Abstract:
A compressor blade and process for inhibiting rub encounters between a blade tip of the blade and an interior surface of a case that surrounds the rotating hardware within a compressor section of a turbomachine. The compressor blade includes a cap that defines the blade tip at a radially outermost end of the blade, and a plurality of flexible elements extending from a surface of the cap that defines the blade tip. The flexible elements extend from the surface in a span-wise direction of the blade, and are operable to become rigid due to centrifugal stiffening at compressor operating speeds and, optionally, cut a groove the interior surface of the case.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to compressors for turbomachinery, such as gas turbine engines. More particularly, this invention relates to a compressor blade whose tip incorporates a flexible cutting element for reducing the risk of damage to the blade tip that can occur due to rub encounters with a case surrounding the compressor. 
     Gas turbine engines generally operate on the principle of compressing air within a compressor section of the engine, and then delivering the compressed air to the combustion section of the engine where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. 
     The compressor includes rotating hardware in the form of one or more disks or rotors from which airfoils (blades) extend radially across the airflow path through the engine. The radially outer limit of the airflow path within the compressor section is defined by a case that surrounds the rotating hardware. The case serves to channel incoming air through the compressor to ensure that the bulk of the air entering the engine will be compressed by the compressor. However, a small portion of the air is able to bypass the compressor blades through a radial gap present between the blade tips and the case at the outer airflow path within the compressor section. Because the air compressed within the compressor section is used to feed the turbine section of the engine, engine efficiency can be increased by limiting the amount of air which is able to bypass the compressor blades through this gap. Accordingly, the rotating hardware and case of a compressor section are manufactured to close tolerances in order to minimize the gap. 
     Manufacturing tolerances, differing rates of thermal expansion and dynamic effects limit the extent to which this gap can be reduced. As an example, the inner diameter of the case is never truly round and concentric with the axis of rotation of the compressor. As a result, there are instances when airfoil-to-case clearances are breached and blade tips rub the case. Blade tip rub damage can vary in form and severity. Damage to the tip of a blade may be in the form of one or more cracks or burrs, which can propagate through local vibratory modes in the tip region of the blade. For example,  FIG. 4  schematically represents a severe tip burr (stress concentrator)  14  resulting from plastic deformation at the tip  12  of a blade  10 . If the tip burr  14  is severe enough, the resulting stress concentration can amplify vibratory stresses due to tip modal vibration and cause degradation in the high cycle fatigue (HCF) life of the blade  10 . Localized frictional heating also occurs from a blade rub, and may result in the formation of a brittle heat-affected zone (HAZ)  16  at the blade tip  12 . 
     Several approaches have been proposed to address the problems of blade tip damage and air leakage at the outer airflow path. One approach involves applying an abradable material to the inner diameter of the compressor case so that the abradable material will sacrificially abrade away when rubbed by the blade tips. Another approach is to incorporate a cutting edge (“squealer tip”) at the blade tip. In each case, the blade tips cut a groove in the inner diameter of the case during initial engine operation, creating a more tortuous path between the case and blade tips at the outer airflow path. Though effective, both techniques are expensive to implement. As an example, a cutting edge of a blade tip is typically formed by a coating, which can be difficult to deposit to a sufficient thickness to survive severe rub encounters often seen in field hardware. On the other hand, deposition of an abradable coating on the inner diameter of a compressor case requires close quality control to produce a suitable composition, including particle/void ratio and distribution, that will exhibit a proper hardness capable of avoiding blade tip damage during rub events. Rub encounters with an abradable coating that is excessively hard will cause scratches or cracks at the blade tip, and continued operation of the engine can cause scratches to serve as initiation sites for subsequent cracks due to vibratory stresses. Conversely, an abradable coating that is too soft can be eroded away by the high velocity gas flow in the compressor section. 
