Experimental investigation on mixed refrigerant Joule-Thomson cryocooler with flammable and non-flammable refrigerant mixtures

The mixed refrigerant Joule-Thomson (MR J-T) cryocoolers have a wide application area covering the temperature range from 80 K to 200 K. The significant advantages of the system are simplicity of its design and working reliability with high level performance. The present paper discusses the experimental results of MR J-T cooler with different flammable and non-flammable mixture compositions. The work highlights the use of pressure-enthalpy and temperature-enthalpy diagrams for these mixtures to support the experimental results. A record lowest temperature of 65 K and a cooling capacity of 6 W at 80 K are obtained for a single stage MR J-T system starting at 300 K. Further, using a mixture of minimum flammable refrigerants, temperatures below 100 K is achieved.     

Development of mechanical cryocoolers for Astro-H/SXS

The Soft X-ray Spectrometer (SXS) is a high-resolution spectrometer with an X-ray micro-calorimeter array onboard the Japanese X-ray astronomy satellite Astro-H, planned for launch in 2013. The micro-calorimeter is operated at cryogenic temperature of 50 mK provided by the Adiabatic Demagnetization Refrigerator (ADR) with a heat sink of 1.3 K liquid helium stored in the SXS Dewar. To extend the liquid helium lifetime to over 3 years in orbit, two types of mechanical cryocoolers are installed: 20 K-class double-staged Stirling (2ST) coolers and a 1 K-class Joule-Thomson (JT) cooler. Improvement of mechanical cryocoolers has been investigated and verified for higher reliability and cooling performance. The engineering model (EM) of upgraded mechanical cryocoolers was fabricated for a long lifetime test. The required cooling power of 200 mW at 20 K for the 2ST cooler and 10 mW at 1.7 K for the JT cooler are achieved by EM test.     

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.     

Heat exchanger versus regenerator: A fundamental comparison

Irreversible processes in regenerators and heat exchangers limit the performance of cryocoolers. In this paper we compare the performance of cryocoolers, operating with regenerators and heat exchangers from a fundamental point of view. The losses in the two systems are calculated from the entropy productions due to the various irreversible processes. Whether an optimized regenerator or heat exchanger performs better depends on the system parameters (molar flux, temperature, and pressure). At temperatures below 200 K the losses due to heat conduction in the axial direction are dominant.     

Proposed designs for a “dry” dilution refrigerator with a 1 K condenser

Recent development of “dry” dilution refrigerators has used mechanical cryocoolers and Joule-Thomson expansion stages to cool and liquefy the circulating ³He. While this approach has been highly successful, we propose three alternative designs that use independently-cooled condensers. In the first, the circulating helium is precooled by a mechanical cooler, and liquified by self-contained ⁴He sorption coolers. In the second, the helium is liquefied by a closed-cycle, continuous flow ⁴He refrigerator operating from a room temperature pump. Finally, the third scheme uses a separate ⁴He Joule-Thomson stage to cool the ³He condenser. The condensers in all these schemes are analogous to the “1-K pot” in a conventional dilution refrigerator. Such an approach would be advantageous in certain applications, such as instrumentation for astronomy and particle physics experiment, where a thermal stage at approximately 1 K would allow an alternative heat sink to the still for electronics and radiation shielding, or quantum computer research where a large number of coaxial cables must be heat sunk in the cryostat. Furthermore, the behaviour of such a refrigerator is simplified due to the separation of the condenser stage from the dilution circuit, removing the complex interaction between the 4-K, Joule-Thomson, still and mixing chamber stages found in current dry DR designs.     

High-power Stirling-type pulse tube cooler working below 30 K

High-power Stirling-type pulse tube coolers (PTCs) are promising candidates for cooling HTS devices and gas liquefaction or separation applications. Nevertheless, till now most high-power Stirling-type PTCs are not able to reach a refrigeration temperature below 35 K. Here, a high-power two-stage Stirling-type PTC was designed, manufactured and experimentally investigated. In order to realize a convenient coupling with a thermal load, U-shape configuration is adopted in both stages, which makes it more challenging to distribute the gas flow and reduce dead volume in the cold end heat exchanger. By optimizing operating conditions, flow straightener, and double-inlet opening, the cooler has reached no-load refrigeration temperatures of 29.6 K and 27.1 K at 55 Hz and 40 Hz, respectively. Furthermore, the cooler is able to provide cooling powers of 50 W at 45.6 K and 100 W at 59.3 K when input pV powers are 4.77 kW and 4.59 kW, respectively.     

