Abstract:
Reducing the tip leakage for an existing compressor or turbine blade ( 10 ), including internally cooled blades, can be achieved by retrofitted the blade with a tip leakage-reducing device in the form of a winglet ( 12 ). The winglet ( 12 ) can be fixed onto the tip portion by bonding the winglet onto the face or faces of the blade by a key and slot arrangement.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to European application no. 08101944.0, filed 25 Feb. 2008, the entirety of which is incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Field of Endeavor 
         [0003]    The invention relates to turbine and compressor blades, more specifically to an apparatus and method for reducing the tip leakage of such blades. 
         [0004]    2. Brief Description of the Related Art 
         [0005]    Compressor and turbine blades are characterised by being subject to extreme operating conditions. These include, in the case of compressor blades, operating temperatures of typically up to 500° C. and, for turbine blades, typically above 500° C. for last stage blades, and greater than 1000° C. for first stage blades. These high temperatures can necessitate the inclusion of internal cooling. Further, compressor/turbine blades are subject to high centrifugal loads operating typically at minimum speeds of between 3000-3600 min −1 . The special consideration of material selection, manufacturing technique and design required to enable operation under these environments differentiate these blades from other types of blades. 
         [0006]    A known compressor/turbine inefficiency is gas leakage across a blade tip, commonly known as tip leakage, from the pressure side to the suction side of the blade. To overcome this problem, tip arrangements have been developed that rely on increasing flow resistance across the blade tip. A common arrangement, referred to as a winglet or mini shroud, includes ribs or brackets formed at the blade tip. An example can be found in U.S. Pat. No. 6,565,324. The problem with the application of that disclosure is the acceptance that, due to the exacting engineering requirements as a result of harsh operating environments, blade improvements must be formed during the manufacture of the blades and cannot be retrofitted. Therefore, in order to take advantage of the solutions in that patent, existing blades must be replaced at significant cost. 
         [0007]    While it is known to modify fan blades as described in U.S. Pat. No. 7,252,478, those solutions are not applicable to compressor or turbine blades due to very different operating environments. Fan blades of the type described in U.S. Pat. No. 7,252,478 typically are subject to temperatures around 50° C and low revolution speed. 
       SUMMARY 
       [0008]    One aspect of the present invention deals with the problem of reducing the tip leakage of compressor or turbine blades that are subject to high centrifugal forces and thermal stresses. It addresses this problem by providing a method and an apparatus that attempts to overcome the apparent deficiencies of known winglet forming methods. 
         [0009]    Another aspect of the present invention is based on the general idea of modifying existing compressor/turbine blades to retrofit tip leakage prevention designs in such as way as not to compromise the mechanical integrity of existing internal cooling arrangements. 
         [0010]    Another aspect provides a method for modifying a turbine or compressor blade comprising a tip portion and having a tip wall, a pressure face, and a suction face. The method locates a winglet on one or each of the faces of the tip portion such that, in use, tip leakage is reduced, and includes the steps of: 
         [0011]    A. machining one or more slots through one or each of the faces 
         [0012]    B. manufacturing the winglet such that the winglet comprises one or more keys on an inner edge face such that the one or more keys are receivable by one or more of the slots therein; 
         [0013]    C. engaging the inner edge face of the winglet with the one or each of the faces by inserting the one or more of the keys in the one or more slots; and 
         [0014]    D. bonding the winglet to the one or each faces. 
         [0015]    A further aspect provides that the blade is a turbine blade having internal cooling and that the winglet is bonded such that the internal cooling is not disrupted by arrangement of the winglet adjacent to the tip wall. 
         [0016]    Another aspect provides a turbine or compressor blade comprising a tip portion having a pressure face, and a suction face, and a winglet with an inner edge face located on the blade such that in use blade tip leakage is reduced. An exemplary blade can be characterised by having: one or more slots through one or each of the faces; one or more keys on the inner edge face of the winglet wherein the one or more keys are receivable by one or more of the slots therein; and one or more keys received in the one or more slots therein enabling engagement of the inner edge with the tip portion face, wherein the winglet is bonded to the one or each tip portion faces. 
