Patent Publication Number: US-7217093-B2

Title: Rotor blade with a stick damper

Description:
The invention was made under a U.S. Government contract and the Government has rights herein. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   This invention applies to rotor blades in general, and to apparatus for damping vibration within a rotor blade in particular. 
   2. Background Information 
   Turbine and compressor sections within an axial flow turbine engine generally include a rotor assembly comprising a rotating disc and a plurality of rotor blades circumferentially disposed around the disk. Each rotor blade includes a root, an airfoil, and a platform positioned in the transition area between the root and the airfoil. The roots of the blades are received in complementary shaped recesses within the disk. The platforms of the blades extend laterally outward and collectively form a flow path for fluid passing through the rotor stage. The forward edge of each blade is generally referred to as the leading edge and the aft edge as the trailing edge. Forward is defined as being upstream of aft in the gas flow through the engine. 
   During operation, blades may be excited into vibration by a number of different forcing functions. Variations in gas temperature, pressure, and/or density, for example, can excite vibrations throughout the rotor assembly, especially within the blade airfoils. Gas exiting upstream turbine and/or compressor sections in a periodic, or “pulsating”, manner can also excite undesirable vibrations. Left unchecked, vibration can cause blades to fatigue prematurely and consequently decrease the life cycle of the blades. 
   It is known that friction between a damper and a blade may be used as a means to damp vibrational motion of a blade. How much vibrational motion may be damped depends upon the magnitude of the frictional force between two surfaces. The frictional force is a function of the amount of surface area in contact between the two surfaces, the frictional coefficients of the two surfaces, and the normal force keeping the surfaces in contact with each other. If the spring rate of the damper (i.e., the normal force) decreases because of fatigue in the spring and/or the thermal environment, the amount of vibrational motion that may be damped similarly decreases. If the surface against which the damper acts decreases in area or wears away from the damper, the effectiveness of the damper is also negatively effected. 
   In addition to the damping requirements, dampers must also be able to perform and last in a very high temperature environment. In some applications it is possible to cool the damper to enhance its durability within the high-temperature environment For example, it is known to cool a stick damper by disposing cooling holes along the radially extending length of the damper. It is also known to dispose slots within the contact surfaces of a damper spaced along the entire length of the damper. Features that enhance heat transfer such as cooling apertures and slots create stress concentration factors (“KT”) that negatively affect the durability of the damper. 
   In short, what is needed is a rotor blade having a vibration damping device that is effective in damping vibrations within the blade, one that can be effectively cooled, and one that provides desirable durability. 
   DISCLOSURE OF THE INVENTION 
   According to the present invention, a rotor blade damper is provided. The damper includes a body having a base, a tip, a first contact surface, a second contact surface, a trailing edge surface, and a leading edge surface. The trailing edge and the leading edge surfaces extend between the contact surfaces. The first contact surface, second contact surface, trailing edge surface, and leading edge surface all extend lengthwise between the base and the tip. The body includes at least one cooling aperture disposed adjacent the base, that has a diameter that is approximately equal to or greater than the width of the trailing edge surface adjacent the tip. The body tapers between the base and the tip such that a first widthwise cross-sectional area adjacent the base is greater than a second widthwise cross-sectional area adjacent the tip. 
   According to an aspect of the present invention, a rotor blade is provided having a passage, and the above-described rotor blade damper is disposed within the damper. 
   According to an embodiment of the present invention, the body includes at least one cooling channel disposed in each contact surface adjacent the tip. 
   An advantage of the present invention is that the present invention damper permits the rotor blade to have a desirable narrow thickness adjacent the tip of the blade. The present damper is tapered, decreasing in cross-sectional area between the base and the tip. The tip end of the damper is sized so that it may be disposed within a narrow tip region of a rotor blade. The thickness of many prior art dampers prohibits the use of a damper within a rotor blade having a narrow tip region. Durability requirements required prior art damper designs to be relatively “thick” at the tip end. Durability is a function of the thermal environment and stress to which the damper is exposed. The present invention provides enhanced cooling and decreased stress relative to prior art dampers of which we are aware. As a result, it is possible to use a damper having a narrow tip, within a rotor blade having a narrow thickness adjacent the tip. 
