Patent Publication Number: US-7219712-B2

Title: Reduced shedding regenerator and method

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. DE-AC03-02SF22491 awarded by the United States Department of Energy. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is directed generally to machines and, more particularly, to thermal regenerative machines. 
   2. Description of the Related Art 
   A conventional thermal regenerative machines including Stirling cycle engines and coolers use a working fluid, such as a gas. Portions of the working fluid travel in passageways between a hot area and a cold area. As the working fluid travels from the hot area to the cold area, it passes through a conventional random fiber mesh material called a regenerator that retains heat from the working fluid. As the working fluid returns from the cold area back to the hot area, it receives some heat back from the regenerator thereby resulting in increased efficiency. Unfortunately, the mesh material of the regenerator can shed small particles, which migrate within the machine to become undesirably located in other regions and parts of the machine, thereby introducing a potential cause of damage or malfunction. Conventional approaches to reduce shedding have included addition of screen components to end surfaces where working fluid either flows into or out of the regenerator. These approaches have had limited success. Other surfaces of the regenerators remain unprotected and the additional screen components increase piece counts for manufacturing. 
   BRIEF SUMMARY OF THE INVENTION 
   Aspects according the present invention for a thermal regenerative machine having a first temperature area and a second temperature area are directed to a regenerator comprising a first layer portion having a first thickness, a first porosity, and a first material composition; and a second layer portion adjacent the first layer, the second layer having a second thickness, a second porosity, and the first material composition, the second thickness being greater than the first thickness, the second porosity being greater than the first porosity, the regenerator configured to be positioned within the thermal regenerative machine such that the first layer portion is nearer the first temperature area than the second layer portion. 
   Other aspects include in some implementations the first layer portion being made from a first number of sheets of random fiber material and the second layer portion being made from a second number of sheets of random fiber material, the first number being smaller than the second number. Other aspects include the first number of sheets being one. Other aspects include the random fiber material of the first layer portion being sintered a first number of times and the random fiber material of the second layer portion being sintered a second number of times, the first number being greater than the second number. Other aspects include wherein the second number is one. 
   Other aspects include a third layer portion adjacent the second layer portion on a side thereof away from the first layer portion, the third layer having the first material composition and a thickness less than the second layer portion, the third layer portion having a porosity less than the second layer portion, the regenerator configured to be positioned within the thermal regenerative machine such that the third layer portion is nearer the second temperature area than the first and second layer portions. 
   Other aspects include the first layer portions and the third layer portions being constructed from a twice sintered sheets of random fiber material and the second layer portion being constructed from a once sintered sheets of random fiber material. Other aspects include the second layer portion being configured to shed particles sized with respect to the second porosity of the second layer portion that they pass through the second layer portion toward the first layer portion, and the first porosity of the first layer portion being sufficiently small to prevent at least a majority of the shed small particles from passing through the first layer portion. 
   Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       FIG. 1  is a schematic of a conventional thermal regenerative machine. 
       FIG. 2  is a schematic of an exemplary engine implementation of a thermal regenerative machine. 
       FIG. 3  is a schematic of an exemplary cooler implementation of a thermal regenerative machine. 
       FIG. 4  is a schematic of an exemplary pulse tube cooler implementation of a thermal regenerative machine. 
       FIG. 5  is an isometric diagram of a reduced shedding regenerator  FIG. 6  is a cross-sectional schematic of the reduce shedding regenerator of  FIG. 2 . 
       FIG. 7  is an isometric diagram of a stack of un-sintered inner sheets sandwiched between a first sintered end sheet and a second sintered end sheet. 
       FIG. 8  is an isometric diagram of a block of continuously porous material having a core layer of once sintered material sandwiched between a first layer and a second layer of twice sintered material. 
       FIG. 9  is an isometric diagram of a post-machined block including the core, the first layer and the second layer. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As will be discussed in greater detail, a reduced shedding regenerator and method are disclosed herein with regenerator surfaces to minimize shedding of particles from the regenerator thereby alleviating a source of potential damage and malfunction of a thermal regenerative machine using the regenerator. 
   A simplified view of an exemplary conventional thermal regenerative machine  10  using a Stirling cycle module  12  and a power module  14  is shown in  FIG. 1 . In the implementation depicted, the Stirling cycle module  12  has a displacer  16  that is moved by pressure differences, which also cause working fluid to move between a first temperature area  18  and a second temperature area  20  by passing through a first passageway  22  coupled to the first temperature area  18  and a second passageway  24  coupled to the second temperature area  20 . Typically a thermal source (not shown) is positioned adjacent the first passageway  22  and a thermal sink (not shown) is positioned adjacent the second passageway  24 . 
   An exemplary conventional regenerator  26  is annular and has a first end surface  28 , a second end surface  30 , an interior surface  32 , and an exterior surface  34 . The regenerator  26  is positioned between the first passageway  22  and the second passageway  24  such that the first end surface  28  of the regenerator is adjacent the first passageway  22  and the second end surface  30  of the regenerator is adjacent the second passageway  24 . Other conventional implementations of thermal regenerative machines utilize regenerators of other shapes that may not have hollow annular cores such as having solid cylindrical or other shapes that depend upon the configuration of the particular Stirling cycle module involved. 
