GB2077363A - Wafer tip cap for rotor blades - Google Patents

Abstract

The tip cap is fabricated from a plurality of chordwisely extending wafers 30, 32. The outermost wafer 30 being of an abrasive material such as silicon carbide embedded in a metal matrix. Convective cooling of the tip is established through the incorporation of laterally extending passages 34 between adjacent wafer elements. <IMAGE>

Description

SPECIFICATION Abrasive tip cap of coolable wafer construction for rotor blades Technical Field This invention relates to blades of axial flow rotary machines, and more particularly to abrasive tip caps of coolable geometry affixed to the ends of the blades.

The concepts were developed in the gas turbine engine industry for use in the turbine section of engines, but have suitability to other high temperature, rotary machine applications.

Background Art In modern gas turbine engines, very high temperature working medium gases are produced within a combustion chamber and are flowed through a turbine section downstream thereof. At the upstream end of the turbine section, the medium gases have characteristic temperatures on the order of twenty-five hundred degrees Fahrenheit (25000 F) or greater. The temperature of the medium gases in many cases exceeds the capabilities of the materials from which the components of the turbine are fabricated, and such components are cooled with lower temperature air to prevent deterioraticn or destruction of the components.

Turbine rotor blades at the upstream end of the turbine section are among the components commonly cooled. The blades extend outwardly on the engine rotor across the working medium flowpath and are opposed at the radially outward end thereof by a circumscribing shroud on the engine stator. The clearance between the tips of the blades and the shrouds is made necessarily small to discourage the leakage of working medium gases over the tips and the decrease in aerodynamic efficiency which results therefrom.

With such small clearance at the tips of the blades, particularly under transient engine conditions, the blades occasionally rub against the circumscribing shroud. A potential for destructive interference exists at such a rub both from mechanical deformation of the blade or shroud and from the generation of excessive heat loads during the period of the rub. Rotor blade tips are conventionally cooled as is illustrated in representative U.S. Patents 3,899,267 to Dennis et al entitled "Turbomachinery Blade Tip Cap Configuration"; 3,994,622 to Schultz et al entitled "Coolable Turbine Blade"; and 4,010,531 to Andersen et al entitled "Tip Cap Apparatus and Method of Installation".

Circumscribing shrouds are commonly formed of abradable or deformable materials designed to accommodate rubbing contact with the blades without destructive interaction. U.S. Patents 3,042,365 to Curtis et al entitled "Blade Shrouding"; 3,854,842 to Caudil entitled "Rotor Blade Having Improved Tip Cap"; 3,817,719 to Schilke et al entitled "High Temperature Abradable Material and Method of Preparing the Same"; 3,879,831 to Rigney et al entitled "Nickel Base High Temperature Abradable Material"; and 3,918,925 to McComas entitled "Abradable Seal" are representative of such structures.

Notwithstanding the structures and material systems developed in the past, scientists and engineers in the industry continue to search for yet improved blade and shroud cOmponents. Rotor blades having high resistance to destructive deformation modes and effective cooling in the tip region are particular objects.

Disclosure of Invention According to the present invention the tip of a rotor blade is formed of a wafer structure in which the outermost wafer is fabricated of an abrasive substance capable of grinding material from a circumscribing shroud without generating excessive heat loads and in which substantial convective heat transfer capability is provided in proximity to the potential rubbing surface at the tip of the blade.

A primary feature in the structure of the present invention is the wafer elements forming the tip cap. The wafers extend chordwisely across the tip of the blade to cover the cooling air cavity contained therein. The outermost wafer is fabricated from an abrasive material which is capable of wearing abradable material from a circumscribing shroud without the generation of excessive heat loads. Cooling air passages between the wafers establish a thermal sink for the conduction of heat energy away from rubbing surfaces at the blade tip. Additional passages extend in the essentially spanwise direction across the wafer stack for additional cooling.

A principal advantage of the present invention is enhanced capability of providing effective cooling capacity in close proximity to the source of generated heat energy. Substantial convective cooling is employed to assure uninterrupted cooling flow. The wafer type structure enables economical formation of cooling holes having a large surface area to flow area relationship.

