Patent Publication Number: US-6709231-B2

Title: Stator of a variable-geometry axial turbine for aeronautical applications

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claim priority under 35 U.S.C. §119 of Italian application number TO2001A 000445, filed May 11, 2001. 
    
    
     BACKGROUND OF INVENTION 
     This invention relates to a stator of a variable-geometry axial turbine for aeronautical applications and, in particular, for aeronautical engines. 
     As is known, an axial turbine for an aeronautical engine determines an annular duct with increasing diameter and comprises at least one stator and one rotor arranged axially in succession to each other, and comprising respective arrays of airfoil profiles housed in the annular duct and between them circumferentially delimiting associated spaces through which a flow of gas can pass. 
     In aeronautical engines, it has been found necessary to use axial turbines having the highest possible efficiency in all operating conditions and, therefore, over a relatively wide range of values for the rate of flow of the gases that pass through the turbine itself. 
     This requirement could be met by producing variable-geometry turbines, i.e. turbines comprising at least one stator in which, in use, it is possible to vary the transverse area of the associated spaces, in particular by adjusting the angular position of the airfoil profiles about respective axes incident to the axis of the turbine. 
     In stators of axial turbines of known type, the annular duct is delimited radially by conical surfaces while the airfoil profiles have a relatively long length in the direction of travel of the gases, because of which any displacement of these profiles would cause jamming against the above-mentioned conical surfaces or else excessive radial clearances and therefore considerable leakage of gas between adjacent spaces, because of which the flow of the gases in the spaces themselves would become non-uniform, with a consequent drastic reduction in the efficiency of the turbine. 
     SUMMARY OF INVENTION 
     The purpose of the invention is to produce a stator of a variable-geometry turbine for aeronautical applications, which enables the problems set out above to be solved simply and functionally. 
     According to the present invention, a stator of a variable-geometry axial turbine for aeronautical applications is produced; the stator having an axis and comprising an annular duct delimited radially by an annular outer and an annular inner surface; an array of airfoil profiles housed in the duct in positions angularly equidistant from each other about said axis and each comprising an associated pair of end edges opposite each other and coupled with said outer and inner surfaces, characterised in that said airfoil profiles are rotatable with respect to said outer and inner surfaces about respective axes of adjustment incident to said axis, and in that it comprises means for maintaining said airfoil profiles a predetermined clearance from said outer and inner surfaces to maintain a substantially constant clearance between said outer and inner surfaces and said end edges when the angular position of said airfoil profiles is varied. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will now be described with reference to the attached drawings, which show a non-limiting embodiment of the invention, in which: 
     FIG. 1 is a schematic radial section of a preferred embodiment of the stator of a variable-geometry axial turbine for aeronautical applications, produced according to the invention; 
     FIG. 2 shows, in radial section and at a larger scale, a detail of the stator in FIG. 1; and 
     FIG. 3 is a perspective view, with parts cut away for clarity, of the detail in FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, the number  1  indicates a variable-geometry axial turbine (shown schematically and in part), which constitutes part of an aeronautical engine, not shown. 
     The turbine  1  is axially symmetrical with respect to an axis  3  coinciding with the axis of the associated aeronautical engine and comprises an engine shaft  4  rotatable about the axis  3  and a case or casing  8  housing a succession of coaxial stages, only one of which is shown as  10  in FIG.  1 . 
     With reference to FIGS. 1 and 2, the stage  10  comprises a stator  11  and a rotor keyed to the engine shaft  4  downstream from the stator  11 . The stator  11  in turn comprises a hub  16  (shown schematically and in part), which supports the engine shaft  4  in a known manner and is integrally connected to the casing  8  by means of a plurality of spokes  17  (FIG. 2) angularly equidistant from each other about the axis  3 . 
     As shown in FIG. 2, the stator  11  also comprises two annular platforms or walls  20 ,  21 , which are arranged in an intermediate radial position between the hub  16  and the casing  8 , have the spokes  17  passing through them and are coupled, one with the casing  8  and the other with the hub  16  in substantially fixed datum positions by means for connecting devices  24  that allow the walls  20 ,  21  themselves the possibility of axial and radial displacements of relatively limited amplitude with respect to the casing  8  and the hub  16  in order to compensate, in service, for the differences in thermal expansion between the components. 
     The walls  20 ,  21  have respective surfaces  27 ,  28  facing each other and radially delimiting an annular duct  30  with a diameter increasing in the direction of travel of the gas flow. 
     With reference to FIGS. 2 and 3, the walls  20 ,  21  carry an array of vanes  32  (only one of which is shown) angularly equidistant from each other about the axis  3  with the spokes  17  passing through them and comprising respective airfoil profiles  33 , which are housed in the duct  30  and between them delimit circumferentially a plurality of spaces through which the gas flow passes (not shown in the attached figures). 
     Each vane  32  also comprises a pair of cylindrical tubular hinge flanges  36 ,  37  arranged at opposite ends of the associated profile  33  and coaxial with each other along an axis  40 , which is incident to the axis  3  and substantially orthogonal to the surfaces  27 ,  28  so as to form an angle other than 90° with the axis  3 . 
