Abstract:
A turbomachine rotor includes a rotor blade mounted to a rotation element. The rotor blade includes a root for mounting the rotor blade to the rotation element. The root has a protrusion structure forming a stop face supporting the mounted root against the rotation element under action of a radially inwardly directed force. The protrusion structure defines a maximum clearance between the stop face and the rotation element. The root is radially moveable to a certain extent where in a radially outermost position the protrusion structure has the maximum radial clearance from the rotation element. The rotation element includes a groove therein. The groove has a groove face bearing the stop face of the rotor blade under action of a radially inwardly directed force. The groove is a circumferential groove extending in a circumferential direction with regard to an axis of rotation of the rotation element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2011/065460 filed Sep. 7, 2011 and claims benefit thereof, the entire content of which is hereby incorporated herein by reference. The International Application claims priority to the European application No. 10187227.3 EP filed Oct. 12, 2010, the entire contents of which is hereby incorporated herein by reference. 
     FIELD OF INVENTION 
     The present invention relates to the field of turbomachines, and the assembly of bladed rotors. 
     ART BACKGROUND 
     EP 0 757 749 B1 relates to gas turbine engines. A pair of root rails is provided on the bottom of a dovetail-shaped root portion of a gas turbine engine blade to minimize reciprocating tangential motion of the blades within the dovetail shaped slots in which the root portions of the blades are retained. Each root rail is wedge shaped, tapering in a decreasing cross section from the base of the root toward the aerofoil platform. 
     In view of the above-described situation, there exists a need for an improved technique that enables to provide for an efficient assembly of a turbomachine. 
     SUMMARY OF THE INVENTION 
     This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the herein disclosed subject matter are described by the dependent claims. 
     The inventor found that is beneficial to allow for a small relative movement of the compressor blades with respect to a rotation element of a compressor rotor to which they are mounted. This may be the case e.g. to cater for relative variations in material thermal expansion rates which otherwise could damage the mechanical integrity of the compressor rotor. Further, in order to obtain a small clearance between a compressor blade tip and a surrounding casing, it is beneficial to perform a final machining operation of the compressor blade tips of the assembled compressor rotor blade. This machining operation in combination with the machined compressor casing inner diameter defines the desired tip-to-casing clearance. However during the machining of the compressor blade tip the cutting forces push the compressor blade down into the rotation element while thereafter in operation under centrifugal loading the blade moves radially outwards. The inventor found that a large variation between the two conditions may hinder the ability to accurately control the desired tip clearance to the casing. 
     According to a first aspect of the invention there is provided a rotor blade, the rotor blade comprising a root for mounting the rotor blade to a rotation element of a turbomachine. The root comprises a protrusion structure forming a stop face supporting the mounted root against the rotation element under action of radially inwardly directed forces, the protrusion structure defining a maximum clearance between the stop face and the rotation element. 
     This aspect of the invention is based on the idea that a fine adjustment of the root dimension is easier and faster if not a full surface but only a protrusion structure of the root has to be machined or trimmed. 
     Generally herein, the term “radially inwardly” or “radially outwardly” refers to a direction with regard to a rotor blade mounted in a rotation element of a turbomachine rotor. That is, radially inwardly refers to a direction opposite the centrifugal forces that arise upon rotation of the turbomachine rotor. Radially outwardly refers to the opposite direction, i.e. to the direction of such centrifugal forces. In another, equivalent definition, “radially inwardly” refers to a direction from a tip to the root of the rotor blade and “radially outwardly” refers to a direction from the root to the tip of the turbomachine blade. 
     Further generally herein, the term “maximum clearance between the stop face and the rotation element” relates to the clearance (or, in other words, to the distance) between the stop face of the root and the rotation element in case the rotor blade is in its radially outermost position that is allowed by the rotation element to which the rotor blade is mounted. 
     According to an embodiment, the turbomachine is a gas turbine. According to a further embodiment, the turbomachine rotor is a compressor rotor. 
