PULSE TUBE COOLER
There is disclosed a cold head for a pulse tube cooler, comprising: a regenerator having a first end connectable to a compressor; a pulse tube having a first end and a second end; a heat exchanger connected between a second end of the regenerator and the first end of the pulse tube; and a phase control device connected at the second end of the pulse tube for controlling the flow dynamics in the pulse tube to provide cooling at the heat exchanger, thereby maintaining a negative temperature gradient between the first and second ends of the regenerator, wherein: the pulse tube comprises a wall having a porous portion for allowing a working gas to enter or leave the pulse tube directly through the porous portion, the porous portion being nearer to being parallel than perpendicular to the temperature gradient between the first and second ends of the regenerator.
The present invention relates to pulse tube coolers that have improved efficiency. Pulse tube coolers have evolved as an alternative to Stirling cycle coolers. Their operation is very closely related; the main difference is that instead of using a “solid” piston or displacer to provide an expander component, this function is provided by a column of gas that acts as a springy, deformable displacer.
The motion of the gas column in a pulse tube can be controlled mechanically by a warm end piston/displacer but more often the motion is controlled by fluid “phase control” components. These avoid the need for extra moving components and control the mass flow in the way that they respond to the applied pressure variation. Various mechanisms have been employed to provide this function. These include designs that use orifices or inertance tubes in combination with reservoir volumes. In addition a second inlet and orifice combination can be connected between the compressor input and the warm end of the pulse tube to further modify the response.
Pulse tube coolers can be used in the same wide range of applications as Stirling cycle coolers. Examples include those for cooling optical components both for space and terrestrial applications. Their main advantage over Stirling cycle coolers is the elimination or simplification of the moving expander/displacer components. This can improve overall reliability and reduce vibration. Potentially it also offers lower cost as a pulse tube cold head does not require high precision.
A good general description of present pulse tube technology is given in: Proceedings of the Institute of Refrigeration (London) 1999-2000, “Development of the Pulse Tube Refrigerator as an Efficient and Reliable Cryocooler, Ray Radebaugh, NIST.
In operation, the movement of the expander piston 113 is arranged to be out of phase with the pressure variation so that net work is done by the gas on the piston 113. This loss of energy from the gas causes the gas temperature to decrease which in turn allows heat to be absorbed from the cold end heat exchanger 110. Over a cycle the net result is that heat can be absorbed from an external load via the heat exchanger 110 and this energy is transported to the warm end of the cold head (at temperature Th) as work done on the expander piston 113, which may in turn do work on gas in a compression space 115.
In the gamma configuration shown, the expander piston 113 becomes what is termed a displacer. The displacer takes energy from the gas at the cold end as an expander and directly transfers it back into a gas volume at the ambient end as a compressor. The additional gas volume is usually connected to the input from the compressor so that the expansion work is “recycled” to improve efficiency. This energy recovery is not essential; for low temperature coolers the expansion power is relatively small and can be just dissipated through a damping arrangement.
In
In discussing Stirling cycle coolers and particularly pulse tube coolers, it is useful to consider the operation in terms of the phase relationship between the applied pressure variation and the mass flow of the gas between the cold end heat exchanger and the expansion space/volume.
The work done W by the expander piston is given by
-
- W=∫P.dV over a cycle P is pressure, V is expander volume
For simplicity, sinusoidal* variations are assumed for pressure and volume. This integral has a maximum when the pressure and first derivative of volume are in phase. This is equivalent to requiring that the pressure variation and mass flow are in phase. (*Note: Actual pressure and mass flow variations will generally have additional harmonics but their magnitude will be small and they will not fundamentally change the operating mechanism).
It is noted that while this relationship will give the highest cooling effect it does not generally give the highest efficiency because losses in the cooler are also phase dependent. However it is generally the case that if the pressure and mass flow are 90 degrees out phase then there is no net work done on the gas and hence no cooling.
Cooling is generated by appropriate phasing of the volume 121 with respect to the pressure variation. Achieving this is a more complex task than for a Stirling cycle machine and this aspect will be described more fully below.