     In view of the above, improved techniques for reducing blade tip damage and air leakage at the outer airflow path of a compressor are desired. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides a compressor blade suitable for use as a component of rotating hardware within a compressor section of a turbomachine, and a process for inhibiting rub encounters between a blade tip of the blade and an interior surface of a case that surrounds the rotating hardware. 
     According to a first aspect of the invention, the compressor blade includes a cap that defines a blade tip at a radially outermost end of the blade, and a plurality of flexible elements extending from a surface of the cap that defines the blade tip. The flexible elements extend from the surface in a span-wise direction of the blade and are operable to become rigid due to centrifugal stiffening at compressor operating speeds. The flexible elements are optionally operable to cut a groove in the interior surface of the case at compressor operating speeds, or may be formed of a lubricious non-cutting material. 
     Another aspect of the invention is a process that includes fabricating a compressor blade to have a first joint interface at a radially outermost end thereof, fabricating a cap to have a second joint interface that has a complementary shape to the first joint interface of the blade, and providing a plurality of flexible elements extending from a surface of the cap that is oppositely-disposed from the second joint interface of the cap. The cap is then joined to the blade so that the first and second joint interfaces form a metallurgical joint, the surface of the cap defines a blade tip of the blade, and the flexible elements extend from the blade in a span-wise direction of the blade. The flexible elements are optionally operable to cut a groove in the interior surface of a case that surrounds the blade and the other rotating hardware of the compressor section, or may be formed of a lubricious non-cutting material. 
     A technical effect of the invention is the ability of the flexible elements to eliminate or at least drastically reduce the risk of blade tip damage from rub encounters with a compressor case that surrounds the blade and the remainder of the compressor rotating hardware. For example, the flexible elements may be adapted to cut a groove in the interior surface of the case. As a result of being cut by the flexible elements, the groove is substantially coaxial with the axis of rotation of the rotating hardware, and is radially spaced from the blade tip of the blade. The groove may be further capable of reducing air leakage through the outer airflow path of the compressor by improving outer flowpath sealing between the blade tips and the interior surface of the case. Alternatively, the flexible elements may be limited to forming a seal with the interior surface of the case. 
     Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a front view of a compressor blade having a blade tip configured in accordance with an embodiment of this invention, and an adjacent portion of a compressor case that surrounds the compressor rotating hardware of which the blade is a component. 
         FIG. 2  is a detailed view of a blade tip cap and an adjacent portion of the blade of  FIG. 1  prior to attaching the cap to the blade to form the blade tip of  FIG. 1 . 
         FIG. 3  is a detailed perspective view of the blade tip cap of  FIG. 2 , and represents a technique for retaining elements in the cap. 
         FIG. 4  represents a blade tip region of a prior art compressor blade and depicts several types of damage that can occur to the blade tip from rubbing encounters with a compressor case. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically represents a portion of a compressor section  20  of a turbomachine, for example, an industrial or aircraft gas turbine engine. A single compressor blade  22  of the compressor section  20  is shown, though it should be understood that the blade  22  is one of a number of blades  22 . The blades and a disk (not shown) to which they are attached form part of the rotating hardware within the compressor section  20 . As also shown in  FIG. 1 , the rotating hardware of the compressor section  20  is circumscribed by a case  24 , a portion of which is represented in close proximity to the radially outermost tip  26  of the blade  22 . The case  24  serves to channel the air flowing through the compressor so as to ensure that the bulk of the air entering the engine will be compressed within the compressor section  20 . (In the orientation of  FIG. 1 , the direction of air flow would be directed into the plane of the page.) A small radial gap is present between the blade tip  26  and the case  24 . Minimizing this gap promotes the efficiency of the compressor section  20  and the engine as a whole. 
     According to a preferred aspect of the invention, the blade  22  is provided with what will be referred to as a blade tip cap  28 , which forms the outer radial extremity (tip  26 ) of the blade  22 . The cap  28  incorporates cutting elements  30  intended to prevent or at least minimize rubbing between the blade tip  26  and the compressor case  24  that can lead to degradation of the HCF life of the blade  22 . The cutting elements  30  can also serve to promote outer flowpath sealing with the case  24  by creating a more tortuous flow path between the blade tip  26  and the case  24 . 
