Gas gap thermal switches using neon or hydrogen and sorption pump

The very low pressure obtained thanks to adsorption phenomenon at low temperature can be used to build cryogenic heat switches, which offer the possibility to make or break thermal contact between two parts of a cryogenic system. The ON (conducting) and OFF (insulating) states of the switch are obtained by varying the gas pressure between two copper blocks separated by a gap of 100 μm. This pressure is controlled by acting upon the temperature of a small sorption pump (activated charcoal) connected to the gap space. For a “high” sorption pump temperature, the gas previously adsorbed in the sorption pump is released to the gap between the two blocks, allowing a good thermal conduction through the gas (ON state). On the opposite, cooling the sorption pump allows a very good vacuum between the copper blocks, which efficiently break the thermal contact (OFF state). Experimental thermal characteristics (Conductance in the ON and OFF state, ON-OFF switching temperature) of such a “Gas Gap Heat Switch” are described using hydrogen or neon as exchange gas and are compared with theoretical calculations.     

Forced flow boiling heat transfer of liquid hydrogen for superconductor cooling

Heat transfer from inner side of a heated vertical pipe to liquid hydrogen flowing upward was first measured at the pressure of 0.7 MPa for wide ranges of flow rates and liquid temperatures. The heat transfer coefficients in non-boiling regime for each flow velocity were well in agreement with the Dittus–Boelter equation. The heat fluxes at the inception of boiling and the departure from nucleate boiling (DNB) heat fluxes are higher for higher flow velocity and subcooling. It was found that the trend of dependence of the DNB heat flux on flow velocity was expressed by the correlation derived by Hata et al. based on their data for subcooled flow boiling of water, although it has different propensity to subcooling.     

Performance measurements of a 7 mm-diameter hydrogen heat pipe

A gravity assisted heat pipe with 7-mm diameter has been developed and tested to cool a liquid hydrogen target for extracted beam experiments at COSY. The liquid flowing down from the condenser surface is separated from the vapor flowing up by a thin wall 3 mm diameter plastic tube located concentrically inside the heat pipe. The heat pipe was tested at different inclination angles with respect to the horizontal plane. The heat pipe showed good operating characteristics because of the low radiation heat load from the surroundings, low heat capacity due to the small mass, higher sensitivity to heat loads (to overcome the heat load before the complete vaporization of the liquid in the target cell) due to the higher vapor speed inside the heat pipe which transfers the heat load to the condenser.     

A very light and thin liquid hydrogen/deuterium heat pipe target for COSY experiments

A liquid hydrogen/deuterium heat pipe (HP) target is used at the COSY external experiments TOF, GEM and MOMO. The target liquid is produced at a cooled condenser and guided through a central tube assisted by gravitation into the target cell. An aluminum condenser is used instead of copper, which requires less material, improves conductivities and provides shorter cooling down time. Residual condenser temperature fluctuations in the order of ≈0.4 K are reduced by using thermal resistances between the cooling machine and the condenser of the heat pipe combined with a controlled heating power. A new design with only a 7-mm-diameter HP has been developed. The diameter of the condenser part remains at 16 mm to provide enough condensation area. The small amount of material ensures short cooling down times. A cold gas deuterium HP target has been designed and developed which allows protons with energy ?1 MeV to be measured. A 7-mm-diameter HP is used to fill a cooling jacket around the D₂ gas cell with LH₂. The D₂ gas is stabilized at 200 mbar to allow for thin windows. Its density is increased by factor 15 compared to room temperature.     

Cryogenic propellant liquefaction and storage for a precursor to a human Mars mission

An analysis of cryogenic liquefaction and storage methods for in-situ produced propellants (oxygen and methane) on Mars is presented. The application is to a subscale precursor sample return mission, intended to demonstrate critical cryogenic technologies prior to a human mission. A heat transfer analysis is included, resulting in predicted cryogenic tank surface temperatures and heat leak values for different conditions. Insulation thickness is traded off against cryocooler capacity to find optimum combinations for various insulation configurations, including multilayer insulation and microspheres. Microsphere insulation is shown to have promise, and further development is recommended.     

