Patent Publication Number: US-9896950-B2

Title: Turbine guide wheel

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to German Patent Application No. 10 2013 217 997.9 filed on Sep. 9, 2013, the entirety of which is incorporated by reference herein. 
     BACKGROUND 
     This invention relates to a turbine stator wheel, in particular to a high-pressure turbine stator wheel of a gas turbine, especially for being used with a gas-turbine engine. 
     It is known from the state of the art that the stator vanes of a turbine stator wheel in particular must be designed according to aerodynamic requirements. The contouring of the vane cross-section on the suction side and on the pressure side plays a major role here, but also important is the design of the vane passage between adjacent stator vanes, since the available flow cross-section through the passage partly determines the efficiency of the stator wheel. 
     The aerodynamic design must however take particular account of ensuring the burn-back capability of the stator vanes of the stator wheel. The burn-back capability of the stator wheel must be understood in this connection as the characteristic that during operation of the gas turbine the trailing edge in particular of the first stator wheel of the high-pressure turbine can burn off under the extreme thermal loads occurring. This means that the stator vane, starting from the vane trailing edge, is shortened by the burn-back. Since the first stator wheel of a high-pressure turbine mainly determines the flow rate through the entire turbomachine, maintenance of the flow rate (capacity) of the first stator wheel is of crucial importance so that the entire turbomachine and all individual components can continue to operate with a nominal mass flow at the design point. It is thus necessary that the flow rate (capacity) of the turbine does not substantially change due to the burn-back. 
     To ensure the burn-back criterion of a turbine stator wheel, the passage cross-section inside the stator wheel upstream of the narrow cross-section (i.e. in the direction of the progressing burn-back of the vane trailing edge) must remain approximately constant, so that even in the case of burn-back of the thermally highly loaded trailing edge, the narrow passage cross-section then effective also remains approximately constant. It is thereby ensured that the flow rate remains similar even in the case of burn-back. An embodiment of this type is known for example from FIG. 4 of DE 10 2005 025 213 A1. 
     The disadvantage of the stator wheel designs known from the state of the art is that the aerodynamic design cannot be made loss-optimized, since the generally advantageous design with heavy aerodynamic loading in the rear suction-side area (“Rear-Loaded-Design”) greatly infringes the burn-back criterion. It is therefore always necessary to compromise in the aerodynamic design so that the burn-back capability is ensured. This in turn reduces the turbine efficiency and increases the specific fuel consumption (SFC) of the turbomachine. 
     SUMMARY 
     An object underlying the present invention is to provide a turbine stator wheel of the type specified at the beginning, which while being simply designed and simply structured, has a high efficiency and at the same time ensures the above mentioned burn-back criterion. In the event of a burn-back in particular, the turbine capacity should remain largely unchanged, so that the overall engine with its individual components can continue to be operated at the design point. 
     It is a particular object of the present invention to provide solution to the above problems by a combination of features as described herein. 
     Accordingly, the solution in accordance with the invention considers a turbine stator wheel in which two adjacent stator vanes each form a passage including a constant passage portion. This constant passage portion is characterized in that it has a substantially constant passage cross-section. The constant passage portion has an inlet area into said constant passage portion and an outlet area. The outlet area is located at the vane trailing edge and is as a rule identical to the narrowest cross-section (narrow cross-section) of the passage. Each stator vane forms on the pressure side a rear area which extends from the vane trailing edge adjoining the constant passage portion as far as the inlet area of the passage portion, and a front area extending upstream of the rear area. The rear area is thus that area of the pressure side of the stator vane that delimits the constant passage portion. 
     It is provided in accordance with the invention that the stator vanes have a convex pressure-side contour on the pressure side which provides a transition from the rear area of the stator vane to the front area of the stator vane. 
