Abstract:
A combustion chamber is supported in a gas turbine engine by a mounting having a first attachment assembly secured to a wall of the combustion chamber, and a second attachment assembly secured to a support structure rigidly mounted from an engine housing. The first attachment assembly includes a pair of clamp surfaces pressed by a spring to grip parallelly-spaced inner and outer surfaces of the wall. Radial thermal expansion and contraction of the wall, relative to the first attachment assembly, are accommodated by allowing radial slippage between the clamp surfaces and the gripping surfaces. The second attachment assembly includes a spring which permits the combustion chamber and the mounting to tilt relative to the support structure. By accommodating both differential radial movement and tilting, the thermal stresses in the material forming the combustion chamber are reduced.

Description:
FIELD OF THE INVENTION 
     This invention is concerned with the mounting of a combustion chamber in a gas turbine engine, and also to a combustion chamber mounting for supporting a combustion chamber within a gas turbine engine. More particularly, this invention is concerned with providing a combustion chamber mounting that will relieve stress resulting from differential thermal movement between a combustion chamber and surrounding structures to which the combustion chamber is attached. 
     BACKGROUND OF THE INVENTION 
     In designing a gas turbine engine, it is desirable to increase the firing temperature to achieve greater thermal efficiency, and this entails the use of high temperature materials for the combustion chambers and associated transition ducts. High temperature alloys are commonly used to form a combustion chamber, but firing temperatures have risen above the highest operational temperature of these special alloys, necessitating either the provision of a cooling system (which increases manufacturing cost and reduces thermal efficiency), or forming the combustion chamber from a ceramic material capable of operating at the higher temperature. 
     A gas turbine combustion chamber is usually supported from a support structure, such as an engine or compressor casing, by at least one mounting. Under operating conditions, all of these components undergo thermal expansion which has to be accommodated by the design in order to avoid excessive stresses and strains. 
     Ceramic materials typically have a lower coefficient of thermal expansion than the materials forming the mounting and the associated support structure, with the consequence that substantial differential thermal expansion occurs between a ceramic combustion chamber and its mounting 
     Although a ceramic combustion chamber enables a higher firing temperature to be used, the stresses and strains, caused by thermal expansion and distortion of the combustion chamber and also by differential thermal expansion between the combustion chamber and its mounting, can result in failure of the combustion chamber by cracking due to the inherent brittleness of the ceramic material. 
     It is known from UK Patent Specification GB 1,476,414 for a ceramic combustion chamber to have a generally cylindrical side wall and a discharge end located, with freedom for axial expansion and contraction, within ducting for receiving the combustion products. An upstream end of the chamber is closed by an integral dome portion defining a central circular opening for an annular abutment which locates the fuel spray nozzle. This annular abutment is rigidly secured by studs to a combustion chamber cover and includes an annular flange which supports a slightly yieldable or resilient gasket positioned to react against the inner surface of the dome portion around the central circular opening. This inner surface of the dome portion is urged towards the rigid annular flange by a spring which reacts between structure rigidly mounted from the combustion chamber cover, and a rigid washer which bears against the outer surface of the dome portion around the central circular opening. There is no teaching concerning the accommodation of differential radial movement between the ceramic combustion chamber, the annular abutment and the combustion case cover. To the contrary, the spring urges the dome portion against the slightly yieldable or resilient gasket which is rigidly supported by the combustion case cover. The force exerted by this spring is clearly provided to cause the gasket to effect a seal between the rigidly mounted annular abutment and the dome portion of the combustion chamber. 
     SUMMARY OF THE INVENTION 
     This invention is based on the realization that stresses caused by thermal expansion and contraction of a combustion chamber, relative to the structures to which it is attached, can be relieved by permitting differential radial movement between the combustion chamber and its mounting, and by also permitting the combustion chamber to tilt relative to the structure from which it is supported. Such tilting can be caused by the thermal gradient between the cooler upstream end and the much hotter downstream end of the combustion chamber, and particularly by any thermal gradient transverse to the combustion chamber. Such transverse thermal gradients can be significant in a gas turbine having an annular array of combustion chambers, the combustion chamber walls adjacent the turbine axis being hotter. 
     According to one aspect of the invention a gas turbine has a combustion chamber secured to a mounting by a first attachment means arranged to accommodate differential radial movement between the combustion chamber and the mounting, the mounting being secured to a support structure by a second attachment means arranged to permit the combustion chamber to tilt relative to the support structure. In this manner the combustion chamber is positively located and supported by the mounting, which accommodates differential tilting and radial expansion and contraction of the combustion chamber, thereby avoiding the generation of excessive thermal stresses and strains in the material forming the combustion chamber. Whilst reduction of thermal stresses and strains is desirable for most combustion chambers, it is particularly beneficial when the combustion chamber is formed from a ceramic material. 
     The first attachment means preferably extends through an aperture in a wall of the combustion chamber and defines a pair of opposed surfaces shaped respectively to engage inner and outer surfaces of the combustion chamber wall adjacent the aperture, and the first attachment means also includes a biasing means operative to cause the opposed surfaces to grip the inner and outer surfaces of the combustion chamber wall with a force sufficient to secure the combustion chamber to the mounting whilst permitting differential radial movement between the combustion chamber wall and the opposed surfaces. With this arrangement, one of the opposed surfaces may be defined by a first member that is axially secured to the mounting, the other opposed surface being defined by a second member that is mounted for axial movement relative to the mounting, and the biasing means being arranged to react between the mounting and the second member. In this case an axial adjustment device may be arranged operatively between the first member and the mounting to enable the position of the said one opposed surface to be adjusted axially of the mounting, Preferably the first member is positioned within the combustion chamber. 
     The second attachment means preferably includes a second biasing device operative to oppose movement of the mounting relative to the support structure. 
     The mounting may be tubular and surrounds a fuel burner. In this case the support structure is preferably an air inlet guide vane communicating with the combustion chamber through the tubular mounting. 
     The mounting may include at least one duct for the passage of cooling air. 
     The combustion chamber is preferably formed of a ceramic material which may comprise woven continuous fibers embedded in a silicon carbide matrix. The surfaces of the first attachment means that are to contact the ceramic combustion chamber are preferably covered with an abradable metallic coating. 
     According to another aspect of the invention a combustion chamber mounting has: 
     first attachment means for securing a combustion chamber to the mounting, said first attachment means being capable of accommodating limited differential radial movement between the combustion chamber and the mounting, and 
     second attachment means for securing the mounting to a support structure, said second attachment means being capable of permitting the mounting, and hence the combustion chamber, to tilt a limited amount relative to the support structure. 
     Further aspects of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic axial section through part of a gas turbine, illustrating the mounting of a ceramic combustion chamber, and 
     FIG. 2 is a enlarged view of the combustion chamber mounting illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a combustion chamber  10  has a cylindrical wall  11  of which the downstream end  12  is radially located within a transition duct  13  arranged in known manner to conduct the combustion gases to a compressor turbine and, if appropriate, a power turbine. A piston ring sliding seal  14  is axially located by an internal annular groove  15  formed in the transition duct  13  to permit limited relative axial movement between the combustion chamber  10  and the transition duct  13  in the direction of the cylindrical axis X—X. The upstream end  16  of the combustion chamber  10  is partially closed by an integral radial wall  17  formed with a circular aperture  18  which is preferably coaxial with the axis X—X. Although the wall  17  is illustrated as extending in a radial plane, it may instead be frusto-conical or of another configuration provided that it defines spaced-apart inner and outer surfaces  19 ,  20  extending substantially parallel to each other and surrounding the edge of aperture  18 . In the event that the wall  17  is frusto-conical, it is advantageous for the portions of the inner and outer surfaces  19 ,  20  immediately surrounding the edge of the aperture  18  to be radial. 
     The combustion chamber  10  is formed from an appropriate ceramic material, for instance, woven continuous fibers embedded in a silicon carbide matrix. If desired, a thermal insulation layer may be fixed to the internal surfaces of the combustion chamber. However, the combustion chamber could instead be made of any suitable material. 
     The radial wall  17  is secured by a first attachment means  21  to a tubular mounting  22  which is in turn secured by a second attachment means  23  to a support structure  24  in the form of an inlet guide vane. The support structure  24  is rigidly mounted from an unshown engine housing and serves both to support the mounting  22 , and as a duct D to direct a flow of air from the compressor into the mounting  22  which also acts as a housing for a fuel burner B. The second attachment means  23  therefore attaches the mounting  22  to the support structure  24  and constitutes a support structure attachment for the mounting  22 . The first attachment means  21  attaches the mounting  22  to the combustion chamber  10  and constitutes a combustion chamber attachment for the mounting  22 . 
     The construction and operation of the first attachment means  21 , the mounting  22  and the second attachment means  23  are now described with reference to FIG. 2 which shows the various components to a larger scale. 
     From FIG. 2 it will be noted that the external cylindrical surface of the mounting  22  is formed externally with a first series of longitudinal slots  25  for mounting the first attachment means  21 , and with a second series of longitudinal slots  26  for mounting the second attachment means  23 . The external cylindrical surface of the mounting  22  is also formed with a screw thread  27 . 
     The first attachment means  21  comprises a downstream or inner clamp plate  28 , an upstream or outer clamp plate  29 , a locking nut  30 , a lock washer  31  and a biasing means in the form of a frusto-conical, or Belleville, spring  32 . The inner clamp plate  28  has an internal screw thread engaging the screw thread  27  and is thereby secured radially to the mounting  22  whilst permitting relative axial adjustment. The inner clamp plate  28  defines a spigot  33  which fits, as illustrated, within the slightly larger diameter of the circular aperture  18 , leaving a clearance C whose size relative to adjacent components is shown exaggerated for clarity of illustration. The combustion chamber  10  is thereby located radially from both the inner clamp plate  28  and the mounting  22 , with clearance C allowing for limited differential radial movement (expansion or contraction) between the ceramic combustor wall  17  and the combination of the mounting  22  and the attachment means  21 . The combustion chamber  10  is axially located, relative to the mounting  22 , by a clamp surface  34  which is defined by the inner clamp plate  28  and abuts the inner surface  19  of the radial wall  17 , the relative axial position of the combustion chamber  10  being adjustable by rotating the inner clamp plate  28  relative to the mounting  22 . The lock washer  31  is formed with inwardly directed integral tangs  35  which engage the first series of longitudinal slots  25 , thereby preventing relative rotation between the lock washer  31  and the mounting  22  whilst permitting relative axial movement. When the inner clamp plate  28  has been adjusted to the required axial position, it is axially secured to the mounting  22  by sliding the locking washer  31  to the right, as seen in FIG. 2, until axially-directed pins or projections  36 , formed integral with the inner clamp plate  28 , are engaged within radial-slots  37  formed through the locking washer  31 . 
     The outer clamp plate  29  is mounted, for axial movement, on a cylindrical hub  38  formed integral with the inner clamp plate  28 , and defines a second clamp surface  39  which abuts the outer surface  20  of the radial wall  17 . In this manner, the inner and outer surfaces  19 ,  20  of the radial wall  17  are gripped between the clamp surfaces  34 ,  39  under the action of the spring  32 , thereby securing the combustion chamber  10  axially to the mounting  22 . The locking nut  30  is also mounted on the screw thread  27  so that the force exerted by the spring  32  can be adjusted to a desired value by rotating the locking nut  30  relative to the mounting  22 . After such adjustment, the locking nut  30  is secured to the locking washer  31  by known means, for example, either by wiring, or by deforming an outer edge portion of the locking washer  31  into an unshown detent in the adjacent peripheral edge of the locking nut  30 . It will be noted that the locking nut  30  additionally serves to retain the locking washer  31  in engagement with the pins  36 . The spring  32  is located by an annular lip  40  formed integral with the outer clamp plate  29 . 
     In addition to securing the combustion chamber  10  to the mounting  22 , the first attachment means  21  is designed to permit differential radial movement between the radial wall  17  and the clamp plates  28  and  29 . This is achieved by choosing the force exerted by the spring  32  to permit radial slippage between the clamp plates  28 ,  29  and the abutting surfaces  19 ,  20  of the radial wall  17 , thereby limiting the stresses that would otherwise have arisen due to radial expansion or contraction of the combustion chamber  10  relative to the first attachment means  21 . This slippage is enhanced by coating the clamp surfaces  34 ,  39  with an abradable material such as that marketed under the designation METCO  314  NS. As the surfaces  19 ,  20  are formed of ceramic material, they are much harder and rougher than the metal clamp surfaces  34 ,  39 . As a consequence, the ceramic surfaces  19 ,  20  become loaded with abraded particles of the coating thereby generating a smoother surface on the ceramic surfaces  19 ,  20  and facilitating relative movement in the plane of the radial wall  17 . Other surface treatments may be used to reduce the friction between the clamp surfaces  34 ,  39  and the inner and outer walls  19 ,  20  irrespective of whether the combustion chamber is formed from ceramic or other material. 
     The second attachment means  23  comprises a clamp ring  41 , a locking ring  42 , a biasing means in the form of a frusto-conical spring  43 , a spacer  44 , and a series of bolts  45  which pass through and secure the clamp ring  41 , the locking ring  42  and the spacer  44  to the support structure provided by the support structure  24 . 
     The second series of longitudinal slots  26  engage tangs  46  formed integral with the locking ring  42 , thereby preventing rotation of the mounting  22  relative to the support structure  24 . The left-hand end of the tubular mounting  22  is enlarged, as shown, and is retained by an inwardly-directed annular flange  47  which is formed integral with the clamping ring  41  and resists the net axial combustion force applied to the combustion chamber  10 . 
     The spring  43  reacts between the support structure  24  and the left-hand end of the mounting  22  with a force determined by the spring rating and the thickness of the spacer  44 . In this manner, the mounting  22  and the attached combustion chamber  10  are located axially in the position shown in FIG. 2, whilst the spring  43  permits tilting movement of the combustion chamber  10  relative to the support structure  24 ; such tilting movement being caused by differential thermal movement between its downstream end  12  and its upstream end  16 . The rating of spring  43  is chosen to be less than that of spring  32  to ensure that such tilting is only permitted in the region of spring  43 . Thus, spring  32  accommodates only differential radial movement whilst spring  43  permits only tilting movement, the ratings of these springs being chosen to limit the stresses within the material forming the combustion chamber  10  to an acceptable level. 
     A working clearance  48  is provided between the flange  47  and the outer diameter of the mounting  22  to accommodate the required range of tilting movement of the mounting  22  relative to the support structure  24 . 
     By way of an overview of the working of the above-described assembly, it may be noted that the first attachment means  21  accomniodates limited differential radial movement between the combustion chamber  10  and the mounting  22  in that the spring  32  biases the members  28 ,  30  together to clamp the combustor wall  17  between them in radially slideable fashion while simultaneously restraining axial movement of the combustor relative to the mounting  22 . At the same time, the second attachment means  23  permits the mounting  22  (and hence the combustion chamber) to tilt a limited amount relative to the support structure  24 , in that the spring  43  biases these two components apart by exerting a separation force between them, while simultaneously restraining relative radial movement between them, due to the abutment of the radially inner and outer edges of the spring  43  against the mounting  22  and the spacer  44  respectively. 
     Although the springs  32 ,  43  are illustrated as single frusto-conical spring washers, other forms of biasing means, such as wave springs, may be used. 
     In view of the high firing temperature within the combustion chamber  10 , it may be necessary to cool at least some of the components of the first and second attachment means  21 ,  23  and the mounting  22 . This can be achieved by arranging air passages in appropriate locations through the mounting. 
     In addition to controlling the level of stresses caused by thermal expansion and contraction, it should be noted that the axial forces exerted by the springs  32  and  43  should also be chosen to take due account of the oscillating combustion forces within the combustion chamber  10  and the range of the residual axial force exerted on, the combustion chamber  10 , to ensure that the system will not vibrate.