Abstract:
A turbine blade includes a blade portion, the blade portion comprising a tip outer wall and a trailing edge, an internal cooling circuit, the internal cooling circuit being configured for directing cooling air within the blade portion, and a tip trailing edge slot positioned adjacent to the tip outer wall and the trailing edge, the tip trailing edge slot being fluidly connected to the internal cooling circuit. The tip outer wall is recessed at the tip trailing edge slot such that the tip outer wall is not provided over the trailing edge slot, thereby allowing cooling air to flow from the cooling circuit, into the trailing edge slot, and radially over the tip outer wall.

Description:
TECHNICAL FIELD 
     The inventive subject matter relates to turbine blades and, more particularly, to improved trailing edge blade tip cooling for high temperature cooled turbine blades. 
     BACKGROUND 
     Gas turbine engines, such as turbofan gas turbine engines, may be used to power various types of vehicles and systems, such as aircraft. Typically, these engines include turbines that rotate at a high speed when blades (or airfoils) extending therefrom are impinged by high-energy compressed air. Consequently, the blades are subjected to high heat and stress loadings which, over time, may reduce their structural integrity. 
     To improve blade structural integrity, a blade cooling scheme is typically incorporated into the turbines. The blade cooling scheme is included to maintain the blade temperatures within acceptable limits. In some cases, the blade cooling scheme directs cooling air through an internal cooling circuit formed in the blade. The internal cooling circuit may include a simple channel extending through a length of the blade or may consist of a series of connected, serpentine cooling passages, which incorporate raised or depressed structures therein. The serpentine cooling passages increase the cooling effectiveness by extending the length of the air flow path. In this regard, the blade may have multiple internal walls that form the intricate cooling passages through which the cooling air flows. 
     As the desire for increased engine efficiency continues to rise, engine components are increasingly being subjected to higher and higher operating temperatures. For example, newer engine designs may employ operating temperatures that are over 1100° C. However, current engine components, such as the blades, may not be adequately designed to withstand such temperatures over time. Hence, designs for improving cooling of the blades may be desired. 
     Turbine blade tips (at the extreme outer radial region) are difficult to cool due to geometry, manufacturing constraints, and the high velocity air that migrates from the pressure side of the airfoil to the suction side via the gap between the rotor tip and the turbine shroud. The trailing edge of the blade tip is particularly difficult to cool in a manner that does not detrimentally affect the turbine performance or introduce risk. 
     Hence, there is an unmet need in the art for a turbine blade having a cooling system that is capable of cooling the blade tip in high-temperature operating environments. The present disclosure addresses at least this need. 
     BRIEF SUMMARY 
     Disclosed are cooled turbine blades for gas turbine engines having improved blade tip cooling. In one embodiment, a turbine blade includes a blade portion, the blade portion including a tip outer wall and a trailing edge, an internal cooling circuit, the internal cooling circuit being configured for directing cooling air within the blade portion, and a tip trailing edge slot positioned adjacent to the tip outer wall and the trailing edge, the tip trailing edge slot being fluidly connected to the internal cooling circuit. The tip outer wall is recessed at the tip trailing edge slot such that the tip outer wall is not provided over the trailing edge slot, thereby allowing cooling air to flow from the cooling circuit, into the trailing edge slot, and radially over the tip outer wall. 
     In another embodiment, a gas turbine engine includes a plurality of turbine blades mounted radially about a turbine rotor, each of the plurality of turbine blades including a turbine blade that includes a blade portion, the blade portion including a tip outer wall and a trailing edge, an internal cooling circuit, the internal cooling circuit being configured for directing cooling air within the blade portion, and a tip trailing edge slot positioned adjacent to the tip outer wall and the trailing edge, the tip trailing edge slot being fluidly connected to the internal cooling circuit. The tip outer wall is recessed at the tip trailing edge slot such that the tip outer wall is not provided over the trailing edge slot, thereby allowing cooling air to flow from the cooling circuit, into the trailing edge slot, and radially over the tip outer wall. 
