Joule-Thomson Cryocooler Research Articles

AFRL cryogenic technology development programs

This is an overview of the cryogenic refrigerator and cryogenic integration programs in development and characterization under the Cryogenic Technologies Group, Space Vehicles Directorate of the Air Force Research Laboratory (AFRL). The vision statement for the Air Force Research Laboratory Cryogenic Technologies Group is to support the space community as the center of excellence for developing and transitioning space cryogenic thermal management technologies. The primary customers for the AFRL cryogenic technology development programs are Ballistic Missile Defense Organization (BMDO), the USAF SBIRS (space based infrared) Low program office, and DoD space surveillance programs. This describes a variety of Stirling, pulse tube, Reverse Brayton, Joule-Thomson, and Sorption cycle cryocoolers currently under development to meet current and future Air Force and DoD requirements. The AFRL customer single stage cooling requirements at 10 K, 35 K, 60 K, 150 K, and multi-stage cooling requirements at 35/60 K are addressed. In order to meet these various requirements, the AFRL Cryogenic Technologies Group is pursuing various strategic cryocooler and cryogenic integration options. The Air Force Research Laboratory is also developing several advanced cryogenic integration technologies that will result in the reduction in current cryogenic system integration penalties and design time. These technologies include the continued development of the cryogenic systems integration model (CSIM), 60 K, and 100 K thermal storage units and heat pipes, cryogenic straps, thermal switches, and development of an integrated lightweight cryogenic bus (CRYOBUS).

Closed-Cycle Joule–Thomson Cryocooler for Resistance Thermometer Calibration down to 0.65K

A closed-cycle Joule–Thomson cryocooler for resistance thermometer calibration has been developed. It consists of a Gifford–McMahon mechanical refrigerator and a closed-cycle 3He Joule–Thomson expansion circuit that utilizes the isenthalpic expansion of 3He for cooling. The developed cryocooler can reach temperatures as low as 0.6K and can operate for months with a simple procedure. The typical cooling power of the cryocooler is 1mW at 0.65K with a molar flow rate of 160μmol ·s−1 through the 3He Joule–Thomson circuit. The possible mechanical vibration level experienced by the resistance thermometers was measured with a laser vibrometer. It was confirmed that the maximum acceleration level is 0.1m· s−2 and will not cause a problem for thermometer calibration.

Development of a stainless steel check valve for cryogenic applications

This paper describes the development of a check valve for use in a sorption compressor that will drive a 10 mW 4.5 K Joule–Thomson cryocooler. For the check valve extremely low backflow rates are tolerable at an operating temperature of the valve of 50 K. To fulfill these requirements, the sealing mechanism of the valve is based on a full metal to metal contact. In order to obtain sufficiently low leak rates, both parts were machined to a surface flatness in the order of 100 nm. In addition, the closing plate (boss) of the valve deforms (bends) slightly under pressure, forming itself to the opposite valve seat and thus reducing the gap between these parts. The measured leak flow at 50 K was 1.6 μg/s helium @ 16 bar pressure difference, which is well below the aim of 3 μg/s. The valve was subjected to an accelerated lifetime test of 300,000 cycles. It was observed that the leak flow through the valve during this test steadily decreased to a level of 0.15 μg/s after 100,000 cycles.

18.1 T cryocooled superconducting magnet with a Bi2223 high- [formula omitted] insert

We have successfully developed a cryocooled superconducting magnet which generates the world’s highest magnetic field of 18.1 T. This conduction-cooled superconducting magnet has a 52-mm room temperature bore. The magnet consists of three sections; a high- T c superconducting insert coil, four Nb 3 Sn coils, and an outer NbTi coil. Each section was energized by an individual power supply. The superconducting coils are cooled conductively by a Gifford-McMahon–Joule-Thomson (GM–JT) cryocooler with a cooling capacity of 4.3 W at 4.3 K. The ramp-up time to 18 T is 60 min. The high- T c insert coil is composed of 25 double- pancake coils using Ag/(Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 (Bi2223) high- T c superconducting tape with stainless steel tape reinforcement. The reinforcement co-winding reduces the effective hoop stress down to 48 MPa, and, hence, Bi2223 tape can be used. The innermost of the Nb 3 Sn section is made of an internal-tin-processed Nb 3 Sn wire because this wire can withstand high critical current density in high magnetic fields. Other three coils require high mechanical strength. They experience hoop stress of 230 MPa, and in this case, bronze-processed high-strength Nb 3 Sn wires were used. The magnet has a distinctive feature, that is the high- T c insert coil is replaceable. It indicates that the magnet can be used as a 15.5 T back-up magnet for a new insert coil. A Y123 coil is a candidate for the replacement.

