Cryogenic refrigerator

A disclosed device cryogenic refrigerator includes a first stage displacer; a first stage cylinder configured to form a first expansion space between the first stage cylinder and the first stage displacer; a second stage displacer connected to the first stage displacer; and a second stage cylinder configured to form a second expansion space between the second stage cylinder and the second stage displacer, wherein the second stage displacer includes a helical groove formed on an outer peripheral surface of the second stage displacer so as to helically extend from the second expansion space, a flow resistor communicating with a side of the first stage displacer in the helical groove, and a flow path connecting the flow resistor to a side of the first expansion space, wherein the flow resistor is always positioned on a side of the second expansion space relative to the first expansion space.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based upon and claims the benefit of priority of Japanese Patent Application No. 2011-221266 filed on Oct. 5, 2011 the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a cryogenic refrigerator which generates cold (a cold thermal energy by causing an ultra-low temperature) by generating Simon expansion using a high-pressure refrigerant gas supplied from a compression device.

2. Description of the Related Art

For example, the Patent Document 1 discloses a Gifford-McMahon (GM) refrigerator causing a gas existing inside a gap between a piston of the GM refrigerator and a cylinder of the GM refrigerator to expand. This GM refrigerator has a linear groove functioning as a phase shifting mechanism.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a regenerative type refrigerator including: a first stage displacer; a first stage cylinder configured to form a first expansion space between the first stage cylinder and the first stage displacer; a second stage displacer connected to the first stage displacer; and a second stage cylinder configured to form a second expansion space between the second stage cylinder and the second stage displacer, wherein the second stage displacer includes a helical groove formed on an outer peripheral surface of the second stage displacer so as to helically extend from the second expansion space, a flow resistor communicating with a side of the first stage displacer in the helical groove, and a flow path connecting the flow resistor to a side of the first expansion space, wherein the flow resistor is always positioned on a side of the second expansion space relative to the first expansion space.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the above structure disclosed in the Patent Document 1, a high temperature side of the linear groove repeatedly enters into and exits from an expansion space on a side of a first stage while a two-stage type displacer reciprocates. Therefore, the flow path resistance of a flow resistor changes. Therefore, there is a problem that refrigeration efficiency is not enhanced.

The object of the embodiments of the present invention is to provide a cryogenic refrigerator which can effectively enhance refrigeration efficiency solving one or more of the problems discussed above.

Preferred embodiments of the present invention are explained next with reference to accompanying drawings.

First Embodiment

A cryogenic refrigerator1of the first embodiment may be of a Gifford-McMahon (GM) type. Referring toFIG. 1, the cryogenic refrigerator1includes a first stage displacer2, a first stage cylinder4forming a first expansion space3between the first stage cylinder4and the first stage displacer2, a second stage displacer5connected to the first stage displacer2, and a second stage cylinder7forming a second expansion space6between the second stage cylinder7and the second stage displacer5.

Further, the cryogenic refrigerator1includes a helical groove8formed on an outer peripheral surface of the second stage displacer5and helically extends from the second expansion space6, a flow resistor9communicating with a side of the first stage displacer2in the helical groove8, and a flow path10communicating with a side of the first expansion space3in the flow resistor9. The flow resistor9is always positioned on a side of the second expansion space6relative to the first expansion space3.

The first stage displacer2and the second stage displacer5have cylindrical outer peripheral surfaces, respectively. A first regenerator11is provided inside the first stage displacer2. A second regenerator12is provided in the second stage displacer5. A sealing portion13is provided between a portion on the high temperature side of the first stage displacer2and the first stage cylinder4. A supply and discharge pipe is connected to the upper end of the first stage cylinder4. The supply and discharge pipe is included in pipes for connecting various parts of a supply and discharge system such as a compressor14, a supply valve15, and a return valve16.

An axis member (not illustrated) is connected to the upper end of the first stage displacer2. The axis member protrudes from the upper end of the first stage cylinder4and is connected to a driving motor (not illustrated) via a crank mechanism (not illustrated). The axis member, the crank mechanism, and the driving motor form a driving mechanism for reciprocating the first stage displacer2and the second stage displacer5in the axial directions.

The first stage displacer2is accommodated in the first stage cylinder4, which is shaped like an upside down bottomed cylinder (a capped cylinder) having an opened lower end. The second stage displacer5is accommodated inside the second stage cylinder7, which is shaped like an upside down bottomed cylinder (a capped cylinder) having an opened lower end. The first stage cylinder4and the second stage cylinder7are integrally formed.

