Abstract:
A high-efficiency compressor section ( 10 ) for a gas turbine engine is disclosed. The compressor section includes a vane carrier ( 12 ) adapted to hold ring segment assemblies ( 16 ) that provide optimized blade tip gaps ( 28,29 ) during a variety of operating conditions. The ring segment assemblies include backing elements ( 30 ) and tip-facing elements ( 32 ) urged into a preferred orientation by biasing elements ( 40 ) that maintain contact along engagement surfaces ( 44,46 ). The backing and tip-facing elements have thermal properties sufficiently different to allow relative growth and geometric properties strategically selected to strategically form an interface gap therebetween ( 42,43 ), resulting in blade tip gaps that are dynamically adjusted operation.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to an apparatus for optimizing the performance of gas turbine compressors. In particular, the invention relates to improving compressor efficiency via an adaptive blade tip seal assembly to adjust a gap between a turbine ring segment and an associated blade tip during engine operation. 
       BACKGROUND OF THE INVENTION 
       [0002]    In gas turbine engines, multi-stage axial compressors include sets of alternating fixed vanes and rotating blades that, during operation, cooperatively produce a flow of compressed air for downstream use as a component of combustion. 
         [0003]    As a byproduct of the compression process, components in the compressor are subjected to temperatures which vary not only in location, but also temporally, as the gas turbine progresses through a variety of operating modes, including cold start, steady state, and any number of transition conditions. Over time, these temperature differences impart varying degrees of thermal growth to the compressor components, and gaps required to allow relative motion during operation are designed to avoid unnecessary component rubbing, while minimizing leakage. 
         [0004]    Gas turbines used for power generation may encounter particularly-difficult operating conditions, since they are often stopped and restarted in response to varying demands for power production. Engine operation in these settings may require that an engine be restarted before compressor components have uniformly cooled—known as a “hot restart.” Compressors that passively accommodate hot restarts are often designed to strike a balance between either (1) using component gaps that, particularly between rotating blade tips and associated ring segments, bigger than needed during most steady-state conditions or (2) using relatively-small gaps and abradable coatings that are sacrificially worn down during component contact. Neither of these approaches is optimal; accordingly, there exists and a need in this field for an improved compressor design capable of accommodate hot restarts without unnecessarily reducing operational efficiency. 
       SUMMARY OF THE INVENTION 
       [0005]    A gas turbine engine having a compressor section optimized to provide enhanced efficiency during several operating conditions, said compressor section comprising: 
         [0006]    a vane carrier; 
         [0007]    a ring segment assembly disposed within said vane carrier, said ring segment assembly characterized by a radially-outward backing element, a radially-inward tip-facing element, and at last one biasing element adapted and arranged to dynamically position said tip-facing element with respect to said backing element, said ring segment assembly being characterized by an arcuate ring segment angle; 
         [0008]    wherein said backing element is characterized by a first coefficient of thermal expansion and said tip-facing element is characterized by a second coefficient of thermal expansion, said first coefficient of thermal expansion being higher than said second coefficient of thermal expansion; 
         [0009]    wherein said backing element includes a first mating surface characterized by an interface angle and said tip-facing element includes a second mating surface, said mating surfaces adapted and arranged to provide positive engagement of said engage said first engagement notch; 
         [0010]    wherein said at least one biasing element is positioned and adapted to cooperatively urge said tip-facing element against said backing element; 
         [0011]    wherein said at least one biasing element and said interface angle are selected to provide a biasing force sufficient to overcome a friction force generated along the first and second mating surfaces; 
         [0012]    whereby said tip-facing element and said backing element, are alternately in contact along an interface disposed therebetween during a first operating condition and spaced apart along an interface an interface gap disposed therebetween during a second operating condition, and whereby said at least biasing element maintains contact between said first and second mating surfaces during both operating conditions. 
         [0013]    Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0014]      FIG. 1  is a side elevation of a gas turbine engine compressor section employing the ring segment assembly of the present invention; 
           [0015]      FIG. 2  is a side sectional view of a blade tip, ring segment assembly, and blade tip gap of the present invention during an initial, cold build condition; 
           [0016]      FIG. 3  is a side sectional view of a blade tip, ring segment assembly, and blade tip gap of the present invention during a steady-state operating mode; 
           [0017]      FIG. 4  is a close-up view of the ring segment assembly of the present invention, taken along cutting line IV-IV′; 
           [0018]      FIG. 5  is schematic diagram showing force resolution within the ring segment assembly of the present invention; 
           [0019]      FIG. 6  is a table showing a relationship among allowable angles and coefficients of friction within the ring segment assembly of the present invention; 
           [0020]      FIG. 7  is a side sectional view of a blade tip, ring segment assembly, and blade tip gap of the present invention during an initial, cold build condition, taken along cutting line VII-VII′; 
           [0021]      FIG. 8  is an alternate side sectional view of a blade tip, ring segment assembly, and blade tip gap of  FIG. 7 , shown in a steady-state operating mode; 
           [0022]      FIG. 9  is a close-up view of the ring segment assembly of the present invention, taken along cutting line IX-IX′; 
           [0023]      FIG. 10  is schematic diagram showing force resolution within the ring segment assembly of the present invention; and 
           [0024]      FIG. 11  is a table showing a relationship among allowable angles and coefficients of friction within the ring segment assembly of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Reference is now made in general to the Figures, and to  FIG. 1 , in particular, wherein the compressor section  10  of the present invention is shown. The compressor section  10  includes several stages of fixed vanes  18  and rotating blades  20 —the vanes  18  are fixed within vane mounting slots  22  in vane carriers  12 , and blades  20  are fixed within a longitudinally-aligned rotor  24  that spins about a central axis during operation. In a longitudinal, flow wise direction, the vane carriers  12  typically span several stages. As shown in  FIG. 4 , each is vane carrier has generally arcuate cross section when cut in a plane perpendicular to the center axis of the compressor rotor  24 , and several are distributed circumferentially around the rotor  24  to form a bounded flow path  25  for compressed air to follow during operation. Although only one blade  20  and vane  18  is shown per stage, each stage will contain multiple blades and vanes distributed circumferentially within the bounded flow path  25 . 
         [0026]    Ring segment assemblies  16  are also mounted within the vane carriers  12 . As shown more fully in  FIGS. 2 and 3 , the ring segment assemblies  16  are multi-layered and include a radially-outward backing element or plate  30  and a radially-inward tip-facing element  32  positioned proximate the tips  26  of the rotating blades  20  during operation. An optional abradable coating layer  34  may be positioned radially inward of the tip-facing element  32  to accommodate occasional blade tip contact. With continued reference to  FIGS. 2 and 3 , the radial space between the ring segment assemblies  16  and blade tips  26  defines a performance-impacting blade tip gap  28 . As will be described more fully below, optimizing the size of these blade tip gaps  28  during the several engine operation modes improves engine overall efficiency and is an object of this invention. 
         [0027]    In  FIG. 2 , a blade tip  26  is shown proximate a ring segment assembly  16  in a steady-state operating condition. In this condition, compressor components are generally considered to be thermally saturated, with the compressor components having reached an optimized level of thermally-driven component growth. In this steady state condition, a desired tip gap  28  exists between the ring segment assembly  16  and the various blade tips  26  of the blades  20  mounted on the circumferentially spinning rotor  24 . 
         [0028]    In  FIG. 3 , the blade tip  26  is shown proximate a ring segment assembly  16  in a hot restart operating condition. In this condition, compressor components are no longer considered to be thermally saturated: due to variations in thermal growth tendencies, some components (like the ring segment assemblies  16 ) will have partially cooled and shrunk radially inward, while other components (like the rotating blades  20 ), will likely not have cooled. In this condition, a hot restart blade tip gap  29  exists, but it is typically smaller than the steady-state blade tip gap  28 . 
         [0029]    In one embodiment of this invention, the backing element  30  and tip-facing element  32  are adapted and arranged to passively optimize the tip gaps  28 ,  29  present during steady-state (shown in  FIG. 5 ) and hot restart conditions (shown in  FIG. 6 ). In a preferred embodiment, the backing element  30  is more thermally reactive than the tip-facing element  32 . In one arrangement, the backing element is made from a high alpha material (such as 304 stainless steel or thermal equivalent), while the tip-facing element is made from a low alpha material (such as 410 stainless steel or thermal equivalent). Additionally, with collective reference to  FIGS. 4, 5, and 6 , each backing element  30  and tip-facing element  32  respectively include positioning notches  44 ,  46  that, together with biasing elements  40 , urge the backing and tip-facing elements into a tip-gap optimizing arrangement during the various operating conditions, as described more fully below. 
         [0030]    During operation, the backing element  30  adopts several orientations due to differing thermal loads. For example the backing element shifts from a circumferentially-expanded and radially-compact orientation in the steady state condition shown in  FIG. 5 , to a circumferentially compact and radially expanded orientation in the hot restart condition shown in  FIG. 6 . 
         [0031]    During steady state operating conditions, the backing elements  30  and tip-facing element  32  are spaced apart by an interface gap  42 , and the associated positioning notches  44 , 46  cooperate with the biasing elements  40  shown in  FIG. 2  to urge the backing elements and tip-facing element into positive engagement. This positive engagement creates and maintains a desired steady-state tip gap  28  that is large enough to avoid component damaging contact while small enough to provide efficient compressed airflow. 
         [0032]    During hot restart conditions, the backing elements  30  and tip-facing element  32  are spaced apart by an interface gap  43 , and the associated positioning notches  44 , 46  cooperatively urge the backing elements and tip-facing element into positive engagement. This positive engagement creates and maintains a desired hot restart tip gap  29  that is large enough to avoid component damaging contact while small enough to provide efficient compressed air flow. 
         [0033]    With reference to  FIG. 7 , a blade tip  26 , ring segment assembly  16 , and blade tip gap  29  of the present invention will be described in a initial, cold build condition. The abradable coating layer  34  is safely spaced away from the blade tips  26 , but the tip gap is 29 it too large for efficient operation. 
         [0034]    Now with reference to  FIG. 8 , a blade tip  26 , ring segment assembly  16 , and blade tip gap  28  of the present invention will be described during steady-state operating condition. The abradable coating layer  34  is safely spaced away from the blade tips  26 , but thermal growth of the backing element  30  has caused the backing element positioning notch  44  to shift circumferentially away from tip-facing element positioning notch  46 , thereby allowing the biasing element  40  to force the tip-facing element  32  away from the  30 , creating an interface gap  42  and reducing the blade tip gap by an amount substantially equal to the radial height of interface gap  42 . 
         [0035]    Operation of this invention benefits from properly matching aspects of the backing element notch  44  and tip-facing notch  46 . This concept will be described in more detail here, with additional reference to  FIGS. 9, 10, and 11 . In particular, is important that angles α and β be selected as compatible pairs, as follows: to avoid thermal lockup between the interface positioning notches  44 , 46 , the following equations must be satisfied. The normal components from spring load Ps equals that of thermal contact load P, then N=Ps*cos (α−β)=P*sin (α−β) Then the shear component of P must be greater than the shear component of Ps plus the friction component, μN, or P*cos (α−β)≧Ps*sin (α−β)+μ*Ps*cos (α−β) Solving for μ, the friction coefficient is μ≦c tan(α−β)−tan(α−β), where α is the wedge angle, and β is ½ of the ring segment angle (the ring segment angle is the arcuate distance in degrees between ring segment ends  48 ,  50  shown in  FIG. 4 ). For a 45 degree ring segment and 45 degree wedge angle, the allowable friction coefficient, μ, must be less than c tan(22.5)−tan(22.5)=2. For a 60 degree ring segment and 50 degree wedge angle, the friction coefficient, μ, must be less than c tan(20)−tan(20)=2.38. As μ of the components chosen decreases, likelihood of notch lockup increases, as the urging capacity of the biasing elements is no longer sufficient to overcome the friction between the backing element positioning notch  44  and the tip-facing element positioning notch  46 , at which point the design would not function reliably. A value (α−β) of 35 degrees or lower is preferred and corresponds to a coefficient of friction of 0.73, although other values may be selected, if chosen to match the biasing characteristics of the biasing elements  40 . 
         [0036]    It is to be understood that while certain forms of the invention have been illustrated and described, it is not to be limited to the specific forms or arrangement of parts herein described and shown. It will be apparent to those skilled in the art that various, including modifications, rearrangements and substitutions, may be made without departing from the scope of this invention and the invention is not to be considered limited to what is shown in the drawings and described in the specification. The scope if the invention is defined by the claims appended hereto.