Model for transient behavior of pulse tube cryocooler

This article describes an investigation of the transient behavior of a small (2.0 W at 85 K) pulse tube cryocooler operating at 120 Hz with an average pressure of 3.5 MPa, capable of relatively fast cool-down from ambient to about 60 K. In a series of experiments, the cold end temperature was measured as a function of time in a complete cool-down and subsequent warm-up cycle, with no heat load and different quantities of excess mass at the cold end. A transient heat transfer model was developed, that considers the effects of the cooling power extracted at the cold end and that of the heat gain at the warm end on the cool-down time. The heat gain factor was calculated from warm-up data, and found to be approximately the same for all experiments. Using the same model with cool-down data enables a determination of both the gross and net cooling power as functions of time, but more importantly – as functions of the cold end temperature. An expression was derived for the cold end temperature as a function of time for any amount of excess mass, including zero. The cool-down time of the “lean” cryocooler (with no excess mass) was found to be less than 50 s.This cool-down/warm-up method for evaluating the cooling power of a cryocooler seems simpler than steady-state experiments with a heater simulating load at the cold end. Use of the heat transfer model with data from one or two good experiments conducted in the above manner, can yield both the gross and net cooling powers of a cryocooler as functions of the cold end temperature, and allow the determination of cool-down time with any amount of excess thermal mass. While the net cooling power during cool-down differs somewhat from that under steady-state operation, the former can serve as a good measure for the latter.     

Characteristics of the double inlet pulse tube

The performance of the double inlet pulse tube (DIPT) is analyzed using a linearized model that takes account of the void volume of the regenerator. The maximum rate of refrigeration obtainable with the regenerator is determined as a function of frequency and void volume. This rate can be achieved by a DIPT with infinitely large reservoir volume. Corrections resulting from a finite reservoir volume are important only at low frequency. The coefficient of performance of a DIPT with optimized rate of refrigeration is less than half of the thermodynamic maximum. The results obtained for the DIPT are compared with corresponding results for the optimized orifice pulse tube refrigerator (OPTR). The large improvements in performance obtained with the DIPT over the OPTR are due primarily to an increase in the pulse tube pressure. The maximum rate of refrigeration decreases as the temperature at the cold side decreases. This is caused primarily from the resulting decrease in cold side flow rate. At given temperature ratio, addition of the second inlet reduces the flow rate through the regenerator over a range of intermediate frequencies.     

Vibration analysis of cryocoolers

The vibrations of Gifford-McMahon (GM) and pulse-tube (PT) cryocoolers were measured and analyzed. The vibrations of the cold-stage and cold-head were measured separately to investigate their vibration mechanisms. The measurements were performed while maintaining the thermal conditions of the cryocoolers at a steady state. We found that the vibration of the cold-head for the 4 K PT cryocooler was two orders of magnitude smaller than that of the 4 K GM cryocooler. On the other hand, the vibration of the cold-stages for both cryocoolers was of the same order of magnitude. From a spectral analysis of the vibrations and a simulation, we concluded that the vibration of the cold-stage is caused by an elastic deformation of the pulse tubes (or cylinders) due to the pressure oscillation of the working gas.     

低温制冷机冷头换热器设计与分析

提出了一种低温制冷机冷头换热器新的结构形式,通过有限元软件分析计算,获得了换热器的温度场分布,并经过分析提出了改进方法,得到改进后的冷头换热器结构形式,通过有限元分析计算验证了改进后的换热器可以满足应用要求,证明了换热器结构的合理性。     

Low-frequency magnetic interference in a SQUID gradiometer caused by a Cryotiger gas-mixture cooler

A Cryotiger^® gas-mixture cooler was applied for cooling of three high-T_c SQUID magnetometers. These SQUID magnetometers were mounted on an alumina holder in an axial gradiometer configuration. From 20 Hz upward, the system noise was about 0.1 pT/√Hz. Below this frequency, the noise gradually increased to a level of 10 pT/√Hz at 1 Hz. This low-frequency excess noise appeared to be due to remnant magnetization of the Cryotiger cold head. Movement of magnetic cold-head parts with respect to the SQUIDs are induced by pressure fluctuations in the heat exchanger lines. By using one SQUID as a reference for the cooler noise, a first-order gradiometer can be formed in which the cooler noise is eliminated. To establish a proper second-order gradiometer either a fourth SQUID has to be added, or the spatial separation between cold head and SQUIDs has to be increased significantly.     

Cryocooler reliability issues for space applications

Analyzing cryocooler reliability has been problematic from the beginning. Classic reliability analyses rely on statistical sampling and comparing failure modes to other similar systems where statistical results are available. These approaches do not apply to cryocoolers, particularly cryocoolers for aerospace applications. The industry has not built enough total cryocoolers, let alone a single type of cooler, to provide any meaningful statistical sample. This forces us to rely on comparing failure modes of similar systems to that of the cryocooler, which leads to the next problem; today's aerospace cryocooler is designed to have no failure modes. What can it be compared to? Any classic reliability study performed on a cryocooler makes several critical assumptions that completely dominate the results. Change the assumptions and you get a different answer; the results are dictated by the assumptions not the hardware. There are no easy answers to these problems. This paper attempts to show why classic reliability studies do not apply to cryocoolers and that as an industry we must work together to show that all cryocoolers have high levels of reliability.     

