Abstract:
An annular heat exchanger suitable for a Stirling engine is provided. The heat exchanger has helical fins, and an outer reinforcing sleeve about the fins. The sleeve improves the pressure resisting ability of a thin separating wall between a pressurized fluid and an outside working environment, resulting in a high-pressure heat exchanger with high heat transfer efficiency. In addition, the sleeve and helical fins together define fluid passages for the flow of heating fluid. The heat exchanger according to the invention has the ability to resist high pressures at high temperatures without distortion, has improved heat transfer capability, better reliability, and lower production cost than prior art heat exchangers.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates broadly to heat exchange devices. More particularly, the invention relates to devices that exchange heat energy from one fluid to another where one or both fluids may be pressurized (above or below atmospheric pressure) and do not mix, such as in a Stirling engine.  
           [0003]    2. State of the Art  
           [0004]    Frequently heat energy must be exchanged between two or more fluids which do not mix and which may be flowing or stagnant. The heat energy is transferred from the hotter fluid to a separating wall by convection and/or radiation. Heat energy is conducted through the wall from the hot side to the cold side. Heat energy is then transferred from the separating wall to the cooler fluid by convection and/or radiation. The purpose of the heat exchanger may be to raise the temperature of a relatively cool fluid (as a heater) or to lower the temperature of a relatively hot fluid (as a cooler).  
           [0005]    Except for radiative only heat exchangers, all heat exchangers have large surfaces where heat energy is absorbed or given off by the surface contacted by the fluids. There are basically three types of fluid heat exchangers for Stirling engines defined by the fluid interfacing configurations. Heat exchangers for Stirling engines may be annular, finned, or tubular, or various combinations of these. Annular heat exchangers consist of concentric tubes with the fluids contained in or between them. The tubes may be cylindrical or of other closed cross sections. One tube separates the fluids and provides the surface area and conductive path required for heat exchange. Finned heat exchangers increase the surface area exposed to one or both fluids by providing finned structures on one or both sides of the wall, which effectively increase the surface area of the wall thus improving heat transfer. Tubular heat exchangers contain one fluid within relatively small diameter tubes that are surrounded by the other fluid. Heat is conducted through the tube wall. Various combinations of these three types may also be used in a heat exchanger. For example, fins may be added to the tubes of an annular heat exchanger to increase the contacted surface area.  
           [0006]    Annular (with and without fins) and tubular heat exchangers have been used for Stirling engines. Tubular heat exchangers (with and without fins) have been traditionally used for engines with power outputs greater than 1 kW mechanical. Many small diameter tubes provide large surface area and the small diameters have lower stress at high pressures. Tubular heat exchangers are the most expensive to produce and are susceptible to burnout due to uneven heating and high stresses at the attachment points due to thermal expansion deformation of long tubes.  
           [0007]    Often one or more of the fluids may be pressurized to a relatively high level. In such case, the separating wall must structurally resist the difference in pressure between the fluids. For high heat exchanger efficiency, large fluid contacted surfaces and low thermal resistance through the separating wall are desired. Low thermal resistance is achieved by using a thin separating wall, large contact area, and a material with high thermal conductivity. On the other hand, high structural strength to resist deformation by pressure is achieved by using thick walls, small surface areas, and high strength materials. In general materials with high thermal conductivity do not have high strength and high strength materials have low thermal conductivity. Thus, the desired characteristics of heat exchanger designs assuring high thermal efficiency and high strength conflict.  
           [0008]    With particular reference to Stirling engines, such engines are typically provided with four heat exchangers: a heater, a regenerator, a cooler, and an exhaust/inlet air preheater. A more detailed explanation of the respective functions of the heat exchangers of Stirling engines can be found in G. Walker in “Stirling Engines”, Clarendon Press, 1980, pp. 124-126, 133-144, and 156-159, which is hereby incorporated by reference herein in its entirety. The above described annular, tubular, and finned heat exchangers, as well as combinations thereof, have all been used in various Stirling engines for heaters and coolers. For example, U.S. Pat. No. 4,671,064, which is hereby incorporated by reference herein in its entirety, describes an annular heat exchanger for a Stirling engine. C. M. Hargreaves in “The Philips Stirling Engine”, Elsevier, 1991, pp. 185-187, describes finned heat exchangers (referred to as “concertina” and “partition” heaters) in Stirling engines.  