     The solution in accordance with the invention provides a convex pressure-side contour on the pressure side of the stator vane such that due to said convex pressure-side contour a transition is made from a rear area of the stator vane, in which a constant passage portion is present, to a front area of the stator vane. The rear area of the stator vane is thus connected to the front area of the stator vane via the convex pressure-side contour. 
     The convex pressure-side contour, or the convex curvature of the pressure side provided by this contour, enables the passage between two stator vanes to be designed constant over a certain length, even if the adjacent stator vane is, in order to obtain a loss-optimized turbine stator wheel, provided on the suction side with a considerably convex curvature which—without compensation by the convex pressure-side contour—would lead to a considerable widening of the passage. The invention thus ensures a burn-back capability even in the event that a loss-optimized turbine stator wheel is provided that has stator vanes with a considerably convex curvature of the suction side in the area of the narrow cross-section. 
     Whereas in designs known in the state of the art the walls of the suction side and of the pressure side adjoining the vane trailing edge are designed substantially straight or with an even curvature and thus form a wedge-shaped cross-sectional area of the stator vane, the solution in accordance with the invention thus provides that the wall of the pressure side of the stator vane forms a convex pressure-side contour, i.e. a convex curvature, which forms the transition between the rear area of the stator vane adjoining the constant passage portion and the front area extending upstream from it. 
     The invention ensures the burn-back capability, due to convex contouring of the pressure side of the stator vane of the stator wheel, without the aerodynamic design of the suction side of the stator vane being affected. It is thus possible in accordance with the invention to freely define the suction side of the stator vane of the stator wheel and design it loss-optimized while achieving stator vanes with a considerably convex curvature of the suction side in the area of the narrow cross-section or adjacent to the narrow cross-section. It is ensured by the embodiment in accordance with the invention of the pressure-side contour of the stator vane that in the event of burn-back the cross-section of the passage between adjacent stator vanes remains substantially constant, so that the flow rate (capacity) of the turbine and hence the efficiency of the overall engine are affected not at all or only to a minor extent by a burn-back. 
     According to an embodiment of the invention, it is provided that the profile thickness of the stator vanes rises or is constant or decreases to a lesser extent in the direction of the vane trailing edge upstream of the rear area of the stator vanes than in the rear area of the stator vane. In other words, this embodiment provides that the profile thickness rises or is constant or decreases to a lesser extent in the direction of the vane trailing edge upstream of the inlet area into the passage than in the area of the constant passage portion. This corresponds to the design of the convex pressure-side contour on the pressure side of the stator vane, which is precisely what ensures that the profile thickness of the stator vanes rises, is substantially constant or decreases only slightly upstream of the constant passage portion when compared with a subsequently sharper decrease of the profile thickness in the rear area of the stator vane up to the vane trailing edge. 
     According to a further embodiment of the invention, it is provided that the convex pressure-side contour at or upstream of the inlet area into the constant passage cross-section forms a maximum. It can further be provided that the convex pressure-side contour at or upstream of the inlet area into the constant passage cross-section forms a maximum in the curvature. The maximum in the curvature is here close to the point locally projecting furthest from the pressure side or close to the line of the pressure-side contour locally projecting furthest from the pressure side. The maximum and/or the maximum in the curvature are thus located not in the rear area of the stator vane, but in the front area of the stator vane, however preferably at a short distance from the rear area (e.g. at a distance corresponding to a maximum of 10% of the length of the skeleton line) or directly at the transition of the two areas. 
     A further embodiment of the invention provides that the convex pressure-side contour on the pressure side of the stator vanes is formed predominantly or completely in the front area of the stator vane. It can be provided here that part of the convex pressure-side contour is additionally formed in the rear area of the stator vane. Generally speaking, a straight or even a concave curvature merging into the convex pressure-side contour can however also be provided in the rear area of the stator vane that delimits the constant passage portion. 
