaporated per unit time - r latent heat of helium vapor generation - T _h temperature of helium vapors - c _p heat capacity of gaseous helium - G weighted quantity of helium evaporated per unit time due to heat supply only along the helium reservoir suspension tube All-Russian Scientific Center S. I. Vavilov State Optical Institute, St. Petersburg, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 5, pp. 760–766, September–October, 1995.     

Performance comparisons of space borne cryostats

We discuss the performance of liquid helium cryostats that have flown in space or are planned for space flight. Usual figures of merit are total cryostat mass or depletion rate of the cryogen. These often fail to accurately represent other important characteristics which affect cryostat performance such as the method used to survive launch lock-up or the temperature of the cryostat vacuum shell obtained by radiative cooling. To address these issues, we define the ratio, H/R in W day/l as metric to judge cryostat performance. The parameter H=σ_BA_tank(T⁴_shell−T⁴_tank) is the Stefan Boltzmann law for energy transfer to the helium tank, where σ_B is the Stefan Boltzmann constant, A_tank is the surface area of the cryogen tank and T_shell and T_tank are the vacuum shell and cryogen temperatures. The average cryogen depletion rate R=V_f/t is computed using the total cryogen volume, V_f, at the last fill before launch, including the volume of ‘booster tank' cryogen if used and the cryogen lifetime, t, to depletion on-orbit. Cryostats launched on the Space Shuttle have the same H/R≈60 W day/l whether the cryogen was liquid helium or solid neon, and for a broad range of vacuum shell temperatures 113<T_shell<300 K, cryogen volumes 2200>V_f>85 l, and mission times, 9 days to >2 years. Cryostats launched on unmanned rockets have a higher H/R≈300 W day/l. Only one, the X-Ray Spectrometer (XRS), out of the four solid neon and two solid hydrogen cryostats showed a clear advantage of using a cryogen other than liquid helium.     

Steady state heat transfer of helium cooled cable bundles

In the present study nucleate and film boiling heat transfer characteristicsof horizontal conductor bundles are investigated at steady state conditions. The effect of gaps between wires, number of wires, wire position, wire size and bundle orientation on the departure from nucleate boling and transition to film boiling is studied. For gaps close to the bubble departure diameter, the critical heat flux can approach up to 90% of the single wire value. Consequently, the maximum stable current for a given bundle can be significantly increased above the single conductor value for the same cross-sectional area.     

Compact dilution refrigerator with a cryogenic circulation cycle of He3

A He³-He⁴ dilution refrigerator has been constructed which circulates He³ by using two alternately operating adsorption pumps which are in the same cryostat as the dilution refrigerator. The scheme described here organised the low temperature circulation cycle such that the construction and cryostat communications were essentially simplified, the dimensions of the arrangement were decreased which resulted in a circulation velocity of 10^?4 mole s^?1, which was sufficient to obtain a temperature in the order of 10 mK.     

Optimal sizes of suspension tubes in nitrogen-free helium cryostats

An analytical method is proposed for the computation of the optimal size of suspension tubes of nitrogen-free helium cryostats.     

基于遗传算法的20 K氦制冷机的优化设计及其热力学分析

通过计算机程序实现10 k W@20 K的大型氦制冷机的热力学过程,在此基础上分析了流程中压缩机和透平膨胀机入口压力对系统制冷系数和效率的影响。同时与Matlab编写的遗传算法接口相连交互数据,从而实现针对该制冷机热力学参数的优化。选取制冷量和系统效率的加权值为优化目标,压缩机和透平膨胀机入口压力为待优化参数,通过遗传算法(GA)的优化计算得到了稳定的最优解,表明遗传算法在优化大型制冷系统过程中可以快速高效搜索到满足目标要求的全局最优解。     

Calculation of the emergency regimes of cryostats

Analytical expressions are obtained that allow one to evaluate the dependence between the time of pressure growth in cryostats containing cryogenic fluids and the thermal energy entering the cryostat. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 3, pp. 413–419, May–June, 1997.     

氦液化循环中透平流量的优化及影响因素热力学分析

为了在氦液化循环中获得最大的液化率,采用MATLAB编程实现流程中的热力学计算,针对带液氮预冷的Collins循环中的透平流量(me_1、me_2)进行优化分析。首先根据前人已得到的结论验证了模型的正确性,在此基础上进行深入研究。结果显示,当固定流程中的高低压力、换热器有效度以及透平的等熵效率时,透平的分流量为压缩机入口总流量的74%并且两台透平均分时系统可获得最大的液化率,且这一最优流量并不随透平的等熵效率和换热器的有效度改变。     

周期性冲击对大型超导核聚变低温系统氦透平膨胀机影响研究

超导核聚变装置磁体会因"交流损耗"而产生大量的热,迫使磁体内通道压力升高,导致与之直接相连的氦透平膨胀机的运行稳定性与运行效率受到严重影响,从而影响超导核聚变实验。详细分析超导核聚变装置用低温氦透平膨胀机受周期性冲击的机理与不可回避性,并用提高氦透平膨胀机本身性能、改善制动调节与氦透平膨胀机出口压力调节的反馈控制思想,得出一些提高氦透平膨胀机稳定性、降低冲击的方法。高效气体轴承与优化的转子结构可以有效提高氦透平膨胀机本身的抗干扰能力;快速响应制动功率,使之与负载匹配,可有效改善外界干扰程度;氦透平膨胀机出口压力快速调节,可降低氦透平膨胀机进出口参数,降低影响。     

Study of heat transfer in the gas-cooled necks of helium vessels and cryostats

A method of calculating heat flow along helium-cooled necks is refined on the basis of an experimental study.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 5, pp. 749–753, November, 1984.     

Numerical investigation of radiative heat exchange in the throats of helium cryostats

A mathematical model of radiative heat exchange in the throats of wide-necked helium cryostats is analyzed. A comparison of the numerical results with known calculated and experimental data shows their good agreement.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 4, pp. 627–634, October, 1985.     

The prediction of steady state heat flux and related temperature profiles in pressurized superfluid helium II

To design superconducting magnets cooled by pressurized superfluid helium II, it is necessary to know the heat transfer characteristics of steady state heat flux in the cryogen. This study shows the calculation used to obtain these characteristics using the universal Gorter-Mellink equation proposed by Soloski et al. The calculated results agree fairly well with the data obtained in this study.