Time dependent heat transport in subcooled superfluid helium

The authors present an extensive study on the behaviour of time dependent heat transport in subcooled He II under conditions which are closely related to the cooling problem of superconducting magnets. Experimental results on the delay for onset of burnout and on the transient recovery from burnout are discussed. A theoretical model is derived from the assumption that heat diffusion characterized by the Gorter-Mellink equation is the dominant mode of heat transport and that thermal waves play no direct role in this connection. The comparison of experimental and calculated results shows a very satisfactory agreement which fully validates the model.     

Peak and recovery heat flux densities in bath of subcooled superfluid helium

This paper describes the measurement of the peak and recovery heat flux densities (q_P, q_R) in a bath of subcooled superfluid helium. A RhFe wire (d = 51 μm, L = 34.8 mm) was horizontally immersed in a He Ilp cryostat, which served as the sample, the heater and the temperature sensor. For the steady state measurement a programmed voltage method with a triangular waveform of 20 s was applied. The measured q_P and q_R values cover the temperature range 1.81–2.1K at a pressure of 0.1 MPa. On the basis of the heat and mass transfer through phase interfaces, theoretical relations (without geometrical parameters) of q_P and q_R are derived, which have very wide application range due to the relative critical heat flux density method used for the calculations. The theoretical results fit the experimental results well for both q_P and q_R.     

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.     

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.     

Thermal design and tests of a subcooled superfluid helium refrigerator

Design methods for a subcooled superfluid helium refrigerator with high efficiency are studied. The design details are presented for a He I heat exchanger (perforated disk stack type), a He II heat exchanger (low fin tube type), an expansion valve, a safety valve, an initial filling valve and a pumping system. Heat loss, cooling capacity and cooldown time are also calculated. The test results for the trial-manufactured refrigeration system are presented in comparison with the designed values.     

ADS 2K低温系统超流氦换热特性研究

以ADS 2K低温系统为研究平台,分析了恒温器结构参数中上升管与峰值热量的关系,测量了超导腔液氦池中不同工作温度下的超流氦峰值热量,最后给出了恒温器上升管的设计方法,并验证了ADS低温恒温器上升管设计的合理性。     

Numerical study of flow and heat transfer of superfluid helium in capillary channels

A modified two-fluid model is adopted to study flow and heat transfer of superfluid helium in a microchannel with a diameter as small as that of a superleak in a fountain effect pump. Variable properties of superfluid helium and energy dissipations due to the two-fluid mutual friction and the friction at the channel wall are fully taken into consideration. It is found that the normal fluid component flow is not trivial even in a channel with diameter of a micrometre, and that there exists an optimum diameter for the maximum mass flow rate. The flow of superfluid helium through a channel with different temperatures at the ends differs considerably from that of a Newtonian fluid. The strong dependence of the thermodynamic properties on temperature and pressure, as well as the internal-convection mechanism are found to be the causes of the unique flows.     

Criterion for quantum turbulence onset after rectangular heat pulse in superfluid helium

On the basis of transient heat transfer measurements of the film boiling onset time in a He II bath, a criterion correlated to the bath temperature and pressure for quantum turbulence onset after a rectangular heat pulse in superfluid helium is shown. Using the criterion parameter α it is possible to determine the time τ needed to initiate quantum turbulence in the He II bath after a rectangular heat pulse. It is found that the criterion value in a He II bath is much smaller than that for He II confined in small channels. A strong pressure effect is also exhibited.     

AMS-02超导磁体中超流氦加注过程研究

论述了AMS-02超导磁体中超流氦的加注过程,进行了各个环节压降和温降的计算分析,并用图表表示了在不同质量流量加注超流氦工况下管路系统中氦工质的速度、压力、温度和含气量的变化情况.计算结果表明,氦工质从液氦主杜瓦加注到磁体杜瓦的过程中,其压力和温度不断降低,而含气量不断增加.同时也表明在节流阀VVP9中实现了常流氦向超流氦的转变和质量流量的控制.     

Experimental investigation of heat transfer through porous media in superfluid helium

An experimental investigation of heat transfer through porous media in superfluid helium has been conducted in the framework of the development of porous electrical insulations for superconducting magnet cables cooled by superfluid helium. Several types of porous media with different characteristics were tested and, in particular, samples with pore size diameters of 0.1 μm, 1 μm, 2 μm, 10 μm and 20 μm. Temperature and pressure were measured between an insulating inner bath and the cryostat bath, communicating only through the porous medium. The cryostat bath is held constant all along the measurement and, for each sample, the tests are performed for bath temperature from 1.4 K to 2.1 K with 0.1 K increment. Depending on the porous medium average pore size diameter, different flow regimes are observed: for porous media with a pore diameter of 0.1 and 1 μm, only the Landau regime is observed whereas for porous media with a pore diameter of 2 μm, we observed the Landau regime and the Gorter-Mellink regime. For samples with a pore diameter of 10 and 20 μm, measurements only permitted to detect the Gorter-Mellink regime. In the laminar regime, the permeability of the samples was determined and it was found that the permeability is constant for bath temperature above 1.9 K whereas it increases as the bath temperature decreases from 1.8 K to 1.4 K. For samples with a pore size diameter of 10 and 20 μm, measurement permits only to observe the turbulent regime and the analysis exhibits a constant average tortuosity for each samples, independently of the bath temperature.     