         [0017]    A further aspect provides that the blade is a cooled turbine blade with internal cooling and the slots are arranged so that the winglet does not disrupt internal cooling flow of the blade by arrangement of the winglet adjacent to the tip wall. 
         [0018]    Other aspects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    By way of example, an embodiment of the invention is described more fully hereinafter with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1  is a perspective view of a compressor or turbine blade with a retrofitted winglet according to a preferred embodiment of the invention; 
           [0021]      FIG. 2  is a perspective view of the compressor or turbine blade showing slots according to a first preferred embodiment of the invention; 
           [0022]      FIG. 3  is a sectional view through III-III in  FIG. 1  showing a winglet fitted to the pressure side of the compressor or turbine blade; and 
           [0023]      FIG. 4  is a sectional view through III-III in  FIG. 1  showing a winglet fitted to the suction and pressure side of the compressor or turbine blade. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0024]    Preferred embodiments of the present invention are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It may be evident, however, that the invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the invention. 
         [0025]    As seen in  FIG. 1 , disclosed is an exemplary embodiment of a manufactured compressor or turbine blade  10  that has been subsequently modified by the retrofitting of a winglet  12  on either or both of the pressure face  16  or suction face  18  of a tip portion  14 . In a preferred embodiment the blade  10  is also an internally cooled turbine blade  10 . 
         [0026]    The winglet  12  is retrofitted to the compressor or turbine blade  10  in such a manner that it is capable of withstanding the extreme operating conditions it will be exposed to during operating. According to one preferred embodiment of the invention, the winglet  12  is retrofitted by being fixed to the blade  10 . 
         [0027]    An exemplary method of fixing includes machining slots  20 , see  FIG. 2 , through either or both the pressure face  6  and suction face  18  of the tip portion  14 . By providing the winglet  12  at the tip portion  14 , tip leakage is minimized. The slots  20  provide a mechanism for holding the winglet  12  in place. The dimensions and number of slots  20  machined is dependent on winglet  12  design and blade  10  operating conditions. Preferably there is more than one slot  20 , as in this arrangement operating stresses are more evenly distributed. 
         [0028]    As shown in  FIG. 3 , a winglet  12  is manufactured to include keys  30  on an inner edge face  32 , wherein the keys  30  are insertable into the slots  20  such that, after insertion, not only is the winglet  12  held in place but also the inner edge  32  of the winglet  12  engages with a face  16 ,  18  of the tip portion. In another preferred arrangement,  FIG. 4 , the winglet  12  engages the pressure face  16  and suction face  18  so as to further reduce tip leakage. 
         [0029]    Bonding by welding the held winglet  12  to the blade tip portion  14  after insertion of the keys  30  and engagement of the winglet  12  and tip portion  14  face  16 , 18  then completes fixing of the winglet  12 . Bonding is, however, not limited to welding but can also include brazing and other known processes. 
         [0030]    The skilled person would also recognize that variations of the fixing method that results in the fixing of a winglet  12  with sufficient integrity to withstand the extreme operating conditions of a compressor or turbine blade  10  could be used without detracting from the intent of the invention. 
         [0031]    As the winglet  12  is retrofitted rather than being formed as one piece with the tip, is it possible to use different materials to manufacture the tip portion  14  and winglet  12 . This characteristic can be advantageously exploited, for example, to improve the oxidation or corrosion resistance of the winglet  12 . Operating and design considerations could suggest other useful exploits of this characteristic. 
         [0032]    In a further exemplary embodiment, the blade  10  is an internally cooled blade  10 , and the slots are preferable arranged adjacent to the tip wall  22  so that when the winglet  12  is fixed the keys  30  of the winglet  12  do not disrupt the internal cooling flow of the blade  10  as shown in  FIGS. 1 and 2 . 
       REFERENCE NUMBERS 
       [0033]      10  Blade 
         [0034]      12  Winglet 
         [0035]      14  Blade tip portion 
         [0036]      16  Pressure face 
         [0037]      18  Suction face 
         [0038]      20  Slot 
         [0039]      22  Tip wall 
         [0040]      25  Replacement tip portion 
         [0041]      30  Winglet Keys 
         [0042]      32  Winglet inner edge face 
         [0043]    While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.