EP0614059B1 - Cooler with a cold finger of pulse tube type - Google Patents

Description

The present invention relates to coolers based on the Stirling cycle. These coolers allow to reach cryogenic temperatures.

They are used in particular to cool electronic components such as infrared detectors which operate at these temperatures.

These coolers include an oscillator electromechanical which generates, in an active enclosure containing a fluid, a pressure wave. The enclosure has a part equipped with a mobile regenerator or displacer which uses the expansion and compression cycles of the fluid to achieve Stirling cycle.

The fluid used is generally helium, under an average pressure of several hundred kilopascals. The oscillator can be rotary or linear.

The cold part generally has, to limit the heat losses by conduction and facilitate its manufacture, shape of a very elongated cylinder which earned him the name of "cold finger". The free end of the cold finger provides cooling capacity created by the expansion of the fluid. The base cold finger connected to the oscillator, dissipates heat created by compression of the fluid. The cold finger is usually immersed in a cryostat, such as a vase of Dewar for example, which contains the device to be cooled. The interior of the cryostat is generally subjected to vacuum to limit heat input.

There are two families of coolers: monoblock coolers and "separate" coolers.

In a monobloc cooler, the oscillator and the cold finger constitute one piece. The movement of regenerative is generally ensured, in the case of a rotary oscillator, by the oscillator piston. This configuration is very compact, it limits losses of charge between the oscillator and the cold finger. But the vibrations induced by the oscillator are transmitted to the device to be cooled.

In a "separate" cooler, the oscillator and cold finger are distant but connected by a pneumatic conduit which ensures the transfer of the pressure wave between the oscillator and the cold finger. The movement of the regenerator can be provided by a specific motor or by the effects tires generated by the pressure wave. The first one configuration is typically used in applications spatial. These two configurations make it possible to separate the oscillator and the equipment to be cooled which facilitates integration of the chiller into the environment of the device to be cooled. In addition, these two configurations significantly reduce vibrations at the level of the device to be cooled.

Mobile regenerative coolers come at a cost relatively high due to the required machining tolerances to realize the regenerator and the cold finger which are in movement relative to each other.

Pulsed tube type cold finger coolers are also known. The cold finger instead of containing a mobile regenerator contains a fixed regenerator and a tube pulsed.

The advantages of this kind of coolers are numerous. The lifespan of the cold finger is almost unlimited because removing movement reduces wear. Vibrations induced in the cold finger are much weaker than those induced in cold fingers with mobile regenerator. The costs are also much lower due to a operation less sensitive to geometric tolerances of manufacturing. Fixed regenerator reduces losses efficiency linked to the shuttle effect and fluid leaks between the wall outside of the cold finger and the regenerator. These coolers, in the as they use a double orifice pulsed tube, have a yield substantially equivalent to that of a cold finger cooler fitted with a mobile regenerator. A double orifice pulsed tube cooler is described later in Figure 1.

The major drawback of this type of cooler is related to the U shape of the cold finger. One of the branches of the U is made by the regenerator and the other by the pulsed tube. The base of the U which is also the free end of the cold finger is formed of an integral end piece on one side of the regenerator and on the other of the pulsed tube.

This cold finger imposes a specific cryostat and therefore prevents its implantation in a cryostat intended for a cold finger with regenerator mobile or Joule-Thomson type. On-site maintenance of the cold finger with pulsed tube is not possible while intervention on a cold finger with mobile regenerator is easy. Indeed, it would be necessary to break the vacuum cryostat to disassemble the pulsed tube or the regenerator.

The patent GB-A-1 202 203 and the article CRYOGENICS 1988, Vol. 28 August show pulsed tube type coolers in which the regenerator is tubular and is mounted coaxially around the pulsed tube.

The present invention aims to remedy these drawbacks. She offers a cooler based on the Stirling cycle, fitted with a cold finger pulsed tube type. This cooler can be installed in a cryostat classic and it has good thermodynamic performance.

More specifically, the cooler according to the invention comprises means for generating and transmitting a pressure wave in a fluid to a cold finger of the pulsed tube type. The cold finger has a regenerator pneumatically connected to the pulsed tube, this regenerator is tubular and is mounted coaxially around the pulsed tube. The regenerator is contained in an outer tube, the inner surface of the regenerator serving as a pulsed tube.

We can also consider that when the cold finger is immersed in a cryostat, the outer tube serves as the inner wall of the cryostat. This configuration that removes the inner wall of the cryostat improves cooling performance by reducing conduction losses.