     In the meaning of the present invention, a substantially constant passage cross-section is present for example when the passage cross-section diverges no more than 20% from the narrow cross-section in the area of the vane trailing edge. This divergence from the narrow cross-section is preferably lower, and less than 10%, 5% or 2% of the narrow cross-section. Ideally, the passage cross-section in the constant passage portion is exactly constant. It can further be provided that the constant passage portion extends over a chord length which is for example in a range between 5% and 40% of the total chord length and is for example approximately 5%, 10%, 15%, 20%, 25%, 30%, 35% or 40% of the total chord length. 
     Generally speaking, it can be provided that the convex pressure-side contour extends over the entire height of the stator vane. It can furthermore be provided that the pressure-side contour extends at least over a partial area of the vane height (for example over at least 50% or at least 70% of the vane height). It is furthermore possible for the design of the curvature to vary over the vane height. 
     It is particularly favourable when the stator vane, starting from the vane trailing edge, is provided with a concave area adjoining the convex area. This embodiment leads in particular to an optimum surface pressure distribution on the vane surface. 
     In accordance with the invention the following advantages are achieved: 
     In accordance with the invention there is an increase in the aerodynamic efficiency, since compared with an embodiment of the stator vanes according to the state of the art an increase in the stage efficiency is achieved. 
     A further advantage results with regard to mechanical stability. The convex pressure-side contour of the stator vane results, when compared with the state of the art, in a substantially higher wedge angle adjoining the vane trailing edge. Hence the profile in the trailing edge area is thicker. This in turn leads to an increased mechanical stability, which results in a far lower deformation of the trailing edge under thermal load in operation. 
     The turbine stator wheel in accordance with the invention has considerable advantages with regard to cooling air consumption too. Since the vane contour has a greater thickness in the trailing edge area, it is possible to extend the internal cooling geometry further in the direction of the vane trailing edge. This can for example be achieved by so-called pedestal banks positioned further back. This results in the possibility of saving on cooling air, since the trailing edge overhang that is difficult to cool and subjected to the highest thermal load can be reduced in length. 
     Due to the mechanically more stable and easier to cool trailing edge area, a longer service life results from the embodiment in accordance with the invention of the cross-section of the stator vanes. 
     A further advantage is achieved with regard to the stability of the engine characteristics and to turbine efficiency in long-term operation. The engine flow rate changes less strongly in long-term operation due to the more stable and easier to cool vane trailing edge. The drop in high-pressure turbine efficiency due to the rise in trailing edge losses as a result of burn-back is reduced. 
     A further substantial advantage results from cost savings due to the longer service life and due to reduced engine development costs. The engine development costs can be reduced due to the reliable capacity forecast, since the necessity for subsequent capacity change is reduced. The engine development time too can be shortened. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described in the following in light of the accompanying drawing, showing exemplary embodiments. 
         FIG. 1  shows a schematic representation of a gas-turbine engine in accordance with the present invention. 
         FIG. 2  shows a partial view of a turbine stator wheel in accordance with the state of the art. 
         FIG. 3  shows a view of a first exemplary embodiment in accordance with the present invention. 
         FIG. 4  shows a view of a second exemplary embodiment in accordance with the present invention. 
         FIG. 5  shows a comparative view of the embodiment in accordance with the state of the art (left-hand half of figure) and the exemplary embodiment in accordance with the invention of  FIG. 4  (right-hand half of figure). 
         FIG. 6  shows in the upper half of the figure the static surface pressures of the embodiment in accordance with the state of the art (as per  FIG. 5 , left-hand side) and in the lower half of the figure the static surface pressures of the embodiment according to an exemplary embodiment in accordance with the invention (as per  FIG. 5 , right-hand side). 
     
    
    
     DETAILED DESCRIPTION 
     The gas-turbine engine  10  in accordance with  FIG. 1  is a generally represented example of a turbomachine where the invention can be used. The engine  10  is of conventional design and includes in the flow direction, one behind the other, an air inlet  11 , a fan  12  rotating inside a casing, an intermediate-pressure compressor  13 , a high-pressure compressor  14 , a combustion chamber  15 , a high-pressure turbine  16 , an intermediate-pressure turbine  17  and a low-pressure turbine  18  as well as an exhaust nozzle  19 , all of which being arranged about a center engine axis  1 . 