Convective and film cooling are combined in some embodiments. The employment of an abrasive material at the outermost wafer decreases the likelihood of closure of the spanwisely extending holes as a result of material deformation into the holes. The incorporation of differing materials at the tip region is readily enabled by manufacturing techniques which are synergistically suited to the economical formation of intricate cooling passages.

The foregoing, and other features and advantages of the present invention, will become more apparent in the light of the following description and accompanying drawing.

Brief Description of the Drawing Fig. 1 is a perspective view of a gas turbine rotor blade showing the tip cap of the present invention; Fig. 2 is an enlarged view of the tip region of the Fig. 1 blade showing the chordwisely extending wafers which comprise the tip cap, with cut-away portions revealing a convective cooling configuration incorporated in an abrasive tip design; and Fig. 3 is a sectional view taken between wafers at the leading edge of the blade and showing conformance of the lateral cooling passages to the contour of the blade leading edge.

Best Mode for Carrying Out the Invention The rotor blade 10 of Fig. 1 is representative of rotor blades of the type to which the present concepts apply. The blade has three principal sections: a root section 12, a platform section 14 and an airfoil section 1 6. The outward end 1 8 of the airfoil section is referred to as the tip region.

The blade has one or more cavities contained therein, such as the leading edge cavity 20 and the trailing edge cavity 22, which extend from the root section of the blade through the airfoil section for the distribution of cooling air thereabout. The cavities are closed at the outward end of the airfoil section by a tip cap 24 having an outwardly facing surface 26.

In its operative environment the root section 12 of the blade is attachable to a rotor disk (not shown) of a gas turbine engine with the airfoil section 1 6 extending outwardly across an annular flowpath for working medium gases within the engine. The plafform section 14 forms a portion of the inner boundary of the flowpath. The outer boundary of the flowpath is formed in part by a circumscribing shroud (also not shown) which opposes the outer surface of the blade tip and is closely spaced therefrom. The circumscribing shroud typically will have a coating of abradable material deposited thereon and into which the blade tip wears under transient operating conditions.

In the embodiment of the invention illustrated by Fig. 2, the airfoil section 1 6 is fabricated of a plurality of spanwisely extending wafers 28 which are joined by a suitable technique such as diffusion bonding. Similarly, the tip cap 24 is fabricated of a plurality of chordwisely extending wafers, including an outermost wafer30 and one or more inner wafers 32, which are stacked in the spanwise direction. Two inner wafers are illustrated. The tip cap closes the outward ends of the leading edge cavity 20 and of the trailing edge cavity 22. The inner wafers 32 are formed of a metallic composition comparable to or identical to the material from which the airfoil section of the blade is fabricated. The outermost wafer 30 is formed of an abrasive material, such as silicon carbide embedded in a metallic matrix.The outwardly facing surface 26 is suited for rubbing engagement with the blade opposing shroud. The wafers are joined, for example, to each other and to the spanwisely extending wafers of the airfoil, also by a suitable technique such as by diffusion bonding.

Lateral passages 34 for the flow of cooling air from the internal cavities are formed at the interface 36 between a pair of adjacent wafer elements. The lateral passages of the illustrated embodiment extend from the interior of the blade at one of the cooling air cavities and laterally to the pressure side surface 38 of the airfoil. In some embodiments the lateral passages may extend to the suction side of the blade as well, although as later described, a combination of radial discharge and pressure side discharge is thought to be preferable from an airfoil aerodynamic performance standpoint. The passages cover a significant portion of the airfoil cross section at the tip cap to establish substantial cooling in proximity to the outwardly facing and potentially rubbing surface 26.Additional passages 40 extend in the essentially spanwise direction from one or more df the cooling cavities. The passages may be canted where practical to increase the amount of convective cooling capacity in the tip cap structure. In order to establish significant convective cooling capacity at the trailing edge of the tip region 18, a channel 42 is provided.

Passages 44 extend in an essentially spanwise direction across the tip cap from the channel. As illustrated, the passages may also be canted to increase the amount of convective cooling capacity. In a preferred form as illustrated, the channel 42 is tapered to a decreased crosssectional area at the trailing edge. The decreasing cross-sectional area helps preserve the pressure differential across the tip between the channel and the ambient condition externally of the airfoil.

