Patent Publication Number: US-6217282-B1

Title: Vane elements adapted for assembly to form a vane ring of a gas turbine

Description:
FIELD OF THE INVENTION 
     The invention relates to a vane element for a gas turbine, particularly a low-pressure turbine, in which the vane element has a vane blade, which extends between an inner platform and an outer platform, the outer platform being secured to a housing by fastening means, the inner platform being adapted for being coupled to the inner platforms of adjacent vane elements. 
     The invention relates further to a vane segment assembled from at least three vane elements as well as to a ring of the vane elements formed by assembling the vane segments. 
     BACKGROUND 
     Vanes of low-pressure turbines have been made of metal and are generally soldered together first to form segments of three or six vanes and then to form the annular ring. For investigations of aerodynamic dimensioning of the profile of the vane blades as well as for incorporation in an actual gas turbine drive mechanism, a number of vane segments are secured together to form a ring of blades in which individual vane segments are not rigidly connected to one another in order to accommodate thermal expansion. The vanes are fastened by their outer platforms in a drive housing and are detachably connected to one another at their inner platforms by metal clamps or the like. By clamping the vanes at the inner platforms, the vibration behavior of the vane blades is improved. The use of clamps as additional components, however, has the disadvantage of more extensive assembly and higher costs. 
     For calibrating the aerodynamic dimensioning process and for rapid determination of test data for alternately measuring pressure, velocity and flow quantity, vanes made substantially of carbon-fiber-reinforced plastics are utilized and are tested in so-called aerodynamic “cold” test stands. Such vanes can be produced considerably more rapidly and at lower cost than corresponding vanes made of metal and thus are preferably utilized in these investigations. Vanes of plastic are loaded in the test stand at considerably lower temperatures (approximately 130° C.) compared to actual operation, but are subjected to forces of the same order of magnitude as in the actual drive turbine. 
     It is thus a problem to adopt the geometry of the outer and inner platforms of the vanes made of metal for those made of plastic, since the latter cannot withstand the high loads produced by the actual gas forces. Gluing or bonding together three or six vane elements into individual vane segments, from which the vane ring is formed has only slightly met this problem. Similarly, securing the vane elements by means of bolts at lateral front surfaces of the inner and outer platforms has not proven effective because the amplitude of vibration of the vanes was too high and led to breakage of the vanes. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a vane element of the above type which has an improved dynamic vibration behavior, which limits vibration amplitudes of adjacent vanes and can be produced by manufacturing technology in a simple and cost-favorable manner. 
     The above and further objects are achieved according to the invention in that at least the inner or outer platform has a projection with a lateral surface at a first lateral front surface, and a recess at an opposite lateral front surface, the recess being shaped to correspond to the projection, such that the projection of one vane element can be form-fit into the recess of an adjacent vane element so that the inner or outer platforms of adjacent vane elements are flush with one another. 
     This construction has the advantage that vane elements of adjacent vane segments, (formed, for example, by joining three vane elements together) are coupled in a form-fitting manner with one another in the axial direction and a damping effect is produced at the contact surfaces due to friction therebetween. The projections also seal any gaps that occur between the platforms. 
     In a preferred embodiment, the lateral surfaces of the projection extend at right angles to the first front surface of the outer or inner platform generally extending in the radial direction. The lateral surfaces of the projection therefore extend in the circumferential direction that is intensely loaded by the gas flow and achieve damping as a consequence of friction at the contact surfaces. 
     Preferably, the lateral surfaces of the projections project at least 3 mm from the first front surface of the inner or outer platform, so that a sufficiently high friction or contact surface is present in the recess of the adjacent vane element. 
     It is advantageous for the projection to constitute at least 30% and preferably at least 50% of the area of the first front surface in order to limit the vibration amplitude of the vane elements. 
     Further, it is preferred that the projection fits with a small play in the recess of the adjacent vane element so that, for example, the vane elements of adjacent vane segments can move towards and away from one another to accommodate thermal expansion. 
     Most preferably, the inner surfaces of the recess at the second front surface extend parallel to the lateral surfaces of the projection, so that secure friction contact is assured between the inner surfaces of the recess and the lateral surfaces of the projection. 
     It is preferred that the outer platform comprises a flange integral with the vane and an outer reinforcing wall, which are joined by means of two transverse webs extending circumferentially at axially spaced locations, and/or that the inner platform comprises a flange integral with the vane and a reinforcing wall. In this way, resistance to bending and twisting of the platforms is obtained. The resistance to bending and twisting is particularly effective for the outer platform consisting of the flange, the outer reinforcing wall, the two transverse webs, and the fastening means attaching the vane element to the drive housing. Due to this arrangement, the vane element also resists relatively high actual gas forces, even when made from relatively weak materials, such as, for example, fiber-reinforced plastic. 
     In a preferred embodiment, the vane element is formed as an integral one-piece body so that it can be made inexpensively from metal or by injection molding processes. 
     It is particularly preferred that the vane element be made of carbon-fiber-reinforced plastic, so that it can be investigated in so-called aerodynamic “cold” test stands. In this way, test data for the calibrating of aerodynamic dimensioning processes can be determined more rapidly and in a more cost-favorable manner, than is the case of the vane elements made of metal. Such test vane elements of plastic also can be joined by bonding or gluing three or six vane elements together to form a vane segment. The lateral surfaces of the projections serve for the application of adhesive, which, in contrast to the first and second front surfaces of the conventional inner or outer platforms, resist the circumferentially applied forces in shear rather than in tension or compression. This is clearly a more favorable form of loading for glued joints. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING 
     The invention will be described hereafter with reference to an embodiment illustrated in the drawing wherein: 
     FIG. 1 is a longitudinal sectional view of a vane element according to an embodiment of the invention shown in perspective; 
     FIG. 2 is a side elevational view of the vane element of FIG. 1; 
     FIG. 3 is a side view of the vane element diagrammatically illustrating the vibration thereof; 
     FIG. 4 is a longitudinal section of a vane segment comprised of three vane elements shown in perspective; and 
     FIG. 5 is a perspective view of the vane segment in FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an embodiment of a vane element  1  according to the invention. The vane element  1  is made of carbon-fiber-reinforced plastic and is produced by injection molding. The vane element  1  is utilized in so-called aerodynamic “cold” test stands in order to rapidly and inexpensively determine test data from pressure, velocity, and flow quantity measurements for calibrating aerodynamic dimensioning processes. 
     Vane element  1  comprises a vane blade  2  integrally formed with an outer platform  3  and an inner platform  5 . The outer platform  3  comprises a flange  3 ′ integral with the vane blade  2  and an outer reinforcing wall  4  spaced outwardly from flange  3 ′. The inner platform  5  comprises a flange  5 ′ integral with the vane blade  2 . The inner and outer platforms  3  and  5  each has a first front surface  7  provided with a projection  8 , which forms a lateral surface  9  projecting from the first front surface  7  by approximately 3 mm. In the case of development of very high forces, the lateral surface  9  can project a distance of 5 mm or more. 
     A number of vane elements  1  are assembled as an annular ring and are attached in a drive housing. The vane blade  2  extends between the inner and outer platforms  5  and  3  essentially in a radial direction of the drive assembly. 
     FIG. 2 shows the wall  4  extending essentially parallel to flange  3 ′ of the outer platform. Wall  6  at the inner platform  5  extends substantially parallel to flange  5 ′. Two hook-type projections  11  are formed on outer platform  3  for engaging the vane element  1  on a drive housing (not shown). A hollow space or recess  12  is formed between flange  3 ′, wall  4  and a transverse web  10  extending between projections  11  at the outer platform, the flange, wall and web having substantially equal thickness. A similar space or recess  12 ′ is formed in the inner platform between flange  5 ′ and wall  6 . The inner and outer platforms  3  and  5  are formed with second front surfaces  13  at the openings of recesses  12  and  12 ′. The recesses  12  and  12 ′ have shapes corresponding to their opposite respective projections  8  so that the projections  8  of an adjacent vane element can be form-fitted in the recesses to interlock the vane elements together. 
