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.     

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.     

Pressure loss effect on recuperative heat exchanger and its thermal performance

Most heat exchangers are designed in pursuit of minimum pressure loss and maximum heat transfer conductance, and the pressure drop of heat exchanger is sometimes neglected intentionally or unintentionally in the designing stage. However, the actual pressure drop should be considered when the requirement of heat exchanger is very strict. Different from the most previous researches that examine the hydraulic aspect, this paper describes the thermal aspect of the pressure loss and its relevance to the basic model of heat exchanger. The analytical result reveals the indirect effect of pressure loss on the thermal performance of heat exchanger. The whole impact on the effectiveness of counter-flow heat exchanger is discussed in terms of newly defined relevant dimensionless parameters.     

Observation and control of temperature instabilities in a four-valve pulse tube refrigerator

The use of a pulse tube cryocooler in an application requires temperature stability at the cold end. In our four-valve pulse tube refrigerator we have observed long-term temperature instabilities lasting some days and short-term instabilities lasting some hours or even minutes. Investigations have shown that the latter anomaly is caused by the dc-flow. The negative influence on the stability is due to an additional mass flow (dc-flow) to the cold end of the pulse tube, which results in a parasitic heat input.In this paper we present an actively controlled dc-flow suppression device, which uses a temperature gradient in the regenerator as a control parameter. This device enables us to eliminate the temperature instabilities.     

Effect of flow mal-distribution on effective NTU in multi-channel counter-flow heat exchanger of single body

Most heat exchangers in services belong to multi-channel heat exchangers. The working fluid is distributed among the multiple channels. This kind of heat exchangers usually has a flow distribution problem. In general, the flow is not evenly distributed and, as a result, the thermal performance of the heat exchanger degrades due to it. In this paper, such flow distribution effect in a single body multi-channel heat exchanger is evaluated in an analytic way. Transverse conduction through the heat exchanger body which is caused by the mal-distributed flow is formulated and included in NTU evaluation. The NTU relationship between the well-balanced flow distribution and the ill-balanced one is obtained. According to the analysis result, the heat exchanger has the best performance in case of the well-balanced flow distribution, as is expected, and has the performance degradation in case of the ill-balanced flow. The performance degradation is very severe in the heat exchanger with poor transverse conduction.     

A small helium-3 pulse-tube refrigerator

A new three-stage pulse-tube refrigerator (PTR) is developed by scaling down a previous PTR by 50%. The new system is small in size and weight, capable of operating using little input power, and uses a small amount of working gas and regenerator material. In addition to that the system is flexible and convenient for modifications. The volume of the low-temperature part of the new PTR (pulse tubes + regenerator) is as small as 0.28 l. With ³He as a working fluid a no-load temperature of 1.73 K is reached and a cooling power of 124 mW at 4.2 K is realized.     

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.     

Vibration spectrum of a pulse-tube cryostat from 1 Hz to 20 kHz

The vibrations of the cold finger of a low-vibration helium pulse-tube cryostat are measured from 1 Hz to 20 kHz using an optical interferometer specially designed to measure small amplitude vibrations at high frequencies in the presence of large vibrations at lower frequencies. While the vibrational amplitude is dominated by the contribution at the fundamental compressor frequency of 1.4 Hz, the pulse tube contributes mechanical noise at frequencies up to 15 kHz, where the spectral density is measured to be 4 × 10⁻¹² m/Hz^1/2. Root-mean-squared vibration amplitudes of 5.2 μm and 3 μm are measured along perpendicular axes in the horizontal plane, and 1.0 μm in the vertical direction. The effect of a suspended sample holder for the purpose of attenuating high-frequency vibrations is evaluated. Finally, the cryostat is shown to be considerably noisier than typical laboratory floors.     

Two-stage pulse tube refrigerator in an entire coaxial configuration

In this paper, we introduce a new kind of two-stage pulse tube refrigerators. The chosen entire coaxial configuration combines the advantages of the coaxial design with the two-stage pulse tube concept. Lead coated screens build the inhomogeneous regenerator matrix of the second stage. Without any rare earth compounds the refrigerator reaches a no load temperature of 6.6 K at the second stage cold tip. The active type of phase shifting is generated by a rotary valve combined with two needle valves at the hot end of each pulse tube (compressor Leybold RW 6000, 6 kW input power). This paper focuses on the design parameters and first performance measurements.     

