Development and performance test of a cryoprobe with heat transfer enhancement configuration

In the present study, a cryoprobe with heat transfer enhancement configuration (HTEC) is developed and its freezing performance is evaluated experimentally. Two kinds of heat transfer enhancement configurations, i.e., coiled wire insert and helical mesh insert, are proposed and used in the cryoprobe. Furthermore, in order to ensure that vapor can be discharged freely and only liquid nitrogen reaches the cryogenic section of the cryoprobe, a vapor–liquid separator is employed upstream of the cryoprobe. It is found that the precooling time of the cryoprobe is significantly shortened with the vapor–liquid separator. The enhancement of the freezing capacity with the helical mesh insert is slightly superior to that with the coiled wire insert. With the helical mesh insert, the freezing capacity of the cryoprobe can be enhanced by 41%, and the wall temperature can reach 111.3 K in about 10 min. Significant temperature oscillations are observed in the precooling stage of the cryoprobe with HTEC, while only slight temperature oscillations can be found without HTEC.     

Accidental release of hydrogen from a cryogenic tank

Liquid hydrogen at 20 K was harmlessly released at Turin’s Porta Susa station over a period of seven hours on 9 July 1991 through the safety valve of a dewar-type tank on a railway wagon following the loss of the vacuum between its two walls. Commercially available programs were unable to model this type of release in the unusual conditions in which this hydrogen had been stored. A model illustrating the course of the accident was therefore worked out. A start was made by examining the changes in the physical and thermodynamic properties of the hydrogen progress in the dewar to find out how long it had taken to build up the pressure needed to open the safety valve.Owing to the complex geometry of the insulating layer in the interspace of the dewar on which the liquefaction of the air took place, the heat exchange coefficient could not be determined a priori. It was therefore assumed and subsequently quantified by means of an iterative process. The thermodynamic data were then used to examine the outflow of the hydrogen from the venting line. Flow dynamic calculations showed that the hydrogen was entirely lost through the safety valve and that pressure losses along the approx. 3-m line were negligible. The model also showed that the speed of the outflow was subsonic. The speed evaluated will enable the dispersion of the hydrogen and hence the areas at risk to be evaluated in the subsequent stages of the study.     

Acoustic emission and fracture behavior of GFRP woven laminates at cryogenic temperatures

This paper describes an experimental and analytical study on fracture and damage behavior of GFRP woven laminates at cryogenic temperatures. CT (compact tension) tests were carried out at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) to evaluate the critical values of the fracture mechanics parameters. During the CT tests, AE (acoustic emission) method was implemented. AE signals can identify the critical load at which gross failure occurs. A FEA (finite element analysis) was also applied to calculate the fracture mechanics parameters. The failure criteria (Hoffman criterion and maximum strain criterion) or the damage variable based on the continuum damage mechanics was incorporated into the model to interpret the experimental measurements and to study the damage distributions within the specimen. Several methods of calculating J-integral are discussed.     

Fracture and deformation properties of Ni-Fe superalloy in cryogenic high magnetic field environments

The present paper includes experimental and analytical data on the fracture properties of a nickel-iron superalloy, a ferromagnetic austenite, at 4 K in magnetic fields of 0 and 6 T. The tensile, notch tensile and small punch tests are employed. A finite element analysis is also performed to convert the experimentally measured load-displacement data into useful engineering information. To interpret the results we review the available theory of the influence of magnetic field on the stress intensity factor for a crack in ferromagnetic materials.     

Three-dimensional analysis for liquid hydrogen in a cryogenic storage tank with heat pipe–pump system

This paper presents a study on fluid flow and heat transfer of liquid hydrogen in a zero boil-off cryogenic storage tank in a microgravity environment. The storage tank is equipped with an active cooling system consisting of a heat pipe and a pump–nozzle unit. The pump collects cryogen at its inlet and discharges it through its nozzle onto the evaporator section of the heat pipe in order to prevent the cryogen from boiling off due to the heat leaking through the tank wall from the surroundings. A three-dimensional (3-D) finite element model is employed in a set of numerical simulations to solve for velocity and temperature fields of liquid hydrogen in steady state. Complex structures of 3-D velocity and temperature distributions determined from the model are presented. Simulations with an axisymmetric model were also performed for comparison. Parametric study results from both models predict that as the speed of the cryogenic fluid discharged from the nozzle increases, the mean or bulk cryogenic fluid speed increases linearly and the maximum temperature within the cryogenic fluid decreases.     

