Characterisation and optimisation of flexible transfer lines for liquid helium. Part I: Experimental results

The transfer of liquid helium (LHe) into mobile dewars or transport vessels is a common and unavoidable process at LHe decant stations. During this transfer reasonable amounts of LHe evaporate due to heat leak and pressure drop. Thus generated helium gas needs to be collected and reliquefied which requires a huge amount of electrical energy. Therefore, the design of transfer lines used at LHe decant stations has been optimised to establish a LHe transfer with minor evaporation losses which increases the overall efficiency and capacity of LHe decant stations. This paper presents the experimental results achieved during the thermohydraulic optimisation of a flexible LHe transfer line. An extensive measurement campaign with a set of dedicated transfer lines equipped with pressure and temperature sensors led to unique experimental data of this specific transfer process. The experimental results cover the heat leak, the pressure drop, the transfer rate, the outlet quality, and the cool-down and warm-up behaviour of the examined transfer lines. Based on the obtained results the design of the considered flexible transfer line has been optimised, featuring reduced heat leak and pressure drop.     

纳米水平热量传输中温度定义的讨论

纳米水平热量传输明显不同于宏观和微观系统的热量传输。由于热量携带子——电子、光子、声子或分子的平均自由行程与被研究物体的特征长度之间具有可比性，经典理论已经不再适用，人们必须采用新的理论来预测纳米水平的热量传输。在许多仿真模型的研究中存在对温度定义的分歧，因此出现不同的研究结论。针对于在仿真模型研究中可能出现的有关温度的定义给出了明确阐述。     

Ultra-sensitive NEMS-based cantilevers for sensing, scanned probe and very high-frequency applications

Scanning probe microscopies (SPM) and cantilever-based sensors generally use low-frequency mechanical devices of microscale dimensions or larger. Almost universally, off-chip methods are used to sense displacement in these devices, but this approach is not suitable for nanoscale devices. Nanoscale mechanical sensors offer a greatly enhanced performance that is unattainable with microscale devices. Here we describe the fabrication and operation of self-sensing nanocantilevers with fundamental mechanical resonances up to very high frequencies (VHF). These devices use integrated electronic displacement transducers based on piezoresistive thin metal films, permitting straightforward and optimal nanodevice readout. This non-optical transduction enables applications requiring previously inaccessible sensitivity and bandwidth, such as fast SPM and VHF force sensing. Detection of 127 MHz cantilever vibrations is demonstrated with a thermomechanical-noise-limited displacement sensitivity of 39 fm Hz(-1/2). Our smallest devices, with dimensions approaching the mean free path at atmospheric pressure, maintain high resonance quality factors in ambient conditions. This enables chemisorption measurements in air at room temperature, with unprecedented mass resolution less than 1 attogram (10(-18) g).     

Study of a liquid helium jet pump for circulating refrigeration systems

A basic diagram of the refrigeration system, in which liquid helium is circulated by means of a jet pump, is described. The equations have been derived to design jet devices which operate with liquid helium.In the experimental setup with a jet pump the flow rate of liquid was 5 to 10 times larger than that of compressed gas in a direct stream of the refrigerator. After pumping the pressure was equal to (0.15–0.40) × 10⁵Nm^?2. A circulating stream of liquid helium at supercritical pressure, due to the heat load corresponding to the refrigerator capacity, was heated to 0.15–0.25 K.The results of our studies permit one to determine circulation loop parameters and main geometric dimensions of the jet pump.     

A correlation for heat transfer to superphysical helium in turbulent flow in small channels

This paper presents a new correlation for the deterioration in heat transfer to turbulent flows of helium, relative to standard constant property correlations. The correlating parameters are arrived at by analogy between the heat transfer process to a gas with high thermal expansion and that which takes place in the presence of gas injection through a porous wall, the idea having been originally proposed by Kruzhilin. The idea appears to be applicable to the ideal gas state as well as the supercritical state. Comparison with available experimental data is made. Not only is the correlation of the data good, but the actual injection equation may be used to predict deterioration by as much as an order of magnitude.     

