Abstract:
An airfoil ( 101 ) is provided which includes a suction side and a pressure side joined along a trailing edge ( 106 ), wherein a trailing edge portion of the airfoil is configured to take a wavelike form along a radial direction of the airfoil, thereby improving the radial bending strength of the airfoil and reducing the magnitude of fluid flow wakes ( 128′ ) formed in a working fluid flowing over the airfoil.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to airfoils. More specifically, this invention relates to a modified external surface of an airfoil, to enhance a resistance to bending of the airfoil and to reduce instances of downstream wakes generated by the airfoil while being used in an aerodynamic system. 
       BACKGROUND OF THE INVENTION 
       [0002]    The design of the trailing edge of an airfoil is preferably dictated by aerodynamic considerations. For improved aerodynamic performance, it is commonly preferred to provide a thin trailing edge in an airfoil used in an aerodynamic system, such as a gas turbine, for example. However, the thinness of the trailing edge may result in physical weakness, and such structural limitations often limit the trailing edge design and necessitate the use of a design that is less than optimal from an aerodynamic perspective. 
         [0003]      FIG. 1  illustrates a known arrangement for an airfoil  10  which may be oriented in a radial direction  23  (perpendicular to the plane of  FIG. 1 ) in a gas turbine engine (not shown). A cord line  12  extends in an axial direction through the airfoil  10 , from a leading edge  14  to a trailing edge  16  of the airfoil  10 . Based on an angle of attack  18  between the incident fluid flow and the cord line  12 , high pressure working fluid is configured to travel over a pressure side  20  of the airfoil  10 , while lower pressure fluid is configured to travel over a suction side  22  of the airfoil  10 , thereby generating a lifting force or stress on the airfoil  10 , including on the trailing edge  16 . As illustrated in  FIG. 1 , the conventional airfoil  10  features a trailing edge  16  having a noticeable thickness  21  (somewhat exaggerated in the figure for illustration purposes) such that the high pressure fluid from the pressure side  20  and low pressure fluid from the suction side  22  merge over this noticeable thickness  21  and generate planar wakes  28  incident from the trailing edge  16  The wakes  28  move with the working fluid to impact a subsequent airfoil (not illustrated) of the gas turbine engine. The intensity and/or instance of these planar wakes  28  may reduce the aerodynamic performance of the engine and/or result in undesirable mechanical effects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention is explained in the following description in view of the drawings that show: 
           [0005]      FIG. 1  is a cross-section view of a prior art gas turbine engine airfoil; 
           [0006]      FIG. 2  is a partial top view of an embodiment of an improved airfoil; 
           [0007]      FIG. 3  is a cross-sectional view of the improved airfoil taken along the line  3 - 3  in  FIG. 2 ; 
           [0008]      FIG. 4  is an end view of the improved airfoil taken along the line  4 - 4  in  FIG. 2 ; 
           [0009]      FIG. 5  is a cross-sectional view of the improved airfoil taken along the line  5 - 5  in  FIG. 2 ; 
           [0010]      FIG. 6  is a cross-sectional view of the improved airfoil taken along the line  6 - 6  in  FIG. 2 ; 
           [0011]      FIG. 7  is an end view of an alternate embodiment of the improved airfoil; and 
           [0012]      FIG. 8  is a cross-section view of the improved airfoil illustrated in  FIG. 2  used within a gas turbine engine showing a downstream airfoil. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    In order to address the shortcomings of the conventional airfoil addressed above, the present inventor has developed an improved airfoil including a modified trailing edge, where the thickness of the trailing edge is minimized such that the aerodynamic performance of the airfoil is maximized, while the trailing edge retains a capability of resisting axial stress imposed during a typical operation of the airfoil within a gas turbine engine. Additionally, the modified trailing edge of the improved airfoil reduces the intensity of planar wakes incident from the trailing edge during use of the airfoil in a working fluid flow steam. Hence, the aerodynamic performance/efficiency of a gas turbine incorporating the improved airfoil is improved. Although some embodiments of the present invention discuss an airfoil used within a gas turbine engine, the present invention is not limited to airfoils used within gas turbines, and may be applied to any airfoil used in any aerodynamic application during which stress/force is imposed on the airfoil in a direction perpendicular to the radial orientation of the airfoil and/or in any aerodynamic application during which planar wakes are created as the high pressure fluid and low pressure fluid merge at the trailing edge of the airfoil. 
         [0014]      FIG. 2  illustrates an exemplary embodiment of a system  100  including an airfoil  101 , such as a stationary airfoil (vane) used within a gas turbine engine. The airfoil  101  has a longitudinal axis oriented in a radial direction  110  and includes a leading edge  107  and a trailing edge  106  which are separated by a cord length  116 . A trailing edge portion  108  of the airfoil  101  extends from the trailing edge  106  toward the leading edge  107  by a distance  124  which is a subset of the cord length  116 . 
