Patent Publication Number: US-2012027578-A1

Title: Systems and apparatus relating to diffusers in combustion turbine engines

Description:
BACKGROUND OF THE INVENTION 
     This present application relates generally to turbine diffuser design, which, as used herein and unless specifically stated otherwise, is meant to include all types of combustion turbine or rotary engines, including gas turbine engines, aircraft engines, and others. More specifically, but not by way of limitation, the present application relates to turbine diffuser design providing robust diffuser and CDC performance. 
     In general, a turbine engine includes a compressor that delivers a supply of highly compressed air to a combustor for combustion with a fuel. The resulting flow of hot gases from the combustor drives the turbines from which work may be extracted. Turbine engines may be configured with an axial compressor that is mechanically coupled by a common shaft or rotor to a downstream turbine, with a combustor positioned between the compressor and the turbine. Air leaves the compressor with a relatively high velocity and, conventionally, a diffuser is utilized for initially decreasing the velocity of the compressed airflow and minimizing subsequent pressure losses. The diffuser may include splitter vanes that divide the airflow into separate diffuser passages. A diffuser dump region or cavity receives airflow from the diffuser, and further deceleration occurs there before the air is directed to annular channels surrounding the combustor. As is conventional, the compressor is provided with an inner compressor discharge inner barrel and a compressor discharge casing (CDC). The CDC interconnects the inner barrel and a first-stage nozzle. 
     A primary source of loss and turbulence in diffusers is vortex generation as flow enters the diffuser dump cavity. The diffuser dump cavity has the highest diffusion gradient, leading to vortex formation. As the fluid flow moves forward, the vortex grows and begins interacting with the upstream sections of the diffuser. Vortex growth elevates the fluid flow upstream of diffuser dump region and results in high loss and poor pre-diffuser and CDC performance. 
     As a result, there is a need for improved systems and apparatus that trap the vortex and arrest its growth in the dump cavity of the diffuser, thus reducing overall losses and ensuring robust diffuser performance. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present application thus describes a discharge diffuser that includes: a forward section and a dump cavity, the forward section being configured to direct discharge from the compressor to the dump cavity; an inner diffuser wall that defines an inner radial flowpath of the upstream section; and an outer diffuser wall that defines an outer radial flowpath of the upstream section; wherein at an aft lip of the inner diffuser wall, the discharge diffuser comprises an overhanging step. 
     The present application further describes a discharge diffuser that includes: a forward section configured to direct compressor discharge from the compressor to a dump cavity; wherein: the forward section includes an inner diffuser wall and an outer diffuser wall, the outer diffuser wall flaring outwardly to define a widening flowpath therethrough; the dump cavity comprises a region of increased volume positioned downstream of the upstream section, the dump cavity being configured to surround at least a portion of a combustor; and at an aft lip of the inner diffuser wall, the discharge diffuser comprises an overhanging step, the overhanging step including a step wall that, from the aft lip, slants radially inward and in an upstream direction so that the step wall undercuts a portion of the inner diffuser wall. 
     These and other features of the present application will become apparent upon review of the following detailed description of the preferred embodiments when taken in conjunction with the drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will be more completely understood and appreciated by careful study of the following more detailed description of exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an exemplary discharge diffuser for a compressor in a combustion turbine engine that includes a vortex trap or over hanging step according to an embodiment of the present application; 
         FIG. 2  illustrates an exemplary discharge diffuser that includes a vortex trap or overhanging step according to an alternative embodiment of the present application; 
         FIG. 3  depicts exemplary dimensions of the various parts of the a vortex trap or overhanging step according to an exemplary embodiment of the present application; 
         FIG. 4  illustrates a fluid flow pattern within a conventional diffuser; and 
         FIG. 5  illustrates a fluid flow pattern within the discharge diffuser having a vortex trap or overhanging step according to an exemplary embodiment of the present application. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As an initial matter, to communicate clearly the invention of the current application, it may be necessary to select terminology that refers to and describes certain parts or machine components of a combustion turbine engine. Whenever possible, common industry terminology will be used and employed in a manner consistent with its accepted meaning. However, it is meant that any such terminology be given a broad meaning and not narrowly construed such that the meaning intended herein and the scope of the appended claims is unreasonably restricted. Those of ordinary skill in the art will appreciate that often a particular component may be referred to using several different terms. In addition, what may be described herein as a single part may include and be referenced in another context as consisting of several component parts, or, what may be described herein as including multiple component parts may be fashioned into and, in some cases, referred to as a single part. As such, in understanding the scope of the invention described herein, attention should not only be paid to the terminology and description provided, but also to the structure, configuration, function, and/or usage of the component, as provided herein. 
