Abstract:
A compressor section includes a rotor platform; a rotor blade extending radially outwardly from the rotor platform, the rotor blade including a pressure sidewall and a circumferentially opposing suction sidewall extending in a radial direction between a root and a tip and in an axial direction between a leading edge and a trailing edge; a casing having an inner surface surrounding the tip and spaced radially outwardly thereform to define a gap therebetween; a plurality of grooves disposed in the inner surface of the casing and extending in a generally circumferential direction, the plurality of grooves including a first groove and a second groove; and a channel system comprising at least a first channel positioned within the casing and configured to provide fluid communication between the first groove and the second groove.

Description:
TECHNICAL FIELD 
     The present invention generally relates to compressors of gas turbine engines, and more particularly relates to casing treatments in compressors of gas turbine engines. 
     BACKGROUND 
     A gas turbine engine may be used to power various types of vehicles and systems. A gas turbine engine may include, for example, five major sections: a fan section, a compressor section, a combustor section, a turbine section, and an exhaust nozzle section. The fan section induces air from the surrounding environment into the engine and accelerates a fraction of this air toward the compressor section. The remaining fraction of air induced into the fan section is accelerated through a bypass plenum and exhausted through the mixer nozzle. The compressor section raises the pressure of the air it receives from the fan section and directs the compressed air into the combustor section where it is mixed with fuel and ignited. The high-energy combustion products then flow into and through the turbine section, thereby causing rotationally mounted turbine blades to rotate and generate energy. The air exiting the turbine section is exhausted from the engine through the exhaust nozzle section. 
     In some engines, the compressor section is implemented with one or more axial and/or centrifugal compressors. A compressor typically includes at least one rotor blade that is rotationally mounted on a hub within a casing. The portion of casing in closest proximity to the rotor blades is referred to as the end wall. From a high pressure efficiency perspective, it is advantageous to minimize the distance between the outer tips of the fan blades and the end wall. However, in some conventional engines, minimizing this distance may increase the likelihood of a stall condition. Engine stall is a phenomenon that occurs as a result of certain engine operating conditions and, if not properly addressed, may adversely impact engine performance and durability. 
     During operation of the compressor, stall occurs when the stream momentum imparted to the air by the blades is insufficient to overcome the pressure rise across the compressor to result in a reduction in airflow. The compressor stall may propagate through several compressor stages, starving the gas turbine of sufficient air to maintain engine speed. This decreases the turbine&#39;s ability to create power, further reducing the output of the engine. To avoid stall, operating limits may be placed on the engine to define a safe operating range in which stall is unlikely. The difference between the safe operating limits and a normal or desired operating condition is often referred to as the stall margin. As in many systems, greater efficiency and overall performance are achieved at higher operating conditions, and thus, to that extent, some conventional compressors may sacrifice engine efficiency to obtain safer operating conditions. To maintain adequate stall margin, the compressor must either operate in a less than optimally efficient manner or mechanisms may be devised to extend the stable operating range of the compressor. 
     Accordingly, it is desirable to provide casing treatments that increase the stall margin while not adversely affecting engine performance. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
     BRIEF SUMMARY 
     In accordance with an exemplary embodiment, a compressor section includes a rotor platform; a rotor blade extending radially outwardly from the rotor platform, the rotor blade including a pressure sidewall and a circumferentially opposing suction sidewall extending in a radial direction between a root and a tip and in an axial direction between a leading edge and a trailing edge; a casing having an inner surface surrounding the tip and spaced radially outwardly thereform to define a gap therebetween; a plurality of grooves disposed in the inner surface of the casing and extending in a generally circumferential direction, the plurality of grooves including a first groove and a second groove; and a channel system comprising at least a first channel positioned within the casing and configured to provide fluid communication between the first groove and the second groove. 
     In accordance with another exemplary embodiment, a casing treatment is provided for a compressor having a rotor blade with a pressure sidewall and a circumferentially opposing suction sidewall extending in a radial direction between a root and a tip and in an axial direction between a leading edge and a trailing edge. The casing treatment includes an inner wall; a plurality of grooves disposed in the inner wall and extending in a generally circumferential direction, the plurality of grooves including a first groove and a second groove; and a channel system comprising at least a first channel positioned within the inner wall and configured to provide fluid communication between the first groove and the second groove. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a cross-sectional view of a gas turbine engine in accordance with an exemplary embodiment; 
         FIG. 2  is a partial cross-sectional view of a compressor of the gas turbine engine of  FIG. 1  in accordance with an exemplary embodiment; 
         FIG. 3  is a more detailed cross-sectional view of a portion of the compressor of  FIG. 2  in accordance with an exemplary embodiment; and 