Elevated-pressure mixed-coolants Joule–Thomson cryocooling

This paper explores the potential of mixed coolants at elevated pressures for Joule–Thomson cryocooling. A numerical model of a Joule–Thomson cryocooler is developed that is capable of simulating operation with mixtures of up to 9 components consisting of hydrocarbons, non-flammable halogenated refrigerants, and inert gases. The numerical model is integrated with a genetic optimization algorithm, which has a high capability for convergence in an environment of discontinuities, constraints and local optima. The genetic optimization algorithm is used to select the optimal mixture compositions that separately maximizes following two objective functions at each elevated pressure for 80, 90 and 95 K cryocooling: the molar specific cooling capacity (the highest attainable is 3200 J/mol) and the produced cooling capacity per thermal conductance which is a measure of the compactness of the recuperator. The optimized cooling capacity for a non-flammable halogenated refrigerant mixture is smaller than for a hydrocarbon mixture; however, the cooling capacity of the two types of mixtures approach one another as pressure becomes higher. The coefficient of performance, the required heat transfer area and the effect of the number of components in the mixture is investigated as a function of the pressure. It is shown that mixtures with more components provide a higher cooling capacity but require larger recuperative heat exchangers. Optimized mixtures for 90 K cryocooling have similar cooling capacity as those for 80 K. Optimized compactness for 80 K is about 50% higher than can be achieved by pure nitrogen. For 90 K, no mixture provides a more compact recuperator than can be achieved using pure argon. The results are discussed in the context of potential applications for closed and open cycle cryocoolers.

Maximizing production rates of the Linde–Hampson machine

In contrast to the ideal case of unlimited size recuperator, any real Linde–Hampson machine of finite size recuperator can be optimized to reach the extreme rates of performance. The group of cryocoolers sharing the same size recuperator is optimized in a closed form by determining the corresponding flow rate which maximizes its rate of cold production. For a similar group of liquefiers an optimal flow rate is derived to maximize the rate of production of liquid cryogen. The group of cryocoolers sharing a constant and given flow rate is optimized by shortening the recuperator for reaching a maximum compactness measured by the cooling power per unit size of the recuperator. The optimum conditions are developed for nitrogen and argon. The relevance of this analysis is discussed in the context of practice of fast cooldown Joule–Thomson cryocooling.

Real gas choked flow conditions at low reduced-temperatures

Real gas choked mass flux is calculated for a frictionless stream expanding isentropically until it reaches the speed of sound and without phase changes. The other parameters associated with the choked state are the pressure, density, temperature ratios, and the speed of sound. Departure of the choked mass flux from the ideal gas model is discussed first in absolute terms and then in relative terms, using the Principle of Corresponding States, for gases with boiling points in the low temperature range. Reduced-stagnation pressures are examined up to values of 30 for hydrogen, neon, nitrogen, argon, methane, krypton, xenon, and R-14 and up to 100 for 4He. The corresponding reduced-stagnation temperatures go down to 1.4 and in some cases down to 1.2 for nitrogen and argon. Also discussed are the limiting values of stagnation parameters for which no phase change occurs in the choked state. Compared to the ideal gas, the mass flux may almost double and the critical pressure ratio may decrease by an order of magnitude. The relevance of results is discussed qualitatively and quantitatively for Joule–Thomson cryocooling.

A numerical study of the Hampson-type miniature Joule–Thomson cryocooler

The Hampson-type miniature Joule–Thomson (J–T) cryocooler is widely used in electronic cooling. We develop the geometry model of the Hampson-type cryocooler to better understand the double helical tube-and-fin heat exchanger. The steady-state governing equations of the cryogen, helical tube and fins, and shield are solved numerically and yield good agreements with experiment data. The conventional way of simulating a Hampson-type J–T cryocooler, which is accompanied by a host of empirical correction factors, especially vis-à-vis the heat exchanger geometry could now be superseded. The effort and time spent in designing a Hampson-type J–T crycooler could be greatly reduced.

Heat transfer coefficient measurements for mixed gas working fluids at cryogenic temperatures

The use of mixed gas working fluids has become common in Joule–Thomson type cryocoolers for a variety of applications. However, there is very little data or theory currently available regarding the heat transfer coefficient associated with these multi-component, multi-phase mixtures at cryogenic temperatures. This paper describes an experimental test facility and procedure that has been used to make careful measurements of the horizontal, flow boiling heat transfer coefficient for several hydrocarbon mixtures that are nominally optimal for small, Joule–Thomson cryocoolers in the 80 K to 120 K operating range. Data are presented over a range of temperatures from 100 K to room temperature and for several pressures and mass flow rates. The results indicate that quality and mass flux are the most important parameters governing the heat transfer coefficient among those that were varied. The experiment is verified by carrying out tests using single-phase, pure nitrogen gas and comparing the results with the Dittus–Boelter equation. The experimental uncertainty of the measurements is estimated from 1st principles; additionally, the repeatability of the experimental measurements was investigated by replicating tests at a nominal set of operating conditions and composition on separate days. The measurements presented here are intended to aid in the design of small, mixed-gas Joule–Thomson cryocoolers.

