Abstract:
A method and apparatus for varying the effective area of a jet engine exhaust nozzle. The apparatus includes a center body disposed upstream of the nozzle throat and configured to form a recirculation wake extending to and through the nozzle throat. The conditions of fluid flow are selectively variable, for example, by axial movement of a portion of the center body, to vary that portion of the nozzle throat which is effectively blocked by the recirculation wake so as to be unavailable for downstream motive fluid flow.

Description:
This is a continuation-in-part of application U.S. Ser. No. 333,501, filed 22 Dec. 1981, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to jet engines. More particularly, this invention relates to a novel method and apparatus for varying the effective fluid flow area defined at the exhaust nozzle throat of a jet engine. 
     Jet engines provide motive power by accelerating a flow of high-temperature, high-pressure motive fluid through an exhaust nozzle to create a high-velocity fluid jet. The thrust provided by the fluid jet is approximately equal to its mass flow rate multiplied by its velocity. During high-power operation of the jet engine the throat of the exhaust nozzle must define an area sufficiently large to accommodate the mass flow of the fluid jet without undue restriction. However, during intermediate-power operation of the engine, during which the mass flow rate of motive fluid is reduced, the effective area of the exhaust nozzle throat must be reduced in order to accelerate the reduced flow of motive fluid to an adequate velocity. Thus, in order for a jet engine to efficiently provide varying thrust levels, the effective area of the exhaust nozzle throat of the engine must be variable. 
     A conventional method of varying the area of a jet engine exhaust nozzle throat is to provide the core structure of the engine with an axially movable tail piece. The tail piece is telescopically arranged with the remainder of the engine core structure and is movable in and out of the exhaust nozzle throat to vary the effective area of the nozzle throat. U.S. Pat. No. 2,405,723, granted 13 August 1946 to S. Way illustrates a jet engine having a movable tail piece. 
     A jet engine having a movable tail piece has a number of recognized deficiencies. Among these recognized deficiencies is the necessity for the core structure of the engine to extend to and through the exhaust nozzle throat. As a result, when the engine includes a long tail pipe the core structure of the engine must be made undesirably long. Therefore, the weight of the engine may be undesirably increased. 
     SUMMARY OF THE INVENTION 
     In view of the deficiencies of conventional jet engines having variable nozzle throats, it is an object for this invention to provide a method of varying the effective throat area of a jet engine exhaust nozzle by moving a recirculation fluid wake into and out of the nozzle throat. 
     To this end, the invention provides a method of varying the effective area of a jet engine nozzle throat by forming a recirculation fluid wake in the flow of motive fluid upstream of and extending downstream toward the nozzle throat, and moving the fluid wake in and out of the nozzle throat to vary the effective fluid flow area at the nozzle throat. 
     According to a preferred embodiment of the invention, the core structure or center body of the engine includes a truncated tail cone. The tail cone carries an axially movable sleeve member which defines the origin of the recirculation fluid wake. Axial movement of the sleeve member moves the fluid wake in and out of the nozzle throat to vary the effective throat area. 
     Another preferred embodiment of the invention includes a center body having a multitude of variable-angle guide vanes protruding from the aft end thereof into the fluid flow. The angle of incidence of the guide vanes is variable to increase the tangential velocity of the flowing motive fluid. The magnitude of the tangential velocity of the flowing fluid effects the recirculation fluid wake to move the wake in and out of the nozzle throat. 
     Yet another preferred embodiment of the invention includes an annular scroll chamber circumscribing the flow path for the motive fluid. Passages communicate the scroll chamber with the flow path so that pressurized fluid supplied to the scroll chamber communicates tangentially into the flow path. The pressurized fluid is effective to increase the tangential velocity of the flowing motive fluid. Thus, the recirculation fluid wake moves in and out of the nozzle throat in response to the supply of pressurized fluid to the scroll chamber. 
     An advantage of the invention is that the center body of the engine need not extend to the nozzle throat. A change in configuration of the center body, effected either by movable guide vanes or by elongation of the center body, is sufficient to move the recirculation fluid wake in and out of the exhaust nozzle throat. The fluid wake may also be moved relative to the nozzle throat by injecting pressurized fluid tangentially into the flowing motive fluid. The fluid wake may be moved by comparatively small, light-weight actuators. That is, the movement of the sleeve member and of the variable-angle guide vanes may be achieved by the exertion of relatively small forces. For example, relatively small hydraulic or electrically powered actuators are sufficient to move the fluid wake. Similarly, the injecting of pressurized fluid into the flow path requires only the opening of a valve device controlling the flow of pressurized fluid from a source thereof into the scroll chamber. Such a valve device may also be actuated by a relatively small, light-weight actuator. 
