Patent Application
20050079058
The present invention relates generally gas turbine engines, and more particularly to shrouded turbine blades therefor.
Numerous problems face the designer of a shrouded gas turbine blade as a result of the high heat and high speed environment in which the shrouded blade must operate. Vibration damping, creep curling, bending stresses, contact stress wear, shroud misalignment and dynamic effects are just a few of the demons facing the designer. And, as if these design problem were not enough, in airborne applications excess weight in itself is also a penalty.
Much attention has been paid in the prior art to improving the damping and bending strength of shrouded blades. However, one area where further improvement is needed is the reduction of contact-related wear between adjacent shrouded blades.
A shrouded rotor blade assembly typically comprises a plurality of airfoil blades extending radially from a rotor having a central axis, and a shroud portion which, as an assembly, forms an annulus around the axis and circumscribing all or a portion of the blades. Throughout this specification and the attached claims, the term “generally perpendicular” is used to refer to the angle of intersection of the annular segmented shroud with the radially-extending blades, and the term “generally planar” is used to refer to the annular planar section (or a segment portion thereof), rotated about the central axis point. Examples of such configuration for shrouded blades are common in the prior art, as shown in U.S. Pat. Nos. 3,576,377, and 4,243,360 for example. In contrast from this typical configuration, FR 1,252,763 in one embodiment, for example proposes a non-annular shroud arrangement which extends acutely (i.e. not generally perpendicularly) from the blades.
Typical prior art shrouded turbine blades generally have a shroud having opposed bearing or contact faces which may be shaped to facilitate interlocking of adjacent shrouds. These shrouds may include different variations in thickness, such as, for example, stiffening rails used to reduce centrifugal deflection, and gradual changes in thickness across the width of the shroud used to reduce bending stresses in the shroud. These features, however, come at the price of increases in shroud weight.
In use, fretting can occur on contract surfaces of abutting turbine shrouds, which is of course undesirable. Prior art such as U.S. Pat. Nos. 3,576,377, 4,822,248 and 6,164,916 teach that the wear resistance of the contact faces may be improved by the introduction of special wear resistant coatings or inserts. While perhaps effective, these solutions introduce manufacturing steps and materials, and therefore cost and reliability issues as well. Further improvement is accordingly needed to improve the contact wear resistance of turbine shrouds.
It is an object of the present invention to provide a shrouded turbine blade having improved contact wear performance. The invention lowers contact stresses and thereby provides, among other things, a design which is less sensitive to shroud misalignment due to shroud wear, deflection or tolerance stack-up at assembly.
Therefore, in accordance with the present invention, there is provided a one-piece blade for a turbine section of a gas turbine engine, the blade comprising a root, an airfoil and a shroud, wherein the shroud extends generally perpendicularly from a tip of the airfoil and is defined by a pair of opposed bearing faces and a pair of opposed non-bearing faces, the bearing faces each having a contact portion adapted to contact a shroud of an adjacent blade, the shroud having a substantially constant nominal thickness and the bearing faces having a substantially constant face thickness across the contact portion, the face thickness being greater than the nominal thickness, the transition between the face thickness and the nominal thickness being substantially discontinuous.
There is also provided, in accordance with the present invention, a blade for a turbine section of a gas turbine engine, the blade comprising: an airfoil portion extending from a root portion to a tip portion; and a shroud portion extending laterally from the airfoil portion, the shroud portion having a body portion having a substantially constant thickness and a pair of opposed bearing faces each having contact portions adapted to matingly contact a bearing face contact portion of a shroud portion of an adjacent turbine blade, wherein the body portion is generally planar and has an increase in thickness immediately adjacent the contact portion of at least one of the opposed bearing faces to thereby provide substantially all of the contact portion of said bearing face with an increased surface area associated with said increased thickness, and wherein said increased surface area is adapted to lower contact stresses arising from contact with at least one mating bearing face of said adjacent turbine blades.
There is further provided, in accordance with the present invention, a method of reducing face contact stress in a shroud contact face of a shrouded turbine blade, the method comprising the steps of: determining a desired shroud design for a given turbine blade design, the shroud design including a nominal thickness; determining a desired face contact stress for at least one shroud contact face of the shroud, the at least one shroud contact face having a contact portion length; and providing a local increase in the shroud nominal thickness to thereby increase the area of the at least one shroud contact face along said contact portion length, wherein the increase in area corresponds to the desired face contract stress, and wherein the local increase is limited to a region immediately adjacent the at least one shroud contact face.
Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:
Referring now to
Although the turbine shroud 26 is preferably cast with the rest of the turbine blade 20 as a single element, the local bearing face edge projections 28 can also be incorporated onto existing shrouded turbine blades, to reduce shroud contact face fretting and increase the contact face life. Existing cast shrouded turbine blades could easily include such edge projections 28, through a relatively minor casting tool change. Further, the edge projections 28 can also be added as a post-production add-on or blade repair process, being added to the turbine shroud using methods which are known to one skilled-in the art, such as braze or weld material build-up or other method. Accordingly the present invention also permits increases to the shroud contact face surface area to reduce contact stress between already-manufactured turbine shrouds.
Further, although the bearing face edge projections 28 are preferably disposed on both ends of the shroud 26 as depicted in
As mentioned, in an alternate embodiment, two abutting contact face edge projections 56/58 of uneven size, as depicted in
Accordingly, increasing the bearing face surface area of the turbine shrouds, as per the present invention, is the key to reducing contact stress between abutting shrouds. This invention, however, is counter-intuitive especially in aero-applications since weight increase itself is almost always a taboo topic. Also in the particular instance of shrouded rotating blades, since any weight increase in the shroud increases dynamic deflections due to the extremely high rate of rotation (e.g. above 20,000 rpm), additional weight misaligns the contact faces and will lead to a yet further increase in contact stress. For this reason, previous attempts to reduce contact stress between abutting shrouds have all generally involved using surface coatings or other inserts which do not significantly add weight to the shroud. However, the present invention is surprising in its results, as a relatively minimal weight increase allows a significantly increased bearing face wear life. Accordingly, the weight added is intentionally minimal to achieve considerable reductions in bearing face contact stresses. For example, by extending the bearing face edge projections along the full length of the contacting portion of the bearing face, the contact stresses can be reduced with only a very minimal weight penalty.
Further, the simple geometry of the shrouds of the present invention make them relatively easy to design and produce, which of course can result in significant cost and time savings. Unlike the prior art, the turbine shroud of the present invention does not compromise the shroud stiffness, nor does it significantly increase the shroud to airfoil interface stress concentrations, which are produced in all shrouded turbines by centrifugal force. Unlike the prior art, stress concentrations are minimized in the present invention by the shroud shape. Some known prior art shrouds (see, for example, FR 1,252,763) are designed with inherent shroud flexibility relative to the airfoil, such that the blades can be assembled with a given level of flexion, permitting the shroud to airfoil interface stress to be reduced by centrifugal force. Such prior art is not directed to reducing contact stress, and in fact generally leads to an undesirable increase in shroud face contact stresses. For example, the shroud of one embodiment of FR'763 is acutely angled relative to the blade to permit the shroud to be flexible in response to dynamic forces, in an effort to reduce bending stresses at the blade root. To accommodate such flexion, FR'763 provides long contact faces on the shroud as a means to ensure that contact between adjacent shroud faces is maintained as inevitable non-uniform shroud flexion occurs. The flexing of adjacent shrouds is never identical (and hence the need to the long contact faces) and the rotating nature of the flexion causes point contact (as opposed to face contact) to occur between adjacent shrouds. Thus the FR'763 design inevitably results in serious local stress concentrations on the shroud contact faces, which is of course undesirable and certainly does not minimize contact face stress. The shrouds of the present invention, however, extend generally perpendicularly from the airfoil and are designed to be substantially rigid relative thereto. Accordingly, significant displacement of the shroud contact faces need not be accommodated.
The embodiments of the invention described above are intended to be exemplary. For example, the invention may be applied to mid-span shrouds, and may incorporate a projection 28 that projects radially inwardly from the shroud, or inwardly and outwardly, as desired. Still other modifications are available, and those skilled in the art will therefore appreciate that the forgoing description is illustrative only, and that various alternatives and modifications can be devised without departing from the spirit of the present invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.