     Further, as used herein, “downstream” and “upstream” are terms that indicate a direction relative to the flow of working fluid through the turbine. As such, the term “downstream” refers to a direction that generally corresponds to the direction of the flow of working fluid, and the term “upstream” generally refers to the direction that is opposite of the direction of flow of working fluid. The terms “aft” and “forward” may be used to describe relative position within the turbine engine. It will be appreciated that the compressor is generally referred to as residing on the “forward” side of the turbine engine while the turbine section resides on the “aft” side. Accordingly, as used herein, “forward” describes a position closer to the compressor and “aft” describes a position closer to the turbine. The term “radial” refers to movement or position perpendicular to an axis. It is often required to describe parts that are at differing radial positions with regard to an axis. In this case, if a first component resides closer to the axis than a second component, it may be stated herein that the first component is “radially inward” or “inboard” of the second component. If, on the other hand, the first component resides further from the axis than the second component, it may be stated herein that the first component is “radially outward” or “outboard” of the second component. The term “axial” refers to movement or position parallel to an axis. Finally, the term “circumferential” refers to movement or position around an axis. 
       FIG. 1  illustrates an exemplary compressor discharge diffuser  100  in a combustion turbine engine that includes a vortex trap or overhanging step according to an exemplary embodiment of the present application. The discharge diffuser  100  may direct compressed fluid from a compressor (not shown) to a combustor (not shown). In general, air in the turbine engine leaves the compressor with relatively high velocity and enters the diffuser  100 , where it is then decelerated. 
     As shown by arrows in  FIG. 1 , the compressed fluid initially enters a forward section  101  of the discharge diffuser  100 . The forward section  101  may include an inner diffuser wall  102  and an outer diffuser wall  104 . It will be appreciated that the outer diffuser wall  104  flares outwardly such that the diffuser walls  102 ,  104  define a widening flow path through the forward section  101 , which decelerates the incoming compressed air. Further, an annular splitter vane  105  may be included (as shown). The splitter vane  105  splits the forward section  101  of the discharge diffuser  100  into two passages—a first passage  106  and a second passage  108 , which direct the compressor discharge to a dump cavity  110 . (Note that only a portion of the dump cavity  110  is shown in  FIGS. 1 through 3 , whereas additional areas of the dump cavity  110  are shown in  FIGS. 4 and 5 ). In certain implementations of the disclosure, the discharge diffuser  100  may include any number of splitter vanes or, alternatively, may include a single widening annular passage without splitter vanes. 
     Generally, the discharge diffuser  101  includes a dump cavity  110 , which receives airflow from the forward section  101  (shown by arrows). It will be appreciated that within the dump cavity  110 , air is directed into the annular channels surrounding the combustor.  FIG. 1  shows the dump cavity  110  as a region of increased volume that is generally positioned downstream of the aft termination points of the inner diffuser wall  102  and the outer diffuser wall  104 . The dump cavity  110  may include an inner cavity wall  112  that defines an inner radial boundary of the dump cavity  110  and, as stated, the dump cavity  110  may surround at least a portion of the combustor. The dump cavity  110  has a substantially high diffusion gradient, which, in operation, typically leads to the formation of a vortex or vortices (not shown). It will be appreciated by those of ordinary skill in the art that the formation of such vortices is a primary source of loss and turbulence, which negatively impact the efficiency of the engine. Typically, as the fluid flow moves downstream, the vortices grow and begin interacting with the upstream section  101  of the discharge diffuser  100 , causing further efficiency losses. 
     The inner diffuser wall  102  terminates at an aft lip  113 . As used herein, the aft lip  113  is the downstream or aft termination point of the inner diffuser wall  102 , as indicated in  FIGS. 1 and 2 . In some embodiments, the inner diffuser wall  102  may include a transition step  116  that is positioned just forward of the aft lip  113 , as shown. It will be appreciated that the aft lip  113  marks the transition point from the forward section  101  to the dump cavity  110  of the discharge diffuser  100 . Conventional design, as shown in  FIG. 4 , provides a radial step at this location, with the step having a step wall that is substantially aligned with the radial direction. 
     According to embodiments of the present application, the discharge diffuser  100  includes an overhanging step  116 , which, as discussed more below, serves to minimize or trap or arrest the growth of vortices in this region. The overhanging step  116  generally includes a step wall  118  that slants radially inward and in an upstream direction, thereby undercutting an aft portion of the inner cavity wall  112 , as shown in the cross-sectional views of  FIGS. 1 through 3 . More specifically, the overhanging step  116  includes a step wall  118  that, from a beginning point at the aft lip  113  of the inner diffuser wall  102 , slants in a direction that includes both an inboard directional component and an axial-upstream directional component. At one end, the step wall  118  connects to the inner diffuser wall  102  and, at the other end, connects to the inner cavity wall  112  at a forward edge (a location that is referred to herein as the dump cavity forward edge  119 ). It will be appreciated that, according to embodiments of the present application, the axial position of the dump cavity forward edge  119  is forward of the axial position of the aft lip  113 . Of course, it is this configuration that creates the overhanging step  116 , which, in turn, produces the flow dynamics that reduce the formation of vortices through the diffuser  100  during operation of the engine. 
     The step wall  118 , as shown, may be planar, and, in cross-section, generally forms an angle  306  with a radial reference line, which is specifically illustrated in  FIG. 3 . As used herein, this angle will be referred to as the step wall angle  306 . Conventionally, as stated, the step wall  118  is aligned with the radial reference line and, thus, the step wall angle  306  is approximately 0°. In other conventional arrangements (not shown), the step wall  118  forms a positive angle with the radial reference line, which, as used herein, refers to a configuration wherein the step wall  118  slants inward radially in the aft direction. As taught in the present application, however, the step wall  118  creates a negative angle with the radial reference line, which, as stated, creates an overhang or undercut. It has been discovered that configurations having certain step wall angles  306  or step wall angles  306  within a certain range provide enhanced performance. For example, in one preferred embodiment, the step wall angle  306  comprises a range between approximately −20° and −60°. More preferably, the step wall angle  306  comprises a range between approximately −30° and −50°. In some applications, an ideal step wall angle  306  comprises approximately 40°. As discussed in more detail below, the slant of the step wall  118  forms a vortex trap that arrests the growth of vortices and prevents the vortices from interacting with the fluid in the forward section  101  of the diffuser  100 . 
