Abstract:
The present invention relates to a two-spool gas turbine engine including an HP turbine stator ring and an exterior wall of the transition channel between the HP and LP stages, a first enclosure for controlling the stator ring, and a second enclosure for distributing air for blowing the exterior wall of the transition channel. The engine is characterized in that the two enclosures are placed in communication via an orifice controlled by a valve adapted to be open when the pressure P 1  in the first enclosure is greater than the pressure P 2  in the second enclosure, and closed when P 1 &lt;P 2.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns the field of gas turbine engines and is directed to means for controlling the circulation of air between two enclosures inside the engine, the relative pressure between the two enclosures varying as a function of the operating conditions. 
     2. Description of the Related Art 
     A gas turbine engine comprises at least three parts: an air compressor, a combustion chamber and a turbine, the compressor feeding the combustion chamber, which produces hot gases driving the turbine. The turbine is connected to the compressor by a shaft through which it drives the latter. The engine can comprise a number of spools each with a rotor formed of a compressor, a turbine and a shaft mechanically connecting them. In the aeronautical field engines generally have two or three spools. They therefore comprise at least one rotary spool using a low-pressure (LP) drive fluid and one rotary spool using a high-pressure (HP) drive fluid, the two spools being mechanically independent of each other and turning at different speeds. 
     The search for ever higher efficiency leads to the development for the same engine of low-pressure turbines the average radius of which increases in particular relative to that of the high-pressure turbine, with the aim of reducing the aerodynamic load. There follows the necessity of providing a transition conduit of appropriate geometry between the stages of the high-pressure turbine and the inlet of the low-pressure turbine. This transition conduit remains relatively short because of the aeronautical application of the engine. Such conduits impose on the gases that travel through them a large deflection over a short distance, and therefore have high slopes and high diffusion. To conserve satisfactory flow quality in the swan-neck formed by the transition channel, means for blowing air along the exterior wall of the stream are provided, to avoid thickening and even separation of the boundary layer. The present applicant has developed a solution related to this problem. It is described in patent application FR 0654139 in the name of the present applicant. An enclosure for distribution of blowing fluid is provided between the exterior wall of the transition channel and an element of the turbine casing. The enclosure communicates via a fluid feed orifice with an intake area upstream of the transition channel. This intake area is preferably in the compressor so that the air injected forms a film for thermal protection of the wall. 
     Moreover, upstream of this transition channel, the annular stream of driving gas is delimited externally by a stator ring. The clearance between the tips of the blades of the HP turbine and the internal face of this ring is kept as small as possible, in all operating phases of the engine, because the efficiency of the turbine depends on it. The HP rotor and stator combination being subjected in operation to different relative radial and axial displacements, there follows a variation of the clearance, which has to be controlled. Air taken from the upstream end of the engine, in the compressor, is used for this purpose to ventilate the stator ring support and to control its expansion as a function of the operating conditions. The air circulating in the ventilation enclosure is then evacuated in the stream. This is known in itself. Note that the control function entails non-continuous circulation of ventilation air. This flow of air is reduced and interrupted, in particular when the operating conditions have stabilized. 
     If the engine comprises both such means for controlling expansion of the turbine stator ring with a flow of ventilation air circulating in a ventilation enclosure and, immediately downstream thereof, a blowing air distribution enclosure formed around the wall of the transition channel, it would be desirable to use that ventilation air as at least part of the blowing air for the exterior wall of the stream in the transition channel. However, in operation, the differential pressure between said ventilation enclosure and the blowing air distribution enclosure may change. Thus if the circulation of ventilation air is interrupted or reduced, the pressure in the ventilation enclosure falls below that of the distribution enclosure. If there were communication between the two enclosures, an unwanted reflow of gas from the distribution enclosure would occur, interfering with control of the clearance between the stator ring and the tips of the turbine blades. 
     BRIEF SUMMARY OF THE INVENTION 
     The present applicant has set itself the following objectives:
         Recovering the HP turbine stator ring support ventilation air;   Ensuring that the ventilation air contributes to blowing the exterior wall of the transition channel whilst preventing reflow of air from the blowing air distribution enclosure.       

     According to the invention, the above objectives are achieved with a two-spool gas turbine engine including an HP turbine stator ring and an exterior wall of the transition channel between the HP and LP stages, a first enclosure for controlling the stator ring, and a second enclosure for distributing air for blowing the exterior wall of the transition channel, characterized in that the two enclosures are placed in communication via an orifice controlled by a valve adapted to be open when the pressure P 1  in the first enclosure is greater than the pressure P 2  in the second enclosure, and closed when P 1 &lt;P 2 . 
     The invention is advantageous with an engine the two enclosures whereof are separated by a partition pierced by said orifice. 
     In a preferred embodiment, the valve includes a tubular element engaged in the orifice, with a flared part, a closure slider mobile in the tubular element between a closure position bearing against the flared part and an open position away from the flared part. 
     Because of the different areas on which the pressures P 1  and P 2  act, this solution has the additional advantage of ensuring opening of the valve and consequently stable operation of the device when there is a significant pressure difference between the two enclosures. 
     The tubular element can be fixed in the orifice or alternatively be formed in one piece with the partition. 
     According to another feature, the valve includes a perforated cover attached to the tubular element against which the slider bears in the open position. 
     According to a further feature, the valve includes a closure slider with a leakage orifice ensuring a reduced flowrate between the distribution enclosure and the ventilation enclosure in the closed position. 
     This solution is advantageous because it prevents too high a pressure difference between the enclosures. 
     According to a further feature, the valve includes a tubular element including a part with a small diameter, a part of greater diameter, the two parts being connected by the flared part, the slider including a guide surface portion cooperating with the larger diameter part to guide the slider inside the tubular element. 
     This ensures flexible operation of the slider and reduces the risk of jamming in one position or the other. 
     Alternatively, the valve includes a tubular element including a part with a small diameter, a part of greater diameter, the two parts being connected by the flared part, the slider including a guide surface portion cooperating with the small-diameter part to guide the slider inside the tubular element. 
     Other features and advantages will emerge from the following description of nonlimiting embodiments of the invention with reference to the appended drawings: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an engine diagrammatically in axial section; 
         FIG. 2  represents the part of the casing of the engine in the area of the HP turbine and the transition channel provided by the invention; 
         FIG. 3  represents the valve of the invention in axial section; 
         FIGS. 4 to 7  represent in axial section variants of the valve of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  represents diagrammatically an example of a turbomachine in the form of a two-spool turbofan (bypass turbojet) engine. A fan  2  at the front feeds air to the engine. Air compressed by the fan is divided into two concentric flows. The secondary flow is evacuated directly into the atmosphere, with no other input of energy, and provides an essential portion of the drive thrust. The primary flow is guided through a number of compression stages to the combustion chamber  5  where it is mixed with fuel and burnt. The compression is effected in succession by a booster compressor constrained to rotate with the fan rotor and forming part of the LP rotor and then an HP compressor. The hot gases from the combustion chamber feed the various turbine stages, the HP turbine  6  and the LP turbine  8 . The LP and HP turbine rotors are attached to the LP and HP compressor rotors, respectively, and thus drive the fan and the compressor rotors. The gases are then evacuated into the atmosphere. 
     The HP turbine is a single-stage turbine whereas, in the LP turbine, expansion is divided between a number of stages on the same rotor. A transition channel is formed between the HP and LP sections, to be more precise between the rotor of the HP turbine and the inlet distributor of the LP turbine. Because of the expansion of the gases, the volume increases and also the average diameter of the stream. This increase remains compatible with undisturbed flow conditions, however. 
     To increase the efficiency of the low-pressure turbine, the profile of the aerodynamic channel is optimized. Such optimization includes increasing the low-pressure turbine inlet slope in the transition channel, which enables a rapid increase in the average radius of the low-pressure turbine. Moreover this increase in the low-pressure distributor inlet section generated by increased diffusion in the channel generates an increase in performance of the first stage with better acceleration in the distributor. 
     However, a steep low-pressure turbine inlet slope creates a risk of separation of the boundary layer along the exterior wall of the main flow coming from the high-pressure turbine. Such separation strongly degrades the performance of the LP turbine. 
     One solution is to inject a significant flow of gas via the wall at the outlet of the high-pressure turbine. This injection of air is commonly called blowing. 
       FIG. 2  represents a portion of the casing of a gas turbine engine in the region of the HP turbine and of the inlet of the transition channel downstream of the latter. 
     The rotor of the HP turbine, of which the blade  14  can be seen, is rotatable inside an annular space defined externally by a stator ring  15  forming sealing means. Downstream of the turbine, the drive gas stream is delimited externally by the wall  20 . This wall is formed of annular sector platforms extending axially between the turbine stator ring  15  and the distributor of the first stage of the LP turbine, which cannot be seen in the figure. 
     The stator ring  15  is itself formed of sectors mounted in an annular intermediate part  16 . The sectors of the ring  15  are retained here by tongue and groove connections on the upstream side and by clamps on the downstream side. The intermediate part  16  is mounted in an internal casing element  17  housed inside the exterior casing  11 . 
     The internal casing  17  includes two radial ribs  17   a  and  17   b  disposed annularly in two transverse planes passing through the rotor of the HP turbine. An annular plate  12  covers the ribs  17   a  and  17   b  and has a radial rim  12   r  that bears against the internal face of the exterior casing  11 . A ventilation enclosure  19  is therefore formed between the plate  12  and the internal casing  17 . The ribs  17   a  and  17   b  are pierced by axial orifices  17   a   1  and  17   b   1  enabling circulation of gas between the area upstream of the ribs and the area downstream of the ribs. The ventilation is provided by a gaseous flow F coming from an appropriate passage formed upstream of the ventilation enclosure  19 . 