   When the first temperature area  18  is significantly higher in temperature than the second temperature area  20 , the thermal source inputs heat H in  to the working fluid in the first passageway  22  and the thermal sink takes heat H out  from the working fluid in the second passageway  24 . The regenerator  26  receives heat from the working fluid as the working fluid passes from the first passageway  22  to the second passageway  24 . The regenerator  26  returns some of this received heat back to the working fluid as the working fluid passes back from the second passageway to the first passageway. 
   The second temperature area  20  is in fluid communication with a power piston  36 , which is part of the power module  14 . The power piston  36  of the power module  14  is connected to a conventional linear electrodynamic system  38  through a shaft  40  coupled to a mover  42 . The conventional linear electrodynamic system  38  further includes a stator  44  and an electrical line  46  to furnish electrical power when the thermal regenerative machine  10  is used as an electrical generator and to receive electric power when the electrothermal system is used as a cooler. Other thermal regenerative machines can have other electromotive configurations besides a moving iron linear alternator or motor such as those utilizing moving magnets. 
   As described, the regenerator  26  serves to receive heat from the working fluid, retain the heat, and then return the retained heat back to the working fluid. Conventional random fiber mesh material has been conventionally found to be effective for these functions of the regenerator  26 . Other materials have been used including wire screens and/or woven screens, porous materials, and those using short fibers, metals, plastics, powdered metals, and so forth. A typical method of conventional construction of the regenerator  26  involves sintering a compressed stack of loosely woven random fiber mesh sheets to form a porous unit of material commonly referred to as a brick. Other approaches exist such as those using pre-sintered sheets to form porous brick units. The brick is then machined to form the proper shape for the regenerator  26 . In other implementations stacked screens are used or short fiber is poured into a mold to form a sintered ring. Unfortunately, the regenerator  26  conventionally formed from loose pre-sintered sheets of random fiber mesh material or the many other conventional means has a proclivity to shed small particles that can then migrate into seals, voids, and other areas of the Stirling cycle module  12  with potentially detrimental consequences. 
   As shown in  FIGS. 2–4 , the regenerator  26  is used in many different applications such as with the regenerative thermal machine  10  being used as an engine  50  to produce electrical power ( FIG. 2 ), as a cooler  52  ( FIG. 3 ) and as a special form of cooler called a pulse tube cooler  54  ( FIG. 4 ) in which the displacer  16  is not present but a buffer chamber  56  is used according to conventional practice. As depicted, the regenerator  26  is typically positioned, in terms of working fluid flow, between a heat acceptor heat exchanger  58  that receives heat from a heat source and a heat rejecter heat exchanger  60  that outputs heat to a heat sink. The power module  14  of the engine  50  is typically an alternator  62 . The power module  14  of the cooler  52  and pulse tube cooler  54  is typically a motor  64 . 
   To reduce this shedding problem, one implementation of a reduced shedding regenerator, shown in  FIG. 5  as an annular reduced shedding regenerator  100 , is formed as having a first end surface  102 , a second end surface  104 , an exterior surface  106 , and an interior surface  108 . The regenerator  100  may be used in place of the conventional regenerator  26  shown in  FIG. 1 . Generally, the regenerator  100  is constructed so that a first end layer  110 , better shown in  FIG. 6 , includes the first end surface  102  and extends longitudinally a relatively small amount toward the second end surface  104 . A second end layer  112  includes the second end surface  104  and extends longitudinally a relatively small amount toward the first end surface  102 . 
   The first layer  110  and the second layer  112  of the regenerator have a porosity less than the conventional regenerator  26  to substantially reduce the number of particles that escape from within the regenerator  100  out the first end surface  102  and the second end surface  104 . On the other hand, since working fluid must flow through the first layer  110  and the second layer  112 , their porosities are sufficiently large not to detrimentally limit the flow of the working fluid so that performance of the regenerator is not adversely impacted. 
   The regenerator  100  further has an exterior wall or layer  114  that extends radially inward a relatively small amount from the exterior surface  106  toward the interior surface  108  and an interior wall or layer  116  that extends radially outward from the interior surface toward the exterior surface. In some implementations discussed below, the exterior layer  114  and the interior layer  116  are impermeable. In other implementations, the exterior layer  114  and the interior layer  116  have at least reduced porosities similar to the first end layer  110  and the second end layer  112 . Consequently, with both the impermeable cases and the reduced porosity cases of the exterior layer  114  and the interior layer  116 , particles are substantially prevented from exiting the regenerator  100  through the exterior surface  106  and the interior surface  108 . 
   Sandwiched between the first end layer  110  and the second end layer  112  in an axial or longitudinal direction, z, and between the exterior layer  114  and the interior layer  116  in a radial direction, r, is a core  118  having an exterior surface  118 ′ and an interior surface  118 ″, also shown in  FIG. 6 . The core  118  is made of continuous material having a porosity higher than the first end layer  110 , the second end layer  112 , the exterior layer  114  and the interior layer  116 . The majority of flow of the working fluid occurs through the core  118 . In order to encourage flow of the working fluid through the regenerator  100 , the first end layer  110 , the second end layer  112 , the exterior layer  114 , and the interior layer  116  are relatively thin to accommodate their lower porosities compared to the core  118  with its higher porosity. 
   In an exemplary implementation, the first layer  110  and the second layer  112  each originate from a sheet of loose random fiber mesh that is doubly sintered. The core  118  originates from a stack of sheets of loosely woven random fiber mesh singly sintered rather than doubly sintered to maintain a higher porosity for the core compared to the porosities of the first layer  110  and the second layer  112 . 
   One method of constructing the regenerator  100  will now be described in greater detail. The regenerator  100  is constructed from a stack of sheets  120 , shown in  FIG. 7 , having a plurality of un-sintered inner sheets  122  of loosely woven random fiber mesh sandwiched between a first sintered end sheet  124  and a second sintered end sheet  126  as a combination. The first sintered end sheet  124  and the second sintered end sheet  126  are each formed from a sheet of the loosely woven random fiber mesh that has been already sintered. Both the first sintered end sheet  124  and the second sintered end sheet  126  are sintered to reduce their porosities before they are added to the un-sintered inner sheets  122  to complete the stack of sheets  120 . 