     In yet another embodiment, a turbine blade includes a blade portion, the blade portion including a tip outer wall and a trailing edge, an internal cooling circuit, the internal cooling circuit being configured for directing cooling air within the blade portion. The internal cooling circuit includes a channel positioned radially inward from the tip outer wall and fluidly connected to the tip trailing edge slot for providing cooling air to the tip trailing edge slot. The channel also includes a first portion and a second portion, the first portion being oriented generally parallel to the tip outer wall, the second portion being angled radially with respect to the tip outer wall for guiding the cooling air flow in the radial direction. The turbine blade further includes a plurality of trailing edge slots being fluidly connected to the internal cooling circuit and positioned along the trailing edge, the plurality of trailing edge slots including a tip trailing edge slot positioned adjacent to the tip outer wall and the trailing edge. The tip outer wall is recessed at the tip trailing edge slot such that the tip outer wall is not provided over the trailing edge slot, thereby allowing cooling air to flow from the cooling circuit, into the trailing edge slot, and radially over the tip outer wall. The tip trailing edge slot includes a tapered lower wall, the tapered lower wall being configured to direct cooling air flow around the trailing edge. 
     Furthermore, other desirable features and characteristics of the cooled turbine blades will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the preceding background. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a cross-sectional side view of a portion of a turbine section of an engine, according to an embodiment; 
         FIG. 2  is a perspective view of a turbine blade, in accordance with an embodiment; 
         FIG. 3  shows the perspective view of the turbine blade of  FIG. 3 , expanded in the area of the turbine blade trailing edge tip; 
         FIG. 4  is a cross-sectional view of a portion of the turbine blade of  FIG. 2 ; and 
         FIG. 5  shows the cross-sectional view of the turbine blade as in  FIG. 4 , expanded in the area of the turbine blade trailing edge tip. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the inventive subject matter or the application and uses of the inventive subject matter. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
     As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Thus, any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Furthermore, as used herein, numerical ordinals such as “first,” “second,” “third,” etc., such as first, second, and third turbine blades, simply denote different singles of a plurality unless specifically defined by language in the appended claims. 
       FIG. 1  is a cross-sectional side view of a portion of a turbine section  100  of an engine, according to an embodiment. The turbine section  100  receives high temperature gases from an upstream engine combustor (not shown) to produce energy for the engine and/or components coupled to the engine. In an embodiment, the turbine section  100  includes a turbine nozzle  104  that has a plurality of static vanes  106  mounted circumferentially around a ring  108 . The static vanes  106  direct the gases from the combustor to a turbine rotor  110 . According to an embodiment, the turbine rotor  110  includes a plurality of blades  112  (only one of which is shown) that are attached to a hub  114  and retained in axial position by a retention plate  116 . When the blades  112  are impinged upon by the gases, the gases cause the turbine rotor  110  to spin. According to an embodiment, an outer circumferential wall  118  surrounds the static vanes  106  and the plurality of blades  112  to define a flowpath  122 . The circumferential wall  118  also defines a portion of a compressor discharge plenum  120  that is disposed radially outwardly relative to the flowpath  122 . The compressor discharge plenum  120  receives bleed air from a compressor section (not shown), which may be directed through one or more openings in the outer circumferential wall  118  towards the plurality of blades  112  to cool the blades  112 . 
       FIG. 2  is a perspective view of a turbine blade  200 , in accordance with an embodiment. The blade  200  may be implemented into a turbine rotor (e.g., turbine rotor  110  in  FIG. 1 ) and may include a single crystal blade including a nickel-based superalloy, in an embodiment. Suitable nickel-based superalloys include, but are not limited to, MAR-M-247EA, MAR-M-247DS, or SC180. In other embodiments, the blade  200  may include a different superalloy. According to an embodiment, the blade  200  may be cast as an equi-axed, directionally solidified, or single crystal blade. 
