Abstract:
A method of manufacturing a recuperator for a gas turbine engine comprises the steps of stamping a plurality of spaced integral ribs on a metal plate so as to define a plurality of high pressure channels, stamping a plurality of spaced integral ribs on a second metal plate so as to define a plurality of low pressure channels, joining said first and second metal plates to form a cell, and joining a plurality of said cells to one another.

Description:
The present invention relates generally to gas turbine engines and more particularly to a method of manufacturing a recuperator for a gas turbine engine. 
     Gas turbine engine recuperators transfer heat from the relatively hot, low pressure engine exhaust gas to incoming, relatively cold, high pressure combustion air. Use of a recuperator enables the gas turbine engine to approach the fuel economy of a diesel engine as well as to exhibit emission levels below ULEV standards. Moreover, relatively low emission levels can be achieved by recuperator equipped gas turbine engines without the use of a catalytic converter. 
     However, due to manufacturing complexity, known recuperators, particularly metallic plate-and-fin recuperators of the type disclosed in U.S. Pat. No. 3,507,115, are one of the most expensive components of a gas turbine engine. Moreover, known gas turbine engine recuperators, generally employ conventional seals between the hot and cold sections of the recuperator which rapidly degrade in the gas turbine engine environment. 
     SUMMARY OF THE INVENTION 
     The aforesaid problems are solved by the manufacturing method of the present invention. Specifically, the recuperator comprises an annular matrix of cells each of which comprises a low and a high pressure plate, Each plate is made from stainless steel foil that is embossed with an array of undulating ribs. The ribs stop short of the plate edges to facilitate welding to one another along the periphery thereof. The undulating ribs on the plates have different patterns to preclude nesting. 
     Specifically, a high pressure plate is welded to a low pressure plate by laser or resistance welding, to form a modular cell. The weld joins the flat edge portion of the low pressure plate to an offset flange on the high pressure plate that encompasses three sides thereof. The high pressure plate is provided with a short spacer bar on the radially inner edge that is also welded to the low pressure plate. A relatively longer spacer bar is welded to the exterior of the low pressure plate at the radially inner edge thereof for welding to an adjacent cell. Each cell is subsequently formed to the involute curve of the inside diameter of the annular recuperator. 
     A plurality of cells are assembled in an annular array to form the recuperator. After the individual cells are assembled, the protruding ends of the spacer bars on the low pressure plates define ring supports that protrude axially at each end of the recuperator annulus. Rings are then shrunk around the protruding ends of the spacer bars to fix the radially inner edges of the cells to one another but leaving radially outer portions of individual recuperator cells free to radially expand and contract. 
     In accordance with one feature of the invention, only the radially inner edges of the cells are brazed to one another thereby to effect joining and sealing of the cells to one another. Brazing is accomplished by rotating the assembly in a fixture at low speed while a brazing alloy, preferably a nickel or silver manganese braze alloy powder suspended in a suitable liquid carrier, is applied by a spray nozzle to the central cavity of the annular matrix of cells. The nozzle is directed radially outwardly and is moved axially of the annulus to apply a uniform coating of braze alloy to the inside diameter of the annular cell matrix. 
     The assembled cell matrix is then placed in either a vacuum chamber, a chamber filled with dry hydrogen, or a chamber filled with an inert gas such as argon or nitrogen. A radiation-type heating source is then inserted into the annular cell matrix which is rotated to effect uniform heating of the inner surface thereof. Distortion is minimized by heating only the inside bore of the annular cell matrix to a temperature sufficient only to melt the braze alloy and cause it to flow into the metal-to-metal contact surfaces between adjacent cells at the radially inner edges thereof, thereby allowing the bulk of the cell matrix to remain cool and unaffected by the brazing process. 
     After brazing, the radially inner surface of the recuperator is machined to define seating surfaces to facilitate assembly of the recuperator to an engine. It is to be noted that the radially outer surfaces of the recuperator cells are not brazed to one another, allowing relative motion between cells to minimize stress caused by thermal gradients and transients. 
     A stainless steel cylinder is then telescoped over the annular cell matrix so as to retain the cells in the involute shape when pressurized internally. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary sectional elevation of a gas turbine engine having a recuperator manufactured in accordance with the present invention; 
     FIG. 2 is an exploded perspective view of the high and low pressure plates of the recuperator of FIG. 1 prior to being welded to one another to form a cell; 
     FIG. 3 is a cross sectional view of the plates of FIG. 2 assembled to form a cell prior to being formed to the involute curve; 
     FIG. 4 is a view taken in the direction of the arrow  4  of FIG. 3; 
     FIG. 5 is a view taken in the direction of the arrow  5  of FIG. 3; and 
     FIG. 6 is a view taken in the direction of the arrow  6  of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As seen in FIG. 1 of the drawings, a recuperator  10 , in accordance with a preferred constructed embodiment of the present invention, is shown in the environment of a conventional gas turbine engine  12 . The engine  12  comprises a shaft assembly  14 , that extends along a central axis  16  of the engine  12  and connects a forwardly disposed compressor section  18 , to a rearwardly disposed turbine section  20 . An annular combustor  22  is disposed radially outwardly of the shaft assembly  14  between the compressor section  18  and turbine section  20 . The shaft assembly  14  is adapted to be connected to, for example, power generation equipment (not shown) to utilize shaft horsepower generated by the engine  12 . A conventional rotatable fuel slinger  30  is disposed on a cylindrical axially extending slinger sleeve  32  which is part of the rotating shaft assembly  14 . 