• la figure 1: un refroidisseur connu, muni d'un doigt froid de type tube pulsé ;
• la figure 2 : un refroidisseur à régénérateur tubulaire connu monté coaxialement autour du tube pulsé ;
• la figure 3 : des courbes des températures du tube pulsé et du fluide qu'il contient en fonction de la position le long du tube ;
• les figures 4, 5 et 7 : des variantes d'un refroidisseur à régénérateur tubulaire connu monté coaxialement autour du tube pulsé ;
• la figure 6 : un refroidisseur selon l'invention.

The characteristics and advantages of the invention will appear on reading the following description, given by way of examples and illustrated by the appended figures which represent:

• Figure 1: a known cooler, provided with a cold finger of the pulsed tube type;
• FIG. 2: a known tubular regenerator cooler mounted coaxially around the pulsed tube;
• FIG. 3: curves of the temperatures of the pulsed tube and of the fluid which it contains as a function of the position along the tube;
• Figures 4, 5 and 7: variants of a known tubular regenerator cooler mounted coaxially around the pulsed tube;
• Figure 6: a cooler according to the invention.

In these figures the same elements bear the same references. For the sake of clarity, the ribs are not respected.

FIG. 1 represents a cooler based on the cycle of Stirling provided with a cold finger 1 of the pulsed tube type 5, according to known art. The cold finger 1 is connected to a pressure oscillator 2 through a base 3. The base 3 provides the mechanical interface and the seal between the cold finger 1 and the cryostat in which we usually plunge the cold finger 1. The base 3 forms the base of the cold finger 1. The cryostat is not shown for reasons of clarity.

The pressure oscillator 2 generates a pressure wave in a fluid and the fluid is successively compressed and expanded.

The cold finger 1 comprises a fixed regenerator 4 contained in a tube 7 and a pulsed tube 5 which form the two branches of a U. Regenerator 4 has the shape of a cylinder full. The base of the U is made by an end piece 6 cold which pneumatically connects regenerator 4 and the tube pulsed 5.

The tube 7 containing the regenerator has one end hot end fixed to the base 3 and a cold end fixed to the end piece 6. The end piece 6 constitutes the free end of the cold finger 1. This is the coldest point from the cooler. It is also used to transmit the device to cool, placed nearby, the frigories returned available through the expansion of the fluid.

The fixed regenerator 4 works in the same way than a mobile regenerator. It is made of a material porous fluid permeable. Regenerator 4 is connected pneumatically with oscillator 2.

The regenerator's function is to capture cold at fluid when the latter is sucked in by oscillator 2 during the relaxation phase and to dissipate heat towards this fluid when it is driven back during the compression phase.

The pulsed tube 5 simply consists of a tube substantially parallel to the tube 7 containing the regenerator 4. It is attached to a cold end of the end piece 6 and at the other hot end of the base 3.

A pneumatic circuit 8 is used to connect the oscillator 2 pressure to the regenerator 4 and to the pulsed tube 5. A tank buffer 9 is also provided and connected to the pneumatic circuit 8. It has sufficient volume for the fluid it contains to remain at a substantially constant pressure whatever the phase of the pressure oscillator 2. When the oscillator sucks the fluid, the fluid in the buffer tank 9 feeds the pulsed tube 5 and when oscillator 2 discharges, the discharged fluid fills the buffer tank 9.

The pneumatic circuit includes a first conduit 81 connecting the regenerator 4 to the oscillator 2, a second conduit 82 connecting the hot end of the pulsed tube 5 to the buffer tank 9 and a third conduit 83 connecting the hot end of the pulsed tube to the pressure oscillator 2. The pulsed tube is connected to both the oscillator and the buffer tank.

The fluid passes through a hot heat exchanger 10 between the hot end of the pulsed tube 5 and the oscillator 2 and / or the buffer tank 9. This hot exchanger 10 can be housed in the base 3.

In FIG. 1, the third conduit 83 is arranged between the first conduit 81 and the second conduit 82 and there reaches the second conduit 82 between the buffer tank 9 and the hot heat exchanger 10.

The hot heat exchanger 10 ensures the capture of the heat of compression of the fluid leaving the pulsed tube and its discharge, via base 3, to the outside of the cooler.

The movement of the fluid in the cold finger is out of phase with the pressure wave generated by the oscillator 2. The phase shift and the flow rates at the ends hot from pulsed tube 5 and tube 7 containing the regenerator 4 are a function of the pneumatic impedance of the conduits 81, 82 and 83 and the volume of the buffer tank 9. The setting of the pneumatic impedance of the conduits can be done by a choice adequate their section, their length. Ducts can also have simple pinches or holes 11 calibrated as in Figure 1 or even valves.

