GB2488604A

GB2488604A - Cryogenic Cooler Arrangement with Improved Heat Transfer

Info

Publication number: GB2488604A
Application number: GB1103782.7A
Authority: GB
Inventors: Thomas William Bradshaw; Tristan Davenne; Christopher James Pulker; Martin Crook
Original assignee: Science and Technology Facilities Council
Current assignee: Science and Technology Facilities Council
Priority date: 2011-03-04
Filing date: 2011-03-04
Publication date: 2012-09-05
Anticipated expiration: 2031-03-04
Also published as: GB2488604B; GB201103782D0

Abstract

There is disclosed a displacement type Stirling cycle cryogenic cooler which has a displacer cylinder 34 extending axially between warm and cold ends 44, 42, respectively. The cooler has a regenerator 40 arranged to reciprocate axially within the displacer cylinder and to provide a heat exchanging fluid path between the warm and cold ends. The arrangement can further include a flow shaper element 70, for example an aperture, provided at the cold end of the regenerator, the flow shaper being arranged to increase forced convection in the cold end of the cylinder by shaping the flow of fluid entering the cold end of the cylinder from the regenerator. Alternatively, the arrangement can include a thermal block 68 provided at the cold end of the displacer cylinder, the thermal block having an exposed face 74 arranged to deliver heat to the operating fluid, and the exposed face is patterned to increase heat transfer from the thermal block to the operating fluid.

Description

Cryogenic Cooling The invention relates to cryogenic cooling using a Stirling cycle.

Introduction

Cryogenic coolers which use a displacement type Stirling cycle are frequently used in cooling subsystems for sensors and focal plane arrays in commercial, military and laboratory applications. Lightweight, maintenance free and long life versions of such coolers have been developed, for example, for use in space based applications.

Displacement type Stirling cycle cryogenic coolers generally make use of a cylinder extending axially between cold and warm ends. A compressor is used to drive pressure oscillations in the cylinder typically with a frequency of a few Hz to a few tens of Hz. A regenerator assembly is fitted closely within the cylinder and driven to reciprocate axially. Typically the regenerator assembly occupies most of the length of the cylinder and reciprocates by only a small fraction of the cylinder length, but allows fluid within the cylinder to pass axially within it while at the same time acting as a heat source/sink to the passing fluid. By setting an appropriate phase difference between the compressor oscillations and the reciprocation of the regenerator assembly, heat is pumped from the cold end of the cylinder to the warm end.

Typical operating temperatures of the cold and warm ends are a few tens of Kelvin and around 300 Kelvin respectively, and commercially available displacement type cryocoolers are typically capable of pumping up to a few Watts under such conditions.

A displacement type Stirling cycle cryogenic cooler is described in US 5,317,878, in which the Stirling cycle stage is used as a pre-cooler to a Joule-Thomson stage.

It would be desirable to improve the performance and/or efficiency of displacement type Stirling cryogenic coolers of the prior art.

Summary of the invention

Accordingly, the invention provides a Stirling cycle type cryogenic cooler in which the displacer comprises a cylinder extending between warm and cold ends, a regenerator arranged to reciprocate axially within the cylinder and to provide a heat exchanging fluid path for an operating fluid between the warm and cold ends, and a flow shaper element provided at the cold end of the regenerator, the flow shaper being arranged to increase turbulence or forced convection in the cold end of the cylinder by shaping or restricting the flow of fluid entering the cold end of the cylinder from the regenerator. Preferably, the flow shaper element is arranged to generate turbulence, in the space between the regenerator and the cold end of the cylinder, which has similar length scales to the internal size of the cold end, or to the reciprocating stroke length of the regenerator.

Increasing forced convection at the cold end of the displacer cylinder significantly increases the rate of heat transfer into the operating fluid, thereby increasing the cooling power of the cryogenic cooler without requiring more complex supplementary heat exchanger apparatus which would increase the complexity and weight of the apparatus.

The flow shaper element may be provided by a wall extending transversely across the regenerator at the cold end of the displacer cylinder. This can be implemented in a number of ways, for example as a discrete component stacked within the displacer cylinder, as an element fixed to the cold end of the displacer cylinder for example by gluing or welding, or by being integrally formed with the cold end of the displacer cylinder.

To provide the increase in forced convection, the flow shaper element may be provided with one or more apertures for fluid flow into and out of the regenerator from the cold end of the displacer cylinder. Constricting the flow using such apertures leads to turbulence in the cold end with eddies at similar length scales to the sizes of the apertures.

