Abstract:
Impingement style heat exchanger through which significant heat transfer improvements can be obtained. The heat exchanger of the present invention operates such that the bulk of heat transfer between the heat source and the working fluid occurs during the portion of the Stirling cycle in which the working fluid impinges upon the pressure vessel surface. Either or both of two heat exchanger configurations may be used. In a first, the impingement heat transfer occurs while the fluid is traveling in the forward direction and towards the expansion space in the vessel. In contrast, and in connection with the second configuration of the heat exchanger of the present invention, the impingement heat transfer occurs while the working fluid is traveling in the backward direction and toward the compression space of the vessel.

Description:
RELATED APPLICATIONS  
       [0001]     This patent application claims priority from Provisional Application Ser. No. 60/484,589, filed on Jul. 1, 2003, the contents of which are hereby incorporated by reference. 
     
    
     FIELD  
       [0002]     The present invention relates to Stirling cycle machines and more particularly to heat exchangers in Stirling cycle machines which are used to transfer heat to and from the working fluid during operation.  
       BACKGROUND  
       [0003]     The Stirling cycle engine was originally conceived during the early portion of the nineteenth century by Robert Stirling. During the middle of the nineteenth century, commercial applications of this hot gas engine were devised to provide rotary power to mills. The Stirling engine was ignored thereafter until the middle of the twentieth century because of the success and popularity of the internal combustion engine. Stirling cycle machines, including engines and refrigerators, are described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference.  
         [0004]     The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: 1) isovolumetric heating of a gas within a cylinder, 2) isothermal expansion of the gas (during which work is performed by driving a piston), 3) isovolumetric cooling and 4) isothermal compression. Additional background regarding aspects of Stirling cycle machines and improvements thereto are discussed in Hargreaves, The Phillips Stirling Engine (Elsevier, Amsterdam, 1991), incorporated herein by reference.  
         [0005]     The high theoretical efficiency of the Stirling engine has attracted considerable interest in recent years. The Stirling engine adds the additional advantages of easy control of combustion emissions, potential use of safer, cheaper, and more readily available fuels and quiet running operation, all of which combine to make the Stirling engine a highly desirable alternative to the internal combustion engine for many applications.  
         [0006]     Despite these advantages, development of the Stirling engine has proceeded at a much slower rate than might otherwise be expected. Some of the more acute problems include the need to seal the working gas at a high pressure within the working space, the requirement for transferring heat at high temperature from the heat source to the working gas through the heater head, and a simple, reliable and inexpensive means for modulating the power as the load changes.  
         [0007]     One design, which is well suited to a variety of applications, is the free-piston Stirling engine. The free-piston Stirling engine uses a displacer that is mechanically independent of the power output member. Its motion and phasing relative to the power output member is accomplished by the state of a balanced dynamic system of springs and masses, rather than a mechanical linkage.  
         [0008]     Stirling engines have been proposed for use in a wide range of applications. Examples include automotive applications, refrigeration systems and applications in outer space. The need to power portable electronics equipment, communications gear, medical devices and other equipment in remote field service presents yet another opportunity, as these applications require power sources that provide both high power and energy density, while also requiring minimal size and weight, low emissions and cost. To date, batteries have been the principal means for supplying portable sources of power. However, the time required for recharging batteries has proven inconvenient for continuous use applications. Moreover, portable batteries are generally limited to power production in the range of several milliwatts to a few watts and thus cannot address the need for significant levels of mobile, lightweight power production.  
         [0009]     Small generators powered by internal combustion engines, whether gasoline- or diesel-fueled have also been used. However, the noise and emission characteristics of such generators have made them wholly unsuitable for a wide range of mobile power systems and unsafe for indoor use. While conventional heat engines powered by high energy density liquid fuels offer advantages with respect to size, thermodynamic scaling and cost considerations have tended to favor their use in larger power plants.  
         [0010]     In order to execute the Stirling cycle, either for the purpose of making power as in an engine embodiment or for the purpose of refrigeration as in a cooler embodiment, the machine must be provided with both an external heat source and an external heat sink. Heat transfer between the external pressure vessel wall of the machine and the working fluid is typically accomplished through the use of internal heat exchangers. Maximum efficiency is obtained when as much heat as possible is transferred to the working fluid rather than to engine components or other heat absorbers.  
