Abstract:
A seal system between a row of buckets supported on a machine rotor and a surrounding stationary casing or stator includes a tip shroud secured at radially outer tips of each of the buckets, the tip shroud formed with a radially-projecting rail. A cellular seal structure is supported in the stationary stator in radial opposition to the tip shroud and the rail. The seal structure has an annular array of individual cells formed to provide continuous, substantially horizontal flow passages devoid of any radial obstruction along substantially an entire axial length dimension of the cellular seal structure to prevent flow about the tip shroud from turning radially inwardly.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This present invention relates generally to turbines and turbine blades and more particularly, to tip-shrouded turbine blades and associated cellular seal structures. 
         [0002]    An axial gas turbine stage consists of a row of stationary blades followed by a row of rotating blades or buckets in an annulus defined by the turbine casing or stator. The flow is partially expanded in the vanes which direct the flow to the rotating blades where it is further expanded to generate required power output. For safe mechanical operation, there exists a minimum physical clearance requirement between the tip of the rotating blade and the casing or stator wall. Honeycomb strips on the casing wall are generally used to minimize running tip clearance of the rotating bucket at all operating conditions. To achieve tighter clearance, a rail on the tip shroud is allowed to rub and cut a groove in the honeycomb strip during transient operations. The shape and depth of this groove depends on the rotor dynamics and thermal behavior, i.e., differential radial and axial thermal expansion of the rotor and casing. 
         [0003]    The high energy flow escaping over the bucket tips and its subsequent interaction with the downstream main flow is one of the major sources of loss in the turbine stage. Typically, these tip clearance losses in turbines constitute 20 to 25 percent of the total losses within a given stage. Due to the inherent shape of the groove cut in the honeycomb seal structure, the overtip leakage flow turns downward (i.e., radially inward) and penetrates deep into the main flow path causing excessive mixing losses. Accordingly, any design which minimizes this mixing loss will improve the turbine stage efficiency. In addition, the turning inward of high temperature, overtip leakage flow due to the groove shape and honeycomb seal configuration, causes the tip leakage flow to touch the aft side of the bucket tip shroud, exposing it to a relatively hotter operating environment compared to a non-grooved seal configuration. Since the bucket shroud is one of the life-limiting components of the turbine machine, any design which reduces shroud temperature will enhance bucket life. 
       SUMMARY OF THE INVENTION 
       [0004]    In accordance with an exemplary but nonlimiting embodiment, the invention provides a seal system between a row of buckets supported on a machine rotor and a surrounding stationary casing comprising: a tip shroud secured at radially outer tips of each of the buckets, the tip shroud formed with a radially-projecting rail; and a cellular seal structure supported in the stationary casing in radial opposition to the tip shroud and the rail, the seal structure having an annular array of individual cells formed to provide continuous, substantially horizontal flow passages devoid of any radial obstruction along substantially an entire axial length dimension of the cellular seal structure. 
         [0005]    In another exemplary but nonlimiting aspect, the invention provides a seal system between a row of buckets supported on a machine rotor and a surrounding stationary casing comprising: a tip shroud secured at radially outer tips of each of the buckets, the tip shroud formed with a radially-projecting rail; a cellular seal structure supported in the stationary casing in radial opposition to the tip shroud and the rail, the seal structure having an annular array of individual cells formed to provide substantially horizontal, closed-periphery flow passages extending continuously between forward and aft ends of the seal structure, the individual cells oriented substantially parallel to a rotation axis of the rotor, plus or minus 45 degrees. 
         [0006]    In still another exemplary but nonlimiting aspect, the invention provides a method of reducing mixing losses caused by tip leakage flow at a bucket tip/shroud-stator seal interface mixing with a main flow of combustion gases in a turbine engine, the method comprising: providing a cellular seal structure in a stator surface surrounding an annular bucket tip shroud; providing a rail on the radially outer surface of the bucket tip shroud adapted to penetrate the cellular seal structure during transient operating conditions of the turbine engine due to differential thermal expansion properties of the rotor and stator; and forming the cellular seal structure to include an annular array of individual cells arranged to provide substantially horizontal, closed-periphery flow passages extending continuously and unobstructed between forward and aft ends of the seal structure so that, upon penetration of the seal structure by the rail, tip leakage flow around the tip shroud will be confined to the substantially horizontal, closed-periphery flow passages and thus be prevented from turning radially inwardly into the main flow along an entire axial length dimension of the seal structure. 
         [0007]    The invention will now be described in detail in connection with the drawing figures identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic side elevation illustrating a tip shrouded bucket and a known honeycomb seal structure on the surrounding stationary shroud; 
           [0009]      FIG. 2  is a schematic side elevation similar to  FIG. 1  but incorporating a cellular seal structure in accordance with a first exemplary but nonlimiting embodiment of the invention; 
           [0010]      FIG. 2A  is a schematic flat projection of the cellular seal structure of  FIG. 2  as viewed in the direction of arrow A in  FIG. 2 ; 
           [0011]      FIG. 3  is a schematic side elevation similar to  FIG. 2  but showing an alternative cellular seal structure having an exit end aligned with a downstream diffuser component; 
           [0012]      FIG. 4  is a schematic side elevation similar to  FIG. 2  but illustrating a variation where coolant is supplied to the seal structure in accordance with another exemplary embodiment of the invention; 
           [0013]      FIGS. 5-9  represent schematic flat projections of cellular structures within the scope of the invention and taken from the same perspective as  FIG. 2A ; and 
           [0014]      FIGS. 10-12  represent schematic representations of the cellular structures at different axial orientations to the rotor axis. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring now to  FIG. 1 , a typical tip-shrouded turbine bucket  10  includes an airfoil  12  which is the active component that intercepts the flow of gases and converts the energy of the gases into tangential motion. This motion, in turn, rotates the rotor to which the buckets  10  are attached. 
         [0016]    A shroud  14  (also referred to herein as a “tip shroud”) is positioned at the tip of each airfoil  12  and includes a plate supported toward its center by the airfoil  12 . The tip shroud may have various shapes as understood by those skilled in the art, and the exemplary tip shroud as illustrated here is not to be considered limiting. Positioned along the top of the tip shroud  14  is a seal rail  16  which minimizes passage of flow path gases through the gap between the tip shroud and the inner surface of the surrounding components. The rail  16  typically provided with a cutting tooth (not shown) for a purpose described below. 
         [0017]    As shown in  FIG. 1 , the surrounding stationary stator shroud  18  mounts a honeycomb seal structure  20  confined within a recessed portion of the stationary shroud as defined by wall surfaces  22 ,  24  and  26 . 
         [0018]    Operating at transient conditions (e.g., during start-up, during significant load changes, and during shut-down), and prior to reaching a state of thermal equilibrium among the turbine hot gas path components, different axial and radial thermal expansion properties of the buckets or blades  10  relative to the stator will cause the rail  16  and its cutting tooth to cut through the honeycomb seal structure  20 , forming a substantially C-shaped groove  30 . Because the honeycomb seal structure is formed at least in part by radially-extending wall surfaces  28  that extend radially and substantially transverse to the rotor axis, the combustion gas leakage flow crossing over the rail  16  turns radially inwardly to the main flow passage (as shown by the flow arrows F) as it enters and exits the groove  30  cut through the honeycomb seal structure. This inward turning causes the leakage flow and the main flow to interact in the area designated  32 , thus creating a relatively large mixing loss. 
         [0019]    To more fully understand this phenomenon, the construction of the honeycomb seal structure  20  includes, in addition to the annular (or part-annular) radially-extending, axially-spaced walls  28 , plural axially-extending, circumferentially-spaced walls that combine with the walls  28  to form individual cells. The shape and arrangement of the walls  28  and  34  may vary but in all cases, it is the presence of axially-spaced, radially-extending annular or part-annular wall portions  28  in the individual cells, that are substantially transverse to the rotor axis, that force the tip leakage flow about the rail  16  to turn radially inwardly to interact with the main flow as previously described. 
         [0020]    With reference now to  FIG. 2 , an exemplary but nonlimiting embodiment of the present invention is illustrated. For convenience, reference numerals as used in  FIG. 1 , but with a prefix “1” added, are used in  FIG. 2  to indicate corresponding components. The difference lies in the construction of the cellular structure  120 . Initially, it is noted that in the prior arrangement described above, the seal structure is properly characterized as a “honeycomb” configuration. As will become apparent below, however, the seal structure need not be of honeycomb configuration and, in fact, may take on any number of cellular configurations so long as certain criteria are met as explained below. 
         [0021]    More specifically, the honeycomb structure  20  of  FIG. 1  has been discarded in favor of a cellular seal structure  120  as shown in  FIG. 2 . Of significance to the modified design is the absence of any axially-spaced, radially-inwardly extending annular or part-annular walls that are substantially transverse to the rotor axis, and that would otherwise obstruct and turn radially inwardly the tip leakage flow.  FIG. 2A  is a schematic reference view of the new cellular (or cell) structure  120  as viewed in the direction of arrow A in  FIG. 2 . It will be understood that the structure is shown in a flat projection but, in fact, has an arcuate cross-section, the arcuate length of which is determined by the arcuate length of the stator segment supporting the seal. The cellular structure  120  is comprised of circumferentially-spaced, axially-extending, radial partitions  134  and plural, substantially concentric, radially spaced and axially-extending annular walls  136 . The combination of walls  134  and  136  create individual cells or passages  138  that extend in a substantially horizontal, (or axial) direction continuously along the cellular seal structure  120 , without obstruction, from one end of the seal structure at wall  122  to the opposite end of the seal structure indicated at wall  126 . This means that when the groove  130  is cut into the cellular structure  120  by the rail  116  (and, specifically, the rail&#39;s cutting tooth, not shown), the tip leakage flow, once it crosses over the bucket tip rail  116 , will flow in an axial direction without obstruction and with the concentric, radially-spaced walls  136  preventing the tip leakage flow from turning radially into the main flow, hence avoiding or at least minimizing the previously-described mixing losses. 
         [0022]    Additional benefits of the above-described cellular structure are illustrated in  FIGS. 3 and 4 . In  FIG. 3 , similar reference numerals but with the prefix “2”, are used to designate corresponding components where applicable. For a last stage row of buckets, the high energy tip leakage flow can be aligned with an exhaust diffuser  240  by altering the exit angle of the cell walls  242  at the downstream end of the cell structure  220  (and downstream of the aft edge of the bucket) to align the tip leakage flow with the angle of the exhaust diffuser, and thereby attach the flow to the diffuser. This can improve the performance of the diffuser apart from improving the stage performance mixing loss reduction. 
         [0023]      FIG. 4  illustrates yet another advantage of the axially-oriented cell structure in that it provides relatively better insulation for the stationary shroud or stator from the hot gas path. This may also be utilized as an improved cooling circuit for the stationary shroud. Here again, similar reference numerals as applied in  FIGS. 2 and 3 , but with the prefix “3”, are used to indicate corresponding components, again where applicable. More specifically a coolant flow conduit  344  and suitable supply means are used to supply coolant to the passage  346  in the cellular structure  320 , closest to the stator wall  348 , thus cooling the stator or shroud wall  348 , by convection. The cooling air then joins with the main flow in a smooth transition, with little or no disruptive mixing. 
         [0024]      FIGS. 5-10  illustrate exemplary but nonlimiting alternative cell configurations within the scope of the present invention. These alternative cell constructions are viewed from the same perspective as  FIG. 2A . In each case, an array of unobstructed, axially-oriented cells are created by the internal structure to cause tip leakage flow to remain in a substantially axial or horizontal orientation, so as to be prevented from turning radially inward into the main flow. Thus, in  FIG. 5 , a combination of alternating “corrugated” walls  410  and radially-spaced, annular concentric walls  412  create a plurality of triangular cells  414  extending continuously without obstruction in the axial or horizontal direction between the radial walls  122  and  126  of the stationary shroud  118  ( FIG. 2 ). 
         [0025]    In the cellular structure shown in  FIG. 6 , alternating corrugated walls  510 ,  512  are inverted relative to each other so that, when combined with the radially-spaced, annular concentric walls  514  the triangular cells  516  are substantially identical to those formed in the  FIG. 5  construction, but the cells are aligned differently with the cells in adjacent rows. 
         [0026]      FIG. 7  illustrates another example embodiment where the individual cells  610  are created by an array of oppositely-oriented, angled (or criss-crossed) walls  612 ,  614  creating axially- or horizontally-extending diamond-shaped cells  616  (but modified along the margins as shown). 
         [0027]    In  FIG. 8 , the cells  710  are created by an array of axially- or horizontally-extending tubes  712 , each of which has a polygonal shape and which are engaged by like tubes in both circumferential and radial directions. 
         [0028]      FIG. 9  illustrates a construction generally similar to that shown in  FIG. 8  but wherein the cells  810  are circular in shape as defined by the array of circular tubes  812  which, again, are engaged both circumferentially and radially. Note that in both embodiments illustrated in  FIGS. 8 and 9 , additional axial cells are created at  714 ,  814 , respectively, at the interstices between the tubes  712 ,  812 . 
         [0029]    Other cell constructions are contemplated by the invention, the significant design feature being the creation of axially-extending, unobstructed cells to cause the tip leakage flow to remain in a substantially axial direction, so as to prevent radially inward turning and subsequent mixing of the tip leakage flow with the main combustion gas flow. In this regard, the individual cells in any given cellular structure need not be of uniform size and shape, so long as the design feature mentioned above is satisfied. 
         [0030]    To this point, the various cell constructions have been shown to extend substantially parallel to the rotation axis of the rotor. However, as shown in  FIGS. 10 ,  11 , and  12 , the cell arrays (using cells  138  as an example) may be slanted in an axial direction at an angle to one side of the rotor axis of up to about 45° ( FIG. 10 ), parallel to the rotor axis ( FIG. 11 ) or slanted to the opposite side ( FIG. 12 ), again up to about 45°. The orientation will depend on the direction of the main combustion gas flow. By aligning the tip leakage flow with the main gas flow, it is expected that an even further decrease in air mixing losses will be achieved. 
         [0031]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.