Mixed refrigerants for a glass capillary micro cryogenic cooler

Optimized mixed refrigerants are applied in Joule-Thomson (JT) micro cryogenic coolers (MCC) to enhance efficiency. Mixed refrigerants deliver equivalent refrigeration power with much lower pressure ratio and flow rate compared to pure nitrogen refrigerant. To determine the behavior of mixtures in MCCs, the normal boiling points of the components, mixture solubility, and refrigeration loss due to pressure drop on the low-pressure side of the heat exchanger are evaluated. The MCC discussed here was designed to operate at 77 K with the heat exchanger warm end precooled to 240 K by a thermo-electric cooler. An optimized five-component mixed refrigerant was calculated to provide a minimum isothermal enthalpy difference of 1.35 kJ/mol between 77 K and 240 K with a high pressure of 1.6 MPa and a low pressure of 0.1 MPa. Experimentally, a stable temperature of 140 K was achieved with a flow rate of 11 μmol/s. A transient temperature of 76 K was observed.     

ADR design for the Soft X-ray Spectrometer instrument on the Astro-H mission

In its instrument suite, the Japanese Astro-H mission will include the Soft X-ray Spectrometer (SXS), whose 36-pixel detector array of ultra-sensitive X-ray microcalorimeters will be cooled to 50 mK. This will be accomplished using a two-stage adiabatic demagnetization refrigerator (ADR). A complicating factor for its design is that the ADR will be integrated into a superfluid helium dewar at 1.3 K that will be coupled to a 1.8 K Joule-Thomson (JT) stage through a heat switch. When liquid helium is present, the coupling will be weak, and the JT stage will act primarily as a shield to reduce parasitic heat loads. When the liquid is depleted, the heat switch will couple more strongly so that the ADR can continue to operate using the JT stage as its heat sink. A two-stage ADR is the most mass efficient option and it has the operational flexibility to work well with a stored cryogen and a cryocooler. The ADR’s design and operating modes are discussed, with emphasis on how they reflect the capabilities and limitations of the hybrid cryogenic system.     

On the performance of copper foaming metal in the heat exchangers of pulse tube refrigerator

As key components in pulse tube refrigerators (PTRs), heat exchangers have great influence on the performance of the PTRs, especially the cold end heat exchangers which dominate the cooling effect between the cold gas and heat load. Filling copper screens are widely used to improve the performance of heat exchange and laminar flow. Whereas, the heat transfer rate of copper screens is still not good enough for the actual requirements of PTRs. Furthermore, the flow resistance of the copper screen is growing up quickly with the increase of screen mesh. In this paper, we propose a new type of copper foaming metal with high heat transfer area and low flow resistance in the heat exchanger instead of the copper screens. The heat transfer performances of the copper screens and the copper foaming metal are firstly compared by theoretical calculation, which shows that the performance of the copper foaming metal with 600 μm pore size is better than that of 20 and 80 mesh copper screens, verified by experimental results. A four valve pulse tube refrigerator (FVPTR) with copper foaming metal of 600 μm pore size as filling material of the heat exchanger achieved 69.5 K, 2.5 K lower than that of using 20 mesh copper screens, 1.7 K lower than that of using 80 mesh copper screens.     

Performance of a cryogenic vacuum system (COLDEX) with an LHC type beam

The cold bore experiment installed in the super proton synchrotron has been used to study the performance of a vacuum system operating at cryogenic temperatures in the presence of a large hadron collider (LHC) type proton beam. The ∼2 m long cryostat, which can be cooled below 3 K, is fitted with an actively cooled beam screen which can be temperature controlled between 5 and 100 K. Molecular desorption and deposited heat load measurements, with or without gas pre-condensation, have been performed. Implications to the LHC design and operation will be discussed.     