Cryogenic helium gas circulation system for advanced characterization of superconducting cables and other devices

A versatile cryogenic test bed, based on circulating cryogenic helium gas, has been designed, fabricated, and installed at the Florida State University Center for Advanced Power Systems (FSU-CAPS). The test bed is being used to understand the benefits of integrating the cryogenic systems of multiple superconducting power devices. The helium circulation system operates with four sets of cryocooler and heat exchanger combinations. The maximum operating pressure of the system is 2.1 MPa. The efficacy of helium circulation systems in cooling superconducting power devices is evaluated using a 30-m-long simulated superconducting cable in a flexible cryostat. Experiments were conducted at various mass flow rates and a variety of heat load profiles. A 1-D thermal model was developed to understand the effect of the gas flow parameters on the thermal gradients along the cable. Experimental results are in close agreement with the results from the thermal model.     

Operational Characteristics of the 200-m Flexible Cryogenic Transfer System

A proper cryogenic environment is essential for the operation of superconducting devices. A test area for the superconducting radio-frequency modules (SRF) has been established in the RF laboratory at National Synchrotron Radiation Research Center in Taiwan; these modules require much liquid helium during conditioning and performance tests; a cooling capacity of 120 W is expected for the acceptance test of the SRF module. The cryogenic environment of the test area is completed on transferring the liquid helium over a remarkable length of 205 m from the two cryogenic plants at Taiwan Light Source, with a valve box located at each end to control and to measure the cryogenic flow. Flexible cryogenic transfer lines of concentric four-tube type are chosen for both the supply of liquid helium and the return of cold helium gas. Functional examination of this long transfer system was first achieved with a 500-L Dewar in the radio-frequency laboratory; an SRF module was then installed in the test area for practical operation. The primary concern about the cryogenic transfer system is the heat loss; a measurement technique based on the principle of thermodynamics is developed and proposed herein. With the available sensors inside the valve boxes and the heaters inside the 500-L Dewar and the test SRF module, this technique has proved promissing from the measured results.     

Plane thermal transpiration of a rarefied gas in the presence of gravitation

Plane thermal transpiration of a rarefied gas that flows horizontally in the presence of gravitation is studied based on the Boltzmann equation. Assuming that the temperature gradient along the walls is small, the asymptotic analysis for a slow variation in the flow direction is conducted. The semi-analytical solution that is valid for arbitrary values of the mean free path and the gravitational strength is derived, and the problem is reduced to solving the spatially one-dimensional Boltzmann equation. This reduced problem is solved numerically for a hard-sphere molecular gas for small values of gravitational strength, and the behavior of the flow is studied based on the numerical solution. The effect of weak gravitation is no longer negligible when the gas is so rarefied that the mean free path is comparable to the maximum range that the molecules travel along the parabolic path within the channel. This phenomenon has been observed in the plane Poiseuille flow of a highly rarefied gas, and a similar phenomenon also occurs in the plane thermal transpiration considered in the present paper.     

Experimental investigation and optimization of small-scale helium liquefaction with multi-cryocoolers

Small-scale helium liquefiers using regenerative cryocoolers with cooling power up to 1.5 W at 4.2 K could be used to re-liquefy evaporated helium gas of small- and medium-sized cryogenic devices such as MEG and PPMS. A serial–parallel-path helium liquefier with a liquefaction rate of 83 Litres per day (L/d) using five 4 K G-M cryocoolers is developed, and has been applied to the Wuhan National High Magnetic Field Center (WHMFC) in China. Different from parallel-path helium liquefier, the helium gas is effectively, stepwise precooled by heat exchangers on multi-cold flanges, and thus the additional purifier and precooling coil heat exchangers on the thinner part of the cold head cylinder containing the 2nd stage displacer could be removed to simplify the construction. Through theoretical calculation and conclusive analysis, an optimum configuration is proposed and makes a reference to the design of serial–parallel-path helium liquefier with multi-cryocoolers.     