As is illustrated in Fig. 3, the most forward of the lateral passages at the leading edge of the blade illustrated is curved in the chordwise direction to closely follow the leading edge contour of the airfoil. High heat transfer rates and enhanced convective cooling result.

The cap structures of the present invention are highly advantageous in ability to establish substantial convective cooling in close proximity to the points of potential heat generation. A thermal sink for drawing heat energy away from the points of generation is established by the multiple passages and particularly by the laterally extending passages 34 of Fig. 2. A planar zone of substantially uniform convective cooling is therein provided. The wafer structure is well adaptable to the fabricatibn of such passages particularly at the interfaces between wafers where passages of intricate geometry and contour may be formed, such as by etching, before the wafers are bonded.

Locating the heat sink close to the potential rubbing surfaces as in the present invention enables the dissipation of heat without destruction of the tip material.

A high density of passages is required to assure adequate convective cooling. Although lesser numbers of passages have been shown by prior art structures not of the wafer design, such structures typically rely on at least a substantial contribution from film cooling techniques to protect the outer surfaces of the blade from high temperature working medium gases. The structure herein disclosed recognizes that upon the occurrence of a rub, film cooling over the tip is interrupted precisely at the time when maximum heat removal capacity is required. Such interruption of cooling capacity is substantially avoided through reliance on cooling in the lateral passages.

In accordance with one remaining aspect of the invention it is recognized that the discharge of cooling air to the suction side of a rotor blade degrades the aerodynamic performance of the blade by encouraging the buildup of boundary layer air along the suction side of the blade. To the extent possible such discharge has been avoided in the present structure through the effective combination of the lateral passages 34 to the pressure side of the blade and the spanwise passages 40 and 44 out of the tip of the blade.

Only limited discharge is made to the suction side of the blade at the leading edge.

The tip cap embodiments of the present invention are illustrated in combination with a blade manufactured by radial wafer techniques.

The concepts are highly compatible with materials systems and manufacturing techniques employed with radial wafer structures and, as such, are likely to find their greatest utility in that field. Known concepts in the radial wafer blade field include those disclosed in U.S. Patent 3,872,563 to Brown et al entitled "Method of Blade Construction" and U.S. Patent No. 4,203,706 to Hess entitled "Radial Wafer Airfoil Construction".

Although the invention has been shown and described with respect to detailed embodiments thereof, it should be understood by those skilled in the art that various changes and omissions in form and detail may be made therein without departing from the spirit and the scope of the invention.

Claims (7)

1. A rotor blade of the type having at least one spanwise cavity extending from the root to the tip of the blade for distributing cooling air, wherein the improvement comprises: a cap disposed over the tip end of the cavity and formed of a plurality of chordwisely extending wafers stacked in the spanwise direction wherein the outermost wafer of said plurality of wafers is fabricated of an abrasive material and wherein at least two of said wafers have a plurality of laterally extending cooling passages formed therebetween for the flow of cooling air from spanwise cavity to the pressure side of the blade.

2. The rotor blade according to claim 1 wherein said wafers have at least one additional laterally extending cooling passage formed therebetween which is oriented so as to be capable of discharging cooling air from the spanwise cavity to the suction side of the blade.

3. The rotor blade according to claim 1 wherein at least one of said one of said laterally extending cooling passages is disposed at the leading edge of the blade and is contoured in geometry to approximate the contour of the leading edge along the pressure side of the blade.

4. The rotor blade according to claim 2 wherein at least one of said one of said laterally extending cooling passages is disposed at the leading edge of the blade and is contoured in geometry to approximate the contour of the leading edge along the suction side of the blade.

5. The rotor blade according to claim 1, 2, 3, or 4 wherein said wafers further include additional passages extending in the essentially spanwise direction across the cap from one of the spanwise cavities.

6. The rotor blade of claim 5 which further includes a channel in the wafers extending in the chordwise direction from the cavity toward the trailing edge of the tip and second additional cooling passage extending in an essentially spanwise direction from said channel so as to enable the discharge of cooling air from the cavity via the channel in the spanwise direction from the tip at the trailing edge.

7. The rotor blade of claim 5 wherein said channel is tapered in decreasing cross section area toward the trailing edge of the blade.