For efficient operation of the pulse tube 114 it is desirable to minimise any turbulence so that the gas column remains stratified and convection by mixing is avoided. Flow straighteners 120 may be provided for this purpose, at either or both ends of the pulse tube 114 (as shown in the example of
In the above description of the Stirling cycle cooler it was shown that a key requirement for cooling is a combination of pressure and mass flow variations that are in phase. In a pulse tube the cooling process is more complicated for two reasons:
-
- Firstly, the deformation of the “gas displacer” (labelled 123 in the example shown in
FIG. 2 ) with pressure requires additional mass flows into the pulse tube. - Secondly, as one of the main attractions of a pulse tube is the elimination of moving components, it is very desirable to control the mass flow using a system of fluid phase control components rather than using pistons/displacers.
- Firstly, the deformation of the “gas displacer” (labelled 123 in the example shown in
In the following, the term “phase control device” is used to refer to any device which allows the relative phases of the pressure and mass flow variations to be such as to allow cooling to take place. The phase control device may operate using pistons/displacers, a system of fluid phase control components that may not comprise any moving parts, or a combination of the two.
The mass flows required into the pulse tube can be divided into two components: those in phase with the pressure pulse and those out of phase. If it is assumed that the pressure variation has a sinusoidal waveform:
P=P0 Sin(ωt)
(Mean pressure level is ignored; pressure is assumed to be AC component only) then a general expression for the mass flow variation is:
{dot over (m)}=A. Sin(ωt)+B. Cos(ωt)
The first component is in phase with the pressure pulse and this has already been established as the “cooling” component. The second component is mass flow required by the “deformation” of the “gas displacer”. This component does not give rise to any work input or output; instead it acts as a spring component that stores energy during half the cycle and releases it in the other half.
This combination of two components of mass flow is illustrated in
-
- Reversibly expanding and contracting—the spring component
- Displacing the tidal gas volumes into and out of the pulse tube
The operation of the pulse tube shown inFIG. 3A can be imagined as the combination of the separate processes illustrated inFIGS. 3B and 3C .
In
One of the earliest designs used in pulse tube coolers is the orifice pulse tube, as shown in
For
P=P0 Sin(ωt)
{dot over (m)}spr=B. Cos(ωt)
W=∫P.dV=0
If the orifice 122 is opened (
{dot over (m)}orif=const.P. Sin(ωt)=A. Sin(ωt)
The mass flow into the pulse tube at the cold end is a combination of the spring mass flow and the orifice mass flow:
{dot over (m)}coldend={dot over (m)}spr+{dot over (m)}orifA. Sin(ωt)+B. Cos(ωt)
The cold end now has a mass flow component that is in phase with the pressure variation and there is a net cooling effect at the cold end.
(Note: As might be expected, the cooling is equal to the work lost in pumping gas backwards and forwards through the orifice 122)
Other Types of Pulse TubeThe orifice pulse tube is probably the simplest design that has succeeded in producing reasonable levels of cooling at low temperatures. However it will be seen from
In the double inlet pulse tube configuration there is an additional connection between the ambient end of the pulse tube to the compressor via a second orifice. The effect of this is to cause a mass flow into the ambient end of the pulse tube that is in phase with the pressure drop across the regenerator and cold end heat exchanger. This can be used to allow some of the “spring” flow to enter from the ambient end. The reduction in flow through the regenerator reduces the regenerator loss and allows an overall improvement in performance.
Although good performance has been achieved with this arrangement it has been found that the second orifice is inclined to be asymmetric in its operation. This attribute tends to generate net DC flows that circulate around the pulse tube and regenerator. These flows do not undergo thermal regeneration as intended and even at low levels can produce unacceptable losses.
Inertance TubeAn alternative to the Double Inlet configuration that attracted interest is the inertance tube configuration. In this configuration the orifice of the Single orifice configuration described above is replaced by a tube or assembly of tubes that connect the ambient end of the pulse tube to a reservoir volume. The tube or assembly of tubes is a further example of a fluid phase control component(s).
The tube assembly is arranged to have both damping and significant inertia. It terms of an electrical analogy the inertance tube assembly is represented by a series combination of a resistor and an inductor whereas an orifice is represented by just a resistor. The inductive component has the effect of allowing some of the “spring” mass flow to enter the pulse tube from ambient end. This helps to reduce the thermal load on the regenerator without setting up the circulating flows that tend to occur with the Double Inlet design.
