Low Heat Leak Research Articles

Novel hybrid structures to improve performance of miniature flat evaporator loop heat pipes for electronics cooling

• Two novel hybrid structures have been proposed to improve performance of miniature LHPs. • A new and simple fabrication process has been proposed for fabrication of hybrid structures. • Hybrid structures have high bond strength, over 100 kPa. • The maximum heat flux is around 7 times higher than that of other polymer heat pipes. Two miniature loop heat pipes (LHPs) with novel hybrid structures of evaporators are developed to cool electronic devices. These structures ensure good heat conduction between evaporators and heat sources and low heat leak from evaporators to compensation chambers through these cases. LHP 1 uses a polymer-glass hybrid structure while LHP 2 uses a polymer-stainless steel hybrid structure. Hydrophilic polytetrafluoroethylene membranes are used as wicks and the working fluid is pure water. The bond strength of these hybrid structures was evaluated quantitatively. Then the effect of hybrid structures on the LHP thermal performance was investigated by measuring temperature at each point. The maximum heat transfer capacity reached 9 W (4.5 W / c m 2 ) and 14 W (7.0 W / c m 2 ) while the thermal resistances were 4.8 K / W and 2.1 K / W for LHP 1 and LHP 2 respectively. Using the polymer-stainless steel structure, the maximum heat flux is around 7 times higher while the thermal resistance is smaller than the corresponding values of other polymer heat pipes.

Thermal management at microscale level: Detailed study on the development of a micro loop heat pipe

• A novel design of micro loop heat pipe is demonstrated. • Temperature reduction of more than 28 ° C is attained. • A new methodology for selection of working fluid is proposed. This study aims to address the problem of removing excessive heat generated in electronic and microfluidic systems. For this purpose, a new design of a micro loop heat pipe is proposed. It is fabricated on a 2-inch silicon wafer, having a cross-section of 35 mm × 17 mm and an overall device depth of 150 μm. Experimental investigations showed that the proposed design has a low heat leak (undesirable heat conduction from the evaporator to the compensation chamber) compared to the various micro heat pipe designs presented in the literature. A temperature reduction of 28.6 °C at 3.78 W heat load is recorded using methanol as a working fluid. This study also reveals that the present methodologies to select working fluids for conventional loop heat pipes cannot be applied to micro loop heat pipes. Hence, a new methodology is proposed for the same, based on four parameters of the working fluid: cooling capacity, dry-out limit, operation in anti-gravity regimes, and force exerted on the evaporator.

Liquid hydrogen fuel system design and demonstration in a small long endurance air vehicle

Abstract Autonomous systems have advanced substantially, yet on-board energy capacity still constrains endurance. Liquid hydrogen (LH2) fuel storage coupled with a high efficiency fuel cell may offer greater endurance. An LH2 storage system has been developed, featuring a lightweight dewar with active pressure control. The dewar has 20.46 L internal volume, yielding ≤7.6 kWh kg−1 specific energy and ≤1.2 Wh L−1 energy density (LHV basis) depending on fuel withdrawal rate and ambient conditions. It demonstrated 48 h continuous flight in the Naval Research Laboratory's Ion Tiger unmanned air vehicle, which is ∼85% longer than prior gaseous hydrogen (GH2) fueled flights, with the same dry mass as the GH2 system. Design considerations included matching the LH2 evaporation rate and variable consumption rate, and satisfying constraints imposed by the existing aircraft. Heat transfer models were used to design for low dewar heat leak, which is necessary to balance evaporation and consumption. The heat leak depends on ambient temperature, with evaporation ranging from 16 g h−1 at 240 K to 29 g h−1 at 300 K, at 275 kPa storage pressure. The LH2/GH2/dewar system takes ≥4 h to reach thermodynamic equilibrium after fueling. LH2 can extend the endurance of small autonomous systems, but at the expense of greater design effort and reduced operational flexibility.

DC Superconducting Cable Using $\hbox{MgB}_{2}$ Wires

Tsinghua University, University of Cambridge, and MIT are collaborating to design, construct, and test an <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{MgB}_{2}$</tex></formula> dc cable for microgrid applications. Two-stage current leads with nitrogen vapor cooled copper and gaseous helium cooled high-temperature superconductor will be used. The two-stage cryostat will be cooled using a single helium fan with a recuperator. The helium gas used to cool the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{MgB}_{2}$</tex></formula> to 20–25 K will be cooled with a two-stage GM cryocooler. The cryogenic stability of the cable will be presented. Designs of cryostats are being investigated. Semi-rigid cryostats, with low heat leak, are being designed and will be tested as part of this program. The system will operate up to 1 kV and 1 kA, limited by the present power equipment. However, the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{MgB}_{2}$</tex></formula> conductor is being designed for 5 kA, and may be tested at low voltage with upgraded current leads. A 30-m cable will be tested using facilities at Tsinghua University in Beijing.

Novel load responsive multilayer insulation with high in-atmosphere and on-orbit thermal performance

Aerospace cryogenic systems require lightweight, high performance thermal insulation to preserve cryopropellants both pre-launch and on-orbit. Current technologies have difficulty meeting all requirements, and advances in insulation would benefit cryogenic upper stage launch vehicles, LH2 fueled aircraft and ground vehicles, and provide capabilities for sub-cooled cryogens for space-borne instruments and orbital fuel depots. This paper reports the further development of load responsive multilayer insulation (LRMLI) that has a lightweight integrated vacuum shell and provides high thermal performance both in-air and on-orbit.LRMLI is being developed by Quest Product Development and Ball Aerospace under NASA contract, with prototypes designed, built, installed and successfully tested. A 3-layer LRMLI blanket (0.63cm thick, 77K cold, 295K hot) had a measured heat leak of 6.6W/m2 in vacuum and 40.6W/m2 in air at one atmosphere. In-air LRMLI has an 18× advantage over Spray On Foam Insulation (SOFI) in heat leak per thickness and a 16× advantage over aerogel. On-orbit LRMLI has a 78× lower heat leak than SOFI per thickness and 6× lower heat leak than aerogel.The Phase II development of LRMLI is reported with a modular, flexible, thin vacuum shell and improved on-orbit performance. Structural and thermal analysis and testing results are presented. LRMLI mass and thermal performance is compared to SOFI, aerogel and MLI over SOFI.

Theoretical Model of a Cold Mass Strap Suspension System for Superconducting Magnets

Strap suspension structures are widely used in superconducting magnet systems for high strength and low heat leak. A theoretical model is developed to design and analyze strap suspension structures. The supported cold mass, the tensile stress due to a cooldown, and a quasi-static acceleration load are considered in the model. In addition, the load conditions during transportation and operating the superconducting magnet system are analyzed. Design rules composed of mechanical strength, minimum resonant frequency, and heat load are presented. A suspension structure consisting of eight strap assemblies for a superconducting magnet is analyzed based on the model. The maximum pretension force is calculated, and the allowable accelerations for the maximum pretension are given. The vertical dynamic performance is better than the horizontal directions for the given strap configuration.

Analysis of AC Losses in a Tri-axial Superconducting Cable

High temperature superconducting (HTS) cables have been studied because of low loss and compactness, compared with conventional copper cables. Three-phase cables are usually composed of three single-phase concentric cables. Recently, a tri-axial cable, composed of three concentric phases, has been intensively developed, because it has advantages such as reduced amount of HTS tapes, low leakage fields, low heat leak and compactness, compared with the three single-phase cables. We analysed the three-phase current distributions in the tri-axial cable as functions of winding pitches of three concentric phase layers, and showed the balanced three-phase current distributions in the tri-axial cable. The each layer supplies a transport current under external magnetic field with the same frequency and different phase. We formulate the general form of AC loss of the transport current in combination with the external field with different phase, and analyse the AC loss of the tri-axial cable.

Development of a Low Heat Leak Current-Lead System

This paper describes a low heat leak current-lead system for a high temperature superconducting magnet which is refrigerated by conduction cooling and operated in a persistent-current mode. Under the condition that the current lead is cooled by conduction without gas cooling, we investigated the current-lead structure with which the heat load to the cryocooler is sufficiently low. The current lead consists of two parts. One of them spans the temperature interval between room temperature and around 70 K. The other spans the temperature interval between around 70 K and the lower end temperature. The former is made of copper alloy and the latter is made of a high temperature superconductor. To decrease the heat leak to a thermal anchor in the 70 K region, a detachable joint is installed in the copper alloy part. The driving mechanism for the joint is set in the room temperature region. An ultrasonic rotation motor, which works in the vacuum and the magnetic field, is adopted for the driving mechanism. We designed and constructed a detachable current lead of 600 A class, and obtained a test result that the heat leak to the thermal anchor is sufficiently low.

