de Atam Fons Waele

Design of thermoacoustic refrigerators

In this paper the design of thermoacoustic refrigerators, using the linear thermoacoustic theory, is described. Due to the large number of parameters, a choice of some parameters along with dimensionless independent variables will be introduced. The design strategy described in this paper is a guide for the design and development of thermoacoustic coolers. The optimization of the different parts of the refrigerator will be discussed, and criteria will be given to obtain an optimal system.

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 (2.5 deltak) is optimum for the cooling power. A spacing of 0.4 mm (4 deltak) leads to the lowest temperature. The optimum spacing for the performance is about 0.3 mm (3 deltak). It is concluded that a plate spacing in the stack of about three times the penetration depth should be optimal (3 deltak) 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.

A pulse tube refrigerator below 2 K

Up to now, all pulse tube refrigerators operating at the liquid helium temperature range use 4He as the working fluid. However, the lambda transition of 4He is a barrier for reaching temperatures below 2 K. Theoretical analysis in this paper shows that, using 3He, the temperature limit is below 2 K, and the efficiency of a 4 K pulse tube refrigerator can be improved significantly. A three-stage pulse tube refrigerator is constructed. A compressor with input power of 4 kW and a rotary valve are used to generate the pressure oscillations. With 4He, a minimum average temperature of 2.19 K was reached. Replacing 4He by 3He, at the same valve settings and operating parameters, the minimum average temperature goes down to 1.87 K and the cooling power at 4.2 K is enhanced about 60%. After fine tuning of the valves, a minimum average temperature of 1.78 K was obtained. This is the lowest temperature achieved by mechanical refrigerators.

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 be 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.

Spinglass behaviour of Zn1−xMnxSe and Zn1−xMnxTe semimagnetic semiconductors

Abstract The magnetic susceptibility of Zn1−xMnxSe (0.02 ≤ × ≤ 0.53) and Zn1−xMnxTe (0.07 ≤ × ≤ 0.21) was investigated in the temperature range 10 mK

Regenerator Dynamics in the Harmonic Approximation

We extend our previous work on the dynamics of regenerators in the harmonic approximation by including the enthalpy flow and nonideal gas behavior. The derivation is based on the energy conservation of the matrix and of the gas, the flow resistance of the matrix, and mass conservation. The model is one-dimensional. Expressions are derived for the gradients of the regenerator parameters which can be integrated directly from the boundary conditions. It is not necessary to assume an initial temperature profile. This provides a very fast and convenient tool for designing cryocoolers such as pulse-tube refrigerators.

Thermodynamic properties of liquid 3He-4He mixtures at zero pressure for temperatures below 250 mK and 3He concentrations below 8%

We calculated the thermodynamic quantities of dilute liquid 3He-4He mixtures, starting from experimental values of the specific heat and the osmotic pressure. The calculations are confined to temperatures below 250 mK and 3He concentrations below 8% at zero pressure. Some results are especially useful for dilution refrigeration. Contrary to the calculations previously performed by Radebaugh, our results are in good agreement with the experimental date on both the osmotic pressure and the osmotic enthalpy.

A gas-spring system for optimizing loudspeakers in thermoacoustic refrigerators

Moving-coil loudspeakers are appropriate drivers for thermoacoustic refrigeration. They are cheap, commercially available, compact, light, and can be adapted to meet specific requirements. This paper deals with the optimization of loudspeakers for thermoacoustic refrigeration. Using an electrical model that describes the refrigerator, it is concluded that the electroacoustic efficiency can be maximized over a wider frequency range by matching the mechanical resonance frequency of the driver to the acoustic resonance frequency of the resonator. A gas-spring system is introduced as a practical tool to shift the mechanical resonance frequency of the driver. An electroacoustic efficiency of 35% is obtained when the mechanical resonance frequency of the driver and the acoustic resonance frequency are equal. Additionally, the efficiency is constant over a relatively wide frequency range. This has advantages for thermoacoustic refrigeration. During cool-down, the operating acoustic frequency decreases so that the refrigerator will keep near the optimum performance.

Optimization of pulse tubes

The coefficient of performance (COP) of pulse tube coolers is mainly determined by irreversible processes in the regenerator. In this paper, a general and basic definition for the efficiency of regenerators is proposed. The efficiency of the regenerator, and consequently of the cooler as a whole, is affected by the system which is used to control the pressure in the tube. For a general pressure-wave form, it is shown that in the optimum situation, under certain conditions, the variation of the pressure in the tube is proportional to the variation of the pressure in the compressor. In that case the COP is independent of the form of the pressure wave.