A. Khalil

Operational Characteristics of Loops with Helium II Flow Driven by Fountain Effect Pumps

Forced flow cooling with helium II is being considered with respect to the design of superconducting magnets with internally cooled conductors. An experimental loop to test such coolant circuits has been investigated. Here, the flow of pressurized helium II is driven by so called self-sustained fountain effect pumps. Such pumps consist of porous plug superfilters where the temperature difference necessary to cause sufficiently high pressures is maintained by the flow of heated helium through appropriately arranged heat exchangers. The test loop has been operated at up to 8 W thermal load. This causes helium II flow rates of about 3 g/s, and the pressure difference ranges up to 0.3 bar. The measured operational characteristics of such loops are discussed. They compare well with theoretical predictions.

Experimental measurement of void fraction in cryogenic two phase upward flow

Abstract Experimental measurements of the void fraction in two-phase single component upward flow of both helium and nitrogen were carried out using the capacitance probe technique. An electronic circuit was designed and constructed to accurately detect small changes in the dielectric constant resulting from changes in density in the two-phase flow. A coaxial capacitance probe at the exit of the test section was used as the sensing element to pick up changes in void fraction. Tests were carried out at both adiabatic and diabatic conditions on two test section sizes (6.35 and 2.75 mm id and each 1000 mm long) with exit quality ranging from 0 to 100%, liquid flow of 5 to 35 lh −1 and inlet pressures of 1 to 2 atm. Analysis of the data indicates that the slip ratio (obtained from quality and void fraction measurements) is affected primarily by pressure, mixture quality, superficial velocity and to a lesser degree by the size of the test section. The data of helium and nitrogen is also compared with the predictions of Lockhart-Martinelli, Levy and the homogeneous models.

Mechanical and Thermal Properties of Glass-Fiber-Reinforced Composites at Cryogenic Temperatures

Because of the large forces encountered in large superconducting energy storage magnets, forces have to be transmitted through high strength struts to the room temperature structure. Besides strength, the material of these struts has to have a low thermal conductivity in order to minimize the refrigeration requirement for the energy storage system. The candidate materials that possess high strength-to-weight ratios and low thermal conductivities are fiberglass-epoxy and polyester composites. However, the cost of the selected material is also an important factor because of the large amount required in large superconducting magnet energy systems (SMES). Therefore, the choice of a strut material has to be based on a careful balance between the different factors involved. Fiberglass-reinforced epoxy composites have been successfully employed in different applications in fusion and magnetohydrodynamic (MHD) programs and several studies were carried out to investigate the mechanical and thermal properties of these composites at low temperatures.1–3 On the other hand, fiberglass-reinforced polyester composites are cheaper.

UWTOR-M-a conceptual design study of a modular stellarator power reactor

A preliminary design of a 5500 MWth modular stellarator power reactor, UWTOR-M, is presented. Discrete twisted coils are used in an l = 3 configuration with maintainability as a prime consideration. The natural steliarator divertor is used for impurity control in conjunction with innovative high performance divertor targets. A unique blanket design is proposed which minimizes the overall tritium inventory in the reactor. Finally, a scheme for maintaining the first wall/blanket and other reactor components is discussed.

Heat exchangers for he II cooling loops with self-sustained fountain effect pumps☆

Abstract A cooling cycle with He II convection driven by self-sustained fountain effect pumps is being investigated. Special attention is drawn to the problem of heat transfer at both ends of the superfilter of this loop. The heat exchanger requirements are derived from theoretical considerations on the degradation of the cooling characteristic effected by non-perfect heat exchangers. A shell and tube type heat exchanger, optimized for the warm end of the filter has been operated in this loop with a thermal load of up to 9 W, with 2.8 g s −1 maximum helium flow rate and with inlet temperatures between 1.8 and 3.4 K. Its performance is well described by computations. A different heat exchanger design with finned Cu walls is suggested for the cold end of the pump. Some considerations on its optimization are given.

HEAT TRANSPORT IN FLOWING SUBCOOLED HELIUM II

There are definite limitations to the maximum heat flux that can be axially transported in a He II filled channel. The present study investigates the effect of forced convection on these limitations defined by the destruction of superfluidity at the heated surface and the jump to film boiling. Three regions of heat transfer and transport are identified in the experiment: (1) non-boiling He II the behavior of which is not affected by velocity of flow, (2) two-phase boiling He I and He II and (3) boiling He I. Results are analyzed in terms of heat balance and two fluid hydrodynamics.

Fabrication and Performance of an Aluminum-Stabilized Composite Superconductor

In a number of cases,1,2 practical superconductors have been fabricated with aluminum as the chief stabilizer. Aluminum was chosen because it is lightweight, can be readily purified to a high degree for modest cost,3 has a low magnetoresistance at high fields,4 and is quite radiation transparent.5 Applications for such superconductors include thin-walled magnets for physics experiments, small lightweight magnets for aerospace applications, and large magnets that require massive amounts of conductor.

HelioTrough thermal performance compared to EuroTrough

Parabolic troughs are the most mature technology and have lowest cost compared to other technologies. A comparison between EuroTrough and HelioTrough is carried out in this work. An energy balance model for the troughs absorber tube is used to estimate the optical and thermal losses from the collector. Engineering Equation Solver (EES) was used as a tool to simulate the trough. Results show that the thermal losses from the HelioTrough are more that loss from EuroTrough. Although this fact, the total energy gained from the HelioTrough is much more than EuroTrough.

Effect of channel restrictions on the axial heat transport in subcooled superfluid helium

Abstract The present study investigates the influence of partial restrictions on the axial heat transport and critical heat flux limits in subcooled superfluid helium (helium II) channels. Different size orifices are used to simulate partial plugging of superconducting magnets cooling channels by frozen oxygen, nitrogen, hydrogen, neon or moisture during the cool down process. Thin stainless steel sharp edged orifices of sizes 0.5, 1, 2 and 5 mm id are mounted between stainless steel flanges attached to 9 mm diameter (helium II) channel. The helium II channel is heated at one end with a copper block heater while the other end heat sinks to an atmospheric superfluid helium heat exchange. Temperature drop across the restriction is measured by two calibrated carbon resistors. Measurements are carried out at both temperatures ranging from 1.8 to 2.2 K. As the orifice/channel area ratio decreases, data show a considerable decrease in the axial heat transported by internal convection process resulting in lower critical heat flux at the phase transition from helium II to helium I by the destruction of superfluidity and inititation of boiling. A linear correlation between critical channel heat flux and orificeI channel area ratio gives a good fit to the experimental data. For heat fluxes higher than the critical heat flux, transient temperature measurements for a step heat input are correlated with the time required to reach the phase transition.

Steady state heat transfer of helium cooled cable bundles

Abstract In the present study nucleate and film boiling heat transfer characteristicsof horizontal conductor bundles are investigated at steady state conditions. The effect of gaps between wires, number of wires, wire position, wire size and bundle orientation on the departure from nucleate boling and transition to film boiling is studied. For gaps close to the bubble departure diameter, the critical heat flux can approach up to 90% of the single wire value. Consequently, the maximum stable current for a given bundle can be significantly increased above the single conductor value for the same cross-sectional area.