Influence of regenerator flow resistance on stability of pulse tube cooler

Based on the fluid network theory,the possibility of utilizing regenerator flow resistance to suppress the direct current （DC） flow induced by the introduction of a double-inlet in a pulse tube cooler is investigated theoretically. The calculation results show that increasing regenerator flow resistance can lead to a smaller extent of DC flow.Therefore,a better stability performance of the cooler can be realized.On this basis,the stability characteristics of the cooler with various regenerator matrix arrangements are studied by experiments.By replacing 30% space of 247 screens of stainless steel mesh at the cold part of the regenerator by lead balls of 0.25 mm diameter,a long-time stable temperature output at 80 K region is achieved. This achievement provides a new way to obtain stable performance for pulse tube coolers at high temperature and is helpful for its application.

Experimental Investigation of a Single-Stage Four-Valve Pulse Tube Refrigerator

In order to simplify the structure of the cold end of the pulse tube refrigerator (PTR) and have a better utilization of the cold energy of the system, a one-stage four-valve pulse tube refrigerator (FVPTR) with an "L" type pulse tube structure and two orifice valves at the hot end of pulse tube has been constructed. Verification experiments show that the two orifice valve structure performs better than one orifice valve structure. A lowest temperature of 67.5 K was obtained at a frequency of 2.5 Hz under a system average pressure of 1.5 MPa with 200 mesh bronze screens as regenerator material, 80 mesh copper screens as stuffing material of heat exchanger. Due to the difficulty in manufacturing the thin "L" type pulse tube, the wall thickness of the pulse tube in the experiment was relatively difficult for us to reach 0.5 mm as that of the ordinary pulse tube, which resulted in relatively big system loss and affected the minimum temperature of the system to some degree.

Study on a 5.0 W/80 K single stage Stirling type pulse tube cryocooler

A single stage Stifling pulse tube cryocooler was designed based on REGEN 3.2 and fabricated for testing. The experimental results show that the cooler can provide a cooling capacity of 5.0 W at 79.1 K, and produce a no-load temperature of 57.0 K, operating with an average pressure of 2.50 MPa and a frequency of 60 Hz, performance results that are very close to the calculated values. The cryocooler can be cooled from room temperature to 80 K in 8.5 min. The fast cooldown time is a result of the small regenerator.

Gas Flow in Unilateral Opening Pulse Tubes Based on Real Gas Equation of State

The real gas effect is dominant at high pressure and low temperature, and it is modeled by complex equations of state other than perfect gas law. In the vicinity of liquid-vapor critical point, the real gas exhibits unusual gas dynamic behavior. In the present work, a transient wave fields in unilateral opening pulse tube is simulated by solving the Navier-Stokes equations incorporated with the Peng-Robinson thermodynamic model. The computational fluid dynamics （CFD） results show a remarkable deviation between perfect gas model and real gas model for contact interface and shockwave. The wave diagram based on the real gas model can help to solve the problem of offset design point.

An Improved Water-filled Pulse Tube Method Using Time Domain Pulse Separation Method

Based on existing low-frequency water-filled impedance tube testing facilities, which is a part of the Low Frequency Facility of the Naval Undersea Warfare Center in Beijing, an improved water-filled pulse tube method is presented in this short paper. This proposed study is significantly different from the conventional pulse tube method because of the capability for a single plane damped sine pulse wave to generate in the water-filled pulse tube with a regular waveform and short duration time of about 1ms. During the generation process of the pulse, an inverse filter principle was adopted to compensate the transducer response. The effect of the characteristics of tube termination can be eliminated through the generation process of the pulse. Reflection coefficient from a water/air interface was measured to verify the proposed method. When compared with the expected theoretical values, a relatively good agreement can be obtained in the low frequency range of 500-2 000 Hz.

Computational fluid dynamic simulation of an inter-phasing pulse tube cooler

An inter-phasing pulse tube cooler (IPPTC) consists of two pulse tube units, which are connected to each other at hot ends of the pulse tubes through a needle valve. This paper presents the computational fluid dynamic (CFD) results of an IPPTC using a 2D axis-symmetrical model. General results such as the phase difference between pressure and velocity at cold end and hot end, the temperature profiles along the wall, the available lowest temperature as well as its oscillations and the coefficient of performance (COP) for IPPTC are presented. The formation of DC flow and its effects on the performance of the cooler are investigated and analyzed in detail. Turbulence, which is partially responsible for the poor overall performance of a single orifice pulse tube cooler (OPTC), is found to be much reduced in IPPTC and its performance is improved significantly compared with the single OPTC.

DC FLOW SUPPRESSION IN A SINGLE-STAGE G-M TYPE PULSE TUBE COOLER

An experimental investigation on DC flow suppression in a single-stage G-M type pulse tube cooler is made. The influence of DC flow induced by the introduction of the double-inlet on the refrigeration performance of the cooler is experimentally examined. Two parallelplaced needle valves with an opposite flow direction called as double-valved configuration, instead of conventional single-valved configuration as the double-inlet is used to reduce the DC flow. With the double-valved configuration, the minimum temperatures of 18.4 K and 14.7 K, and the cooling powers of 11.5 W and 29.5 W are also obtained by RW2 and CP4000, respectively.