   The effectiveness of the present tapered damper is a result of the stiff, larger cross-sectional area base and the smaller cross-sectional area tip. The stiff base provides desirable frictional contact under load, while the relatively narrow tip permits greater centrifugal loading between the damper and the blade in a blade area subject to high cycle fatigue. 
   The tapered body of the damper is subjected to less stress than would be a damper having a body with a constant cross-section. The taper reduces the mass of the damper increasingly in the direction from the base to the tip. Consequently, stress that is attributable to mass located at the radial end of the damper (i.e., the tip) is reduced. 
   The tapered body of damper also facilitates cooling of the damper and adjacent airfoil along the length of the damper without substantially affecting the ability of the damper to provide the desired damping. The greater widthwise cross-sectional area adjacent the base end of the damper permits cooling apertures disposed within the damper extending between the leading edge and trailing edge surfaces of the damper. The diameter of the cooling holes is large enough to accommodate most debris encountered within the turbine blade, and thereby prevent blockage. The cooling channels disposed adjacent the second end of the body permit cooling of the second end of the damper. 
   The prior art teaches that cooling channels may be disclosed within the contact surfaces, spaced apart along the length of the damper. In an embodiment of the present invention, cooling channels are disposed within the contact surfaces of the damper adjacent the tip and cooling apertures are disposed within the damper adjacent the base. The cooling apertures disposed within the base region create a stress concentration factor (KT) within the base that is less than the stress concentration factor (KT) typically associated with cooling channels disposed within the contact surfaces of a damper. Consequently, the amount of low cycle fatigue experienced by the damper within the base region is less than that which would be present if cooling channels were used in place of the cooling apertures. 
   The cooling channels disposed within the contact surfaces of the damper adjacent the tip, provide cooling in a region of the damper where it is not possible to utilize cooling apertures having a diameter the same as or larger than the diameter of the cooling apertures disposed within the base. The diameter of the cooling apertures within the base are approximately equal to or greater than the width of the trailing edge surface adjacent the tip. Consequently, a cooling aperture of the same diameter disposed adjacent the tip would either break through the contact surfaces of the damper, or would leave an unacceptable wall thickness adjacent the trailing edge surface between the aperture and each contact surface. A smaller diameter cooling aperture would be more susceptible to blockage by debris traveling within the cooling air. 
   These and other objects, features and advantages of the present invention will become apparent in light of the detailed description of the best mode embodiment thereof, as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial perspective view of a rotor assembly. 
       FIG. 2  is a cross-sectional view of a rotor blade. 
       FIG. 3  is a diagrammatic cross-sectional view of a rotor blade section. 
       FIG. 4  is a diagrammatic cross-sectional view of a rotor blade section. 
       FIG. 5  is a diagrammatic perspective view of an embodiment of the present damper. 
       FIG. 6  is a diagrammatic perspective partial view of an embodiment of the present damper. 
       FIG. 7  is a diagrammatic planar view of a damper having wavy contact surfaces. 
       FIG. 8  is a diagrammatic cross-sectioned damper. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   Referring to  FIG. 1 , a rotor blade assembly  10  for a gas turbine engine is provided having a disk  12  and a plurality of rotor blades  14 . The disk  12  includes a plurality of recesses  16  circumferentially disposed around the disk  12  and a rotational centerline  18  about which the disk  12  may rotate. Each blade  14  includes a root  20 , an airfoil  22 , a platform  24 , and a damper  26  (see  FIG. 2 ). Each blade  14  also includes a radial centerline  28  passing through the blade  14 , perpendicular to the rotational centerline  18  of the disk  12 . The root  20  includes a geometry (e.g., a fir tree configuration) that mates with that of one of the recesses  16  within the disk  12 . The root  20  further includes conduits  30  through which cooling air may enter the root  20  and pass through into the airfoil  22 . 