     The intermediate-pressure compressor  13  and the high-pressure compressor  14  each include several stages, of which each has an arrangement extending in the circumferential direction of fixed and stationary guide vanes  20 , generally referred to as stator vanes and projecting radially inwards from the core engine casing  21  in an annular flow duct through the compressors  13 ,  14 . The compressors furthermore have an arrangement of compressor rotor blades  22  which project radially outwards from a rotatable drum or disk  26  linked to hubs  27  of the high-pressure turbine  16  or the intermediate-pressure turbine  17 , respectively. 
     The turbine sections  16 ,  17 ,  18  have similar stages, including an arrangement of fixed stator vanes  23  projecting radially inwards from the casing  21  into the annular flow duct through the turbines  16 ,  17 ,  18 , and a subsequent arrangement of turbine rotor blades  24  projecting outwards from a rotatable hub  27 . The compressor drum or compressor disk  26  and the blades  22  arranged thereon, as well as the turbine rotor hub  27  and the turbine rotor blades  24  arranged thereon rotate about the engine axis  1  during operation. 
       FIG. 2  shows a view of a turbine stator wheel known from the state of the art when viewing the front sides of adjacent stator vanes  23 , which each have a pressure side  30  and a suction side  31  and form a passage  29  through which flow the hot gases exiting the combustion chamber.  FIG. 2  shows that in the area of a vane trailing edge  32  the passage  29  has a narrowest cross-section (narrow cross-section  36 ) which is formed to match the required profile shape of the stator vanes  23 . The thermal load during operation results in a burning off of the area of the vane trailing edge  32 , so that a burn-back  35  results. This means that the hatched surface of the vane profile burns off. This results in an effective passage cross-section  37  which is considerably wider compared with the narrow cross-section  36  and consequently leads to a marked reduction in efficiency. The widening of the passage cross-section is accompanied by a change in flow rate and capacity. 
     The problem described is all the greater the more the turbine stator wheel is designed as a loss-optimized turbine stator wheel and to do so has stator vanes  23  which are provided on the suction side  31  in the area of the narrow cross-section  36  or adjoining the narrow cross-section  36  with a considerably convex curvature, which in the case of a burn-back leads to a considerable widening of the passage. 
       FIG. 3  shows a view of an exemplary embodiment in accordance with the invention. The stator vanes  23  have in turn a pressure side  30  and a suction side  31 , where two adjacent stator vanes  23  form a passage  29  between the suction side  31  of the one stator vane and the pressure side  30  of the other stator vane starting from the vane trailing edge  32  through which passage  29  flow the hot gases exiting the combustion chamber. It is provided here that the passage  29  includes a constant passage portion  29   a  in which the passage  29  has a substantially constant passage cross-section  37 . 
     The constant passage portion  29   a  has an inlet area  38  and an outlet area  36  which have substantially the same passage cross-section. The outlet area  36  is delimited here by the vane trailing edge  32 , so that the outlet area  36  matches the narrow cross-section of the passage  29 . 
     The statement that the passage cross-section  37  in the constant passage portion  29   a  is substantially constant means that the divergence of the passage cross-section  37  from the narrow cross-section in this constant passage portion  29   a  is less than a defined value, which is for example defined as 20% of the narrow cross-section. Alternatively, a constant passage cross-section  29   a  can for example be defined in that the divergence from the narrow cross-section is less than 15%, 10% or 5% of the narrow cross-section. 
     The stator vane  23  furthermore forms on the pressure side a rear area  320  that extends, starting from the vane trailing edge  32  adjoining the constant passage portion  29   a  as far as the inlet area  38  of the constant passage portion  29   a . The pressure-side rear area  320  of the stator vane is therefore that area which delimits the constant passage portion  29   a  on the pressure side. Upstream of the rear area  320  a front area  310  extends generally speaking as far as the vane leading edge, but for the purposes of the present invention only that part of the front area adjoining the rear area  320  is considered in detail. 
