Abstract:
A turning strut for use in a diffuser of a turbine engine has a leading edge with first and second opposing surfaces depending therefrom that terminate at a trailing edge. Slots extend through the turning strut and reduce in volume from the first surface to the second surface. During turn down operation of the turbine engine, exhaust flow impacts the leading edge at a deviated swirl angle. This results in exhaust flow at the first surface being at a higher pressure than at the second surface, which causes exhaust flow to be induced through the slots. The reduction in slot volume causes exhaust flow through the slots to accelerate. This exhaust flow from the slots is combined with exhaust flow at the second surface. Thusly, momentum of exhaust flow at the second surface is increased to maintain the second laminar boundary layer at the second surface.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter described herein relates to turbine engines, and, more specifically, to turning struts in a diffuser of a turbine engine. 
         [0002]    A gas turbine engine includes a compressor having a plurality of compressor blades disposed on a shaft, with the compressor blades and shaft configured to define a decreasing volume. Airflow ingested into the gas turbine is compressed as it passes through the compressor. A plurality of combustors are disposed downstream of the compressor, where air and fuel are mixed and the fuel is ignited, as is known. A multi-stage turbine is disposed downstream of the combustors. First stages of the multi-stage turbine are defined by a plurality of turbine vanes disposed on the shaft of the compressor. Final stages of the multi-stage turbine are defined by a plurality of turbine vanes disposed on an output drive shaft, which rotates independently of the shaft of the compressor. The heated compressed air flow from the combustors turns the multi-stage turbine. The rotation of the first stages of the multi-stage turbine rotates the shaft of the compressor. The rotation of the final stages of the multi-stage turbine rotates the output drive shaft, which in turn drives a generator. A diffuser is disposed aft of the final stages of the multi-stage turbine and is configured to decelerate the exhaust flow and convert dynamic energy to a static pressure rise. The diffuser includes a plurality of turning struts that consist of a support strut encased by an aerodynamic faring. The turning struts turn a flow from the multi-stage turbine towards the axial direction when the gas turbine engine is operated within a designed performance range. The turning struts are disposed circumferentially within the annulus of the diffuser. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    According to one aspect of the invention, a turning strut for use in a diffuser of a turbine engine has a curved leading edge, a first tapered surface depending at one end thereof from the curved leading edge, a second tapered surface depending at one end thereof from the curved leading edge, and a trailing edge defined at the other ends of the first and second tapered surfaces. The second tapered surface is disposed opposite the first tapered surface. At least one slot extends through the turning strut from the first tapered surface to the second tapered surface. The at least one slot reduces in volume from the first tapered surface to the second tapered surface. The at least one slot is disposed proximate the curved leading edge. 
         [0004]    According to another aspect of the invention, a method of turning a flow in a diffuser of a turbine engine includes impacting the flow at a leading edge of the turning strut, at a swirl angle that is deviated from a design point swirl angle. The method further includes defining a first laminar boundary layer at a first tapered surface of the turning strut from the flow thereat and defining a second laminar boundary layer at a second surface of the turning strut from the flow thereat. The first tapered surface depending at one end thereof from the leading edge. The second tapered surface depending at one end thereof from the leading edge. The second surface is disposed opposite the first surface. The deviated swirl angle results a pressure differential between the flow at the first and second surfaces. The flow at the first surface is at a higher pressure than the flow at the second surface. The method includes inducing the flow through at least one slot that extends through the turning strut from the first surface to the second surface. The flow through the at least one slot is from the first surface to the second surface and is a result of the pressure differential between the flow at the first and second surfaces. The method further includes accelerating the flow through the at least one slot by a reduction of volume of the at least one slot from the first surface to the second surface. The method still further includes combining the flow from the at least one slot with the flow at the second surface. Momentum of the flow at the second surface is increased to maintain the second laminar boundary layer at the second surface. 