The design and fabrication of a reverse Brayton cycle cryocooler system for the high temperature superconductivity cable cooling

A high temperature superconductivity cable must be cooled below the nitrogen liquefaction temperature to apply the cable to power generation and transmission systems under superconducting state. To maintain the superconducting state, a reliable cryocooler system is also required. The design and fabrication of a cryocooler system have been performed with a reverse Brayton cycle using neon gas as a refrigerant. The system consists of a compressor, a recuperator, a cold-box, and control valves. The design of the system is made to have 1 kW cooling capacity. The heat loss through multilayer insulators is calculated. Conduction heat loss is about 7 W through valves and access ports and radiation heat loss is about 18 W on the surface of a cryocooler. The design factors are discussed in detail.     

逆布雷顿循环微型低温制冷机

在目前广泛采用的低温制冷循环中,以透平机械为主的逆布雷顿循环制冷机有望能够满足卫星,宇宙飞船等航天器上低冷量,高性能,高效率的特殊要求,成为空间制冷的一种选择,总结了近几年空间用逆布雷顿循环制冷机的研究成果,表明由ＤＣ－ＡＣ转换器,微型离心式压缩机,微型透平膨胀机及高效紧凑式换热器组成的微型逆布雷顿循环制冷机是一种有前途的微低温制冷机。     

液氮作动力的VM制冷机的理论分析及实验研究

在采用回热器整机模型进行分析计算的基础上，对液氮作动力的ＶＭ制冷机在结构和工作参数上进行了优化。达到零负荷温度１０．５５Ｋ，在１５Ｋ时有２．３６Ｗ制冷量输出，液氮消耗约为３Ｌ／ｈ，并给出了制冷量Ｑｃ同冷头温度Ｔｃｗ，及１５Ｋ时制冷量Ｑｃ１５同两个推移活塞的相角θ、转速ｎ、平均工作压力Ｐａｖ及比体积γ（ＶＨＯ／ＶＣＯ）等实验和计算结果。另外还进行了四组不同的冷端回热器填料结构的实验及分析计算。     

Efficient helium recondensing using a 4 K pulse tube cryocooler

This paper introduces helium recondensing in a 4000 l dewar using a 4 K pulse tube cryocooler at Amundsen-Scott research station at the South Pole. The helium dewar has a normal boil-off rate of 14 l/day. Two features of cooling the dewar neck by helium vapor and precooling helium gas to be liquefied ensured high efficiency of the pulse tube recondenser in this application. The liquefier/recondenser has being successfully operating in the dewar at South Pole station since February 2005. It not only maintains zero boil-off of the dewar, but also liquefies helium gas supplied from outside of the dewar with a rate around 2.7 l/day.     

Theoretical study on a Miniature Joule-Thomson & Bernoulli Cryocooler

In this paper, a microchannel-based cryocooler consisting of a compressor, a recuperator and a cold heat exchanger has been developed to study the feasibility of cryogenic cooling by the use of Joule-Thomson effect and Bernoulli effect. A set of governing equations including Bernoulli equations and energy equations are introduced and the performance of the cooler is calculated. The influences of some working conditions and structure parameters on the performance of coolers are discussed in details.     

A pulse-tube refrigerator using variable-resistance orifice

In the present study, we propose a new design of orifice pulse-tube refrigerator (VROPT) using a variable-resistance valve to replace the conventional orifice. The variable-resistance orifice (VRO) is basically a high-speed solenoidal valve similar to the fuel jet device widely used in automobile engines. By changing the frequency and periods of ON and OFF of the valve through an electronic device, we can change the flow resistance of the VRO. This thus provides a possibility for an OPT to be controlled on-line during operation. From the results obtained in the present study, we have shown that VROPT is able to achieve on-line control by regulating the duty cycle d or frequency f_v of the VRO. We also show that VROPT will not loss its thermal performance as compared to conventional OPT.     

空间机械制冷机与红外探测器的耦合技术

在空间红外遥感中，红外探测器是通过与制冷机的耦合获取冷量的。着重讨论为满足红外探测的需要，如何最大限度地消除空间的振动、电磁干扰等不利因素的影响。同时介绍降低漏热，提高冷量传输效率，从而保证制冷机与红外探测器良好耦合的技术。     

Analysis of DC gas flow in GM type double inlet pulse tube refrigerators

In a GM type double inlet pulse tube refrigerator, a DC gas flow is an intrinsic phenomenon. It is important to understand the characteristics of the DC gas flow. In this paper, the relation between the DC gas flow, valve operating time intervals, and flow patterns in the bypass of the GM type double inlet pulse tube refrigerator is studied with a numerical simulation when a symmetric bypass is used. The governing equations of the numerical simulation based on the nodal analysis are discretized with an implicit finite volume method. The simulation result shows that the valve opening angle difference is the main parameter having influence on the DC gas flow, and the effect depends on the flow patterns in the bypass.