US20090084116A1

US20090084116A1 - Gas phase shifting multistage displacer cryocooler

Info

Publication number: US20090084116A1
Application number: US11/904,680
Authority: US
Inventors: Sidney W.K. Yuan; David G.T. Curran
Original assignee: Aerospace Corp
Current assignee: Aerospace Corp
Priority date: 2007-09-28
Filing date: 2007-09-28
Publication date: 2009-04-02

Abstract

A displacer cryocooler including a multistage heat exchanger is modified with a phase shifting device connected between stage volumes for gas phase shifting of gas pressure in the cryocooler for improving the efficiency and temperature range of expander cryocoolers.

Description

REFERENCE TO RELATED APPLICATION

The present application is related to applicant's copending applications entitled Gas Phase Shifting Multistage Displacer Cryocooler, Ser. No. xx/xxx,xxx, filed xx/xx/xx, Controlled and Variable Gas Phase Shifting Cryocooler Ser. No. yyy/yyy,yyy, filed yy/yy/yy, and Gas Phase Shifting Inertance Gap Pulse Tube Cryocooler, Ser. No. zz/zzz,zzz, filed zz/zz/zz, by the same inventors.

FIELD OF THE INVENTION

The invention relates to the field of refrigeration systems. More particularly, the invention relates to cryocoolers providing phase shifting of gas pressures using controlled gas phase shifting devices in a multiple stage expander cryocooler and multiple stage pulse tube cryocooler for improved energy transfer and cooling efficiencies.

