Finite heat-capacity effects in regenerators

This paper deals with the influence the finite heat capacity of the matrix of regenerators on the performance of cryocoolers. The dynamics of the various parameters is treated in the harmonic approximation focussing on the finite heat-capacity effects, real-gas effects, and heat conduction. It is assumed that the flow resistance is zero, that the heat contact between the gas and the matrix is perfect, and that there is no mass storage in the matrix. Based on an energy-flow analysis, the limiting temperature, temperature profiles in the regenerator, and cooling powers are calculated. The discussion refers to pulse-tube refrigerators, but it is equally relevant for Stirling coolers and GM-coolers.     

US Navy programme in small cryocoolers

If superconducting and cryogenic electronic instrumentation are to be deployed in future US Naval operational systems, there is a strong need for compact, highly reliable cryogenic refrigerators. Accordingly, several years ago, a programme was initiated to develop fractional-watt cryocoolers capable of operating below 10 K. Several varieties of Stirling coolers have been built and are under evaluation. In addition, helium gas compressors designed for use with small, closed cycle Joule- Thomson coolers are under development. An overview of the technical aspects of the programme are presented.     

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.     

太阳同步轨道辐射制冷技术的发展

辐射制冷装置由于具有寿命长、无机械振动、无电磁干扰、本身无功耗等非常适合于空间应用的一系列特点、优点，因而得到了广泛的应用。随着各国空间项目的实施，辐射制冷技术得到了长足的发展，并且逐渐成熟。迄今为止，这种类型的制冷器在空间应用中还是最广泛和最实用的。     

Thermal conductance characterization of a pressed copper rope strap between 0.13 K and 10 K

Mechanically pressing the ends of a copper braid in solid copper is an effective way of constructing solderless conductive straps for cryogenic applications. In this paper we present thermal conductance data of such a copper strap measured using the two-heater one-thermometer method. The measurements span a wide temperature range of 0.13–10 K applicable to a variety of cryogenic systems employing liquid helium, pulse tube coolers, adiabatic demagnetization refrigerators, and others. Above ≈1.5 K, the braid thermal conductivity dominates the strap conductance resulting in a near-linear dependence with temperature. The variation with temperature below ≈1.5 K is near-quadratic indicating dominance of the pressed contact conductance at the strap ends. Electron-beam welding the braid to the strap ends is shown to be a promising solution for improving sub-Kelvin thermal conductance of the strap.     

Application of hydrocarbon mixtures in small refrigerating and cryogenic machines

Application of hydrocarbon mixtures enables the creation of simple, reliable and durable refrigerating and cryogenic Joule–Thomson micro coolers for the temperature range of −73 to −183 °C. The temperature, thermal, power and hydraulic performances of a series of prototypes are presented. The results of tests demonstrate that small, single stage, sealed, lubricated compressors can be applied to these purposes. The start up and steady operation hydraulic performance of those machines are quite similar to the performance of domestic refrigerators. The last, together with the fact that in the studied micro coolers the lubricated compressors are used at temperatures down to −183 °C, ensures a large resource of operation. That is just the reason that holds out a hope for prospects in a broad field of applications for the studied prototypes, despite their lower power performances in contrast to gas micro coolers.     

The multiple mixing chamber

This paper deals with the multiple mixing chamber in which He³, circulating in a dilution refrigerator, is diluted in steps.The double mixing chamber in the stationary and in the dynamic state is described and the important design features are given. The triple mixing chamber is also discussed. A comparison is made between the performance of dilution refrigerators operating with a single mixing chamber and with a double mixing chamber.     

Cryocoolers near their low-temperature limit

This paper analyses the recently-observed temperature–time dependence in a GM-cooler near its low-temperature limit. The paper mainly focusses on GM-coolers with ⁴He as the working fluid, but some attention is also paid to pulse-tube refrigerators (PTR’s) using ³He and many features of the treatment equally apply to Stirling coolers. Ample attention is paid to the thermodynamics of the cycle by considering the isentropes in the Tp-diagrams of ⁴He and ³He. The role of the line, where the thermal expansion coefficient is zero, is emphasized. Some fundamental thermodynamic relationships are derived.     

A review of pulse tube refrigeration

The pulse tube belongs to the class of miniature cryogenic refrigerators usually referred to as cryocoolers. In common with Stirling and Gifford-McMahon machines, operation depends on a regenerative gas expansion cycle but unlike these coolers the pulse tube has no moving parts at low temperature and hence offers the potential for high reliability. Although comparisons may be drawn between the operation of a pulse tube and that of a Stirling cooler, the exact nature of the working cycle is far from clear and the device continues to intrigue. We provide here an introduction to the device and attempt to explain it's operation as a conventional second law system.     

Effects of reservoir volume on performance of pulse tube cooler

A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.     

