Performance characteristics of pulse tube refrigerators

In the present study experiments were carried out to investigate the performance characteristics of pulse tube refrigerators. It was found that the cool-down time t_c during the transient or start-up period is dominated by the time constant of the pulse tube wall τ_pt and that the dynamics of a basic pulse tube (BPT) refrigerator approaches that of a first-order system. For steady state operation, the cold-end temperature T_L was found to vary with τ_pt, and the cooling load Q_L increases monotonically with increasing τ_pt. This indicates that heat pumped by the gas from the cold to the hot end increases with decreasing h_pt (i.e. less energy exchange between the gas and wall). The process of heat storage or release of the pulse tube wall is thus shown to have a negative effect on the performance of a BPT refrigerator. It was thus found experimentally that the gas compression/expansion process inside the pulse tube, which is similar to a Brayton cycle but lies between isothermal and adiabatic, can explain the performance of BPT refrigerators. The present experiment also shows that the performance of a pulse tube refrigerator at transient and steady states is mainly dominated by the time constant of the pulse tube wall τ_pt.     

Multi-dimensional flow effects in pulse tube refrigerators

Pulse tube cryocoolers are often modeled as one-dimensional flow fields. We examine the adequacy of this assumption in this study. Two entire inertance tube pulse tube refrigerator (ITPTR) systems operating under a variety of thermal boundary conditions are modeled using a computational fluid dynamics (CFD) code. Each simulated ITPTRs includes a compressor, an after cooler, a regenerator, a pulse tube, cold and hot heat exchangers, an inertance tube, and a reservoir, and the simulations represent fully coupled systems operating in steady-periodic mode. The objectives are to ascertain the suitability of CFD methods for ITPTRs, and examine the extent of multi-dimensional flow effects in various ITPTR components. The results confirm that CFD simulations are capable of elucidating complex periodic processes in ITPTRs. The results also show that one-dimensional modeling is appropriate only when all the components in the system have large length-to-diameter (L/D) ratios. Significant multi-dimensional flow effects occur at the vicinity of component-to-component junctions, and secondary-flow recirculation patterns develop when one or more components have relatively small L/D ratios. Parameters in need of experimental measurement are discussed.     

Acoustic streaming in pulse tube refrigerators: tapered pulse tubes

Acoustic streaming is investigated in tapered tubes with axially varying temperature, in the boundary layer limit. By appropriately shaping the tube, the streaming can be eliminated. Experimental data demonstrate that an orifice pulse tube refrigerator with a conical pulse tube whose cone angle eliminates streaming has more cooling power than one with either a cylindrical pulse tube or a conical pulse tube with twice the optimum cone angle.     

Investigation on the internal thermal link of pulse tube refrigerators

Within a pulse tube refrigerator (PTR) in coaxial configuration the pulse tube is located inside the regenerator matrix in axial direction. An internal thermal contact between these two main components of the coldfinger occurs. The experimental investigation of the direction and the quantity of transferred heat is in focus of this paper. Intermediate cooling of the regenerator by the corresponding part of its own pulse tube can improve the cooling performance of a PTR. Therefore, a well-adapted geometrical arrangement between the pulse tube and the regenerator is essential, considering the temperature distribution inside the coldfinger. We deduce design parameters to optimise the configuration of coaxial PTRs.     

Performance analysis and optimization of high capacity pulse tube refrigerator

High capacity pulse tube refrigerator (HCPTR) is a new generation of cryocoolers tailored to provide more than 250 W of cooling power at cryogenic temperatures. The most important characteristics of HCPTR when compared to other types of pulse tube refrigerators are a powerful pressure wave generator, and an accurate design. In this paper the influence of geometrical and operating parameters on the performance of a double inlet pulse tube refrigerator (DIPTR) is studied. The model is validated with the existing experimental data. As a result of this optimization, a new configuration of HCPTR is proposed. This configuration provides 335 W at 80 K cold end temperature with a frequency of 50 Hz and COP of 0.05.     

A thermodynamic model based on exergy flow for analysis and optimization of pulse tube refrigerators

A thermodynamic model based on exergy flow through pulse tube refrigerators (PTRs) is developed. An exergetic efficiency parameter representing the losses in the pulse tube itself is proposed. The effects of control parameters representing a general phase shifter and their effect on the system performance are discussed. Analytical solutions representing important parameters in the design of PTRs such as the load curve, cooling power and efficiency in terms of basic system input parameters are developed. It is shown that the analytical model is powerful and convenient for optimization of PTRs and in quantifying its operational bound and important losses. Results indicating a compromise between cooling power and efficiency in PTRs under certain conditions are presented and discussed.     