         [0015]      FIG. 3  (taken along line  3 - 3  of  FIG. 2 ) illustrates that a suction side  102  and a pressure side  104  of the airfoil  101  are joined along the leading edge  107  and trailing edge  106 .  FIG. 3  also illustrates that the trailing edge portion  108  of the airfoil  101  thickness varies from a thickness  118  at the trailing edge  106  to a greater thickness  126  at a predetermined distance  124  from the trailing edge  106 . As discussed in the following description, the trailing edge portion  108  takes a wavelike form along the radial direction  110  of the airfoil  101 . 
         [0016]      FIG. 4  (taken along line  4 - 4  of  FIG. 2 ),  FIG. 5  (taken along line  5 - 5  of  FIG. 2 ) and  FIG. 6  (taken along line  6 - 6  of  FIG. 2 ) illustrates the wavelike form of the trailing edge portion  108  at respective distances from the trailing edge  106  along the cord length  116 . As discussed below, an amplitude  112 , 113  of the wavelike form and the thickness  118 , 119 , 126  of the trailing edge portion  108  varies based on the respective distance from the trailing edge  106  along a cord length  116  of the airfoil  101 . 
         [0017]      FIG. 4  illustrates the wavelike form of the trailing edge portion  108  at the trailing edge  106  may take a sine wave form, for example. However, the wavelike form may take any known wave form, such as the illustrated sine-wavelike form configuration or a non-sine wavelike form, for example. The amplitude  112  (centerline peak to peak distance) of the wavelike form at the trailing edge  106  may be in a range of 0.2-2 times the thickness  118  of the trailing edge  106  in some embodiment, although is not necessarily so limited. In an exemplary embodiment, the amplitude  112  of the wavelike form at the trailing edge  106  is approximately equal (i.e. within normal manufacturing tolerances for the material of construction of the airfoil) to the thickness  118  of the trailing edge  106 . Although  FIG. 4  illustrates only a portion of the trailing edge  106  extending in the radial direction  110 , the entire trailing edge  106  extending in the radial direction  110  may have the wavelike form. However, the entire trailing edge  106  need not take the wavelike form. Additionally, as illustrated in  FIG. 4 , a wavelength  120  (peak to peak) of the wavelike form along the trailing edge  106  may be in a range of 2-4 times the thickness  118  of the trailing edge  106 . However, the wavelength  120  of the wavelike form at the trailing edge  106  need not be within any particular multiple range of the thickness  118  of the trailing edge  106  and may vary along the radial direction  110 . 
         [0018]      FIG. 5  illustrates a cross-sectional view at a distance  114  from the trailing edge  106  along the cord length  116 . The amplitude  113  of the wavelike form of the trailing edge portion  108  at the distance  114  is less than the amplitude  112  of the wavelike form of the trailing edge portion  108  at the trailing edge  106  ( FIG. 4 ). The amplitude  113  may be equal to or greater than the amplitude  112 , however, for the embodiments currently envisioned for gas turbine applications, the amplitude of the wavelike form will gradually reduce to zero along a distance  124  defining the trailing edge portion  108 . Additionally, the thickness  119  of the trailing edge portion  108  at the distance  114  from the trailing edge  106  along the cord length  116  is greater than the thickness  118  of the trailing edge  106 . In an exemplary embodiment, the trailing edge portion  108  may be designed such that the amplitude of the wavelike form at a respective distance from the trailing edge  106  along the cord length  116  is inversely proportional to the thickness of the trailing edge portion  108  at the respective distance and/or to the respective distance itself. 
         [0019]      FIG. 6  illustrates a cross-sectional view at an outermost boundary of the trailing edge portion  108  from the trailing edge  106 , at the predetermined distance  124  from the trailing edge  106  along the cord length  116 . As illustrated in  FIGS. 4-6 , the amplitude  112 ,  113  of the wavelike form along the trailing edge portion  108  decays from a maximum amplitude  112  at the trailing edge  106  to the amplitude  113  at the distance  114  from the trailing edge  106  along the cord length  116 , and to zero at the predetermined distance  124  long the cord length  116 . Although  FIGS. 4-6  illustrate discrete amplitudes of the wavelike form along the trailing edge portion  108  at respective distances from the trailing edge  106 , the amplitude actually continuously varies as the distance from the trailing edge  106  along the cord length  116  increases. Additionally, as illustrated in  FIG. 6 , the thickness  126  of the airfoil  101  at the predetermined distance  124  from the trailing edge  106  may be in a range of 2-3 times the thickness  118  of the trailing edge  106 , although it is not necessarily so limited. 