The behavior of the fluid in the pulsed tube is the next: consider a volume A of fluid which transits between the cold end of the pulsed tube 5 and the end piece 6. From phase shift of fluid movement in the cold finger with the expansion and compression phases of oscillator 2, this fluid when it relaxes passes to the end piece 6 while cooling and when it compresses, enters the pulsed tube 5 where it heats up almost adiabatically.

Now consider a volume B of fluid which passes between the hot end of the pulsed tube 5 and the exchanger hot 10. Due to the phase shift of the movement of the fluid in the cold finger with the relaxation and compression phases of oscillator 2, this fluid when it compresses transits to the hot exchanger by giving it its heat and when it relaxes enters the pulsed tube where it cools so almost adiabatic.

Let us consider a volume C of fluid which remains in permanence in the pulsed tube 5. This volume plays a role in buffer volume between the two previous volumes. He has the same shuttle effect as the mobile cooler displacer classics. It undergoes compression and expansion cycles adiabatic and reversible. He participates in exchanges mainly via the wall of the pulsed tube 5.

The cooler in Figure 2 is comparable to that of figure 1. The main difference is at cold finger level 21 which instead of having a regenerator and a pulsed tube configured in U has a tubular regenerator 24 mounted coaxially around the tube pulsed 25. The cold finger 21 is always connected to an oscillator 2 through a base 3. Base 3 plays the same role as on Figure 1.

The free end of the cold finger always ends by an end piece 26. It is always the most cooler.

Regenerator 24 plays the same role as in art known. Instead of having the shape of a full cylinder it has now the shape of a tube. Regenerator 24 is contained between an outer tube 27 and an inner tube 28. The outer tube 27 cylindrical has a hot end fixed tightly to the base 3 and a cold end fixed in a sealed manner to the end piece 26. It forms the outer surface of the finger cold 21. This outer tube 27 preferably has a thickness as fine as possible to limit thermal input on along the cold finger. It is preferably carried out in a material with a thermal conductivity as low as possible, for example stainless steel. This outer tube 27 as well as its attachments to the base 3 and to the end piece 26 must seal the inside of the cold finger vis-à-vis the external environment. The cold finger is usually immersed in a cryostat subjected to vacuum. This cryostat is represented with the reference 30 in FIG. 4 in the form of a Dewar vase.

The inner tube 28 serves both as a pulsed tube and inner wall of tubular regenerator 24. He is willing coaxially in the outer tube 27 and has a hot end fixed to the base 3. Its other end which is cold opens out in the end piece 26. This internal tube 28 is not like the outer tube 27 subjected to pressure differences important. It doesn't have to be strictly waterproof like the outer tube. It avoids a direct passage of the fluid from regenerator 24 to pulsed tube 25 without passing through the end piece 26. The design of this internal tube 28 with regard to the choice of the constituent material, its mode of fixing, and its thickness can be more easily optimized. According to the invention we physically remove the inner tube 28, the inner surface of the regenerator 24 being waterproof. In this case, it is the inner surface of the regenerator which serves as a pulsed tube.

End piece 26 looks like parts end of cold fingers with mobile regenerator. The regenerator and the pulsed tube communicate pneumatically thanks to her. To facilitate heat exchange between the fluid and the cooling device arranged near the end piece 26, the thickness of the end piece 26 will be as low as possible. It will be carried out in a material with a thermal conductivity as high as possible: copper for example. We can consider that the end piece contains a cold exchanger 29 formed by example of copper grates brazed at their periphery. This cold exchanger 29 improves the heat exchange between the fluid and the end piece 26.

The material of the end piece 26 will have, from preferably a coefficient of expansion as low as possible when cold finger 21 is used to cool a device placed directly on the end piece 26 (technique known by the Anglo-Saxon name of Integrated Dewar Cooler Assembly).

The end piece 26 could, for example, be provided with a tranquilizer to ensure a level the lowest possible turbulence in the pulsed tube 25. It it is indeed desirable to maintain a thermal gradient important between the two hot and cold ends of the tube pulsed. This transquillizing device can be produced by a honeycomb or cold exchanger part 29. The others elements of the cooler, namely oscillator 2, the circuit pneumatic 8, the buffer tank 9 and the hot exchanger 10 are comparable to those in Figure 1.

The hot exchanger 10 if it is configured with grids or a honeycomb material also has a role in tranquilizer.

Optimal adjustment of phase shift and amplitude of fluid flows in the regenerator 24 and the pulsed tube 25 will depend on the volume of the buffer tank 9 and the characteristics of the conduits 81, 82, 83 as before.

