S. W. Van Sciver

Design, development and testing of the cryogenic system for the 45-T hybrid

The cryogenic system for the 45-T Hybrid superconducting magnet employs an innovative design primarily driven by the requirement that it be a user-friendly, reliable facility. To achieve this requirement, the superconducting magnet is being built with highly stable cable-in-conduit conductors, operating at 1.8 K and internally cooled with static He II. Further, the magnet is housed in a separate cryostat connected to the refrigeration system through a services duct to the supply cryostat. This design allows the magnet cryostat to be free of penetrations that may interfere with the magnet user. The 45-T Hybrid cryostat was delivered to the NHMFL in March of 1995. The present paper reports on the design, development, fabrication, installation and preliminary testing of this unique system.

Forced flow He II heat exchangers

Abstract A parallel/counter-flow He II heat exchanger of total length 711.2 mm and flow area 2 mm × 4 mm has been designed, fabricated and tested under a variety of conditions in saturated He II. In the present paper, the experimentally measured temperature profiles for both parallel and counter-flow configurations are compared to a numerical model which is based on a combination of He II internal convection heat transfer and ordinary forced convection mechanisms. The numerical model used to fit the experimental results has one adjustable parameter, the heat transfer coefficient, h , which is dominated by the Kapitza conductance between He II and a solid surface. The results show that the value for h is independent of fluid velocity in agreement with the findings of previous studies. The He II heat exchanger investigated in this configuration exhibits an unusual temperature distribution not seen for classical fluids. For the parallel-flow case, the temperature profiles can cross at low fluid velocity, signalling a reversal of heat transfer. This result comes about because of the high heat conductivity of He II. At high velocities and in the counter-flow configuration, the behaviour is more similar to that commonly observed with classical, low thermal conductivity fluids. The features of the He II system that bring about these unique effects and a suggested approach to the design of such He II heat exchanger systems are presented.

Design, Development and Testing of the JT Refrigerators For The 45-T Hybrid Magnet

The 45-T Hybrid Facility requires 1.8 K refrigeration for extended periods of time. The JT refrigeration system design is fully redundant to guarantee continuous cooling of the super-conducting magnet. Each refrigerator is equipped with a removable instrumentation probe and JT valve plug. These removable components make service and repair possible at 4.2 K. In June of 1995, preliminary testing of the JT refrigerators was performed during the initial cool down of the Hybrid Cryostat. Details of the design, development, and fabrication will be presented along with the results of these early tests.

Heat and Mass Transfer Between Two Saturated He II Baths

Publisher Summary This chapter examines the heat and mass transfer processes between two saturated He II baths. The two baths, formed from two stainless steel cans of 50 mm ID and 660 mm length, are connected at the top and the bottom with two 100 mm long tubes. The bottom tube, with a 5 mm ID, is filled with liquid He II; the top tube, with a 1.3 mm ID, is filled with saturated He vapor. Heat transfer between the two baths is then governed by the counterflow process in the bottom tube and vapor mass transfer process in the top tube. Steady state and dynamic models, based on energy and momentum equations, are presented and agree very well with experimental results. The chapter also demonstrates that the mass transfer process is a far more efficient heat transport mechanism than the counterflow process in vapor/He II two-phase systems.

New calorimetric AC loss measurement technique involving superfluid helium

We have developed a new calorimetric AC loss measurement facility involving superfluid helium (He II). At present, the Test of AC Loss (TACL) facility performs AC loss measurements on Cable-in-Conduit Conductors (CICC) under development for the NHMFL 45 Tesla hybrid superconducting outsert magnet. TACL can handle large scale conductors up to one meter in length. Measurements utilize the exceptional high heat conductivity of He II, which provides an isothermal environment and is the dominant enthalpy in the system. The test conductors are placed in an independent cryostat containing He II which is inserted in a superconducting dipole magnet producing a transverse magnetic field up to 7 T. For a change of the magnetic field and associated AC loss, the temperature variation of the He II surrounding the conductor is measured and directly converted to enthalpy variation of the He II. This paper describes the measurement technique and compare its resolution to that of more conventional calorimetric AC loss measurements.

Heat Transfer from Aluminum Surfaces to Pool Boiling He I

Heat transfer measurements between different aluminum surfaces and pool boiling He I at 4.21 K are reported. The samples are contained in a channel configuration similar to what might be found in a large bath cooled superconducting magnet. Results include heat transfer coefficients in the nucleate and film boiling regimes as well as values for the peak nucleate and minimum film boiling heat flux. The effects of sample orientation and surface condition are also studied.

Thermal Hydraulic Characteristics of a Prototype CEA Cable-in-Conduit Conductor

The thermal hydraulic characteristics of a prototype CEA Cable-in-Conduit Conductor (CICC) have been studied in steady state and transient conditions. The supercritical helium velocity in the central channel was measured with a Pitot tube located at the down stream end of the conductor. An inductive heater, located at the center of the conductor, initiated thermally induced transient flow of the helium within the conductor. The induced flow velocity was measured as a function of Reynolds number and heat input. A calorimetric calibration technique was used to estimate the total heat input to the conductor. In a separate part of the experiment, a thermometer array was installed in the central channel to record the helium temperature. The associated reduction of central channel flow area significantly affects the thermal hydraulic characteristics of the conductor.

Localized Heat Transfer to Vertical Forced Flow Two-Phase Helium

Localized heat transfer measurements in vertical two-phase helium are reported. The test loop contains two short heat transfer sections made of 5 mm thick oxygen-free high conductivity (OFHC) copper discs. These test sections were installed in a U-shaped vertical flow loop driven by a single-stroke bellows pump. The surface temperature of each test section is measured with two germanium thermometers placed on different radial positions in each test section. With one test section placed on the downflow side and one on the upflow side of the loop, the effect of flow orientation on heat transfer characteristics in vertical two-phase helium flow is investigated. The study includes the effects of system pressure, mass flow rate, and geometry on the heat transfer coefficient, critical heat flux, and recovery heat flux.

A continuously cooled mechanical support for cryogenic equipment

A scheme is presented for interception of the heat flowing into a cryogenic environment, through its mechanical support. In the present method, the mechanical support is cooled continuously by means of a coolant-carrying tube, wound in a helix around the support. A numerical simulation of the present support cooling method has been conducted. An experimental study of this support-cooling method is proposed.