Adiabatic expansion of \u3csup\u3e3\u3c/sup\u3eHe in \u3csup\u3e4\u3c/sup\u3eHe

\u3cp\u3eIn 1951 H. London proposed the adiabatic expansion of \u3csup\u3e3\u3c/sup\u3eHe in \u3csup\u3e4\u3c/sup\u3eHe to obtain cooling in the millikelvin range. In this paper the feasibility of the technique is investigated to provide a cooling power of 1 μW at 50 mK. Injecting \u3csup\u3e4\u3c/sup\u3eHe in a fixed amount of \u3csup\u3e3\u3c/sup\u3eHe is, in principle, a single-cycle method but it can produce continuous cooling by using two such expanders working in opposite phase. It is independent of gravity, so it may be a candidate for cooling in space. In contrast to the Planck mission it is a closed system which uses fixed amounts of \u3csup\u3e3\u3c/sup\u3eHe and \u3csup\u3e4\u3c/sup\u3eHe.\u3c/p\u3

Basic treatment of onset conditions and transient effects in thermoacoustic Stirling engines

This paper treats the basics of thermoacoustic engines. The set of differential equations, which describes the dynamics of the individual components, is condensed in a single high-order differential equation which determines the time dependence of all dynamic variables. From this relation analytical expressions are obtained for the damping coefficient, the oscillation frequency, and the onset temperature that allows stable oscillations. Also transient effects are discussed based on numerical integration of the dynamic equations. © 2009 Elsevier Ltd. All rights reserved

Vijftig jaar in de kou

Na een beschrijving van de schoonheid van de verschijnselen bij lage temperaturen zal professor De Waele een schets geven van de ontwikkelingen van de koeltechniek gedurende de laatste vijftig jaar. Ook zal hij aandacht besteden aan het universitair onderwijs

Thermodynamics and hydrodynamics of \u3csup\u3e3\u3c/sup\u3eHe-\u3csup\u3e4\u3c/sup\u3eHe mixtures

\u3cp\u3eThe specific heat of liquid \u3csup\u3e3\u3c/sup\u3eHe–\u3csup\u3e4\u3c/sup\u3eHe mixtures is usually written in terms of the sum of the specific heat of a \u3csup\u3e3\u3c/sup\u3eHe-quasiparticle gas and the specific heat of the pure \u3csup\u3e4\u3c/sup\u3eHe component. The thermodynamics based on this starting point is derived. Relations of important quantities and their low- and high-temperature limits are given. These are used to derive expressions for the velocity of second sound. This latter quantity is a very important source of information for the Fermi gas properties. Finally, the Fermi gas parameters are summarized in the chapter. The experimental aspects of the \u3csup\u3e3\u3c/sup\u3eHe–\u3csup\u3e4\u3c/sup\u3eHe hydrodynamics are treated. The appearance of mutual friction that has long been neglected in this field is discussed, together with the properties of the critical velocities. The phenomenological equations of motion are given. The occurrence of mutual friction is a strong indication that \u3csup\u3e4\u3c/sup\u3eHe vortices play an important role in \u3csup\u3e3\u3c/sup\u3eHe–\u3csup\u3e4\u3c/sup\u3eHe hydrodynamics. From the equation of motion of quantized \u3csup\u3e4\u3c/sup\u3eHe vortices, the observed cubic velocity dependence of the \u3csup\u3e4\u3c/sup\u3eHe chemical potential difference is explained on purely dimensional grounds. A differential equation is given from which the temperature profile in a cylindrical tube in which \u3csup\u3e3\u3c/sup\u3eHe flows through superfluid \u3csup\u3e4\u3c/sup\u3eHe can be calculated.\u3c/p\u3

Counterflow pulse-tube refrigerators

The regenerators in standard pulse-tube refrigerators are large, heavy, expensive, and are a source of losses. In this contribution we avoid using regenerators by combining two pulse-tube refrigerators which operate in opposite phase. The regenerators are replaced by a counterflow heat exchanger. We treat the basic thermodynamic equations, make some design considerations, and report the results of some preliminary experiments. ©2002 American Institute of Physics

Stability of split Stirling refrigerators

In many thermal systems spontaneous mechanical oscillations are generated under the influence of large temperature gradients. Well-known examples are Taconis oscillations in liquid-helium cryostats and oscillations in thermoacoustic systems. In split Stirling refrigerators the compressor and the cold finger are connected by a flexible tube. The displacer in the cold head is suspended by a spring. Its motion is pneumatically driven by the pressure oscillations generated by the compressor. In this paper we give the basic dynamic equations of split Stirling refrigerators and investigate the possibility of spontaneous mechanical oscillations if a large temperature gradient develops in the cold finger, e.g. during or after cool down. These oscillations would be superimposed on the pressure oscillations of the compressor and could ruin the cooler performance

Thermodynamical aspects of pulse tubes II

In this paper, the dynamic behaviour of the temperature profiles in the regenerator and in the gas near the hot and cold ends of the tube are discussed. The operation of a single-orifice pulse tube is described in general thermodynamical terms. This approach is extended to the double-inlet pulse tube. Expressions for the various sources of entropy are derived which lead to a relation for the coefficient of performance. It is determined when the second inlet improves the operation of the pulse tube and what are the optimum settings

Prandtl number and thermoacoustic refrigerators

From kinetic gas theory, it is known that the Prandtl number for hard-sphere monatomic gases is 2/3. Lower values can e realized using gas mixtures of heavy and light monatomic gases. Prandtl numbers varying between 0.2 and 0.67 are obtained by using gas mixtures of helium-argon, helium-krypton, and helium-xenon, This paper presents the results of an experimental investigation into the effect of Prandtl number on the performance of a thermoacoustic refrigerator using gas mixtures. The measurements show that the performance of the refrigerator improves as the Prandtl number decreases. The lowest Prandtl number of 0.2, obtained with a mixture containing 30% xenon. leads to a coefficient of performance relative to Carnot which is 70% higher than with pure helium

The optimal stack spacing for thermoacoustic refrigeration

The characteristic pore dimension in the stack is an important parameter in the design of thermoacoustic refrigerators. A quantitative experimental investigation into the effect of the pore dimensions on the performance of thermoacoustic devices is reported. Parallel-plate stacks with a plate spacing varying between 0.15 and 0.7 mm are manufactured and measured. The performance measurements show that a plate spacing in the stack of 0.25 mm (2.5k) is optimum for the cooling power. A spacing of 0.4 mm (4k) leads to the lowest temperature. The optimum spacing for the performance is about 0.3 mm (3k). It is concluded that a plate spacing in the stack of about three times the penetration depth should be optimal (3k) for thermoacoustic refrigeration

Construction and performance of a thermoacoustic refrigerator

This paper deals with the construction and performance of a thermoacoustic refrigerator. The manufacturing of the different components of the apparatus will be explained along with the reasons for using specific materials. The setup consists of three major parts: The refrigerator which is contained in a vacuum vessel, the electronic apparatus necessary for the measurements and acquisition of the experimental data, and the gas-control panel which is used to fill and purge the system and to prepare gas mixtures. The system is assembled and the first measurements show a good behavior. A low temperature of -65 °C is achieved which is one of the lowest reported temperatures up to date