     According to an embodiment, the protrusion structure comprises at least one rail. For example, according to an embodiment, the protrusion structure comprises two rails. In one embodiment the rails run parallel and are radially curved to match the diameter at the face of the rotation element. However, other orientations of the rails are also possible and may or may not be curved to match at the face of the rotation element. Rails facilitate machining of the rails and hence a fine adjustment of a maximum radial clearance between the stop face of the rotor blade and the rotation element of the turbomachine. 
     According to a further embodiment, the protrusion structure comprises a circumferential rail, such as an annularly closed rail. 
     According to a further embodiment, the root further comprises a base portion located laterally adjacent the protrusion structure. The protrusion structure protrudes with regard to the base portion. According to an embodiment, the base portion of the root comprises or consists of a flat surface. A flat surface may facilitate machining operations. According to a further embodiment, the protrusion structure defines a stop plane of the root. In an embodiment, the flat surface and the stop plane are parallel. For example, if the bottom of the root comprises a flat surface or if the bottom of the root is a flat surface, in an embodiment the protrusion structure defines a flat bottom plane of the root. These embodiments may again facilitate machining of the protrusion structure. 
     According to an embodiment, the protrusion structure is located at a bottom of the root. According to other embodiments, the protrusion structure is provided at other locations of the root. 
     According to an embodiment, the stop face of the protrusion structure is curved. According to a further embodiment, the curvature of the stop face mates with the curvature of a surface of the rotation element that is opposite the protrusion structure in a mounted state. 
     According to a second aspect of the herein disclosed subject matter, a turbomachine rotor is provided, the turbomachine rotor comprising a rotation element and a rotor blade mounted to the rotation element, wherein the rotor blade is configured according to the first aspect or an embodiment thereof. 
     For example, in an embodiment, the turbomachine rotor comprises a rotation element and a rotor blade wherein the rotor blade comprises a root for mounting the rotor blade to a rotation element of a turbomachine. In accordance with aspects and embodiments of the herein disclosed subject matter the root comprises a protrusion structure forming a stop face supporting the mounted root against the rotation element under action of radially inwardly directed forces, i.e. forces towards the rotation element. Further in accordance with aspects and embodiments of the herein disclosed subject matter, the protrusion structure defines a maximum clearance between the stop face and the rotation element. According to embodiments of the herein disclosed subject matter the maximum clearance between the stop face and the rotation element is greater zero. The maximum clearance may be adjusted depending on the size of the turbomachine and the thermal expansion coefficients of the root of the rotor blade and the rotation element. 
     It should be understood that while only a single rotor blade is referenced in many embodiments described herein in order to illustrate the basic concept of these embodiments, a rotation element usually has a plurality of such rotor blades mounted thereto. 
     According to an embodiment, the protrusion structure defines a maximum radial clearance between the root and the rotation element. Hence, according to an embodiment, the root is radially moveable to a certain extent where in a radially outermost position the protrusion structure has the maximum radial clearance from the rotation element. 
     According to an embodiment, the rotation element comprises a groove therein, wherein the groove has a groove face bearing the stop face of the rotor blade under action of radially inwardly directed forces. Hence, in an embodiment the maximum radial clearance between the protrusion structure and the rotation element is the minimum distance between the protrusion structure and the groove face when the rotor blade is in its radially outermost position with regard to the rotation element. 
     According to an embodiment, the rotation element is a single piece which has the groove therein. According to other embodiments, the rotation element comprises two pieces configured for axial abutment wherein each piece forms part of the groove and the two pieces together form the groove when abutting each other. 
     According to a further embodiment, the rotor blade is mounted in the groove. Accordingly, in an embodiment, the groove has a cross section that is capable of retaining the rotor blade against radially outwardly directed forces such as centrifugal forces that arise upon rotation of the turbomachine rotor. 
     According to a further embodiment, the protrusion structure defines a stop plane (e.g. as described above with regard to the first aspect) and the stop plane of the protrusion structure and the groove face are parallel. This allows for a good support of the rotor blade on the groove bottom if radially inwardly directed forces are applied to the rotor blade. 
     According to a further embodiment, the groove is a circumferential groove extending in a circumferential direction with regard to an axis of rotation of the rotation element. 
     According to a further embodiment, the rotor blade has an further stop face for retaining the rotor blade against a radially outwardly directed force. 
