Abstract:
A method of designing an airfoil for a gas turbine engine according to one embodiment of this disclosure can include localizing a sweep angle at a leading edge of a tip region of the airfoil, and localizing a dihedral angle at the tip region of the airfoil. The dihedral angle can be applied by translating the airfoil in direction normal to a chord of the airfoil.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 12/336,610, filed Dec. 17, 2008 now U.S. Pat. No. 8,167,567. 
    
    
     BACKGROUND 
     This disclosure generally relates to a gas turbine engine, and more particularly to rotor blades that improve gas turbine engine performance. 
     Gas turbine engines, such as turbofan gas turbine engines, typically include a fan section, a compressor section, a combustor section and a turbine section. During operation, air is pressurized in the compressor section and mixed with fuel in the combustor section for generating hot combustion gases. The hot combustion gases flow through the turbine section which extracts energy from the hot combustion gases to power the compressor section and drive the fan section. 
     Many gas turbine engines include axial-flow type compressor sections in which the flow of compressed air is parallel to the engine centerline axis. Axial-flow compressors utilize multiple stages to obtain the pressure levels needed to achieve desired thermodynamic cycle goals. A typical compressor stage consists of a row of moving airfoils (called rotor blades) and a row of stationary airfoils (called stator vanes). The flow path of the axial-flow compressor section decreases in cross-sectional area in the direction of flow to reduce the volume of air as compression progresses through the compressor section. That is, each subsequent stage of the axial flow compressor decreases in size to maximize the performance of the compressor section. 
     One design feature of an axial-flow compressor section that may affect compressor performance is tip clearance flow. A small gap extends between the tip of each rotor blade and a surrounding shroud in each compressor stage. Tip clearance flow is defined as the amount of airflow that escapes between the tip of the rotor blade and the adjacent shroud. Tip clearance flow reduces the ability of the compressor section to sustain pressure rise and may have a negative impact on stall margin (i.e., the point at which the compressor section can no longer sustain an increase in pressure such that the gas turbine engine stalls). 
     Airflow escaping through the gaps between the rotor blades and the shroud can create gas turbine engine performance losses. In the middle and rear stages of the compressor section, blade performance and operability of the gas turbine engine are highly sensitive to the lower spans (i.e., decreased size) of the rotor blades and the corresponding high clearance to span ratios. Disadvantageously, prior rotor blade airfoil designs have not adequately alleviated the negative effects caused by tip clearance flow. 
     SUMMARY 
     A method of designing an airfoil for a gas turbine engine according to one embodiment of this disclosure can include localizing a sweep angle at a leading edge of a tip region of the airfoil, and localizing a dihedral angle at the tip region of the airfoil. The dihedral angle can be applied by translating the airfoil in direction normal to a chord of the airfoil. 
     In a further embodiment of the foregoing method embodiment, the sweep angle can include a forward sweep angle. 
     In a further embodiment of either of the foregoing method embodiments, the step of localizing the sweep angle can include displacing a plurality of airfoil sections of the airfoil parallel to the chord relative to a base-line rotor blade design. 
     In a further embodiment of any of the foregoing method embodiments, the dihedral angle can include a positive dihedral angle. 
     In a further embodiment of any of the foregoing method embodiments, the step of localizing the dihedral angle can include displacing a plurality of airfoil sections of the airfoil tangentially to the chord relative to a base-line rotor blade design. 
     In a further embodiment of any of the foregoing method embodiments, the sweep angle and the dihedral angle can be extended over a distance of the airfoil equivalent to about 10% to about 40% of a span of the airfoil. 
     In a further embodiment of any of the foregoing method embodiments, the sweep angle and the dihedral angle can be extended from an outer edge of the tip region radially inward along a radial axis over a distance equal to about 10% to about 40% of the span. 
