Ch. Haberstroh

A Superconductive MGB2 Level Sensor for Liquid Hydrogen

With upcoming use of liquid hydrogen for alternative fuel applications increasing requirements on sensor technology are accompanied. Concern often is focussed on the remaining liquid content, which usually can be derived from the position of the vapor/liquid interface in a given container.Measurement methods utilized here at present (e.g. with capacitive sensor types) are tainted with certain deficiencies.For liquid helium on the other hand since a long time superconductive level probes are applied very successfully. A small heat input in e.g. a vertically mounted NbTi wire keeps the section situated in the vapor phase above transition temperature. Measurement of the remaining electric resistance allows a continuous reading of the liquid level.For liquid hydrogen until recently a superconducting material with a suitable transition temperature was missing. With the discovery of the superconductive properties of magnesium diboride up to 39 K and the availability of first tangible wires now a new approach wa...

Further Development of a Mixed Gas Joule Thomson Refrigerator

A mixed gas Joule Thomson refrigerator has distinct advantages over other cryocooler for certain applications. The system is compact, has good thermodynamic efficiency and low levels of vibration and noise. Further development of this refrigeration machine is possible by optimization of the gas mixture composition. A method was developed which permits a reduction by a large factor in the number of simulation calculations and thus shortens the optimization time considerably.

Visualization of magnetoconvection

The opacity of liquid metals represents a seemingly insurmountable barrier for visualizing flows of electrically conducting fluids under the influence of a magnetic field. We demonstrate that this fundamental limitation of experimental magnetohydrodynamics (MHD) can be overcome by using (transparent) electrolytes in place of (opaque) liquid metals if a superconducting magnet with field strength of the order of 10 Tesla is used instead of traditional electromagnets. We study the Rayleigh–Benard convection as a prototype problem under a horizontal magnetic field using a shadowgraph method and tracer particles. It is shown that recent observations of Burr and Muller [J. Fluid Mech. 453, 345 (2002)] of an increased heat transfer in magnetoconvection can be attributed to the two-dimensionality and a smaller length scale of motion due to the magnetic field. The present approach provides a rational framework for the application of modern optical flow measurement techniques like laser Doppler velocimetry and part...

Low Cost Mixture Joule Thomson Refrigerator

Publisher Summary This chapter focuses on the development of the closed cycle Joule Thomson Refrigerator for cooling electronic devices, which operates in the temperature range from 70 K to 95 K, which uses refrigerant mixtures and a single stage oil lubricated compressor. It provides high reliability and no maintenance. It describes the system and its performance characteristics. This system is compact, has a good thermodynamic efficiency and low levels of vibration and noise. The system and its performance characteristics is addressed in the chapter.

Mixed Gas J-T Cryocooler with Precooling Stage

A mixed gas Joule Thomson refrigerator offers decisive advantages for several applications. Requirements such as an extreme long MTBF and long life, low levels of vibration and noise can be satisfied by such a refrigerator. However, the known coolers of this type have a relative low thermodynamic efficiency. An improvement of the efficiency is possible by introducing a precooling stage. This method will be discussed in the paper. A mixed gas JT refrigerator with precooling stage has been tested. The efficiency proved to be 1.5 times better compared with a Gifford-McMahon refrigerator.

Natural convection in inclined pipes - A new correlation for heat transfer estimations

Heat intake minimization is one of the main challenges during the design process of cryogenic storage tanks. It is widely known that connection pipes significantly contribute to this residual heat transfer from ambient temperature conditions to the cold inner vessel. A certain pipe inclination can cause a convective flow field within the fluid. This effect usually increases the total heat transfer much more dramatically than anticipated. In several previous papers we discussed the impact of pipe geometry as well as boundary conditions intensively. However, there is no suitable correlation in literature available which could be used to estimate the total heat transfer properly. The large number of experimental data we gained during our investigations allows us to propose a new correlation in order to predict the total heat transfer through an inclined pipe in function of the inclination angle. In this paper we derivate this new correlation and show its application for heat transfer estimations. Several com...

Heat transfer in gas filled pipes with closed warm end under different orientations

Publisher Summary In cryogenics it is a standard arrangement, that pipes filled with a stagnant gas, which lead from a high temperature to a low temperature, are installed with the warm end up. But sometimes the overall configuration does not allow this and pipes have to be arranged, for example, horizontally or even with the warm end down. Based on a straight pipe with a closed warm end and an open cold end, a model for a CFD-simulation is created. The model contains the fluid region as well as the solid walls. The warm and cold temperatures are applied to the pipe ends. A structured grid is applied to the model and the basic parameters are set. The warm end is connected to the surrounding temperature at 298 K, the cold end corresponds to a liquid hydrogen tank at 28 K and 6 bar absolute pressure. The pipe is filled with gaseous para-hydrogen and the wall consists of stainless steel. CFD simulations show that the heat flux through pipes by convection compared to conduction can be up to 30 times higher. A doubling of the wall thickness increases the heat flux for positive inclinations. The cross section for heat conduction becomes higher. A doubling of the gas cross section gives a more than 4 times higher heat flux if convection occurs. As a result from these CFD-simulations more detailed hints for construction can be deduced. Usual design rules can be optimized more precisely. Moreover the influence of the inclination can be taken into account.

Safe and Efficient Operation of Multistage Cold Compressor Systems

Large refrigeration rates in the temperature range of super fluid helium can only be obtained with the help of centrifugal cold compressors. For the large 2 K systems, four compression stages are necessary to reach atmospheric pressure. Centrifugal cold compressors are quite sensitive to mass flow and suction temperature variations; but these have to be expected in a real system. The first step in the systems design is to find safe and efficient quasi-stationary modes of operation.

Operating parameters of liquid helium transfer lines used with continuous flow cryostats at low sample temperatures

Continuous flow cryostats are used to cool samples to a variable temperature level by evaporating a cryogen, e.g. liquid helium (LHe). For this purpose LHe is usually stored outside the cryostat in a mobile dewar and supplied through a transfer line. In general, the complete setup has to be characterised by the lowest possible consumption of LHe. Additionally, a minimum sample temperature can be favourable from an experimental point of view. The achievement of both requirements is determined by the respective cryostat design as well as by the transfer line. In the presented work operating data, e.g. the LHe consumption during cooldown and steady state, the minimum sample temperature, and the outlet quality are analysed to characterise the performance of a reference transfer line. In addition, an experimental transfer line with built-in pressure sensors has been commissioned to examine the pressure drop along the transfer line, too. During the tests LHe impurities occurred which restricted a steady operation.

CRYOGENIC SUPPLY FOR THE GERDA EXPERIMENT

In the GERDA experiment (GERmanium Detector Array for the search of neutrinoless double beta decay of 76Ge) germanium diodes are suspended in a superinsulated cryostat filled with 70 m3 of liquid argon. The cold medium is required since the diodes have to be operated at low temperatures, and furthermore for shielding against background radiation. For the same reason the whole experiment will be placed in the underground laboratories in the Gran Sasso mountains, Italy. In order to avoid any detrimental perturbation inside the dewar vessel, the liquid-argon (LAr) inventory in the main tank will be kept in a subcooled state at a working pressure of 0.12 MPa absolute at the surface. At the TU Dresden an appropriate cryogenic arrangement was designed to match these requirements. Liquid nitrogen (LN2) is used as a cooling fluid. Special care was taken to cope with the narrow temperature span between the LAr boiling temperature and triple point. In the proposed solution a subcooler located close to the cryostat ...