A reciprocating liquid helium pump used for forced flow of supercritical helium

The performance of a small double acting piston pump for circulating helium in a closed heat transfer loop is described. The pump was manufactured by LINDE AG, Munich, West Germany. The measured flow rate of supercritical helium was about 17 gs^?1 (500 lhr^?1) with a differential pressure of Δρ = 0.5 × 10⁵Nm^?2 at a working pressure of p = 6 × 10⁵Nm^?2. At differential pressures beyond 0.5 × 10⁵Nm^?2 the volumetric efficiency decreases.     

40-T混合磁体外超导线圈迫流氦流动摩擦系数分析

40-T混合磁体外超导磁体使用4.5 K超临界氦进行迫流冷却。迫流氦在管内电缆导体(CICC)内的流动过程受到摩擦阻力的影响会造成一定的压降和热量产生,同时由外界带来的热量也会由迫流氦带走来让磁体保持在超导温区。使用了超导磁体实际运行过程中的实验数据计算不同导体结构下雷诺数与摩擦系数的变化关系,利用Katheder经验公式对实验的摩擦系数进行了重新拟合,给出了适用于该导体摩擦系数的经验公式。     

氦冷却法制备氮浆中液滴凝固过程的模拟与分析

采用氦冷却法制备氮浆的工艺所制氮浆颗粒尺寸均匀,制备过程中杜瓦内可维持正压,从而防止周围空气的漏入。通过对氦冷却法制备氮浆过程中液滴凝固时固氮颗粒的形成过程进行分析,建立起相应的物理模型以及微分方程,并对微分方程进行数值求解。着重考察了液滴的凝固过程,并对液滴群完全凝固时的氦气温度、压力、流速以及扩散腔长度进行了研究,进而探讨了采用该方法制备氮浆时氦气工作参数、喷管尺寸以及扩散腔长度等的选择问题。     

A superconducting coil indirectly cooled by forced helium flow

A small superconducting coil is indirectly cooled by a forced flow of helium. The coil, wound from Nb-Ti-Zr multifilament superconducting composite, is 15 mm id, 24 mm od and 30 mm long. The maximum central field is 30.3 kG at 4.2 K. Contact between the cooling tube and the coil is achieved using grease.The experimental results are expressed by a relation of critical current and energizing rate. Typical transient temperatures of the coil and coolant during energizing and after quenching are presented.The analysis shows the design of coils of this type is possible using hysteresis loss and heat conduction analysis.     

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.     

Heat flow transients in Si/Pt systems

The energy deposition of pulsed (50 ns) electron beams of energy ranging between 18 and 30 keV in an Si/Pt system is calculated using a Monte Carlo computer program. On the basis of a numerically solved heat diffusion equation, the temperature profiles into the sample and the time evolution of the temperatures are discussed and compared with experimental results.     

Pressure, flow and temperature transients in refrigeration systems

The pressure, flow and temperature transients which occur in simple refrigeration systems (incorporating both dry expansion and flooded evaporators), when subjected to disturbances such as control and load inputs and when defrosting, are described. The effects of oil in such systems are also considered. It is concluded that such transients have a significant influence on system reliability and that system design still contains a significant element of art as well as technology.     

An efficient continuous flow helium cooling unit for mossbauer experiments

A Mossbauer continuous flow cooling unit for use with liquid helium over the temperature range 4.2–300 K is described. The cooling unit can be used for either absorber or source studies in the horizontal plane and it is positioned directly on top of a helium storage vessel. The helium transfer line forms an integral part of the cooling unit and feeds directly into the storage vessel so that helium losses are kept to the minimum. The helium consumption is 0.12 / h^-1 itat 4.2 K decreasing to 0.055 / h-1 at 40 K. The unit is top loading and the exchange gas cooled samples can be changed easily and quickly.     

On the cooling of forced flow cable conductors for superconducting magnets

An exact analytical solution is presented for the cooling of forced-flow, long, cable-conductors for large superconducting magnets. It is shown that, when the temperature profiles for the coolant fluid and the cable material exhibit a wave-front like pattern in their propagation along the conduit, the cooling time for the cable can be determined with a very simple analysis.     

Review of hydrodynamics and heat transfer for large helium cooling systems

This paper is a review of recent experimental results and computational methods needed for the fluid engineering aspects of helium cooling systems for power applications. The discussion is illustrated by three different applications: large magnets; power transmission lines and ac generators. An attempt is made to put recent research into perspective, considering the requirements of each application, and problems in need of further research are pointed out.     

A cold box for the forced cooling systems of superconducting magnets

A cold box, which does not have temperature expanders, designed for a forced cooling system is described. The cold box can be used to maintain superconducting magnets at 4.5 ? + 5 K for a long period of time.Tests on the cold box have shown that the mass flow rate of forced cooling flow is 6 + ? 10 kg h^?1. For this flow rate, 1.1 – 1.3 kg h^?1 of liquid He from the transport dewar is required.     

Numerical simulation of cavitating flow of liquid helium in venturi channel

The fundamental characteristics of the two-dimensional cavitating flow of liquid helium through a venturi channel near the lambda point are numerically investigated to realize the further development and high performance of new multi-phase superfluid cooling systems. First, the governing equations of the cavitating flow of liquid helium based on the unsteady thermal nonequilibrium multi-fluid model with generalized curvilinear coordinates system are presented, and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two-dimensional structure of the cavitating flow of liquid helium though venturi channel is shown in detail, and it is also found that the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid-to-gas phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase.     