     In  FIGS. 1 and 2 , the cutting elements  30  are represented as multiple wires or fibers that are spaced apart from each other in a chord-wise direction of the blade tip  26  and extend from the blade tip  26  in a direction essentially parallel to the span-wise axis of the blade  22 . The elements  30  are adapted to cut the inner surface  42  of the case  24  surrounding the blade  22 , yet are preferably lightweight so as contribute minimal parasitic loading to the blade  22 . As represented in phantom in  FIG. 2 , the elements  30  are preferably flexible, but then become rigid at compressor operating speeds due to the physics of “centrifugal stiffening.” The elements  30 , when stiffened at compressor operating speeds, are able to act as cutting elements against the inner surface  42  of the case  24 , and in doing so cut a groove  44  in the case inner surface  42  that is more nearly coaxial with the axis of rotation of the rotating hardware of the compressor than the inner surface  42 . In effect, the elements  30  serve to bring the inner surface  42  of an otherwise out-of-round case  24  into concentricity with the axis of rotation of the compressor rotating hardware. As evident from  FIG. 1 , the groove  44  is radially spaced from the blade tip  26  of the blade  22 , roughly corresponding to the lengths of the elements  30 , such that the risk of blade tip damage from rub encounters with the case  24  is eliminated or at least drastically reduced. While  FIGS. 1 and 2  depict the presence of five elements  30 , a lesser or greater number of elements  30  could be employed. Generally speaking, it is believed that at least one hundred elements  30  per square inch (at least about fifteen elements  30  per square centimeter) should be present at the blade tip  26  in order to achieve an adequate cutting efficiency. The number of elements  30  is preferably limited so that adjacent elements  30  are spaced apart from each other at their respective points of attachment to the cap  28 , so that the elements  30  retain their ability to flex. As an example, it may be necessary to limit the number of elements  30  to about six hundred elements  30  per square inch (about one hundred elements  30  per square centimeter). 
     The elements  30  can be formed of a variety of materials, notable examples of which include stainless steel wires, carbon steel wires, carbon fibers, aramid (for example, Kevlar®) fibers, alumina fibers, and silicon carbide fibers. To enhance their cutting capability, the elements  30  may be coated with an abrasive coating formed of, for example, cubic boron nitride, alumina, diamond, tungsten carbide or another hard abrasive material. Currently, alumina fibers and carbon fibers with a cubic boron nitride coating are believed to be preferred. Suitable processes for producing the elements  30  include such conventional methods as wire drawing for carbon steels and stainless steels, and spinning sol-gels or other chemical precursors to produce ceramic fibers. Abrasive coatings or particles can be applied by various techniques, for example, plating, brazing, or resin bonding. Suitable lengths and diameters for the elements  30  will depend in part on the particular application. However, the lengths and diameters of the elements  30  affect the flexibility and cutting capability of the elements  30 , and therefore certain limits are believed to exist. For example, it is believed that the elements  30  should have lengths of at least 2.5 millimeters and may be as long as about 8.5 millimeters, with a preferred range being about 4 to about 6 millimeters. Furthermore, it is believed that the elements  30  should have diameters of at least 17 micrometers and may be as large as about 500 micrometers, with a preferred range being about 125 to about 300 micrometers. 