Preferably, the one or more apertures have a total area which is less than 90 % of the inside bore area of the displacer cylinder, and/or which is more than 5% of the inside bore area of the displacer cylinder. More preferably, the one or more apertures have a total area which is less than 75 % of the inside bore area of the displacer cylinder, and/or which is more than 15% of the inside bore area of the displacer cylinder.

The apertures may be round, rectilinear, triangular or of other shapes.

There may be one, two or more apertures.

The invention also provides a displacement type Stirling cycle cryogenic cooler comprising a displacer cylinder extending axially between warm and cold ends, a regenerator arranged to reciprocate axially within the displacer cylinder and to provide a heat exchanging path for an operating fluid between the warm and cold ends, and a thermal block provided at the cold end of the displacer cylinder, the thermal block having an exposed face arranged to deliver heat to the operating fluid, the exposed face being patterned to increase heat transfer from the thermal block to the operating fluid.

This is another way in which the rate transfer of heat into the operating fluid at the cold end can be increase, increasing the efficiency and/or power or the apparatus. In particular, the exposed face may patterned to increase the surface area of the exposed face by at least 20% more than the area of a corresponding planar face.

The exposed face may be patterned in a variety of ways, for example being formed with a plurality of grooves to increase the surface area of the exposed face.

A Stirling cycle cryogenic cooler as set out above may use, for example, either beta type or gamma type Stirling cycle configurations. in a gamma type Stirling cycle a separate compressor drives pressure oscillations in the displacer cylinder, while in a beta type Stirling cycle the pressure oscillations are typically driven by a piston within the displacer cylinder.

The invention also provides a displacement type Stirling cycle cryogenic cooler comprising: a regenerator having warm and cold ends; and a flow shaper element provided at the cold end of the regenerator, the flow shaper being arranged to increase forced convection by shaping the flow of fluid exiting the cold end of the regenerator. Other aspects of the flow shaper element described herein also apply to this arrangement, which can be used in configurations where the regenerator is located externally to the cold end of the cryogenic cooler.

The invention also provides methods corresponding to the above apparatus.

Brief description of the drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings of which: figure 1 shows a cryogenic cooler according to the invention; figure 2 shows the displacer cylinder and related components of the displacement Unit of figure 1; figure 3 shows the cold end of the regenerator of figure 2 in perspective view; figures 4a -4d show various configurations of the flow shaper element of figure 3; and figure 5 is a perspective view of the cold end thermal block of figure 2.

Detailed description of embodiments

Referring to figure 1 there is shown a displacement type Stirling cycle cryogenic cooler embodying the invention. The cooler comprises a compressor unit 10 which provides an oscillating driving pressure in an operating fluid via a transfer pipe 12 to a displacer unit 14, although in a beta type implementation of this embodiment the compressor function would be provided directly within the displacer unit 14. Thedriving pressure may typically oscillate at a few tens of Hz over a pressure range of a few atmospheres. The compressor unit comprises a reciprocating compressor shaft 16 supported on spiral springs 18. The compressor shaft is driven by a linear compressor motor 22, and drives a piston to provide the oscillating pressure.

The displacer unit 14 comprises a reciprocating displacer shaft 28 supported on spiral springs 30. The displacer shaft 28 is driven by a linear displacer motor 32 and drives reciprocating movement of a regenerator assembly 40 within a displacer cylinder 34, sometimes referred to as the "cold finger", containing the operating fluid subject to the oscillating driving pressure. When the compressor motor and displacer motor are driven a the same frequency, but with a suitable phase difference between each other, heat is pumped from the distal end of the displacer cylinder 34 to the proximal end, which ends are therefore referred to herein as the cold end 42 and warm end 44 of the cylinder respectively.

The operating fluid pumped by the compressor and providing the thermodynamic cycling in the displacer unit 14 is typically helium, at a mean pressure of about 15 atmospheres, although other suitable fluids may be used.

Figure 2 shows the structure of the displacer cylinder 34 and regenerator assembly 40 in more cross sectional detail. The regenerator assembly 40 comprises a regenerator tube 54 having a wall thickness of about 1.5 mm and an outside diameter of about 14 mm. The tube may be formed of polyamide for example, and may be coupled to the displacer shaft 28 by a titanium sleeve 50 which fits within a slightly larger inside diameter portion of the polyamide tube, and an apertured coupling block 52. Within the regenerator tube 54 are stacked about a thousand disks of fine stainless steel wire mesh. The wire mesh may typically be of wire having a diameter of around 50 pm, and a pitch of about 100 wires per cm. The stack of wire mesh disks provides a very effective heat source/sink to the operating fluid moving within the regenerator assembly, while providing a minimal barrier to flow and therefore causing minimal pressure drops along the axis of the regenerator, which would otherwise lead to a loss of efficiency.

The regenerator assembly is a close fit within the cylinder 34. Ideally the regenerator assembly and cylinder 34 should not touch, and a spacing of about pm is achievable. The displacer cylinder 34 is provided by a lightweight titanium tube having a wall thickness of about 100 pm. The thin wall reduces the weight of the displacement unit, but importantly minim ises heat transfer through the cylinder between the cold and warm ends. To improve the rigidity and strength of the titanium tube, stiffener flanges 62 are provided at regular intervals.

At the warm end 44 the cylinder 34 is mounted to a warm end thermal block 66, typically made of titanium, from which the heat pumped to the warm end can be dissipated or further transferred.

The cold end of the cylinder 34 is terminated by a cold end thermal block 68, typically made of copper, into which heat to be transferred by the cryogenic cooler is coupled. The stroke distance of the regenerator assembly as it reciprocates within the cylinder 34 may typically be about I to 4 mm, within an overall length of the cylinder of about 100 mm, and preferably the cold end of the regenerator assembly moves as close as practical to the cold end thermal block during reciprocation without risking collision, for example within about 0.1 mm, to maximise efficiency.

Working heat transfer from the cold end thermal block 68 into the operating fluid is limited by heat gradients within the operating fluid in the space between the cold end thermal block 68 and the end of the regenerator assembly 40. A small amount of turbulence may be induced in the operating fluid in this space by the wire mesh of the regenerator, but typically such eddies are of the same scale as the thickness of the wires of the mesh. Since a fine mesh is very desirable to optimise the regenerator, these eddies will tend to be on a very small scale, with wires of 50 pm giving rise to eddies having a lifetime of only a few milliseconds and a length scale of about 50 pm.

Figure 3 illustrates in perspective view the cold end of the regenerator assembly 40 according to one example. A flow shaper element 70 is provided at the cold end of the regenerator assembly to shape the flow of the operating fluid as it passes from the inside of the regenerator tube 54 towards the cold end thermal block 68 shown in figure 2. Shaping the flow using a flow shaper element or component increases turbulence in the cold end of the displacer cylinder 34 and therefore reduces the heat gradient, by increasing forced convection, in this way, the performance of the cryogenic cooler can be increased significantly.

In figure 3, the flow shaper element 70 is provided by a transverse wall disposed across the end or within the bore of the regenerator tube 54. One or more apertures are provided in the transverse wall. In figure 3, four circular apertures each having a diameter of about 2 mm are illustrated, and the total area of the apertures is about 10% of the area of the bore. The flow shaper element of figure 3 is provided as a separately formed component, for example a disk of stainless steel stacked within the regenerator tube 54 alongside the wire -7-.

mesh disks. However, the flow shaper element 70 may take a variety of forms in order to provide the desired effect of increasing the forced convection in the cold end of the cylinder and thereby enhancing heat transfer from the cold end thermal block 68 to the operating fluid.

Figures 4a to 4d show some other possible forms for the flow shaper element, in plan view. In figure 4a a single central circular aperture is provided, and in figure 4b four triangular apertures are provided, within a circular transverse flow shaper element wall. In figure 4c a non-circular component having apertures in the edge of the flow shaper element is provided. Three rectilinear slots are used in the circular flow shaper element of figure 4d.

Typically, the apertures in the flow shaper element 70 will occupy less than 90%, and preferably less than 75%, but more than 5% and preferably more than 15% of the area of the bore of the regenerator tube 54. The number of apertures and the shape of the apertures may be chosen to for convenience of manufacture, strength, and rigidity to the reciprocating motion of the regenerator assembly, and to optimise the forced convection within the cold end of the cylinder 34.

In order to optimise the forced convection while minimising restriction of fluid flow, typically the diameter of one or more of the apertures in the flow shaper element may be between about quarter and twice the distance of travel of the reciprocating motion of the regenerator assembly. If the travel is about 4 mm, therefore, the aperture diameter may preferably be between about 1 mm and 8 mm.

The flow shaper element may be provided as a separate component, for example to be fitted within the regenerator tube 54. In particular, such a flow shaper component may be an apertu red disc, washer, or similar component stacked within the bore of the regenerator, for example at the end of the stack of wire mesh discs as shown in figure 3. Such a component could be secured in a variety of ways, for example using a circlip or similar, or by laser welding. Such a component could be formed, for example, from stainless steel or titanium, for example with a thickness of about 0.1 mm, or from the same material as the regenerator tube which may be of a plastic such as a polyamide plastic.