     The various features and advantages of this disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an example gas turbine engine; 
         FIG. 2  illustrates a portion of a compressor section of the example gas turbine engine illustrated in  FIG. 1 ; 
         FIG. 3  illustrates a schematic view of a rotor blade according to the present disclosure; 
         FIG. 4  illustrates another view of the example rotor blade illustrated in  FIG. 3 ; 
         FIG. 5  illustrates an airfoil designed having a sweep angle S and a dihedral angle D; 
         FIG. 6  illustrates a sectional view through section  6 - 6  of  FIG. 5 ; 
         FIG. 7  illustrates yet another view of the example rotor blade having a redesigned tip region merged relative to a base-line design of the rotor blade; and 
         FIG. 8  illustrates another view of the rotor blade illustrated in  FIG. 5  as viewed from a leading edge of the rotor blade. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example gas turbine engine  10  that includes a fan  12 , a compressor section  14 , a combustor section  16  and a turbine section  18 . The gas turbine engine  10  is defined about an engine centerline axis A about which the various engine sections rotate. As is known, air is drawn into the gas turbine engine  10  by the fan  12  and flows through the compressor section  14  to pressurize the airflow. Fuel is mixed with the pressurized air and combusted within the combustor  16 . The combustion gases are discharged through the turbine section  18  which extracts energy therefrom for powering the compressor section  14  and the fan  12 . Of course, this view is highly schematic. In one example, the gas turbine engine  10  is a turbofan gas turbine engine. It should be understood, however, that the features and illustrations presented within this disclosure are not limited to a turbofan gas turbine engine. That is, the present disclosure is applicable to any engine architecture. 
       FIG. 2  schematically illustrates a portion of the compressor section  14  of the gas turbine engine  10 . In one example, the compressor section  14  is an axial-flow compressor. Compressor section  14  includes a plurality of compression stages including alternating rows of rotor blades  30  and stator blades  32 . The rotor blades  30  rotate about the engine centerline axis A in a known manner to increase the velocity and pressure level of the airflow communicated through the compressor section  14 . The stationary stator blades  32  convert the velocity of the airflow into pressure, and turn the airflow in a desired direction to prepare the airflow for the next set of rotor blades  30 . The rotor blades  30  are partially housed by a shroud assembly  34  (i.e., outer case). A gap  36  extends between a tip region  38  of each rotor blade  30  to provide clearance for the rotating rotor blades  30 . 
       FIGS. 3 and 4  illustrate an example rotor blade  30  that includes unique design elements localized at tip region  38  for reducing the detrimental effect of tip clearance flow. Tip clearance flow is defined as the amount of airflow that escapes through the gap  36  between the tip region  38  of the rotor blade  30  and the shroud assembly  34 . The rotor blade  30  includes an airfoil  40  having a leading edge  42  and a trailing edge  44 . A chord  46  of the airfoil  40  extends between the leading edge  42  and the trailing edge  44 . A span  48  of the airfoil  40  extends between a root  50  and the tip region  38  of the rotor blade  30 . The root  50  of the rotor blade  30  is adjacent to a platform  52  that connects the rotor blade  30  to a rotating drum or disk (not shown) in a known manner. 
     The airfoil  40  of the rotor blade  30  also includes a suction surface  54  and an opposite pressure surface  56 . The suction surface  54  is a generally convex surface and the pressure surface  56  is a generally concave surface. The suction surface  54  and the pressure surface  56  are designed conventionally to pressurize the airflow as airflow F is communicated from an upstream direction U to a downstream direction DN. The airflow F flows in an axial direction X that is parallel to the longitudinal centerline axis A of the gas turbine engine A. The rotor blade  30  rotates in a rotational direction (circumferential) Y about the engine centerline axis A. The span  48  of the airfoil  40  is positioned along a radial axis Z of the rotor blade  30 . 