Cooling system for the soft X-ray spectrometer onboard Astro-H

The Soft X-ray Spectrometer (SXS) is a cryogenic high resolution X-ray spectrometer onboard the X-ray astronomy satellite Astro-H which will be launched in 2014. The detector array is cooled down to 50 mK using an adiabatic demagnetization refrigerator (ADR). The cooling chain from the room temperature to the ADR heat-sink is composed of superfluid liquid He, a Joule-Thomson cryocooler, and double-stage Stirling cryocoolers. It is designed to keep 30 l of liquid He for more than 5 years in the normal case, and longer than 3 years even if one of the cryocoolers fails. Cryogen-free operation is also possible in the normal case. It is fully redundant from the room temperature to the ADR heat-sink.     

In-tube condensation of mixture of R134a and ester oil: empirical correlations

This paper presents a comparative study of the condensation heat transfer coefficients in a smooth tube when operating with pure refrigerant R134a and its mixture with lubricant Castrol “icematic sw”. The lubricant is synthetic polyol ester based oil commonly used in lubricating the compressors. Two concentrations of R134a-oil mixtures of 2% and 5% oil (by mass) were analysed for a range of saturation temperatures of refrigerant R134a between 35 °C and 45 °C. The mass flow rate of the refrigerant and the mixtures was carefully maintained at 1 g/s, with a vapour quality varying between 1.0 and 0. The effects of vapour quality, flow rate, saturation temperature and temperature difference between saturation and tube wall on the heat transfer coefficient are investigated by analysing the experimental data. The experimental results were then compared with predictions from earlier models [Int J Heat Mass Transfer (1979), 185; 6th Int Heat Transfer Congress 3 (1974) 309; Int J Refrig 18 (1995) 524; Trans ASME 120 (1998) 193]. Finally two new empirical models were developed to predict the two-phase condensation heat transfer coefficient for pure refrigerant R134a and a mixture of refrigerant R134a with Castrol “icematic sw”.     

A study of transcritical carbon dioxide flow through diabatic capillary tubes

An experimental and theoretical study of the diabatic flow of carbon dioxide through lateral capillary tube suction line heat exchangers is outlined. The influence of both operating conditions (capillary tube inlet and outlet pressures, capillary tube inlet temperature and suction line inlet temperature) and tube geometry (heat exchanger length and position, suction line diameter and capillary tube length) on the heat and mass flow rates was experimentally evaluated using a purpose-built testing facility. In total, 75 tests were carried out with heat fluxes spanning from 1 to 11 kW m⁻² and refrigerant mass flow rates ranging from 12 to 26 kg h⁻¹. In addition, the mathematical model of Hermes et al. (2008) was adapted to run with carbon dioxide as working fluid. The model was validated against experimental data, and a good agreement between the experimental and calculated mass flow rates was achieved with 85% and 98% of the data points being within ±5% and ±10% error bounds, respectively.     

Improving adsorption cryocoolers by multi-stage compression and reducing void volume

Adsorption cryocoolers offer the potential of operating reliably in space for > 10 years, far longer than present mechanical systems. They have no wear-related moving parts, are relatively simple and quiet, generate negligible mechanical vibration and electromagnetic interference, are easily scalable by combining additional compressor modules, can be combined in cascade refrigeration systems to produce temperatures < 4 K, and can be powered by waste heat from a nuclear or solar thermal energy source. This Paper describes how the performance of gas adsorption cryocoolers can be greatly improved by using adsorbents with low void volume within and between individual adsorbent particles, reducing void volumes in plumbing lines, and by compressing the working fluid in more than one stage. An analytical model was used to determine refrigerator specific power requirements and compressor volumetric efficiencies in terms of adsorbent and plumbing line void volumes and operating pressures for various charcoal adsorbents. Operating pressure optimization curves for 80 and 117.5 K charcoal/nitrogen adsorption cryocoolers are also presented, for both single stage and multi-stage compressor systems.     