Test results of 60-kA HTS current lead for fusion application

A 60-kA high-temperature-superconductor (HTS) current lead has been fabricated and tested aiming at the application to a fusion magnet system. The HTS current lead consists of a forced flow cooled copper part and a conduction cooled HTS part. The HTS part is composed of 288 Ag-alloy sheathed Bi-2223 tapes, which are cylindrically arrayed on a stainless steel tube. For the Ag alloy, Ag–10at.%Au is used to obtain low thermal conductivity. Operation of a 60-kA current, which is the world record, was successfully achieved at a cooling condition of 20 K, 3.2 g/s for the copper part, and a low heat leak of 4.8 W at 4.2 K was demonstrated. This result indicates that the electric power of a refrigerator to cool the current lead can be reduced by 1/3 of that in a conventional current lead. In conclusion, technology of a large HTS current lead for fusion application is established.

Development of Bi-2223 HTS tape and its application to coil and current leads

We are developing Bi-2223/Ag tapes with a high engineering critical current density by optimizing the powder-in-tube process and are studying its application to coil and current leads. We have fabricated 250 m-long tape and investigated optimized processing conditions to enhance engineering critical current density. More bubbling was found when the tape was heat-treated with a higher heating rate. Different kinds of superconducting joints were fabricated with multi-filamentary Bi-2223/Ag tapes, and 58% of retained I c was achieved using the insertion of Bi-2223 core between two exposed tapes. Current decay property of the persistent mode HTS coil was investigated. Rapid current decay was observed when the operating current is in a flux-flow range. We could successfully fabricate a low heat leak type HTS current lead with Bi-2223/Ag–Au tapes by employing a stepped geometry. Using this lead, safe operation of 2 kA current transport was confirmed.

The effect of flux diverters on AC losses of a 10 kVA high temperature superconducting demonstrator transformer

In this paper the design, construction and characterization of a 50 Hz single phase high temperature superconducting (HTS) transformer is presented. The transformer is designed to deliver 10 kVA at 77.4 K and 20 kVA at 65 K. The transformer is made of two windings positioned at the middle limb of a three-limb iron core. The secondary winding is made of nine double pancake superconducting coils, using silver sheathed BPSCCO-2223 multifilamentary tapes with nominal critical current of 20 A (at 77.4 K and self field). The primary winding is made of copper to reduce the cost of HTS material. Utilizing a superinsulated nonmetallic doughnut shaped cryostat with low background heat leak, the AC losses in the superconducting winding have been measured by calorimetric method. Two powdered iron flux diverters have been used inside the cryostat at both ends of the superconducting winding in order to reduce the radial component of leakage flux density in the HTS tapes. The use of these flux diverters is shown to be very effective in reducing the AC losses by 40% at the nominal current.

Charging magnet for the floating coil of LDX

The charging coil (C-coil) for the joint Columbia University/MIT Levitated Dipole Experiment (LDX) is under development jointly by MIT and the Efremov Institute. The NbTi superconducting C-coil serves to charge/discharge inductively the floating superconducting magnet to/from 2277 A when it is resting in the charging port at the bottom of the LDX vacuum vessel. The C-coil is designed for 3200 charge-discharge cycles. The solenoid magnet is installed in a low heat leak liquid helium cryostat with a warm bore of more than 1 m. The magnet protection system has an external dump resistor, which dissipates most of the 12 MJ stored during a quench.

13 kA current lead with textured YBCO plates

At Haldor Topsoe A/S, the authors have developed a texturing process by which they produce YBCO high temperature superconductor plates with high current densities and low thermal conductivity suitable for current leads between 77 K and 4 K. In their design of such a lead, the superconductor is quench protected by a shunt with low heat leak and the brittleness of the YBCO material has been overcome by reinforcement. The lead has been tested by CERN and has fulfilled CERN's specifications concerning current capacity, total heat leak, contact resistances and quench protection of the superconductor.

An advanced dilution refrigerator designed for the new lancaster microkelvin facility.

We have constructed a large new dilution refrigerator for use with the new Lancaster nuclear cooling facility. The machine is housed in a purpose-rebuilt suite of rooms and has been designed to have a very low base temperature, a very low heat leak environment and to run for long periods between refrigerant refills. The machine has been operated in continuous mode down to similar to 1.75 mK and can run for 10 days between refills. Preliminary nuclear cooling experiments suggest that even with an experiment attached the mixing chamber can still run below 2 mK.

97/00693 Measurements of low heat leak demountable seals for use in superfluid helium

97/00693 Measurements of low heat leak demountable seals for use in superfluid helium

Current status of superconducting synchronous motor in Saga University

Since there are no iron cores in the rotor and stator windings of superconducting synchronous motors, some reactances are intrinsically smaller than those of conventional motors. Low synchronous reactance would be expected to improve the torque-ampere ratio stability and the overload capacity of superconducting machines. Two types of 30 kVA synchronous motors, such as rotating armature and rotating field types, have been constructed. The rotating armature type motor had been fabricated and tested. The results show the difficulty of holding a stator cryostat in a low-heat leak condition. The low synchronous impedance requires some control of the armature current and/or the motor field current. The rotating field type motor encountered balancing problems due to the triple-pipe configuration of the helium supplier in the torque tube.