Coaxial DesignsIn
There are four main loss mechanisms directly associated with the operation of a pulse tube:
-
- Heat transfer through the bulk of the gas: The minimum value is set by thermal conduction in stratified layers but this can be greatly increased if there is significant mixing/turbulence.
- Natural Convection: The temperature gradient produces a corresponding density gradient. The latter tends to drive a circulation of gas within the pulse tube. This effect causes pulse tube performance to be very dependent on orientation—pulse tubes generally work better with their cold ends pointing down.
- There are two recognised losses associated with the interaction of the axial oscillation of the gas and the pulse tube wall.
- One loss, referred to as a type of shuttle loss, is due to the constantly reversing temperature gradient between the gas and the wall.
- A second loss that has been termed “Streaming convection” does not have any simple explanation but appears to derive from the velocity boundary conditions imposed at the interface between the gas and wall.
The “Streaming convection” loss is discussed in: J. R. Olsen, G. W. Swift, Acoustic Streaming in Pulse Tube Refrigerator: Tapered Pulse Tubes, Cryogenics, Volume 37, Issue 12, December 1997.
It is an object of the present invention to provide an improved pulse tube cooler which at least partially address one or more of the problems with the prior art discussed above, for example by reducing losses.
According to an aspect of the invention, there is provided a cold head for a pulse tube cooler, comprising: a regenerator having a first end connectable to a compressor; a pulse tube having a first end and a second end; a heat exchanger connected between a second end of the regenerator and the first end of the pulse tube; and a phase control device connected at the second end of the pulse tube for controlling the flow dynamics in the pulse tube to provide cooling at the heat exchanger, thereby maintaining a negative temperature gradient between the first and second ends of the regenerator, wherein: the pulse tube comprises a wall having a porous portion for allowing a working gas to enter or leave the pulse tube directly through the porous portion, the porous portion being nearer to being parallel than perpendicular to the temperature gradient between the first and second ends of the regenerator.
Thus, an arrangement is provided in which a porous portion allows gas to enter or leave the pulse tube substantially laterally (which may also be referred to as “radially”). This provides a number of advantages relative to the prior art.
For example, there is a greater range of possibilities for the distribution of both the refrigeration process and the “spring” flows required:
i) In prior art arrangements such as that shown in
ii) In the prior art the gas “spring” flows can only enter from the cold end or warm end (or both). However, the actual flow requirement is distributed along the length of the pulse tube. The provision of a porous wall allows the flow requirement to be fulfilled with more favourably distributed gas flows. The resulting pulse tube can be likened to a “multiple” inlet pulse tube.
A further advantage is related to the ability to reduce gas velocities and gas movement with respect to the thermal gradients:
i) In a conventional pulse tube all the gas flow has to be distributed across the end faces of the pulse tube. The flow straighteners are designed to provide low velocity, evenly distributed flows but there is clearly a lower limit set by the cross sectional area of the pulse tube. The provision of porous lateral walls allows the gas flows to be distributed over much larger areas allowing gas velocities to be considerably reduced. This helps to maintain streamlined flow with minimal mixing and turbulence
ii) In a conventional pulse tube the gas flows are all axial and this requires the gas to have significant velocities in the direction of the thermal gradient. The porous walls of the present invention allow much of the flow to be radial where the gas velocity is perpendicular to the thermal gradient. This reduces the axial gas velocities and allows the establishment of a more favourable temperature distribution.
A further advantage is that, where there is radial flow, the gas flow will be closer to being perpendicular to the wall than parallel to it. The axial velocity component that interacts with the tube wall will be much smaller than for a conventional axial flow design. This will result in lower losses that are dependent on the gas flow/wall interface. For example the “shuttle” loss will be reduced as this loss is proportional to the square of the axial displacement. The other loss related to the interaction between the pulse tube wall and the axial gas velocity is the “Streaming Convection” and it is also believed that the large reduction in axial gas velocity adjacent to the tube wall will also result in lower values for this loss.