           [0009]    For maximum efficiency, the Stirling engine working fluid temperature should be as high (as close to the heating fluid temperature) as possible at the heater and as low (as close to the cooling fluid temperature) at the cooler as possible. For maximum power production, the working fluid pressure should be as high as possible. This requires high thermal conductivity of the wall separating the fluids and high strength at the operating temperature. Heating fluid temperature should be as high as the heat exchanger construction material can withstand at the working fluid pressure.  
           [0010]    One manner of increasing the pressure-resisting strength of a pressure vessel is to use “orthogonal grillage” about a separating wall; i.e., providing straight internal fins parallel to the cylinder axis combined with disk-like external fins perpendicular to the axis and integral to the separating wall. The straight and disk-like fins cross each other at right angles. “Orthogonal grillage” is described in more detail in J. F. Harvey in “Theory and Design of Modern Pressure Vessels”, 2 nd  Ed., Van Norstrand Reinhold, 1974, pp. 120-122, which is hereby incorporated by reference herein in its entirety. However, orthogonal grillage has the disadvantage in that it is complicated and difficult to move a heating fluid around the pressure vessel to permit the heat exchange.  
         SUMMARY OF THE INVENTION  
         [0011]    It is therefore an object of the invention to provide a heat exchanger for heating a fluid in a high pressure vessel.  
           [0012]    It is another object of the invention to provide a heat exchanger which has a relatively high structural integrity.  
           [0013]    It is a further object of the invention to provide a heat exchanger through which it is relatively easy to circulate heating fluid.  
           [0014]    It is an additional object of the invention to provide a heat exchanger which has a high heat transfer efficiency.  
           [0015]    It is also an object of the invention to provide a heat exchanger which is relatively light weight.  
           [0016]    It is still another object of the invention to provide a heat exchanger which is relatively inexpensive to manufacture.  
           [0017]    It is yet another object of the invention to provide a heat exchanger for a Stirling engine.  
           [0018]    In accord with these objects, which will be discussed in detail below, an annular heat exchanger having helical fins is provided. According to preferred aspect of the invention, an outer reinforcing sleeve is provided about the helical fins. The sleeve improves the pressure resisting ability of a thin separating wall (e.g., the heater wall of a Stirling engine) resulting in a high-pressure heat exchanger with high heat transfer efficiency. In addition, the sleeve and helical fins together define fluid passages for the flow of a heating fluid.  
           [0019]    The heat exchanger according to the invention has an ability to resist high pressures at high temperatures without distortion, has an improved heat transfer capability, better reliability, and lower production cost than prior art heat exchangers. 
       
    
    
       [0020]    Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a partial cut-away side elevation view of a Stirling engine according to the invention;  
         [0022]    [0022]FIG. 2 enlarged partial cut-away side elevation view of a hot end heat exchanger and heating fluid passages of a Stirling engine according to the invention, revealing heating fluid passages;  
         [0023]    [0023]FIG. 3 is a section view across line  3 - 3  in FIG. 2;  
         [0024]    [0024]FIG. 4 is a section view across line  4 - 4  in FIG. 2; and  
         [0025]    [0025]FIG. 5 an enlarged section through a cylinder wall, and heat wall fins and outer sleeve of the heat exchanger according to the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    Referring now to FIG. 1, a Stirling engine  10  generally includes a pressure vessel  12 , a hot end heat exchanger (heater)  16 , a cold end heat exchanger (cooler)  18 , a regenerator  20 , a piston  22 , a displacer  24 , and a crank assembly  25 . The pressure vessel  12  defines a working space containing a pressurized working fluid (not shown). The heater  16  (described in detail below) adds heat to the working fluid in the pressure vessel (to increase total working fluid pressure in the system). The cooler  18  removes heat from the working fluid (and decreases total working fluid pressure in the system). The regenerator  20  serves as a thermal storage medium and increases the engine efficiency by reducing energy losses as the working fluid is alternately transferred between the hot and cold ends. The heater  16  is preferably integrated with the regenerator  20 , and both are preferably positioned on top of the cooler  18 .  
         [0027]    The working space, mentioned above, is defined as all of the space or internal volume occupied by the working fluid, and includes the fixed internal volumes of the heater  16 , regenerator  20 , and cooler  18  as well as any connecting ducts or passageways. The working space also includes a variable compression space  26  and a variable expansion space  27 . The compression space  26  is the volume contained between the displacer  24  and the piston  22  that varies as the displacer  24  and piston  22  move axially in a cylinder  29  (discussed below) relative to each other. The expansion space  27  is the volume contained between the displacer  24  and a closed hot end of the pressure vessel (end cap  38 , discussed below).  