Measurements of low heat leak demountable seals for use in superfluid helium

In an experiment using superfluid helium, a successful method has been developed for sealing a G-10 to G-10 interface within the superfluid which satisfies the following requirements: (1) provision for a very good (though imperfect) thermal barrier to heat flow, (2) easy disassembly, and (3) allowance for significant roughness or mismatch between the mating surfaces. Test results using a variety of sealing methods and materials are presented; these reveal that a combination of furniture wax applied to the G-10 surfaces and a subsequent epoxy seal between the waxed surfaces provides the best means for satisfying the above requirements.

Cryogenic skirt support post

The cold masses of cryostats having vertical axes, like vertical pressure vessels, can be effectively supported by means of a cylindrical skirt that wraps concentrically around the cold mass. The skirt is a cryogenic support post connected at its upper end to the cold mass and at its lower end to the cryostat vacuum vessel. A heat intercept connection to an intermediate temperature refrigeration source can be employed to control heat leak. The support post consists of a composite; e.g. epoxy fibreglass, or cylinder with bolted or thermal interference fit end connections. The support post, being a single element support, simplifies cryostat assembly and alignment. The composite cylinder, with a relatively large diameter, lends itself to structural soundness and stability under both static and dynamic loading conditions. Its relatively long length and intermediate temperature heat intercept allows low heat leak to the cold mass. The details of the design of a cryogenic skirt support post as applied to a superconducting magnetic energy storage cryostat are presented. Included are support post fabrication, cryostat assembly, and predicted structural and thermal performance. Fabrication of and operational experiences with a prototype support post assembly are discussed.

Two-stage magnetic refrigerator for astronomical applications with reservoir temperatures above 4 K

We propose a novel adiabatic demagnetization refrigerator (ADR) to produce temperatures as low as 100 mK starting from a high temperature reservoir between 4 and 8 K. The high temperature reservoir for the ADR can be provided by a mechanical cooler or an unpumped liquid helium bath. This refrigerator can be used to cool bolometric infrared detectors for low background astronomy from mountain tops, balloons or satellites, as well as to cool cryogenic X-ray detectors. The two-stage ADR consists of a single magnet with a paramagnetic chromic caesium alum (CCA) salt pill to produce the low temperature and paramagnetic gadolinium gallium garnet (GGG) as the first stage to intercept heat from the high temperature reservoir. Thermal contact between the paramagnets and the reservoir during magnetization is made with a mechanical heat switch. The ADR is suspended with Kevlar cords under tension for high mechanical stiffness and low parasitic heat leak. In a single cycle, the ADR maintains a temperature of 100 mK for 10–100 h. This time depends strongly on the magnetic field and reservoir temperature but not on the volume of the paramagnetic material as long as the heat leak is dominated by the suspension.

A Cryocondensation Pump for the DIII-D Advanced Divertor Program

A cryocondensation pump was designed for the baffle chamber of General Atomics DIII-D tokamak and will be installed in the fall of 1992. The purpose of the pump is to study plasma density control by pumping of the divertor. The pump is toroidally continuous, approximately 10 m long and located in the lower outer corner of the vacuum chamber of the machine. It consists of a 1 m2 liquid helium-cooled surface surrounded by a liquid nitrogen-cooled shield to limit the heat load on the helium-cooled surface. The liquid nitrogen-cooled surface is surrounded by a radiation/particle shield to prevent energetic particles from impacting and releasing condensed water molecules. A thermal enhancement coating was applied to the nitrogen shell to lower the maximum temperature of the shell. The coating is non-continuous to keep the toroidal electrical resistance high. The whole pump is supported off the water-cooled vacuum vessel wall. Supports for the pump were designed to accommodate the thermal differences between the 4K helium surface, the 77 K nitrogen shells, and the 300 K vacuum vessel supporting the pump and to provide a low heat leak structural support. Disruption loading on the pump was analyzed and a finite element structural analysis of the pump was completed.A testing program was completed to evaluate coating techniques to enhance heat transfer and emissivity of the various surfaces. Fabrication tests were performed to determine the best method of attaching the liquid nitrogen flow tubes to their shield surfaces and to determine the best alternative to fabricating the different shells of the pump. A prototype sector of the pump was built to verify fabrication and assembly techniques.