Furthermore, there is the effect the radial gas velocity components have on natural convection losses. Natural convection occurs when buoyancy forces drive a circulation of gas. Typically this occurs when a hot surface is positioned below a cold surface—in a simple model the heated less dense fluid rises and the cooled denser fluid descends. The magnitude of the natural convection heat transfer is dependent on the Grashof Number—a measure of the balance between the buoyancy forces that drive the circulation and resistive processes that reduce it. In a conventional axial flow pulse tube, the tube's walls are the principal source of damping—the smaller the diameter the more the circulation is suppressed. The axial gas flows do not generally have much influence as they will add to the circulation on one side and subtract on the other. In the radial flow pulse tube it will be seen that there are two effects that will tend to reduce natural convection: firstly, the radial flows close to the wall will disturb the axial flow and will tend to suppress it; and, secondly, the core of the gas displacer where circulation could become established has a significantly reduced diameter which will also tend to suppress circulation.
In an embodiment, a phase control device is provided that comprises a piston configured to move within a cylinder. In comparison with prior art Stirling cycle coolers, the piston can be confined to a greater extent to the warm end of the tube than can the displacer of the Stirling cycle cooler. This allows the arrangement to be made lighter and/or easier to manufacture, while also tending to lower vibration and/or manufacturing cost. In comparison with prior art pulse tube coolers, this approach allows for a more compact arrangement because there is no need for a reservoir volume to be provided to implement the phase control.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols indicate corresponding parts, and in which:
In an embodiment, a cold head for a pulse tube cooler is provided in which the flow of at least a portion of the working gas between the pulse tube and other components is diverted through a porous portion of a wall that is nearer to being parallel with the temperature gradient of the regenerator than being perpendicular to it. Preferably, the porous portion is within 5 degrees of being parallel, preferably within 1 degree of being parallel. The result of this arrangement is that the working gas enters or leaves the pulse tube in a predominantly lateral direction (perpendicular to the temperature gradient). As discussed above, this allows operational losses to be reduced. An example of such an arrangement is shown in
In this particular embodiment, the cold head 2 comprises a regenerator 4 which has a first end 6 and a second end 8. In use a temperature gradient will be maintained along the regenerator 4 such that the second end 8 will be colder (at Tc) than the first end 6 (at Th). The second end 8 may therefore be referred to as the cold end and the first end 6 may be referred to as the warm end. In an embodiment, the warm end 6 is at ambient temperature in which case it may be referred to as the ambient end.
The regenerator 4 is connectable via passageway 7 to a compressor (not shown in
The second end 16 of the pulse tube 14 is connected to a phase control device via connection 26, optionally via flow straightener 20. As described above with reference to
In the embodiment shown, the pulse tube 14 has a cylindrical form and comprises a wall 15 defining a pulse tube volume. In other embodiments, the pulse tube may take other forms. In the case where the pulse tube is cylindrical it will be understood that references to the axial direction refer to the axis of the cylinder. In the case where the pulse tube has an annular cross-section, the axial direction will refer to the axis of the cylinder formed by the inner or outer surfaces of the annular cross-section. In embodiments where the pulse tube is not cylindrical and does not have an annular cross-section it will be understood that references to the axial direction refer to an axis of elongation or “long axis” of the pulse tube. Typically, the axis of the pulse tube 14 will be substantially parallel to the direction of the temperature gradient in the regenerator (i.e. the direction of steepest temperature gradient—the vertical direction in the orientation of the figures).
In the particular embodiment shown the regenerator 4 coaxially surrounds the pulse tube 14 (as in the embodiment of
In an embodiment, the pulse tube shares a wall with one or both of the regenerator 4 and the heat exchanger 10 and/or, where provided, a flow distributer 11 between the second end of the pulse tube 14 and the phase control device (see
In an embodiment, at least a portion (the uppermost region in the marked range 28 in the example shown) of the shared wall 15 is porous to a working gas such that a portion of the working gas can enter or leave the pulse tube 14 (indicated by arrows 30) through the porous portion of the wall 15. Allowing some of the gas to enter into the pulse tube 14 directly from the regenerator in this way does not change the distinction between “spring” type mass flows that are out of phase with the pressure variation and “displacement” flows that are in phase, it just alters the way in which the gas displacer deforms and the distribution of the cooling effect. In the example shown, the additional radial inward flows in the range 28 effectively change the shape of the first tidal volume (the equivalent of the volume 121 of
In an embodiment, the flows through the porous portion of the wall are such as to cause minimal mixing or turbulence. The ideal is that the gas flows follow streamlines so that over a cycle they tend to return to the point where they entered the pulse tube 14. This (or close to it) is achieved by having low gas velocities and well distributed apertures/pores.
