Abstract:
A heat transfer head ( 10 ) for a stirling engine is disclosed. The heat transfer head ( 10 ) comprises a plurality of external circumferential fins ( 30 ) projecting out from the heat transfer head ( 10 ). At least two of the adjacent fins ( 30 ) are arranged to be substantially parallel and such that some of the radiant heat received by one of the fins is reflected onto the other fin. A second set of fins ( 40 ) is provided above the circumferential fins. The second set of fins are arranged to enable combustion gases from a burner to pass upwards therebetween to enable heat to be absorbed from the passing combustion gases.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present patent application claims priority to International Application No. PCT/GB01/03637 filed Aug. 14,2001, and Great Britain Patent Application No. 0020012.1, filed Aug. 15, 2000, each of which is incorporated by reference as if fully set forth herein. 
    
    
     FIELD OF THE INVENTION 
     The present application relates generally to a Stirling engine. More specifically, the invention relates to a heat transfer head for use with a Stirling engine. 
     BACKGROUND OF THE INVENTION 
     A Stirling engine is an externally heated reciprocating engine. It has a cylinder within which oscillation of a piston or pistons serves to move a working fluid between hot and cold heat exchangers and provide power output. 
     Heat is generally supplied to one end of the cylinder of the Stirling engine in order to heat the gas inside and drive the working piston. The end of the Stirling engine which is heated is called a heat transfer head and is generally surrounded by an annular heater or burner, which supplies heat to the heat transfer head. The cylinder is generally arranged vertically, with a burner surrounding it to supply heat so that hot exhaust gases from the burner can escape upwards. In order to enhance the transfer of heat to the cylinder of the Stirling engine to increase its efficiency the heat transfer head can be provided with fins to increase-its surface area. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to enhance the transfer of heat to the heat transfer head. 
     According to the present invention there is provided a heat transfer head for a Stirling engine having an axis of reciprocation, the head comprising a plurality of external fins providing a first set with each fin projecting out from the heat transfer head and having a length, a first side and a second side and at least two of the adjacent fins being arranged such that at least a portion of their lengths are substantially parallel and arranged such that at least a portion of any radiant heat received on a side of one of the two adjacent fins is reflected onto a side of the other fin, wherein a second set of fins is provided above the first set, each of the second set of fins having a length, a first side and a second side and the length of each of the second plurality of fins extending in a plane, in use, extending in the direction of the axis of the heat transfer head. 
     Heat is absorbed by the first set of fins at each reflection, so that by arranging the fins such that radiant heat is reflected between two substantially parallel adjacent fins, more of the heat energy is absorbed due to the multiple reflections providing a more efficient heat transfer. The arrangement of the second set of fins enables combustion gases from the burner to pass upwards therebetween to enable heat to be absorbed from the combustion gases. 
     At least a portion of the radiant heat reflected onto the side of the other substantially parallel fin is preferably reflected back to the one fin to further enhance the heat transfer. 
     The neighbouring sides of two adjacent fins may diverge as they extend away from the heat transfer head to provide an overall saw tooth cross-section with each fin having a substantially triangular or truncated triangular cross-section. This reduces radiant heat transfer from the fin tips back to the burner, external walls and hot gases. 
     The length of the fins may extend around the periphery or circumference of the Stirling engine which increases the physical strength of the heat transfer head. 
     At least some of the external fins which are arranged to reflect radiant heat between each other are preferably arranged to be positioned substantially opposite a source of radiant heat such as a heater or a burner. 
     In order to improve the heat transfer, each of the second set of fins is attached, in use, to a cylindrical part of the Stirling engine and has an extended portion extending above the cylindrical part but not being directly attached to the Stirling engine. In order to improve the stability of this and other arrangements, the second set of fins are preferably connected by a circumferential ring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of heat transfer heads illustrating the present invention will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a heat transfer head; 
         FIG. 2  is across-sectional side view of the fins of the heat transfer head shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional side view of an alternative set of fins for the heat transfer head; and 
         FIG. 4  is across-sectional side view of a second alternative set of fins for the heat transfer head. 
     
    
    