         [0005]    According to yet another aspect of the invention, a turning strut for use in a diffuser of a turbine engine has a generally elongated tear drop shape defined by a leading edge having first and second surfaces depending therefrom, the second surface being disposed opposite the first surface, and the first and second surfaces terminating at a trailing edge. At least one slot extends through the turning strut from the first surface to the second surface. The at least one slot reduces in volume from the first surface to the second surface. The at least one slot is disposed proximate the leading edge. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a diagrammatic cross sectional view of a typical gas turbine engine with a diffuser; 
           [0009]      FIG. 2  is a diagrammatic cross sectional view of a turning strut of the prior art operating at a design point swirl angle; 
           [0010]      FIG. 3  is a diagrammatic cross sectional view of the turning strut of  FIG. 2  operating at a swirl angle that is deviated from the design point swirl angle; 
           [0011]      FIG. 4  is a diagrammatic cross sectional view of an embodiment of a turning strut of the invention operating at a design point swirl angle; 
           [0012]      FIG. 5  is a partial side view of the turning strut of  FIG. 4 ; 
           [0013]      FIG. 6  is another partial side view of the turning strut of  FIG. 4 ; and 
           [0014]      FIG. 7  is a diagrammatic cross sectional view of the turning strut of  FIG. 4  operating at a swirl angle that is deviated from the design point swirl angle. 
       
    
    
       [0015]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring to  FIG. 1 , a heavy-duty gas turbine engine is shown generally at  10 . The gas turbine engine  10  has a generally annular shape defined by an outer turbine casing  12 . An inlet  14  is defined at one end of the gas turbine engine  10 . The inlet  14  leads to a compressor  16  that is defined by and a plurality of compressor blades  18  disposed within the casing  12 . The compressor blades  18  are disposed on a shaft  20  that extends along a centerline  22  of the casing  12 , with the compressor blades  18  and shaft  20  configured to define a decreasing volume. Airflow ingested into the gas turbine engine  10  at the inlet  14  is compressed as it passes through the compressor  16 . A plurality of combustors  24  are disposed downstream of the compressor  16 , and are positioned axially about the shaft  20 . The combustors  24  have a premixing chamber and a combustion chamber (both of which are not shown). The airflow from the compressor  16  is ingested through entry ports  26  into the premixing chamber. Also, fuel from a fuel inlet  28  is delivered into the premixing chamber. This air and fuel are mixed within the premixing chamber to form a fuel and air mixture that flows into the combustion chamber where it is ignited, as is known. A multi-stage turbine  30  is disposed within the casing  12  downstream of the combustors  24 . First stages  32  of the multi-stage turbine  30  are defined by a plurality of turbine vanes  34  disposed on the shaft  20 . Final stages  36  of the multi-stage turbine  30  are defined by a plurality of turbine vanes  38  disposed on an output drive shaft  40 . The output drive shaft  40  also extends along the centerline  22  of the casing  12 , as it is axially aligned with the shaft  20 , but rotates independently thereof. The heated compressed air flow from the combustors  24  turns the multi-stage turbine  30 . The rotation of the first stages  32  of the multi-stage turbine  30  rotates the shaft  20 , which in turn drives the compressor  16 . The rotation of the final stages  36  of the multi-stage turbine  30  rotates the output drive shaft  40 , which in turn drives a generator (not shown). A diffuser  42  is disposed aft of the final stages  36  of the multi-stage turbine  30  and is configured to decelerate the exhaust flow and convert dynamic energy to a static pressure rise. The diffuser  42  includes a plurality of turning struts  50  that consist of a support strut encased by an aerodynamic faring. The turning struts  50  turn a flow  44  from the multi-stage turbine  30  towards the axial direction, resulting in a flow  46 , when the gas turbine engine  10  is operated within a designed performance range. The turning struts  50  are disposed circumferentially within the annulus of the diffuser  42 . 
         [0017]    Referring to  FIG. 2 , in the prior art turning struts designated  50 ′ are shown in a diagrammatic cross sectional view as having a generally elongated tear drop shape. The turning strut  50 ′ has a curved leading surface  52  that leads to tapered surfaces  54  and  56 , which meet at trailing edge  58 . The turning struts  50 ′ are designed to turn a flow  60  of exhaust with limited efficiency drop when the gas turbine  10  is operated within its a designed performance range. More specifically, the flow  60  impacts the curved leading surface  52  of the turning strut  50 ′ at a design point inlet swirl angle, such that the flow  60  around the turning strut  50 ′ is uniform. Thusly defining laminar boundary layers  62  and  64  at the surfaces of the turning strut  50 ′. This is reliably achieved when the gas turbine engine  10  is operated within its designed performance range. 
         [0018]    However, it is at times desirable to operate the gas turbine engine  10  below its designed performance range. This would include operation during off peak energy demand or other low energy demand conditions. When operated in this manner, often referred to as “turn-down” operation, the flow of exhaust about the turning struts  50 ′ is less than optimal. 