In an embodiment, the porous portion of the wall 15 extends from the second end 8 (the cold end) of the regenerator 4 along a portion, but not all, of the regenerator 4 towards the first end 6 (the warm or ambient end) of the regenerator 4. This is the case for example in the arrangement of
In the arrangement of
In the example of
In the embodiment shown, the flow distributer 11 comprises a wall having a porous portion that allows for a radial flow between the pulse tube 14 and the phase control device (via the flow distributer 11). In the arrangement of
All the pulse tube arrangements described above have assumed the use of a phase control device comprising a fluid phase control component, which avoids additional moving components. For example, a combination of inertance tube and gas reservoir may be used.
This use of a warm end displacer 32 can be regarded as a gamma configuration Stirling cycle cooler in which the thermal insulation function of a conventional displacer is transferred to a deformable “gas displacer”—the “gas displacer” effectively becomes an insulating piston crown. One advantage of this arrangement over a conventional Stirling cycle cooler is that the displacer 32 is confined to the warm end. This allows it to be both lighter and easier to make with lower vibration and lower manufacturing costs.
At the cold end the operation of the pulse tube 14 shown in
The warm end displacer arrangement will not be the first choice for all pulse tube applications because of the additional moving components. However it possibly more compact as there is no longer a requirement for a reservoir gas volume which can occupy a significant volume.
In the example shown, the compressor 45 (which may also be referred to as a linear pressure wave generator) is configured to impart a modulating pressure on the cold head 2 via connector 7 (in this case a length of connecting tube). The compressor 45 comprises a piston cylinder assembly driven by a linear motor 50. The moving components are mounted on flexures (suspension springs) 46 so as to allow a close but non-touching fit between the compression piston 52 and cylinder 53. This arrangement is generally referred to as a “clearance seal”—there is leakage but it is small enough to be acceptable. The compressor 45 does not have valves and the modulating pressure can be regarded as analogous to a voltage that has both AC and DC components. The basic operating frequency may typically be in the range 50 to 100 Hz.
The configuration of the phase control device of the example of
Implementation of Radial Flows
For the radial flows into the pulse tube 14 there are number of approaches that can be used.
For radial flows between the regenerator 4 and the pulse tube 14 the porosity of the pulse tube walls 16 needs to be controlled so as to give the required flow distribution. As the pressure drop across the pulse tube wall 15 varies along the regenerator 4 it is expected that porosity will also need to vary significantly. The gas flows within the regenerator 4 and porous wall 16 are expected to be laminar and this allows them to be modelled using simplified methods—e.g. software intended for mathematically similar processes.
A finite element thermal conduction model was used to estimate the range of permeability that would be needed to achieve the required gas flows. For a pulse tube 14 where the radial flow was comparable with the axial flow as per the arrangement shown in
If the volume flow dV/dt through the regenerator 4 is given by:
where Af is flow area, κr is permeability of regenerator 4, μ is viscosity of gas and dP/dx is pressure gradient, the permeability of the pulse tube wall 15 would need to vary in the range 0.01κr to κr. The regenerator mesh used in pulse tube coolers is usually very fine. A typical mesh specification is:
The permeability of porous material is generally:
-
- Proportional to flow area and hence porosity
- Inversely proportional to hydraulic diameter/aperture size.
As it is required to reduce the permeability by a factor of 100 from material that already has only 34 micron pore size it will be seen that it is necessary to produce a tube wall 16 with apertures of 1 micron or larger. A variation of 1 to 34 microns does not give the required permeability range so it is also necessary to alter the effective porosity.
Although there are a number of technologies that can be used to produce apertures in sheet material, many are not suited to the small dimensions required. One approach that is suited is the technology of electroforming. For example an electroformed screen can be made to give a varying permeability by controlling both the size and density of the apertures (the density effectively determines the flow area/porosity). The thickness of the screen needs to be between 5 and 10 microns to allow apertures of ˜1 micron to be defined. The screen is sandwiched between two layers of fine mesh which can then be formed into a tube and installed between the regenerator and pulse tube volume. The mesh is used for two reasons:
-
- To produce a more robust assembly that can be handled
- To give a symmetric flow characteristic so as to avoid any tendency to produce net circulations between the pulse tube and the regenerator as these will tend to generate losses.
