Abstract:
A sealing system for reducing a gap between a tip of a shrouded turbine blade and a stationary shroud of a turbine engine. The sealing system includes one or more seal lands extending from a shrouded turbine blade toward a stationary shroud of a turbine engine. During operation of the turbine engine, the seal lands straighten and extend towards the stationary shroud of the turbine engine, thereby reducing the leakage of air past the shrouded turbine blades and increasing the efficiency of the turbine engine. The sealing system may also include one or more protrusions extending from the stationary shroud towards the tips of the shrouded turbine blades.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention is directed generally to turbine engines, and more particularly to systems for sealing gaps between shrouded blade tips and stationary shrouds in turbine engines so as to improve turbine engine efficiency by reducing leakage.  
       BACKGROUND  
       [0002]     Typically, gas turbine engines are formed from a combustor positioned upstream from a turbine blade assembly. The turbine blade assembly is formed from a plurality of turbine blade stages coupled to discs that are capable of rotating about a longitudinal axis. Each turbine blade stage is formed from a plurality of blades extending radially about the circumference of the disc. Each stage is spaced apart from each other a sufficient distance to allow turbine vanes to be positioned between each stage. The turbine vanes are typically coupled to the shroud and remain stationary during operation of the turbine engine.  
         [0003]     The tips of the turbine blades are located in close proximity to an inner surface of the shroud of the turbine engine. There typically exists a gap between the blade tips and the shroud of the turbine engine so that the blades may rotate without striking the shroud. During operation, high temperature and high pressure gases pass the turbine blades and cause the blades and disc to rotate. These gases also heat the shroud and blades and discs to which they are attached causing each to expand due to thermal expansion. After the turbine engine has been operating at full load conditions for a period of time, the components reach a maximum operating condition at which maximum thermal expansion occurs. In this state, it is desirable that the gap between the blade tips and the shroud of the turbine engine be as small as possible to limit leakage past the blade tips.  
         [0004]     However, reducing the gap cannot be accomplished by simply positioning the components so that the gap is minimal under full load conditions because the configuration of the components forming the gap must account for emergency shutdown conditions in which the shroud, having less mass than the turbine blade and disc assembly, cools faster than the turbine blade assembly. In emergency shutdown conditions, the diameter of the shroud reduces at a faster rate than the length of the turbine blades. Therefore, unless the components have been positioned so that a sufficient gap has been established between the turbine blades and the turbine shroud under operating conditions, the turbine blades may strike the stationary shroud because the diameter of components of the shroud is reduced at a faster rate than the turbine blades. Collision of the turbine blades and the shroud often causes severe blade tip rubs and may result in damage. Thus, a need exists for a system for reducing gaps between turbine blade tips and a surrounding shroud under full load operating conditions while accounting for necessary clearance under emergency shutdown conditions.  
       SUMMARY OF THE INVENTION  
       [0005]     This invention relates to a sealing system for reducing a gap between a tip of a shrouded turbine blade and a stationary shroud of a turbine engine. As a turbine engine reaches steady state operation, components of the sealing system reach their maximum expansion and reduce the size of the gap located between the blade tips and the engine shroud, thereby reducing the leakage of air past the turbine blades and increasing the efficiency of the turbine engine. In at least one embodiment, the sealing system includes a turbine blade assembly having at least one stage formed from a plurality of turbine blades.  
         [0006]     The sealing system may also include one or more seal lands coupled to a turbine blade with an integral tip shroud and extending from a tip of the turbine blade toward a stationary shroud of the turbine engine. The seal land may be coupled to the turbine blade by sliding the seal land into a slot and by peaning the seal land to keep the seal land from sliding out, by brazing the seal land onto the turbine blade shroud, or through any other appropriate connection method. The seal land may also have a curved configuration such that while the turbine engine is at rest, the seal land is curved and does not contact the shroud. The seal land may be curved such that the tip of the seal land may face into the gas flow, thereby enabling the seal land to deflect the incoming tip leakage flow upstream and thus, improve the effective sealing ability of the seal land. The seal land is adapted to straighten during operation of the turbine engine due to at least centrifugal forces such that the seal land is closer to the stationary shroud than when the turbine engine is in a resting state. In at least one embodiment, the seal land may be formed from two or more materials having different coefficients of thermal, expansion. The seal land may be formed from a first material forming an outer perimeter of the seal land and from a second material forming an inner perimeter of the seal land. The second material forming the inner perimeter may have a coefficient of thermal expansion that is greater than coefficient of thermal expansion for the first material forming the outer perimeter. When heated, the second material extends a greater distance than the first material, which causes the seal land to straighten.  