Low-heat input cryogenic temperature control with recuperative heat-exchanger in a Joule Thomson cryocooler

The control of cryogenic temperatures is usually accomplished by a passive stage, exploiting the combined effect of a thermal mass connected to a thermal resistance; by an active control, often of a PID type, based on the combination of a dedicated sensor, a heater and a controller; or by a combination of the two. Such a system typically uses a controlled stage that is isolated from the source of the fluctuations by a thermal isolator. Controlled insertion of heat into this stage counters the temperature fluctuations reaching the stage. Inherent to this type of system is the insertion of heat into the controlled stage that eventually reaches the cold end of the cooler, reducing the net heat lift available. The larger the thermal isolation, the smaller the reduction of the net heat lift, but with the attendant increase in the interface temperature. Any scheme that can reduce the penalty associated with the loss of heat lift or the temperature offset would be attractive in terms of cooler performance. If the cooler system has a recuperative heat exchanger between the coldest heat sink and a higher temperature precooler, a different approach can be used. In this paper we describe a novel control approach capable of passively damping low frequency fluctuations, requiring minimal reduction of cooler heat lift and minimal temperature increase of the cold end interface. This alternative scheme is based on the idea of controlling the temperature of a section of the recuperative heat exchanger between the coldest precooler and the cold end of the cooler and it has been tested on a 20 K hydrogen sorption JT cooler.

Development of metal hydride beds for sorption cryocoolers in space applications

The development of hydrogen sorption cryocoolers over the past 30 years is briefly reviewed. The behavior of the metal hydride sorbent beds used in the sorption compressors dominates both the performance and reliability of these closed-cycle Joule–Thomson cryocoolers. Improved compressor elements have been recently designed to minimize their input power requirements and to enhance hydride durability during extended temperature cycling while in operation. ZrNi hydride is used to provide variable gas pressure in the gas-gap heat switches for each compressor element. Characterization tests have been performed on the compressor elements built for an engineering breadboard (EBB) cryocooler to evaluate the behavior of both the sorbent bed and gas-gap switches under conditions simulating flight operation for the future Planck mission of the European Space Agency (ESA). Operation of the Planck EBB sorption cryocooler has produced a cooling capacity of 1.0–1.7 W at a temperature of 17.7 K during initial laboratory tests.

A high- Tc SQUID-based sensor head cooled by a Joule–Thomson cryocooler

The goal of the so-called FHARMON project is to develop a high- T c SQUID-based magnetometer system for the measurement of fetal heart activity in standard clinical environments. To lower the threshold for the application of this fetal heart monitor, it should be simple to operate. It is, therefore, advantageous to replace the liquid cryogen bath by a closed-cycle refrigerator. For this purpose, we selected a mixed-gas Joule–Thomson cooler; the APD Cryotiger ©. Because of its magnetic interference, the compressor of this closed-cycle cooler will be placed at a distance of ≈2 m from the actual sensor, which is an axial second order gradiometer. The gradiometer is formed by three magnetometers placed on an alumina cylinder, which is connected to the cold head of the cooler. This paper describes the sensor head in detail and reports on test experiments.

Aircraft wheel testing with machine-cooled HTS SQUID gradiometer system

For eddy current detection of deep-lying flaws in large aircraft wheels, an automated airplane wheel inspection system using a HTS SQUID gradiometer sensor is being developed. Wheel drums made of aluminum alloys have to be tested frequently since they are subject to enormous dynamic loads and very high braking temperatures at landing. For economic reasons, testing should be performed from the outside without removing the inner ferromagnetic keys which fit the brake system. In order to operate the sensor in hostile environments such as airport maintenance hangars, a planar rf double hole SQUID gradiometer was used. SQUID cooling is performed by a closed cycle Joule-Thomson cryocooler, equipped with flexible plastic gas lines. The wheel testing is being performed on an automated test stand with the wheel slowly rotating and a robot with the SQUID enclosure scanning stepwise along the wheel axis. Additional signals due to inner cracks of 10 mm length, penetrating 25 percent of the 10 mm thick wall, are easily identifiable in the periodic signal background due to the presence of ferromagnetic keys. In comparative measurements, the prototype SQUID system clearly exhibited advantages over conventional techniques, with optimization reserve still at hand.