         [0019]    Referring to  FIG. 3 , turning strut  50 ′ is shown during turn down operation, where the flow  60  impacts the curved leading surface  52  of the turning strut  50 ′ at a swirl angle that is significantly deviated from the design point inlet swirl angle ( FIG. 2 ). The flow  60  at this deviated swirl angle impacts the curved leading surface  52  at the turning strut  50 ′. The flow  60  continues along surface  54  defining the laminar boundary layer  62 . The flow  60  continues along surface  56  defining the laminar boundary layer  64 , which separates downstream at a point of separation  66 . When the momentum of flow in the laminar boundary layer  64  is reduced to the point where it is zero; the laminar boundary layer  64  then separates from the surface  56 . When the laminar boundary layer  64  separates from the surface  56 , it then causes reverse flow over the surface  56 . When the laminar boundary layer  64  separates, it produces a wake  68  that causes an increase of pressure drag, which adversely affects the efficiency of the system. Within the wake  68  a plurality of vortices  70  are created. When the vortices  70  begin to shed off the surface  56  they do so at a certain frequency. The shedding of the vortices  70  can cause vibrations in the strut  50 ′, further adding to inefficiencies of the system, such as increased noise and back pressure. 
         [0020]    Referring to  FIGS. 4 ,  5  and  6 , in an embodiment turning struts designated  50 ″ are shown in a diagrammatic cross sectional view ( FIG. 4 ) as having a generally elongated tear drop shape. The turning strut  50 ″ has a curved leading edge  72  that leads to tapered surfaces  74  and  76 , which meet at trailing edge  78 . A plurality of slots  80  extend through the turning strut  50 ″ from an inlet  82  at surface  74  ( FIG. 5 ) to an outlet  84  at surface  76  ( FIG. 6 ). The slots  80  are disposed proximate the leading edge  72  of the turning strut  50 ″. The slots  80  are aligned and are shaped generally rectangular with rounded ends. The area of the inlet  82  is greater than that of the outlet  84 . Consequently the volume of the slots  80  reduces from inlet  82  to outlet  84 . A diagrammatic cross sectional view of the slots  80  ( FIG. 4 ) shows a generally compound curve that is generally Serpentine shaped. The turning struts  50 ″ are designed to turn a flow  86  of exhaust with limited efficiency drop when a gas turbine engine  10  is operated within its designed performance range. More specifically, the flow  86  impacts the curved leading surface  72  of the turning strut  50 ″ at a design point inlet swirl angle, such that the flow  86  around the turning strut  50 ″ is uniform. Laminar boundary layers  88  and  90  are formed at the surfaces  74  and  76 , respectively, of the turning strut  50 ″. At the design point swirl angle the pressure differential of the flow from surface  74  to surface  76  is negligible, whereby any flow through slots  80  is de minimis. 
         [0021]    Referring now to  FIG. 7 , turning strut  50 ″ is shown during turn down operation, where the flow  86  impacts the curved leading edge  72  of the turning strut  50 ″ at a swirl angle that is significantly deviated from the design point inlet swirl angle ( FIG. 4 ). The flow  86  continues along surface  74  defining the laminar boundary layer  88 . The flow  86  continues along surface  76  defining a laminar boundary layer  92 . The deviated swirl angle results a pressure differential between the flow at surfaces  74  and  76 . A high-pressure flow is established at the surface  74  and low-pressure is established at surface  76 . This pressure differential between the surfaces  74  and  76  creates suction at the slots  80  resulting in flow through the slots. The reducing volume of the slots  80  causes the flow therethrough to increase in speed due to Bernoulli&#39;s principle. Bernoulli&#39;s principle states that the correlation between the velocity and pressure of a fluid; when fluid velocity increases pressure falls and likewise. Therefore, the flow entering the slots  80  at the inlet  82  accelerates through the slots  80  to the outlet  84 . The flow exiting the slots  80  is faster than the flow at the upstream end of surface  76 . These flows combine at the outlet  84 , whereby the flow at surface  76  downstream of the outlet  84  is accelerated. Unlike the above discussed prior art, this accelerated flow at surface  76  does not separate. The momentum of this flow is not reduced to zero; uniformity of the flow between the surfaces  74  and  76  is maintained. As described above, the slots  80  are Serpentine shaped in order facilitate flow into the inlet  82  and out of outlet  84 . More specifically, the direction of flow at the surface  74  leads to the inlet  82  and the outlet  84  follows the direction of flow at surface  76 . Accordingly, pressure drag is reduced, which will increase the efficiency of the diffuser  42  as well as reducing part stress. 
         [0022]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but it is only limited by the scope of the appended claims.