         [0007]     The sealing system may also include one or more protrusions extending from the shroud of the turbine engine towards the tips of the turbine blades. The protrusions may extend circumferentially around the turbine blade assembly and may be positioned downstream of a seal land. In at least one embodiment, a protrusion may be positioned between two adjacent seal lands. The protrusions act as a dam to enhance the sealing ability of the sealing system.  
         [0008]     These and other embodiments are described in more detail below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the presently disclosed invention and, together with the description, disclose the principles of the invention.  
         [0010]      FIG. 1  is a perspective view of an embodiment of this invention.  
         [0011]      FIG. 2  is a side view of an embodiment of this invention shown in a resting state of a turbine engine.  
         [0012]      FIG. 3  is a side view of the embodiment of this invention shown in  FIG. 2  and shown in  FIG. 3  in an operating state of a turbine engine with the lands deflected outward.  
         [0013]      FIG. 4  is a side view of an alternative embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     As shown in  FIGS. 1-4 , this invention is directed to a sealing system  10  usable in a turbine engine. In particular, the sealing system  10  is operable to reduce a gap  12  between one or more tip shrouds  14  of a turbine blade  16  in a turbine engine  18  and a surrounding stationary shroud  20  while the turbine engine  18  is operating. The sealing system  10  reduces the gap  12  to the gap  48 . The gap  48  exists in the turbine engine  18  so that the tip shrouds  14  do not contact the stationary shroud  20  while the turbine engine  18  is at rest or is operating, or during assembly. In at least one embodiment, the turbine engine  18  includes a turbine blade assembly  22  formed at least in part from a plurality of turbine blades  16  coupled to a disc  24 . The blades  16  may be coupled to the disc  24  at various points along the disc  24  and may be assembled into rows, which are commonly referred to as stages  26 , having adequate spacing to accommodate stationary vanes (not shown) between adjacent stages of the blades  16 . The stationary vanes are typically mounted to a casing of the turbine engine  18 . The disc  24  may be rotatably coupled to the turbine engine  18  enabling the turbine blades  16  to move relative to the turbine vanes. Each tip shroud  14  may extend the width of one pitch of a turbine blade segment  16 . In at least one embodiment, the tip shrouds  14  may generally form a ring around the turbine blade assembly  22  having small openings at the junctions between adjacent tip shrouds  14 .  
         [0015]     The sealing system  10  may be formed from one or more seal lands  28  extending from the turbine blade  16  toward the stationary shroud  20 . The seal land  28  may extend the width of the tip shroud  20  to form a relatively continuous ring around the tip shrouds  20  of the turbine blades  16  and may include spaces between adjacent seal lands  28 . In at least one embodiment, the seal land  28  may have a flange  30  on bottom portion  32  for attaching the seal land  28  to the tip shroud  14  of the turbine blade  16 . The seal land  28  may be inserted into a slot  34  in the tip shroud  14  of the turbine blade  16 . In some embodiments, the seal land  28  is not inserted directly into the tip shroud  14  of the turbine blade  16 . Instead, the seal land  28  may be attached to other portions of the turbine blade  16  in any fashion allowing the seal land  28  to extend beyond the tip shroud  14  toward the stationary shroud  20 . In other embodiments, the seal land  28  may be coupled to the turbine blade  16  using brazing, welding, or other methods of mechanically fastening the seal land  28  to the turbine blade  16 . Still yet, in other embodiments, the seal land  28  may be integrally formed with the turbine blade  16  in the same casting process and machined into the proper shape and configuration.  
