Abstract:
A Stirling cycle cryocooler is disclosed that includes a displacer unit having a cold end and a hot end. The displacer unit includes a cold cylinder housing and a displacer liner disposed on the inner surface of the housing. A displacer assembly lies within the displacer liner and is slidable with respect to the lengthwise axis of the housing. The displacer unit also includes a regenerator unit. A heat acceptor is affixed to the cold end of the displacer unit. The heat acceptor transfers heat from a device such as a High Temperature Superconducting Filter to a gas such as helium located within the displacer unit. The heat acceptor preferably includes a radial component and an annular component. The heat acceptor advantageously decreases the heat transfer resistance between the heat acceptor and the helium gas. The Stirling cycle cryocooler is thus able to operate with reduced input power to achieve a desired lift level.

Description:
FIELD OF THE INVENTION 
     The field of the invention relates generally to cryocoolers. More particularly, the field of the invention relates to Stirling cycle cryocoolers. 
     BACKGROUND OF THE INVENTION 
     Recently, substantial attention has been directed to the field of superconductors and to systems and methods for using such products. Substantial attention also has been directed to systems and methods for providing a cold environment (e.g., 77° K. or lower) within which superconductor products such as superconducting filter systems may function. 
     One device that has been widely used to produce a cold environment within which superconductor devices may function is the Stirling cycle refrigeration unit or Stirling cycle cryocooler. Such units typically comprise a displacer assembly and a compressor assembly, wherein the two assemblies are in fluid communication and are driven by one or more linear or rotary motors. Conventional displacer assemblies generally have a “cold” end and a “hot” end, the hot end being in fluid communication with the compressor assembly. Displacer assemblies generally include a displacer having a regenerator mounted therein for displacing a fluid, such as helium, from one end, i.e., the cold end of the displacer assembly, to the other end, i.e., the hot end, of the displacer assembly. The piston assembly functions to apply additional pressure to the fluid, when the fluid is located substantially within the hot end of the displacer assembly, and to relieve pressure from the fluid, when the fluid is located substantially within the cold end of the displacer assembly. In this fashion, the cold end of the displacer assembly may be maintained, for example, at 77° K., while the hot end of the displacer assembly is maintained, for example, at 15° K. above ambient temperature. Devices such as superconducting filter systems are then typically placed in thermal contact with the cold end of the displacer assembly. 
     Current Stirling cycle cryocooler designs employ a heat acceptor positioned at the cold end of the displacer assembly. The heat acceptor is typically in thermal contact with the device that is to be cooled, such as a High Temperature Superconducting Filter (HTSF) system. Heat is transferred from the device and to the heat acceptor. The heat transferred to the heat acceptor then passes to the helium gas contained in the displace assembly. The transfer of heat from the heat acceptor to the helium gas typically is the most difficult since the resistance to heat transfer is greatest in this step. 
     In current cryocooler designs, the ineffective transfer of heat from the heat acceptor to the helium gas results in additional power requirements. In essence, a greater amount of input power is needed to achieve the desired refrigeration lift. The lower heat transfer rate is due, in large part, to the relatively small surface area and low convective heat transfer coefficient. 
     There is a need for a cryocooler design that decreases the heat transfer resistance between the heat acceptor and the helium gas. The cryocooler design would advantageously require less input power to provide an equivalent amount of refrigeration as compared to prior designs. 
     SUMMARY OF THE INVENTION 
     In a first aspect of the invention, a displacer unit for use in a Stirling cycle cryocooler is disclosed. The displacer includes a housing, a displacer liner adjacent to the inside of the housing, a displacer assembly, a regenerator unit, and a heat acceptor. The displacer assembly is located inside the displacer liner and is axially slidable with respect to the housing. The heat acceptor includes a radial component and an annular component. The heat acceptor is affixed to the cold end of the displacer unit. 
     In a second separate aspect of the invention, a heat acceptor for use on the cold end of a displacer unit is disclosed. The heat acceptor includes a radial component including a radially located inner face that is perpendicular to the long axis of the displacer unit. In addition, the heat acceptor includes an annular component including an inner circumferential face. 
     In a third aspect of the invention, the displacer unit according the first aspect of the invention further includes a plurality of radial holes in the displacer assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a side view of the Stirling cycle cryocooler. 
     FIG. 2 shows an enlarged side view of the cold end of the displacer unit. 
     FIG. 3 shows a graph illustrating the heat lift vs. input power for the present cryocooler and the conventional cryocooler. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a Stirling cycle cryocooler  2  in accordance with a preferred form of the present invention. As seen in FIG. 1, the Stirling cycle cryocooler  2  preferably includes a displacer unit  4 , a heat exchanger unit  6 , a compressor and a linear motor assembly  8 . 