         [0011]     Heat transfer to the working fluid is affected by three heat exchanger characteristics: 1) the surface area of the heat exchanger that is in contact with the heat source/sink and the working fluid, 2) the heat transfer coefficient between the working fluid and the surface, and 3) the temperature differential between the heat exchanger surface and the working fluid. Improved heat transfer can be effected by increasing any or all of these three parameters.  
         [0012]     The desire for high thermal efficiencies in Stirling engines dictates high regenerator effectiveness and as a result, the fluid exiting the regenerator and entering the hot end heat exchanger, henceforth referred to as the “heater”, is at or near the temperature of the heater walls. Similarly, the temperature of the working fluid exiting the regenerator and entering the cold end heat exchanger, henceforth referred to as the “cooler”, is at or near the temperature of the cooler walls. Further, since engine pressure variations are typically low, particularly in free-piston Stirling machine embodiments, end state expanded or compressed fluid temperatures tend towards the heater and cooler wall temperatures, respectively. Additionally, working fluid temperatures within conventional heater or cooler heat exchangers vary spatially with the maximum temperature differential between the working fluid and the heat exchanger walls at the respective heat exchanger inlets, and decreasing along the lengths of the heat exchangers until reaching a minimum at the heat exchanger exit where, if the heat exchangers are reasonably well designed, the working fluid has reached very nearly the heat exchanger wall temperature. As a result, the effective temperature differentials between the heater and cooler heat exchangers and working fluid in well designed Stirling cycle machines are, by design small.  
         [0013]     Conventional heat exchanger designs for Stirling cycle machines typically employ slots or holes inside of thick walls, tubes, or alternatively, extended surfaces within channels such as fins of various types. Traditional heat exchanger designs which are used within Stirling cycle machines operate with a low temperature differential between the heat source and the working fluid as discussed above. In order to compensate for the low temperature differentials between the heat source or sink and the working fluid, traditional heat exchanger applications in connection with Stirling cycle machines have suffered from other tradeoffs.  
         [0014]     For example, in some cases heat exchanger structures must be larger than desirable in order to provide the necessary increased surface area for effective heat transfer. This, in turn results in larger engine sizes, less space for other engine components, or both. Additionally, in some cases solutions designed to achieve the necessary heat transfer have required the use of expensive, and sometimes exotic, materials as well as expensive, time-consuming and sometimes less than reliable manufacturing processes and designs.  
         [0015]     Other drawbacks also exist with prior art heat exchanger designs. For example, the requirement for metal-to-metal contact between the pressure vessel and the heat exchanger walls to achieve minimal thermal resistance results in designs that are difficult to fabricate and thus expensive.  
         [0016]     In lieu of providing a large surface area to achieve the required heat transfer, the heat exchanger may be designed to generate high heat transfer coefficients, albeit at the expense of somewhat higher pressure drops in the heat exchanger. However, significant manufacturing, assembly and cost benefits accrue from eliminating the need for extended surface heat exchangers.  
       SUMMARY  
       [0017]     One aspect is to provide a heat exchanger for use with Stirling cycle machines which provides the required heat transfer capability.  
         [0018]     Another aspect is to provide a such a heat exchanger which is cost efficient and functionally reliable.  
         [0019]     Yet another aspect is to provide such a heat exchanger which is easily manufactured and installed.  
         [0020]     A still further aspect is to provide a heat exchanger which is inexpensively manufactured and installed.  
         [0021]     A still further aspect is to provide a heat exchanger that offers the ability to locally vary heat transfer in order to match the spatial heat transfer characteristics of the external heat exchangers to maximize heat transfer and control temperature gradients.  
         [0022]     A still further aspect is to provide a heat exchanger that offers the ability to transfer heat to and from the working fluid at significantly different rates depending upon the direction of flow so as to enhance and/or modify the thermodynamic cycle for improved efficiency and/or output.  
         [0023]     A preferred form of the present invention comprises an impingement style heat exchanger, the use of which can provide significant heat transfer improvements and cost reductions. The heat exchanger of the present invention operates such that the bulk of heat transfer between the heat source and the working fluid occurs during the portion of the Stirling cycle in which the working fluid impinges upon the pressure vessel surface. The impingement heat exchanger may be configured so that the impingement of the working fluid upon the vessel surface occurs in either flow direction. For the heater, the impingement heat exchanger may be configured so that the bulk of the heat transferred to the fluid during the cycle occurs either as the fluid enters or exits the expansion space. Similarly for the cooler, the impingement heat exchangers may be configured so that the bulk of the heat transferred from the fluid during the cycle occurs either as the fluid enters or exits the compression space.  
         [0024]     According to the teachings of the present invention, two different impingement heat exchanger configurations are possible. In a first, referred to herein as the forward flow impingement heat exchanger (“FFIHX”) configuration for both heater and cooler applications, the impingement heat transfer occurs while the fluid is traveling in the forward direction and towards the expansion space in the vessel. In contrast, and in connection with the second configuration of the heat exchanger of the present invention, referred to herein as the backward flow impingement heat exchanger (“BFIHX”) configuration for both heater and cooler applications, the impingement heat transfer occurs while the working fluid is traveling in the backward direction and toward the compression space of the vessel.  
         [0025]     The FFIHX configuration of the heat exchanger of the present invention may be used in connection with either the cooler or the heater. In one heater embodiment, the FFIHX is positioned adjacent to the expansion space of the vessel with a manifold between the heat exchanger and the vessel wall. In this way, working fluid from the regenerator may be heated as it enters the forward flow heat exchanger and passes through the impingement ports so as to impinge upon the heated surface of the vessel wall. When the working fluid is forced from the expansion space toward the compression space (through the regenerator) and cooling is desired, the working fluid passes through the forward flow heat exchanger and impinges on the cooler interior surface of the heat exchanger. With the FFIHX heater configuration, the bulk of the cycle heat transferred to the working fluid is accomplished when the flow is towards the expansion space. The fraction of the total heat that is transferred from the pressure vessel wall to the working fluid in the FFIHX heater during the forward flowing portion of the cycle, as opposed to the backward flowing portion of the cycle, may be tailored through the particular design of the FFIHX.  
         [0026]     The BFIHX configuration of the heat exchanger of the present invention may be used in connection with either the cooler or the heater. In the case of the former, the BFIHX is positioned below the regenerator adjacent to the compression space. As the working fluid, in a relatively heated state, is forced from the compression space toward the expansion space of the engine, it passes through the BFIHX and the fluid impinges on the relatively hotter surface of the heat exchanger which is adjacent to the compression space. When fluid flows in the other direction from the expansion space to the compression space and cooling is desired, the fluid impinges on the pressure vessel wall which is the relatively cooler surface of the cooler mechanism. With the BFIHX cooler configuration the bulk of the cycle heat extracted from the working fluid is accomplished when the flow is towards the compression space. The fraction of the total heat that is transferred from the working fluid to the pressure vessel wall in the BFIHX cooler during the backward flowing portion of the cycle, as opposed to the forward flowing portion of the cycle, may be tailored through the particular design of the BFIHX.  
         [0027]     The BFIHX configuration may also be used in a heating capacity. In this case, this form of the heat exchanger of the present invention is located in the upper portion of the pressure vessel adjacent to the expansion space. When working fluid is flowing from the compression space towards the expansion space and after it has passed through the regenerator, the fluid obtains heat transfer by virtue of the fact that it is flowing between the relatively hotter surface of the pressure vessel wall and the impingement baffle prior to entering the expansion space via the impingement ports. When working fluid is forced from the expansion space towards the compression space the fluid impinges on the pressure vessel wall and the fluid is heated. As the fluid continues to flow toward the compression space it passes through the regenerator where it gives up much of this heat which it can recapture when flow reverses.  
         [0028]     Other embodiments of the present invention are also possible as described in further detail below and as will be understood by one of skill in the art. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     The invention will now be described in more detail with reference to preferred forms of the invention, given only by way of example, and with reference to the accompanying drawings, in which:  
         [0030]      FIG. 1  is a sectional view of a portion of a Stirling engine according to the present invention showing BFIHX heater and BFIHX cooler embodiments;  
         [0031]      FIG. 2  is a detailed sectional view of the BFIHX heater embodiment of a heat exchanger of the present invention;  
         [0032]      FIG. 3  is a detailed sectional view of the BFIHX cooler embodiment of a heat exchanger of the present invention; and  
         [0033]      FIG. 4  is a detailed sectional view of the FFIHX heater embodiment of a heat exchanger of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0034]     Reference is now made to the embodiments illustrated in  FIGS. 1-4  wherein like numerals are used to designate like parts throughout. It will be understood by one of skill in the art that, although the invention is described below in the context of a free piston Stirling engine, its application is not necessarily limited thereto and the invention is defined only by the appended claims. It will further be understood that various other applications of the invention, including, for example, and not by way of limitation, applications in connection with various heat engines and cooler machines whether or not such engines or machines operate based upon the Stirling cycle.  
         [0035]      FIG. 1  is a sectional view of a portion of a free piston Stirling engine (FPSE)  100  designed according to the teachings of the present invention. FPSE  100  includes cylinder  170  within which displacer piston  150  reciprocates axially. The displacer piston  150  defines an expansion chamber  180  of variable volume between displacer piston  150  and cylinder head  140 . The volume of expansion chamber  180  changes during engine operation as displacer piston  150  reciprocates toward and away from cylinder head  140 . Displacer piston  150  rides on a displacer piston rod  160 . Compression chamber  190 , below displacer piston  150  also varies in volume in respect of the movements of displacer piston  150  and power piston(s) (not shown). Compression chamber  190  is generally defined on one end by the bottom of displacer piston  150  and on the other end by the top of the power piston(s) (not shown).  
         [0036]     The operation of FPSE  100  shown in  FIG. 1  generally proceeds as follows. A heat source is applied as shown to the cylinder head of FPSE  100 . The resulting thermal energy is transferred through the pressure vessel wall at cylinder head  140  and is imparted to the working fluid via heat exchanger  130  as discussed in greater detail below. Movement of the working fluid through the heat exchangers and the compression and expansion volumes within the machine to accomplish the Stirling cycle is predominantly driven by the motion of the displacer piston  150 . At one point during the Stirling cycle, as the displacer piston  150  moves upward, the working fluid in expansion chamber  180  is displaced in the “backward” direction from expansion chamber  180 , through the heat exchanger  130 , through regenerator  110 , through heat exchanger  120  and into compression chamber  190 . In the ideal Stirling cycle, the power piston (not shown) is moved to compress the working fluid when the maximum quantity of the working fluid resides in the compression space  190  following the upward motion of displacer piston  150 . Those practiced in the art will recognize that in practical embodiments of the Stirling cycle, the motions of the displacer piston  150  and power piston(s) are neither discontinuous nor completely out of phase with one another.  
         [0037]     As displacer piston  150  moves downward, the working fluid in compression chamber  190  is forced in the “forward” direction through heat exchanger  120 , through regenerator  110 , through heat exchanger  130  and into expansion chamber  180 . During the movement of the working fluid in the forward direction, the working fluid is heated, and as a result of working fluid expansion, mechanical work may be extracted from the cycle as the power piston is pushed by the working fluid in the direction of expansion.  
         [0038]     The particular embodiment shown in  FIG. 1  employs a BFIHX as heat exchanger  120  and a BFIHX as heat exchanger  130 . For purposes of the following discussion, heat exchanger  120  is referred to herein as “cooler” and heat exchanger  130  is referred to herein as “heater” in keeping with their respective functions in this embodiment. The operation of each heat exchanger and each heat exchanger in connection with the operation of FPSE  100  overall is now discussed in detail.  
         [0039]     The BFIHX heater  130  in  FIG. 1  is shown in greater detail in  FIG. 2 . Referring specifically to  FIG. 2 , BFIHX impingement baffle  215  is fastened to regenerator  210  and is supported thereby. Copper plating (not shown) is optionally placed along the inner surface of the pressure vessel wall  240  as required or desired to assist in moderating “hot spots” along the inner surface of cylinder head  240 .  
         [0040]     According to a preferred embodiment of the present invention, impingement baffle  215  is formed with a plurality of apertures  265  to provide jet impingement heat transfer either against the inner surface of the pressure vessel wall  240  or against the surface of the displacer  250 . As discussed in greater detail below, the direction of fluid flow, whether “forward” or “backward”, determines the surface upon which the working fluid impinges.  
         [0041]     In a preferred embodiment of the present invention, BFIHX impingement baffle  215  is formed with a specific number of apertures  265  and a particular aperture spacing and pattern so as to maximize heat transfer through jet impingement. There exist many papers and other sources of information concerning jet impingement techniques such as, for example, “Enhanced Jet Impingement Heat Transfer with Crossflow at Low Reynolds Numbers” by G. Failla, et al. (published in the Journal of Electronics Manufacturing, Vol. 9, No. 2, June 1999) and “Heat Transfer by a Square Array of Round Air Jets Impinging Perpendicular to a Flat Surface Including the Effect of Spent Air” by D. M. Kercher and W. Tabakoff, (published in the Journal of Engineering Power, January 1970) which describe techniques for maximizing jet impingement function. For purposes of the present invention, aperture diameters ranging from 1 to 3 mm and spaced in a relatively uniform fashion over the surface of the impingement baffle with center-to-center spacing ranging from 6 to 10 mm may employed. The impingement baffle  215  may be fabricated from stainless steel, and may be formed using various techniques such as spinning, drawing, deep drawing, hydro-forming or machined from solid stock.  
         [0042]     According to the construction of the novel heat exchanger design of the present invention, the amount of heat transfer to and from the working fluid varies depending upon the direction of the working fluid (i.e. “forward” toward expansion chamber  280  or “backward” away from expansion chamber  280 ). With the BFIHX heater configuration, the bulk of the external heat transferred to the working fluid during the cycle occurs when the flow is in the backward direction. When flowing in the backward direction from the expansion space  280  and through the impingement baffle  215 , the working fluid impinges upon the pressure vessel wall  240 . The high heat transfer rates achievable by impingement combined with the relatively high temperature difference between the expanded working fluid moving from the expansion space  280  and the pressure vessel wall  240  result in the post-impinged working fluid reaching a temperature near that of the pressure vessel wall temperature. The working fluid then proceeds into regenerator  210  where it gives up much of its energy before entering BFIHX cooler  120 . In a well designed machine, this heat is returned to the working fluid by regenerator  210  when the fluid returns to the expansion space  280 .  
         [0043]     When flowing in the forward direction from regenerator  210 , through manifold  235 , through impingement baffle  215 , the working fluid either impinges upon displacer  250 , or the jets dissipate the fluid within the expansion space  280 . In either case, less heat is transferred to the working fluid during this portion of the cycle because channel heat transfer between the working fluid in manifold  235  and the pressure vessel wall  240  is low and the subsequent impingement heat transfer either occurs between the working fluid and the displacer  250 , at low temperature differential, or not at all.  
         [0044]     The BFIHX cooler  120  comprises various functional components. Referring specifically to  FIG. 3 , BFIHX impingement baffle  315  is oriented between cylinder  370  and pressure vessel wall  340  so as to divide BFIHX cooler  120  volume into inner manifold  325  and outer manifold  335 . The inner manifold  325  is open to regenerator  310  and in communication with the outer manifold  335 , which is open to compression space  390 , via the impingement ports  365 .  
         [0045]     With the BFIHX cooler configuration, the bulk of the heat extracted from the working fluid during the cycle occurs when the flow is in the backward direction. When flowing in the backward direction from regenerator  310 , through inner manifold  325  and through impingement baffle  315 , the working fluid impinges upon the pressure vessel wall  340 . The high heat transfer rates achievable by impingement combined with the relatively high temperature difference between the working fluid moving from regenerator  310  and the pressure vessel wall  340  result in the post-impinged working fluid within outer manifold  335  reaching a temperature near that of the pressure vessel wall temperature. The working fluid then proceeds into the compression space  390 .  
         [0046]     As is explained below, the exit temperature from regenerator in the backward flow direction is significantly higher than the pressure vessel wall temperature. Post compression working fluid flowing in the forward direction from compression space  390 , through outer manifold  335  and through impingement baffle  315  impinges upon the wall of inner manifold  325 , which is at a higher temperature than the pressure vessel wall  340 . Less heat is extracted from the working fluid during this portion of the cycle because channel heat transfer between the working fluid in outer manifold  335  and the pressure vessel wall  340  is low and the subsequent impingement heat transfer occurs between the working fluid and the wall of inner manifold  325  at low temperature differential. Subsequently, regenerator  310  is charged with a higher temperature fluid at its cold end than if a conventional heat exchanger were employed. In a preferred embodiment of the BFIHX cooler of the present invention, the gas is delivered to the cold end of regenerator  310  from the compression space  390  at the post compression temperature rather than the pressure vessel wall  340  temperature as it would in the preferred embodiments of conventional heat exchangers.  
         [0047]     Turning now to  FIG. 4 , the FFIHX embodiment of the heat exchanger is next discussed. In this embodiment, as in the BFIHX cooler configuration, two manifolds are used to control heat transfer in both the forward and the backward direction. With the BFIHX heater configuration, the bulk of the external heat transferred to the working fluid during the cycle occurs when the flow is in the forward direction. When flowing in the forward direction from regenerator  410 , through inner manifold  435  and through impingement baffle  415 , the working fluid impinges upon the pressure vessel wall  440 . The high heat transfer rates achievable by impingement combined with the relatively high temperature difference between the compressed working fluid moving from regenerator  410  and the pressure vessel wall  440  result in the post impinged working fluid reaching a temperature near that of the pressure vessel wall temperature. The fluid then travels from outer manifold  425  into the expansion space  480 . The BFIHX heater embodiment depicted in  FIG. 4  also preferably includes a cavity  445  used to isolate volume from interacting in the cycle pressure variations.  
         [0048]     As is explained below, the exit temperature from regenerator  410  in the forward flow direction is significantly below the pressure vessel wall temperature. Post-expansion working fluid flowing in the backward direction from the expansion space  480 , through outer manifold  425 , through impingement baffle  415 , through inner manifold  435 , and entering regenerator  410 , acquires less heat from the source, pressure vessel wall  440 . In the backward flow direction, channel heat transfer within the manifolds is low and impingement heat transfer occurs between the post expanded working fluid and inner manifold wall  435 , at low temperature differential. Subsequently, regenerator  410  is charged with a lower temperature fluid at its hot end than if a conventional heat exchanger, or the BFIHX of the present invention, were employed. Thus, in a preferred embodiment of the FFIHX heater, the working fluid is delivered to the hot end of regenerator  410  from expansion space  490  at the post expansion temperature, rather than the pressure vessel wall  440  temperature as it would in the preferred embodiments of conventional heat exchangers.  
         [0049]     Use of the FFIHX embodiment as a heater provides a greater thermodynamic cycle advantage over use of the BFIHX as a heater. As such, with the FFIHX, the fluid is cooler by the time it reaches regenerator  410  allowing for the use of a smaller and less expensive regenerator. In addition, less of a pressure drop penalty is incurred through the use of the FFIHX than with the BFIHX embodiment.  
         [0050]     It will be understood by one of skill in the art that while the BFIHX is generally less effective in terms of minimizing heat transfer to the working fluid in the backward flow direction when compared to the FFIHX embodiment, the BFIHX embodiment presents the advantages of requiring less metal for construction as well as a much simpler and more reliable fabrication and installation process.  
         [0051]     The disclosed heat exchangers of the present invention provide significant advantages including, for example, significantly reduced cost in terms of construction, additional reliability in operation, and enhanced heat transfer characteristics per unit of size. The heat exchanger of the present invention in its various embodiments may be constructed from relatively inexpensive 300 series stainless steels.  
         [0052]     Additionally, high cost and risky brazing operations involved in the construction and installation of the heat exchangers can be eliminated particularly through the use of the BFIHX embodiment of the present invention. Instead, low cost machining and forming techniques may be used and the FFIHX and BFIHX embodiments of the present invention may be easily inserted into the pressure vessel assembly.  
         [0053]     A novel heat exchanger design in various embodiments and for use both in connection with the heater head and the cooling segment of a Stirling cycle machine has been disclosed herein. As will be understood by one of skill in the art, the invention is not necessarily limited to the particular embodiments disclosed herein and that various other embodiments are possible while still remaining within the scope and spirit of the present invention.