开架式气化器竖直圆管内超临界氮换热实验研究

目前国内外对液化天然气(LNG)接收站的开架式气化器中超临界天然气的流动换热实验研究非常少,本文为了研究开架式气化器中竖直管内超临界流体的流动换热特性,搭建了竖直单管超临界流体换热实验平台。以液氮代替液化天然气,研究了氮入口压力、水温和水流量等不同参数对换热的影响。结果表明:在拟临界温度以下,表面传热系数随着压力的增大逐渐减小,但拟临界温度以后,这种趋势相反;当水流量足够大时,氮出口温度取决于管外水温而不是水侧流量。最后,基于实验数据拟合出了适用于竖直圆管内超临界低温流体流动换热的半经验关联式,关联式预测值和实验值的平均绝对偏差为8.42%,可以准确预测竖直加热管中超临界氮的表面传热系数。     

Measurement of isobaric heat capacity of R125

The specific isobaric heat capacity (c_p) was measured for R125 (pentafluoroethane) in the gas phase by using a flow calorimeter. Twelve measurements for R125 were obtained at temperatures from 313 to 333 K and at pressures from 0.8 to 2.4 MPa. Some of them are close to the saturation curve. The expanded uncertainty (k = 2) of the temperature measurements is estimated to be less than 23 mK, and that of the pressure measurements is less than 14 kPa. The expanded uncertainty for c_p is estimated to range from 12 to 22 J kg⁻¹ K⁻¹. Also, the experimental data were compared with available equations of state. From the results, it became clear that these data will be essential to improve available models so as to represent more reliable thermodynamic properties of R125 and refrigerant mixtures with R125 that are used for refrigeration and air conditioning systems.     

CO2 and propane blends: Experiments and assessment of predictive methods for flow boiling in horizontal tubes

The CO₂ and propane blends are an interesting alternative to solve technical and safety issues related to the use of pure CO₂ or pure hydrocarbons. These mixtures of pure fluids are environmentally friendly and have a large glide, that affects remarkably heat transfer.In this paper a review of works and predictive methods on flow boiling of wide-boiling mixtures is first presented. Experiments during flow boiling in a smooth horizontal tube with an internal diameter equal to 6.00 mm of CO₂ and propane mixtures (with 83.2/16.8% and 70.0/30.0% in mass concentrations) are reported. The experiments are related to the following operating ranges: mass fluxes from 200 to 350 kg m⁻² s⁻¹, heat fluxes from 10 to 20.2 kW m⁻², temperatures of the mixture from 6.9 to 14.0 °C in the whole range of vapor qualities.An assessment of predictive methods based on the present and independent databases is reported.     

Flow boiling in microgravity condition investigation using inverse techniques

The objective is to provide a method to obtain local heat transfer coefficients in small channels when flow boiling occurs. The experimental device has been developed to perform investigations in parabolic flights campaigns on board A300-ZéroG. Simultaneously flow visualization and thermo-hydraulic measurements are carried out to investigate the two phase flow and heat transfer in minichannels. The experiments are conducted with HFE-7100 in several operating conditions for three hydraulic diameters. The investigations concern flow pattern and the associated heat transfer coefficient during convective for several gravity levels. We mainly on the thermal measurements which consists in inversing experimental temperature measurements (thermocouples) to derive the local surface temperature and heat flux. For the investigated operating conditions, the heat transfer coefficient is found to vary along the flow axis especially at the channel entrance zone.     

Helium liquefaction with a commercial 4 K Gifford-McMahon cryocooler

This paper describes helium liquefaction using a commercial cryocooler with 1.5 W cooling power at 4.2 K (Sumitomo model RDK415D with compressor CSW-71D, consuming 6.5 kW electrical power), equipped with heat exchangers for precooling the incoming gas. No additional cooling power of cryoliquids or additional Joule-Thomson stages were utilized. Measurements of the pressure dependence of the liquefaction rate were performed. A maximum value of 83.9 g/h was obtained for 2.25 bar stabilized input pressure. Including the time needed to cool the liquefied helium to 4.2 K at 1 bar after filling the bottle connected to the cold head, and correcting for heat screen influences, this results in a net liquefaction rate of 67.7 g/h. Maintaining a pressure close to 1 bar above the bath during liquefaction, a rate of 55.7 g/h was obtained. The simple design enables many applications of the apparatus.     

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.