     DETAILED DESCRIPTION 
     The heat transfer head  10  illustrated in  FIG. 1  forms the top of a Stirling engine. The head  10  is in the form of a cylinder  11  with a dome  12  at one end. An annular burner  20  is shown schematically. The. burner  20  surrounds the heat transfer head  10  and is arranged to supply it with heat to make the Stirling engine operate. The burner  20  in this example is powered by natural gas. 
     The heat transfer head  10  is provided with a plurality of elongate first fins  30  the length of each of which extends circumferentially around the cylindrical portion of the heat transfer head  10 . The aspect ratio of the first fins  30  is such that the gap  4  between adjacent fin tips is small compared to the fin height  5 . The heat transfer head  10  is also provided with a plurality of second fins  40 , the length of each of which extends longitudinally in the direction of the axis of the cylindrical portion  11  of the heat transfer head. The second fins  40  are arranged in radial planes around the heat transfer head  10 . 
     As shown in  FIG. 2  each fin  30  has a first under side  31  and a second upper side  32 . The sides  31 ,  32  of each fin are substantially flat and converge towards each other as they extend away from the heat transfer head  10 . The cross-section of the tip  33  of each fin  30  where the two sides  31 ,  32  converge is shown in  FIG. 5  as being curved but could be a point or any other suitable cross-section. The cross-section of the area  34  where the bases of two adjacent fins  30  are joined to the heat transfer head  10  is also shown in  FIG. 2  as being curved but it could be any suitable cross-section. Each fin  30  in this example has a height  5  of about 25 mm from the wall of the heat transfer head  10  to its tip  33  and the gap  4  between the tips  33  of adjacent fins  30  is about 5 mm. The height  5  of each fin is preferably more than twice the distance between tips  33  of adjacent fins  30  to promote reflection of radiant heat on the fins. However, the height  5  of each fin may be three, four or five times the gap  4  between tips  33  of adjacent fins  30 . 
     A source of radiant heat, in this case a burner  20 , is arranged opposite to the plurality of fins  30  such that radiant heat is directed into the spaces between adjacent fins  30 . As can be seen in  FIG. 2 , radiant heat impinging on one of the sides  31 ,  32  of a fin  30  is reflected to the neighbouring side of the adjacent fin  32 ,  31 . At each reflection heat is absorbed by the fin  30  of the heat transfer head  10  and used to operate the Stirling engine. The triangular structure of each fin  30  producing an overall saw tooth shape may produce multiple reflections of radiant heat between each pair of fins  30  with each reflection enabling the heat transfer head  10  to absorb more heat. 
     When radiant heat reaches the point  34  at which two adjacent fins  30  meet, the radiant heat is reflected away from the heat transfer head  10  and may encounter more reflections against the fins  30  on its passage away from the heat transfer head  10  enabling the heat transfer head  10  to absorb yet more heat. 
     Above the first fins  30 ,  FIG. 2  shows the cross-section of one of the second fins  40  in a plane extending in the direction of the axis of the cylindrical portion of the heat transfer head  10 . Hot combustion gases from the burner  20  pass upwards between adjacent second fins  40  and much of the heat from the combustion gases is absorbed by the fins  40 . 
     Fins  40  are arranged to maintain high gas velocity and have a high convective heat transfer coefficient. The fins  40  preferably have a rectangular cross-section to increase fin efficiency. The heat transfer head  10  is arranged relative to the heat source  20  such that radiant heat is directed from the heat source into the spaces between adjacent fins  30 . The second fins are preferably arranged above the heat source  20  to absorb heat from rising combustion gases. In this arrangement a greater proportion of heat from the heat source  20  is absorbed by the heat transfer head  10 , increasing efficiency. 
       FIG. 3  shows an alternative arrangement of fins for a heat transfer head  10 . The fins shown in  FIG. 3  are identical to those shown in  FIG. 2  except that channels have been made transversely into the sides of fins  30  at points spaced circumferentially around the heat transfer head  10 . The channels extend in the direction of the axis of the cylindrical portion of the heat transfer head  10 . The channels extend further into the sides of the fins  30  nearer to the top of the heat transfer head  10  to accommodate an increasing flow of combustion gases. The channels may be cut in the fins  30  using a circular saw. 
     The fins may be made from any suitable heat conducting material such as metal, usually steel. The heat conducting head  10  and fins  30 ,  40  may be formed from the same integral piece or separate pieces of material brazed to the cylindrical part of the head to enhance heat conduction from the fins  30 ,  40  to the heat conducting head  10 . 
     Additional circumferential slots may be used in the fin section  40  for stress relief purposes depending upon cylinder design and operating pressures and temperatures. 
       FIG. 4  shows an alternative arrangement of fins for a heat transfer head  10 . This arrangement is broadly the same as that shown in FIG.  2 . In addition, this arrangement may incorporate channels as shown in FIG.  3 . In this arrangement, the fins  40  have an extended portion  40 A extending above the cylindrical portion of the heat transfer head. However, the fins  40  are only attached to the cylindrical portion of the head and there is a clearance between the dome  12  and the extended portion  40 A of the fins  40 . 
     A ring  41  extends around the outer peripheral edge of the upper surfaces of the extended portion  40 A thereby connecting the fins  40  together. A second circumferential ring  42  of triangular cross-section extends around the lower inner part of the fins  40 . This provides a rigid structure allowing the fins to be manufactured and installed as a single component. The extended fins allow a higher level of heat transfer from the burner gases with all of this conduction occurring through the lower part of the fin which is attached to the head  10 .