     The stator vane  23  furthermore has on the pressure side  30  a convex pressure-side contour  33  creating a transition from the rear area  320  to the front area  310 . This means that the convex pressure-side contour  33  is provided in the transition area between the two areas  310  and  320 , and can extend exclusively in the front area  310  or alternatively over both areas  310 ,  320 . The convex pressure-side contour  33  has a maximum M, which in the cross-sectional view of  FIG. 3  indicates the point at which the curvature provided by the convex pressure-side contour  33  projects locally furthest from the pressure side  30 . 
     Accompanying the convex pressure-side contour  33  is a certain course of the profile thickness d of the stator vane  23 . If the course of the profile thickness d in the direction of the vane trailing edge  32  is viewed, the situation is such that the profile thickness d upstream of the rear area  320  (or upstream of the inlet area  38 ) rises or is constant, as is illustrated by the profile thicknesses d1 and d2 of  FIG. 3 . In the rear area  320  of the stator vane, by contrast, the profile thickness d drops relatively sharply, as shown by way of example by the profile thickness d3. Alternatively, it can also be provided that the profile thickness upstream of the rear area  320  does not rise or is constant, however decreases only to a lesser extent (i.e. by a smaller value per unit of length) than in the rear area  320 . This course of the profile thickness d corresponds to the provision of a maximum M for the curvature provided by the convex pressure-side contour  33  upstream of or at the inlet area into the constant passage portion  29   a.    
     The provision of a convex pressure-side contour  33  leads on the one hand to an increase of the wedge angle between the surfaces of the pressure side  30  and the suction side  31  in the area adjoining the vane trailing edge  32 , and in particular to an avoidance of any widening of the passage cross-section in the event of burn-back. This widening is prevented precisely because the solution in accordance with the invention provides a constant passage portion  29   a , so that the narrow cross-section does not change in the event of burn-back  35  in the area of this constant passage portion  29   a . A burn-back  35  is drawn heavily exaggerated in  FIG. 3  in order to better explain the effectiveness of the invention. The result is that the passage cross-section  37  inside the constant passage portion  29   a  remains substantially the same in the event of burn-back, since the narrow cross-section  36  in this portion is substantially equal to the passage cross-section  37 . 
       FIG. 4  shows a further exemplary embodiment in accordance with the invention of two stator vanes  23  of a turbine stator wheel. Generally speaking, the exemplary embodiment matches the embodiment of  FIG. 3 , to which reference is made with regard to the reference numerals used. It is in turn provided that by the provision of a convex pressure-side contour  33  on the pressure side  30  of the stator vane  23 , the rear area  320  of the stator vane  23  is given a shape that permits the provision of a constant passage portion  29   a  with substantially constant passage cross-section  37  between an inlet area  38  and an outlet area  36  of this constant passage portion  29   a.    
     The corresponding curvature of the convex pressure-side contour  33  leads to the profile thickness d of the stator vane  23  rising or remaining substantially constant upstream of the rear area  320  and decreasing sharply only in the rear area  320  of the stator vane (cf. profile thicknesses d1, d2 and d3 in  FIG. 4 ). 
     One difference in the embodiment of  FIG. 4  from the embodiment of  FIG. 3  is in the curvature of the pressure side  30  of the stator vane in the rear area  320 . Whereas this curvature in  FIG. 3  is designed at least approximately concave, it is in the exemplary embodiment of  FIG. 4  designed convex, so that the rear area  320  forms a partial area of the convex pressure-side contour  33  and contributes to the latter. The maximum M of the convex pressure-side contour  33  is located however upstream of the constant passage portion  29   a  in the front area  310 . The convex pressure-side contour  33  here forms the transition from the rear area  320  of the stator vane to the front area  310  of the stator vane. 