BACKGROUND OF THE INVENTION

Cryocoolers are mechanical machines used for cooling, heating, and thermal transfer. The cryocoolers typically have multiple internal volumes for heating and cooling. Multistage coolers are coolers with more than one cooling or heating stage having more than one volume. Mechanical cryocoolers can be classified according to the type of heat exchangers used, that is, regenerative versus recuperative. Regenerative mechanical cryocoolers can be further classified according to the presence or absence of valves, thermal compressors, or mechanical compressors, and the presence or absence of displacers, such as pulse tubes. Multistage cryocoolers are routinely used for reaching temperatures below what a single stage cryocooler can achieve. The staging of cryocoolers can be done in parallel or in series.
Relevant cryocoolers include displacer cryocoolers and pulse tube cryocoolers. A displacer cryocooler is generally comprised of a compressor connected to an expander. The expander may be a multistage heat exchanger. The compressor and expander are connected together with or without transfer tubes. A pulse tube cryocooler includes a stationary regenerator connected to a pulse tube that includes a long inertance tube.
The displacer cryocooler is driven by a compressor, which sends a pressure wave to the displacer. In a multistage displacer cryocooler, the first stage of the expander pre-cools the gas that enters the second stage, and the second stage pre-cools the gas that enters the third stage, and so on. The cooling capacity at each stage is directly proportional to the swept volume of the expansion space. Because the cross-sectional areas of the expansion spaces are fixed in a given multistage mechanical cryocooler design, the ratio of heat loads among the stages that the cryocooler can cool is also fixed. Limited shifting of loads can be achieved by changing the frequency, charge pressure, or temperature at each stage.
The efficient pulse tube cryocooler consists of a long inertance tube that may be up to several meters in length between the warm end of the pulse tube and a buffer volume. Pulse tube coolers are reliable primarily because pulse tube coolers do not have any cold moving parts, or displacers, or valves that can break. A compressor drives a piston that provides the energy needed for the refrigeration. As the piston compresses, a parcel of warm gas travels through the regenerator. The gas is cooled by the matrix of the regenerator, that is, the heat exchanger. Part of the heat from compression Q_ois removed at ambient temperature. The gas is then expanded through the pulse tube and the orifice into the buffer or reservoir volume. Expansion provides cooling Q_cthat takes place at temperature T_c. Because the pulse tube does not have a displacer, phase shifting is accomplished by a combination of the orifice and the buffer volume. Theoretically, maximum cooling efficiency is accomplished with the pressure wave in phase with respect to mass flow at the cold end of the cryocooler. The performance of the orifice pulse tube can be enhanced by incorporating double inlets or bypasses. Moreover, in replacing the orifice with a long capillary known as the inertance tube, the performance of the cooler can be improved by avoiding irreversible losses associated with a sharp edged orifice. With a sharp edged orifice, the only variable parameter is the diameter of the orifice, limiting heat exchange control. With an inertance tube, there are two variables consisting of the length and diameter of the tube. The phase shift mechanism can be modeled using an LRC circuit analogy. Inductance L=4L_t/(πd_t ²) is analog to a flow inertance. Resistance R is analogous to flow impedance. Capacitance C=Mv_t/(γRT) is analog to the fluid heat capacity. The tube geometry can be defined where L_tis the gap length, d_tand v_tare the length, diameter and internal volume of the inertance tube, η is viscosity, ρ is density, and γ is the specific heat ratio, T is the temperature, and M is the molecular weight. The long inertance tube is used to optimize the gas phase shift. Unfortunately, the long tube geometry is also cumbersome, heavy, and bulky. The long inertance tube does not readily provide the means for optimizing the gas phase shift between the compressor and reservoir for various applications, which requires different cooling capacity. The tuning of the gas phase shift is only by setting the length and diameter of the inertance tube. Accordingly, there are two ways to add inductance, that is, the analogous inertance. One way to add inertance is by increasing the length L_tor by decreasing the diameter d_t. Because resistance is inversely proportional to the fourth power of diameter d_t, decreasing d_twill increase resistance substantially. Additionally, inertance can be increased by increasing the length of tube, but this disadvantageously results in a long and slender tube.
Pulse tube systems with inertance tubes have been studied both theoretically and experimentally. The flow circuit in a pulse tube is analogous to that of an electrical circuit. The optimum phase shifting can be obtained by introducing an inertance term into the circuit, instead of relying on a pure resistance circuit caused by a pressure drop in a sharp edged orifice used for phase shifting. The pulse tube system includes an exchanger stage coupled to a pulse tube that is coupled to a reservoir. The pulse tube system disadvantageously includes an elongated inertance tube having a length that is longer than a meter in most applications. The packaging of a long tube presents problems as well as in applications where vibration, such as during a launch, may cause failure of the cooler. US Patent Publication No. 20050022539 teaches incorporating a hybrid cryocooler with a first stage expander and a second stage pulse tube design. Because the pulse tube cryocooler uses an orifice or inertance tube between the pulse tube and the surge volume to optimize cooling, the hybrid design provides an extra parameter for load shifting. Expander cryocoolers are in general more efficient than pulse tube cryocoolers, however, the displacer cryocoolers are less reliable due to the presence of a moving displacer. The hybrid design tends to combine the main disadvantage of a expander cryocooler with that of a pulse tube cryocooler.
Mechanical cryocoolers are used extensively for cooling purposes. Multistage coolers are generally used to reach temperatures below 35° K. The shifting of loads between stages is not flexible in a multistage mechanical cryocooler. Thus, there is disadvantageously a limited range of temperature that each stage can achieve relative to other stages. These and other disadvantages are solved or reduced using this invention.

SUMMARY OF THE INVENTION

An object of the invention is to provide gas phase shifting in a mechanical device.
Another object of the invention is to provide controlled gas phase shifting using a flow impedance device in a cryocooler.
Yet another object of the invention is to provide controlled gas phase shifting using a flow inertia device in a cryocooler.
Also another object of the invention is to provide gas phase shifting using an inertance gap in a pulse tube cryocooler.
Still another object of the invention is to provide gas phase shifting using an impedance device in a multistage displacer cryocooler.
Furthermore, another object of the invention is to provide gas phase shifting using an inertance device in a multistage displacer cryocooler.
The invention is directed to gas phase shifting in cryocoolers for improved cooling efficiency. In a first aspect, a valve is disposed between stage volumes in a multistage displacer cryocooler. In a second aspect, inertance gaps are disposed in line of a pulse tube cryocooler. A gas phase shifting means is installed between different volumes of the stages of the cryocooler allowing the cryocooler to operate with wider heat loads and temperature ranges. The gas phase shifting device provides for load shifting between the stages in a multistage mechanical cryocooler. In a displacer cryocooler, the gas phase shifting can be achieved by installing a phase shifting device between the expansion volumes. The gas phase shifting device can be a flow impedance device, for example, a valve, a sharp-edged orifice, or a porous medium in an expander cryocooler. The gas phase shifting device can also be a flow inertia device, for example, an inertance gap in a pulse tube of a cryocooler. The gas phase shifting device phase shift the gas pressure in phase with mass flow at the cold end of the cryocooler for maximum cooling. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a two-stage gas phase shifting cryocooler.

