Patent Publication Number: US-7214034-B2

Title: Control of leak zone under blade platform

Description:
The invention relates to controlling the leakage zones under the platforms of the blades of a blade-wheel in a turbomachine. 
   More precisely, the invention relates to a turbomachine blade-wheel comprising a disk presenting a plurality of substantially axial slots on its periphery, a plurality of blades having roots that are retained in said slots, and which blades present platforms for defining the stream of gas on the radially inner side, and upstream and downstream radial walls which extend from said platforms towards the periphery of said disk, inter-blade cavities defined by said platforms and the periphery of said disk, and sealing devices for sealing the inter-blade spaces, the sealing devices being made in the form of liners having edges that flare radially inwards and that are disposed in said cavities against the walls of the platforms of two adjacent blades. 
     FIG. 1  is a perspective view showing a sealing liner  1  of the prior art which presents an upstream edge  2  and a downstream edge  3  that flare radially inwards, and also two curved longitudinal flaring edges which fit closely against the flanks of the blades under the platforms. The upstream and downstream edges  2  and  3  are designed to come into the immediate vicinity of the adjacent upstream and downstream radial walls of two adjacent blades, in order to limit leakage through the space separating the adjacent lateral walls. The top wall  6  of each liner bears against the bottom faces of two adjacent platforms under the action of centrifugal forces when the wheel is rotating and seals the gap between the adjacent platforms. By construction, it is practically impossible for the flared edges to be deformed under the action of centrifugal forces, and it is impossible to ensure that the upstream and downstream flared edges ( 2  and  3 ) are pressed effectively against the upstream and downstream radial walls of the blades. As shown in  FIG. 2 , those edges may be spaced apart from the adjacent radial walls, which results in an air leak f between the cavity under the platform and the stream of gas in these zones, which is prejudicial to the efficiency of the wheel. 
   The object of the invention is to have better control over the leakage zone under a blade platform, particularly in the gaps between the under-platform radial walls. 
   The invention achieves this object by the fact that each liner presents an elastic zone on one of its upstream and downstream flared edges, and the radial walls adjacent to said edges are connected to the platforms by inside surfaces that are inclined relative to a radial plane, and against which edges said elastic zone bears, in such a manner that said elastic zone can slide radially inwards in the event of said wheel ceasing to rotate, and radially outwards under the action of centrifugal forces in order to urge said liner to move axially towards the radial walls distant from said elastic zone so as to improve sealing in said zone. 
   In the event of the blade-wheel ceasing to turn, the elastic zone slides radially inwards and the liner relaxes, moving itself away from the bottom walls of the two platforms, at least in the regions adjacent to the elastic zone. When the blade-wheel starts to rotate, the centrifugal forces press the liner against the bottom walls of the platforms, and the elastic forces push the corresponding flared edge towards the lateral walls facing the elastic edge, in order to improve sealing in this location. Since the elastic zones are still bearing against the adjacent lateral walls, sealing in this zone is guaranteed. 
   Advantageously, the radial walls that are spaced apart from the elastic zones include abutments to limit the axial movement of the liners under the action of centrifugal forces. 
   The lateral walls that are adjacent to the elastic zones also include abutments to limit inward sliding of said elastic zones. 
   According to an advantageous characteristic of the invention, the elastic zones are circumferentially defined by two notches that are cut in the corresponding flared edges of the liners. This disposition facilitates implementation of the invention at no additional cost. 
   The invention applies particularly to turbine blade-wheels. 
   In this specific example, the elastic zone is provided on the upstream edge, and the angle of the surface that is inclined relative to the radial plane is greater than the slope of the platform relative to the axis of rotation of the turbomachine. 

   
     Other characteristics and advantages of the invention appear on reading the following description, given by way of example and with reference to the accompanying figures, in which: 
       FIG. 1  is a view from below and in perspective of a sealing liner of the prior art; 
       FIG. 2  is a side view in section of a liner edge and of a radial edge of a blade, of the prior art; 
       FIG. 3  is a view from above and in perspective of a sealing liner of the invention; 
       FIG. 4  is a view from below and in perspective of the sealing liner in  FIG. 3 ; 
       FIG. 5  is a section on a plane containing the axis of the blade-wheel, showing the disposition of the sealing liner of the invention in the under-platform cavity, after assembly and in the absence of centrifugal forces; and 
       FIG. 6  is similar to  FIG. 5  and shows the position of the sealing liner, when it is subjected to centrifugal forces as a result of the blade-wheel rotating. 
   