     According to a further embodiment, the root of the rotor blade is movable within the rotation element between the stop face and the further stop face. With the rotor blade contacting the further stop face, the protrusion structure has the maximum clearance (distance) from the rotation element. 
     According to a third aspect of the herein disclosed subject matter, a method of assembling a turbomachine rotor is provided, the method comprising: (a) providing a rotor blade according to the first aspect or an embodiment thereof; (b) machining the protrusion structure to adjust the maximum clearance between the stop face and the rotation element; (c) mounting the rotor blade to the rotation element. 
     Due to the protrusion structure the adjustment of the maximum radial clearance between the stop face and the rotation element is facilitated and can be completed in a shorter time period. 
     According to a further embodiment, the method further comprises machining a radially outer portion of the rotor blade after mounting the rotor blade to the rotation element. Having adjusted the maximum clearance between the stop face and the rotation element to a desired, specific value, machining of a tip portion of the rotor blade can be performed so as to achieve a high accuracy of the distance between the rotor blade and a turbomachine casing that surrounds the turbomachine rotor with the rotor blade. 
     In the above there have been described and in the following there will be described exemplary embodiments of the subject matter disclosed herein with reference to a compressor blade, a compressor rotor and a method of assembling a compressor rotor. It has to be pointed out that of course any combination of features relating to different aspects of the herein disclosed subject matter is also possible. In particular, some embodiments have been described with reference to rotor blade claims whereas other embodiments have been described with reference to turbomachine rotor claims or method claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one aspect also any combination between features relating to different aspects or embodiments, for example even between features of the rotor blade claims and features of the turbomachine rotor claims or between features of the apparatus type claims and features of the method type claims is considered to be disclosed with this application. 
     The aspects and embodiments defined above and further aspects and embodiments of the present invention are apparent from the examples to be described hereinafter and are explained with reference to the drawings, but to which the invention is not limited. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross sectional view of part of a compressor of a gas turbine in accordance with embodiments of the herein disclosed subject matter. 
         FIG. 2  shows a cross sectional view of part of a compressor of a further gas turbine in accordance with embodiments of the herein disclosed subject matter. 
         FIG. 3  shows a larger part of the compressor rotor of  FIG. 1 . 
         FIG. 4  shows a larger part of the compressor rotor of  FIG. 2 . 
         FIG. 5  shows a partially cross-sectional view of the rotation element with mounted rotor blades of  FIG. 4  along line V-V. 
         FIG. 6  shows a perspective view of a rotor blade in accordance with embodiments of the herein disclosed subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     The illustration in the drawings is schematic. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit. The description of such elements is not repeated. Rather only differences between different figures are emphasized. 
       FIG. 1  shows a cross sectional view of part of a compressor of a gas turbine  100  in accordance with embodiments of the herein disclosed subject matter. In accordance with an embodiment, the compressor section of gas turbine  100  comprises a casing  102  and a rotor  104 . The rotor comprises a rotation element  106  and a rotor blade  108 . The rotor blade  108  comprises a root  110  for mounting the rotor blade  108  to the rotation element  106  of the gas turbine  100 . In accordance with an embodiment, the root comprises a protrusion structure  114  and a base portion  112  located laterally adjacent the protrusion structure  114 . The protrusion structure  114  protruding with regard to the base portion  112 . According to an embodiment shown in  FIG. 1 , the protrusion structure  114  protrudes over the base portion  112  in a direction towards the rotation element  106 . 
     In accordance with an embodiment the protrusion structure  114  forms a stop face  116  supporting the mounted root  110  against the rotation element  106  under action of radially inwardly directed forces, indicated at  118  in  FIG. 1 . 
     As shown in  FIG. 1 , in the vicinity of the protrusion structure the base portion  112  is flat and parallel to a stop plane  120  of the root  110 , the stop plane  120  being defined by the protrusion structure  114 . 
     In accordance with an embodiment shown in  FIG. 1 , the rotation element  106  comprises a groove  122 , the groove  122  having a groove face  124  bearing the stop face  116  of the rotor blade  108  under action of radially inwardly directed forces  118 . The rotation element  106  is formed by two discs  128 ,  130 . 