Alternatively, the flow shaper element 70 may be formed by a cap secured to the end of the regenerator tube 54, for example by welding or gluing, or by machining the regenerator tube to include an end wall which when suitably apertured provides the flow shaper element, Heat transfer from the cold end thermal block 68 into the operating fluid can also be improved by suitable configuration and shaping of the face of the cold end thermal block 68 exposed to the operating fluid, referred to herein as the exposed face 74, and so labelled in figure 2. Figure 5 illustrates one way in which the exposed face 74 can be configured to achieve such an effect. In figure 5 the exposed face is machined with an array of crossed grooves to thereby increase the surface area of the face. The patterning or texture may also beneficially increase local turbulent heat transfer. Of course, other types of patterning and texturing can be provided in a variety of ways to achieve similar effects.

In order to improve heat transfer by patterning the exposed face 74, the surface area of the exposed face is preferably increased above the area which would be exposed if the face was fIat and/or unpatterned. At the same time, the patterning should avoid undue increase in the volume of the cold end of the cylinder which is unavailable for reciprocation by the regenerator assembly.

Preferably, the patterning increases the surface area of the exposed face by at least 20%, and more preferably by at least 50%. Preferably also, however, the void volume which the patterning makes unavailable to reciprocation of the regenerator assembly should be less than 10%, and more preferably less than 2% of the regenerator stroke volume.

Although specific examples of the invention have been described it will be apparent that a number of modifications and variations can be made by the skilled person while not departing from the scope of the invention as defined by the claims. For example, in the described embodiments the regenerator is provided within the displacer piston, but could be placed externally to the displacer piston instead, with suitable connective fluid passages.

Claims

CLAIMS: 1. A displacement type Stirling cycle cryogenic cooler comprising: a displacer cylinder extending axially between warm and cold ends; a regenerator arranged to reciprocate axially within the displacer cylinder and to provide a heat exchanging fluid path between the warm and cold ends; and a flow shaper element provided at the cold end of the regenerator, the flow shaper being arranged to increase forced convection in the cold end of the cylinder by shaping the flow of fluid entering the cold end of the cylinder from the regenerator.
2. The cryogenic cooler of claim 1 wherein the flow shaper element is provided by a wall extending transversally across the regenerator at the cold end of the displacer cylinder.
3. The cryogenic cooler of claim 1 or 2 wherein the flow shaper element is a discrete component stacked within the displacer cylinder.
4. The cryogenic cooler of claim I or 2 wherein the flow shaper element is fixed to the cold end of the displacer cylinder.
5. The cryogenic cooler of claim 1 or 2 wherein the flow shaper element is integrally formed with the cold end of the displacer cylinder.
6. The cryogenic cooler of any preceding claim wherein the flow shaper element is provided with one or more apertures for fluid flow into and out of the regenerator from the cold end of the displacer cylinder.
7. The cryogenic cooler of claim 6 wherein the one or more apertures have a total area which is less than 90 % of the inside bore area of the displacer cylinder.

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8. The cryogenic cooler of claim 6 or 7 wherein the one or more apertures have a total area which is more than 5% of the inside bore area of the displacer cylinder.
9. The cryogenic cooler of any preceding claim wherein the flow shaper element is arranged to generate turbulence having length scales similar to the distance of travel of the regenerator within the displacer cylinder.
10. A displacement type Stirling cycle cryogenic cooler comprising: a displacer cylinder extending axially between warm and cold ends; a regenerator arranged to reciprocate axially within the displacer cylinder and to provide a heat exchanging path for an operating fluid between the warm and cold ends; and a thermal block provided at the cold end of the displacer cylinder, the thermal block having an exposed face arranged to deliver heat to the operating fluid, the exposed face being patterned to increase heat transfer from the thermal block to the operating fluid.
11. The cryogenic cooler of claim 10 wherein the exposed face is patterned to increase the surface area of the exposed face by at least 20% more than the area of a corresponding planar face.
12. The cryogenic cooler of claim 10 or 11 wherein the exposed face is formed with a plurality of grooves to increase the surface area of the exposed face.
13. A method of increasing heat transfer into the cold end of a displacement type Stirling cycle cryogenic cooler comprising a displacer cylinder extending axially between warm and cold ends and a regenerator arranged to reciprocate axially within the displacer cylinder and to provide a heat exchanging path for an operating fluid between the warm and cold ends, comprising: -:ii -increasing forced convection in the operating fluid at the cold end of the cylinder by constricting the flow of the operating fluid entering the cold end of the cylinder from the regenerator.
14. The method of claim 13 comprising constricting the flow using a transverse wall across the cold end of the regenerator, the transverse wall comprising one or more apertures allowing the operating fluid to pass thereth rough.
15. Apparatus substantially as herein described with reference to the accompanying drawings.