A coupled numerical analysis of shield temperatures, heat losses and residual gas pressures in an evacuated super-insulation using thermal and fluid networks: Part III: Unsteady-state conditions (evacuation period)

This paper analyses the evacuation period of a 300 L super-insulated cryogenic storage tank for liquid nitrogen. Storage tank and radiation shields are the same as in part I of this paper. The present analysis extends application of stationary fluid networks to unsteady-states to determine local, residual gas pressures between shields and the evacuation time of a multilayer super-insulation. Parameter tests comprise magnitude of desorption from radiation shields, spacers and container walls and their influence on length of the evacuation period. Calculation of the integrals over time-dependent desorption rates roughly confirms weight losses of radiation shields obtained after heating and out-gassing the materials, as reported in the literature. After flooding the insulation space with dry N₂-gas, the evacuation time can enormously be reduced, from 72 to 4 h, to obtain a residual gas pressure of 0.01 Pa in-between shields of this storage tank. Permeation of nitrogen through container walls is of no importance for residual gas pressures. The simulations finally compare freezing H₂O-layers adsorbed on shields, spacers and container walls with flooding of the materials.     

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.     

Thermal contact resistance of different materials between 0.3 K and 4.5 K

The choice of a good bonding agent often depends on the temperature range of use and on its corresponding thermal contact resistance. Measurements obtained between 0.3 K and 4.5 K were made applying the steady state heat flow method across the interfaces of small crystals of quartz (SiO₂) sandwiched by two copper blocks and attached with various types of contact materials. Below 1 K, we found that for one crystal attached with indium solder its thermal contact resistance was . This value in comparison with those attached with a conductive and an insulating epoxies, turns to be the lowest.     

Energy storage unit: Solid state demonstrators at 20 K and 6 K

Cryocoolers’ vibrations prevent them from being used in some sensitive applications. Stopping the cryocooler even for a short period gives rise to a steep drift of temperature. Such a drift can be significantly attenuated by adding an enthalpy reservoir to the cryocooler, separated from its cold finger by a heat switch. The whole assembly for the enthalpy reservoirs and switch is here called ESU, the energy storage unit. Two units have been built and tested based on solid state materials. One unit was designed to work up to 20 K, the other up to 6 K. This paper presents the experimental results obtained for both ESUs.Lead was used for the 20 K ESU while the ceramic GOS (Gd₂O₂S) was found adequate for the 6 K ESU. After stopping the cryocooler, a fairly slow temperature drift was measured at each ESU (from 11 to 20 K or from 3 to 6 K, respectively) while applying 10 mW for one hour, for instance. Otherwise, a temperature controlled platform experiment can use an ESU as a cold source allowing a constant temperature. Input power to the ESUs was monitored along with temperature and time. In the case of the 20 K ESU, calculations match the stored amount of energy as well as the temperature drift of the energy reservoir.A study for a high enthalpy intercept at the middle of the switch is also presented here. This intercept shall allow the attenuated temperature drift to hold for longer times. A cryogenic experiment can then be carried on using a cryocooler in a completely silent environment.     

Thermal design of the CFRP support struts for the spatial framework of the Herschel Space Observatory

Thermal finite element (FE) models, of low thermal conductance struts which are required to provide support for the low temperature components of the Herschel Space Observatory, have been validated by measurements at temperatures below 20 K. The Herschel Space Observatory structure is introduced. FE modelling of two designs of support strut is briefly discussed and the final designs presented. Validation of the design models was made in two experiments. The first of these provided specific thermal conductivity data for component CFRP materials, whose composition was initially designed on the basis of data available in the literature. The second experiment was performed to confirm the thermal conductance (Q′/ΔT), of the completed struts. The validation test rigs are described together with details of the experimental methods employed. Values of conductance were at the level of 5 × 10⁻⁵ W/K at a mean temperature of 6 K. The measured data are presented and discussed with reference to the thermal models. Sources of measurement inaccuracy, are also discussed.     

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.     

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.     

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.     