     The aft lip  113  of the inner diffuser wall  102  may include several preferred configurations. In one configuration, as shown in  FIG. 1 , the aft lip  113  may have a smooth, rounded edge. In another preferred embodiment, as shown in  FIG. 2 , the aft lip  113  may have a sharp edge. In yet another preferred embodiment, as  FIG. 3  shows, the aft lip  113  includes a flat surface, aligned in a substantially radial direction. These aft lip  113  alternatives have been proven effective at trapping vortices and reducing aerodynamic losses. While these described configurations for the aft lip  113  represent preferred embodiments, it will be appreciated that other configurations are possible. 
     In a preferred embodiment, the shape of the connection made between the step wall  118  and the inner cavity wall  112 , as shown, includes a rounded, fillet region. As will be appreciated, this may prevent stress concentrations. Other configurations are also possible. 
     Further, it has been discovered through experimentation and computer modeling of flow patterns that certain dimensions are particularly more effective at controlling or limiting vortex formation than others.  FIG. 3  assists in describing the exemplary dimensions of the various parts of an exemplary vortex trap  300  within the diffuser  100 . For example, the radial height  312  of the overhanging step  116  may be between approximately 4 and 6 inches. More preferably, this radial height  312  may be approximately 4.4 inches. In some embodiments, the distance  302  between the transition step  114  and the aft lip  113  may be approximately 3.5 to 4.5 inches. More preferably, this distance  302  may be about 4 inches. In some embodiments, the height  304  of the flat edge of the aft lip  113  (see  FIG. 3 ) may be between approximately 0 and 1 inches. More preferably, this height  304  may be approximately 0.5 inches. In some embodiments, the radius  308  of the arc formed between the step wall  118  and the inner cavity wall  112  may be between approximately 0.5 and 2 inches. More preferably, this radius  308  may be approximately 1 inch. Further, the height  310  of the transition step  114  may be between approximately 0.2 and 1 inches. More preferably, this height  310  may be around 0.5 inches. The above dimensions are optimized for minimizing overall losses due to vortex (not shown) growth and limiting vortex interaction with upstream fluid flow. It will be appreciated that these dimensions may be altered depending on the application and that they represent only a preferred method of practice. 
       FIG. 4  illustrates a fluid flow pattern within a conventional diffuser  400  and experimental results of same. The conventional diffuser  400  includes the first passage  106  and the second passage  108  through which the compressed fluid travels to the dump cavity  110 , as shown by arrows. A step  402 , which has a step wall oriented substantially in the radial direction, connects the inner diffuser wall  102  to the inner cavity wall  112  of the dump cavity  110 , which is inboard in relation to the inner diffuser wall  102 . The dump cavity  110  has the highest diffusion gradient, leading to vortex generation. A vortex  404  (which is represented by the shaded region) forms in proximity to the step  402  as fluid enters the dump cavity  110 . The vortex  404  grows as the fluid moves downstream and begins interacting with the fluid in the forward section of the conventional diffuser  400 . Fluid flow reversal in the dump cavity  110  climbs the step  402 , which is a substantially vertical wall, and consequently, interacts with the upstream flow and aids to the further growth of the vortex  404 . The interaction can be seen in  FIG. 4 , where the fluid is shown entering the second passage  108 . This growth in the vortex  404  elevates the flow upstream of the dump cavity  110 , leading to heavy loss and poor pre-diffuser and CDC performance. 
       FIG. 5  depicts a fluid flow pattern within the diffuser  100  according to an embodiment of the present application and experimental results of same. As discussed, the first leg  118  may slant radially inward and in an upstream direction so that the first leg  118  undercuts a portion of the inner diffuser wall  102 . Further, the aft lip  113  may have an axial position that is aft relative to the axial position of the dump cavity forward edge  119 .  FIG. 5  shows a mitigated vortex  502  formed in close proximity to the step wall  118 . The slant of the step wall  118  creates a vortex trap in the dump cavity  110 , arresting the growth of the mitigated vortex  502  and preventing its interaction with the forward section  101  of the diffuser  100 .  FIG. 5  shows the mitigated vortex  502  being significantly disengaged from the second passage  108 , as compared to the vortex  404  in  FIG. 4 , which has significant interaction with the second passage  108 . This comparison illustrates the manner in which the present design of the diffuser  100  may prevent the growth of the mitigated vortex  502  and may substantially disengage the mitigated vortex  502  from the upstream fluid flow. 
     It will be appreciated that the design of the diffuser  100  also facilitates uniform flow distribution across a transition piece  504  and prevents the formation of hot spots. The resulting flow field reduces overall losses and improves the diffuser  100  performance. Further, containment of the mitigated vortex  502  relieves the stringent need of having a uniform flow profile at the compressor, without negatively affecting performance. The reduced losses in the CDC may also allow a higher margin of loss during compressor or combustor design, providing significant performance and financial benefit. 
     Table 1 compares the performance of the conventional diffuser  400  with that of the diffuser  100 . Four scenarios are considered, having different leakage levels set at the fourteenth stator (S 14 ) of the compressor, the leak being between the airfoil at S 14  and the CDC. 0.3% leak at S 14  is the design point for the present example. The pressure loss is measured according to the following equation 1: 
     