     In accordance with a further embodiment, the rotor blade  108  has a further stop face  132  for retaining the rotor blade  108  against a radially outwardly directed force  126 . In accordance with embodiments of the herein disclosed subject matter, root  110  of the rotor blade  108  is radially movable within rotation element  106  (in the depicted case within the groove  122 ) between the stop face  116  and the further stop face  132 . Such a movability of the rotor blade  108  (in particular the root  110  thereof) allows to cope with different thermal expansion coefficients of the rotation element  106  and the root  110 . 
     By appropriate machining of the protrusion structure, a maximum clearance  134  between the stop face  116  and the groove face  124  of the rotation element  106  can be adjusted to a desired value in a short time period, shorter than the time that would be necessary to machine a plain surface to obtain the same clearance  134 . Precise adjustment of the clearance  134  provides necessary movability of the rotor blade  108  within the groove while at the same time providing sufficient accuracy in machining a tip  136  of an aerofoil  138  of the rotor blade  108  so as to achieve a desired clearance  140  between the tip  136  and the housing  102 . By decreasing the clearance  140 , the efficiency of the gas turbine  100  can be increased. 
       FIG. 2  shows a cross sectional view of part of a compressor of a further gas turbine  200  in accordance with embodiments of the herein disclosed subject matter. 
     In contrast to the gas turbine  100  shown in  FIG. 1 , the rotation element  206  of the rotor  204  is made of a single piece which comprises the groove  222 . The groove  222  comprises a groove face  224 . In contrast to  FIG. 1 , the groove face  224  is located at the bottom of the groove  222 . Accordingly, the stop face  216  of the rotor blade  208  is provided by a protrusion structure  214  at the bottom  242  of the root  210 . According to an embodiment the protrusion structure  214  comprises two rails that extend in parallel over the bottom  242  of the root  210 . Between the rails, the bottom  242  comprises a base portion  212  forming a generally flat surface. 
     According to an embodiment, a further stop face  232  for retaining the rotor blade  208  against a radially outwardly directed force  226  is provided at an angle to a radial direction, indicated by the arrow  226  in  FIG. 2 . According to an embodiment shown in  FIG. 2 , the angle is different from 90 degrees, e.g. in a range from 30 to 60 degrees. 
     Likewise, also the corresponding bearing face  244  on the rotation element  206  is provided at an angle (e.g. the same angle as the further stop face  232 ) with regard to the radial direction indicated at  226 . According to other embodiments, the further stop face and the corresponding bearing face of the rotation element are provided at an angle of 90 degrees with regard to the radial direction. 
     The rotor blade configuration shown in  FIG. 2  also allows for a precise adjustment of the maximum radial clearance  234  between the root  210  and the rotation element  206 . Hence the turbomachine  200  allows a precise machining of the blade tip in order to adjust the clearance  240  between the blade tip  236  and the casing  202 . 
     It should be noted that root  210  may comprise a further face  246  that opposes a further face  248  of the rotation element. However, in accordance with embodiments of the herein disclosed subject matter, these opposing faces  246 ,  248  do not limit the radial movability of the root  210  in the groove  222  of the rotation element  206 . In other words, the distance  250  between the opposing further faces  246 ,  248  is larger than the maximum radial clearance  234 . 
       FIG. 3  shows a larger part of the compressor rotor  104  of  FIG. 1 . As is apparent from  FIG. 3 , the compressor rotor  104  comprises a plurality of rotation elements. Each of the rotation elements is formed by two discs, of which two are indicated at  128  and  130 . Each rotation element comprises a plurality of rotor blades, one of which is indicated at  108  in  FIG. 3 . An axis of rotation of the compressor rotor  104  is indicated at  152  in  FIG. 3 . 
       FIG. 4  shows a larger part of the compressor rotor  204  of  FIG. 2 . As is apparent from  FIG. 4 , the compressor rotor  204  comprises a plurality of rotation elements. Each of the rotation elements is formed by a single disc, one of which is indicated at  206 . Each rotation element  206  comprises a plurality of rotor blades, one of which is indicated at  208  in  FIG. 4 . An axis of rotation of the compressor rotor  204  is indicated at  252  in  FIG. 4 . 