     The example rotor blade  30  includes a sweep angle S (See  FIG. 3 ) and a dihedral angle D (See  FIG. 4 ) that are each localized relative to the tip region  38  of the rotor blade  30 . The term “localized” as utilized in this disclosure is intended to define the sweep angle S and the dihedral angle D at a specific portion of the airfoil  40 , as is further discussed below. Although the sweep angle S and the dihedral angle D are disclosed herein with respect to a rotor blade, it should be understood that other components of the gas turbine engine  10  may benefit from similar aerodynamic improvements as those illustrated with respect to the rotor blade  30 . 
     Referring to  FIG. 5 , the sweep angle S, at a given radial location, is defined as the angle between the velocity vector V of incoming flow relative to the airfoil  40  and a line tangent to the leading edge  42  of the airfoil  40 . In one example, the sweep angle S is a forward sweep angle. Forward sweep usually involves translating an airfoil section at a higher radius forward (opposite to incoming airflow) along the direction of the chord  46 . 
     As illustrated in  FIGS. 4 ,  5  and  6 , the dihedral angle D is defined as the angle between the shroud assembly  34  and the airfoil  40 . In this example, the dihedral in the tip region  38  of the airfoil  40  is controlled by translating the airfoil  40  in a direction perpendicular to the chord  46 . A measure of the dihedral angle D is performed at the center of gravity C of the airfoil  40 . In one example, the dihedral angle D is a positive dihedral angle. Positive dihedral increases the angle between the suction surface  54  of the airfoil  40  and an interior surface  58  of the shroud assembly  34 . That is, positive dihedral angle results in the suction surface  54  pointing down relative to the shroud assembly  34 . In another example, the suction surface  54  forms an acute dihedral angle D relative to the shroud assembly  34 . 
     The amount of sweep S and dihedral D included on the rotor blade  30  is defined at the tip region  38  of the rotor blade  30  and merged back to a baseline geometry (see  FIGS. 7 and 8 ). In one example, the sweep angle S and the dihedral angle D extend over a distance of the airfoil  40  that is equivalent to about 10% to about 40% of the span  48  of the rotor blade  30 . That is, the sweep S and dihedral D are positioned at a distance from an outer edge  39  of the tip region  38  radially inward along radial axis Z by about 10% to about 40% of the total span  48  of the airfoil  40 . The term “about” as utilized in this disclosure is defined to include general variations in tolerances as would be understood by a person of ordinary skill in the art having the benefit of this disclosure. 
       FIGS. 7 and 8  illustrate the example rotor blade  30  superimposed over a base-line design rotor blade (shown in shaded portions). The base-line design rotor blade represents a blade having sweep and dihedral as a result of stacking airfoil sections in a conventional way. A conventional stacking is such that the center of gravity of airfoil sections are close to being radial with offset as a result of minimizing stress caused by centrifugal force acting on the airfoil when the rotor is rotating. In the illustrated example, a plurality of airfoil sections  60  of the rotor blade are tangentially and axially restacked relative to the base-line design rotor blade to provide tip region  38  localized forward sweep S and positive dihedral D, for example. The amount of sweep S and dihedral D and the corresponding tangential and axial offsets are defined at the tip region  38  and merged back to the base-line design rotor blade over a distance equivalent to about 10% to about 40% of the span  48  of the rotor blade  30 , in one example. 
     Providing localized sweep S and dihedral D at the tip region  38  of the rotor blade  30  results in airflow being pulled toward the tip region  38  relative to a conventional rotor blade without the sweep and dihedral described above. This reduces the diffusion rate of local flow, which tends to have a lower axial component and is prone to flow reversal. Simulation using Computational Fluid Dynamics (CFD) analysis demonstrates that an airfoil with local sweep and dihedral reduces the entropy generated by the tip clearance flow. At the same time, tip clearance flow through the gaps  36  is reduced. Therefore, the radial distributions of blade exit velocity and stagnation pressure are improved, thus maintaining higher momentum in the region of the tip region  38 . The negative effects of stall margin are minimized and gas turbine engine performance and efficiency are improved. 
     The foregoing description shall be interpreted as illustrative and not in any limiting sense. A person of ordinary skill in the art would understand that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope and content of the disclosure.