Patent Publication Number: US-6910854-B2

Title: Leak resistant vane cluster

Description:
TECHNICAL FIELD 
     This invention relates to shrouded vane clusters of the type used in turbine engines, and particularly to a cluster with a leak resistant, segmented shroud. 
     BACKGROUND OF THE INVENTION 
     The compressor section of a typical gas turbine engine comprises a case circumscribing an engine axis and axially alternating arrays of stationary vanes and rotatable blades. Each vane array may be constructed of multiple vane clusters distributed circumferentially about the interior of the case with each cluster being supported by the case. Each vane cluster comprises a radially inner shroud, a radially outer shroud, and two or more airfoils extending between the shrouds. Collectively, the inner and outer shrouds define the inner and outer boundaries of part of an annular flowpath for a working medium fluid. 
     During engine operation, the vane clusters are subject to nonuniform heating and cooling. The accompanying temperature gradients can cause damage by overstressing the clusters. To help alleviate these thermally induced stresses, one of the two shrouds may be divided into segments by slots that sever the shroud at locations circumferentially intermediate two neighboring airfoils. Since the outer shroud of each cluster connects the cluster to the case, it is conventional to segment the inner shroud rather than the outer shroud. The slots reduce the risk of damage by allowing the shroud segments to expand and contract independently of each other. 
     One technique for forming the slots is wire electro-discharge machining (EDM). Wire EDM uses an electrically charged electrode in the form a wire wound around a source spool and extending to a take-up spool. The vane cluster shroud is exposed to the wire between the spools. During the EDM operation, the wire travels from the source spool to the takeup spool and simultaneously advances toward the shroud. The difference in electrical charge between the wire electrode and the shroud causes an electrical discharge that removes material from the shroud. As material is removed, the wire advances through the shroud until the slot is completely formed. 
     One drawback of the shroud slots is that they provide a path by which working medium fluid can leak out of the flowpath during engine operation or by which non-working medium fluid can leak into the flowpath. Leakage can be mitigated, to some extent, by using a small diameter EDM wire to cut a thin slot, i.e. one with a correspondingly narrow kerf. However the use of thin EDM wire leads to increased machining time. Moreover, thin EDM wire is more susceptible to breakage than thick EDM wire during the EDM operation. Thin EDM wire is also more likely than thick EDM wire to be stalled by the presence of minute particulate impurities trapped in the vane cluster. Finally, commercially available EDM equipment capable of using thin wire is more specialized than EDM equipment capable using thicker wire. As a result, a manufacturer may find it economically unattractive to invest in the more specialized, thin wire equipment. Accordingly, it may be desirable to avoid thin slots in favor of relatively wider slots 
     One way to reduce leakage through a wide slot is to provide a recess in the interior of the slot and install a seal in the recess. U.S. Pat. Nos. 3,728,041, 3,970,318, and 5,167,485 show arrangements of this type. Although such seals may be easily installable between the shrouds of individual vanes, or between the circumferential extremities of adjacent vane clusters, they are not easily installable in the inter-airfoil shroud slots of an otherwise unitary vane cluster. In addition, forming the intra-slot recess increases manufacturing cost and decreases manufacturing throughput. Another possible way to mitigate leakage is to install an external seal, such as the sealing strip 78 shown in U.S. Pat. No. 4,422,827, to bridge across each slot. However such external seals also increase manufacturing cost. 
     What is needed is a vane cluster with thermally independent shroud segments and which is economical and easy to manufacture. 
     SUMMARY OF THE INVENTION 
     According to the invention, a vane cluster includes a shroud with a nonlinear slot extending therethrough to divide the shroud into thermally independent shroud segments. The slot is bordered by matching nonlinear surfaces that are easy and inexpensive to produce with conventional wire EDM equipment. The nonlinear slots effectively resist fluid leakage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a vane cluster whose radially inner shroud is segmented by slots embraced by the present invention. 
         FIGS. 2-5  are enlarged views of the slots shown in FIG.  1 . 
         FIGS. 6-8  are enlarged views of slots having curved portions. 
         FIG. 9  is a view similar to  FIG. 1  showing a vane cluster with a nonlinear profile at the lateral extremities of its inner shroud. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring to  FIGS. 1-5 , a vane cluster for a turbine engine compressor includes a radially outer shroud  10 , a radially inner shroud  12  and two or more airfoils  14  extending radially or spanwisely between the shrouds. Hooks  16  at the axial extremities of the outer shroud facilitate its attachment to an engine case, not shown. Feet  18  at the axial extremities of the inner shroud accommodate an inner airseal, also not shown. The cluster extends circumferentially between lateral extremities  20 . When several such clusters are installed in a turbine engine, the shrouds define the radially inner and outer boundaries of a portion of an annular fluid flowpath  22 . The flowpath circumscribes an engine axis, not shown. The vane cluster itself is typically a cast metallic article finish machined to prescribed dimensions. 
