REGENERATIVE CRYOGENIC MACHINE

Abstract

The application relates to a cryogenic machine of the regenerative type, comprising: a pressure oscillator,at least one cold finger (20) in fluid connection with the pressure oscillator,

Description

TECHNICAL FIELD

The application relates to a cryogenic machine of the regenerative type (for example pulsed gas tube, Stirling, Gifford-McMahon, etc.).

BACKGROUND

There are different types of cryogenic machines. These cryogenic machines are classified according to two types: recuperative coolers and regenerative coolers.

Recuperative coolers (Joule-Thomson or inverted Brayton cycles) are based on a continuous flow of the working fluid, usually a gas, which is compressed and expanded, the expansion taking place continuously in an orifice for the Joule-Thomson cycle or in a turbine for the Brayton cycle. The term “recuperative” comes from the fact that an exchanger, generally countercurrent, is used to recover the enthalpy of the cold gas coming from the expansion to pre-cool the hot gas coming from the compressor.

For regenerative coolers, the flow of the working fluid is alternating. Gas compression and expansion take place cyclically at a frequency of a few Hertz for the coolers called “low-frequency” coolers and at several tens of Hertz for the coolers called “high-frequency” coolers.

As with recuperative coolers, the enthalpy of the cold gas derived from the expansion must be recovered. However, it is not possible to use an exchanger in the case of a cyclic operation. A regenerator is then used to perform this function. The regenerator allows transferring the enthalpy from the cold gas to the hot gas between two cycles. The regenerator also implements a thermal storage to ensure this heat transfer at two different times.

FIGS. 1 and 2 are block diagrams of a regenerative cryogenic machine of the pulsed gas tube type operating at high frequency (i.e. above 20 Hz).

FIG. 3 is a block diagram of a regenerative cryogenic machine of the Stirling-type operating at high frequency.

The diagrams in FIGS. 1 to 3 represent the general topology of these coolers, i.e. in a schematic configuration called “in line” configuration of the cold fingers. In other embodiments, the topology of these cold fingers can also be “U” shaped or coaxial, while keeping the same operating principle and the same components.

The machine comprises an oscillator 1′ and a cold finger 20 in fluid connection with the oscillator. The machine contains a working fluid, usually helium.

The oscillator 1′ takes the form of a piston with a back-and-forth motion schematized by the bidirectional arrow, thus generating a pressure wave in the working fluid. This is referred to a “pressure oscillator” because the back-and-forth motion of the piston allows generating a pressure oscillation and not a pressure difference as in the recuperative machines.

The cold finger 20 (which can in particular be of the pulsed gas tube, Stirling or Gifford-McMahon type) allows the production of the cooling effect.

In the case of a pulsed gas tube (cf. FIGS. 1 and 2), the cold finger comprises a first heat exchanger 2, a regenerator 3, a second heat exchanger 4, a pulse tube 5, a third heat exchanger 6 and a phase shifter 7, 8 or 9.

When the piston moves to the right of FIG. 1 or 2, the working fluid is compressed and passes through the first heat exchanger 2 and the regenerator 3. The regenerator having a high specific heat and being thermally insulated from the outside of the machine, the temperature of the working fluid switches from a first temperature T1, which is generally the ambient temperature to which the machine is exposed, to a second temperature T2 lower than T1. The second temperature is a cryogenic temperature, i.e. typically below 120K. During this phase, the working fluid gives energy to the regenerator 3 which stores it due to its high specific heat.

The working fluid enters the pulse tube 5 through the second heat exchanger 4. In the tube 5, which is thermally insulated from the outside of the machine, the working fluid undergoes compression and adiabatic expansion successive to the operating frequency of the oscillator 1′.

The compression work is discharged to the end of the pulse tube 5, in a third heat exchanger 6 operating at ambient temperature while at the other end of the pulse tube 5, the expansion allows lowering the temperature of the gas in the second exchanger 4, reaching a cryogenic temperature.

This thermal decoupling effect on either side of the cold finger is ensured by a phase shift system or phase shifter 7, 8. This system ensures the necessary phase shift between the pressure wave and the flow rate in the cold finger so that the expansion takes place at the cold exchanger 4. The phase shifter generally consists of an inertance 7 and a buffer tank 8. The inertance has a small passage section compared to that of the pulse tube 5, and the buffer tank 8 has a high volume compared to that of the tube and of the inertance; consequently, the pressure within the buffer tank 8 is substantially constant.

In some cases, as illustrated in FIG. 2, this phase shifter is in the form of an expansion piston 9, similar to the pressure oscillator 1′ but of different volume and power.

After the expansion phase producing the cooling effect, the working fluid passes through the regenerator 3 in the opposite direction and this time it is the regenerator that gives the energy stored during the compression to the working fluid cooled during the expansion.