Practical data on steady state heat transport in superfluid helium at atmospheric pressure

Experimental results covering a technically interesting range of temperatures (1.4 K ? T ? T_λ) and channel dimensions (L ? 100 cm, 0.2 ? id ? 5 mm) have been complied into a collection of practical data on steady state heat transport in superfluid helium at atmospheric pressure. The data, given in the form of diagrams, allow all the problems of linear steady state heat transport in superfluid helium at atmospheric pressure to be solved directly.     

Heat transfer through subcooled He I layer from distributed heat source in a pressurized He II channel

The subcooled He I layer, in contact with a large heated surface in a channel filled with the pressurized superfluid He II (He II_p), expands the non-boiling region above the Kapitza region up to q_n, above which nucleate boiling sets in. As the bath temperature decreases, q_n is increased more rapidly than q_λ at which the superfluidity is broken at the centre of the heated surface. The value of q_n is increased as the channel gap increases, and is independent of the channel orientation as well as q_λ. Metastabilization of superconducting coils may be enhanced by taking the non-boiling limit q_n into account.     

超流氦低温系统发展及涡轮冷压缩机的应用

2 K下大型氦低温系统已采用离心式涡轮冷压缩机在低温低压下对饱和液氦槽减压操作,以获得超流氦或过冷氦.介绍了2K温度级超流氦制冷机发展情况和涡轮冷压缩机在氦制冷系统中的应用,以及中国科学院等离子体物理研究所EAST超导托卡马克氦低温制冷机中过冷氦的制取过程.     

弦月形通道在太阳能空调系统中的强化传热研究

研究表明,弦月形通道的特殊结构可以有效促进Li—Br溶液的沸腾,降低溶液过热度,强化传热、传质过程,相同工况下的换热量是环形通道的2～3倍,使太阳能小型溴化铿吸收式空调系统在70～80℃宅加热水温度下就可以正常运行。弦月形通道偏心特性尺寸S对通道内Li-Br溶液的流动与换热影响很大,实验结果表明：di＝19mm和do＝32mm的铜管构成的弦月形通道的提升效果较好。     

Effect of number of turns and configurations on the heat transfer performance of helium cryogenic pulsating heat pipe

The pulsating heat pipe (PHP) is a potential alternative to highly conductive metals such as copper for long distance heat transfer. Effective thermal conductivity and heat transfer capacity of a PHP are two of the most critical factors for practical applications. In this paper, a helium based PHP, which consists of 48 parallel tubing sections, was developed. The lengths of the evaporator, adiabatic and condenser sections are 50 mm, 100 mm and 50 mm respectively. The condenser section was thermally anchored to a Gifford-McMahon cryocooler (GM cryocooler) with a cooling capacity of 1.5 W at 4.2 K. A maximum effective thermal conductivity of 12330 W/m∙K was obtained when 1.1 W heat was applied to the evaporator section at a fill ratio of 70.5%. With the same geometric parameters and operational parameters, the effect of the number of turns on the heat transfer performance was figured out by comparing the 48-turn PHP with an 8-turn PHP. The results show that the temperature difference between the evaporator and condenser sections of the 48-turn PHP is much smaller than that of the 8-turn PHP. The dry-out temperature response, effective thermal conductivity and heat transfer capacity of them are obtained and analyzed. Furthermore, two configurations of the 48-turn PHP, a parallel configuration and a series configuration, are defined. An optimum configuration is proposed and makes a reference to the design of a cryogenic PHP for applications.     

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.     

Measurement of the transient heat transfer to liquid helium from a thin metal film

A method to measure transient heat transfer to liquid helium from a thin metal film heater under the condition of pulsed heating during τ ≤ 400 ns is proposed. The experimental equipment used for the measurements is described. The method is based on the comparison of heat pulses transfered from the heater into a monocrystal substrate which is surrounded either by vacuum or by liquid helium. The method can also be used to investigate the heat flux density transmitted into liquid helium over a wide region of thermal loads. Experimental results showing the heat flux density radiated from a Cu heater into liquid helium at 3.8 K as a function of the electric power fed into the heater by pulses of 200–400 ns duration are demonstrated.     

Film boiling on a vertical plate in subcooled helium II

Film boiling heat transfer coefficients were measured on 10, 30 and 50 mm long vertical plates in subcooled He II for bulk liquid temperatures from 1.8 to 2.1 K. A film boiling model on a vertical plate in subcooled He II was presented based on convection heat transport in the vapor film, radiation heat transport, and heat transport in He II. The numerical solutions of the model were obtained and an equation which can express the numerical solutions within ±5% difference was derived. The equation predicted well the experimental data for lower ΔT range but significantly under-predicted the data for higher ΔT. A correlation of film boiling heat transfer including radiation contribution was presented by modifying the equation based the experimental data. This correlation can describe the experimental data within ±20% difference.     

不同金属换热面对总传热系数的影响

铜是重大战略物资,我国每年约60%的铜需要进口,制冷空调机组中的两器(蒸发器和冷凝器)是耗铜大户,用其他金属代替铜意义重大。本文整理有关金属替代的实践成果,证明换热面采用的金属的导热系数大小对换热器的总传热系数的影响不大;而换热器的结构型式、流体的流动状态、是否采取强化措施对总传热系数有显著的影响。为制冷空调机组中两器实现以铝代铜和以不锈钢代铜,提高两器的性价比提供实践和理论依据。     

过冷段翅片管换热器的试验研究

对过冷段翅片管换热器对空气源热泵冷热水机组系统性能的影响进行试验研究。过冷段翅片管换热器通过横向导热和强制对流换热可增大制冷剂液体过冷度，制热工况下，可使系统性能系数COP提高约19％，并有效降低翅片表面结霜。