Normal form analysis of stochastically forced dynamical systems

The possibility of reducing the dynamics of a system undergoing a Hopf bifurcation and forced by multiplicative noise to a universal normal form is examined on a simple model of geophysical interest. It is shown that the normal form equations and the stationary probability distribution depend on the way the noise is coupled to the original system. The universality of the normal forms is thus severely limited in the presence of noise.     

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.     

Numerical analysis of pressure transients in bubbly two-phase mixtures by explicit-implicit methods

Summary The one-dimensional equations for transient two-phase flow are a system of nonlinear hyperbolic partial differential equations, expressible, under certain assumptions, in conservation form. Inasmuch as the use of the method of characteristics becomes complicated if shock waves are present, it is easier to follow a gas-dynamics approach and employ one of the available procedures for solving one-dimensional systems of conservation equations. A recently introduced technique, due to McGuire and Morris [1, see also 12] and known as an Explicit-Implicit method, is used here for a simple boundary-value problem of wave propagation in bubbly two-phase mixtures, and is found to be simple and versatile. A comparison of this method with the well-known Lax-Wendroff (two-step) scheme demonstrates that shock fronts are simulated better, oscillations behind the shocks are smoothable by parameter adjustment, and computation time is reduced when the Explicit-Implicit method is employed.     

Modeling of thermohydraulic transients in a boiling helium natural circulation loop

Boiling helium natural circulation loops are a cooling option for superconducting magnets. Previous studies on the field have provided a thorough understanding of their steady state behavior in all boiling regimes. Recent experimental research has lead to the understanding of their transient behavior. In particular, it highlights the impact of the thermohydraulic evolution of the circuit on the onset of transient boiling crisis, which represents a limitation of the cooling system. Hence, the need of modeling this aspect of these systems. In this work we present modeling options of two-phase helium loops departing from the homogeneous equilibrium two-phase flow model. Reasonable additional assumptions are introduced to obtain a simplified model and the effect of these assumptions is evaluated by comparison with the solution of the non-simplified equations system. These methods are compared to experimental data to analyze their success and limitations.     

Numerical analysis of a near-room-temperature magnetic cooling system

In this study, for a near-room-temperature magnetic cooling system, a decoupled multi-physics numerical approach (Magnetism, Fluid Flow, and Heat Transfer) is developed using a commercial CFD solver, ANSYS-FLUENT, as a design tool. User defined functions are incorporated into the software in order to take into account the magnetocaloric effect. Magnetic flux density is assumed to be linear during the magnetization and demagnetization processes. Furthermore, the minimum and maximum magnetic flux densities (B_min and B_max) are defined as 0.27 and 0.98, respectively. Two different sets of analyses are conducted by assuming an insulated cold heat exchanger (CHEX) and by defining an artificial cooling load in the CHEX. As a validation case, experimental work from the literature is reproduced numerically, and the results show that the current methodology is fairly accurate. Moreover, parametric analyses are conducted to investigate the effect of the velocity of heat transfer fluid (HTF) and types of HTF on the performance of the magnetic cooling system. Also, the performance metrics of the magnetic cooling system are investigated with regards to the temperature span of the magnetic cooling unit, and the cooling load. It is concluded that reducing the cycle duration ensures reaching lower temperature values. Similarly, reducing the velocity of the HTF allows reducing the outlet temperature of the HTF. In the current system, the highest temperature spans are obtained numerically as around 6 K, 5.2 K and 4.1 K for the cycle durations of 4.2 s, 6.2 s and 8.2 s, respectively.     

热声系统内交变流动特性的数值模拟

对热声系统中交变流动的声场与流场分布进行了数值模拟,得到了热声谐振管和回热器(板叠)内的压力和速度分布及二者之间的相位关系.数值模拟表明:在谐振管内,在模拟的边界条件下,径向速度分布出现"环形效应",流动分为核心区的完全湍流流动和湍流边界层内的粘性流动;对于水力半径和粘性渗透层深度相当的热声板叠,其内速度分布逐步出现"环形效应",流动介于层流和湍流的过渡区,为过渡态流动,在流道的大部分区域压力和速度振荡的相位差趋向于π/2;对于水力半径和粘性渗透层深度之比较小的热声回热器,其内交变流动的流动特性与稳态流动的相似,速度的径向分布为抛物线形,类似于定常流动的层流速度分布.     

BEPCII SSM磁体控制阀箱内氦冷却管线热应力分析

北京正负电子对撞机重大改造工程(BEPCII)超导螺线管磁体(SSM)的控制阀箱是该磁体低温冷却系统的主要设备之一,用来分配、调节和控制超导磁体及其电流引线的冷却介质流量。低温下磁体超导导线及其冷却管会产生冷缩变形,变形量要在控制阀箱内给与补偿。同时阀箱内来流和回流氦冷却管线布置紧凑,需要判断是否要采取措施来补偿冷缩变形。本文运用有限元分析软件ANSYS对BEPCII SSM控制阀箱内冷却管线的热应力进行模拟及分析,从而为该阀箱内冷却管线的设计及布局提供理论依据。