For example, stainless steel may be used as a material of the first and second stage cylinders4and7to achieve high strength, low thermal conductivity, and sufficient helium interruption capability. For example, a phenol resin including fabric or the like is used for the first stage displacer2to obtain a lighter specific gravity, more sufficient wear resistance, higher strength, and lower thermal conductivity. The second stage displacer5is a metallic cylinder on the outer periphery of which a coating such as fluorine contained resin having high wear resistance is provided. The first regenerator11is formed of a first regenerative material such as a screen. The second regenerator12is formed by holding a regenerative material such as lead spheres in the axial directions by felt and screens.

The helical groove8is formed on the outer peripheral surface of the second stage displacer5. The helical groove8has a start end communicating with the second expansion space6, helically extends toward the side of the first expansion space3, and has a final end ending at a middle position in the axial direction of the second stage displacer5.

Further, the flow resistor9in a groove-like shape is formed on the outer peripheral surface of the second stage displacer5so as to extend from the end of the helical groove8in an axial direction (a longitudinal direction) of the second stage displacer5. Referring toFIG. 1, the end of the flow resistor9is positioned lower than the bottom of the first stage cylinder4when the first stage displacer2and the second stage displacer5are positioned at their upper dead centers. A flow path10is further formed on the outer peripheral surface of the second stage displacer5and extends from the end of the flow resistor9to the top of the second displacer5so as to communicate with the first expansion space3.

As described, the flow resistor9always exists on the side of the second expansion space6relative to the first expansion space3. When the first stage displacer2is positioned at the upper dead center where the first expansion space3becomes maximum, the flow resistor9is entirely positioned on the side of the second expansion space6relative to an exposing portion of the outer peripheral surface of the second stage displacer5exposed to the first expansion space3. Referring toFIG. 1, the upper end of the flow resistor9is positioned below the bottom of the first cylinder4forming the first expansion space3.

The flow path10is formed on the outer peripheral surface of the second stage displacer5so as to extend in the axial direction of the second stage displacer5. A cross-sectional area A10(a corresponding area is indicated by a dashed oval inFIG. 1) of the flow path10in a cross-sectional view perpendicular to the axial direction is greater than a cross-sectional area A9(a corresponding area is indicated by a dashed oval inFIG. 1) of the flow resistor9in a cross-sectional view perpendicular to a direction of extending the flow resistor9(A10>A9).

When the compressor14is operated and the supply valve15is opened, a high pressure helium gas is supplied from the above supply and discharge pipe into the first stage cylinder4via the supply valve15. The high pressure helium gas is further supplied to the first expansion space3via a communicating passage for communication of the refrigerant gas between an upper end inside the first stage displacer2and the first regenerator11and a communicating passage for communication of the refrigerant gas between the first regenerator11and the first expansion space3.

The high pressure helium gas supplied to the first expansion space3is further supplied to the second regenerator12via a communicating passage for communication of the refrigerant gas between the first expansion space3and the second regenerator12, and is supplied to the second expansion space6via a communicating passage for communication of the refrigerant gas between the second regenerator12and the second expansion space6. A part of the high pressure helium gas other than that supplied to the first expansion space3is supplied to the high pressure side of the helical groove via the flow path10and the flow resistor9formed on the outer peripheral surface of the second displacer5as a gas passage. A part of the high pressure helium gas supplied to the second expansion space6is supplied to a low temperature side inside the helical groove8.

FIG. 2schematically illustrates a flow of a gas in which a side clearance of the cryogenic refrigerator is regarded as a pulse tube of a virtual pulse tube refrigerator. The flow resistor9corresponds to an orifice arranged on a communicating passage connecting the supply and discharge pipe to the high pressure side of the helical groove8. The refrigerant gas inside the helical groove8corresponds to a virtual gas piston8P (a substantially center portion of the helical groove8in the axial direction).

The length of the virtual gas piston8P in the axial direction and the phase of the virtual gas piston8P may be adjusted so that the virtual gas piston8P is always accommodated inside the helical groove8during reciprocation of the helical groove8and so that a high temperature side space8H exists on the high temperature side of the virtual gas piston8P and a low temperature side space8L exists on the low temperature side of the virtual gas piston8P. The length of the virtual gas piston8P in the axial direction and the phase of the virtual gas piston8P are adjusted by changing the cross-sectional area and the length of the flow resistor (the orifice)9functioning as a phase shifting mechanism.