     Other objects and advantages of this invention will appear in light of the following detailed description of three preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary diagrammatic longitudinal view, partially in cross section, of a jet engine embodying the invention; 
     FIG. 2 is a view like FIG. 1 and showing the jet engine in an alternative operational configuration; 
     FIG. 3 is a view like FIG. 1, but showing an alternative construction for a jet engine embodying the invention; 
     FIG. 4 is a view like FIGS. 1 and 3 but showing yet another alternative construction for a jet engine embodying the invention; 
     FIG. 5 is a transverse cross-sectional view taken along the line 5--5 of FIG. 4; and 
     FIGS. 6 and 7 are graphs illustrating changes in operating characteristics of jet engines incorporating the invention with changes in the physical parameters of the engines. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show the aft end of a jet engine 10 having an engine housing 12. The engine housing 12 defines a duct 14 which defines a flow path 16 for a flow of motive fluid (represented by arrows F). The motive fluid typically originates with a gas generator (not shown) which may include a compressor (also not shown) and one or more combustors 18. A center body 20 is disposed within the duct 14 and includes a turbine 22 and a truncated tail cone 24. The tail cone 24 is supported concentrically within the duct 14 by a plurality of radially extending struts 26 (only two of which are visible in FIG. 1) which engage the housing 12. Because of the presence of the center body 20 within the duct 14, the flow path 16 is annular upstream of the tail cone 24. The housing 12 and tail cone 24 cooperate downstream of the turbine 22 to define an annular diffuser section 28 for the motive fluid flowing from the turbine. Downstream of the diffuser 28, the housing 12 converges to define a nozzle throat 30 (illustrated by dashed line) having a diameter A forming an appropriate area for the flow of motive fluid. The center body 20 causes a fluid wake 32 to extend downstream from the tail cone 24. The fluid wake 32 terminates at a point 34 upstream of the nozzle throat 30. A relatively small core of turbulence 36 extends downstream from the terminus point 34 of the wake 32. Thus, substantially all of the area of the nozzle throat 30 is available for the flow of motive fluid. 
     In order to vary the flow area of the nozzle throat 30, an annular sleeve member 38 is movably carried by the tail cone 24. The sleeve member 38 is axially movable with respect to a recess 40 defined by the tail cone 24. An extensible actuator 42 is secured to an end wall 44 of the sleeve member and to the tail cone 24, viewing FIG. 2. Consequently, the sleeve member 38 is axially movable between a first retracted position, illustrated in FIG. 1, and an extended position, viewing FIG. 2. 
     When the sleeve member 38 is moved rightwardly from the retracted position to an extended position, the fluid wake 32 and the terminus point 34 also move rightwardly. 
     FIG. 2 illustrates that the fluid wake 32 is movable through the nozzle throat 30. At the nozzle throat, the fluid wake defines a diameter B. Thus, the effective area available for the flow of motive fluid is reduced to a value less than that defined by the diameter A. The reduced effective flow area at the nozzle throat is substantially that of an annulus having an outer diameter A and an inner diameter B. 
     It will be recognized by those skilled in the art to which the present invention pertains that the truncated tail cone 24 originates a classical base wake in fluid F. That is, the wake 32 is a recirculation or reverse flow type of wake which is generally bounded by a mixing region and which includes a core within which the fluid velocity is substantially zero at the outer regions thereof, and has an upstream or negative velocity within the core, as is represented by arrows 33, viewing FIGS. 1 and 2. Moreover, the wake 32 because of its recirculation reverse flow is clearly distinguishable from fluid boundary layer wakes wherein the flow always has a positive downstream velocity, or a zero velocity. Because the wake 32 comprises fluid flowing in the upstream direction (leftwardly viewing FIGS. 1 and 2) extension of wake 32 into the nozzle throat 30 decreases the area thereof which is available for downstream flow of fluid F. 
     FIG. 3 illustrates an alternative construction for a jet engine embodying the invention. In order to obtain reference numerals for use in FIG. 3, features illustrated in FIG. 3 are referenced with the same numerals used in FIGS. 1 and 2 with a prime added. 
     Viewing FIG. 3, it will be seen that a jet engine 10&#39; includes a housing 12&#39;. A duct 14&#39; extends through the housing 12&#39; to define a flow path 16&#39; for a flow of motive fluid (represented by arrows F). Combustors 18&#39; and a center body 20&#39; are disposed within the duct 14&#39; along with a turbine wheel 22&#39;. The center body 20&#39; includes a truncated tail cone 24&#39; supported in the duct 14&#39; by struts 26&#39;. 
     The tail cone 24&#39; cooperates with the housing 12&#39; to define an annular diffuser section 28&#39;. Downstream of the diffuser section 28&#39;, the housing 12&#39; converges to define a nozzle throat 30&#39; having a diameter A. Downstream of the tail cone 24&#39;, the flowing fluid F forms a fluid wake 32&#39; having a terminus point 34&#39;a. 
     A multitude of radially outwardly extending variable-angle guide vanes 44&#39; are carried by the tail cone 24&#39; of the center body 20&#39;. The guide vanes 44&#39; are movable about respective radially extending axes so that their angle of incidence with respect to the flowing motive fluid is selectively variable. An actuator (not shown) within the tail cone 24&#39; is drivingly coupled with the guide vanes 44&#39; to collectively vary their angle of incidence in response to an input signal. Because mechanisms for collectively moving an annular array of vanes is well known in the jet engine art, further explanation of the structure illustrated in FIG. 3 is deemed unnecessary. 