       
         
           
             
               
                 
                   
                     DPt 
                     Pt 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           Pressure 
                            
                           
                               
                           
                            
                           at 
                            
                           
                               
                           
                            
                           diffuser 
                            
                           
                               
                           
                            
                           input 
                         
                         - 
                         
                           Pressure 
                            
                           
                               
                           
                            
                           at 
                            
                           
                               
                           
                            
                           diffuser 
                            
                           
                               
                           
                            
                           output 
                         
                       
                       
                         Pressure 
                          
                         
                             
                         
                          
                         at 
                          
                         
                             
                         
                          
                         diffuser 
                          
                         
                             
                         
                          
                         input 
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 DPt/Pt for 
                   
               
               
                 S14 Leak 
                 conventional diffuser 
                 DPt/Pt for diffuser 100 
               
               
                 (%) 
                 400 (%) 
                 (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 0.1 
                 1.17 
                 1.16 
               
               
                 0.3 
                 1.13 
                 0.99 
               
               
                 (design point) 
               
               
                 0.4 
                 1.19 
                 1.03 
               
               
                 0.8 
                 1.42 
                 1.3 
               
               
                   
               
            
           
         
       
     
     It should be noted that, typically, significant effort is invested in uniformly maintaining such low levels of leakage. The reduced losses in the diffuser  100  may impart some flexibility during compressor or combustor design and may further relax the stringent requirements for maintaining leakage levels. 
     Table 1 shows that the diffuser  100  lowers the pressure loss due to vortex growth compared to the conventional diffuser  400 . It should be noted that the claimed diffuser design provides robust performance not only at design point, but also across various operating conditions. Thus, the diffuser  100 , according to the embodiments of the present disclosure, restricts vortex growth and limits upstream flow interaction with the vortex, leading to substantial improvements in diffuser and CDC performance. 
     As one of ordinary skill in the art will appreciate, the many varying features and configurations described above in relation to the several exemplary embodiments may be further selectively applied to form the other possible embodiments of the present invention. For the sake of brevity and taking into account the abilities of one of ordinary skill in the art, all of the possible iterations is not provided or discussed in detail, though all combinations and possible embodiments embraced by the several claims below or otherwise are intended to be part of the instant application. In addition, from the above description of several exemplary embodiments of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are also intended to be covered by the appended claims. Further, it should be apparent that the foregoing relates only to the described embodiments of the present application and that numerous changes and modifications may be made herein without departing from the spirit and scope of the application as defined by the following claims and the equivalents thereof.