         FIG. 4  is a partial view of a portion of the compressor through line  4 - 4  of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Thus, any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. All of the embodiments described herein are exemplary embodiments provided to enable persons skilled in the art to make or use the invention and not to limit the scope of the invention which is defined by the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description. 
     Broadly, exemplary embodiments discussed herein provide casing treatments for compressor end walls. The casing treatments include a number of circumferential grooves that are connected via channels to enable fluid communication between the grooves. As such, secondary flows occurring at the blade tip may be removed by the casing treatment and injected in a location that does not impact mainstream air flow to thus improve the stall margin of the compressor. 
       FIG. 1  is a cross-sectional view of a gas turbine engine  100  in accordance with an exemplary embodiment. The gas turbine engine  100  includes a fan section  102 , a compressor section  104 , a combustion section  106 , a turbine section  108 , and an exhaust section  110 . The fan section  102  includes a fan  112  mounted in a fan case  114  that induces and accelerates ambient air into the compressor section  104 . 
     The compressor section  104  includes at least one compressor and, in the depicted embodiment, includes an intermediate pressure compressor  120  and a high pressure compressor  122  that raise the pressure of the air and directs it into the combustion section  106 . In the combustion section  106 , which includes an annular combustor  124 , the high pressure air is mixed with fuel and combusted. The combusted air is then directed into the turbine section  108 . The turbine section  108  includes a number of turbines disposed in axial flow series, including a high pressure turbine  126 , an intermediate pressure turbine  128 , and a low pressure turbine  130 . The combusted air from the combustion section  106  expands through each turbine  126 ,  128 ,  130 , causing them to rotate. As the turbines  126 ,  128 ,  130  rotate, each respectively drives equipment in the engine  100  via concentrically disposed shafts or spools  134 ,  136 ,  138 . The air is then exhausted through the exhaust section  110 . 
       FIG. 2  is a partial cross-sectional view of a compressor  200  that may be incorporated, for example, into the compressors  120 ,  122  discussed above in reference to  FIG. 1  or any type of compressors, including those in an auxiliary power unit (APU). In the depicted embodiment, the compressor  200  includes one or more rotor assemblies  202  that each include a number of rotor blades  210  (one of which is shown) mounted on platform  230 , which in turn, is coupled to a hub  234  mounted on a shaft (not shown). The blades  210  extend in a radial direction and are generally spaced apart from one another around the circumference of the hub  234 . Each rotor blade  210  includes a generally concave, pressure sidewall  212  and a circumferentially opposite, generally convex suction sidewall (not shown). The two sidewalls extend radially between a root  214  and an outer tip  216  and axially between a leading edge  218  and a trailing edge  220 . The blade  210  is typically solid and has a plain, generally flat tip  216 , although other configurations may be provided. 
     The compressor  200  further includes one or more stator assemblies  260  with stator vanes  262  (one of which is shown) mounted a platform  264 . A generally circumferentially arcuate casing  270  surrounds the rotor blades  210  and stator vanes  262  to at least partially define the compressor flow path with the platforms  230 ,  264 . The portion of casing  270  that is in closest proximity to the blade tip  216  is referred to as an end wall  272 . During operation, the rotor assembly  202  rotates, and the rotor blades  210  draw mainstream air  280  through the compressor  200 . As the mainstream air  280  flows axially downstream between the blades  210  and the stator vanes  262 , it is pressurized and directed through additional compressor or fan stages, as desired, for further compression. 
     As mentioned above, it is generally advantageous from an efficiency standpoint to minimize a gap  290  between the blade tip  216  of the rotor blade  210  and the end wall  272  of the casing  270  to avoid excess leakage of the mainstream air  280  across the tip  216  of the rotor blade  210 . However, conventional compressors may be subject to a stall condition as the gap  290  approaches zero. In such conditions, the air flow between the tips and the end wall may undergo secondary flows, including formation of vortices, that reduce flow momentum of the end wall boundary layer, thus reducing pressure and increasing the likelihood of an undesirable stall. Generally, engine designers determine the mass flow and pressure of operating conditions in which a stall may occur, as well as a stall margin that corresponds to the difference between normal operating conditions and stall operating conditions. As such, it is generally desirable to increase the stall margin either to enable increased normal operating conditions or to increase the margin of safety for existing conditions. 
     One mechanism for increasing the stall margin includes a number of circumferential grooves  301 - 305  on an inner surface of the end wall  272 . The grooves  301 - 305  are illustrated in greater detail in  FIG. 3 , which is a cross-sectional view of a portion  300  of the compressor  200  of  FIG. 2  in accordance with an exemplary embodiment. In particular,  FIG. 3  illustrates the tip  216  of the rotor blade  210  and the end wall  272  of the casing  270 . In the exemplary embodiment, the grooves  301 - 305  include a first groove  301 , a second groove  302 , a third groove  303 , a fourth groove  304 , and a fifth groove  305  extending sequentially in a downstream direction. Generally, the extent  310  of the grooves  301 - 305  corresponds to the length of the blade  210 , i.e., from the leading edge  218  to the trailing edge  220 , although in other embodiments, the extent  310  of the grooves  301 - 305  extend from upstream of the leading edge  218  to downstream of the trailing edge  220 . Each groove  301 - 305  may be defined by a length  311 , a depth  312  and a width (not shown in  FIG. 3 ). Although the depicted grooves  301 - 305  extend in a circumferential direction that is perpendicular to the flow of mainstream air  280 , the grooves  301 - 305  may also extend in a general circumferential direction that is not exactly perpendicular, such as in a helical arrangement. Each of the grooves  301 - 305  is separated from adjacent grooves  301 - 305  by a circumferential wall or rib  321 ,  322 ,  323 ,  324 . The grooves  301 - 305  are generally inverted U-shaped with a flat bottom, although other embodiments may have different shapes, including rounded bottoms or inverted V-shapes. 
     The end wall  272  further includes a channel system defined by one or more channels  331 - 334  that extend in an axial direction, i.e., in a direction parallel to the flow of mainstream air  280 , between the grooves  301 - 305 . In the cross-sectional view of end wall  272  of  FIG. 3 , one channel  331 - 334  extends between each pair of adjacent grooves  301 - 305 . For example, channel  331  extends between groove  301  and groove  302 . As illustrated in  FIG. 3 , the grooves  301 - 305  and channels  331 - 334  provide fluid communication between grooves  301 - 305  such that air flows  350  circulate into and through the grooves  301 - 305  and channels  331 - 334 , as discussed in greater detail below. The grooves  301 - 305  and channels  331 - 334  may be collectively referred to as an interconnected casing treatment. Such casing treatments may be machined directly into the end wall  272  or designed as a discrete insert into the end wall  272 . Although not shown, in some exemplary embodiments, the channels  331 - 335  extend to the end wall  272  to form a series of cross-hatched grooves. 
     As noted above, mainstream air  280  within the gap  290  may form secondary flows, such as vortices, that may otherwise disrupt the flow of mainstream air  280  between the tip  216  and end wall  272 . However, in the exemplary embodiment, any such occurrences may flow into and out of the grooves  301 - 305  to avoid disruptions. In particular, individual grooves  301 - 305  tend to reduce the impact of secondary flows in localized areas. For example, groove  301  tends to prevent secondary flow in the area immediately downstream of the leading edge  218 . Additionally, since the grooves  301 - 305  may communicate with one another via the channels  331 - 334 , the collection of connected grooves  301 - 305  tends to prevent secondary flows across the length  310  of the tip  216  to a greater extent than a collection of individual grooves. This may be due to the fact that the magnitudes of secondary flow vary along the tip  216 . In various areas, it may be advantageous to either inject or bleed air flows  350  from the mainstream air  280 . For example, in one exemplary embodiment, air flow  350  captured by groove  305  tends to flow upstream through the channels  331 - 334  to one or more of the upstream grooves  301 - 304  before being injected back into the mainstream air  280 . As such, air flow  350  may travel from the most downstream groove  305  to the most upstream groove  301  such that air from the trailing edge  320  may be circulated to the leading edge  218 . 
       FIG. 4  is another view of the grooves  301 - 305  and channels  331 - 334  from the perspective of line  4 - 4  in  FIG. 3 . As shown in  FIG. 4 , the channels  331 - 334  may further include additional channels that are circumferentially offset from the channels  331 - 334  shown in  FIG. 3 . In general, the channels  331 - 334  may be in any suitable configuration. For example,  FIGS. 3 and 4  depict channel  331  and channel  332  as being circumferentially aligned, although in other configurations, such channels may be circumferentially offset. Similarly, each channel  331 - 334  may have any suitable height  390  ( FIG. 3 ), width  391 , length  392 , and separation  393 . The channels  331 - 334  may start at any suitable depth  394  ( FIG. 3 ). Although the cross-sectional shape of the channels  331 - 334  are depicted as rectangular, any cross-sectional shape may be provided. The depicted channels  331 - 334  extend in an axial direction, although other configurations may be provided, including those other than axial. Similarly,  FIGS. 3 and 4  illustrate that the channels  331 - 334  and grooves  301 - 305  are perpendicular to each other, although other configurations may be provided. Design constraints may be determined, for example, with CFD analysis. In general, the particular dimensions and arrangement of the grooves  301 - 305  and channels  331 - 334  are selected in order to obtain the proper stall margin without adversely affecting fan performance, as will now be discussed. 
     The grooves  301 - 305  and channels  331 - 334  improve the stall margin by capturing secondary flows, efficiently removing any swirl component, and reintroducing the air back into the flow of mainstream air  280  at a point that results in minimal disturbance. Particularly, the channels  331 - 334  ensure that this is done in an efficient manner. Air with weak axial flow velocity tends to be captured by the grooves  301 - 305 , selectively recirculated and injected back into the mainstream air  280 , while air with strong axial flow velocity tends to remain within the gap  290  and is not captured as flow  350 . This tends to avoid recirculating air more than necessary to avoid a detrimental impact to efficiency. 
     Table 1, below, illustrates an improvement in stall margin for an exemplary casing with grooves and channels, such as that shown in  FIGS. 3 and 4 , relative to a casing with grooves but no channels. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Configuration 
                 Stall Margin 
               
               
                   
                   
               
             
             
               
                   
                 Grooves, no Channels 
                 0.33% 
               
               
                   
                 Grooves and Channels 
                 4.02% 
               
               
                   
                   
               
             
          
         
       
     
     As shown, the exemplary grooves and channels demonstrate a substantially improved margin without a decrease in efficiency. This enables a higher efficiency operation at higher pressures and minimized end wall gaps and/or enhanced safety. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.