Abstract:
A heat shield for a turbine shroud in a gas turbine engine. The heat shield takes the form of an annular U-shaped shell, with the open part of the U facing radially inward. The shell covers an annular flange, or other body, and is mounted to that body, or an associated body. Pleats, bellows, convolutions, or other deformations in the shell reduce the axial modulus of elasticity of the shell. Thus, thermal expansion and contraction of the shell apply reduced forces to the body to which the shell is mounted.

Description:
TECHNICAL FIELD  
       [0001]     The invention concerns a heat shield for a turbine casing in a gas turbine engine.  
       BACKGROUND OF THE INVENTION  
       [0002]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine. Turbine  3  is surrounded by a shroud  6 .  FIG. 2  is a simplified perspective view of the shroud  6 .  FIG. 3  is a cross-sectional view, in the direction of arrows  3 - 3  in  FIG. 2 .  
         [0003]     Each part  6 A and  6 B of the shroud  6  in  FIG. 3  contains an annular flange  9 A and  9 B. Holes  12  extend through the flanges, as also indicated in  FIG. 4 , and the parts  6 A and  6 B in  FIGS. 3 and 4  are assembled together by bolts (not shown).  
         [0004]     In some designs, a heat shield  18  in  FIG. 5 , shown in partial exploded form, surrounds the flanges  9 A and  9 B, to control temperature attained by the flanges  9 A and  9 B. Some features of the heat shield  18  will be explained.  
         [0005]     In many instances, the heat shield  18  is constructed in segments, as in  FIG. 5 . This segmentation can cause the problem illustrated in  FIG. 6 , which shows the segmented heat shield  18  alone, without the shroud  6 . Hot or cold air, indicated by dashed arrow  21 , can penetrate the joint between adjacent segments  18 A and  18 B.  
         [0006]     In addition, the assembled combination the heat shield and the shroud can act as a bi-metallic thermal element, as will be explained with reference to  FIGS. 7-9 .  FIG. 7  shows a segment  18 A of the heat shield, and part of the shroud  6 , which bears part of the flange  9 .  
         [0007]      FIGS. 8 and 9  show the segment  18 A of the heat shield connected to the flange  9 . Circles  25  represent bolts, which attach the segment  18 A to the flange  9 .  
         [0008]     If the segment  18 A is hotter than the shroud/flange assembly, the system will bend into the phantom shape  27  indicated in  FIG. 8 .  
         [0009]     Conversely, if the segment  18 A is cooler than the shroud/flange assembly, the system will bend into the phantom shape  30  indicated in  FIG. 9 .  
         [0010]     The deformations of  FIGS. 8 and 9 , and the leakage of  FIG. 6 , are not desirable in many situations. The deformations can increase clearances between the rotating and static components, which is not desirable. For example, if the space between the outer tip of a turbine blade and the shroud surrounding the blade increases, then additional leakage occurs, which causes a penalty in efficiency.  
       SUMMARY OF THE INVENTION  
       [0011]     In one form of the invention, an annular hollow heat shield surrounds an annular flange in a turbine shroud in a gas turbine engine. Deformations in the walls of the heat shield allow the heat shield to change in circumference in response to changes in temperature, without applying significant force to the shroud. The deformations can take the form of convolutions, pleats, bellows, and the like. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  illustrates a schematic cross-sectional view of a gas turbine engine.  
         [0013]      FIG. 2  illustrates a schematic view of the turbine shroud  6  of  FIG. 1 .  
         [0014]      FIG. 3  illustrates a cross-sectional view of the shroud  6  of  FIG. 2 , taken in the direction of arrows  3 - 3 .  
         [0015]      FIG. 4  illustrates bolt holes  12  formed in flanges of the shroud  6 .  
         [0016]      FIG. 5  illustrates a heat shield commonly used to protect the flanges  9 A and  9 B.  
         [0017]      FIG. 6  illustrates infiltration of air  21  at the junction between adjacent sections  18 A and  18 B of the heat shield.  
         [0018]      FIG. 7  illustrates an exploded view of a segment  18 A of a heat shield and part of the shroud  6 .  
         [0019]      FIGS. 8 and 9  illustrate two types of deformation which can occur when the heat shield  18 A and the shroud  6  reach different temperatures.  
         [0020]      FIGS. 10-12  illustrate one form of the invention.  
         [0021]      FIG. 13  illustrates an insulating blanket  65 , provided by one form of the invention.  
         [0022]      FIG. 14  illustrates a circular array of shells  50  and  51 , surrounding a shroud  40 .  
         [0023]      FIG. 15  illustrates an assembly of shells  50  and  51  in their normal state.  
         [0024]      FIG. 16  illustrates an assembly of shells  50  and  51  in an expanded state.  
         [0025]      FIG. 17  illustrates an assembly of shells  50  and  51  in a compressed state.  
         [0026]      FIG. 18  illustrates another form of the invention.  
         [0027]      FIGS. 19 and 22  illustrate other forms of the invention.  
         [0028]      FIG. 20  illustrates schematically the apparatus of  FIG. 14  installed in a gas turbine engine  100 .  
         [0029]      FIG. 21  illustrates one approach to sealing adjacent sectors of a heat shield.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     For ease of explanation, one type of heat shield embodied by the invention will be constructed conceptually in stepwise fashion. The sequence of construction of an actual heat shield will not necessarily correspond to the conceptual steps discussed.  
         [0031]      FIG. 10  illustrates a two-part turbine shroud  40 , having flanges  43 . Bolt holes  45  will contain bolts (not shown) which hold the flanges  43  together. For simplicity, the shroud  40  is shown as linear, although, in practice, it will assume the shape of a hoop, with flanges  43  on the radially outer side.  
         [0032]     Channels, or housings,  50  and  51  represent the heat shield, and are constructed of known heat-shield material. Two types of channel are present: channel  50 , which is smaller, and channel  51 , which is larger.  
         [0033]      FIG. 11  shows the channels  50  and  51  positioned next to each other.  FIG. 12  shows bulkheads  55  added to the ends of the larger channels  51 .  FIG. 13  shows a larger number of channels  50  and  51 , positioned on the flanges  43 .  
         [0034]      FIG. 14  shows the channels  50  and  51 , and the shroud  40 , in their actual circular configurations. Outer surfaces  57  are shown as arcuate, but they may be flat. That is, the individual channels  50  and  51  may be box-like, with flat sides.  
         [0035]     The heat-shield channels  50  and  51  form a circular array surrounding the flanges  43 . This arrangement provides several advantageous features, some of which will now be explained.  
         [0036]     As shown in  FIG. 15 , the smaller channels  50  contain holes  60 . Bolts, not shown, extend through the holes  60  to connect the smaller channels  50  to the flanges  43  in  FIG. 10 . When connected, the smaller channels  50  are in good thermal contact with the flanges  43  in  FIG. 10 . From another perspective, the inner surfaces  63  in  FIG. 10  of the smaller channels  50  are in physical contact with the flanges  43 .  
         [0037]     In another embodiment, the inner surfaces  63  are not in thermal contact with the flanges  43 , but are separated from the flanges  43 , as by an intervening layer of material (not shown). In yet another embodiment, bushings  125  in  FIG. 22  are placed around the bolts, to separate the inner surfaces  63  in  FIG. 10  from the flanges  43 , although the bushings themselves do contact the flanges  43 . In this latter embodiment, an air space is created between the inner surfaces  63  and the flanges  43 , except at the bushings.  
         [0038]     The larger channels  51  in  FIG. 13  are separated from the flanges  43 . Both channels  50  and  51  together encapsulate the flanges  43 . The larger channels  51  cooperate with the flanges  43  to define an air space, or blanket,  65  adjacent the flanges  43 , as indicated by exploded channel  51 A. Preferably, this blanket  65  is at least one millimeter in thickness, represented by dimension  70 . One specific thickness contemplated is 12 millimeters, or about {fraction (1/2)} inch. The invention specifically covers all thickness between one millimeter and 60 millimeters, as well as larger thicknesses.  
         [0039]     The question of thickness of blanket  65  can be viewed from another perspective. In general, when two flat materials are placed into contact, such as two flat pieces of glass, some air molecules generally remain between the two materials. Those air molecules could be termed a “blanket.” But, in this glass-example, some atoms of one material (one glass sheet) are nevertheless in contact with atoms of the other material (the other glass sheet).  
         [0040]     This contact may be illustrated by common sandpaper. If the rough sides of two sheets of sandpaper are placed together, then the tips of sand grains of one sheet will contact either the sand grains or the paper of the other sheet. Air will surround the sand grains, and could be termed a “blanket.” 
         [0041]     At the microscopic level, the sheets of glass resemble the sheets of sandpaper.  
         [0042]     However, in one form of the invention, this type of contact is preferably not present inside larger channels  51 . Blanket  65  completely separates the channel  51  from the flanges  43 , except possibly at bulkheads  55  in  FIG. 12 . No atoms of the flange  43  extend through the blanket  65  and contact the inner surface of the channel  51 , except possibly at the bulkheads  55 .  
         [0043]     Since the blanket  65  in  FIG. 13  is constructed of air, which is a very good insulator, the heat-shielding properties of the channel  51  are enhanced by the blanket  65 .  
         [0044]     Another advantageous feature resides in a physical characteristic of bulkheads  55  in  FIG. 12 . The bulkheads  55  act as flexible diaphragms. They remove, or reduce, the deformations illustrated in  FIGS. 7-9 .  
         [0045]     For example,  FIG. 15  illustrates the bulkheads  55  in their undeformed state. If the turbine shroud (not shown) should undergo thermal expansion, relative to the channels  50  and  51 , then bulkheads  55  bow outward, as indicated in  FIG. 16 . The overall length of the assembly of channels  50  and  51  increases.  
         [0046]     Conversely, if the turbine shroud (not shown) should undergo thermal contraction, the bulkheads  55  bow inward, as in  FIG. 17 . The overall length of the assembly of channels  50  and  51  decreases.  
         [0047]     Thus, the bulkheads  55  allow an accordion-style, or bellows-style, expansion and contraction of the assembled channels  50  and  51 . This expansion and contraction reduces, or eliminates, the deformations illustrated in  FIGS. 8 and 9 .  
         [0048]     A numerical value for the reduction of deformation will be given for one embodiment. The heat shield  72  in  FIG. 12  is a shell-like structure. It is hollow. The modulus of elasticity of the overall shell-like structure is determined by the material, and geometry, of the walls of the structure.  
         [0049]     This modulus of elasticity of the shell-structure (as opposed to the modulus of elasticity of the material itself of which the shell-structure is constructed) is less than fifty percent, and preferably ten percent, of the modulus of elasticity of the overall shroud  40  of  FIG. 10 . An example will illustrate the significance of this percentage.  
         [0050]     Assume that a pair of forces  70 A and  70 B are applied to the shroud  40  in  FIG. 11 . Assume that those forces cause a percentage elongation (ie, strain) of 0.01 percent. If the same strain (ie, percentage elongation) is to be attained in the heat shield  72  in  FIG. 12 , then pair of forces  68 A and  68 B are required. Those forces  68 A and  68 B must be about ten percent of the forces  70 A and  70 B in  FIG. 11 , which is the percentage given in the preceding paragraph.  
         [0051]     Stating this another way, assume that the moduli of elasticity of shroud  6  in  FIG. 8  is equal to that of the shell-like heat shield  18 A. A given deformation occurs at a given temperature difference between the shroud  6  and the shield  18 A. However, if the modulus of the shield  18 A is ten percent of that of the shroud  6 , as stated above, then the deformation will, roughly, be about that same percentage, namely ten percent, of the deformation occurring when the moduli are equal.  
         [0052]     The large discrepancy in size between the forces  68 A and  68 B in  FIG. 12  and forces  70 A and  70 B in  FIG. 11  is taken to indicate that the deformation of the type shown in  FIGS. 8 and 9  is effectively eliminated, or substantially reduced.  
         [0053]     The modulus of elasticity under consideration, which is found based on forces  68 A and  68 B in  FIG. 12 , will be termed an axial modulus of elasticity. One reason is that the elongation, or contraction, of the heat shield  72  which occurs in response to the forces does so in the direction of the longitudinal axis of the heat shield  72 . Of course, the heat shield  72  is an annular structure. Nevertheless, short sections can be viewed as linear, and having a longitudinal axis. This concept of axial modulus also applies to the shroud  40  in  FIG. 14 .  
         [0054]      FIG. 18  illustrates another form of the invention. The larger channels  51  can be equipped with depressions  75 , which mate with the flanges  43 , and act as air seals. Stated in other words, the base  76  of shell  51  is equipped with a flange  78  which engages flange  43 , to form a seal.  
         [0055]      FIG. 19  illustrates another form of the invention, wherein a U-shaped channel  80  is formed in some, or all, of the larger shells  51 . Each U-shaped channel  80  adds two additional bulkheads, or diaphragms,  55 . The added diaphragms  55  provide additional flexibility.  
         [0056]     The inner surface of the base  86  of the U-shaped channel  80  may, or may not, contact the flanges  43  (not shown in  FIG. 19 ). In addition, a true bellows may be formed in some, or all, of the larger channels  51 , as indicated by bellows  90 .  
         [0057]      FIG. 22  illustrates an other form of the invention. All sections  51  are of the same cross-sectional size and shape. Adjacent sections  51  are connected by pleats, bellows, or deformations, such as those shown in  FIG. 19 , and indicated as elements  91  in  FIG. 21 . Periodic bolt holes  120  are provided, and bushings  125  space sections  51  from the flanges  43 .  
         [0058]     Some significant features of the invention include the following. One is that the heat shield  72  in  FIG. 14  is a continuous structure, at least in the sense of being impervious to air flow, except possibly at the locations where the heat shield contacts the shroud, namely, at region  76  in  FIG. 18 . That is, unlike the prior-art situation of  FIG. 6 , no leakage exists at junctions between adjacent channels  50  and  51 .  
         [0059]     The heat shield  72  may be constructed in two halves, defined by the split line  68 B in  FIG. 12 . The two halves are mirror images of each other. The single split line, or seam, is less than the number of seams found in the prior art. Thus, opportunities for leakage through the single split line  68 B is less than in the multiple seams in the prior art.  
         [0060]     A second feature is that the heat shield  72  in  FIG. 14  can be viewed as constructed of two types of units. One unit  50  spans a first sector  100  of the shroud  40 , and acts as a mounting unit. This unit is U-shaped, with at least the legs of the U in thermal contact with the flanges  43  of  FIG. 13 . A second unit  51  in  FIG. 14  spans a second sector  105  of the shroud  40 , and contains the blanket  65  of  FIG. 13 . The two units are sealed to each other by bulkheads  55  in  FIG. 12 .  
         [0061]     A third is that the heat shield  72  in  FIG. 14  can be viewed as containing an array of housings  51 , between which are interleaved brackets  50 . The housings  51  and brackets  50  are connected to each other, through bulkheads  55  in  FIG. 12  which act as gas seals. The brackets  50  connect the assembly to the flanges  43  in  FIG. 13 .  
         [0062]     A fourth feature is that the heat shield  72  in  FIG. 14  can be constructed in sectors. The structure shown in  FIG. 12  can represent one sector, though linearized in depiction. Adjacent sectors are sealed to each other, as by overlapping bulkheads  55 A, as in  FIG. 21 . Such seals are known in the arts of sheet-metal working, particularly as applied to metal roofing and heating duct work.  
         [0063]     In the case of  FIG. 21 , it is possible that the axial modulus of elasticity is only defined in tension, and not in compression, if the joint, or seal, used does not resist compression.  
         [0064]     An axial modulus of elasticity of less than fifty percent, and preferably ten percent, for the heat shield was discussed above. Different embodiments can utilize all percentages from one to fifty, respectively.  
         [0065]     Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.