Abstract:
An apparatus for minimizing signature of an engine having an exhaust gas path and a fan gas path includes a casing, and a liner disposed within the casing. The exhaust gas path passes within the liner and the fan gas path passes between the liner and the casing. The casing is disconnected into an upstream portion and a downstream portion, each portion in registration with the liner, such that there are minimum pressure imbalances caused by the exhaust gas path, the fan gas path and ambient on the downstream portion.

Description:
BACKGROUND 
       [0001]    The present invention relates to gas turbine engines and, more particularly, to nozzles therefor. 
         [0002]    A typical gas turbine engine operates in an extremely harsh environment characterized by very high temperatures and vibrations. A conventional gas turbine engine includes a compressor for compressing entering air, a combustor for mixing and burning the compressed gases that emerge from the compressor with fuel, a turbine for expanding the hot gases to generate thrust to propel the engine, and an exhaust nozzle for allowing hot gases to exit the engine. Thus, the exhaust nozzle must accommodate extremely hot gases exiting the engine. 
         [0003]    In military operations, design of planes to avoid detection by radar is an important issue. The ability of the plane to remain undetected depends on the overall geometry of the plane and its engine. To minimize detection, it is preferable to minimize the detectability (i.e., the signature) of the engine. 
       SUMMARY 
       [0004]    According to an embodiment disclosed herein an apparatus for minimizing signature of an engine having an exhaust gas path and a fan gas path includes a casing, and a liner disposed within the casing. The exhaust gas path passes within the liner and the fan gas path passes between the liner and the casing. The casing is disconnected into an upstream portion and a downstream portion, each portion in registration with the liner, such that there are minimum pressure imbalances caused by the exhaust gas path, the fan gas path and ambient on the downstream portion. 
         [0005]    In one example embodiment that includes the elements of the foregoing embodiment, a flexible seal disposed between the upstream portion and the downstream portion. 
         [0006]    In another example embodiment that includes the elements of the foregoing embodiment, the casing and the liner comprise an exhaust nozzle. 
         [0007]    In another example embodiment that includes the elements of the foregoing embodiment, the casing and the liner further comprise a forward portion and an aft portion wherein a flexible seal is disposed between the casing forward portion and the casing aft portion. 
         [0008]    In another example embodiment that includes the elements of the foregoing embodiment, casing and the liner further have a convergent portion disposed between the forward portion and the aft portion. 
         [0009]    In another example embodiment that includes the elements of the foregoing embodiment, the flexible seal is disposed within the casing divergent portion. 
         [0010]    In another example embodiment that includes the elements of the foregoing embodiment, there are no supports between the upstream segment and the downstream segment. 
         [0011]    According to a further embodiment disclosed herein, a nozzle for minimizing signature of a gas turbine engine having an exhaust gas path and a fan gas path, includes a casing and a liner disposed within the casing. The exhaust gas path passes within the liner and the fan gas path passes between the liner and the casing. The casing is disconnected into an upstream portion and a downstream portion, each portion in registration with the liner, such that there are minimum pressure imbalances caused by the exhaust gas path, the fan gas path and ambient on the downstream portion. 
         [0012]    In another example embodiment that includes the elements of the foregoing embodiment, a flexible seal disposed between the upstream portion and the downstream portion. 
         [0013]    In another example embodiment that includes the elements of the foregoing embodiment, the casing and the liner further comprise a forward portion and an aft portion wherein a flexible seal is disposed between the casing forward portion and the casing aft portion. 
         [0014]    In another example embodiment that includes the elements of the foregoing embodiment, the casing and the liner further have a convergent portion disposed between the forward portion and the aft portion. 
         [0015]    In another example embodiment that includes the elements of the foregoing embodiment, the flexible seal is disposed within the casing divergent portion. 
         [0016]    In another example embodiment that includes the elements of the foregoing embodiment, there are no supports upstream of the flexible seal between the casing and the liner. 
         [0017]    In another example embodiment that includes the elements of the foregoing embodiment, there are no supports between the upstream segment and the downstream segment. 