       FIG. 5  shows a partially cross-sectional view of the rotation element  206  with mounted rotor blades  208  of  FIG. 4  along line V-V. According to an embodiment shown in  FIG. 5 , the stop face  216  of the protrusion structure  214  is curved to match the groove face  224  that faces the stop face  216 . Hence, in an embodiment the stop face  216  of the protrusion structure is curved in circumferential direction of the groove face  224  rotation element. In other embodiments, the stop face of the protrusion structure may be flat. For example, in such a case the protrusion structure is tangential to the rotation element  206 . 
       FIG. 6  shows a perspective view of a rotor blade  208  in accordance with embodiments of the herein disclosed subject matter.  FIG. 6  shows in particular the root  210  of the rotor blade  208  which comprises, in accordance with an embodiment, a protrusion structure  214  in the form of two rails with a stop face  216 . Between the rails extends the base portion  212  of the root. According to embodiments of the herein disclosed subject matter, the base portion  212  forms a recess with regard to the protrusion structure. In a further embodiment, the root  210  forms a dovetail shaped bottom profile, as shown in  FIG. 6 . The dovetail shaped bottom profile formed by the protrusion structure  214  and the base portion  212  may be curved to match the disc (rotation element) profile or may be flat, thereby easing manufacturing. In an embodiment only the rails but not the base portion  212  of the protrusion structure  214  have to be machined to match the profile of the rotation element, saving time and costs.  FIG. 6  also shows the further face  246  of the root  210  and the tip  236  of the rotor blade  208 . 
     Although  FIG. 1  and  FIG. 2  show part of a compressor of a gas turbine, it should be noted that aspects, embodiments and examples of the herein disclosed subject matter are as well applicable to other types of turbomachines e g compressors and steam turbines or to other parts of a gas turbine, like a turbine section comprising blades and discs. Protrusion structures according to embodiments of the herein disclosed subject matter may be machined faster than plain surfaces. Hence embodiments of the herein disclosed subject matter may allow for a fast and efficient adaption of the maximum clearance and the maximum movability of rotor blade with respect to a rotation element to which the rotor blade is mounted. As a consequence the machining time required during assembly of the turbomachine can be reduced. Although the protrusion structure intentionally provides a relatively small stop face area, this relatively small stop face area is sufficient to withstand the radially inwardly directed forces that arise during machining of the blade tip of the already mounted rotor blade. 
     It should be noted that the term “comprising” does not exclude other elements or steps and the “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims. 
     In order to recapitulate the above described embodiments of the herein disclosed subject matter one can state: 
     It is described a rotor blade comprising a root for mounting the rotor blade to a rotation element of a turbomachine. The root comprises a base portion and a protrusion structure protruding with regard to the base portion laterally adjacent the protrusion structure. The protrusion structure forms a stop face supporting the mounted root against the rotation element under action of radially inwardly directed forces. Further, a respective turbomachine rotor is provided. 
     In an exemplary embodiment of a gas turbine, one method of constructing the rotor of a compressor of the gas turbine is to assemble several discs tied together with a central tension stud. Rotor blades may be entrapped between two adjacent discs as shown in  FIG. 1  or loaded into a groove within a disc as shown in  FIG. 2 . Both methods provide means of radial location of the rotor blades thus retaining the rotor blades in operation under centrifugal load. It is beneficial to control the amount of radial location accuracy for operation of the gas turbine whereby closer tip clearances of the aerofoil to the outer casing results in improved compressor efficiency. 
     Embodiments of the herein disclosed subject matter describe a rotor blade, a turbomachine rotor and a method of achieving close fitting radial assembly accuracy by enabling fine adjustment at the assembly stage of rotor blade into the respective rotation element prior to final tip diameter machining. Embodiments of the herein disclosed subject matter reduce the reliance on costly tight manufacturing limits that may otherwise be required. Additionally, there is introduced a flexibility desired in a low volume assembly environment where adjustments are normal practice to improve build accuracy at low cost.