Size effects on miniature Stirling cycle cryocoolers

Size effects on the performance of Stirling cycle cryocoolers were investigated by examining each individual loss associated with the regenerator and combining these effects. For the fixed cycle parameters and given regenerator length scale, it was found that only for a specific range of the hydrodynamic diameter the system can produce net refrigeration and there is an optimum hydraulic diameter at which the maximum net refrigeration is achieved. When the hydraulic diameter is less than the optimum value, the regenerator performance is controlled by the pressure drop loss; when the hydraulic diameter is greater than the optimum value, the system performance is controlled by the thermal losses.It was also found that there exists an optimum ratio between the hydraulic diameter and the length of the regenerator that offers the maximum net refrigeration. As the regenerator length is decreased, the optimum hydraulic diameter-to-length ratio increases; and the system performance is increased that is controlled by the pressure drop loss and heat conduction loss. Choosing appropriate regenerator characteristic sizes in small-scale systems are more critical than in large-scale ones.     

Dynamic simulation of the helium refrigerator/liquefier for LHD

A real-time dynamic simulation has been carried out for the 10 kW class helium refrigerator/liquefier of Large Helical Device (LHD) at National Institute for Fusion Science (NIFS). The refrigerator consists of eight screw compressors, seven expansion turbines, fourteen heat exchangers and a 20 m³ liquid helium reservoir. A simulation model was implemented to Cryogenic Process REal-time SimulaTor (C-PREST), developed as a platform for the plant process study and optimization. Validity of the simulation model has been confirmed based on the design values as well as the results of commissioning tests. This paper describes the cooldown process and expansion turbine trips during the operation. Difficulties of dynamic simulation for the large cryoplant are also discussed.     

Simulation of multistream plate-fin heat exchangers of an air separation unit

Hot and cold reversible heat exchangers of an air separation unit are simulated. Five fluid streams exchange heat with six fluid streams in parallel and counter flow. The numerical method employed divides the heat exchanger in a number of sections, for which fluid properties, capacity rates and heat transfer coefficients are considered constant. Single and two-phase streams are taken into account. Results obtained from the model are compared with field data.     

Recommendations for accurate heat capacity measurements using a Quantum Design physical property measurement system

A commercial instrument for determination of heat capacities of solids from ca. 400 K to 0.4 K, the physical property measurement system from Quantum Design, has been used to determine the heat capacities of a standard samples (sapphire [single crystal] and copper). We extend previous tests of the PPMS in three important ways: to temperatures as low as 0.4 K; to samples with poor thermal conductivity; to compare uncertainty with accuracy. We find that the accuracy of heat capacity determinations can be within 1% for 5 K < T < 300 K and 5% for 0.7 K < T < 5 K. Careful attention should be paid to the relative uncertainty for each data point, as determined from multiple measurements. While we have found that it is possible in some circumstances to obtain excellent results by measurement of samples that contribute more than ca. 1/3 to the total heat capacity, there is no “ideal” sample mass and sample geometry also is an important consideration. In fact, our studies of pressed pellets of zirconium tungstate, a poor thermal conductor, show that several samples of different masses should be determined for the highest degree of certainty.     

Regenerator performance improvement of a single-stage pulse tube cooler reached 11.1 K

In order to improve the cooling performance of pulse tube cooler (PTC) at 20-40 K, hybrid regenerators are often employed. In this paper a three-layer regenerator, which consists of woven wire screen, lead sphere and Er₃Ni is optimized to enhance the cooling performance and explore the lowest attainable refrigeration temperature for a single-stage PTC. The efforts focus on the temperature range of 80-300 K, where woven wire screens are used. Theoretical and experimental studies are carried out to study the metal material and the mesh size effect of woven wire screens on the performance of the single-stage G-M type PTC. A lowest no-load refrigeration temperature of 11.1 K was obtained with an input power of 6 kW. The PTC can supply 17.8 W at 20 K and 39.4 W at 30 K, respectively.     