The fact of having placed the pulsed tube 25 outside of the regenerator 24 makes it possible to produce a cooler the efficiency is of the same order as that of the cooler of the figure 1. When we analyzed the behavior of the fluid in the pulsed tube, we have seen that the fluid which remains permanently in the pulsed tube takes part in the heat exchanges essentially via the wall of the pulsed tube. The following numerical example, illustrated by FIG. 3, shows that it is advantageous to limit to maximum this heat exchange because it has a harmful effect on the efficiency of the cooler.

It is assumed that the pulsed tube has a length of 70 mm and that in operation its extreme temperatures are 80 ° K at the cold end and 300 ° K at the hot end. It is assumed that the thermal gradient is linear in the wall of the pulsed tube, that the average pressure in the pulsed tube is 35.10 ⁵ Pa and that it varies more or less 10 ⁶ Pa, because of the pressure wave. We consider a thin slice of fluid located in the middle of the pulsed tube. During the compression and expansion cycles, its displacement between position al and position a2 will have an amplitude of at least 10 mm relative to its position a0 at rest. This physical data is typical of an infrared detector cooler.

The fluid slice will have an average temperature of 190 ° K but during the expansion and compression cycles, it will see its temperature oscillate between 166 ° K and 210 ° K (curve C1). This slice of fluid will be opposite with a section of pulsed tube whose temperatures will be between 158 ° K and 221 ° K (curve C2) because of the thermal gradient linear.

In the expansion phase, the fluid will be in contact with a portion of pulsed tube cooler than him and will tend to give it heat. Symmetrically, in the phase of compression, the fluid will be in contact with a portion of tube pulsed hotter than him and will tend to extract from it heat. This heat pumping effect from the hot end of the tube pulsed towards the cold end is harmful for the cooler efficiency. These heat exchanges with walls only concern part of the fluid: the boundary layer thermal which is close enough to the wall to have the time to exchange, mainly by gas conduction, the heat during a compression-expansion cycle. Such cooler reduces trade between the fluid and the wall of the pulsed tube by placing the tube pulsed inside the regenerator and not around.

If the cold finger is formed of two tubes (thick null to simplify) coaxial diameters 5mm and 3.5mm, in the case of a thermal boundary layer thickness of 0.2 mm, we can estimate that only 20% of fluid participates in heat exchange with the wall of the pulsed tube, if the tube pulse is placed inside the regenerator, while more than 50 of fluid participates in the heat exchange with the walls external and internal of the pulsed tube, if the regenerator is placed inside the pulsed tube.

Figures 4 and 5 show variants of a cooler with tubular regenerator mounted coaxially around the pulsed tube. We refer to figure 4.

The cold finger is immersed in a cryostat such as a Dewar 30 vase with two speakers 31, 32 inserted one inside the other and separated by a vacuum. The internal enclosure 32 has the shape of a well. The device to be cooled referenced 33 is disposed between the external enclosure 31 and the internal enclosure 32. It is fixed to the bottom of the well. A thermal coupler 34 is inserted between the free end of the cold finger 21 and the bottom of the well to optimize the cooling of the device to cool 33.

The pressure oscillator 2 is rotary. The reservoir buffer consists of the housing 35 of the oscillator which saves space.

The second conduit 82 and the third conduit 83 each have a valve 36 instead of a calibrated orifice, and in addition, the second conduit 82 is provided with a pinch 37 between the valve and the hot exchanger 10.

We now refer to Figure 5. The cold finger and the base are represented by a single block 50 for simplify. Now the pressure oscillator 51 is a resonant linear oscillator. The buffer tank 52 is provided with a heater 54. The temperature of the fluid in the buffer tank 52 is adjustable so that it can adjust the average pressure in the cooler and to be able adjust the resonant frequency of the cooler. this is particularly interesting in the case where the cooler is used in a satellite where we are looking for a frequency adjustable to avoid exciting the platform or instruments near the cooler. In this configuration, the hot heat exchanger consists of a device with fins 55 or equivalent. This device 55 is arranged on the second conduit 82 connecting the pulsed tube to the buffer tank 52. The other conduits 81, 83 and the tank 52 could also participate in the evacuation of the heat of compression of the fluid. To this end, they would be fitted with devices, fins for example, improving the dissipation of this heat outwards.

The buffer tank 52 is provided with a device tranquilizer 56 to ensure a level of turbulence too as low as possible in the pulsed tube. These measures tranquilizer 56 may be of the same nature as that described in the end piece 26 of FIG. 2.

