PATENT ABSTRACT
The clearances between an array of turbine blades and its surrounding blade track may be controlled by an expansion control material system supporting the blade tracks. The blade track support hoop is placed in tension by the expansion control material placed therein and the expansion control material is placed in compression.

PATENT DESCRIPTION
RELATED APPLICATIONS  
       [0001]    The present application claims the benefit of U.S. Provisional Application Serial No. 60/302,463, filed Jul. 2, 2001, which is incorporated herein by reference. 
     
    
     GOVERNMENT RIGHTS  
       [0002] The present invention was developed under United States Air Force Contract No. F33615-97-C-2778, and the United States Air Force has certain rights therein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates generally to blade tracks for gas turbine engines. More specifically, the present invention relates to a blade track support assembly having controlled thermal expansion properties.  
           [0004]    A gas turbine engine is typical of the type of machinery which the invention described herein may be advantageously employed. It is well known that a gas turbine engine conventionally comprises a compressor for compressing inlet air to an increased pressure for combustion in a combustion chamber. A mixture of fuel and the increased pressure air is burned in the combustion chamber to generate a high temperature gaseous flow-stream for causing rotation of turbine blades within the engine. In an effort to reduce specific fuel consumption of engines, there has been a move to increase the efficiency of the turbine by decreasing the clearance between the rotating turbine blade tips and the stationary blade track. In designing a gas turbine engine with tighter blade tip clearances, designers must account for transient conditions that the gas turbine engine experiences during operation. During acceleration of the gas turbine engine, the rotor carrying the turbine blades experiences mechanical growth in a radial direction faster than blade track/shroud, thereby allowing the potential for mechanical contact between the blade tips and the blade track/shroud. During deceleration of the gas turbine engine, the blade track/shroud exhibits mechanical shrinkage in the radial direction more quickly than the rotor, thereby allowing the potential for mechanical contact between the blade tips and the blade track/shroud.  
           [0005]    The present invention seeks to control the clearance between the blade tips and the blade track/shroud by lowering the thermal expansion of the blade track support assembly, thereby allowing a reduction in the steady state run clearance between the blade tip and the blade track/shroud. The resulting improvement is manifested as an increase in turbine efficiency and a reduction in specific fuel consumption. The present invention provides a novel and non-obvious blade track/shroud assembly for a gas turbine engine.  
         SUMMARY OF THE INVENTION  
         [0006]    One form of the present invention contemplates a support for a gas turbine engine blade track, comprising: a ring member having a centerline, the ring member having a cavity open towards the centerline; a plurality of circumferentially spaced ceramic members positioned within the cavity; and, a plurality of metallic members positioned within the cavity, the plurality of ceramic members and the plurality of metallic members are positioned so that each of the plurality of ceramic members is located between a pair of the plurality of metallic members.  
           [0007]    Another form of the present invention contemplates a low expansion blade track assembly, comprising: a continuous hoop member having an inner surface; a plurality of blade track segments coupled to the hoop member; and, expansion control means located within the hoop member for supporting the plurality of blade track segments, the expansion control means has a first surface abutting the inner surface.  
           [0008]    Yet another form of the present invention contemplates a blade track support assembly, comprising: a continuous metallic ring member symmetrical about a centerline, the ring member having a circumferential channel opening towards the centerline; a plurality of circumferentially spaced ceramic cylinders extending parallel to the centerline and located within the channel, each of the ceramic cylinders has an outer surface; and, a plurality of circumferentially spaced metallic spacers extending parallel to the centerline and located within the channel, each of the plurality of metallic spacers including a pair of bearing surfaces corresponding to the outer surface, each of the plurality of ceramic cylinders located between a pair of the plurality of metallic spacers, and each of the bearing surfaces abutting one of the ceramic cylinders.  
           [0009]    One object of the present invention is to provide a unique blade track support.  
           [0010]    Further objects and advantages of the present invention will become apparent from the following description of the preferred embodiment.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a partially fragmented side elevational view of a gas turbine engine.  
         [0012]    [0012]FIG. 2 is a side view of a portion of one embodiment of a blade track and support assembly that comprises a portion of the FIG. 2 gas turbine engine.  
         [0013]    [0013]FIG. 3 is a view of the blade track and support assembly of FIG. 2 that has the gas turbine engine blade removed.  