FIG. 2 is a diagram of a pulse tube gas phase shifting inertance gap cryocooler.

FIG. 3A is a diagram of a concentric ring inertance gap.

FIG. 3B is a diagram of a parallel plate inertance gap.

FIG. 4A is a diagram of a motor controlled variable inertance gap.

FIG. 4B is a diagram of a heater controlled variable inertance gap.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to FIG. 1, a two-stage gas phase shifting valve cryocooler is a modified version of a conventional two-stage cooler with a moving porous piston heat exchanger. The porous piston heat exchanger, or simply the regenerator, functions as a heat exchanger. The porous piston is bifurcated for heat exchange into a first stage heat exchanger for exchange heat with a first stage volume and into a second stage heat exchanger for heat exchange with a second stage volume. A compressor provides the pressure and volume work required for cooling, by moving the piston. The compressor injects gas through an intake conduit into an empty volume, known as a plenum. The gas is cooled when passing through the porous heat exchanger. The piston in the cryocooler moves when a parcel of gas is admitted into the plenum through the intake conduit. The parcel of gas then passes through the first stage porous piston heat exchanger being a first stage regenerator into the first stage volume, known as an expander volume. When passing through the piston, the gas exchanges heat with the matrix of the porous piston and is cooled in a first stage regenerator. The gas is cooled further as the expander increases in volume. Part of the admitted gas continues to pass through the second stage porous piston heat exchanger for further cooling into the second stage volume that is also an expander volume for further cooling. When passing through the second stage heat exchanger, the gas exchanges heat with the matrix of the heat exchanger and is further cooled. The motion of the porous piston heat exchanger is driven pneumatically by the compressor or separately by a motor, not shown. The motion of the porous piston heat exchanger is driven with a phase lag relative to the compressor motion. When the compressor compresses, gas is admitted into the plenum. As the porous piston heat exchanger moves towards the plenum, expansion takes place at the first and second stage expander volumes, resulting in cooling. The compression and expansion of the first and second stage volumes are synchronized. Thus, the temperature of the second stage of the cooler is influenced by the temperature of the first stage, and vice versa.
A controlled gas phase shifting device can be a controlled valve or orifice opening, that is a resistance device, or a controlled tube or gap geometry, that is an inertance device. For maximum cooling, the gas pressure is in phase with mass flow at the cold end of the cryocooler. The gas mass flow of gas moving from the first stage to the second stage is in phase with the pressure difference between the first stage and the second stage. The gas phase shifting devices mentioned above are controlled by a controller that controls gas flow through a first stage conduit connected to the first stage volume and a second stage conduit connected to the second stage volume. The valve controls partial gas flows between the first and second expander volumes. The partial gas flows create a phase shift in gas pressures and mass flow between the first and second volumes. The valve or orifice resistance device, or a tube or gap inertance device disposed between the first stage conduit and second stage conduit, functions as a gas phase shifting device. By introducing this phase shifting device between the first and second stage volumes, a wider temperature range of one stage relative to the other can be achieved. The phase shifting device can be a gas resistance device such as valves and orifices, or an inertance device such as tubes and gaps. Different settings of valve opening and orifice size for resistance devices, and diameter, length, width, and thickness of gaps, or the diameter and the length of the tube for inertance can be used to optimize cooler performance. While shown for two stages, one or more phase shifting devices can be applied to any number of expander volumes in including those having more than two stages and respective volumes.
Referring to FIG. 2, a pulse tube cooler is characterized as having a pulse tube coupled between a stationary first stage heat exchanger and a reservoir. A compressor drives gas through the first stage heat exchanger or regenerator. Four tubes are used to couple in line the compressor to the heat exchanger, to the pulse tube, to the inertance gap, and finally to the reservoir. The reservoir is coupled to an inertance gap through a first tube. The inertance gap is coupled to a pulse tube through a second tube. The pulse tube is connected to a heat exchanger through a third tube. The heat exchanger is coupled to a compressor through a fourth tube. The compressor includes a piston driven by a load. The compressor provides the gas pressure work required to achieve cooling. The heat exchanger and third tube can be considered a first stage and the pulse tube and the inertance gap can be considered a second stage. As such, the third tube defines an entrance volume as a first stage volume. The entrance of the fourth tube is a hot end and the exit of the third tube is a cold end of the first stage. The reservoir is an exit volume as a second stage volume.
When the piston in the compressor compresses, a parcel of gas is admitted into the regenerating heat exchanger through the intake conduit, that is, the fourth tube. As the gas passes through the heat exchanger, the gas exchanges heat with the matrix, not shown, within the heat exchanger and is cooled. The cooled gas is then passed to the pulse tube that is an empty tube. As the gas moves from the third tube end into the pulse tube and then to the reservoir through the inertance gap, expansion of the gas occurs resulting in cooling of the gas in the third tube. There is created pressure gas phase lag between the compression of the gas at the compressor and expansion of the gas through the inertance gap to the reservoir for optimal cooling. The inertance gap is used in place of an inertance tube. Variously configured inertance gaps can be used.