         [0028]    The axial position of the displacer  24  in the cylinder  29  is always ahead of the position of the piston  22  with respect to time. Oscillating motion of the displacer  24  transfers or displaces working fluid alternately between the compression space  26  and expansion space  27 . Working fluid flow to and from the compression space  26  and expansion space  27  must flow through the heater  16 , regenerator  20  and cooler  18 .  
         [0029]    In general, the working fluid pressure in the total working space is uniform at any instant in time. When working fluid flow is from the regenerator  20 , through the heater  16 , and into the expansion space  27 , working fluid temperature and pressure increase and the piston  22  is forced out by having a higher pressure on the working fluid side than on the opposite side. When working fluid flow is from the regenerator  20 , through the cooler  18 , and into the compression space  26 , working fluid temperature and pressure decrease and the piston  22  returns. Thus, the oscillating motion of the displacer  24  creates an oscillating pressure wave in the working fluid that moves the piston  22  in and out. The piston, acting on crank assembly  25 , moves the displacer  24  to provide the pressure wave and also produces mechanical energy at an output shaft  28 .  
         [0030]    Before explaining the heater  16  of the invention, it is helpful to more fully understand particular elements of the pressure vessel  12  containing the working fluid. Referring to FIGS. 2 through 5, the pressure vessel  12  includes the cylinder  29 , a tubular wall  30  about the cylinder, preferably axial internal fins  32  between the cylinder  29  and the wall  30 , axial flow fluid passages  34  bounded by the cylinder  29 , wall  30 , and internal fins  32  between the cylinder and the wall, a transition cone  36 , and an end cap  38 . At the location of the transition cone  36 , the cylinder  29  includes radial ports  40  which open into the fluid passages  34 , thereby permitting the working fluid to move from the cylinder  29  to the axial flow fluid passages  34 . The pressure vessel also includes a flange  39  which mates with the cooler  18  and provides a sealed annular opening at the bottom of the regenerator  20  for passage of the working fluid between the regenerator and the cooler.  
         [0031]    The function of the heater  16  is to add heat to the pressurized working fluid within the axial fluid passages  34 . The heater  16  is an annular heat exchanger which, according to a first preferred aspect of the invention, has external helical fins  42  integral with the exterior of the wall  30 . The helical fins  42  preferably taper away from wall  30 . An exemplar size for the fins includes a width of 0.125″ at the root  44   a  of the fin (against the wall  30 ), a width of 0.06″ at the tip  42   b,  and a height  42   c  of 0.5″ (FIG. 5), though fins of other sizes may be used. It will be appreciated that because in FIG. 5 the fins are sectioned at an oblique angle, the exemplar preferred relative dimensions of the fins are distorted. A preferred lay angle for the helical fins  42  is one revolution every 3.5 inches about a 3.5 inch diameter wall  30 . The helical fins  42  increase heat transfer across the wall  30  by effectively increasing the surface area of the wall that can be wetted (contacted) by the heating fluid. It will be appreciated that helical fins  42  are longer than either of annular fins or longitudinal fins, and therefore provide a relatively larger surface over which heat transfer between the heating fluid and the working fluid can occur. Longer fins  42  imply longer passages  48  and therefore more time for heat transfer with the heating fluid at any given heating fluid velocity. Furthermore, the helical fins  42  add substantial structural integrity to the heat exchanger.  
         [0032]    According to a second preferred aspect of the invention, an outer tubular reinforcing sleeve  44  is attached to the outer edges of the helical fins  42 . The resulting unified construction of the wall  30 , axial fins  32 , helical fins  42 , and sleeve  44  provides a composite pressure vessel wall with an effective thickness much greater than the wall  30  alone; in effect, providing a wall with an effective thickness approximating the combined material of the sleeve  44 , the helical fins  42 , axial fins  32 , and the wall  30 , without the weight of a solid wall of that thickness. As such, the sleeve  44  greatly improves the pressure resisting ability of the wall  30  resulting in a high-pressure heat exchanger with high heat transfer efficiency.  
         [0033]    The sleeve  44 , transition cone  36 , lower portion of end cap  38 , and wall  30  define a plenum  46  (FIG. 2) which distributes heating fluid to numerous inlets of the relatively long helical fluid passages  48  defined between the sleeve  44 , the helical fins  42 , and the wall  30 . The number of helical fins  42  and passages  48  are optimized according to a particular application, and is based on factors such as fluid nature (liquid, gas, or a combination), fluid velocity, temperature, viscosity, etc. The thermal and structural properties of the wall  30 , helical fins  42 , axial fins  32 , and sleeve  44  determine the optimum dimension of those components. A preferred material for both of the helical fins and sleeve is a high temperature metal or alloy, such as stainless steel.  
         [0034]    The sleeve  44  is preferably permanently bonded to the ends of the helical fins  42  by welding, casting, brazing, or some other permanent attachment process. The wall  30 , axial fins  32 , and helical fins  42  are also preferably a unitary construction. The cylinder  12  is optionally permanently bonded to the end of the axial fins  32  by welding or brazing to increase the pressure resisting strength of the vessel.  
         [0035]    The heater  16  also includes an insulating barrier  54 , an exhaust cylinder  56 , and an insulating wall  58 . The insulating barrier  54  deflects the heating fluid leaving the helical passages  48  at the bottom of the heater and protects the flange  39  and other engine components from heat. The exhaust cylinder  56  forms an exhaust passage  60  through which the heating fluid exhausts after passing through the helical passages  48 . The exhaust cylinder can be insulated or non-insulated. Once heating fluid is exhausted, it can be directed to another location for use in preheating incoming fluid at  64  (FIG. 1) or other purposes needing heated fluid. The insulating wall  58  surrounds the sleeve  44  and insulates the sleeve from the relatively cooler heating fluid in the exhaust passage  60 , thus maintaining a relatively high temperature at the sleeve.  
         [0036]    The heater  16  is less expensive to produce than the tubular heat exchangers of the prior art, has increased surface area over traditional annular heat exchangers of the prior art, and does not have the thermal expansion and uneven heating problems associated with tubular heat exchangers.  
         [0037]    In operation, heated fluid is created (e.g., as combustion gas) at  66  (FIG. 1). The heated fluid enters the Stirling engine  10 , surrounds the cap  38  (thereby heating the cap), and enters the plenum  46  of the heater  16 . Because the net heat flow in the structure composed of the sleeve  44 , helical fins  42 , and the wall  30  is from the sleeve  44  to the axial fins  32 , there is a temperature gradient where the temperature of the sleeve  44  is higher than the temperature of the wall  30 . As a result, there is heat transfer from the sleeve  44  to the wall  30  to heat the working fluid in the axial passages  34  defined by the axial fins  32 .  
         [0038]    The work output and efficiency of a Stirling engine are directly related to the high working fluid pressure and the temperature differential obtained. In view thereof, it will be appreciated that the ability of the heat exchanger  16  to operate under extremely high working fluid pressures (e.g., 150 psi-450 psi or more) and large temperature differentials (e.g., 1000° F.) permit the realization of a high efficiency heat exchanger and enable a relatively high output and particularly efficient engine. The heat exchanger of the invention can be used anywhere a high efficiency heat exchanger operating with high-pressure fluid is needed.  
         [0039]    There have been described and illustrated herein a Stirling engine and particularly a heat exchanger suitable for a Stirling engine. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while a both helical fins and an outer reinforcing sleeve have been disclosed on the heat exchanger, it is believed that each component provides advantage over prior art heat exchanger, and each component may be used alone without the other. As such, the external fins may be radial or axial in shape with a reinforcing sleeve thereabout. Regardless of which shape, it is preferable that the angle between the internal and external fins should be relatively large (e.g., 70°-110°) such that the strengthening advantage of orthogonal grillage is maintained. In addition, if desired, bumps, wall variations and/or inserts can be added to the helical passages or axial passages to induce turbulence in the fluid flows. Also, while a particular heating fluid (combustion gas) has been disclosed, it will be appreciated that other heating fluids, in gas and liquid form, may be used as well. Furthermore, while the axial internal fins are described as defining axial flow passages, it will be appreciated that such fins may be radial or helical in shape other shaped fluid passages, as this may be an advantage in lengthening the working fluid flow path to give more time for heat exchange at higher fluid velocities. In addition, the heating fluid direction may be reversed with flow through the helical fluid passages in the opposite direction. Flow may also be reversing or oscillating, if desired. Moreover, it will appreciated that the heat exchanger can be configured as a Stirling engine cooler. When used as a cooler, the sleeve and helical fins are preferably made from aluminum. Also, while particular materials have been disclosed, it will be appreciated that other suitable materials may be used. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as claimed.