Thermal Performance of a 4 K High-Frequency Pulse Tube Cryocooler with Different Working Fluids

The high-frequency pulse tube cryocooler(HPTC)represents a promising miniature cryocooling technology due to its compact structure and the absence of low-temperature moving components.However,limited to the non-ideal gas effect of4He,the HPTC is hard to obtain high cooling performance in the liquid helium temperature range.3He as the working fluid can effectively improve the cooling performance of the HPTC,but the high cost hinders its wide application.In consideration of both cooling performance and cost-effectiveness,this paper explores the feasibility of utilizing^(3)He-^(4)He mixtures as the working fluid for HPTCs.Firstly,the experimental results of a developed HPTC based4He are reported.With a total power consumption of 575 W,the lowest temperature of 3.26 K was observed.And the measured cooling power at 4.2 K was 20.8 mW.Then the theoretical utmost efficiency of the cryocooler was calculated in terms of the thermophysical properties of the working fluids,using ^(3)He-^(4)He mixtures with different compositions as the working fluids.The whole machine modeling of the HPTC was further carried out,and the influence of the working fluids with different components on the structural parameters such as double-inlet and inertance tube,and operating parameters such as pressure and frequency were analyzed.The calculated results show that the cooling power is expected to be increased to36 mW and 53 mW if the equimolar ^(3)He-^(4)He mixture and pure ^(3)He are used,respectively.

Optimization of the Swept Volume Ratio between the Compressor and the Active Displacer in an Efficient 20 K Thermal-Coupled Two-Stage Pulse Tube Cryocooler

As an important component of the stirling-type pulse tube cryocooler(SPTC),an efficient phase shifter can significantly improve the cooling capacity.Compared to the common phase shifter,the active warm displacer(AWD)has a wider phase adjustment range and therefore can obtain a better phase relationship easily.Based on a two-stage thermal-coupled SPTC operating in the 20 K range,this paper studied the influence of the swept volume ratio between the compressor and displacer.The research found that the swept volume ratio changes the cooling capacity and efficiency of the cryocooler mainly by changing the phase difference between the pressure wave and the volume flow at the cold end.It was found from the results of the simulation and experiments that there is an optimal displacement of the displacer(Xd)of 2.5 mm and an optimal phase angle of 15°to obtain the highest cooling efficiency while the displacement of the compressor is constant.The cooling capacity at 20 K is 1.3 W while the input electrical power of the second stage compressor is 202 W,which indicates an overall relative Carnot efficiency(rCOP)of 0.055 in terms of input electrical power.In addition,due to the reasonable setting of precooling temperature and capacity,the swept volume ratio and phase at the maximum cooling capacity and maximum efficiency are consistent in this study.The research improves the understanding of phase shifters and has guiding significance for the optimization of the SPTC working below 20 K.

Energy distribution between liquid hydrogen and liquid oxygen temperatures in a Stirling/pulse tube refrigerator

A two-stage gas-coupled Stirling/pulse tube refrigerator(SPR),whose first and second stages respectively involve Stirling and pulse tube refrigeration cycles,is a very promising spaceborne refrigerator.The SPR has many advantages,such as a compact structure,high reliability,and high performance,and is expected to become an essential refrigerator for space applications.In research regarding gas-coupled regenerative refrigerator,the energy flow distribution between the two stages,and optimal phase difference between the pressure wave and volume flow,are two critical parameters that could widely influence refrigerator performance.The effects of displacer displacement on the pressure wave,phase difference,acoustic power distribution,and inter-stage cooling capacity shift of the SPR have been investigated experimentally.Notably,to obtain the maximum first-stage cooling capacity,an inflection point in displacement exists.When the displacer displacement is larger than the inflection point,the cooling capacity could be distributed between the first and second stages.In the present study,an SPR was designed and manufactured to work between the liquid hydrogen and liquid oxygen temperatures,which can be used to cool small-scale zero boil-off systems and space detectors.Under appropriate displacer displacement,the SPR can reach a no-load cooling temperature of 15.4 K and obtain 2.6 W cooling capacity at 70 K plus 0.1 W cooling capacity at 20 K with 160 W compressor input electric power.