Referring to FIGS. 3A, 3B, 4A, and 4B in sequence, the inertance gap can be made of concentric elongated rings. The gap thickness between the rings, the diameter of the rings, and the length of the rings, define the gap geometry, and hence, define the gas phase shifting capability. The gap can also be fashioned out of parallel plates defining a planar inertance gap. In both the concentric ring configuration and the parallel plate configuration, the gap geometry is compact and light in weight. The compact gap design offers the capability of optimizing the phase shift for different applications by varying the gap size. For in-operation adaptations, the gap can be made dynamic and precisely externally controlled when desired. A gap can be defined as between a gap piston and gap housing. A variable gap is realized by moving the piston within a gap housing. A motive means can be used to drive the gap piston toward or away from the housing to vary the thickness of the gap. That is, the position of the gap piston in relation to the gap housing defines the gap thickness. The position of the gap can be varied mechanically by using the motive means that can be in the exemplar forms a motor using electrical power or thermal expander that is powered by differences in thermal contraction coefficients. Likewise, in a controlled inertance tube device, the geometry of the tube can also be changed, for example, by using a bellows tube.
Referring to all of the Figures, the gas phase shifting cryocooler provides more efficient cooling at each stage in a multistage displacer cryocooler. The gas phase shifting in the displacer cryocooler can be achieved by installing a phase shifting device between the expansion spaces. For the displacer cryocooler, the exemplar gas phase shifting device can be a flow resistance device, for example, a valve, a sharp-edge orifice, or a porous medium. The gas phase shifting device can also be a flow inertia device, for example, a long capillary or an inertance gap. Likewise, the gas phase shifting pulse tube cryocooler provides more efficient cooling in a pulse tube cooler. The gas phase shifting in the pulse tube cryocooler can be achieved by an inertance gap installed between the pulse tube and the reservoir. For the pulse tube cryocooler, the exemplar gas phase shifting device is a defined gap preferably having at least three degrees of design freedom for optimizing the gas phase shifting. In both the displacer cryocooler and the pulse tube cryocooler, the gas phase shifting device can be externally controlled such as by using a controlled valve or a controlled inertance gap changing the gas shifting characteristics for improved cooler performance.
In a pulse tube cryocooler, an inertance gap is placed between the warm end of the pulse tube and the surge buffer reservoir. Exemplar gaps include concentric ring inertance gaps and parallel plate inertance gaps. The inertance gap is defined as a geometry with the thickness of the opening much smaller than the width or length of the opening. As such, the inertance gap provides three variables consisting of a thickness Sg, a length Lg, and a width Wg of the gap having a volume Vg. The analogous LRC equations become L=L_g/(W_gS_g), R=12L_gη/ρW_gS³), and C=Mv/(γRT). Relating the inertance gap parameters to the inertance tube parameters, Lg=(4Wt/π) (S_g/d_t ²)L_t. The length of the inertance gap Lg is orders of magnitude (S_g/d_t ²) smaller than the length of the inertance tube L_t. The performance of the inertance gap pulse tube is more efficient than that of the inertance tube pulse tube at high powers, and is slightly less efficient at low powers. Moreover, the design of the inertance gap can be more compact, such as a few inches in length, compared to that of the inertance tube that can be a few meters in length.
The performance of a cryocooler can be predicted using commercial cryocooler software tools. For a two-stage displacer cryocooler, the first stage heat load can be plotted as a function of the first stage temperature and the second stage heat load plotted as a function of the second stage temperature. In both stages, the cryocooler with the phase shifting device offers a wider operating temperature range and a higher cooling capacity.
The gas phase shifting cryocooler provides improved cooler performance. The addition of a control valve or orifice or controlled inertance tube or gap between expansion volumes in a displacer cryocooler, or the placement of an inertance gap in a pulse tube cryocooler, can provide improved cooler performance. The gap design substantially decreases the length of the phase shifting device reducing packaging constraints and the potential for vibration failures. The performance of the inertance gap in the pulse tube cooler is improved at high powers. By further optimizing the performance with a different geometry of the inertance gap, the gas phase shifting cryocooler can approach or surpass the performance of an inertance tube pulse tube cooler. The controlled gas phase shifting cryocooler enables real-time optimization of the performance of the pulse tube subject to different operating conditions by varying the gap size remotely through a thermal or electromechanical means. It is more practical to vary the inertance gap size rather than the dimensions of a long inertance tube. Instead of setting the dimensions of a valve or orifice or the inertance gap at discrete values throughout the entire thermodynamic cycle, the dimensions of the controlled gas phase shifting device can also be varied within the thermodynamic cycle, resulting in a time variant phase shifting device.
A pulse tube with a compact inertance gap, which replaces the long inertance tube offers comparable performance, but is much more compact, easier to package, without any vibration failures while offering real-time performance optimization capability. Various gaps designed can be used in a pulse tube cryocooler between the pulse tube and the reservoir. For example tapered, series, and parallel gaps could be used in various configurations. Various gas phase-shifting devices, such as a controlled valve or orifice, or inertance gap or tube can be used between expander volumes in displacer cryocooler. The preferred forms are cryocoolers and heat pumps. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.