         [0014]    [0014]FIG. 4 is an enlarged view of the blade track support assembly of FIG. 3 from the direction of arrow A with the blade track segments removed.  
         [0015]    [0015]FIG. 5 is an enlarged end view of the blade track support assembly of FIG. 3.  
         [0016]    [0016]FIG. 6 is an illustrative view of the spacers and low expansion members comprising a portion of the blade track support assembly of the present invention. FIG. 7 is an illustrative end view of the spacers and low expansion members being assembled into the blade track support assembly with the sidewall removed for clarity. FIG. 8 is a graph showing the reduction in tip clearance by lowering the thermal expansion coefficient of the blade track support assembly.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.  
         [0018]    Referring to FIG. 1, there is shown an exemplary gas turbine engine  11 . It is understood that such power plants as gas turbine engine  11  may find application in all types of aircraft, including for example, helicopters, fixed wing planes, tactical fighters, trainers, missiles and other related apparatus. Further, the gas turbine engine may be equally suited to be used for a wide variety of industrial applications. Historically, there has been the widespread application of industrial gas turbine engines, such as pumping sets for gas and oil transmission lines, electricity generation and naval propulsion. The gas turbine engine  11  includes a compressor  12 , a combustor  13  and a turbine  14 . This is only an example of a gas turbine engine and it will be understood that there are a variety of ways that the components may be linked together or arranged. The gas turbine engine  11  includes a rotor disk  17 , with a plurality of turbine blades  16  mounted thereto, that is coupled to a shaft (not shown) within gas turbine engine  11 .  
         [0019]    With reference to FIG. 2, there is illustrated a portion of the working fluid sealing system  20 . In one form of the present invention, the sealing system  20  is designed to minimize the leakage of working fluid in the working fluid pathway in the turbine  14 . However, it is contemplated herein that the working fluid sealing system could be utilized in other portions of the working fluid pathway within the gas turbine engine. Controlling the clearance  28  between the tips  27  of turbine blades  26  and surface  25  of the blade track  24  assists in minimizing the bypassing of the rotor  17  and turbine blades  16  by the working fluid. The present application will utilize the term blade track interchangeably with the term shroud and/or air seal.  
         [0020]    In one form of the present invention, the sealing system  20  comprises two components that form a substantial seal between the rotating and static components. The term “seal” as utilized herein includes the reduction and/or the elimination of fluid flow between the rotating and static components. There is no intent herein to limit the term “seal” to a theoretical fluid tight seal. More specifically, the support structure for the stationary components comprises thermal expansion control features that are intended to control the movement of blade track inner surface  25 . In a preferred embodiment, support  22  comprises a continuous hoop member  30  and an inner cavity  34 . In one form the ring member that is symmetrical about a centerline X defines the continuous hoop member  30 . Inner cavity  34  is bounded by inner surface  38  of continuous hoop member  30  and first sidewall  32  and second sidewall  33 . However, other geometries for the ring member are contemplated herein.  
         [0021]    With reference to FIGS. 4 and 5, there is illustrated one embodiment of the first and second side walls  32  and  33  which may be substantially interrupted at various intervals by circular holes  50  and slots  52  extending from the holes to inner edge  53  of support  22 . It will be appreciated that slot  52  permits some movement of the adjacent walls to allow for expansion or contraction. Further, circular opening  50  permits material deformation that may accompany changes in the width of slot  52  to be spread over a greater area to limit stress concentrations. While the particular use of the combination of slots and circular openings are contemplated in a preferred embodiment, it will be appreciated that other side wall interruptions or configurations that allow for at least some expansion or contraction may be utilized with the present invention. In another form of the present invention, the sidewalls are continuous and have no interruption or slots cut therein.  
         [0022]    An expansion control material  36  is disposed within inner cavity  34 . Expansion control material  36  includes an outer surface  40  adapted to bear against inner surface  38  of continuous hoop  30 . As discussed more fully below, expansion control material  36  may be maintained in position by engagement with inner surface  38  under a compressive load. Further sidewalls  32  and  33  inhibit forward and backward movement. Still further, once assembled, blade track  24  includes a finger member  46  adapted to limit inward buckling of the expansion control material. It is understood herein that blade track preferably includes a plurality of blade track segments. The finger member in one embodiment extends continuous along the assembled blade track and in an alternate embodiment is discontinuous.  