Reference is made to FIG. 6 which shows a cooler according to the invention. Cold finger 60, immersed in a cryostat 61, and the rotary pressure oscillator 68 form a monobloc cooler. Compressor housing 69 constitutes the buffer tank. The cold finger 60 has a external tube 62, a tubular regenerator 63 and a pulsed tube 66. In this example, the outer tube 62 serves as a wall inside the cryostat. The inner surface of the regenerator tubular serves as a pulsed tube 66. The removal of the wall inside the cryostat can of course be used in other configurations.

The cooling device 65 is directly attached to the end piece 64 which connects the pulsed tube and the regenerator. The cooling is improved compared to the configuration where the outer tube and the inner wall of the cryostat are separate. In this figure, we placed a cooler 67 with circulating fluid around of the hot exchanger 10. This cooling device is preferably used when the pulsed tube is subjected to significant powers, for example greater than a few watts. Instead of using a circulation of fluid one could have used a device of natural or forced convection cooling with air for example. The configuration shown is particularly compact, it minimizes the losses of load between oscillator 68 and cold finger 60.

Refer to Figure 7. The cooler has now several pressure oscillators 70, 71, 72 mounted in parallel. A switch 73 having several input channels and an exit route allows the cold finger to be connected to one of the pressure oscillators 71. With a cold finger without a coin mobile, the only element presenting a risk of breakdown relatively high is the pressure oscillator which has moving parts. Switching from a pressure oscillator to a other can be controlled by the user or automatically when the operation of the oscillator in service is no longer normal. This switching requires neither intervention nor disassembly on the cold finger and the cryostat, it can be perform instantly and remotely.

The cooler according to the invention can cool any device including sensors or detectors, components electronics, samples, etc ...

Claims (12)

1. Cooler based on the Stirling cycle, which includes means for generating a pressure wave in a fluid and for transmitting it to a cold finger (60) of the pulsed-tube (66) type, comprising a tubular regenerator (63) pneumatically connected to the pulsed tube (66), mounted coaxially about the pulsed tube and placed inside an external tube (62), characterized in that the inner surface of the regenerator (63) serves as the pulsed tube.
2. Cooler according to Claim 1, characterized in that the cold finger is immersed in a cryostat, the external tube (62) forming the inner wall of the cryostat.
3. Cooler according to either of Claims 1 and 2, characterized in that the means for generating the pressure wave and for transmitting it to the cold finger comprise a pressure oscillator (68), a buffer reservoir (69) and a pneumatic circuit (8) which connects the pressure oscillator (68) and the buffer reservoir (69) to the base of the cold finger (60).
4. Cooler according to Claim 3, characterized in that the pneumatic circuit (8) includes a first duct (81) between the pressure oscillator (68) and the regenerator (63), a second duct (82) between the buffer reservoir (69) and the pulsed tube (66) and a third duct between the pressure oscillator (68) and the pulsed tube (66).
5. Cooler according to either of Claims 3 and 4, characterized in that the pressure oscillator (68) has a casing which serves as the buffer reservoir.
6. Cooler according to one of Claims 3 to 5, characterized in that a fluid passes through a heat exchanger (10) between the pressure oscillator (68) and/or the buffer reservoir (69) and the pulsed tube (66).
7. Cooler according to one of Claims 1 to 6, characterized in that the fluid passes through a cold exchanger between the regenerator (63) and the pulsed tube (66) at the free end of the cold finger (60).
8. Cooler according to one of Claims 3 to 7, in which the pressure oscillator (51) is a resonant linear oscillator, characterized in that a device (54) makes it possible to modify the temperature of the fluid in the buffer reservoir (52) and therefore its pressure so that the cooler has an adjustable resonant frequency.
9. Cooler according to one of Claims 3 to 8, in which the pressure wave undergoes a phase shift between the pressure oscillator (68) and the cold finger (60), characterized in that the pneumatic circuit includes means for regulating the said phase shift.
10. Cooler according to Claim 9, characterized in that the means for regulating the phase shift consist in adjusting the length and/or the cross-section of the ducts (81, 82, 83).
11. Cooler according to either of Claims 9 and 10, characterized in that at least one of the ducts (81, 82, 83) includes at least one element such as a means for pinching its wall, a gauged restriction or a valve so as to adjust the phase shift of the pressure wave.
12. Cooler according to one of Claims 3 to 11, characterized in that it includes a plurality of pressure oscillators (70, 71, 72) mounted in parallel, a switch (73) allowing the cold finger to be connected to one of the pressure oscillators (71).

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