The range of permeability that can be defined with a single electroformed screen is large but if necessary more than one screen can be used in conjunction with additional layers of mesh to further reduce the permeability.
The embodiments described above relate to single stage pulse tubes where the regenerator 4 has an annular form and the pulse tube 14 is located concentrically within the regenerator 4. However, these features are not essential. Multi-stage pulse tubes may be provided. Additionally or alternatively, the regenerator and pulse tubes may take different forms. Some example configurations are described below with reference to
The embodiment of
In an embodiment, the pulse tube 14 comprises one or more flow shaping features 70. The flow shaping features 70 are configured to deflect flow towards the axial direction and/or make unavailable to the flow volumes of the pulse tube 14 in which the flow rate would otherwise be relatively low. Such volumes in which the flow rate would be relatively low are sometimes referred to as “dead volumes”. The existence of dead volumes reduces efficiency and can cause undesirable stagnant and/or swirling flow patterns. The flow shaping features 70 improve efficiency by reducing dead volumes and helping to provide a smooth transition (arrows 80) between radial flow into the pulse tube (for example through meshes 64) and the predominantly axial flow (arrows 82) that exists in the bulk of the pulse tube. The embodiment of
In an embodiment, the pulse tube 14 is provided radially outside, optionally coaxially surrounding, the regenerator 4. In an example of such an embodiment the regenerator 4 has a cylindrical form and the pulse tube 14 has an annular form surrounding the regenerator 4. The porous wall through which the working gas can enter or leave the pulse tube 14 may in this embodiment comprise a part of a shared wall (e.g. shared between the pulse tube 14 and the regenerator 4) that is a radially inner wall of the pulse tube 14. An example of such an embodiment is depicted in
In an embodiment, the cold head 2 is configured to operate as a multi stage cooler, with cooling at different temperatures being provided at different heat exchangers. The multi stage cooler may comprise a two stage cooler.
The multi stage cooler comprises at least one additional regenerator 72 (one in the case of a two stage cooler), at least one additional pulse tube 74 (one in the case of a two stage cooler), and at least one additional heat exchanger 76 (one in the case of a two stage cooler). The original pulse tube 14, regenerator 4 and heat exchanger 10 may be referred to as a first stage pulse tube assembly. Each set of additional elements may be referred to as a second (or third, fourth etc.) stage pulse tube assembly. Thus, in the example of
The second stage pulse tube assembly is attached to the cold end of the first stage pulse tube assembly (e.g. to the heat exchanger 10 of the first stage pulse tube assembly). The cold head 2 is configured so that a portion of the working gas from the first stage pulse tube assembly is directed, for example via appropriate passages 86, into the additional regenerator 72 of the second stage pulse tube assembly. The additional pulse tube 74 of the second stage pulse tube assembly is connected directly (i.e. has a continuous fluidic connection) to the pulse tube 14 of the first stage pulse tube assembly and shares the same connection 26 to the phase control device (via the pulse tube 14 of the first stage pulse tube assembly). It is noted that the phasing may not be simultaneously ideal for both stages. However, this is a workable arrangement as the performance of the first stage pulse tube assembly is not overly phase sensitive and the phase can therefore be adjusted to be close to ideal, or ideal, for the second stage pulse tube assembly. The portion of the flow that enters the additional regenerator 72 is cooled and input to the additional pulse tube 74 at the cold end of the additional pulse tube (the top of the additional pulse tube 74 in
Claims
1. A cold head for a pulse tube cooler, comprising:
- a regenerator having a first end connectable to a compressor;
- a pulse tube having a first end and a second end;
- a heat exchanger connected between a second end of the regenerator and the first end of the pulse tube; and
- a phase control device connected at the second end of the pulse tube for controlling the flow dynamics in the pulse tube to provide cooling at the heat exchanger, thereby maintaining a negative temperature gradient between the first and second ends of the regenerator, wherein:
- the pulse tube comprises a wall having a porous portion for allowing a working gas to enter or leave the pulse tube directly through the porous portion, the porous portion being nearer to being parallel than perpendicular to the temperature gradient between the first and second ends of the regenerator; and
- the heat exchanger coaxially surrounds the pulse tube.