Dynamics and thermodynamics of quasi-one-dimensional helium deposited on carbon nano-bundles

No Heading The thermodynamics of monoatomic gas (helium) adsorbed on one-dimensional groove positions on carbon nanolube bundles has been investigated theoretically. The model of lattice gas type is based on account of equilibrium conditions between two subsystems: quasi-one-dimensional helium in grooves and three-dimensional helium atmosphere closed within the free volume of an experimental cell (the total amount of helium atoms in the system is taken to be constant). The analytical calculations of the energy and the heat of adsorption have been compared with the experimental data.¹PACS: 68.35.Dv, 68.35.Ja, 68.65.+g     

Microstructured vacuum gauges and their future perspectives

New prototypes and concepts of microstructured vacuum gauges have been developed by using the fabrication technologies for micro electro mechanical systems (MEMS). The realization of such microstructured gauges requires sophisticated fabrication processes such as thin film deposition, photolithography and etching techniques. This approach of MEMS vacuum gauges is demonstrated by a few examples.Micro-Pirani gauges are based on the principle that the heat transfer between two surfaces is proportional to the number of molecules (and hence the pressure) transferring the heat, when the mean free path in the gas is larger than the distance between the surfaces. In contrast to conventional Pirani gauges with heated thin wires, in a micro-Pirani gauge the heat transfer takes place between an extremely thin heated membrane and the surrounding. The thin membrane (200-nm thick) is realized by deposition of siliconoxide/siliconnitride, photolithography and anisotropic etching of silicon wafers. Heating is performed by a meander-shaped aluminum thin film heater in the center of the membrane. This micro-Pirani gauge shows a high sensitivity in the pressure range between 10⁻⁴ and 100 mbar. By using a silicon “microbridge” with 10 μm small gap between heated membrane and surrounding, the pressure sensitivity of the chip is extended up to 1000 mbar.Similar concepts are presented and discussed with respect to the miniaturization of spinning rotor gauges. The new concept is based on the application of microfabricated disks (instead of spheres) and of electrostatic instead of magnetic driving forces. The extension of the sensitivity range for miniaturized spinning rotor gauges is also discussed.Finally, new perspectives for mechanical vacuum gauges are demonstrated. By application of micromechanical processes, very thin stress-compensated membranes can be fabricated which enable sensitive mechanical gauges even for pressures in the high vacuum range. First, experimental results with respect to these membranes are represented.     

Carbon Aerogel-Based High-Temperature Thermal Insulation

Carbon aerogels, monolithic porous carbons derived via pyrolysis of porous organic precursors synthesized via the sol–gel route, are excellent materials for high-temperature thermal insulation applications both in vacuum and inert gas atmospheres. Measurements at 1773K reveal for the aerogels investigated thermal conductivities of 0.09W · m⁻¹ · K⁻¹ in vacuum and 0.12W · m⁻¹ · K⁻¹ in 0.1MPa argon atmosphere. Analysis of the different contributions to the overall thermal transport in the carbon aerogels shows that the heat transfer via the solid phase dominates the thermal conductivity even at high temperatures. This is due to the fact that the radiative heat transfer is strongly suppressed as a consequence of a high infrared extinction coefficient and the gaseous contribution is reduced since the average pore diameter of about 600nm is limiting the mean free path of the gas molecules in the pores at high temperatures. Based on the thermal conductivity data detected up to 1773K as well as specific extinction coefficients determined via infrared-optical measurements, the thermal conductivity can be extrapolated to 2773K yielding a value of only 0.14W· m⁻¹ · K⁻¹ in vacuum.     

The permeability of alumina over an extended temperature range

A semiempirical mathematical model has been developed to predict the permeability of porous alumina over a wide temperature range at atmospherie pressure. Although the thermal expansion of the alumina is of the order of 1 % over the range of temperatures considered, the apparent permeability of the sample to gas flow varies by over 400%. This behavior is due to the dependence of the mean free path of a gas on temperature and to the corresponding slip conditions that occur in the pores of the solid. The model developed correlates the apparent permeability data with temperature, true permeability, gas viscosity, and gas molecular weight, variables suggested by kinetie theory on mean free path and slip. Apparent permeability was found to be a very strong function of temperature. It exhibited both a direct thermal dependence and an indirect dependence, manifest through thermally driven variations in the gas viscosity. A mathematical model from the literature was used for gas viscosity. The inverse correlation with gas molecular weight, suggested by kinetie theory, is demonstrated. The model covers the temperature range between 250 and 1600 K. Small molelcule gases including air, nitrogen, argon, and helium were used in the development.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Porous plug and superfluid helium film flow suppressor for the soft X-ray spectrometer onboard Astro-H

Suppression of superfluid helium flow is critical for the Soft X-ray Spectrometer (SXS) onboard Astro-H, to achieve a life time of the liquid helium over 5 years. The superfluid film flow must be sufficiently small, compared to a nominal helium gas flow rate of the SXS . For this purpose, four devices composed of a porous plug, an orifice, a heat exchanger, and knife edge devices will be employed based on the experience of the X-ray microcalorimeter (XRS for X-Ray Spectrometer) onboard Suzaku. The porous plug is a phase separator of the liquid and gas helium. A potential film flow leaking from the porous plug is suppressed by the orifice. Almost all the remaining film flow evaporates at the heat exchanger. The knife edge devices stop the remaining film flow by using atomically sharp edges. In this paper, we describe the principle and design of these four devices.     