Small punch testing for determining the cryogenic fracture properties of 304 and 316 austenitic stainless steels in a high magnetic field

This paper examines the effect of magnetic field on the fracture properties of austenitic stainless steels at liquid helium temperature (4 K). Small punch tests were performed on cold-rolled 304 and 316 austenitic stainless steels. Previously proposed correlation for small punch and elastic-plastic fracture toughness test methods was applied to predict a small punch test-based fracture toughness from equivalent fracture strain.     

Transient thermodynamic behavior of cryogenic mixed fluid thermosiphon and its cool-down time estimation

Thermosiphon is an efficient heat transfer device by utilizing latent heat of fluid at liquid-vapor phase change. One of the disadvantages of thermosiphon, however, is that the operational temperature range is fundamentally limited from the critical point to the triple point of the working fluid to maintain two phase state. Nitrogen (N₂) and tetrafluoromethane (CF₄) were selected as the mixed working fluid to widen their original operational temperature range. Thermodynamic behavior of mixture and its effect on the cool-down time were investigated. A simple calculation model was proposed to estimate the cool-down time of the thermosiphon evaporator prior to experiments. The calculated results agreed well with the experimental results within 5% error. The cool-down time reduction was not achieved by mixing two components at once due to the separation of mixture. One idea to avoid this problem was suggested in this paper where the estimated cool-down time was reduced 17.8% compared to pure N₂.     

Heat transfer performance of cryogenic oscillating heat pipes for effective cooling of superconducting magnets

The cryogenic oscillating heat pipe (OHP) for conduction cooling of superconducting magnets was developed and the function was demonstrated successfully. OHP is a highly-efficient heat transfer device using oscillating flow of two-phase mixture. The working fluids that are employed in the present research are Nitrogen, Neon and Hydrogen, and the operating temperatures are 67–91 K, 26–34 K and 17–27 K, respectively. The estimated effective thermal conductivities from the measurement data of the OHP were higher than one of the solids such as copper at low temperature. These results revealed that the cryogenic OHP can enhance the performance of cooling system for magnets.     

Integrated Resonant Self-Pumped Loop and pulse tube cryocooler for cryogenic cooling distribution

Cooling distribution is a vital technology concerning cryogenic thermal management systems for many future space applications, such as in-space, zero boil-off, long-term propellant storage, cooling infrared sensors at multiple locations or at a distance from the cryocooler, and focal-plane arrays in telescopes. These applications require a cooling distribution technology that is able to efficiently and reliably deliver cooling power (generated by a cryocooler) to remote locations and uniformly distribute it over a large-surface area. On-going efforts by others under this technology development area have not shown any promising results.This paper introduces the concept of using a Resonant Self-Pumped Loop (RSPL) integrated with the proven, highly efficient pulse tube cryocooler. The RSPL and pulse tube cryocooler combination generates cooling power and provides a distributive cooling loop that can be extended long distances, has no moving parts, and is driven by a single linear compressor. The RSPL is fully coupled with the oscillating flow of the pulse tube working fluid and utilizes gas diodes to convert the oscillating flow to one-directional (DC) steady flow that circulates through the cooling loop. The proposed RSPL is extremely simple, lightweight, reliable, and flexible for packaging. There are several requirements for the RSPL to operate efficiently. These requirements will be presented in this paper. Compared to other distributive cooling technologies currently under development, the RSPL technology is unique.     

Development of superconducting and cryogenic technology in the Institute for Technical Physics (ITP) of the Research Center Karlsruhe

Since the early 1970s the Institute for Technical Physics of the Research Center Karlsruhe has been involved in the development of superconductivity for research and industrial applications. A broad program with a focus on the superconducting magnet technology was established to include large magnets for nuclear fusion, high-field magnets for nuclear magnetic resonance spectrometers, ore separation and energy storage magnets. Research and development work was performed in collaborative projects with other national as well as international institutions and industry. The success of these projects has been supported by a broad foundation of engineering science in superconductor development, electrical and cryogenic engineering. Several well known test facilities like TOSKA, STAR, HOMER, MTA along with well equipped laboratories for conductor development, materials at cryogenic temperatures, cryogenic high-voltage engineering have made substantial contributions to in-house, national and international projects. A strong cryogenic infrastructure with two refrigerators and sophisticated cooling circuits from about 4.5 K down to 1.8 K assure the reliable operation of these large facilities. Last but not least, cryogenic research, including vacuum pumps for International Thermonuclear Experimental Reactor, improvements in thermal insulation, cryogenic instrumentation and small on board refrigerators has supported progress in this field. High-temperature superconductivity projects for low AC loss conductors, a 70 kA current lead and a fault current limiter are currently in progress.     