       FIG. 2  shows the inner ends of the elements  30  as imbedded in the cap  28  and protruding through the blade tip  26  formed by the cap  28 .  FIG. 3  represents the cap  28  as having been fabricated to contain a surface cavity or slot in the surface that defines the blade tip  26 , and the result of filling the slot with a material  31  that anchors the elements  30  to the cap  28 . For example, the slot can be filled with a resin, braze alloy, or other material capable of securing and retaining the elements  30  under the operating conditions of the blade  10 . Suitable processes for producing the cap  28  include such conventional methods as electro-discharge machining (EDM), grinding, milling, etc. The cap  28  is preferably formed of an alloy that is compatible with the alloy used to form the blade  22 . In compressor blade applications for industrial gas turbine engines, notable examples of blade alloys include chromium-containing iron-based alloys such as GTD-450, AISI 403, and AISI 403+Cb. Chemical compatibility is particularly important in terms of the ability to metallurgical join the cap  28  to the blade  22  using such processes as brazing and welding, including welding techniques that use friction between the parts being welded to generate the welding temperatures. In view of these considerations, alloys that are believed to be particularly suitable for the cap  28  and subsequent joining to a blade formed of an iron-based alloy include GTD-450 and AISI 403+Cb. As noted above, suitable processes for joining the cap  28  and blade end  34  include brazing, welding and friction welding, with brazing currently viewed as the preferred method. 
     The cap  28  is further represented in  FIGS. 1 and 2  as being fabricated to form a double scarf joint  32  with an end  34  of the blade  22  to which the cap  28  is attached. The double scarf joint  32  defines a joint interface  36  and  38  on each of the blade end  34  and cap  28 , respectively. The joint interfaces  36  and  38  have shapes that are complementary to each other, and each joint interface  36  and  38  comprises a pair of faying surfaces that are inclined toward each other and neither parallel nor perpendicular to the span-wise axis of the blade  22 .  FIG. 2  further shows the joint interface  36  of the blade end  34  as incorporating perturbations  40  to promote metallurgical and mechanical interlocking at the joint  32 , providing structural load path redundancy against the typically high centrifugal stress field existing within the blade  22  at compressor operating speeds. Alternatively or in addition, the joint interface  38  of the cap  28  may be formed to include perturbations, similar or complementary to the perturbations  40 . Other known joint configurations are also possible, including forming one of the joint interface  36  and  38  as a dovetail and the other as a complementary dovetail slot. 
     As a result of the elements  30  cutting the groove  44  in the inner surface  42  of the case  24 , the likelihood that the blade tip  26  will be damaged by rub encounters with the case  24  are greatly reduced if not eliminated. As a result, typical forms of damage can be avoided or reduced, including the brittle HAZ  16  and minor and severe tip burrs  14  represented in  FIG. 4 , which can initiate cracks and, with subsequent propagation, can degrade the HCF life of the blade  22  and result in tip fracture driven by airfoil modal vibrations. The flexibility of the elements  30  is believed to be particularly advantageous, since their flexibility enables the elements  30  to be less prone to being completely removed when a severe rub encounter occurs, as often seen in turbomachines such as gas turbine engines. In addition, individual elements  30  are more likely to be lost as opposed to the majority of the elements  30 , such that the cap  28  is able to continue providing a degree of cutting action against the case  24  that may be necessary as a result of subsequent rub encounters. 
     It is foreseeable that, in some situations, the ability of the elements  30  to cut a groove  44  in the inner surface  42  of the case  24  may be unnecessary. Accordingly, an alternative aspect of the invention is to form the flexible elements  30  to be lubricious and non-cutting, and therefore only flex on contact with the case  24 . Lubricious non-cutting elements  30  are believed to be capable of reducing the risk of damage to the tip  26 , as well as seal the radial clearance gap between the blade tip  26  and compressor case  24 . In most cases, suitable lubricious materials for non-cutting elements  30  will be limited to the early stages of an industrial gas turbine compressor. Notable but nonlimiting examples of such materials include fiber materials such as carbon fibers or polymeric fibers, for example, Kevlar® fibers. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the blade tip cap  28  and elements  30  could differ from that shown. It is also foreseeable that this invention could be used in combination with an abradable material incorporated into the region of the case  24  immediately circumscribing the tips of the compressor blades. Therefore, the scope of the invention is to be limited only by the following claims.