Multiple-layer Insulation (MLI) is essential for cryogenics. Without MLI, it is impossible to attain cryogenic temperatures. MLI is one of vacuum thermal insulations that requires a high vacuum to obtain enough performance. MLI working process is important to get high thermal insulation performance of a cryogenic system/device. In order to attain and maintain a good vacuum for MLI, a baking process is required to reduce the Out gas. Reduction of the Out gas is extremely important for maintaining a good insulation vacuum.
The regenerator is an indispensable regenerative heat exchanger in Stirling cycle machines, G-M cryocoolers and pulse tube cryocoolers. Its existence is a key factor in enabling these machines to have the highest thermal efficiency and coefficient of performance. A regenerator is composed of a matrix and housing. The matrices are often made of stacks of fine wire mesh screens. However, an optimum balance between flow loss and heat transfer characteristics is required. In this report, based on the theoretical studies of thermal behavior in wire elements and matrices, relationships between design factors are obtained. An approach for designing regenerator used in Stirling cryocoolers is presented.
Regenerative cryocoolers widely used for advanced applications, such as sensor cooling, superconducting systems and cryopumps, consist mainly of a compressor, regenerator and expander. A working fluid (generally helium gas) flows periodically in these main parts. The regenerator that is located between the compressor and expander is operated in a wide, lowtemperature range from 77 to 4 K, and a wide frequency range from one to a few hundred hertz. The regenerator is one of the most essential parts because its efficiency is directly linked to the performance of the cryocoolers. Therefore, understanding the properties of regenerators (including regenerator material) and helium are important. In this paper, the thermodynamic efficiency of a regenerator, and the temperature dependence of the specific heat of regenerator materials and pressurized helium are presented. Furthermore, several types of simulated temperature and specific heat (heat capacity) distributions within a 4 K regenerator are shown to clarify the intriguing effect of magnetic regenerator materials.
We introduce the basic characteristics of a high-porosity sintered metal fiber (SMF) matrix for the regenerator that plays a key role as a thermal storage regenerative heat exchanger in Stirling cycle machines. We measured the basic characteristics of flow resistance and heat transfer in a comparison with a stacked wire-mesh matrix. The results of our experiment show that the heat transfer characteristics of a SMF matrix is equivalent to that of a stacked wire-mesh matrix, whereas the pressure drop in a SMF matrix is lower than that in a stacked wire-mesh matrix under the condition of equivalent specific surface area. Using the measurement data, we obtained the relational expression between the Nusselt and Reynolds numbers, and between the friction factor and Reynolds number for each porosity level of the SMF matrix. The estimation of COP using a simple Schmidt model shows the positive performance of the SMF matrix.
A simple numerical method to calculate the performance of a stacked-screen regenerator is proposed. The method is based on a well-known thermoacoustic theory but uses some empirical factors that were determined by the measurement of viscous losses in stacked-screen regenerators. The efficiency of the energy conversion in a stacked-screen regenerator is calculated using the method and compared with the experimental results. Furthermore,a comparison of the numerically calculated efficiencies in stack-screen and tube-array regenerators is performed.