Performance of extended surface from a cryocooler for subcooling liquid nitrogen by natural convection

Natural convection of subcooled liquid nitrogen under a horizontal flat plate is measured by experiment. This study is motivated mainly by our recent development of cryocooling systems for HTS power devices without any forced circulation of liquid nitrogen. Since the cold surface of a GM cryocooler is very limited, the cooling plate immersed in subcooled liquid nitrogen is thermally anchored to the cryocooler located at the top in order to serve as an extended surface. A vertical plate generating uniform heat flux is placed at a given distance under the cooling plate so that subcooled liquid may generate cellular flow by natural convection. The temperature distributions on the plates and liquid are measured during the cool-down and in steady state, from which the heat transfer coefficients are calculated and compared with the existing correlations for a horizontal surface with uniform temperature. A fair agreement is observed between two data sets, when the heat flux is small or the plate temperatures are relatively uniform in horizontal direction. Some discrepancy at higher heat flux is explained by the cellular flow pattern and the fin efficiency of the extended surface, resulting in the non-uniformity of the horizontal plate.     

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.     

Effect of flow mal-distribution on effective NTU in multi-channel counter-flow heat exchanger of single body

Most heat exchangers in services belong to multi-channel heat exchangers. The working fluid is distributed among the multiple channels. This kind of heat exchangers usually has a flow distribution problem. In general, the flow is not evenly distributed and, as a result, the thermal performance of the heat exchanger degrades due to it. In this paper, such flow distribution effect in a single body multi-channel heat exchanger is evaluated in an analytic way. Transverse conduction through the heat exchanger body which is caused by the mal-distributed flow is formulated and included in NTU evaluation. The NTU relationship between the well-balanced flow distribution and the ill-balanced one is obtained. According to the analysis result, the heat exchanger has the best performance in case of the well-balanced flow distribution, as is expected, and has the performance degradation in case of the ill-balanced flow. The performance degradation is very severe in the heat exchanger with poor transverse conduction.     

Pressure loss effect on recuperative heat exchanger and its thermal performance

Most heat exchangers are designed in pursuit of minimum pressure loss and maximum heat transfer conductance, and the pressure drop of heat exchanger is sometimes neglected intentionally or unintentionally in the designing stage. However, the actual pressure drop should be considered when the requirement of heat exchanger is very strict. Different from the most previous researches that examine the hydraulic aspect, this paper describes the thermal aspect of the pressure loss and its relevance to the basic model of heat exchanger. The analytical result reveals the indirect effect of pressure loss on the thermal performance of heat exchanger. The whole impact on the effectiveness of counter-flow heat exchanger is discussed in terms of newly defined relevant dimensionless parameters.     

Numerical analysis of two-dimensional steady-state and transient heat transfer in a parallel duct filled with pressurized He II

Analysis on steady-state and transient heat transfer on a flat plate at the middle of a parallel duct immersed in He II was performed for bath temperatures from 1.8 to 2.1 K at 101.3 kPa. Two-dimensional computer code named SUPER-2D developed by the authors based on the two-fluid model and the theory of mutual friction was used. Steady-state critical heat flux (CHF) and the time lag from the application of a step heat input to λ transition, that is called a lifetime, were obtained numerically for various step heat fluxes and for the channel gaps from 2 to 20 mm. Effect of the gap restriction on the CHF and the lifetime were clarified. The solutions were compared with the experimental data for the ducts with the same structures and the corresponding conditions. They agreed well with the experimental data. The heat transport mechanism in the parallel duct was clarified.     

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.     

Kapitza resistance and thermal conductivity of Kapton in superfluid helium

We have determined simultaneously the Kapitza resistance, R_K, and the thermal conductivity, κ, of Kapton HN sheets at superfluid helium temperature in the range of 1.4–2.0 K. Five sheets of Kapton with varying thickness from 14 to 130 μm, have been tested. Steady-state measurement of the temperature difference across each sheet as a function of heat flux is achieved. For small temperature difference (10–30 mK) and heat flux density smaller than 30 W m⁻², the total thermal resistance of the sheet is determined as a function of sheet thickness and bath temperature. Our method determines with good accuracy the Kapitza resistance, R_K=(10540±444)T⁻³×10⁻⁶ K m² W⁻¹, and the thermal conductivity, κ=[(2.28±0.54)+(2.40±0.32)×T]×10⁻³ W m⁻¹ K⁻¹. Result obtained for the thermal conductivity is in good agreement with data found in literature and the Kapitza resistance’s evolution with temperature follows the theoretical cubic law.