       FIG. 4  furthermore shows an additional line  40  and an additional surface  50  which are not actually present in the stator vane  23  and serve only to make the solution in accordance with the invention more clear. The line  40  thus indicates the course of the pressure side of a stator vane designed according to the state of the art, where the wall of the stator vane  23  adjoining the vane trailing edge  32  is designed substantially straight or with a slight and even curvature. The line  40  thus makes clear the pressure-side contour of a conventional stator vane. The surface  50  makes clear a thickening achieved by providing a convex pressure-side contour  33 . By this thickening or provision of a convex pressure-side contour  33  it is possible, even with a loss-optimized turbine stator wheel that has stator vanes  23  with a considerably convex curvature of the suction side  31  in the area of the narrow cross-section and/or adjacent to the narrow cross-section, to provide a constant passage portion  29   a  inside the passage  29 , so that in the event of a burn-back  35  widening of the passage is prevented. 
     A thickening  50  is also present in the exemplary embodiment of  FIG. 3 , but has in the exemplary embodiment of  FIG. 3  a different shape and is not convex overall, but also has a convex portion in the transition from the rear area  320  to the front area  310 . In the embodiment of  FIG. 4 , the thickening  50  is formed completely by the convex pressure-side contour  33 . 
     A further feature specific to the embodiment of  FIG. 4  is that a concave area  34  is provided adjoining the convex area  33  on the pressure side  30  of the stator vane  23 . This leads to a further optimization of the surface pressure distribution on the vane surface. 
       FIG. 5  shows a comparison of the embodiment according to the state of the art as shown in  FIG. 2  (left-hand half of  FIG. 5 ) and an exemplary embodiment of the invention according to  FIG. 4 . The contouring of the pressure side  30  provided in accordance with the invention results in the advantages described above. This is also evident in particular from the comparative representation of the static surface pressures according to  FIG. 6 , where the standardized chord length of 0.0 corresponds to the position of the vane leading edge and the standardized chord length of 1.0 to the position of the vane trailing edge. 
     The upper half of  FIG. 6  shows the surface pressure distribution associated with the geometric embodiment according to the state of the art ( FIG. 5  left-hand side). The lower half of  FIG. 6  shows the surface distribution associated with the embodiment in accordance with the invention ( FIG. 5  right-hand side). Discernible is the advantageous pressure course resulting in accordance with the invention on the suction side ( FIG. 6  bottom) and implementable without infringement of the burn-back criterion. The S-shape of the pressure course on the pressure side in the area of the vane trailing edge for the chord length 0.7 to 1.0 ( FIG. 6  bottom) results from the contouring of the pressure side in accordance with the invention to ensure the burn-back criterion. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Engine axis 
           10  Gas-turbine engine/core engine 
           11  Air inlet 
           12  Fan 
           13  Intermediate-pressure compressor 
           14  High-pressure compressor 
           15  Combustion chamber 
           16  High-pressure turbine 
           17  Intermediate-pressure turbine 
           18  Low-pressure turbine 
           19  Exhaust nozzle 
           20  Guide vanes 
           21  Core engine casing 
           22  Compressor rotor blades 
           23  Turbine stator vanes 
           24  Turbine rotor blades 
           26  Compressor drum or disk 
           27  Turbine rotor hub 
           28  Exhaust cone 
           29  Passage 
           29   a  Constant passage portion 
           30  Pressure side 
           310  Front pressure-side area of pressure side 
           320  Rear pressure-side area of pressure side 
           31  Suction side 
           32  Vane trailing edge 
           33  Convex pressure-side contour/convex area 
           34  Concave area 
           35  Burn-back 
           36  Narrow cross-section/passage outlet area 
           37  Passage cross-section 
           38  Passage inlet area 
           40  Course of conventional stator vane wall 
           50  Thickening 
         d Profile thickness 
         M Maximum of convex pressure-side contour