Type Pulse Tube Cooler Research Articles

Numerical study of a cryogen-free vuilleumier type pulse tube cryocooler operating below 10 K

This paper presents a numerical investigation on a Vuilleumier (VM) type pulse tube cooler. Different from previous systems that use liquid nitrogen, Stirling type pre-coolers are used to provide the cooling power for the thermal compressor, which leads to a convenient cryogen-free system and offers the flexibility of changing working temperature range of the thermal compressor to obtain an optimum efficiency. Firstly, main component dimensions were optimized with lowest no-load temperature as the target. Then the dependence of system performance on average pressure, frequency, displacer displacement amplitude and thermal compressor pre-cooling temperature were studied. Finally, the effect of pre-cooling temperature on overall cooling efficiency at 5 K was studied. A highest relative Carnot efficiency of 0.82 % was predicted with an average pressure of 2.5 MPa, a frequency of 3 Hz, a displacer displacement amplitude of 6.5 mm, ambient end temperature 300 K and pre-cooling temperature 65 K, respectively.

Numerical and experimental study of an annular pulse tube used in the pulse tube cooler

Multi-stage pulse tube coolers normally use a U-type configuration. For compactness, it is attractive to build a completely co-axial multi-stage pulse tube cooler. In this way, an annular shape pulse tube is inevitable. Although there are a few reports about previous annular pulse tubes, a detailed study and comparison with a circular pulse tube is lacking. In this paper, a numeric model based on CFD software is carried out to compare the annular pulse tube and circular pulse tube used in a single stage in-line type pulse tube cooler with about 10 W of cooling power at 77 K. The length and cross sectional area of the two pulse tubes are kept the same. Simulation results show that the enthalpy flow in the annular pulse tube is lower by 1.6 W (about 11% of the enthalpy flow) compared to that in circular pulse tube. Flow and temperature distribution characteristics are also analyzed in detail. Experiments are then conducted for comparison with an in-line type pulse tube cooler. With the same acoustic power input, the pulse tube cooler with a circular pulse tube obtains 7.88 W of cooling power at 77 K, while using an annular pulse tube leads to a cooling power of 7.01 W, a decrease of 0.9 W (11.4%) on the cooling performance. The study sets the basis for building a completely co-axial two-stage pulse tube cooler.

Experimental study of the influence of cold heat exchanger geometry on the performance of a co-axial pulse tube cooler

Improving the performance of the pulse tube cooler is one of the important objectives of the current studies. Besides the phase shifters and regenerators, heat exchangers also play an important role in determining the system efficiency and cooling capacity. A series of experiments on a 10W @ 77K class co-axial type pulse tube cooler with different cold heat exchanger geometries are presented in this paper. The cold heat exchangers are made from a copper block with radial slots, cut through using electrical discharge machining. Different slot widths varying from 0.12mm to 0.4mm and different slot numbers varying from around 20–60 are investigated, while the length of cold heat exchangers are kept the same. The cold heat exchanger geometry is classified into three groups, namely, constant heat transfer area, constant porosity and constant slot width. The study reveals that a large channel width of 0.4mm (about ten times the thermal penetration depth of helium gas at 77K, 100Hz and 3.5MPa) shows poor performance, the other results show complicated interaction effects between slot width and slot number. These systematic comparison experiments provide a useful reference for selecting a cold heat exchanger geometry in a practical cooler.

Improved Performance of an Indigenous Stirling Type Pulse Tube Cooler and Pressure Wave Generator

Sustained efforts have been made in our laboratory to improve the performance of an indigenously developed pressure wave gen- erator by reducing the mechanical losses and the required input power. An acoustically matching pulse tube cooler, with a design target of 0.5W at 80K, was designed using Sage and experience gained from previous studies. The pulse tube cooler was fabri- cated and tested. The effect of regenerator stacking pattern on the cooler performance was studied by filling the regenerator with mesh of the same size #400 and with multi meshes #250, 325, 400. In present experiments, regenerator with #400 mesh at 30bar filling pressure performed better with more energy efficiency. A no load temperature of 74K was achieved with input power of 59W corresponding to a cooling power of 0.22W at 80K. Parasitic heat load to the cooler was measured be 0.68W. This heat load is primarily by heat conduction through the regenerator and pulse tube wall. By reducing the wall thickness from 0.30mm to 0.15mm, the parasitic loads can be reduced by 50%.

A novel effective suppression of natural convection in pulse tube coolers

Due to natural convection, the cooling performance of the pulse tube cooler (PTC) greatly degrades if the pulse tube cold-head is placed upward. In our previous study, it was found that the natural convection in the pulse tube can be well suppressed by inserting a certain number of circular screens. In this paper, a further detailed investigation into this suppression method are made, including the effect of number and material of screens, input acoustic power, and some other factors on suppressing natural convection at different inclination angles θ in the range of 0–180°. In our experiment of a single stage inline type PTC driven by a linear compressor, the orientation-dependence is considerably weakened with 6-layer copper screens in the pulse tube, in the cold temperature range of 40–80 K. Moreover, the cooling performance deterioration at θ = 0° induced by the inserted screens can be kept in an acceptable range. This novel suppression of natural convection greatly improves adaptability of pulse tube cryocoolers’ application.