     The displacer unit  4  preferably includes a cold cylinder housing  10 , a displacer assembly  12 , a regenerator unit  14 , and a displacer rod assembly  16 . A displacer liner  18  is positioned circumferentially about the displacer assembly  12  and inward of the cold cylinder housing  10 . The displacer assembly  12  is slidably mounted in the axial direction within the cold cylinder housing  10 . Preferably, the displacer liner  18  is affixed to the inner surface of the cold cylinder housing  10 . 
     The displacer unit  4  also includes a heat acceptor  20 . Preferably, as shown in FIGS. 1 and 2, the heat acceptor  20  includes a radial component  22  and an annular component  24 . The radial component  22  is generally perpendicular to the long axis of the displacer unit  4 . The long axis lies between the hot and cold ends of the displacer unit  4 . The annular component  24  lies along a circumferential annulus of the displacer unit  4 . Preferably, the annular component  24  extends from the radial component  22  to beyond the edge of the displacer assembly  12 . Even more preferably, the annular component  24  extends axially beyond the edge of the displacer assembly  12  and abuts against a distal end of the displacer liner  18 . The heat acceptor  20  is preferably brazed to the cold cylinder housing  10  to provide a hermetically sealed environment. The annular component  24  opposes, in a co-axial-type manner with the displacer liner  18 . In this regard, the total area of the heat acceptor  20  available for heat transfer is increased. 
     Referring now to FIG. 2, the radial component  22  of the heat acceptor  20  includes a radially located inner face  21 . The radially located inner face  21  is preferably perpendicular to the long axis of the displacer unit  4 . The annular component  24  includes an inner circumferential face  23 . 
     While the heat acceptor  20  has been described as containing two separate components, i.e., a radial component  22  and an annular component  24 , it should be understood that the heat acceptor  20  can be a single unitary component. Preferably, the heat acceptor  20  is made of thermally conductive metal such as copper. Even more preferably, the heat acceptor  20  is made from high purity copper or oxygen-free-high-conductivity (OFHC) copper. 
     In one aspect of the invention, the displacer assembly  12  includes a plurality of radial holes  26 . The radial holes  26  permits additional flow of helium within the cold end  25  of the displacer unit  4 . The helium flowing through the holes  26  will impinge directly on the heat acceptor  20 . The area available for heat transfer, shown by arrow A in FIG. 2, is thus increased. The radial holes  26  assist in decreasing the convective resistance between the heat acceptor  20  and the helium gas within the cryocooler  2 . 
     Still referring to FIG. 1, the displacer rod assembly  16  is coupled at one end to a base section  28  of the displacer assembly  12  and coupled at the other end to a displacer spring assembly  32 . 
     The heat exchanger unit  6 , which is located between the displacer unit  4  and the compressor and linear motor assembly  8 , preferably includes a heat exchanger block  34 , a flow diverter or equivalent structure, and a heat exchanger mounting flange  38 . The heat exchanger mounting flange  38  preferably is coupled to a distal end of a pressure housing  40  of the compressor and linear motor assembly  8 . The heat exchanger block  34  preferably includes a plurality of internal heat exchanger fins  42  and a plurality of external heat rejector fins  44 . Thus, the heat exchanger unit  6  is designed to facilitate heat dissipation from a gas, such as helium, that is compressed in the region P HOT  located at the juncture between the displacer unit  4  and the compressor and linear motor assembly  8  (the region P HOT  also is referred to herein as the compression chamber of the compressor and linear motor assembly  8 ). Preferably, the heat exchanger block  34 , internal heat exchanger fins  42  and external heat rejector fins  44  are made from a thermally conductive metal such as high purity copper. 
     The compressor and linear motor assembly  8  preferably includes a pressure housing  40  that has a piston assembly  46  mounted therein. The piston assembly  46  includes a cylinder  48 , a piston  50 , a piston assembly mounting bracket  54  and a spring assembly  56 . The piston assembly mounting bracket  54  provides a coupling between the piston  50  and the spring assembly  56 . The piston  50  is thus adapted for reciprocating motion within the cylinder  48 . A plurality of gas bearings  58  are provided within the exterior wall  60  of the piston  50 , and the gas bearings  58  receive gas, e.g., helium, from a sealed cavity  62  that is provided within the piston  50 . A check valve  64  provides a unidirectional fluid communication conduit between the sealed cavity  62  and the region P HOT  of the cylinder  48  (i.e., the compression chamber of the cylinder  48 ) when the pressure of the gas within that region exceeds the pressure within the cavity  62  (i.e., exceeds the piston reservoir pressure). 