   The stack of sheets  120  having the un-sintered inner sheets  122  sandwiched between the first sintered end sheet  124  and the second sintered end sheet  126  is then sintered to form a block  128  of continuously porous material having a core layer  130  of once sintered material sandwiched between a first layer  132  and a second layer  134  of twice sintered material, as shown in  FIG. 8 . Since the first layer  132  and the second layer  134  are sintered twice, whereas the core layer  130  is sintered once, the first and second layers of the block  128  have a lower porosity than the core layer of the block. The porosity of the first layer  132  and the second layer  134  is sufficiently low to substantially retain particles inside the core layer  130  that would otherwise tend to migrate from the core layer during assembly or operation. In contrast, the porosity of the core layer  130  provides sufficiently high porosity to allow for proper flow of the working fluid in the regenerator  100 . 
   The block  128  is then machined using conventionally known techniques to a post-machined block  140  including the core  118 , the first end layer  110  and the second end layer  112 , as shown in  FIG. 9 . In some first implementations, additional material is amended to the post-machined block  140  to provide the exterior layer  114  and the interior layer  116  of the regenerator  100 . In these first implementations, the block  128  is machined to accommodate subsequent material amendment without exceeding dimensional requirements of the regenerator  100 . 
   Examples of material amendment include, referring to  FIG. 6 , sealing the exterior surface  118 ′ and the interior surface  118 ″ of the core  118  with a braze material. Braze foil can be bonded, for instance, by spot welding, on the exterior surface  118 ′ of the core  118  to serve as the exterior layer  114  and on the interior surface  118 ″ of the core to serve as the interior layer  116 , and then vacuum brazed to seal the exterior and interior surfaces. 
   Again referring to  FIG. 6 , another example of material amendment involves brazening a first metal ring onto the exterior surface  118 ′ to seal the exterior surface and a second metal ring onto the interior surface  118 ″ to seal the interior surface. The first and second metal rings are then machined to produce thin metal walls as the exterior wall  114  and the interior wall  116 , respectively, sealing the exterior surface  118 ′ of the core  118  and the interior surface  118 ″ of the core, respectively. 
   In another second implementation, after the block  128  is machined into the post-machined block  140 , the exterior surface  118 ′ of the core  118  and the interior surface  118 ″ of the core are treated to seal these surfaces to decrease their porosity and thereby reduce particle shedding. In this implementation, no additional material is amended to the post-machined block  140 , so the post-machined block is near dimensional requirements of the regenerator  100  with an accounting for slight dimensional change due to surface treatment. 
   In yet another implementation, no additional material is added or no treatment of the post-machined block  140  is performed after the block is machined. Accordingly, in this implementation, the post-machined block  140  is machined to dimensional specifications of the regenerator  100 . The machining forms the exterior wall  114  and the interior wall  116  from those portions of the core  118  near the exterior surface  118 ′ and the interior surface  118 ″, respectively, thereby sealing the exterior and interior surfaces of the core. 
   In another implementation, the block  128  is cut by laser to form the post-machined block  140 , which has the dimensional requirements of the regenerator  100 . The laser cutting melts the core  118  at the exterior surface  118 ′ and the interior surface  118 ″ to form the exterior wall  114  and the interior wall  116 , respectively, thereby sealing the exterior and interior surfaces of the core. Another implementation uses a laser and cover gas to locally melt a sprayed braze alloy or a braze foil wrap to form the exterior wall  114  and the interior wall  116  or other layers or walls. 
   In another implementation the first layer  110  and the second layer  112  are not used, but the regenerator  100  still has the exterior wall  114 , the interior wall  116 , and the core  118 . In another implementation the first layer  110  and the second layer  112  are used and the exterior wall  114  and the interior wall  116  are not used. Other implementations are used for other configurations of regenerators where the regenerators are solid without any annulus. The various procedures described above are used to form wall and/or layers for one or more surfaces of these types of regenerators to also reduce shedding. 
   Implementations described herein process end surface portions (such as for the first layer  110  and the second layer  112 ) of the porous material other material components need to be handled for these end surfaces. For other surfaces of the porous material can be either processed or in some implementations can be covered with other material to possibly provide additional resistance to shedding. Whether processing or additional materials are used for these other surfaces, one goal is to have relatively thin barriers. 
   Thin barriers on the regenerator are desirable since the barriers provide a conduction path between hot and cold temperature regions, which causes a parasitic heat loss. Stirling cycle efficiency can be better maintained if this parasitic heat loss can be minimized by keeping barriers thin. Also, thin barriers help to maintain Stirling cycle efficiencies in another way by allowing for greater volume available to the Stirling cycle rather than being taken up by thicker barriers. In contrast to these implementations, conventional approaches seek to cover only end portions with screening material. 
   Table 1 lists some of the methods used by implementations discussed herein to provide barriers for regenerator surfaces such as those for the external wall  114  and the interior wall  116  and other than the end surfaces such as those for the first layer  110  and for the second layer  112 . Also shown are various barrier thickness ranges associated with these implementations. These methods work with regenerators made from various materials including sintered random fiber mesh, sintered woven mesh, rolled mesh and coiled sheet. Additionally, the methods work with regenerators fabricated with end caps of woven screens having a mesh of lower porosity than the regenerator. Other material also include powered metals and plastics. As shown by Table 1 all the thicknesses involved are less than 0.020 inches. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Method 
               Thickness range (inches) 
             