In the case of a Stirling cooler (cf. FIG. 3), the general configuration is similar to that of the pulsed gas tube, with the difference that the cold finger 20 uses a mechanical expander 9 to ensure the expansion of the working fluid. In other words, the pulse tube 5, the exchanger 6 and the phase shifter 7, 8 are eliminated in the case of a Stirling cold finger.

In both cases, and in general for all regenerative machines, the pressure oscillator impacts the following aspects: cost, performance, size, mass, reliability.

Furthermore, it is difficult to offset the oscillator from the cold finger because the addition of volume in the system lowers the amplitude of the pressure wave and the flow rate and thus lowers the cooling capacity.

Finally, these pressure oscillators use linear motors to create the back-and-forth motion of the piston. This type of motor has several drawbacks.

On the one hand, even if they are mechanically balanced, these motors generate problematic vibrations for space applications. They then require binding auxiliary systems and complex, compromising and costly control electronics.

On the other hand, these oscillators are difficult to extrapolate at high powers. Indeed, beyond a few hundred Watts, the size, mass and cost of these oscillators become problematic.

BRIEF DESCRIPTION

One aim of the application is to overcome the aforementioned drawbacks and particularly to design a cryogenic machine in which the generation of the pressure oscillation is carried out by a means that is less expensive, more reliable and generates less vibration than existing oscillators. Furthermore, said machine must be able to be used in a high or low power cooler.

To this end, the application proposes a cryogenic machine of the regenerative type, comprising:

• • a pressure oscillator,
  • at least one cold finger in fluid connection with the pressure oscillator, said machine being characterized in that the pressure oscillator comprises a centrifugal compressor and a fluid distribution member configured to alternately distribute high-pressure and low-pressure working fluid from the centrifugal compressor into said cold finger.

The use of a centrifugal compressor coupled to a fluid distribution member instead of a linear motor pressure oscillator has several advantages.

On the one hand, the centrifugal compressor does not generate vibrations, which is particularly advantageous in the space field and in all applications where vibrations could disturb the operation of devices.

On the other hand, as the transmission of the pressure wave does not depend on the volume between the compressor and the cold finger, the compressor can be offset from the cold finger, which authorizes greater freedom in the design of the machine and in particular greater compactness, which is particularly sought for embedded applications.

Finally, such a compressor is reliable and changing according to the required power.

It is furthermore possible to couple several cold fingers (for example Stirling and/or pulsed gas tube) on the same compressor.

According to optional but advantageous characteristics, possibly combined when technically possible:

• • the compression ratio of the centrifugal compressor is between 1.1 and 1.5;
  • the operating frequency of the pressure oscillator is greater than 10 Hz;
  • the centrifugal compressor is arranged between a buffer tank called low-pressure buffer tank and a buffer tank called high-pressure buffer tank, the fluid distribution member being configured to selectively put in fluid connection the cold finger and one of the low-pressure and high-pressure buffer tanks:
  • the fluid distribution member comprises a rotary valve or a linear distribution valve;
  • the machine comprises a cold finger of the pulsed gas tube type including a pulse tube, an exchanger and a phase shifter;
  • the machine comprises a cold finger of the Stirling type including an expansion piston;
  • the fluid distribution member is configured to be fluidly actuated by the working fluid or mechanically by an outer actuator;
  • the fluid distribution member is configured to be actuated by the control rod of the expansion piston of the cold finger;
  • the machine contains helium as working fluid;
  • the machine comprises several cold fingers, each cold finger being fluidly connected to one or several centrifugal compressors;
  • the machine further comprises a circuit for circulating working fluid from the high-pressure buffer tank to the low-pressure buffer tank, so as to cool a part offset from the cold finger and mechanically decoupled from said cold finger.

Another object of the application relates to a spacecraft comprising a cryogenic machine as described above.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages will emerge from the following detailed description, with reference to the appended drawings, in which:

FIG. 1 is a block diagram of a cryogenic machine of the pulsed air tube type according to the state of the art;

FIG. 2 is a block diagram of another cryogenic machine of the pulsed air tube type according to the state of the art;

FIG. 3 is a block diagram of a cryogenic machine of the Stirling type according to the state of the art;

FIG. 4 is a block diagram of a cryogenic machine of the pulsed gas tube type according to one embodiment;

FIG. 5 is a block diagram of a cryogenic machine of the Stirling type according to one embodiment;

FIG. 6 is a block diagram of a cryogenic machine of the Stirling type according to another embodiment;

FIG. 7 is a block diagram of a cryogenic machine of the pulsed gas tube type integrating a thermal switch function according to one embodiment.

Naturally, these figures are given for illustrative purposes only.