     The piston  50  preferably has mounted thereon a plurality of magnets  66 . Internal laminations  68  are secured to the outside of the cylinder  48 . External laminations  70  are secured within the pressure housing  40  and are located outward of the magnets  66 . The external laminations  70  are preferably secured to a mounting flange  38 . The internal and external laminations  68 ,  70  are preferably made of an iron-containing material. A motor coil  72  preferably lies within the external laminations  70  and surrounds the piston  50 . The motor coil  72  is preferably located outward of the magnets  66  and within recesses formed within the external laminations  70 . Thus, it will be appreciated that, as the piston  50  moves within the cylinder  48 , the magnets  66  move within a gap  74 . 
     During operation, the piston  50  and displacer assembly  12  preferably oscillate at a resonant frequency of approximately 60 Hz and in such a manner that the oscillation of the displacer assembly  12  is approximately 90° out of phase with the oscillation of the piston  50 . Stated somewhat differently, it is preferred that the motion of the displacer assembly  12  will “lead” the motion of the piston  50  by approximately 90°. 
     Those skilled in the art will appreciate that, when the displacer assembly  12  moves to the “cold” end P COLD  of the displacer housing  10 , most of the fluid, e.g. helium, within the system is displaced to the warm end P HOT  of the displacer housing  10  and/or moves around the flow diverter or similar device and through the internal heat exchanger fins  42  into the compression area P HOT  of piston assembly  46 . Due to the phase difference between the motion of the displacer assembly  12  and the piston  50 , the piston  50  should be at mid-stroke and moving in a direction toward the heat acceptor  20  when displacer assembly  12  is located at the cold end of the displacer housing  10 . This causes the helium in the areas P HOT  to be compressed, thus raising the temperature of the helium. The heat of compression is transferred from the compressed helium to the internal heat exchanger fins  42  and from there to the heat exchanger block  34  and external heat rejector fins  44 . From the heat rejector fins  44 , the heat is transferred to ambient air. As the displacer assembly  12  moves to the warm end P HOT  of the displacer housing  10 , the helium is displaced to the cold end P COLD  of the displacer housing  10 . As the helium passes through the displacer cylinder  12 , it deposits heat within the regenerator unit  14 , and exits into the cold end P COLD  of the displacer housing  10  at approximately 77° K. At this time, the compressor piston  50  preferably is at mid-stroke and moving in the direction of the spring assembly  56 . This causes the helium in the cold end P COLD  of the displacer housing  10  to expand further reducing the temperature of the helium and allowing the helium to absorb heat. In this fashion, the cold end P COLD  functions as a refrigeration unit and may act as a “cold” source. 
     By using the heat acceptor  20  with the radial component  22  and the annular component  24 , the lift of the Stirling cycle cryocooler  2  can be increased for any given input power. Generally, during operation of the Stirling cycle cryocooler  2 , helium gas expands at the cold end of the displacer unit  4 , which reduces the temperature of the helium gas, and thus reduces the temperature of the heat acceptor  20 . The temperature gradient in heat acceptor  20  and helium gas causes heat to flow from the device being refrigerated, such as a High Temperature Superconducting Filter (HTSF), to the heat acceptor  20  and helium gas. The heat transfer rate is a function of the temperature difference between the device being refrigerated and the temperature of the heat acceptor  20  and helium gas, the interface resistance between the device being refrigerated and the heat acceptor  20 , the conductive resistance of the heat acceptor  20 , and the convective resistance between the heat acceptor  20  and the helium gas inside the Stirling cycle cryocooler  2 . 
     In actual use, the greatest resistance to heat transfer occurs between the heat acceptor  20  and helium gas. The equation which defines this convective resistance is as follows: 
     (1) Q=h*A*ΔT 
     where Q=heat transfer rate (watts) 
     h=convective heat transfer coefficient (watt/° C. m 2    
     A=heat transfer area (m 2 ), and 
     ΔT=temperature difference (°C.). 
     The Stirling cycle cryocooler  2  reduces the convective resistance by use of the heat acceptor  20 . The heat acceptor  20  accomplishes this by increasing the heat transfer area (A), and increasing the convective heat transfer coefficient (h). In addition, the radial holes  26  aid in increasing the convective heat transfer coefficient (h) between the helium gas and the annular portion  24  of the heat acceptor  20 . By decreasing the overall convective heat resistance, the Stirling cycle cryocooler  2  requires less input power for the same amount of refrigeration. 
     FIG. 3 illustrates the improved performance of the Stirling cycle cryocooler  2  using the modified heat acceptor  20 . As seen from FIG. 3, at 100 watts input power, the lift has increased from 4.25 watts to 5.7 watts, an improvement of about 34%. Consequently, a desired amount of lift can be achieved with reduced input power. 
     While embodiments of the present invention have been shown and described, various modifications may be made without departing from the scope of the present invention. The invention, therefore, should not be limited, except to the following claims, and their equivalents.