跨临界二氧化碳热泵热水系统气冷器的仿真分析

建立了跨临界二氧化碳热泵热水系统中的气体冷却器模型，对管内二氧化碳和水侧的流动和传热进行了数值仿真；并运用该模型分析了系统运行时各有关参数对换热器性能的影响，并结合最优排气压力，研究使系统高效运行的方法，为气冷器的优化设计提供了基础。     

空间辐冷器耦合因子计算软件的开发

介绍了空间辐冷器耦合因子计算软件的开发原理、相应流程及运行结果，对于具有复杂表面的辐冷器，蒙特卡洛法是进行耦合因子计算的有效方法。基于上述原理所开发的计算软件，具有收敛快、精度高的优点，达到了实用要求。     

Performance optimization of irreversible refrigerators based on a new thermo-ecological criterion

A performance analysis and optimization based on a new thermo-ecological optimization criterion has been carried out for refrigerators. The ecological objective function is defined as the ratio of the cooling load to the loss rate of availability (or entropy generation rate). The maximum of the ecological performance criterion and the corresponding optimal conditions have been derived analytically. The optimum performance parameters which maximize the objective function have been investigated and the effects of irreversibility parameters on the general and optimal performances are discussed detailed. The obtained results may provide a general theoretical tool for the ecological design of refrigerators.     

Performance optimization of an irreversible Carnot refrigerator with finite mass flow rate

The present study develops a theoretical model for the optimization of an irreversible Carnot refrigerator subject to a constraint of finite mass flow rate, which includes internal as well as external irreversibilities. By introducing two dimensionless parameters as indicative of the mass flow rates for the refrigerator, the new model allows detailed analyses on the finite mass flow rate allocation problem of working fluids among the hot- and cold-side heat exchangers of Carnot refrigerators. The analytical solutions of the maximum coefficient of performance (COP) for irreversible Carnot refrigerators are obtained under the equivalent of the finite-flow rate constraint. Furthermore, the influences of major parameters on the maximum COP and the corresponding mass flow rate allocation are examined and shown by numerical examples. The obtained results may provide a theoretical guidance for the optimal design and operation of real refrigerators.     

Visualization of refrigerant flow at the capillary tube inlet of a high-efficiency household refrigerator

The subcooled condition at the condenser outlet ensures complete condensation, which is necessary in vapor compression systems to increase the cooling capacity and ensure the liquid conditions at the expansion device inlet. However, in household refrigerators, recent works indicate the presence of two-phase flow at the capillary tube inlet. These systems behave quite differently from other refrigeration systems due to the extremely low capacity. In the present work, a test bench was built to visualize the refrigerant flow at the condenser outlet and at the capillary tube inlet of a commercial household refrigerator. A transparent tube replaced the end of the condenser and three transparent filters were installed with different orientations. Different positions of the capillary tube within the filters were also tested. Despite measuring a certain subcooling, all the reported visualizations showed that the capillary tube was steadily drawing in two-phase flow.     

Measurement of the heat transfer characteristics of gas flow in fine channel heat exchangers used for microminiature refrigerators

The heat transfer characteristics have been measured for the flow of gas in the fine channels of the heat exchangers used for microminiature J-T refrigerators. The test channels were fabricated using a photolithographic technique similar to that used for the fabrication of the refrigerators. The unique features of such channels are their asymmetric roughness, their large relative roughness and a large variation of the heat flux and temperature over the walls of the channels. The tests involved both laminar and turbulent flow regimes. The results we have obtained are useful for designing heat exchangers in microminiature refrigerators.     

Enthalpy flow rate of a pulse tube in pulse tube refrigerator

An integration formula of enthalpy flow rate along a pulse tube in pulse tube refrigerators is described on the assumption of sinusoidal mass flow rate and sinusoidal pressure fluctuation. For ideal double inlet and ideal orifice pulse tube with helium as working medium, it is simplified to a polynomial formula. Polynomial formulas for roughly evaluating the volume of the pulse tube in ideal double inlet and ideal orifice pulse tube refrigerators are also given.     

Cryogen-free cryostat for neutron scattering sample environment

Today all advanced neutron facilities maintain a fleet of Orange cryostats, or similar systems, to provide low temperature sample environment in neutron scattering experiments. However recent liquid helium cost increases, caused by global helium supply problems, have raised significant concern about the affordability of such cryostats. The ISIS facility is carrying out a development programme intended to substitute conventional cryostats with cryogen-free systems preferably based on pulse tube refrigerators. The main aim of the development is to create a cryogen-free system as a potential replacement for the conventional Orange cryostat. This paper describes the design and test results of a cryogen-free cryostat, based on a pulse tube refrigerator, with 50 mm diameter top-loading sample facilities for neutron scattering experiments. The sample temperature range is 1.45–300 K in the continuous flow regime. The cryostat may also be used with ultra-low temperature dilution refrigerator inserts.     

An empirical model for sizing capillary tubes

This paper presents an empirical model that has been developed to size adiabatic and non-adiabatic capillary tubes for small vapour compression refrigeration systems, in particular, household refrigerators and freezers. The model is based on the assumption that the length of a capillary tube is dependent on five primary variables, namely the capillary tube inner diameter, the mass flow rate of the refrigerant in the capillary tube, the pressure difference between highside and lowside, the refrigerant subcooling at capillary inlet and the relative roughness of the capillary tube material. The model is validated with previous studies over a range of operating conditions and is found to agree reasonably well with the experimental data for HFC134a.