Identical reference signs from one figure to the other refer to identical elements or fulfill the same function.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 4 is a block diagram of a cryogenic machine of the pulsed air tube type according to one embodiment. The reference signs identical to those of FIG. 1 refer to identical elements or elements fulfilling the same function. These elements will therefore not be described again in detail.

Particularly, the cold finger 20 is similar to that of existing machines, for example to that of FIG. 1 or to that of FIG. 2.

The oscillator 1 comprises a centrifugal compressor fluidly coupled on the one hand to a buffer tank 10 called low-pressure buffer tank and a buffer tank 11 called high-pressure buffer tank. In the present text, the terms “low-pressure” and “high-pressure” are relative terms, a low pressure being lower than a high pressure.

The oscillator further comprises a fluid circuit connecting the cold finger to each of the buffer volumes 10, 11.

The oscillator finally comprises a fluid distribution member 12 arranged in the fluid circuit, making it possible to selectively and alternately put the cold finger in fluid connection with the buffer tank 10 or the buffer tank 11.

This distribution member 12 can be advantageously a rotary valve or a linear actuator, but any other type of actuator could be used as long as it allows alternately distributing the high-pressure and low-pressure gas in the cold finger. For example, each buffer tank could be fitted with a respective valve, said valves being configured to open or close depending on the phase of the operating cycle of the machine.

By buffer tank, it is meant that the volume of the tanks 10 and 11 is large enough compared to the volume of the fluid circuit which connects the tanks and the cold finger so that the pressure generated by the centrifugal compressor in the said tanks 10, 11 remains substantially constant. These tanks can be possibly eliminated if the volume of the fluid circuit ensures this function or if the performance of the cold finger is not impacted by this pressure fluctuation.

For helium, for example, a compression ratio between 1.1 and 1.5 will be sought to replace the pressure oscillator with a centrifugal compressor and a fluid distribution member. This compression ratio is completely compatible with the compression ratio generated by a centrifugal compressor. The pressure oscillator can therefore be directly replaced by a centrifugal compressor coupled to a fluid distribution member.

In practice, conventional coolers are filled at an average pressure from 20 to 40 bars. Then, the pressure oscillates due to the pressure oscillator around this average pressure with an amplitude from +/− 2 to 5 bars. The average pressure and the amplitude of the pressure wave are parameters specific to each cooler.

The operating frequency of the pressure oscillator is advantageously greater than or equal to 10 Hz.

The operation of the proposed cryogenic machine is as follows.

In a first phase of the cycle, the cold finger 20 is in fluid connection with the high-pressure buffer tank 11 via the valve 12. The working fluid passes through the first exchanger 2, the regenerator 3 and the second exchanger 4 towards the tube 5. The working fluid switches from the ambient temperature T1 to the cryogenic temperature T2; the heat of the working fluid transferred to the regenerator 3 is accumulated therein.

In the tube 5, the working fluid undergoes adiabatic compression.

Under the effect of the compression of the fluid in the tube 5, part of the fluid is pushed towards the buffer tank 8 through the inertance 7.

In a second phase of the cycle, the valve 12 is actuated so as to interrupt the fluid connection between the cold finger and the high-pressure buffer tank 11 to establish a fluid connection between the cold finger and the low-pressure tank 10.

The working fluid undergoes adiabatic expansion in the tube 5. Part of the fluid is drawn from the buffer tank 8 towards the tube 5 through the inertance 7. The working fluid passes through the second heat exchanger 4 and the regenerator 3, which restores the stored heat via the first heat exchanger 2.

Contrary to the case where the compressor is a volumetric compressor such as the piston illustrated in FIGS. 1 and 2, the centrifugal compressor allows decoupling the compression area from the cold finger. Indeed, the pressure wave can be transmitted over a sufficiently long distance and does not depend on the volume of fluid between the compressor and the cold finger.

Consequently, the oscillator is not necessarily aligned with the cold finger as represented in FIG. 4, but can be arranged at another location in the machine, depending on the space constraints encountered.

The same operating principle is applicable to a machine comprising a Stirling cold finger, as illustrated in FIG. 5.

In this machine, the centrifugal compressor 1 and the fluid distribution member are similar to those already described with reference to FIG. 4.

Furthermore, the regenerator 3 and the expander 9, which form the cold finger 20 of the machine, are similar to those of FIG. 3.

As explained above, the centrifugal compressor allows decoupling the compression area from the cold finger. Indeed, the pressure wave can be transmitted over a sufficiently long distance and does not depend on the volume of fluid between the compressor and the cold finger.

Consequently, the compressor is not necessarily aligned with the cold finger as represented in FIG. 3, but can be arranged at another location in the machine, depending on the space constraints encountered.

It is also possible to couple the fluid distribution member with the driving of the expansion piston 9 as shown in FIG. 6. In this embodiment, the cold finger is of the coaxial type, the expansion piston 9 being arranged in the regenerator 3.