Design and prototyping of a large capacity high frequency pulse tube

This document describes the design and the prototyping performed at CEA/SBT in partnership with AIR LIQUIDE of a high frequency large cooling power pulse tube. Driven at 58 Hz by a 7.5 kW flexure bearing pressure wave generator, this system provides a net heat lift of 210 W at 65 K. The phase shift is obtained by an inertance and a buffer volume. This type of cryogenic cooler can be used for on site gas liquefaction or drilling site and for high temperature superconductivity power device cooling (transmission lines, large generators, fault current limiters).In this paper, we focus on two essential points, the regenerator and the flow straightener. The regenerator is a key component for good performance of the pulse tube cooler. It must have a large thermal inertia, a low dead volume, a good heat transfer gas/matrix and at the same time, small pressure drop. In the present case and unlike typical moderate cooling power pulse tubes, the regenerator is very compact. However, the resulting conductive losses remain negligible compared to the cooling power targeted. The goal of the flow straightener is to avoid as much as possible any jet stream effect and to guarantee the uniformity of the velocity field at both ends of the pulse tube. Indeed multi-dimensional flow effects can significantly impact the performances of the machine.     

A high capacity completely closed-cycle 250 mK He refrigeration system based on a pulse tube cooler

We present the design of a completely closed-cycle refrigeration system. The system consists of four stages; the first two, 40.5 and 2.7 K, are provided by a pulse tube cooler, the third and fourth stages by helium adsorption refrigerators. The adsorption refrigerators use ⁴He and ³He with base temperatures of 700 and 250 mK, respectively. Performance tests including some with the cooler rotated from the vertical showed that with a load of 15 μK, a temperature below 286 mK can be maintained for 72 h.     

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.     

Experimental study of staging method for two-stage pulse tube refrigerators for liquid He temperatures

The first two-stage pulse tube refrigerator, providing a lowest temperature of 2.23 K and a cooling power of 370 mW at 4.2 K, employed a parallel arrangement of the two pulse tubes with phase shifters located at room temperature¹. With the aim of increasing the COP at liquid ⁴He temperatures, three modified staging methods were tested in this paper. All refrigerator versions operate with the same two regenerators as already used in the first two-stage setup¹ and also the same 6 kW He-compressor combined with a redesigned G-M rotary valve. The best performance is achieved with a parallel arrangement two-stage refrigerator by introducing proper negative DC flow and impedance tubes. So far the highest cooling power achieved on the second stage at 4.2 K was 0.5 W. With a heat load of 20 W at 67 K on the first stage, the second stage can provide a cooling power of 0.42 W at 4.2 K. Details of the design of the different refrigerators and a comparison of their performance are presented.     

Performance modeling of AMR refrigerators

A system modeling approach for predicting the performance of active magnetic regenerators using a one-phase approximation is presented. The approach is described for an arbitrary AMR device independent of the magnetic refrigerant, thermal losses or magnetic waveform. A general expression for magnetic work is derived which can be used for cycles where the low-field intensity is not zero. Additionally, a means of treating the varying magnetic field waveform as a single high and low field is described. The model is applied to a permanent magnet magnetic refrigerator using water–glycol as the heat transfer fluid. Simulated results are compared to experimental data which vary by heat load, frequency and utilization. A sensitivity analysis is performed using utilization, adiabatic temperature change, effective conductivity and particle size as independent variables. Comparisons to experimental data show that reducing the calculated magnetocaloric effect by 25% provides good agreement between simulations and experimental results.     

Performance modeling of optical refrigerators

Optical refrigeration using anti-Stokes fluorescence in solids has several advantages over more conventional techniques including low mass, low volume, low cost and no vibration. It also has the potential of allowing miniature cryocoolers on the scale of a few cubic centimeters. It has been the topic of analysis and experimental work by several organizations. In 2003, we demonstrated the first optical refrigerator.We have developed a comprehensive system-level performance model of optical refrigerators. Our current version models the refrigeration cycle based on the fluorescent material emission and absorption data at ambient and reduced temperature for the Ytterbium–ZBLAN glass (Yb:ZBLAN) cooling material. It also includes the heat transfer into the refrigerator cooling assembly due to radiation and conduction.In this paper, we report on modeling results which reveal the interplay between size, power input, and cooling load. This interplay results in practical size limitations using Yb:ZBLAN.     

A new computational model for entire pulse tube refrigerators: Model description and numerical validation

In present paper, a new modeling approach for the performance of pulse tube refrigerator is proposed. The new approach combines one-dimensional and two-dimensional models (1-D and 2-DCC model) together, and can be used to simulate the fluid flow and heat transfer processes of the basic type, orifice type and double-inlet type pulse tube refrigerators (PTRs). With the present model, the complicated fluid flow and heat transfer characteristics in the PTR system can be efficiently depicted and the computational time can be greatly reduced. Then based on the approach, the distribution characteristics of the flow and temperature fields of the three types of PTR are numerically analyzed. The complicated fluid flow and heat transfer phenomena in PTR, such as DC flow, velocity and temperature annular effects are presented vividly. The numerical results show that the 1-D and 2-DCC model is reliable and practical, which can be used to explore the physical mechanism of the thermodynamic processes of the PTR system and optimize the design of the PTR system and its components.     