CFD analysis of high frequency miniature pulse tube refrigerators for space applications with thermal non-equilibrium model

High frequency, miniature, pulse tube cryocoolers are extensively used in space applications because of their simplicity. Parametric studies of inertance type pulse tube cooler are performed with different length-to-diameter ratios of the pulse tube with the help of the FLUENT (R) package. The local thermal non-equilibrium of the gas and the matrix is taken into account for the modeling of porous zones, in addition to the wall thickness of the components. Dynamic characteristics and the actual mechanism of energy transfer in pulse are examined with the help of the pulse tube wall time constant. The heat interaction between pulse tube wall and the oscillating gas, leading to surface heat pumping, is quantified. The axial heat conduction is found to reduce the performance of the pulse tube refrigerator. The thermal non-equilibrium predicts a higher cold heat exchanger temperature compared to thermal equilibrium. The pressure drop through the porous medium has a strong non-linear effect due to the dominating influence of Forchheimer term over that of the linear Darcy term at high operating frequencies. The phase angle relationships among the pressure, temperature and the mass flow rate in the porous zones are also important in determining the performance of pulse tuberefrigerator.

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 Er 3Ni 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.

A single-stage pulse tube cooler reached 12.6 K

A single-stage G-M type pulse tube cooler (PTC) was designed and tested to explore the lowest attainable refrigeration temperature and to further improve the cooling performance in the temperature range of 15–40 K. The magnetic material Er 3Ni was used as part of the regenerative material besides the phosphor-bronze and the lead so as to improve the efficiency of the regenerator. With an input power of 6 kW, a lowest no-load refrigeration temperature of 12.6 K was obtained, which is a new record for the single-stage PTC. The cooling capacity at 15–40 K was also significantly improved, which may extend the application of the single-stage PTC for the cooling of superconductors and cryopumps.

A thermoacoustically driven cooler capable of reaching temperature below 77 K with no moving part

The pulse tube cooler has no cryogenic displacer and has attracted lots of attention in the field of cryocooler research. On the other hand, the thermoacoustic engine can generate self-oscillation and output work without moving components. Combining both technologies leads to a cryogenic cooler system with no moving components at all, which has great advantages of high reliability, low manufacturing cost, etc. Limited by largest available pressure ratio of thermoacoustic engines, up to now the best results on such a combined system are 88.6K when standing-wave thermoacoustic engine is used and 80.9K when traveling-wave thermoacoustic engine is used. Both the traveling-wave thermoacoustic engine and pulse tube cooler have oscillating flow inside. The phase relationship between pressure wave and volume flow rate is very important for conversion between heat and acoustic work. To achieve high efficiency, both systems require that traveling-wave acoustic field (which actually means pressure wave and volume flow rate is in phase) be realized somewhere inside the regenerator. To realize the above-mentioned phase relationship in a pulse tube cooler, pressure wave should lead volume flow rate by a certain degree (<90°) at pulse tube hot end due to compliance effect of the pulse tube. The orifice type pulse tube cooler cannot achieve this. Although the double-inlet type pulse tube cooler can realize this, but it has an intrinsic serious DC flow problem. The inertance tube turns out to be a better solution. It makes use of the acoustic characteristic of small diameter tube to generate the phase leading effect. In literature, the inertance tube is always connected with a large volume buffer (namely reservoir). According to our analysis, inertance tube with one end closed could as well provide the phase relationship needed at the pulse tube hot end. The reservoir is no longer necessary. For a traveling-wave thermoacoustic engine, which consists of a feedback loop and a resonance tube connected with the loop, arrangement of the loop components’ impedance is the key to realizing travelingwave acoustic field inside its regenerator. This impedance is related to the frequency of the thermoacoustic selfoscillation. In turn, the frequency is mainly determined by the dimension of the resonance tube. Besides, geometrical shape of the resonance tube is also very important. A suitable shape can eliminate higher harmonics and reduce viscous dissipation, which could lead to higher pressure ratio and efficiency. The inertance tube and resonance tube are generally cylindrical tubes with lengths being several meters. Their proper functioning in respective systems relies on the length and the ratio of radius to viscous penetration depth. Because of this, the inertance tube at pulse tube hot end is chosen among copper tubes with a diameter of several millimeters and length of several meters. The resonance tube of the engine consists of two sections: tapered tube and straight tube, whose details are introduced below. Configuration of the pulse tube cooler is shown in Fig. 1(a). The main dimensions are: regenerator i.d.15 mm, length 50 mm; pulse tube i.d.6mm, length 75 mm. It is designed to work at about 80Hz and needs an input work of about 30 W. We did a series of experiments before coupling the cooler with the thermoacoustic engine to verify the effect of inertance tube with one end closed. When the regenerator is filled with 300# stainless steel mesh, average pressure 2.8 MPa, frequency 84 Hz, and inlet pressure ratio 1.11, comparison results are listed in Table 1. It is apparent that the inertance tube much improves the cooler’s performance. When we finally coupled the cooler with the thermoacoustic engine, some changes were made: the regenerator is re-filled with 400# meshes; the inertance tube is changed into a 1.5 m long i.d.2 mm copper tube connected with a 1.6 m long i.d.4.8 mm copper tube. The tube with smaller diameter has the connection with pulse tube hot end. Configuration of the traveling-wave thermoacoustic engine is shown in Fig. 1(b). The loop is mainly made of i.d.80 mm tube and has a circumference of about 2 m. An

A separate two-stage pulse tube cooler working at liquid helium temperature

A novel 4 K separate two-stage pulse tube cooler (PTC) was designed and tested. The cooler consists of two separate pulse tube coolers, in which the cold end of the first stage regenerator is thermally connected with the middle part of the second regenerator. Compared to the traditional coupled multi-stage pulse tube cooler, the mutual interference between stages can be significantly eliminated. The lowest refrigeration temperature obtained at the first stage pulse tube was 13.8 K. This is a new record for single stage PTC. With two compressors and two rotary valves driving mode, the separate two-stage PTC obtained a refrigeration temperature of 2.5 K at the second stage. Cooling capacities of 508 mW at 4.2 K and 15 W at 37.5 K were achieved simultaneously. A one-compressor and one-rotary valve driving mode has been proposed to further simplify the structure of separate type PTC.