In other embodiments, regardless of the type of cold finger, the fluid distribution member can be actuated fluidly by the working fluid or mechanically by an outer actuator.

In some embodiments, it is possible to couple several cold fingers, of the same type or of different types (pulsed air tube, Stirling, Gifford-McMahon, etc.) to an oscillator or several oscillators each comprising a centrifugal compressor and one or several fluid distribution members.

This coupling is furthermore independent from the configuration of the cold fingers, which can be for example in line, coaxial, with active expander, with inertance, alpha, beta, with free piston, etc. Consequently, the figures should not be construed as limiting the invention to a particular cold finger configuration.

In addition, the oscillator(s) can be offset from the cold finger(s).

The oscillator therefore allows forming a wide variety of regenerative cryogenic machines, with great freedom of choice in the arrangement of the different components.

Referring to FIG. 7, it is also possible to integrate in the cryogenic cooler, an integrated “thermal link” function that allows:

• • offsetting the cold finger 20 to a position away from the part to be cooled which is represented by the exchanger 50;
  • mechanically decoupling the cold finger 20 from the part to be cooled, making it possible to simplify the cold finger and its integration into the application using the cryogenic machine;
  • offering a thermal switch function linked to the inherent operation of the cryogenic cooler.

This thermal link function is performed by using the pressure difference between the buffer tanks 10 and 11 to ensure circulation of working fluid from the high-pressure buffer tank to the low-pressure buffer tank.

The working fluid is cooled to a cold temperature close to T2 by a countercurrent exchanger 40, then to the temperature T2 on an exchanger integrated into the cold exchanger 4. The cold working liquid is then offset at a distance ranging from a few centimeters to several meters to cool the part to be cooled via the exchanger 50. The working fluid is heated in the exchanger 50 and then returns to the countercurrent exchanger 40 to be re-injected into the low-pressure tank buffer 10.

The secondary fluid circuit 51 and 52 constituting the thermal link can be made with small-dimensioned tubes making it possible to limit the mass of the system, to lower the stiffness of the tubes (in order to ensure mechanical decoupling between the components) or to limit the losses by conductions along these tubes.

This thermal link is therefore passive in the sense that, when the cooler is operating, the circulation of working fluid is effective and so is the thermal coupling. Conversely, if the cryogenic cooler is stopped, there is no thermal coupling.

This is then referred to as thermal switch, having a thermal coupling/decoupling function. This function is particularly useful for systems integrating several cold fingers (case of a spacecraft in particular integrating a nominal cooler and a redundant cooler). The non-operating cold fingers are then thermally decoupled from the part to be cooled and thus do not cause heat losses.

Although the cold finger 20 represented in FIG. 7 is of the pulsed gas tube type, it goes without saying that any other type of cold finger could be used in connection with this thermal switch.

Claims

1. A cryogenic machine of a regenerative type, comprising: a pressure oscillator,at least one cold finger in fluid connection with the pressure oscillator,wherein the pressure oscillator comprises a centrifugal compressor and a fluid distribution member configured to alternately distribute high-pressure and low-pressure working fluid from the centrifugal compressor into said cold finger.

2. The cryogenic machine according to claim 1, wherein the compression ratio of the centrifugal compressor is between 1.1 and 1.5.

3. The cryogenic machine according to claim 1, wherein the operating frequency of the pressure oscillator is greater than 10 Hz.

4. The cryogenic machine according to claim 1, wherein the centrifugal compressor is arranged between a buffer tank called low-pressure buffer tank and a buffer tank called high-pressure buffer tank, the fluid distribution member being configured to selectively put in fluid connection the cold finger and one of the low-pressure and high-pressure buffer tanks.

5. The cryogenic machine according to claim 1, wherein the fluid distribution member comprises a rotary valve or a linear distribution valve.

6. The cryogenic machine according to claim 1, comprising a cold finger of a pulsed gas tube type including a pulse tube, an exchanger and a phase shifter.

7. The cryogenic machine according to claim 1, comprising a cold finger of a Stirling type including an expansion piston.

8. The cryogenic machine according to claim 1, wherein the fluid distribution member is configured to be fluidly actuated by the working fluid or mechanically by an outer actuator.

9. The cryogenic machine according to claim 7, wherein the fluid distribution member is configured to be actuated by a control rod of the expansion piston of the cold finger.

10. The cryogenic machine according to any of claim 1, containing helium as working fluid.

11. The cryogenic machine according to claim 1, comprising several cold fingers, each cold finger being fluidly connected to one or several centrifugal compressors.

12. The cryogenic machine according to claim 4, further comprising a circuit for circulating working fluid from the high-pressure buffer tank to the low-pressure buffer tank, so as to cool a part offset from the cold finger and mechanically decoupled from said cold finger.

13. A spacecraft comprising a cryogenic machine according to claim 1.