大冷量高频脉冲管制冷机重力影响的理论分析及实验研究

通过对重力场中脉冲管管内部的流体力学方程进行化简和无量纲化,提出了影响脉冲管制冷机重力特性的无量纲数(ρgl/Pa),并对大冷量高频脉冲管的重力特性进行试验研究.试验结果表明:提高输入功率、减小惯性管的尺寸、减小充气压力、减小脉冲管的尺寸以及提高脉冲管制冷机的运行温区能够改变无量纲数(ρgl/Pa)的各个参数从而减小无量纲数(ρgl/Pa)的大小,进而减弱重力方向性的影响.     

3He/4He dilution refrigerator with high cooling capacity and direct pulse tube pre-cooling

In the article, a ³He/⁴He dilution refrigerator (DR) is described which is pre-cooled by a commercial two-stage pulse tube refrigerator (PTR); cryo-liquids are not necessary with this type of milli-kelvin refrigerator. The simple design of the condensation stage of this so-called dry DR is novel and explained in detail. In most dry DRs the circulating ³He gas is cooled by a two-stage PTR to a temperature of about 4 K. In the next cooling step, the ³He flow is cooled and partially liquefied in a Joule–Thomson circuit, before it is run to the dilution refrigeration unit. The counterflow heat exchanger of the Joule–Thomson circuit is cooled by the cold ³He gas pumped from the still of the DR. In the DR described here, the heat exchanger of the Joule–Thomson stage was omitted entirely; in the present design, the ³He gas is cooled by the PTR in three different heat exchangers, with the first one mounted on the first stage of the PTR, the second one on the regenerator of the second stage, and the third one on the cold end of the second stage. The heat load caused by the ³He flow is mostly absorbed by the first two heat exchangers. Thus the ³He flow presents only a small heat load to the second stage of the PTR, which therefore operates close to its base temperature of 2.5 K at all times. A pre-cooling temperature of 2.5 K of the ³He flow is sufficiently low to run a DR without further pre-cooling. The simplified condensation system allows for a shorter, compacter and more economical design of the DR. Additionally, the pumping speed of the turbo pump is no longer obstructed by the counterflow heat exchanger of the Joule Thomson stage as in our earlier DR design.     

高频脉冲管制冷机特性分析

通过相位理论分析了提高频率可以减小制冷机体积的原理,通过脉冲管制冷机内部损失影响分析了实现微型高效脉冲管制冷机的方法.研究表明:在现有的50 Hz高频下,减小制冷机体积会导致效率降低,提高运行频率,提高充气压力并采用更小水力直径的回热器填料,可以在减小体积的同时获得高效的制冷机.     

Development of high efficiency Stirling-type pulse tube cryocoolers

Thermoacoustic theory is a powerful tool to understand the working mechanism of regenerative thermodynamic systems. In this paper, a modified thermoacoustic model is employed to design three single-stage Stirling-type pulse tube cryocoolers. The first one (PTC-10) is designed with in-line configuration and the second one (CPTC-10) is designed with co-axial configuration. Both of them are able to provide about 10 W cooling power at 77 K with a relative Carnot efficiency of about 18.6%. The third one (PTC-20), designed with in-line configuration, has a twice cross section area of the PTC-10. It can provide more than 20 W cooling power at 77 K with a relative Carnot efficiency of 22%.     

Lifetime prediction and reliability estimation methodology for Stirling-type pulse tube refrigerators by gaseous contamination accelerated degradation testing

Lifetime and reliability are the two performance parameters of premium importance for modern space Stirling-type pulse tube refrigerators (SPTRs), which are required to operate in excess of 10 years. Demonstration of these parameters provides a significant challenge. This paper proposes a lifetime prediction and reliability estimation method that utilizes accelerated degradation testing (ADT) for SPTRs related to gaseous contamination failure. The method was experimentally validated via three groups of gaseous contamination ADT. First, the performance degradation model based on mechanism of contamination failure and material outgassing characteristics of SPTRs was established. Next, a preliminary test was performed to determine whether the mechanism of contamination failure of the SPTRs during ADT is consistent with normal life testing. Subsequently, the experimental program of ADT was designed for SPTRs. Then, three groups of gaseous contamination ADT were performed at elevated ambient temperatures of 40 °C, 50 °C, and 60 °C, respectively and the estimated lifetimes of the SPTRs under normal condition were obtained through acceleration model (Arrhenius model). The results show good fitting of the degradation model with the experimental data. Finally, we obtained the reliability estimation of SPTRs through using the Weibull distribution. The proposed novel methodology enables us to take less than one year time to estimate the reliability of the SPTRs designed for more than 10 years.     

Experimental research of high frequency tapered pulse tube cooler

Tapered pulse tube, corresponding to traditional cylinder tube, is a special design of pulse tube coolers. In this paper, different types of tapered pulse tube arrangements with different angles are tested based on our optimized high frequency PTC. Experiments show that PTCs with tapered PTCs, either to hot or cold end, degrade the performance of PTC in comparison to the cylinder tube. This paper proposes and tests for the first time CD tapered pulse tube. This special tapered pulse tube from two directions, with big diameter at two ends and small diameter in the middle, is similar to converging–diverging nozzle. It improves the performance of PTC over the optimized cylinder tube, though the improvement is only about 1–2 K in minimum temperature.