         [0018]    According to a further embodiment disclosed herein, a method for designing a nozzle for minimizing signature of a gas turbine engine having an exhaust gas path and a fan gas path, includes the steps of providing a casing, providing a liner disposed within the casing, the exhaust gas path passing within the liner and the fan gas path passing between the liner and the casing, wherein the casing is disconnected into an upstream portion and a downstream portion, each portion in registration with the liner, such that there are minimum pressure imbalances caused by the exhaust gas path, the fan gas path and ambient on the downstream portion. 
         [0019]    In another example embodiment that includes the elements of the foregoing embodiment, the method includes providing a flexible seal between the upstream portion and the downstream portion. 
         [0020]    In another example embodiment that includes the elements of the foregoing embodiment, determining an inward load on the liner, determining an outward load on the downstream portion, and determining a net of the inward load and the outward load. 
         [0021]    In another example embodiment that includes the elements of the foregoing embodiment, the method includes adjusting a length of the downstream portion if the net does not approximate zero. 
         [0022]    In another example embodiment that includes the elements of the foregoing embodiment, the method includes adjusting a length of the upstream portion if the net does not approximate zero. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
           [0024]      FIG. 1  is a schematic depiction of a gas turbine engine incorporating an embodiment of a nozzle. 
           [0025]      FIG. 2  is a cross-portional schematic depiction of a prior art nozzle portion of an engine. 
           [0026]      FIG. 3  is a cross-portional schematic view of a nozzle used in in the gas turbine engine of  FIG. 1 . 
           [0027]      FIG. 4  is a perspective view of the nozzle of  FIG. 3 . 
           [0028]      FIG. 5  is a block diagram of a method for designing the nozzle of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Referring to  FIG. 1 , a gas turbine engine  10  includes a compressor  12 , a combustor  14 , and a turbine  16  centered about a central axis  17 . A first air flow  18  passes axially through the compressor  12 , the combustor  14 , and the turbine  16 . As is well known in the art, the first air flow  18  is compressed in the compressor  12 . Subsequently, the compressed first air flow  18  is mixed with fuel and burned in the combustor  14 . The hot gases of the first air flow  18  expand generating thrust to propel the engine  10  and to drive the turbine  16 , which in turn drives the compressor  12  and a fan  26 . The exhaust gases from the turbine  16  exit through the exhaust nozzle  20 . Though a two spool engine  10  is shown herein, one of ordinary skill in the art will recognize that other numbers of spools may be utilized with the teaching provided herein. 
         [0030]    The engine  10  has an exterior casing  22  that extends a length of the engine and an interior casing  24  that encloses the compressor  12 , the combustor  14  and the turbine  16 . The fan  26  drives a second air flow  28  through an area  30  between the exterior casing  22  and the interior casing  24 . A gap  32  exists between the interior casing  24  and a nozzle liner  35 . The second air flow  28  flows between the nozzle liner  35  and a case  40  of the exterior casing  22 . 
         [0031]    Referring now to  FIG. 2 , a prior art version of the exhaust nozzle  20  is shown. The exhaust nozzle  20  includes the nozzle liner  35 , the exterior case  40  and, at an aft end thereof  45 , a convergence portion  50  that may or may not be mobile. The nozzle liner  35  has a liner forward portion  55 , a liner central converging portion  60  and a liner aft portion  65  that is approximately in parallel to the forward portion  55  and disposed radially inwardly thereof. As will be seen infra, this exhaust nozzle  20  flattens so the liner aft portion  65  may vary as to its radial relationship with the liner forward portion  55  about a perimeter of the nozzle  20 . 
         [0032]    Similarly, the exterior case  40  has a cross-portional shape mimicking the shape of the liner  35 . The exterior case  40  has a case forward portion  75 , a case converging portion  80  and a case aft portion  85  that is approximately in parallel to the case forward portion  75  and disposed radially inwardly thereof. As will be seen infra, this exhaust nozzle  20  flattens so the case aft portion  85  may vary as to its radial relationship with the case forward portion  75  about a perimeter of the nozzle  20 . A plurality of supports  90  extend between the liner forward portion  55  and the case forward portion  75 , supports  95  extend between the liner converging portion  60  and the case converging portion  80 , and supports  100  extend between the liner aft portion  65  and the liner aft portion  85 . The supports  90 ,  95 ,  100  act to maintain the nozzle liner  35  and exterior case  40  at a roughly equal distance apart. 