2. A cold head according to claim 1, wherein the porous portion is within 5 degrees of being parallel to the temperature gradient between the first and second ends of the regenerator.
3. A cold head according to claim 1, wherein the pulse tube is cylindrical or has a substantially annular cross-section and the porous portion is within 5 degrees of being parallel to the axial direction of the pulse tube.
4. A cold head according to claim 1, wherein the porous portion is a part of a shared wall that is shared between the pulse tube and one or more of the following: the regenerator, the heat exchanger, a flow distributer between the pulse tube and the phase control device.
5. A cold head according to claim 4, wherein the regenerator coaxially surrounds the pulse tube and the porous portion is part of a shared wall that is a radially inner wall of the regenerator.
6. A cold head according to claim 1, wherein the pulse tube is substantially cylindrical and the regenerator has a substantially annular cross-section.
7. (canceled)
8. A cold head according to claim 4, wherein the pulse tube coaxially surrounds the regenerator and the porous portion is part of a shared wall that is a radially inner wall of the pulse tube.
9. A cold head according to claim 8, wherein the regenerator is substantially cylindrical and the pulse tube has a substantially annular cross-section.
10. A cold head according to claim 1, wherein the porous portion extends from the second end of the regenerator along a portion of the regenerator towards the first end of the regenerator, without reaching the first end of the regenerator.
11. A cold head according to claim 1, wherein the porosity of the porous portion of the wall decreases as a function of increasing separation from the second end of the regenerator.
12. A cold head according to claim 1, wherein the pulse tube is orientated such that the second end of the pulse tube is further from the first end of the regenerator than the first end of the pulse tube.
13. A cold head according to claim 1, wherein the phase control device is configured to control the flow dynamics using one or more fluid phase control components to control the flow of gas into and out of the second end of the pulse tube, the fluid phase control components being configured to operate without using any solid moving parts.
14. A cold head according to claim 1, wherein the phase control device provides either or both of damping and inertia.
15. A cold head according to claim 1, wherein the phase control device comprises a piston and cylinder in fluid communication with the second end of the pulse tube.
16. A cold head according to claim 1, wherein the porous portion of the wall is formed from an electroformed sheet.
17. A cold head according to claim 16, wherein the electroformed sheet is sandwiched between layers of mesh that act to even out and/or straighten the flow of gas passing through the porous portion of the wall.
18. A cold head according to claim 1, wherein the cold head is configured to operate as a multi stage cooler and has a first stage pulse tube assembly comprising:
- a first regenerator having a first end connectable to a compressor;
- a first pulse tube having a first end and a second end; and
- a first heat exchanger connected between a second end of the first regenerator and the first end of the first pulse tube, wherein
- the phase control device is connected at the second end of the first pulse tube for controlling the flow dynamics in the first pulse tube to provide cooling at the first heat exchanger, thereby maintaining a negative temperature gradient between the first and second ends of the first regenerator;
- the first pulse tube comprises the wall having the porous portion for allowing the working gas to enter or leave the first pulse tube directly through the porous portion, the porous portion being nearer to being parallel than perpendicular to the temperature gradient between the first and second ends of the first regenerator; and
- the cold head further comprises a second stage pulse tube assembly comprising:
- an additional pulse tube directly connected to the first pulse tube in the region of the first heat exchanger and in such a way that there is a continuous fluidic connection between the first pulse tube and the additional pulse tube, the additional pulse tube being fluidically coupled to the phase control device via said continuous fluidic connection;
- an additional regenerator configured to receive working gas from the first regenerator; and
- an additional heat exchanger configured to provide cooling at a lower temperature than the cooling provided at the first heat exchanger.
19. A pulse tube cooler comprising a cold head according to claim 1 and a compressor connected to the first end of the regenerator.
20. (canceled)
Type: Application
Filed: Jun 6, 2014
Publication Date: May 12, 2016
Inventor: Michael William Dadd (Oxford, Oxfordshire)
Application Number: 14/895,709