Design and preliminary experimental investigation of a 4K Stirling-type pulse tube cryocooler with precooling

A Stirling-type pulse tube cryocooler （PTC） with precooling was designed and manufactured to investigate its performance at 4 K. Numerical simulation was carried out based on the well-known regenerator model REGEN with an emphasis on the performance of a 4 K stage regenerator of the Stifling-type PTC as influenced by the warm end temperature, pressure ratio, frequency and average pressure with helium-4 and helium-3 as the working fluid respectively. This study demonstrates that the use of a cold inertance tube can significantly improve the efficiency of a 4 K Stirling-type PTC. A preliminary experimental investigation was carried out with helium-4 as the working fluid and a refrigeration temperature of 4.23 K was achieved. The experimental results show that the operating frequency has a significant influence on the performance of the Stirling-type PTC and a relatively low average pressure is favorable for decreasing the loss associated with the real gas effects of a 4 K Stirling-type PTC.

Development of a linear compressor for two-stage pulse tube cryocoolers

A valveless linear compressor was built up to drive a self-made two-stage pulse tube cryocooler. With a designed maximum swept volume of 60 cm3, the compressor can provide the cryocooler with a pressure volume (PV) power of 400 W. Preliminary measurements of the compressor indicated that both an efficiency of 35%～55% and a pressure ratio of 1.3～1.4 could be obtained. The two-stage pulse tube cryocooler driven by this compressor achieved the lowest temperature of 14.2 K.

Advance in research of several types of streaming of pulse tube refrigerators

The pulse tube refrigerator （PTR） is a promising small-scale cryocooler. This paper first briefly introduces the history of the pulse tube refrigerator. It has pointed out that technology improvements and theoretical developments of the pulse tube refrig- erator closely relate with the internal streaming effects. Then the discovering history and classification of the streaming or DC （direct current） flow effect are summarized. It proposes for the first time that the physical significance of the streaming con- tains the driving mechanisms and the transport mechanisms. It demonstrates that the driving mechanisms are the asymmetry of fluid flow and temperature while the transport mechanisms are a loop or vorticity, which transmits nonlinear dissipations. The important advancements have been made over the past two decades all over the world in research of streaming of the pulse tube refrigerator including Gedeon DC flow, Rayleigb streaming, the third type of DC flow and the regenerator circulation. With regard to Gedeon DC flow, theoretical and experimental analyses have been made and different suppression methods are summarized. In the aspect of Rayleigh streaming, it mainly focuses on the analytical solution of the second-order mass flow and the research of tapered pulse tubes. In particular, limited research on the third type of DC flow and regenerator circulation is presented. The experimental measurement techniques of streaming also are summarized. Finally, this paper briefly discusses the key scientific and technical issues of the current research, and foretells the future development trends of streaming research in PTR.

Theoretical Analyses and Design of a 4 K Gas-Coupled Multi-Bypass Stirling-Type Pulse Tube Cryocooler

Gas-coupled Stirling-type pulse tube cryocooler(SPTC) is currently the most compact and simplest configuration among all types of cryocoolers, but it is challenging to achieve a very low temperature. This paper investigates a gas-coupled SPTC which is capable of directly achieving a temperature of around 4 K. Theoretical analyses were performed based on SAGE to study the effects of employing one or more multi-bypass structures on apparent cooling performance, and internal working parameters. The simulation results indicate that the function of the multi-bypass is similar to that of a multi-stage gas-coupled structure, producing a pre-cooling effect on the lower-temperature section by increasing the acoustic power and the enthalpy flow in the pulse tube of the higher-temperature section. The cooperation of two multi-bypass structures can promote a higher enhancement of the cooling performance, but it is difficult to achieve the same cooling performance of a completely multi-stage gas-coupled SPTC due to weak phase-shifting capability and excessive reduction of the mass flow. Based on the model, the developed prototype has achieved a no-load temperature of 4.4 K, which shows the great potential of using a gas-coupled SPTC to obtain a cooling temperature below 4 K.

Dynamic Experimental Study of a Multi-bypass PulseTube Refrigerator with Two-bypass Tubes

A dynamic experimental apparatus to measure the instantaneous velocity and pressure in the multi-bypass pulse tube refrigerator (MPTR) was designed and constructed. Some theortant experimental results of the instantaneous measurements of the velocity and the pressure in the MPTR with two-bypass tubes during actual operation are presented. The effects of the middle-bypass version on the dynamic pressure and mass flow rate at the cold end of the pulse tube are evaluated from experimental measurements.DC-flow phenomena are observed in this MPTR. The reasons of the multi-bypass version improved the performance of pulse tube refrigerator are given.

Influence of input acoustic power on regenerator's performance

Performance of a pulse tube cooler significantly depends on the efficient operation of its regenerator. Influence of input acoustic power on regenerator's performance is simulated and analyzed with simple harmonic analysis method. Given regenera-tor's dimensions and pressure ratio,there is an optimal input acoustic power for achieving a highest coefficient of performance,due to a compromise between relative time-averaged total energy flux in regenerator and relative acoustic power at regenerator's cold end. Additionally,optimal dimensions of regenerator are also estimated and presented for different input acoustic powers. The computed optimal diameter obviously increases with increase of input acoustic power,while the optimal length decreases slightly,and as a result,a larger input acoustic power requires a smaller aspect ratio (length over diameter).