     As is well known in the jet engine art, the motive fluid flowing from the turbine 22&#39; may have a purely axial flow or may have a tangential velocity (swirl) either in the same or opposite direction with respect to turbine rotation. The magnitude of the swirl of the motive fluid influences the fluid wake 32&#39;. The fluid wake 32&#39; elongates with increasing swirl magnitude and shortens with decreasing swirl magnitude. Viewing FIG. 3, the fluid wake 32&#39; has a terminus point 34&#39;a when the guide vanes 44&#39; are positioned to have a zero angle of incidence with respect to the flowing motive fluid. The terminus point 34&#39;a is upstream of the nozzle throat 30&#39; so that substantially all of the throat area is available for fluid flow. When the guide vanes 44&#39; are positioned to impart a swirl to axially flowing motive fluid or to increase the swirl magnitude of swirling motive fluid, the terminus point of the fluid wake moves downstream to a point 34&#39;b, as is shown in dashed lines in FIG. 3. Thus, it will be seen that the fluid wake 32&#39; moves through the nozzle throat 30&#39; in response to an increased swirl of the motive fluid. 
     FIGS. 4 and 5 illustrate yet another alternative construction for a jet engine embodying the invention. In order to obtain reference numerals for use in FIGS. 4 and 5, features illustrated in these Figures are referenced with the same numerals used in FIGS. 1-3 with a double prime added. 
     Viewing FIGS. 4 and 5, it will be seen that a jet engine 10&#34; includes a housing 12&#34; having a duct 14&#34; defining a flow path 16&#34; for a flow of motive fluid (illustrated by arrows F). The engine 10&#34; includes a center body 20&#34; having a truncated tail cone 24&#34; supported by struts 26&#34; and cooperating with the housing 12&#34; to define a diffuser section 28&#34;. The housing 12&#34; defines a circumferentially extending scroll chamber 44&#34;. The scroll chamber 44&#34; communicates with the flow path 16&#34; via an annular passage 46. A multitude of guide vanes 48 are disposed in the annular passage 46. An inlet 50 of the scroll chamber 44&#34; communicates with a source of fluid pressure 52 via a conduit 54. A valve device 56 is disposed in the conduit 54 to control the flow of pressurized fluid into the chamber 44&#34; from the source 52. 
     When the valve 56 is opened to allow pressurized fluid to flow from the source 52, for example, from the compressor discharge area of the engine, into the scroll chamber 44&#34;, the pressurized fluid inherently possesses a tangential velocity with respect to the axis of the duct 14&#34; because of the configuration of the scroll chamber 44&#34;. The pressurized fluid from chamber 44&#34; communicates into the duct 14&#34; via the passage 46 (as represented by arrows g) which in combination with the guide vanes 48 insure that the tangential velocity of the pressurized fluid is employed to best advantage. By admixture, the pressurized fluid g imparts swirl to or increases the magnitude of the swirl of the motive fluid in the duct 14&#34;. Consequently, the fluid wake 32&#34;  may be moved into the nozzle throat 30&#34;. The terminus point of the wake 32&#34; moves between the points 34&#34;a and 34&#34;b in response to the opening and closing of the valve 56. 
     FIG. 6 graphically illustrates computed and actual values for decreases in the effective area (C D  II A 2  /4, where C D  represents flow coefficient) of a nozzle throat with change in the spacing ratio (L divided by A) where L is the axial distance from the aft end of the tail cone to the nozzle throat, and A is the diameter of the nozzle throat. FIG. 7 illustrates changes in the thrust coefficient (C T ) of a nozzle according to the invention with change in the spacing ratio. The family of computed curves on graphs 6 and 7 represent engines having the indicated ratio of &#34;a&#34; to A where &#34;a&#34; is the diameter defined at the origin of the fluid wake i.e., the outer diameter of the sleeve member 38 in the engine illustrated in FIGS. 1 and 2, and A is the diameter of the nozzle throat. 
     As FIG. 6 illustrates, the movement of the fluid wake 32 with respect to the nozzle throat 30 can effect a significant reduction in the effective area of the nozzle throat. FIG. 6 also illustrates two test points 50&#39; which were generated from data collected from an engine test. The test engine was fitted with a center body having an axially movable disc movement of which moved the wake origin in much the same way as does movement of the sleeve member 38, illustrated in FIG. 1. The two test points 50&#39; show a good correlation between the calculated curves and actual test engine performance. 
     FIG. 7 illustrates that the thrust coefficient CT of an engine according to the invention is not significantly decreased as the spacing ratio (L/A) is decreased. Thus, the efficiency of an engine according to the invention remains relatively high as the area of the nozzle throat is decreased. 
     In view of the above, this invention may be of significant value in those jet engines which are required to fulfill a variety of missions. Such engines may benefit greatly from the ability to vary their nozzle flow coefficients to gain increased efficiency and performance.