Stability analysis of the LHC cables

The commissioning and operation of the LHC accelerator calls for better understanding of the stability margin of its superconducting magnets with respect to the perturbation spectrum expected during operation, such as wire motion, AC loss during ramps and in particular beam induced heat depositions. In this paper, the stability analysis of the cables for the LHC dipole for transient heat deposition is reported. The sensitivity of the stability margin to the parameters of the model is investigated and discussed. In particular, the helium content in the cable is shown to have a large impact on stability. In fact, we find that the most important parameter is the heat transfer into helium II and helium I. Furthermore, we quantify the influence of the interstrand thermal and electrical coupling on the cable stability in the several examples reported.     

Development of compact heat exchangers for CO2 air-conditioning systems

Compact and lightweight heat exchangers are needed for motor vehicle air-conditioning systems and for several types of unitary equipment. The high-pressure natural refrigerant CO₂ is now being evaluated for use in such applications, and efficient heat exchangers are being developed and investigated. Carbon dioxide heat exchangers are designed for high refrigerant mass flux and use small-diameter tubes or extruded flat microchannel tubes. Refrigerant-side heat transfer coefficients are higher than with fluorocarbons, and reduced internal surface areas can therefore be tolerated. Both small-diameter mechanically expanded round-tube heat exchangers and brazed microchannel-type units have been built and tested successfully. Results show that compact heat exchangers optimized for CO₂ are very competitive with baseline HFC/HCFC units in terms of physical dimensions, exchanger mass and thermal performance. Smaller tube and manifold dimensions can give reduced size compared with HFC-134a equipment. The temperature approach between air inlet and refrigerant outlet is much lower in CO₂ gas coolers than in baseline system condensers of equal size and capacity, and the reduced refrigerant exit temperature has a marked influence on the coefficient of performance, Microchannel heat exchangers give the best overall efficiency. Refrigerant distribution in multiport manifolds and heat transfer tubes does not seem to be a problem.     

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.     

Rf heating and attainable rf fields of high frequency He II cooled Nb cavities with defect free internal surface

The rf heating of Nb superconducting resonant cavities with defect free internal surface, cooled by a superfluid helium bath, is simulated by the numerical solution of a mathematical model of heat generation and transfer in the cavity wall. The model fully takes into account the nonlinear temperature dependences of the rf heating at the internal surface, the thermal conductivity of Nb, and the heat transfer coefficient at the Nb/He II interface. The simulation results are compared with experimental data reported in the literature and a good agreement is found for maximum attainable rf fields in cavities working above nearly 4 GHz. How the maximum attainable surface magnetic field and general features of the thermal behaviour depend on some cavity parameters in the simulation results is also discussed.     

Hybrid silicon/molecular FETs: a study of the interaction of redox-active molecules with silicon MOSFETs

Redox-active molecular monolayers were incorporated in silicon MOSFETs to obtain hybrid silicon/molecular FETs. Cyclic voltammetry and FET characterization techniques were used to study the properties of these hybrid devices. The redox-active molecules have tunable charge states, which are quantized at room temperature and can be accessed at relatively low voltages. The discrete molecular states were manifested in the drain current and threshold voltage characteristics of the device, confirming the presence of distinct energy levels within the molecules at room temperature. This study demonstrates the modulation of Si-MOSFETs' drain currents via redox-active molecular monolayers. The single-electron functionality provided by the redox-active molecules is ultimately scalable to molecular dimensions, and this approach can be extended to nanoscale field-effect devices including those based on carbon nanotubes. The molecular states coupled with CMOS devices can be utilized for low-voltage, multiple-state memory and logic applications and can extend the impact of silicon-based technologies.