     The flanges  36 ,  37  of each vane  32  engage rotatably in respective circular seatings  41 ,  42  made in the walls  20  and  21  respectively to allow the associated profile  33  to rotate about the axis  40 , project from the profile  33  radially with respect to the associated axis  40  and are delimited by respective surfaces  46  (FIG. 2) and  47 , which are facing each other and extend with no break in continuity as a continuation of the surface  27  and the surface  28 , respectively. 
     With reference to FIG. 2, the flange  36  of each vane  32  ends in a threaded cylindrical section  48  coaxial with the flange  36  itself and caused to rotate in use by an angular positioning unit  50  (partly shown) comprising in particular a motor-driven actuating and synchronising ring  51  designed to rotate the profiles  33  simultaneously about their respective axes  40  through the same angle, keeping the profiles  33  themselves in the same orientation to each other with respect to the surfaces  27 ,  28 . In particular, the maximum angular deflection of each vane  32  about the associated axis  40  is approximately 6°. 
     With reference to FIG. 3, the profile  33  of each vane  32  is of known type, has a convex or dorsal surface  54  and a concave or ventral surface  55 , and comprises a head portion  56  and a tapering tail portion  57 , which define the leading edge and trailing edge respectively of the profile  33 . The head portion  56  is integral with the two flanges  36 ,  37  while the tail portion  57  extends along the duct  30  beyond the flanges  36 ,  37  themselves. 
     In the tail portion  57 , the dorsal face  54  and the ventral face  55  are connected to each other by two flat surfaces  59 ,  60  opposite each other, each of which is facing and at a predetermined clearance from an associated shaped zone  66 ,  67  of the surfaces  27 ,  28 . 
     In fact, each surface  27 ,  28  has an associated conical zone  64 ,  65  that defines a mean course or path of the gases in the duct  30 , while the zones  66 ,  67  have a shape complementary to respective ideal surfaces, which are defined by an envelope of the various angular positions assumed by the surfaces  59 ,  60  about the axis  40 . 
     In the example described, these ideal surfaces are generated by the rotation about the axis  40  of datum lines  69 ,  70 , which are situated on the surfaces  59  and  60  respectively, preferably in the median position between the ventral face  55  and the dorsal face  54 . FIG. 3 shows in section a vane  33  in which only one associated point is shown for each of the median datum lines  69 ,  70 . 
     Still with reference to the illustration in FIG. 3, in order to guide the gas flow progressively in the duct  30 , the surfaces  27 ,  28  comprise, finally, respective pluralities of zones  71 ,  72 , which gradually connect the zones  66 ,  67  to the associated conical zone  64 ,  65 , while the surfaces  46 ,  47  are shaped according to the path followed by the surfaces  27 ,  28  to connect the edges delimiting the seatings  41 ,  42 . 
     In use, it is possible to adjust the geometry or capacity of the spaces by simultaneously rotating the profiles  33  about their respective axes  40  by means of the unit  50 . During this rotation, between the surfaces  59 ,  60  of each profile  33  and the associated zones  66 ,  67  of surfaces  27 ,  28 , the radial clearance remains substantially constant for every angular position assumed by the profile  33  itself by reason of the special shaping of the zones  66 ,  67  themselves described above. 
     In particular, the height of the profiles  33  measured between the surfaces  59 ,  60  and the distance between the walls  20 ,  21  are calibrated in such a way that the surfaces  59 ,  60  co-operate with sliding against the zones  66 ,  67  of the surfaces  27 ,  28  with extremely limited radial clearance to ensure the fluid seal between vanes  33  and walls  20 ,  21  and, consequently, the uniformity of the flow of gas that passes through the stator spaces. 
     From the foregoing it is evident that the special shaping of the surfaces  27 ,  28  of the stator  10  allows relatively high efficiency levels of the stage  10  to be obtained for all angular positions of the vanes  32  and consequently for a relatively broad range of operating conditions of the turbine  1 . 
     The situation just stated is due to the fact that the angular position of the profiles  33  can be adjusted and to the fact that the radial clearance between the profiles  33  and the walls  20 ,  21  is extremely limited and, above all, constant for all angular positions of the vanes  32  about their associated axes  40 , even if the profiles  33  have a relatively long length in the direction of travel of the gases and the diameter of the duct  30  is increasing. 
     Consequently, in the stator  11  the substantially constant clearance and the continuous fluid seal between the vanes  32  and walls  20 ,  21  during adjustment not only prevents jamming or friction occurring between the vanes  32  themselves and the walls  20 ,  21  during adjustment, but above all prevents the formation of unwanted and unpredictable vortex wakes in the gas flow in the stator spaces due to leakage. 
     Moreover, the presence of the connecting zones  71 ,  72  and the special shaping of the vanes  32  and, in particular, the presence of the flanges  36 ,  37  enable the gas flow in the duct  30  to be guided in a gradual and optimum manner for all angular positions of the profiles  33  about their respective axes  40 . 
     Finally, it is evident from the above that changes and variations can be made to the stator  11  described and illustrated, without extending it beyond the scope of protection of the present invention. 
     In particular, the surfaces  59 ,  60  could be shaped rather than flat and therefore the edges of the profiles  33  slidably at a predetermined clearance from the surfaces  27 ,  28  could also be defined by a line or a corner that extends from the hinge portions of the vane  32  as far as the trailing and/or leading edges. 
     Furthermore, the vanes  32  could be hinged to the walls  20 ,  21  or to other structures supporting the stator  11  in a manner different from the one illustrated and described, and/or could be driven in rotation by an angular positioning unit other than the unit  50  illustrated in part.