   Referring to  FIGS. 2 and 3 , the airfoil  22  includes a base  32 , a tip  34 , a leading edge  36 , a trailing edge  38 , a first cavity  40 , a second cavity  42 , and a passage  44  between the first and second cavities  40 ,  42 . The airfoil  22  tapers inward from the base  32  to the tip  34 ; i.e., the length of a chord drawn at the base  32  is greater than the length of a chord drawn at the tip  34 . The first cavity  40  is forward of the second cavity  42  and the second cavity  42  is adjacent the trailing edge  38 . The airfoil  22  may include more than two cavities, however. The second cavity  42  contains a plurality of apertures  46  disposed along the trailing edge  38  through which cooling air may pass. In the embodiment shown in  FIG. 4 , the first and second cavities  40 ,  42  are formed from a single cavity by the damper  48  disposed therebetween. 
   The passage  44  between the first and second cavities  40 ,  42  comprises a pair of walls  50  extending substantially from base  32  to tip  34 . One or both walls  50  converge toward the other wall in the direction from the first cavity  40  to the second cavity  42 . The centerline  52  of passage  44  is skewed from the radial centerline  28  of the blade  14  by an angle α, such that the tip end of the passage  44  is closer to the radial centerline  28  than the base end of the passage  44 . A plurality of tabs  54  may be included in the first cavity  40 , adjacent the passage  44 , to maintain the damper  48  within the passage  44 . In the embodiment shown in  FIG. 2 , an aperture  56  disposed in the platform  24  enables the damper  48  to be inserted into the passage  44 . 
   Referring to  FIGS. 5 and 6 , the damper  48  includes a body  58  having a base  60 , a tip  62 , a first contact surface  64 , a second contact surface  66 , a trailing edge surface  68 , and a leading edge surface  70 . The trailing edge and the leading edge surfaces  68 , 70  extend between the contact surfaces  64 ,  66 . The first and second contact surfaces  64 ,  66 , the trailing edge surface  68 , and the leading edge surface  70  all extend lengthwise between the base  60  and the tip  62 . The contact surfaces  64 ,  66  may be smooth or textured. In some embodiments, the width of the body  58  at the trailing edge surface  68  is less than the width of the body at the leading edge surface  70 . In those embodiments, the body may be described as tapered between the trailing edge surface  68  and the leading edge surface  70 . The body  58  may assume different cross-sectional shapes.  FIGS. 3 and 4  show a damper  48  having a substantially trapezoidal shape.  FIGS. 5 and 6  show a damper  48  having a trapezoidal shape with a relief  72  at each edge. In alternative embodiments, the trailing edge-surface  68  may be arcuately shaped. 
   The body  58  tapers between the base  60  and the tip  62  such that a first widthwise cross-sectional area adjacent the base  60  is greater than a second widthwise cross-sectional area adjacent the tip  62 ; i.e., the body  58  decreases in cross-sectional area between the base  60  and the tip  62 , in the direction from the base  60  to the tip  62 .  FIG. 6  shows an example of a plane  73  in phantom. A sectional cut of the body  58  within that plane  73  would be a widthwise cross-section. In the embodiment shown in  FIGS. 5 and 6 , the taper is substantially linear. Alternative embodiments may have a non-linear taper. 
   Referring to  FIG. 8 , the width of trailing edge surface  68  is defined as the shortest distance along a line  74  extending between a first plane  76  in which the first contact surface  64  is substantially disposed, and a second plane  78  in which the second contact surface  66  is substantially disposed. The line  74  is in contact with the trailing edge surface  68 . The sectioned damper diagrammatically shown in  FIG. 8  has a symmetrical trapezoidal type cross-sectional shape. The line  74  extends between the lines representing the first and second planes  76 ,  78 . The angles between the line  74  and each plane  76 ,  78  are substantially equal. The width of the leading edge surface  70  may be defined similarly, with the exception that the line  74  would be contact with the leading edge surface  70 . 