     In operation, rotation of the shaft assembly  14  and fuel slinger  30  effects the discharge of fuel radially outwardly into the combustor  22 . 
     Primary combustion air, at ambient temperature, is drawn into the compressor section  18 . The relatively cool high pressure air then flows radially outwardly through a high pressure air channel  34  to the recuperator  10 , wherein the air is heated. The heated high pressure air then flows axially to the combustor  22  and, after combustion, through the turbine  20 . 
     The recuperator  10  comprises an annular nested assembly or matrix of cells  36  each of which comprises a high pressure plate  40  and a low pressure plate  42 . 
     Each low pressure plate  42  is provided with a plurality of undulating relatively closely spaced ribs  44  that are stamped into the surface thereof to a height of, for example, 0.040 inches. Channels defined by the ribs  44 , in combination with the high pressure plate  40  of an adjacent cell  36 , communicate at one end with a hot exhaust gas inlet plenum  48  and at an opposite end with a cool exhaust gas outlet plenum  50  which, in turn, vents to atmosphere. 
     Each high pressure plate  40  has a plurality of ribs  52  that are stamped to a height of, for example, 0.024 inches, in a C-shaped array to define channels which, in conjunction with a low pressure plate  42  of the cell  36 , communicate at one end with a compressed air inlet plenum  80  and at the opposite end with a heated compressed air outlet plenum  82 . The stamped ribs  52  in a “bight” portion of the high pressure plate  40  undulate in generally parallel flow relation to the ribs  44  on the low pressure plate  42  but do not nest therein. 
     The high pressure plate  40  has a flanged edge portion  86  of equal height to the ribs  52  thereon which, in combination with a relatively short spacer bar  88 , provides welding surfaces for joining the plates  40  and  42  as well as to direct high pressure air flow first radially outwardly from the compressed air plenum  80 , thence parallel to the central axis  16  of the engine  12 , thence radially inwardly to the heated compressed air exit plenum  82  and combustor  22  of the engine  12 . The low pressure plate  42  has a spacer bar  90  at the radially inner edge thereof to direct low pressure exhaust gas flow between the cells  36  from the exhaust gas inlet plenum  48  to the exhaust gas exit plenum  50 , thence to atmosphere. 
     After the plates  40  and  42  forming the individual cells  36  are welded to one another, the cells  36  are formed to the involute curve  130  of the radially inner diameter of the annular recuperator matrix, then assembled by stacking in an annular array. After assembly, opposite ends  114  and  116  of the spacer bars  90  on the individual cells  36  protrude axially at each end of the recuperator  10 . Rings  120  and  122  are then shrunk around the protruding ends  114  and  116  of the spacer bars  90  to fix the cell matrix for welding leaving the radially outer portions of the individual cells  36  free to expand and contract radially while the inner edges thereof are fixed to one another. 
     The assembled annular cell matrix is then rotated in a fixture at low speed while braze alloy is applied by a conventional spray nozzle inserted centrally of the annular cell matrix. The nozzle is directed radially outwardly and is moved axially of the cell matrix so as to apply a uniform coating of braze alloy to the inside diameter of the annular cell matrix. The braze alloy comprises, for example, a conventional nickel or silver manganese braze alloy that consists of finely divided powder suspended in a liquid carrier. 
     The next step is to place the annular cell matrix in a heated vacuum chamber or a chamber filled with dry hydrogen or an inert gas such as argon or nitrogen. A conventional radiation-type heat source is inserted into the annular cell matrix which is rotated at low speed to effect uniform heating of the radially inner surface thereof. Rotation ensures uniform heating of the inner surface of the annular cell matrix which is continued just long enough to melt the braze alloy and to cause it to flow into the voids between the metal-to-metal contact surfaces between the spacer bars  90  and radially inner edges of the high pressure plates  40 , respectively of the individual cells  36 . In this manner distortion is minimized by heating only the inside surface of the cell matrix allowing the bulk mass thereof to remain cool and unaffected by the brazing process. 
     After brazing, the radially inner surface of the cell matrix is machined to form a cylindrical seating surface on the engine  12  that precludes bypassing of high and low pressure air to and from the recuperator  10 . It is to be emphasized that the surfaces between individual cells  36  radially outwardly of the spacer bars  88  and  90  thereon, are not brazed to one another, allowing relative motion between the cells  36  to minimize stress caused by thermal gradients and transients. 
     A stainless steel cylinder  132  is then telescoped over the annular matrix of cells  36  to restrain the cells  36  from radial expansion when the recuperator  10  is pressurized internally. 
     While the preferred embodiment of the invention has been disclosed, it should be appreciated that the invention is susceptible of modification without departing from the scope of the following claims.