     Downstream of a radial flange  17   c  of the internal casing  17 , a blowing air distribution enclosure  21  is formed by a plate that is conformed to include a substantially radial upstream partition  21   a , a downstream partition  21   b , also oriented globally radially, a radially interior partition  21   c  and a radially exterior partition  21   d . A strip seal  22  is placed between the radial flange  17   c  of the internal casing  17  and the partition  21   a . The enclosure  21  communicates with the enclosure  19  via an orifice  21   a   1  fitted with a valve  30 . The enclosure  21  communicates with the gas stream via an opening  21   c   1  formed in the radially internal partition  21   c , a tube  23 , and openings  20   a  along the wall  20  of the transition channel. 
     The valve  30  is represented in more detail in  FIG. 3 . It comprises a tubular part  31 , a slider  33  and a perforated cover  35 . The tubular part  31  is formed of a first cylindrical part  31   a  of diameter d 1 , a second cylindrical part  31   c  of greater diameter d 2 , d 2 &gt;d 1 , and a flared part  31   b , connecting the two cylinders  31   a  and  31   c . The slider is housed in the large-diameter part  31   c  with one face conformed to cover the flared part. The slider  33  is pierced with annularly disposed orifices  33   a  and a central orifice  33   b . The large diameter of the slider corresponds to the inside diameter of the cylindrical part  31   c . The cover  35  mounted on this part forms an axial abutment for the slider. It is open in its central part at  35   a  facing the orifices  33   a . The slider can assume an open position, bearing against the cover, in which case the orifices  33   a  are uncovered. The slider  33  can assume a closure or blocking position when it bears against the flared part  31   b . In this position the orifices  33   a  are closed by the flared wall. 
     The device operates as follows. 
     To ensure controlled expansion of the internal casing  17 , and thus to ensure control of the clearance at the tips of the blades of the turbine with the stator ring  15 , the air F coming from the compressor is conveyed into the enclosure  19  and sweeps over the ribs. It thus enables expansion of the stator ring  15  of the HP turbine. This controls the clearance by controlling the flowrate and the source of air according to the various phases of operation of the engine. 
     Optimum use is made of this flow of air, after it has swept over the ribs, by sending it into the enclosure  21  located immediately downstream, via the orifice  21   a   1  of the partition  21   a , to participate in blowing the wall  20  of the transition channel. 
     Such circulation between the ventilation enclosure  19  and the blowing air distribution enclosure does not give rise to any problem if the pressure P 1  in the enclosure  19  is greater than that P 2  in the enclosure  21 . 
     If, in certain phases of operation of the engine, it is necessary to cut off or to reduce the feed of ventilation air from the enclosure  19 , and if nothing were to be done about it, circulation of air or gas between the enclosure  21  and the enclosure  19  would occur that would compromise controlling the clearance. 
     The function of the valve is therefore to isolate the enclosure  19  from the enclosure  21  when the pressure P 1  is less than P 2 . The valve  30  is furthermore advantageously configured with a difference between the areas to which the pressures P 1  and P 2  are applied so that it passes from the closed position, i.e. with the slider bearing against the flared part to achieve closure, to the open position only if the pressure P 1  is sufficiently greater than P 2  to ensure stable operation. 
     When the valve is in the closed position, the  FIG. 3  solution comprises a central opening  33   b  that enables limited circulation from the enclosure  21  to the enclosure  19  and ensures pressurization of the latter. Alternatively, the valve has no central orifice. In this case it has only one, non-return, function. 
     Other embodiments of the valves are shown in the subsequent figures. 
       FIG. 4  shows a variant valve  130  with a cover  135  provided with axial projections  135   b  around the central opening  135   a . These projections limit the bearing area of the slider. The other elements of the valve are not changed compared to that of  FIG. 3 . 
     In  FIG. 5 , the valve  230  differs from the preceding valves in that the slider  233  is of smaller diameter than the large-diameter cylindrical part. It moves freely inside the latter. The cover  235  has projections  235   b  as previously. Air circulates around the slider and through the central bore  233   b  and then circumvents the axial projections  235   b  and passes through the central opening  235   a  of the cover  235 . 
     In  FIG. 6 , the valve  330  includes a slider  333  provided with notches  333   b  at its periphery forming air passages. The valve is otherwise similar to the previous valves. 
     In  FIG. 7 , the valve  430  includes a slider  433  with a portion  433   c  engaged in the small-diameter part  431   a  of the tubular element  431 . This part  433   c  includes air passages  433   c   1 . The slider is also guided inside the larger-diameter part  431   c  and comprises openings  433   a  for air to pass through. These openings  433   a  are at the periphery so as to be blocked by the flared part  431   b  when the slider bears against the latter. These openings can be obtained by means of notches as shown in  FIG. 7  or by drilling. 
     The operation of these valve variants is the same as for the valve  30  from  FIG. 3 , for which they can be substituted. The geometry of these valves enables operation without binding regardless of the operating phase of the engine.