J. D. Siegwarth

Feasibility of electrocaloric refrigeration for the 4–15 K temperature range☆

Abstract The feasibility of a solid state type of refrigeration, which utilizes the electrocaloric effect in certain dielectric materials, has been investigated. The study was limited to the temperature range where the refrigerator would absorb heat from a load at about 4 K and reject heat to a reservoir at about 15 K. Heat switches would be required for such a refrigerator and two types were studied. One type was a multiple-leaf contact switch, the other a magnetothermal switch of single crystal beryllium. Many electrocaloric materials were studied but none was found with a sufficiently large reversible electrocaloric effect for a practical refrigerator. The largest effects were seen in a SrTiO 3 ceramic, followed by a KTaO 3 single crystal. Temperature reductions of about 0.3 K at 10 K were observed during depolarization from fields of 20 kV cm −1 . A theoretical model, based on the electret behaviour of impurity-vacancy dipoles is postulated to interpret the anomalous dielectric behaviour of the materials investigated. Another theoretical model, based on the lattice dynamics of displacive dielectrics, is postulated to explain the observed temperature changes seen in such materials. The model points out that at 4 K the entropies of displacive type materials are probably too low for practical refrigeration. An investigation of certain order-disorder dielectrics is suggested.

Convective Heat Transfer Coefficients for Near-Horizontal Two-Phase Flow of Nitrogen and Hydrogen at Low Mass and Heat Flux

Correlations for convective heat transfer coefficients are reported for two-phase flow of nitrogen and hydrogen under low mass and heat flux conditions. The range of flowrates, heat flux and tube diameter are representative of thermodynamic vent systems (TVSs) planned for propellant tank pressure control in spacecraft operating over long durations in microgravity environments. Experiments were conducted in normal gravity with a 1.5° upflow configuration. The Nusselt number exhibits peak values near transition from laminar to turbulent flow based on the vapor Reynolds number. This transition closely coincides with a flow pattern transition from plug to slug flow. The Nusselt number was correlated using components of the Martinelli parameter and a liquid-only Froude number. Separate correlating equations were fitted to the laminar liquid/laminar vapor and laminar liquid/turbulent vapor flow data. The correlations give root-mean-squared (rms) prediction errors within 15%.

Friction and oxidative wear of 440C ball bearing steels under high load and extreme bulk temperatures

Abstract Unlubricated sliding friction and wear of 440C steels in an oxygen environment have been studied under a variety of load, speed and temperature ranging from approximately —185 to 675 °C. A specially designed test apparatus with a ball-on-flat geometry has been used for this purpose. The observed dependencies of the initial coefficient of friction, the average dynamic coefficient of friction, and the wear rate on load, speed, and test temperatures have been examined from the standpoint of existing theories of friction and wear. High contact temperatures are generated during the sliding friction causing rapid oxidation and localized surface melting. A combination of fatigue, delamination, and loss of hardness due to tempering of the martensitic structure is responsible for the high wear rate observed and the coefficient of friction.

A high conductance helium temperature heat switch

Abstract Metallic contact heat switches with high thermal conductances have been designed and tested in the 4 to 15 K temperature range. Both gold plating and solid silver were used for the contact surfaces. Switch conductances greater than 1 W K −1 at 15 K were achieved for a switch closed at this temperature with a force of 54 kg. The conductance is proportional to temperature, and varies approximately as the 2 3 power of the closing force.

Electrocaloric refrigeration at cryogenic temperatures

Abstract Refrigeration utilizing the electrocaloric effect can be produced with no moving parts and thus, promises to be highly reliable. This paper reviews the principles and experimental results on electrocaloric refrigeration, particularly in the 4–15K temperature range. Many electrocaloric materials were studied but none was found with a sufficiently large reversible electrocaloric effect for a practical refrigerator. The largest effects were seen in a SrTiO3 ceramic, followed by a KTaO3 single crystal. Temperature reductions of about 0.3K at 10K were observed during depolarization from fields of 20 kV/cm. A theoretical model based on the lattice dynamics of displacive dielectrics is postulated to explain the observed temperature changes in such materials.

Flexible laminates for thermally grounded terminal strips and shielded electrical leads at low temperatures

Abstract It is shown how to make compact, thermally grounded terminal strips for electrical leads in cryostats from flexible electrical laminates composed of copper foil bonded to a polyimide film. Laminates of lead foil and polyimide film are used to make a superconducting strip line, a type of transmission line in which it is easy to thermally ground both conductor and shield. The thermal resistance across a laminate was measured between 0.02 and 4 K from which the thermal conductivity of polyimide film is deduced between 0.1 and 4 K.

Analysis of Heat Exchangers for Dilution Refrigerators

Numerical calculations of the behavior of dilution refrigerator heat exchangers are discussed and some results for both discrete and continuous exchangers are presented. It is shown that thermal conductance along the stream is negligible for a typical continuous exchanger of the coaxial tube type, but becomes a dominant feature of a typical discrete exchanger operating below about 50 mK and degrades the performance considerably. A simple design change can be made that reduces the conductance along the liquid and improves the performance of such an exchanger. A simple means of determining whether conductivity is important in either continuous or discrete exchangers is given.

Liquid-vapour surface sensors for liquid nitrogen and hydrogen☆

Abstract Tests of resistance thermometers as liquid-vapour interface sensors for LH 2 and LN 2 showed that most could be made to detect the liquid surface, but a tiny silicon sensor developed at NASA Goddard Space Flight Center gave the fastest response. Tests of a commercial optical surface sensor and two modified versions of it showed that optical sensors can reliably and rapidly detect the liquid-vapour interface of both hydrogen and nitrogen.

Portable helium dewars for use with superconducting magnetometers

Abstract Simple helium dewars have been constructed using gas-cooled radiation shields and multi layer aluminized plastic insulation. They will retain one litre of liquid for up to 40 hours The shields may be made mostly non-conducting so the dewar can be used with ultrasensitive superconducting magnetometers. For most other applications thin sheet metal such as copper or aluminium is suitable for the shields.

Tribological behavior of 440C martensitic stainless steel from -184 C to 750 C

Characterization of the coefficient of friction and wear rate of 440C stainless steel is needed to understand the effects of frictional heating in the bearings of the High Pressure Oxygen Turbopump of the Space Shuttle Main Engine. The coefficient of friction and wear rate have been measured over a range of temperature varying from liquid oxygen temperature (−184°C) to 750°C. The normal load has also been varied resulting in a variation of Hertzian stress from 0.915 to 3.660 GPa while the surface velocity has been varied from 0.5 to 2.0 m/s.