Capacitance based mass metering for cryogenic fluids

This paper describes a method for measuring the mass of cryogenic fluids in on-board rocket propellant tanks or ground storage tanks. Linear approximations to the Clausius-Mossotti relationship serve as the foundation for a capacitance based mass sensor, regardless of fluid density stratification or tank shape. Sensor design considerations are presented along with the experimental results for a capacitance based mass gage tested in liquid nitrogen. This test data is shown to be consistent with theory resulting in a demonstrated mass measurement accuracy of ±0.75% and a deviation from linearity of less than ±0.30% of full scale mass. Theoretical accuracies are also shown to be ±0.73% for hydrogen and ±1.39% for oxygen for a select range of pressures and temperatures.     

Development of a stainless steel check valve for cryogenic applications

This paper describes the development of a check valve for use in a sorption compressor that will drive a 10 mW 4.5 K Joule-Thomson cryocooler. For the check valve extremely low backflow rates are tolerable at an operating temperature of the valve of 50 K. To fulfill these requirements, the sealing mechanism of the valve is based on a full metal to metal contact. In order to obtain sufficiently low leak rates, both parts were machined to a surface flatness in the order of 100 nm. In addition, the closing plate (boss) of the valve deforms (bends) slightly under pressure, forming itself to the opposite valve seat and thus reducing the gap between these parts. The measured leak flow at 50 K was 1.6 μg/s helium @ 16 bar pressure difference, which is well below the aim of 3 μg/s.The valve was subjected to an accelerated lifetime test of 300,000 cycles. It was observed that the leak flow through the valve during this test steadily decreased to a level of 0.15 μg/s after 100,000 cycles.     

Equipment using a “static-analytic” method for solubility measurements in potentially hazardous binary mixtures under cryogenic temperatures

A new apparatus designed to study, at cryogenic temperatures, thermodynamic equilibria of potentially explosive binary systems such as hydrocarbon-oxygen mixtures is described herein. This equipment has an equilibrium cell which was especially designed to minimize hazards while allowing accurate phase equilibrium measurements. Reliability of results, obtained with this equipment has been verified by working on the nitrogen-propane system, for which data are already available in literature, over a large range of compositions and at various temperatures. Four isothermal curves describing liquid phase compositions at 109.98, 113.77, 119.75 and 125.63 K have been determined. Our experimental data are represented within 2% in compositions and in pressures through the Peng-Robinson equation of state implying Mathias-Copeman alpha function and Huron-Vidal mixing rule. Comparisons to literature allow pointing out: good agreement is observed with Kremer and Knapp data (1983) while the three sets of Poon and Lu data (1974) presenting systematic positive deviation are consequently judged as suspicious.     

A review of experimental methods for solid solubility determination in cryogenic systems

Over the past years, there have been a number of serious explosions in air industry, which have resulted in workers injuries and fatalities. At the same time, there has been an increase in the use of air separation products for industrial activities.The quality of air entering an air separation plant is of crucial importance for its safe and reliable operation and the interest in the solubility data of solids in cryogenic liquid solvents is closely connected to the problem of impurities accumulation in the process plant and storage tanks. Such accumulations, especially in liquid oxygen, may cause fouling and blockage in heat exchangers and pipelines and they may eventually cause serious explosions. For this reason the air contaminants composition in liquid oxygen must be determined with great precision.This paper aims at reviewing experimental methods for determining the solubility of solid compounds that may be present in the cryogenic liquefaction processing of air distillation. A review of the literature data on solubility of solids in liquid oxygen and nitrogen is included as well.Emphasis is given to the difficulties in setting-up measuring apparatuses working at extreme conditions, i.e. low compositions and low temperatures.     

Development of a valved linear compressor for a satellite borne J-T cryocooler

The provision of temperatures below 12 K is essential for sub-mm and FIR observations from satellite instruments. Historically this has been achieved with stored cryogens, however mechanical coolers could potentially provide higher reliability and flexibility. These cryocoolers typically incorporate a regenerative cold-finger, such as a pulse-tube, however this can be replaced by a recuperative Joule-Thompson stage to obtain the lowest temperatures required. The major change to the compressor is the requirement for steady flow. This paper describes the development of such a compressor using reed valves, based on space-qualified hardware. Long life potential was demonstrated by measuring the motion of the valves during operation. A model was also developed and validated to optimize performance.     