         [0016]     The seal land  28  may have a generally curved shape, as shown in  FIGS. 1-4 . The seal land  28  may be configured in this manner so that as the turbine engine  18  approaches and operates at design load, the seal land  28  straightens, thereby reducing the gap  48  between the seal land  28  and the stationary shroud  20 . The seal land  28  should be sized such that at rest the seal land is not in contact with the stationary shroud  20  and during steady state operation is not in contact with the stationary shroud, but is in very close proximity to reduce the gap  48  to a small distance. At rest and while the seal lands  28  are cold, the seal lands  28  should be able to be installed into the slot  34  relatively easily. The size of gap  48  in both the cold resting state and in the hot operating state depends on, in part, the rotational speed of the turbine blade  16 , the length of the seal land  28 , and properties of the materials forming the stationary shroud  20 , the seal land  28 , the turbine blade  16 , and related components.  
         [0017]     In at least one embodiment, the seal land  28  may be bimetallic, such as formed from two or more materials. The materials may, in at least one embodiment, have different coefficients of thermal expansion. For instance, as shown in  FIG. 4 , the seal land  28  may be formed from a first material  36  on the outer perimeter  38  of the seal land  28  and a second material  40  on the inner perimeter  42  of the seal land  28 . The second material  40  may have a coefficient of thermal expansion that is greater than a coefficient of thermal expansion for the first material  36 . In at least one embodiment, the first material  36  may be, but is not limited to, IN 909 or other appropriate materials, and the second material  40  may be, but is not limited to, A286, IN718, IN738, CM247, or other appropriate materials. As the materials heat up during operation of the turbine engine  18 , centrifugal forces and the configuration of the first and second materials  36  and  40  cause the seal land  28  to straighten and reduce the distance between the seal land  28  and the stationary shroud  20 . The first and second materials  36  and  40  are not limited to any particular material, except that the materials should be able to withstand the hot environment found in the turbine engine  18 .  
         [0018]     The sealing system  10  may also include one or more protrusions  44  extending from the stationary shroud  20  of the turbine engine  18  toward the tip shroud  14  of the turbine blade  16 . In at least one embodiment, the stationary shroud  20  may be, but is not limited to, a honeycomb structure configured to provide little resistance to deformation should a seal land  28  or blade shroud tip  14  contact the stationary shroud  20 . In the event the seal land  28  or blade shroud tip  14  contacts the stationary shroud  20 , the stationary shroud  20  formed from a honeycomb configuration easily deforms to reduce the likelihood of damaging the turbine blade  16 .  
         [0019]     The protrusions  44  may be formed integrally within the stationary shroud  20  or may be attached to the stationary shroud  20  using a weld or other appropriate method of connection. In at least one embodiment, a protrusion  44  may be positioned downstream of the seal land  18 . In yet another embodiment, a protrusion  44  may be attached to a stationary shroud  20  and positioned between two adjacent seal lands  28 , as shown in  FIGS. 1-4 . Specifically, a first seal land  28  may be positioned upstream of the protrusion  44  and a second seal land  28  may be positioned downstream of the protrusion  44 . The protrusion  44  should be positioned between the seal lands  28  so that the seals lands  28  do not contact the protrusions during operation or while in a resting state. The protrusion  44  may extend circumferentially around an axis of rotation  46  of the turbine blade assembly  22 .  
         [0020]     While the turbine engine  18  is at rest, the seal land  28  is not in contact with the stationary shroud  20 , as shown in  FIG. 2 . Rather, a gap  48  exists between the seal land  28  and the stationary shroud  20 . During operation, as shown in  FIG. 3 , the turbine blade assembly  22  rotates relative to the turbine engine  18 , and the turbine engine  18  increases in temperature. Centrifugal forces and differences in coefficients of thermal expansion cause the seal land  28  to straighten and reduce the width of the gap  48  between the seal land  28  and the stationary shroud  20 . The distance that the seal land  28  extends from the tip shroud  14  of the turbine blade  16  should account for thermal expansion of the turbine blade  16  and the stationary shroud  20  so that the seal land  28  does not contact the stationary shroud  20 . During emergency shutdown situations, the seal land  28  returns to its resting position and does not contact with the stationary shroud  20  in doing so. In particular, the seal land  28  cools faster than the stationary shroud  20 , in part, because the seal land  28  has a larger surface area to mass ratio than the shroud. Thus, the temperature of the seal land  28  is reduced at a faster rate than the shroud, which causes the length of the seal land  28  to be reduced at a faster rate than the stationary shroud  20 , thereby withdrawing the seal land  28  from the stationary shroud  20  and towards the blade tip shroud  14 .  
         [0021]     The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention.