Abstract:
A turbine stator of a gas turbine engine includes a turbine casing, a turbine shroud ring and a shroud ring support connecting the shroud ring to the casing. The stator is one wherein the support is provided with a heat shield positioned on the turbine side. This assembly makes it possible to reduce the take-up of play during transient operating phases.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of turbine machines and is aimed at a means for controlling the clearance there is between the tips of the moving turbine blades and the casing. 
     A gas turbine engine conventionally comprises a compressor, in one or more stages, a combustion chamber and one or more turbine stages. The compressor, which is connected to the turbine, supplies the combustion chamber with air and the hot gases produced are directed onto the turbine in order to extract their energy. The compressor and turbine rotors have sets of blades at their periphery moving at right angles to the engine axis inside annular stator components that form shroud rings with respect to which they enjoy an operating clearance. This clearance needs to be large enough that no friction will slow the rotation of the moving parts but needs to be controlled in order to prevent a substantial amount of fluid from being diverted away from the active surfaces of the sets of blades. In order to ensure the highest possible efficiency, it is therefore important to control this clearance. 
     The present invention is concerned with the operating clearance of a turbine motor and more especially of the rotor positioned immediately downstream of the combustion chamber. In a multiple-spool engine, that is to say an engine comprising two or more, generally no more than three, independent shafts, this will be the high-pressure spool. 
     The radial clearances at the blade tips are the result of the various radial thermomechanical movements between the rotors and the stators.  FIG. 1  shows an axial half section of a gas turbine engine  1 , viewed in the region of the high-pressure turbine. The turbine rotor  3  comprises a disk  31 , provided with blades  33  distributed around its rim, and mounted transversely on a central shaft. The rotor is positioned downstream of a nozzle guide vane stage  5  communicating with the combustion chamber  7  only the bottom of which can be seen here. The casing  9  is made up of several shell rings assembled by flanges. There is a distinction between the combustion chamber casing  91  and the high-pressure turbine casing  93 . The two casings are held by a flanged assembly  95 . The casing supports the elements of the combustion chamber, the upstream  5  and downstream  15  nozzle guide vanes and a support  11  for a shroud ring  13 . 
     The radial clearance between the tips of the blades  33  and the shroud ring  11  is thus the resultant of several types of movement:
         thermal displacements resulting from the expansion of the materials as the temperature varies,   mechanical displacements resulting from the variations in centrifugal force applied to the rotating parts, and variations in pressure.       

     The disks, the blades and the stator elements are subjected to both mechanical and thermal displacements. 
     During the various engine operating phases, because of these displacements which will not always be in the same direction, the radial clearance is not therefore constant. In particular, the rotor and the stator do not have displacements of equal amplitude, nor do they have the same thermal response time. 
       FIG. 2  shows the change in displacement of the rotor R and of the stator S respectively as a function of the variation in engine speed over time. Thus, it can be seen that the take-up of transient clearance A is greater than that B obtained after thermal stabilization. The take-up of clearance is to be understood to mean the magnitude of the displacement of the rotor minus that of the stator. 
     DESCRIPTION OF THE PRIOR ART 
     It is known practice to use clearance-control devices comprising ventilation means in order to control the thermal expansion of the elements of which it is made. The ventilation air is bled from the compressor at one or two points with a control in flow rate. A clearance control device such as this is incorporated in order to reduce as far as possible the clearance at the tips of the high pressure turbine blades and increase engine performance. It is generally managed by the full authority digital electronic control, often known by its English-language acronym FADEC. This means controls the temperature and the flow rate of air sent to the stator element concerned in such a way as to act on the thermal displacement thereof. 
     For certain engines, attempts have been made to get around these active clearance control means. The clearance at the blade tips in this case is set in such a way that the maximum blade wear during the life of the engine does not exceed the capabilities of the machine. This maximum wear is determined as a function of the maximum take-up of clearance observed during the life of the engine and is based on the displacements of the stator and of the rotor. This maximum take-up is generally observed during cycles known in the art as critical reburst. A cycle such as this consists, from a stabilized full throttle operating speed, in reducing the speed to low idle in a short space of time then instigating a reacceleration up to full throttle, again in a short space of time. 
     During this cycle, the take-up of clearance is great for the following reasons:
         since the rotor is stabilized at full throttle, the displacements due to thermal expansion of the disk are slow when the rapid variation in operating speed toward low idle is commanded because of the significant mass of this rotor and the ensuing lengthy thermal response time;   the stator elements, which also were stabilized at full throttle speed have a smaller mass and therefore a more rapid thermal response.       

     On immediately reaccelerating to full throttle operating speed, the rotor has not yet become thermally stabilized at low idle because of its long thermal response time. By contrast, the stator will have already reached the low idle operating conditions. It therefore follows that, at this moment, there is a take-up of play and the blade tip clearance is small. 
     Because of the acceleration, the disk experiences a centrifugal displacement leading to a temporary additional take-up of play. This additional take-up of play results in part wear because the blade tips come into contact with the shroud ring. 
     It can therefore be seen that the more rapid the thermal response of the casing with respect to that of the rotor, the more take-up of clearance there is, and the greater the blade tip wear during reacceleration. 
     SUMMARY OF THE INVENTION 
     It is a first objective of the invention to find a solution to this problem. 
     Another objective is to find a solution which does not involve significant modifications to the existing structure and which is inexpensive to implement. 
     According to the invention, the turbine stator of a gas turbine engine comprising a turbine casing, a turbine shroud ring and a shroud ring support connecting the shroud ring to the casing is one wherein the support is provided with an element that forms a heat shield positioned on the turbine side. 
     The solution therefore consists in increasing the thermal response time of the stator by using a heat shield which delays the influence of the temperature of hot gases in the stream from the combustion chamber. This solution is highly advantageous because it has proven to be effective. Furthermore, it can be implemented using relatively simple means. 
     Thus, according to another feature, the element forming a heat shield comprises a sheet forming a space with respect to the support surface. As a preference, the space forms a dead cavity not swept by gases. According to another embodiment, the space contains a thermally insulating material. 
     The invention applies more particularly to a stator the support of which comprises, on one side, a radial flange for securing to the turbine casing and, on the other side, a means for securing the elements of the shroud ring. The support advantageously forms a partition wall of frustoconical overall shape and the means for securing the elements of the shroud ring comprise two radial flanges sandwiching the elements of the shroud ring. 
     According to one particular embodiment, the element forming a heat shield comprises a first sheet fixed between two radial flanges. It also comprises a second sheet positioned axially between the means for securing the elements of the shroud ring and the radial flange for fixing the support to the casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to a nonlimiting embodiment based on the attached drawings in which: 
         FIG. 1  shows an axial half section of one example of part of a gas turbine engine in the region of the high-pressure turbine immediately downstream of the combustion chamber; 
         FIG. 2  shows the displacement D of, respectively, the rotor blade tips and the stator elements that form the operating clearance; 
         FIG. 3  shows in greater detail and in an enlarged view that part of the turbine casing that is provided with an element that forms a heat shield. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 3  shows an enlarged detail of the mounting of the shroud ring  13  in the casing  9 , incorporating the solution of the invention. The ring support  11  according to the example consists of a metal partition wall, such as an annular partition wall, of substantially frustoconical shape with the same axis as the engine. The support here is formed as a single piece but could equally consist of several ring sectors joined together to form an annular entity. The support  11  comprises radial flanges  11   a  and  11   b  for attaching the elements  13  that form the high-pressure or HP turbine shroud ring. Attachment according to this example is of the tongue and groove type. For the upstream fixing, towards the combustion chamber, the back of the elements  13  is shaped to form an axially opening groove  13   a  which collaborates with an axial return  11   b   1  of the radial flange  11   b . The downstream fixing of the elements  13  is provided also by a groove  13   b  the external branch of which bears against an axial return  11   a   1  of the flange  11   a  and is held in position by clamps  17 . 
     The upstream nozzle guide vanes  5  are fixed by bolts to the radial flange  11   b.    
     The support  11  is itself mounted on the turbine casing  93  via a radial transverse flange  11   c . This flange is inserted in the flange assembly  95  which connects the various elements of the casing  9 . The support  11  does not have any active clearance control nor does it have any ventilation means for achieving this. 
     According to the invention, a heat shield has been positioned on the internal face of the support  11 , that is to say on the face facing into the engine gas stream. The heat shield advantageously consists of a first sheet positioned parallel to the support wall  11  between the two radial flanges  11   a  and  11   b . This sheet is secured by welding, brazing, screw-fastening or any other fastening means, to the support. The sheet  21  is distant from the partition wall  11  so as to form a cavity  21   a . This cavity is preferably dead, that is to say that the gases it contains do not circulate. It is, for example, a closed cavity. The cushion of gas thus forms a thermally insulating mass. However, if appropriate, this cavity may contain another thermally insulating material. A second sheet is positioned in the same way, upstream of the flange  11   b , on the internal face of the partition wall  11  some distance therefrom. It is welded, brazed, screw-fastened or the like to the partition wall and forms a dead cavity  22   a  with the partition wall  11 . The mass of gas contained in this dead cavity thus forms a thermally insulating layer. 
     The support  11  is made of metal as are the sheets  21  and  22 . In stabilized speed operation, the clearance between the blade tip  33  and the ring  13  is fixed and of a determined value. This clearance is the result of equilibrium between deformations of mechanical and thermal origin to which the moving and stationary parts are subjected. In transient conditions this equilibrium is upset. Particularly in the case of critical reburst, as explained above, during the phase of rapid reduction in speed, the temperature of the gases in the driving stream drops. Because of the heat shield, the response to the drop in temperature of the support is slowed by comparison with that of the setup of the prior art. This means that during the sudden reacceleration or reburst that follows, the radial displacement of the rotor as a result of the increase in centrifugal forces does not interfere with the elements of the shroud ring. There is no contact between the blade tips and the elements of the shroud rings. No wearing either of the blade tip rub strips or of the abradable surfaces of the elements is observed. 
     The results of testing have demonstrated that the solution is effective and that the machine efficiency is improved as a consequence. Furthermore, attaching sheets is not particularly expensive. Overall, the solution is effective and economical.