     FIGS. 1 and 2  show the prior art which is described above in the present document. 
     FIGS. 3 and 4  show a sealing liner  10  of the invention which has edges that flare radially inwards, that is, an upstream edge  12 , a downstream edge  13 , and between the upstream edge  12  and the downstream edge  13  two longitudinal inwardly curved flaring edges which fit closely to the shape of the flanks of two adjacent blades. 
   The upstream edge  12  presents two notches  16  and  17  which define between them an elastic zone  18  which, at rest, projects forwards from the upstream edge  2  of the prior art liner  1  shown in  FIG. 1 . That is, at rest, the elastic zone  18  lies outside the geometrical surface which would join together the ends  12   a  and  12   b  of the upstream edge  12  smoothly and continuously, which ends are situated beyond the notches  16  and  17 , and connected to the longitudinal edges  14  and  15  respectively via convex surfaces. 
     FIGS. 5 and 6  show a blade-wheel  30  which comprises a disk  31  that presents a plurality of substantially axial slots  32  in its periphery, with each of said slots housing the root of a blade  33 . Each blade  33  presents a platform  34  above its root, which platform defines the radially inner side of the stream of gas F going through the row of blades, the platform  34  being connected to an upstream radial wall  35  and to a downstream radial wall  36  which extend towards the periphery of the disk  31 . Inter-blade cavities  37  are thus formed in the periphery of the disk  31  under the platforms  34 . When the row of blades is observed axially in the direction of the stream of gas F, each blade  33  presents a platform portion on the right and a platform portion on the left. This same applies to the radial walls  35  and  36 . Each under-platform cavity  37  is thus defined by right and left platform portions of two adjacent blades and by their right and left upstream and downstream lateral wall portions. By construction and because of assembly requirements, a gap or clearance separates the right hand portion from the left hand portion, which gap needs to be sealed by a sealing liner. 
   As shown in  FIGS. 5 and 6 , the connection  38  between the upstream radial wall  35  and the platform  34  presents beside the cavity  37 , a surface  39  which makes an angle α with the radial plane that is perpendicular to the axis of rotation of the blade-wheel  30 . The downstream radial wall  36  is connected to the platform  34  by a zone  40  that presents a curved surface  41 , beside the cavity  37 , said surface being complementary to the flaring of the downstream edge  13  of the liner  10 . Moreover, the downstream radial wall  36  presents a protuberance  42  on its inside face, said protuberance serving as an abutment for the downstream shoulder of the liner  10 . The upstream radial wall  35  also presents a protuberance  43  on its face situated beside the cavity  37 . 
   The liner  10  is mounted in the cavity  37  in such a manner that its downstream edge  13  is positioned above the protuberance  42  and its elastic zone  18  is positioned above the protuberance  43 . In this position, the elastic zone  18  of the liner  10  bears against the inclined surface  39 . 
   The angle α of the inclined surface  39  is calculated as a function of the slope of the platform  34  relative to the axis of rotation of the wheel and as a function of the friction angle φ of the liner  10  against the inside surface of the platform  34 , so that, in the absence of any centrifugal force, i.e. when the blade-wheel  30  is stationary, the elastic zone  18  slides radially inwards over the inclined surface  39 . 
   In this position most of the surface of the top wall  19  of the liner is spaced apart from the bottom face of the platform  34 , as can be seen in  FIG. 5 , the liner  10  tilting about an axis intersecting the plane of  FIG. 5  at the point referenced  44 , said axis being situated near the downstream flared edge  13 . The protuberance  43  on the upstream radial wall  35  serves to prevent the elastic zone  18  from sliding too far, and to retain the liner  10  in the top zone of the cavity  37 . 
     FIG. 6  shows the position of the liner  10  while the blade-wheel  30  is rotating. In this position, the liner  10  is subjected to centrifugal forces which tend to press it against the inside face of the platform  34 . The elastic zone  18  is urged radially outwards and slides against the inclined wall  39 . 
   The angle α is advantageously greater than the slope of the platform  34 . When the elastic zone  18  moves outwards, through the fact that the liner  10  tilts about the pivot axis defined by the point referenced  44 , the elastic force exerted by the elastic zone  18  increases and tends to move the liner  10  axially towards the downstream radial wall  36 , thereby improving sealing in the connection zone  40 . The axial movement of the liner  10  is limited by the protuberance  42  which serves as an abutment. 
   When the blade-wheel  30  comes to a stop, the liner  10  will return to the position shown in  FIG. 5 , as soon as the centrifugal forces are insufficient to prevent the elastic zone  18  from sliding over the inclined wall  39 .