     The inner shroud  12  is divided into individual segments  24  by nonlinear slots  26  between circumferentially neighboring airfoils  14 . The slots are installed by wire EDM or other suitable process. Four different types of slots are depicted in  FIG. 1 , however only one type of slot would ordinarily be used in a given cluster. The leftmost portion of the inner shroud is depicted in its “as-cast” state, i.e. without slots. Each slot is nonlinear, but may comprise two or more straight line portions as seen best in  FIGS. 2-4 . Each slot is bordered by a pair of matching surfaces, such as  28   a ,  28   b . As used herein, the term “matching” surfaces refers to surfaces that are substantially exact counterparts of each other, i.e. surfaces that complement each other. This is in contrast to the surfaces shown in U.S. Pat. Nos. 3,728,041, 3,970,318, and 5,167,485, all of which feature intra-slot recesses that render the adjacent slot surfaces non-matching. 
       FIGS. 2-5  show the four slots in greater detail. Referring first to  FIG. 2 , the nonlinear slot  26  comprises three straight line portions  26   a ,  26   b ,  26   c , each of which has a juncture  32  with at least one of the other portions. Each juncture corresponds to a change of angular direction in the slot. For example, one juncture  32  between slot portions  26   a  and  26   b  corresponds to an approximate 90 degree change of angular orientation from the radial direction to the lateral direction. The other juncture  32  between slot portions  26   b  and  26   c  corresponds to another change of approximately 90 degrees from lateral to radial. The accumulated angular change is therefore about 180 degrees. 
       FIG. 3  shows a variant in which the slot comprises three straight line portions and two junctures. Each juncture corresponds to an approximately 120 degree change of angular orientation for an accumulated angular change of about 240 degrees. 
       FIG. 4  shows a variant in which the slot comprises seven straight line portions and six junctures. Each juncture corresponds to an approximately 90 degree change of angular orientation for an accumulated angular change of about 540 degrees. 
     The abrupt changes in angular orientation at the junctures  32  help resist fluid leakage through the slot and therefore permit the use of inexpensively installed, relatively wide slots that might otherwise be unsatisfactory. Each change of orientation increases the resistance to fluid leakage. As a result, larger and/or more abrupt changes are superior to smaller and/or less abrupt changes. Accordingly, although a slot having only two straight line portions and one juncture can be used, it is believed that the most practical and cost effective slots are those with at least three straight line portions and two changes of orientation totaling at least about 180 degrees. A larger quantity of straight line portions would be expected to further increase leak resistance of the slot, but the correspondingly longer slot length would increase the time necessary to cut the slot using wire EDM. The tradeoff between leak resistance and manufacturing complexity is a matter for consideration by the designers and manufacturers of the vane cluster. 
     As seen in  FIGS. 6-8 , the nonlinear slot need not be comprised of linear portions as in the above examples, but may instead be a curved slot having one or more radii of curvature. The average radius of curvature R may vary continuously along the length of the slot ( FIG. 6 ) or may vary discontinuously ( FIG. 7 ) thus defining one or more distinct junctures  32  between individual portions  26   a ,  26   b  of the slot. As seen in  FIG. 8 , a slot may comprise both curved and straight line portions in combination. Since the leak resistance of a slot depends on the abruptness and quantity of directional changes, a smoothly curved slot may provide unsatisfactory leak resistance. A curved slot with an abrupt directional change is expected to be superior to a smooth curve, but may be more difficult to manufacture than a slot comprised of straight line portions. One example of a curved slot having multiple, continuously varing radii of curvature is the serpentine slot of FIG.  5 . 
     The slots need not be installed circumferentially between each and every airfoil, but may instead be installed selectively, for example between every second or third airfoil, to achieve the desired degree of thermal independence. 
     The cluster of  FIG. 1  is one sector of a single array or stage of vanes. In some engines the vane clusters comprise two or more circumferentially aligned sub-clusters, integral with each other but axially separated from each other by an interstage space. In a fully assembled engine, rotor blades extend radially into the interstage space. The invention includes such multi-stage clusters as well as the illustrated single stage cluster. 
     As seen in  FIG. 9 , the nonlinear geometry of the slot  26  may also be employed as the interface between the lateral extremities  20  of adjacent vane clusters. Such a construction includes inner and outer shrouds  10 ,  12  with at least one airfoil extending between the shrouds. The lateral extremities of at least one of the shrouds, e.g. inner shroud  12 , has a nonlinear profile that matches a counterpart nonlinear profile on the extremity of a laterally adjacent vane cluster. 
     Although the invention has been presented in the context of stator vanes for a compressor, it is equally applicable to turbines. In addition, the invention includes clusters in which the outer shroud, rather than the inner shroud is the segmented shroud. It will be understood by those skilled in the art that these and other changes in form and detail may be made without departing from the invention as set forth in the accompanying claims.