         [0020]      FIG. 7  illustrates an alternate embodiment of the present invention, in which the wavelike form of the trailing edge portion may take a non-sinusoidal waveform, such as an imperfect (tapered) square wave, in which the waveform oscillates between alternating flat levels  132 ,  134 , having different relative elevations joined by respective sloped sections  136  joining the two levels. The amplitude  138  (perpendicular distance between the two levels  132 , 134 ) of the wavelike form at the trailing edge  130  may be in a range of 0.2-2 times the thickness  140  of the trailing edge  130  in some embodiments, although is not necessarily so limited. In an exemplary embodiment, the amplitude  138  of the wavelike form at the trailing edge  130  is approximately equal to the thickness  140  of the trailing edge  130 . Although  FIG. 7  illustrates only a portion of the trailing edge  130  extending in the radial direction, the entire trailing edge  130  extending in the radial direction may have the wavelike form, or the entire trailing edge  130  need not take the wavelike form. Additionally, a duty cycle, or ratio of the length  142  of the first level  132  to the length  144  of the second level  134 , may be different in various embodiments to achieve optimal aerodynamic performance of the trailing edge  130 . 
         [0021]      FIG. 8  illustrates a system  100 ′ including a gas turbine  103 ′ which has a stationary airfoil  101 ′ (vane) and a rotating airfoil  105 ′ (blade) positioned downstream from the stationary airfoil  101 ′. The stationary airfoil  101 ′ has properties similar to the stationary airfoil  101  discussed above in the embodiments of  FIGS. 2-6 , including the trailing edge portion  108 ′ and other elements similar to the elements of the stationary airfoil  101  discussed above and numbered with prime notation. Although  FIG. 8  illustrates a gas turbine  103 ′ with one stationary airfoil  101 ′ and one rotating airfoil  105 ′, one skilled in the art will realize that these airfoils are only a portion of respective rows of airfoils and that multiple such vane/blade rows may be used in the gas turbine engine  103 ′. 
         [0022]    The wavelike form of the trailing edge portion  108 ′ provides several performance advantages during operation of the gas turbine  103 ′. Since the wavelike form of the trailing edge portion  108 ′ necessarily displaces material of the trailing edge portion  108 ′ away from the radial axis (see  FIGS. 4-5 ), the moment of inertia of the trailing edge portion  108 ′ about the a radial axis  110  increases as compared to a conventional non-wavy trailing edge portion. Differences in pressure across a typical airfoil result in lateral bending forces on the trailing edge  106 ′, which are more effectively resisted by the wavelike form of the trailing edge portion  108 ′. As previously discussed, the trailing edge  106 ′ resistance to these lateral bending forces may be a limiting factor in minimizing the thickness of the trailing edge  106 ′, and thus a limiting factor to enhancing aerodynamic efficiency. Thus, the wavelike form of the trailing edge portion  108 ′ permits the thickness  118 ′ of the trailing edge  106 ′ to be minimized beyond that of a conventional trailing edge utilizing the same materials, and thereby achieves improved aerodynamic advantages. Such aerodynamic advantages include the ability of the trailing edge  106 ′ of the stationary airfoil  101 ′ to adequately mix the pressurized air from the suction side  102 ′ and the pressure side  104 ′, and to control the direction of air flow incident from the trailing edge  106 ′ to the rotating airfoil  105 ′. The stationary airfoil  101 ′ reduces instances and intensity of planar wakes  128 ′ incident from the stationary airfoil  101 ′ on the rotating airfoil  105 ′ of the gas turbine  103 ′ when compared to a thicker trailing edge of equivalent strength in a prior art non-wavy trailing edge. Such advantages are useful for airfoils of any material of construction, but are particularly useful for airfoils or airfoil trailing edge portions made of ceramic or ceramic matrix composite (CMC) materials where traditionally thicker trailing edge designs have been required than with traditional metal airfoil embodiments. 
         [0023]    An additional performance advantage of the wavelike form of the trailing edge portion  108 ′ is an increased radial compliance of the trailing edge  106 ′ to thermal growth during the operation of the airfoil  101 ′. A ceramic or CMC airfoil is routinely subjected to rapid thermal transients, and because the trailing edge is thinner, it responds faster to these transients than the remainder of the airfoil. The growth of the trailing edge is constrained by the bulkier part of the airfoil, thus concentrating stresses in the trailing edge portion of the airfoil. The increased radial compliance of the trailing edge  106 ′ when compared to a prior art trailing edge provides some compliance in the radial direction and alleviates such thermal stresses in the trailing edge portion  108 ′. 
         [0024]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.