Claims

1. A cryocooler a first volume and a second volume, the cryocooler comprising,

a first stage defining a first volume for containing gas at a first temperature and first pressure, and

a second stage defining a second volume for containing the gas at a second temperature and second pressure, and

a gas phase shifting device between the first volume and the second volume, the gas phase shifting device providing a pressure and temperature phase difference between the first volume and the second volume, wherein,

the cryocooler is a displacer cryocooler,

the first stage is a moving first heat exchanger, and

the second stage is a moving second heat exchanger.

2. The cryocooler of claim 1 wherein,

the cryocooler is selected from a group consisting of coolers, heaters, and heat pumps.

3. The cryocooler of claim 1 wherein,

gas mass flow is in phase with the gas pressure in the first volume and the second volume.

4. The cryocooler of claim 1 wherein,

the cryocooler comprises a compressor,

the moving first heat exchanger and the second stage heat exchanger are portions of a porous piston driving gas pressure and mass flow from the compressor.

5. The cryocooler of claim 1 wherein,

the cryocooler comprises a controller, and the gas phase shifting device is a flow impedance device.

6. The cryocooler of claim 1 wherein,

the gas phase shifting device is a valve.

7. The cryocooler of claim 1 wherein,

the gas phase shifting device is a flow impedance device selected from the group consisting of valves, sharp edge orifices, and porous mediums.

8. The cryocooler of claim 1 wherein,

the gas phase shifting device is a flow impedance device, and

cooling of gas from the first volume to the second volume is controlled by flow impedance of the gas phase shifting device for controlling mass flow and pressure waves between the first volume and the second volume.

9. The cryocooler of claim 1 wherein,

the gas phase shifting device is an inertance gap.

10. The cryocooler of claim 1 wherein,

the gas phase shifting device is a flow inertance device selected from the group consisting of tubes and gaps.

11. The cryocooler of claim 1 wherein,

the gas phase shifting device is a flow inertance device, and cooling of gas from the first volume to the second volume is controlled by flow inertia of the gas phase shifting device for controlling mass flow and pressure waves between the first volume and the second volume.