     When assembling the vane elements  1  to form the annular ring, the projections  8  of one vane element  1  are inserted into the recesses  12  and  12 ′ of an adjacent vane element. The dimensions of the projections and the recesses are closely matched to one another so that the projections fit tightly into the recesses and the surfaces of the inner and outer platforms of adjacent vane elements are flush with one another as seen in FIGS. 4 and 5. Recesses  12  and  12 ′ have inner surfaces  14  open at surface  13 , and in the assembled state, surfaces  14  contact lateral surfaces  9  of projections  8  because of the dimensional conformance of projections  8  and recesses  12  and  12 ′. In this way, a seal is also produced between adjacent vane elements  1 . As can be seen in FIG. 2 the projections  8  and recess  12  and  12 ′ make up more than 50% of the cross-sectional surface area lying in the plane of the first or second surface  7  or  13 . This will insure adequate interfitting of the projections within the recesses. 
     FIG. 3 diagrammatically shows the vibration pattern of a single vane element  1  in the radial and axial directions, whereby two distinct vibration states are obtained. The vibration amplitude that is particularly evident in FIG. 3 at flange  5 ′ or wall  6  at inner platform  5  is effectively limited according to the invention when the vane elements  1  are assembled into the annular ring due to the tight fitting engagement of projections  8  into recesses  12 ′ of adjacent vane elements  1 . Damping of the vibration is also produced by the friction developed between inner surfaces  14  of recess  12 ′ and lateral surfaces  9  of projections  8 . 
     FIG. 4 shows a vane segment  15  comprised of three vane elements  1 . An annular ring is formed from a plurality of interfitted vane segments. Vane elements  1  made of plastic are joined not only by the form-fit between projections  8  and recesses  12 ,  12 ′ of adjacent vane elements  1 , but also by the friction present between the contact surfaces. The adjacent vane elements  1  can also be glued or bonded together at least at lateral surface  9  of projection  8  and inner surfaces  14  of recesses  12 ,  12 ′. The bonded joint between lateral surfaces  9  of projections  8  and inner surfaces  14  of recesses  12 ,  12 ′ is subject to shear stress by the loads produced during operation and thus is capable of resisting much greater loads than when stressed in tension or compression. 
     In order to form an annular ring of vane elements, several vane segments  15  are joined together without being glued or bonded to one another so as to compensate for thermal expansion. Hence, the segments are only coupled together by the form fit between projections  8  and recesses  12 ,  12 ′ of adjacent segments. The form fit, however, produces a damping effect due to the friction between lateral surfaces  9  of projections  8  and inner surfaces  14  of recesses  12 ,  12 ′. This brings about an improvement in the dynamic vibration behavior of vane elements  1 . In addition, the limiting of the vibration amplitude and a sealing of the gaps between adjacent vane segments are produced by the form fit of the projections in the recesses. 
     A hole  16  is provided in each vane element  1  of a vane segment  15  in the wall  4  of outer platform  3 . A bolt (not shown) in the housing (also not shown) engages in hole  16  and supports the vane segment to resist gas flow forces in the circumferential direction. As shown in FIG. 4, wall  4  is thickened locally around hole  16  for reducing stress concentrations. Hole  16  extends in the radial direction of the drive assembly and forces developed by the gas flow acting axially are resisted by the connection of the hook members  11  and not by the bolts. 
     FIG. 5 shows vane segment  15  in a perspective view, in which the profile of vane blades  2  can be clearly seen. It can further be seen that the dimensions of projections  8  and recesses  12 ,  12 ′ closely correspond to one another, so that the outer and inner platforms  3  and  5  are flush in the assembled state. When a number of segments  15  are assembled to form an annular ring, the projections  8  at the left side of segment  15  in FIG. 5 are fitted into the recesses of the adjacent segment without gluing or bonding. 
     Although the invention has been described in relation to specific embodiments thereof, it will become apparent to those skilled in the art that numerous modifications and variations can be made within the scope and spirit of the invention as defined in the attached claims.