         [0033]    Referring now to  FIGS. 3 and 4 , an exemplary embodiment is shown. For ease of illustration, the numbering system for  FIGS. 2 and 3  are consistent to highlight the differences between the prior art shown in  FIG. 2  and the exemplary embodiment shown in  FIGS. 3 and 4 . There are pressure forces on the inner liner  335  and the case  340  that may affect the signature of the engine  10 . For instance, the pressure of the second air flow  328  between the inner liner  335  and the case  340  along the length of the nozzle is generally greater than the pressure of the first air flow  318  along the length of the nozzle  300  and greater than ambient radially outside of the case  340 . Also the pressure of the first air flow  318  is greater than ambient radially outside of the case  340 . The pressures sum to a net outward load on the system. However, it has been discovered that pressure imbalances on the liner aft portion  365  and the case aft portion  385  (and possibly portions of the case converging portion  380  and a liner converging portion  360 ) may provide a signature for the engine  10 . In order to minimize the effect of the pressure imbalances on the liner aft portion  365  and the case aft portion  385  (and possibly portions of the case converging portion  380  and a liner converging portion  360 ), it is preferable to shift the pressure imbalances to the case forward portion  375  and to the liner forward portion  355  (and possibly portions of the case converging portion  380  and a liner converging portion  360 ) as will be discussed infra. 
         [0034]    A flexible seal  405 , made of an elastomer reinforced fabric or the like, is disposed between attached to an upstream side  410  and a downstream side of the case converging portion  380 . The supports  90  that extend between the liner forward portion  55  and the case forward portion  75  in the prior art are removed. A bumper  415  may be disposed between the case forward portion  375  and the liner forward portion to minimize contact between the parts. The supports  395  upstream of the seal  405  on the case converging portion  380  are also removed. 
         [0035]    By separating the case  340  into a first segment  420  upstream of the seal  405  and a second segment  425  downstream of the seal, the second segment  425  is free to move without being affected by the expansion of the first segment  420 . Given particular geometries of the liner  335  and the case  340 ; the relative lengths and circumferences; and the particular pressure differences between the liner  335  and the case  340  regarding the first air flow  318 , the second air flow  328  and ambient, the pressure imbalances on the second segment  125  tend to equalize such that the net load results in a liner displacement which meets a pre-determined criteria. 
         [0036]      FIG. 4  shows that the exhaust nozzle  320  smoothly tends to flatten out through the liner and case forward portions  355 ,  375 , the liner and case converging portions  360 ,  380  and the liner and case aft portions  360 ,  380 . Because exhaust nozzles may have different shapes and geometries, one of ordinary skill in the art will recognize from the teachings herein, that the seal  405  may be placed in other portions of the case so that the forces downstream of the seal  405  result in a liner displacement which meets a pre-determined criteria. For instance and without limitation, the seal  405  may be placed between a first junction  430  between the case forward portion  375  and the case diverging portion  380  or a second junction  435  between the case diverging portion  380  and the case aft portion  385 . The seal may also be placed in the case forward portion  375  and in the case aft portion  385 . 
         [0037]    Referring now to  FIG. 5 , in order to determine where to place the seal  405 , one must: determine the inward load on the liner by multiplying the differences of pressure between the first gas path  318  and the second gas path  328  by the area of the liner along its path in registration with the case  340  (step  200 ); determine the outward load on the second segment  425  by its area along its length (step  210 ); and determine the net load on the system by subtracting the two determinations (step  220 ). If the remainder results in a liner displacement which meets the pre-determined criteria (step  230 ), place the seal  405  where the length of the second segment  425  ends adjacent the first segment  420  (step  240 ). If the remainder results in a liner displacement which does not meet the pre-determined criteria, the relative lengths of the first segment  420  and the second segment  425  are adjusted (step  250 ) until the remainder results in a liner displacement which meets the pre-determined criteria to minimize a signature of the engine  10  and the nozzle  300 . 
         [0038]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.