Next, the operation of the refrigerator is described. At a certain time point of supplying the refrigerant gas, the first stage displacer2and the second stage displacer5are positioned at lower dead centers in the first stage cylinder4and the second stage cylinder7, respectively. At this timing or a timing slightly different from this timing, the supply valve15is opened. Then, a high pressure helium gas is supplied inside the first stage cylinder4from a supply and discharge pipe via the supply valve15. The high pressure helium gas flows inside the first stage displacer2(into the first regenerator11) from an upper portion of the first stage displacer2. The high pressure helium gas flowing inside the first regenerator11is supplied into the first expansion space3via the communicating passage positioned at the lower portion of the first stage displacer2while being cooled by the first regenerative material.

Most of the high pressure helium gas supplied to the first expansion space3is then supplied to the second regenerator12via a communication passage (not illustrated). The residual helium gas which is not supplied to the second regenerator12is supplied from the high temperature side to the helical groove8via the flow path10and the flow resistor9. This gas corresponds to the helium gas existing in the high temperature side space8H inFIG. 2, which functions to prevent the virtual gas piston8P from flowing toward the first expansion space3from the helical groove8. Because the cross-sectional area of the flow path10is sufficiently greater than the cross-sectional area of the flow resistor9, the resistance of the helium gas in flowing through the flow path10is substantially smaller than the resistance of the helium gas on flowing through the flow resistor9. Therefore, the inflow resistance of the helium gas flowing from the first expansion space3to the high temperature side space8H can be adjusted by changing the total length and the cross-sectional area of the flow resistor9.

The high pressure helium gas flowing into the second regenerator12is cooled by the second regenerative material inside the second regenerator and is supplied to the second expansion space6. A part of the high pressure helium gas supplied to the second expansion space6is supplied into the low temperature side helical groove8from the low temperature side. This gas corresponds to the helium gas existing inside the low temperature side space8L inFIG. 3.

As described, since the cross-sectional area of the flow resistor9is smaller the than cross-sectional area of the helical groove8, the inflow resistance of the helium gas flowing into the high temperature side space8H (flowing inside the helical groove8) is greater than the inflow resistance of the helium gas flowing into the low temperature side space8L (inside the helical groove8). Therefore, the amount of the helium gas flowing inside the high temperature side space8H is smaller than the amount of the helium gas flowing inside the low temperature side space8L thereby preventing the gas in the high temperature side space8H from flowing into the second expansion space6.

As described, the first expansion space3, the second expansion space6and the helical groove8are filled with the high pressure helium gas and the supply valve15is closed. At this time, the first stage displacer2and the second stage displacer5are positioned at the upper dead centers in the first stage cylinder4and the second stage cylinder7, respectively. At this timing or a timing slightly different from this timing, the return valve16is opened. Then, the refrigerant gas inside the first expansion space3, the second expansion space6and the helical groove8are depressurized to thereby expand. The helium gas inside the first expansion space3having a low temperature by the expansion absorbs heat of a first cooling stage (not illustrated) and the helium gas in the second expansion space6absorbs heat of a second cooling stage (not illustrated).

The first stage displacer2and the second stage displacer5move toward the lower dead centers thereby reducing the volumes of the first expansion space3and the second expansion space6. The helium gas inside the second expansion space6is recovered into the first expansion space3via the second regenerator12. The helium gas on the low temperature side space8L in the helical groove8is recovered via the second expansion space6.

The helium gas in the first expansion space3returns to the compressor14via the first regenerator11to a suction side of the compressor12. At this time, the first regenerative material and the second regenerative material are cooled by the refrigerant gas. These processes form one cycle. The first cooling stage and the second cooling stage are cooled by repeating the cycle.

The following functions and effects are obtainable by the cryogenic refrigerator1of the first embodiment. The virtual gas piston8P is realized inside the helical groove8forming the side clearance between the second stage displacer5and the second stage cylinder7to cause the gas piston8P to function as the sealing portion for preventing the helium gas from communicating between the high and low temperature sides of the side clearance.

Said differently, the virtual gas piston realized by using the side clearance between the outer peripheral surface of the second displacer5and the inner peripheral surface of the second stage cylinder7prevents the helium gas from bi-directionally moving thereby preventing generation of leakage loss. Thus, the refrigeration efficiency can be enhanced.

Additionally, the side clearance can be regarded as the virtual pulse tube refrigerator due to the virtual gas piston8P. Then, it is possible to use the low temperature side space8L on the low temperature side of the gas piston8P as a third expansion space. Thus, it is possible to enhance refrigeration efficiency.