   Referring to  FIGS. 5 and 6 , one or more cooling apertures  82  are disposed in the body  58  adjacent the base  60 . The cooling apertures  82  have a diameter that is substantially equal to or greater than the width of the trailing edge surface  68  adjacent the tip  62 . In some embodiments, the cooling apertures  82  are uniform in diameter. In other embodiments, there is a plurality of different diameter cooling apertures  82 . The cooling apertures  82  extend between the leading edge surface  70  and the trailing edge surface  68 , thereby enabling passage of cooling air through the damper  48  between the contact surfaces  64 ,  66 . 
   In some embodiments, the damper  48  further includes a plurality of cooling channels  84  disposed in each contact surface  64 ,  66  adjacent the tip  62  of the damper  48 . The cooling channels  84  extend in a direction approximately perpendicular to the lengthwise centerline  80  of the damper  48 .  FIG. 6  shows the cooling channels  84  disposed within the first contact plane  64  offset from the cooling channels  84  disposed within the second contact plane  66  along the lengthwise centerline  80 . The cooling channels  84  within the first and second contact planes  64 ,  66  are not necessarily offset, however. In  FIGS. 5 and 6 , the cooling channels  84  are substantially rectangular in cross-section. The cooling channels  84  are not limited to a rectangular cross-sectional shape. For example, the cooling channels  84  can be formed by a wavy contact surface (see  FIG. 7 ), wherein the valleys  86  form the channels  84  and the peaks  88  form the portion of the contact surface  64 ,  66  operable to be in contact with the blade  14 . The cooling channels  84  may also be formed by protrusions extending out from the contact surfaces  64 ,  66 , wherein the channels  84  extend between the protrusions. 
   In some embodiments, the damper  48  further includes a head  90 , fixed to one end of the body  58 . U.S. Pat. Nos. 5,820,343 and 5,558,497 disclose examples of dampers  48  having a head  90  attached to the body  58  of the damper  48 . U.S. patent application Ser. No. 10/771,587 discloses an alternative damper head embodiment. U.S. Pat. Nos. 5,820,343 and 5,558,497, and U.S. patent application Ser. No. 10/771,587 are hereby incorporated by reference. These head embodiments are examples of damper heads  90  that may be used with the present invention damper  48 . The present damper  48  is not, however, limited to these damper head embodiments. 
   Referring to  FIGS. 1 and 2 , under steady-state operating conditions, a rotor assembly  10  within a gas turbine engine rotates through core gas flow passing through the engine. The high temperature core gas flow impinges on the blades  14  of the rotor assembly  10  and transfers a considerable amount of thermal energy to each blade  14 , usually in a non-uniform manner. To dissipate some of the thermal energy, cooling air is passed into the conduits  30  within the root  20  of each blade  14 . From there, a portion of the cooling air passes into the first cavity  40  and into contact with the damper  48 . The cooling apertures  82  in the damper  48  provide a path through which cooling air may pass into the second cavity  42 . In those embodiments that include cooling channels  48 , the cooling channels  48  also provide a path through which cooling air may pass into the second cavity  42 . 
   Referring to  FIGS. 2–4 , the contact surfaces  64 ,  66  of the damper  48  contact the walls  50  of the passage  44 . Centrifugal forces acting on the damper  48 , created as the disk  12  of the rotor assembly  10  is rotated about its rotational centerline  18 , provide a portion of the force that loads the damper  48  into contact with the blade  14 . In the embodiment shown in  FIG. 2 , the skew of the passage  44  relative to the radial centerline  28  of the blade  14 , and the damper  48  received within the passage  44 , causes a component of the centrifugal force acting on the damper  48  to act in the direction of the blade walls  50 ; i.e., the centrifugal force component acts as a normal force against the damper  48  in the direction of the blade walls  50 . 
   Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and the scope of the invention. For example, it is disclosed as the best mode for carrying out the invention that a damper  48  is disposed between a first and second cavity  40 ,  42  where the second cavity  42  is adjacent the trailing edge  38  of the airfoil  22 . In alternative embodiments, a damper  48  may be disposed between any two cavities within the airfoil  22 .