Analytical and finite element investigations of shear/compression test fixtures

Insulation systems are critical components of the international thermonuclear experimental reactor (ITER). They must meet the super conducting magnets design requirements, including mechanical strength under combined shear and compressive stresses at cryogenic temperatures. Past cryogenic magnet systems often relied on woven glass/epoxy materials for insulation. An important point is to find a reliable shear/compression test method for these materials. The present work investigates a commonly used shear/compression setup and aims at measuring the reliability of the obtained test results. Therefore, the stress and failure analysis is performed analytically and numerically using the finite element method. The model is based on woven glass fiber reinforced materials which are subjected to combined shear and compressive stresses as well as to thermal loading, that results from cooling from 293 K to the test temperature of 77 K. A short analytical section shows the problems of common failure criteria which are used to describe the interaction of the shear and compression stresses. The numerical—finite element—section is based on three-dimensional linear elastic finite element models under thermo-mechanical loading. The locations of high stress gradients are investigated using an average stress criterion. Three different model geometries (15°, 45°, and 70°) are analyzed and finally compared with respect to their reliability.     

Comparative test on two kinds of new compliant foil bearing for small cryogenic turbo-expander

Two kinds of new compliant foil journal bearing for a small cryogenic turbo-expander used in the Brayton cycle cryocooler are presented in this paper. The foil of bearings is supported with copper wires and elastic material respectively, so the bearings are much simpler in structure than previous compliant foil bearing. The performances of the bearings are studied under the same experiment conditions. The results indicate that both bearings have better damp performance and the foil bearing supported with elastic material has preferable stability to the one supported with copper wires.     

Thermal performance of multilayer insulation around a horizontal cylinder

Thermal performance of multiplayer insulation (MLI) is affected by contact pressure between adjacent layers. In order to evaluate the thermal performance of the MLI fabricated in the horizontal cryostats of superconducting magnets, it is important to investigate the contact pressure in the MLI. In case of a horizontal cryostat, the MLI is wound around horizontal cylindrical surface and is compressed at the upper part of the cylinder due to the MLI self-weight. At first, a single thin film wound around the horizontal cylinder was analyzed to evaluate the contact pressure acting on the cylinder. The analysis has been extended to the multiply wound film around horizontal cylinder, in order to investigate the distribution of contact pressure between adjacent layers. By using experimental data obtained with a flat panel calorimeter, the results of this analysis have been applied to evaluate the thermal performance of MLI around a horizontal cylinder. And the non-dimensional contact pressure parameter P^* has been introduced as a useful parameter to evaluate and compare the thermal performance among different kinds of MLI.     

Thermal conductance at millikelvin temperatures of woven ribbon cable with phosphor-bronze clad superconducting wires

Woven Nomex® ribbon cables made up with superconducting niobium-titanium wire are used at millikelvin temperatures in many large cryogenic instruments. It is important to know how much heat in transmitted down such cables. However, the conductivity of the materials used is not well known. Another problem is that the wires are normally clad with alloys which exhibit some magnetism. This is a potential problem for instruments employing superconducting detectors. A safe non-magnetic alternative to the usual materials is phosphor-bronze clad niobium-titanium wiring. However, there is little experience with such wires. We have therefore measured the conductance of a ribbon cable made up with these wires. The measured values are in good agreement with our predictions, suggesting that the values we have used to model the cable are sufficiently accurate, and could therefore be used to predict the performance of ribbon cables using other cladding materials, so long as the conductivity of the cladding is reasonably well known. As part of our analysis, we consider the likely variation in thermal conductivity values for C51000 phosphor bronze caused by legitimate variations in composition.     

Validation of a novel fiber optic strain gauge in a cryogenic and high magnetic field environment

We report on the first operation of an easy to use low cost novel fiber optic strain gauge (FOSG) in cryogenic and magnetic field environments. The FOSGs were mounted on a superconducting coil and resin impregnated. The gauges detected resin shrinkage upon curing. On cooldown, the FOSG monitored the thermal contraction strains of the coil and the electromagnetic strain during energization. The coil was deliberately quenched, in excess of 175 times, and again the FOSG detected the quenches and measured the thermal expansion-induced strains and subsequent re-cooling of the coil after a quench. Agreement with FEA predictions was very good.