Further, a double inlet forming the phase shifting mechanism for adjusting the length of the virtual gas piston8P in the axial direction and the phase of the virtual gas piston8P is realized by the first flow resistor9in a groove-like shape extending in the axial direction on the outer peripheral surface of the second stage displacer5. Therefore, the phase shifting mechanism can be further easily formed. Further, the flow resistor9is formed so as not to enter into the first expansion space3regardless of the above reciprocation of the first stage displacer2and the second stage displacer5. Therefore, the flow rate coefficient as the double inlet is constant along the entire region of the reciprocation to thereby stabilize a phase shifting function.

Within the first embodiment, it is possible to stabilize a phase shifting function. Therefore, it is possible to stabilize the length and the phase of the virtual gas piston8P and the above described function of the sealing portion. The leakage loss can be further secured. Furthermore, the refrigeration efficiency can be further securely enhanced by providing the third expansion space.

Although the flow resistor9extends in the axial direction on the outer peripheral surface of the second stage displacer5, the flow resistor9may instead be a hole communicating with the end of the helical groove8by downwardly extending from the start end of the flow path10.

Second Embodiment

In the above cryogenic refrigerator1of the first embodiment, the high pressure helium gas flows from the first expansion space3to the helical groove8through the flow path10and the flow resistor9. The low pressure helium gas flows from the helical groove8to the first expansion space3. Said differently, the refrigerant gas bi-directionally flows through the flow resistor9functioning as the double inlet. Since a high pressure helium gas has a density higher than that of a low pressure helium gas, the high pressure helium gas has a smaller flow velocity and a smaller pressure loss than the low pressure helium gas. Therefore, the amount of the high pressure helium gas flowing through the flow resistor per cycle is slightly greater than the amount of the low pressure helium gas flowing through the high pressure helical groove8per cycle. Therefore, the gas flow rates in the bi-directional flows are not balanced. As a result, a steady-state flow directed from the high temperature side of the helical groove8to the low-temperature side of the helical groove8may be generated every time the cooling cycles are repeated. Referring toFIG. 2, this flow is a secondary flow along an arrow L in a clockwise direction.

Within the second embodiment, the constant cross-sectional area A10(a corresponding area is indicated by a dashed oval inFIG. 3A) of the flow path10in the extending direction inFIG. 3Aof the first embodiment is changed to a continuously increasing cross-sectional area starting from the flow resistor9as illustrated inFIG. 3Bof the second embodiment. Referring toFIG. 3B, the cross-sectional area A10(a corresponding area is indicated by a dashed oval inFIG. 3B) of the flow path10is adjusted by changing the width perpendicular to the radius direction of the second stage displacer5. However, the depth in the radius direction may be adjusted alone or in addition to the adjustment of the width.

Thus, it is possible to give a resistance of preventing generation of a secondary flow to a flow of the helium gas by reducing the cross-sectional area A10of the flow path10. The flow path resistance of the helium gas caused when the helium gas flows from the first expansion space3through the flow resistor9to the helical groove8is greater than the flow path resistance of the helium gas caused when the helium gas flows from the helical groove8through the flow resistor9to the first expansion space3thereby restricting the generation of the secondary flow. Therefore, a heat loss caused by the secondary flow L can be prevented to thereby enhance refrigeration efficiency.

Third Embodiment

The third embodiment is described in detail. Within the first and second embodiments, the flow path10is provided on the outer peripheral surface of the second displacer5. However, the flow path10may be arranged along the radius direction of the second displacer5.

Except for a flow path10-1, a cryogenic refrigerator1of the third embodiment has a structure basically similar to that of the first embodiment. Therefore, the same reference symbols are applied to the same portions and description of the different portions are mainly described. Referring toFIG. 4, the cryogenic refrigerator1of the third embodiment includes a helical groove8formed on an outer peripheral surface of a second stage displacer5and helically extends from a second expansion space6, a flow resistor9communicating with the helical groove8on a side of the first stage displacer2, and the flow path10-1connecting the flow resistor9to a first expansion space3. The flow resistor9is always positioned inside the second stage cylinder7relative to the first expansion space3.

Within the third embodiment 3, the flow resistor9is shaped like a groove extending on the outer peripheral surface of the second stage displacer5in the axial direction. Referring toFIG. 4, the upper end of the flow resistor9is always positioned lower than a first expansion space3, namely around the bottom portion of the first stage cylinder4.

Within the third embodiment, in a manner similar to the first embodiment, the helical groove8forming the side clearance between the outer peripheral surface of the second stage displacer5and the inner peripheral surface of the second stage cylinder7is regarded as a pulse tube refrigerator as illustrated inFIG. 2to form the virtual gas piston8P inside the helical groove8. Then, the flow resistor9having a constant flow rate coefficient is regarded as the double inlet of which phase and length are appropriately adjustable.

Said differently, it becomes possible to enhance refrigeration efficiency by securely providing a sealing function to the virtual gas piston8P to thereby prevent a leakage loss. Further, the refrigeration efficiency can also be enhanced by additionally cooling using a low temperature side space8L of the helical groove8as a third expansion space.

Further, the helium gas flowing inside the high temperature side space8H flows via the second regenerator12. Therefore, the refrigerant gas cooled to have a lower temperature than that in the first embodiment preferably flows into the helical groove8.

Fourth Embodiment

Within the above first to third embodiments, the flow resistor9extends on the outer peripheral surface in the axial direction as the groove. A flow path may be formed by a hole extending in the radius direction of the second stage displacer5so that the hole functions as a flow resistor. The fourth embodiment is described next.

Except for a flow path10-2, a cryogenic refrigerator1of the fourth embodiment has a structure basically similar to that of the third embodiment. Therefore, the same reference symbols are applied to the same portions and description of the different portions are mainly described.

Referring toFIG. 5, the cryogenic refrigerator1includes a helical groove formed on an outer peripheral surface of a second displacer5and helically extending from the second expansion space6, and the flow path10-2communicating with the helical groove8on the side of a first stage displacer2. The flow path10-2extends in the radius direction of the second stage displacer5and communicates with the second regenerator12. The flow path10-2is always positioned on the side of a second expansion space6relative to a first expansion space3. Therefore, the flow path10-2is not constantly exposed to the first expansion space3regardless of the above reciprocation of the first stage displacer2and the second stage displacer5.

Within the fourth embodiment, a communicating portion8T communicates with the flow path10-2of the helical groove8. The cross-sectional area of the helical groove8perpendicular to a direction along the communicating portion8T becomes continuously small from the side of the helical groove8to the flow path10-2. Thus, a helium gas smoothly flows at the communication portion8T.

Within the fourth embodiment, in a manner similar to the first embodiment, the helical groove8forming the side clearance between the outer peripheral surface of the second stage displacer5and the inner peripheral surface of the second stage cylinder7is regarded as a pulse tube refrigerator as illustrated inFIG. 2to form the virtual gas piston8P inside the helical groove8. Then, the flow path10-2having the flow resistor is regarded as a double inlet of which phase and length are appropriately adjustable to realize a sealing function with the gas piston8P. Said differently, a leakage loss is prevented to enhance refrigeration efficiency. Further, a low temperature side space8L inside the helical groove8ais used as a third expansion space to thereby enhance the refrigeration efficiency.

Further, the flow path10-2is also the flow resistor. The cross-sectional area of the flow resistor is made smaller than the cross-sectional area of the helical groove in order to reduce the flow rate coefficient. Further, by reducing the inner diameter of the flow path10-2to be smaller than outer diameters of lead spheres as a second regenerative material of the second regenerator12, the lead spheres are prevented from intruding from an opening in the flow path10-2on the side of the second regenerator12. Thus, lead spheres are prevented from dropping off the outside of the second regenerator12.

When it is necessary to increase the inner diameter of the flow path10-2to be greater than the outer diameter of the second regenerative material, an appropriate drop preventing part having reticulation smaller than the diameters of the lead spheres is provided on the flow path10-2on the side of the second regenerator12.

Although the cryogenic refrigerator described above has the two stages of the displacers, the number of the stages may be appropriately changed to three or the like.

Within the embodiments, the flow resistor9and the flow path10are shaped like grooves on the outer peripheral surface of the second stage displacer5in the axial direction. However, the shape is not limited to the grooves. For example, the flow resistor9and the flow path10may further extend from the helical groove8in the direction of the helical groove8.

As described, the embodiments provide a cryogenic refrigerator which can reduce the leakage loss in the side clearance and enhance the refrigeration efficiency by using the side clearance as the third expansion space.

As described, within the embodiments, it is possible to securely adjust the length in the axial direction and the phase of the virtual gas piston in using the side clearance as the pulse tube type refrigerator.