             
                 
                 
             
           
          
             
                 
               Sleeve 
               .005–.015 
             
             
                 
               Laser 
               .002–.005 
             
             
                 
               “Slow” EDM pass 
               .001–.005 
             
             
                 
               Braze foil 
               .001–.002 
             
             
                 
               Braze paste 
               .002–.010 
             
             
                 
                 
             
          
         
       
     
   
   The sleeve method refers to brazing or sintering a solid metal sleeve to regenerator surfaces that are other than end surfaces such as described above to form the exterior wall  114  and the interior wall  116  for the annular implementation. The metal sleeve can either be attached at finish dimensions or machined later. Machining the sleeve at a later operation could result in a thinner sleeve. The metal sleeve operation ensures a solid sleeve, however, installation may be complicated, additional one or more parts may require fabrication and a final machining operation may be required after the sleeves are brazed. It is also possible that sleeves being made of other materials could be so fastened and further processed according to technologies adapted to the material such as a ceramic. 
   The laser method refers to regenerator surfaces other than end surfaces are melted using a laser to form a solid surface. The coverage and depth of the melted surface region can be controlled by varying the speed and power of the laser. This method can be time intensive may not result in a completely impervious wall. 
   The slow EDM pass method refers to using electric discharge machining (EDM) to perform a slow pass with an EDM machine to form barriers on the surfaces of the regenerator. An advantage is that the method merely requires a change to operational parameters of the machine that also manufactures the core portions of the regenerators in addition to forming the protective barriers. As with the laser, the EDM process does not guarantee a completely impervious wall. 
   The braze foil and paste method refers to either paste or foil being brazed on to regenerator surfaces other than end surfaces to form an impervious surface. Foil is available in a preset thickness, which can result in a uniform barrier thickness. Application of the paste can be more problematic than the foil to obtain a uniform thickness. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.