Development of a valved linear compressor for a satellite borne J-T cryocooler

The provision of temperatures below 12 K is essential for sub-mm and FIR observations from satellite instruments. Historically this has been achieved with stored cryogens, however mechanical coolers could potentially provide higher reliability and flexibility. These cryocoolers typically incorporate a regenerative cold-finger, such as a pulse-tube, however this can be replaced by a recuperative Joule-Thompson stage to obtain the lowest temperatures required. The major change to the compressor is the requirement for steady flow. This paper describes the development of such a compressor using reed valves, based on space-qualified hardware. Long life potential was demonstrated by measuring the motion of the valves during operation. A model was also developed and validated to optimize performance.     

Effect of convection heat transfer on the design of vapor-cooled current leads

A general optimization method for vapor-cooled current leads is presented with taking into account the effect of convection heat transfer and extended surfaces. This analytical work is considered as a unified design method, since one formulation calculates the minimum heat load and the corresponding optimal design condition for arbitrary heat transfer condition, spanning two limiting cases—the zero convection (or conduction-cooled leads) and the perfect heat transfer. It is clearly shown that the augmentation of the convective cooling can reduce the heat load to a certain extent, but the optimum lead parameter required to minimize the heat load for the finite heat transfer may not exist between the two limiting values. A new dimensionless parameter called the Ch number is introduced to conveniently incorporate the convection effect into the optimization. The present method is demonstrated for two specific lead designs that have been recently developed for 10 kA level of applications.     

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.     

Transient thermodynamic behavior of cryogenic mixed fluid thermosiphon and its cool-down time estimation

Thermosiphon is an efficient heat transfer device by utilizing latent heat of fluid at liquid-vapor phase change. One of the disadvantages of thermosiphon, however, is that the operational temperature range is fundamentally limited from the critical point to the triple point of the working fluid to maintain two phase state. Nitrogen (N₂) and tetrafluoromethane (CF₄) were selected as the mixed working fluid to widen their original operational temperature range. Thermodynamic behavior of mixture and its effect on the cool-down time were investigated. A simple calculation model was proposed to estimate the cool-down time of the thermosiphon evaporator prior to experiments. The calculated results agreed well with the experimental results within 5% error. The cool-down time reduction was not achieved by mixing two components at once due to the separation of mixture. One idea to avoid this problem was suggested in this paper where the estimated cool-down time was reduced 17.8% compared to pure N₂.     

A numerical model of thermal analysis for woven wire screen matrix heat exchanger

Woven wire screen matrix heat exchanger (WSMHE) is a kind of compact, light-weight and high-efficiency matrix heat exchanger (MHE) for cryogenic applications. This paper presented a numerical model for the design and thermal analysis of WSMHE. The influence of wall thermal resistance, axial conduction, parasitic heat load and properties variation was taken into account, which is neglected in the traditional effectiveness-NTU method but important for compact cryogenic heat exchangers. The proposed numerical method is verified by effectiveness-NTU method under specific conditions, then it is tested according to experiment data, and a well agreement was obtained. Based on this model, a detailed analysis was performed on WSMHEs. The analysis results show that the axial conduction might result in evident decrease of effectiveness at low flow rate region; the effectiveness of WSMHEs with large flow rate could be remarkably enhanced by increasing its length; the influence of parasitic heat load varied little throughout of the flow rate region. Furthermore, the numerical model presented in this paper can be developed to the design and thermal analysis of small partition wall type heat exchangers.     

Thermodynamic design of methane liquefaction system based on reversed-Brayton cycle

A thermodynamic design is performed for reversed-Brayton refrigeration cycle to liquefy methane separated from landfill gas (LFG) in distributed scale. Objective of the design is to find the most efficient operating conditions for a skid-mount type of liquefaction system that is capable of LNG production at 160 l/h. Special attention is paid on liquefying counterflow heat exchanger, because the temperature difference between cold refrigerant and methane is smallest at the middle of heat exchanger, which seriously limits the overall thermodynamic performance of the liquefaction system. Nitrogen is selected as refrigerant, as it is superior to helium in thermodynamic efficiency. In order to consider specifically the size effect of heat exchangers, the performance of plate-fin heat exchangers is estimated with rigorous numerical calculations by incorporating a commercial code for properties of methane and the refrigerant. Optimal conditions in operating pressure and heat exchanger size are presented and discussed for prototype construction under a governmental project in Korea.