         [0023]    In a preferred form of the invention, expansion control material  36  comprises multiple materials having different thermal expansion coefficients. Referring to FIGS. 2 and 6, there is illustrated a preferred form of the expansion control material  36  which comprises alternating ceramic members  64  and metallic spacers  60 . The ceramic members  64  and the metallic spacers  60  are preferably elongated in the direction of the centerline X. It is preferred that the ceramic members  64  have a rounded outer surface to minimize point loading and more preferably are cylindrical in shape. In FIG. 2, the ceramic members  64  are shown in phantom lines and the metallic spacers  60  are shown in solid lines. While the combination of ceramic and metallic spacers is shown in the preferred embodiment, other materials may be utilized in accordance with the present invention to tailor the blade track expansion characteristics. Additionally, it is desired that the ceramic members  64  have a low thermal expansion coefficient such that the inner diameter of the support member and attached blade track remains substantially constant in diameter, or with only minor variations, over a wide range of operating conditions for the gas turbine engine. Ceramic members  64  preferably have a thermal expansion coefficient lower than the thermal expansion coefficient of the hoop  30  and the spacers  60 . Further, the expansion control material may be selected to impart greater or lesser expansion forces on the support member to impart the desired blade tip clearances for various applications.  
         [0024]    The blade track assembly according to the present invention may be assembled in the following steps. The assembled fluid sealing system  20  has the ceramic members  64  disposed between metallic spacers  60  and loaded against the inner surface  38  within the cavity  34  of the hoop  30 . A blade track support member with a continuous hoop is provided. Ceramic cylinders  64  and metallic spacers  60  are fitted into position within interior cavity  34  to engage one another and put support ring  30  in tension by pressing against surface  38 . The cylinders  64  and spacers  60  would likewise be loaded in compression against the inner surface  38  of the hoop  30 . During assembly, it is contemplated the last cylinder would be pressed into position. With reference to FIG. 7, there is illustrated one form of the ceramic cylinders  64  and the metallic spacers being finally installed against the continuous hoop member  30 . The hoop member is heated to a predetermined temperature and the ceramic cylinders  64  and metallic spacers  60  are set into position. A load is applied to push the last ceramic cylinder  64  and metallic spacers  60  into position. The blade track segments  24  are then moved axially into position. In one form of the present invention the predetermined temperature is about 500° F.  
         [0025]    As previously mentioned, fingers  46  on the blade track would engage surface  41  on spacers  60  to inhibit the cylinder and spacer assembly forming the expansion control material from buckling inward. The blade tracks  24  are held in place by engagement of flange  42  on wall  33  with recess  44  on the blade track and corresponding structures formed on opposite wall  32  and blade track portion. As will be understood, the combination of metallic and ceramic components in the compression stack placed in the inner cavity  34  permits the expansion control material to be tailored to meet specific thermal expansion characteristics.  
         [0026]    A cylindrical shape for the ceramic cylinders  64  is preferred to decrease the average and peak stresses applied on the surface. Still more preferably, the metal spacers would be shaped and coated to decrease the bearing stresses in the parts. The spacers preferably include a curved portion  100  that corresponds to the shape of the outer surface of the ceramic members  64 . In one form of the present invention the metallic spacers are formed of MAR-M247 and the ceramic members are formed of silicon nitride. In another form of the present invention the metallic spacers are coated with a high temperature dry film lubricant. However, other materials and coatings are contemplated herein. As shown in FIG.  6 , it is contemplated that a slight gap  66  may be created between adjacent spacers  60  and  62 . The gap is preferably within a range of about 0.020 inches to about 0.060 inches. It will be understood that should ceramic cylinder  64  deteriorate, collapse, or otherwise fail, gap  66  may close and the system may continue to operate, at a slightly lower effectiveness.  
         [0027]    Referring to FIG. 8, there is illustrated a graph showing advantages that may be achieved with use of the present invention for controlling blade track expansion. Specifically, in the preferred embodiment illustrated and described herein, the effect on one application was to lower the thermal expansion coefficient of the support assembly from 9.0×10 −6 in/in/deg. F to 5.0×10 −6  in/in/deg. F. As shown in FIG. 7, the tip clearance required due to thermal transients can be reduced from 0.078 inches to 0.032 inches (a 60% reduction) for one application such as shown in FIG. 2.  
         [0028]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least a portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary.