Abstract:
Primary surface recuperators generally undergo severe thermal and pressure cycles. Thermal cycling tends to cause the primary surface recuperator to expand along a central axis. However ducting connected with the primary surface recuperator tends to limit its expansion. Constructing bars in cells of the primary surface recuperator from the same material as the ducting tends to reduce thermal stresses that may otherwise result from difference in thermal expansion.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to a recuperator and more particularly to a cell of the recuperator.  
         BACKGROUND  
         [0002]    Many gas turbine engines use a heat exchanger or recuperator to increase the operating efficiency of the engine by extracting heat from the exhaust gas and preheating combustion air. Typically, a recuperator for a gas turbine engine must be capable of operating at temperatures of between about 500 C. and 800 C. and internal pressures of between approximately 140 kPa and 1400 kPa. During operation, the recuperator experiences repeated cycles of starting and stopping of the gas turbine engine.  
           [0003]    An example of such a recuperator is disclosed in U.S. Pat. No. 5,060,721 issued to Charles T. Darragh on Oct. 29, 1991. Such recuperators include a core which is commonly constructed of a plurality of relatively thin flat sheets having an angled or corrugated spacer fixedly attached therebetween. The sheets are joined into cells, sealed and form passages between the sheets. These cells are stacked or rolled and form alternate air (recipient) cells and hot exhaust (donor) cells.  
           [0004]    During operation, hot exhaust gas expands through a turbine turning a shaft connected with an air compressor. Compressed discharged air from the compressor passes through the air cell while hot exhaust gas flows through the hot exhaust cells. The exhaust gas heats the sheets and the spacers of the hot exhaust cells. Through conduction, heat transfers to the sheets and spacers of the air cells and ultimately the compressed air.  
           [0005]    U.S. Pat. No. 5,918,368 issued to Ervin et al. on Jul. 6, 1999 improves reliability of the recuperator by making each air cell as an individual unit. Each air cell is made of a pair of primary sheets separated by a plurality of bars and a pair of guide strips. Making each cell as an individual unit improves reliability of the air cells. The hot exhaust cells are formed by connecting two air sells separated by a pair of gas guide strips. Generally, a plurality of cells are attached to form a recuperator core.  
           [0006]    Severe environments in gas turbine engines increase stresses in connections between various components. As mentioned above, operating the gas turbine engine increases both temperatures and pressures in both the recuperator core and ducting causing both to expand. Further, these pressures and temperatures are cyclic and may lead to increased loading especially at connections where components have different thermal characteristics such as thermal expansion.  
           [0007]    The present invention is directed to overcoming one or more of the problems as set forth above.  
         SUMMARY OF THE INVENTION  
         [0008]    In one aspect of the present invention a cell for use with a recuperator has a first sheet and a second sheet having generally equivalent dimensions. A bar attaches between the first sheet and the second sheet. The bar is made of a second material having a coefficient of thermal expansion generally equivalent with a duct. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a partially sectioned side view of a gas turbine engine including a primary surface recuperator embodying the present invention;  
         [0010]    [0010]FIG. 2 is a sectioned view of the recuprator taken along line  2 - 2  looking at a recipient side of a sheet as is embodied in the present invention; and  
         [0011]    [0011]FIG. 3 is a view of a cell assembly. 
     
    
     DETAILED DESCRIPTION  
       [0012]    Referring to FIG. 1, a gas turbine engine  5  is shown having a primary surface recuperator  10  with a plurality of cells  12 . The primary surface recuperator  10  has a first surface  16  and second surface  18 . An air inlet duct  20  and air outlet duct  21  are connected proximate the first surface  16  and second surface  18  respectively. Each of the plurality of cells  12  are separated by a respective gas guide strip  22 .  
         [0013]    Further defining the invention, FIGS. 2 and 3 shows one of the plurality of cells  12  having a first sheet  26 , a second sheet  28 , an air guide  30 , an exhaust guide  32 , a first air bar  31 , a second air bar  33 , a first gas bar  34 , and a second gas bar  35 . The first sheet  26  and second sheet  28  each have generally identical dimensions. In this application, the first sheet  26  and second sheet  28  have central portion  36  generally trapezoidal in shape separating a first wing portion  38  from a second wing portion  40 . The central portion  36  is corrugated while the first wing portion  38  and second wing portion  40  are generally flat with respect to the central portion  36 . The first sheet  26  and second sheet  28  are made from a first material that is a thermally conductive, oxidation resistant material such as stainless steel.  
         [0014]    The first gas bar  34  and second gas bar  35  are attached to the first sheet  26  in some conventional manner such as tack welding or adhesive. The air guide  30  is positioned between the first sheet  26  and second sheet  28  on the first wing portion  38  and second wing portion  40  opposite the gas bars  34 ,  35 . In this application, the air guide  30  has a plurality of passages  42  generally perpendicular to the corrugations forming a Z-flow path. The passages  42  may also form other flow paths such as a C-flow wherein the first wing portion  38  and second wing portion  40  would be mirror images of one another. While the passages  42  in this application are shown as trapezoidal, any conventional shape may be used. The air guide  30  is made from an oxidation resistant material such as stainless steel, ceramic, or other conventional materials that maintain their mechanical strength in the gas turbine engine environment.  
         [0015]    Similarly, the exhaust guide  32  is positioned on the first wing portion  38  and second wing portion  40  opposite the air guide  30 . In this application the exhaust guide  32  has a plurality of passages  43  generally parallel with the corrugations. Like the air guide  30 , the exhaust guide is made from an oxidation resistant material such as stainless, steel, ceramic, or other conventional materials that maintain their mechanical strength in the gas turbine environment.  
         [0016]    The first air bar  31  and second air bar  33  further separate the first sheet  26  from the second sheet  28  by running along a periphery of the first sheet  26  and the second sheet  28 . The bars  31 ,  34  sealingly connects the first sheet  26  and second sheet  28  through some conventional manner such as welding leaving only the passages  42  through the cell  12  between sheets  26  and  28 . In the present embodiment, the first air bar  31  and second air bar  33  are L-shaped. Alternatively, the bars may be of different shapes so long as air may be directed through the sheets  26 ,  28  along the corrugations over some predetermined length. The present invention requires that at least the first air bar  31  adjacent the air outlet duct  21  is made from a material having superior oxidation resistance at high temperatures such as a nickel based alloy and the material has a coefficient of thermal expansion similar to that of the air outlet duct. Optionally, the second air bar  33  may have a duct tab portion  48  preferably near the air outlet duct  20 . For simplicity all of the bars  31 ,  33 ,  34 , and  35  may be made of the same material.  
         [0017]    The air inlet duct  20  is connected to the primary surface recuperator  10  proximate the second surface  18 . The air outlet duct  21  is connected to the primary surface recuperator  10  proximate the first surface  16 . In one embodiment of the present invention, the air outlet duct  21  is welded to the duct tab portion  48 . The air outlet duct  21  is made from a first material having similar thermal characteristics as the duct tab portion  48  such as oxidation resistance, thermal conductivity, and coefficient of thermal expansion. Preferably the air outlet duct  21  is make of a second material such as nickel based alloy. In this application the second material has a lower coefficient of thermal expansion than the first material. Alternately, both the air inlet duct  20  and the air outlet duct  21  may be attached to duct tab portions  48  proximate the inlet portion  14  and outlet portion  15  respectively.  
       INDUSTRIAL APPLICABILITY  
       [0018]    As exhaust gases pass through the primary surface recuperator  10 , separate components begin to expand due to increasing temperatures. Each component in the primary surface recuperator  10  may be constrained by interactions with other components.  
         [0019]    The air outlet duct  21  at a minimum must be made to withstand the extremes of the gas turbine engine environment. Using the nickel based alloy or similar material insures good oxidation resistance in the gas turbine environment. Making the bar  34  of the same material increases compatibility of axial thermal expansion between the primary surface recuperator  10  and the air outlet duct  21 . Increased compatibility of axial thermal expansion reduces thermal strains that may otherwise exist if the primary surface recuperator  10  and air outlet duct  21  expanded at different rates.  
         [0020]    In the cells  22 , only the bar  31  needs to be made of the nickel based alloy or similar material. The air bars  31 , 33  determines axial expansion of the first sheet  26  and second sheet  28 . Allowing the second air bar  33  to be made of the first material having a greater thermal expansion may reduce thermal stresses. The second air bar  33  is exposed to lower temperatures and therefore not as likely to created undue expansion. Allowing the second air bar  33  to expand further at lower temperatures than the first air bar  31  increase likelihood of similar thermal growth. Further, the first sheet  26  and second sheet  28  must have good thermal conductivity. Thermal conductivity may not be a consideration in selecting proper materials for making the air outlet duct  21 .  
         [0021]    Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.