Two-phase coexistence analysis of the bellows control mechanism for a J–T cryocooler

The bellows control mechanism is one of the most critical control elements in the Joule–Thomson cryocooler. In the cool-down process, the interior gas of the bellows may reach liquefaction temperature, cause vapour–liquid two phase coexistence phenomena, and achieve the quick open/close control function of a J–T valve. This paper establishes a transient model of a simplified bellows mechanism, investigates the variation of the state properties of nitrogen inside the bellows, and its effects on the performance with different bellows stiffness in the cool-down process. According to the real shape and dimension, the two-phase model is put into a J–T cryocooler integrated model to accomplish the J–T cryocooler system simulation. To adopt the benefit of two-phase coexistence behavior effectively, a revised initial J–T valve design is also proposed in this work.

HTS SQUID system with Joule-Thomson cryocooler for eddy current nondestructive evaluation of aircraft structures

For the detection of deep-lying flaws in aircraft structures, an eddy current system in conjunction with a planar SQUID gradiometer is being developed. The need for a mobile system imposes additional requirements on cooling regarding mobility, operation independent of spatial orientation, and handling. We present results on the operation of HTS SQUIDs with the commercial Joule-Thomson-cryocooler KC 100 ("cryotiger") by APD and report on cooler characteristics and system performance. Magnetometers and gradiometers of both rf and dc type have been integrated with the cryocooler. Noise spectra both in unshielded and shielded environments have been shown to be orientation independent. Compared to stationary operation, the system sensitivity is reduced by additional noise generated by the movement. With an improved integration scheme, this motion-related noise is nearly eliminated. The system was equipped with a differential eddy current excitation and a synchronized digital lock-in detection. Two-dimensional scans are presented that illustrate the system's capability to find fatigue cracks and corrosion pits hidden below several layers of aluminum.

97/00708 A study on the transient characteristics of a self-regulating Joule-Thomson cryocooler

97/00708 A study on the transient characteristics of a self-regulating Joule-Thomson cryocooler

A study on the transient characteristics of a self-regulating Joule-Thomson cryocooler

An experimental and numerical study of transient characteristics for the self-regulating Joule-Thomson (J-T) cryocooler is developed in the present work. The modelling of the bellows control mechanism and the simulation of the cooler system are developed. It is proved by experiment that the most important transient behaviours can be accurately predicted by the present numerical simulation. In order to rectify the instability problem of the bellows controlled J-T cooler, this study also proposes a revised design to improve temperature stability. The revised dual-orifice J-T cooler can effectively reduce the phenomena of unstable temperature and mass flow rate, but sacrifices the expected gas consumption rise by approximately 5 percent. Through experimental evaluation, it becomes obvious that this revised type cooler would be a better design.

Analysis of binary cryogenic mixtures containing nitrogen and Freon in cryocoolers

Several new non-inflammable binary cryogenic mixtures of nitrogen and Freon are suggested in this paper. The working mechanism of using these mixtures in a Joule-Thomson cryocooler is analysed according to thermodynamic theory. The Peng-Robinson equation of state is used to calculate the mixed free enthalpy of these mixtures and a method to calculate the mutual solubility is also suggested. Finally, the vapour-liquid equilibria, liquid-liquid equilibria and vapour-liquid-liquid equilibria of these mixtures are analysed. It is shown that the suggested mixtures can be used as alternatives to mixtures of nitrogen and alkanes or alkenes.

Preliminary experimental and numerical study of transient characteristics for a Joule-Thomson cryocooler

To establish a tool for computer-aided design, a preliminary experimental and numerical study of transient characteristics for a Joule-Thomson (J-T) cryocooler is developed in the present work. A simplified transient one-dimensional model of momentum and energy transport is used to simulate the flow and heat transfer characteristics. The computational results for cool-down time agree with the experimental data. The transient temperature variations of high pressure gas, tube wall, low pressure gas, glass Dewar and mandrel at the grid point near the valve exit are shown. From the spatial temperature distributions of high and low pressure gases along the finned tube at the moment of cool-down, it is proposed that the J-T cooler currently used may be reduced in terms of both size and weight; factors which are very important in a miniature J-T cryocooler.

Performance of ideal flow regulated Joule-Thomson cryocooler

The performance of an ideal flow regulated Joule-Thomson cryocooler is analyzed and formulated in closed form expressions. Two experiments involving nitrogen and argon pressure vessel discharges through a couple of flow regulated cryocoolers are examined in the light of these theoretical results. The reasonable match verifies the validity of the ideal performance analysis.