     The blade  200  includes a blade attachment section  202 , an airfoil  204 , and a platform  206 . The blade attachment section  202  provides an area in which a shape is machined. In an embodiment, the shape corresponds with a shape formed in a respective blade attachment slot (not shown) of the turbine hub (e.g., hub  114  in  FIG. 1 ). For example, in some embodiments, the shape may be what is commonly referred to in the art as a “firtree” shape. In other embodiments, the shape may be a beveled shape. However, in other embodiments, any one of numerous other shapes suitable for attaching the blade  200  to the turbine may be alternatively machined therein. 
     The airfoil  204  has a root  208  and two outer walls  210 ,  212 . The root  208  is attached to the platform  206  and each outer wall  210 ,  212  has outer surfaces that define an airfoil shape. The airfoil shape includes a leading edge  214 , a trailing edge  216 , a pressure side  218  along the first outer wall  210 , a suction side  220  along the second outer wall  212 , a tip outer wall  222 , a plurality of pressure side discharge trailing edge slots  224  (the edge slot at the tip is the tip trailing edge slot  225 ), a tip plenum  226  recessed radially inward from the tip outer wall  222 , and a series of holes  228  (commonly referred to in the art as “film cooling” holes). Holes  228  may be provided along the leading edge  214 , along the first outer wall  210  near the tip outer wall  222 , and/or along the tip plenum  226 . Though not shown in  FIG. 2 , the blade  200  may have an internal cooling circuit formed therein, which may extend from an opening in the platform  206  through the blade  200  and may include various passages that eventually communicate with the plurality of trailing edge slots  224  and the tip trailing edge slot  225 , or other openings (not shown) that may be formed in the blade  200 . In particular, the convex suction side wall  212 , the concave pressure side wall  210 , and the tip  222  each include interior surfaces defining the internal cooling circuit. 
     With reference now to  FIG. 3 , an expanded view of the region of the blade  200  surrounding the tip trailing edge slot  225  is provided. As shown therein, in accordance with an embodiment, the tip outer wall  222  is removed from the region over the tip trailing edge slot  225 . From the pressure side  218 , the perimeter of the tip outer wall  222  is recessed or angled inward from the outer wall  210  at the tip trailing edge slot  225  such that no tip outer wall is formed over the tip trailing edge slot  225 . As such, a narrow tip outer wall portion  222   a  is formed proximate to the suction side  220  portion of the blade adjacent to the tip trailing edge slot  225 . In one exemplary embodiment, the tip outer wall  222  is removed from the area over the tip trailing edge slot  225  by machining after a standard airfoil-shaped blade with trailing edge slots has been cast. In another exemplary embodiment, the blade is cast in the configuration shown in  FIG. 3  (i.e., without the tip outer wall  222  being present over the tip trailing edge slot  225 ) using a mold that has been pre-configured to provide for this feature. 
     The narrowing of the tip outer wall  222  so as to avoid covering the tip trailing edge slot  225  can be accomplished generally with any pattern that deviates from the normal tapering of the blade (as in a traditional airfoil shape). For example, as shown in  FIG. 3 , the tip outer wall  222  narrows toward the suction side  220  with a tip outer wall perimeter edge  252  that is generally perpendicular (or otherwise angled) with respect to the outer wall  210 . The tip outer wall  222  perimeter then curves (referring to tip outer wall perimeter curve feature  254 ) to become parallel with the outer wall  212  as it extends along the tip trailing edge slot  225  in portion  222   a  thereof (referring to tip outer wall perimeter edge  256 ). Other patterns to remove the tip outer wall  222  over the tip trailing edge slot  225  are possible, and those having ordinary skill in the art will be readily able to design other patterns in accordance with the teachings of the present disclosure. 
     Without the presence of the tip outer wall  222  over the tip trailing edge slot  225 , the tip trailing edge slot is “open” in the radial direction, i.e., air can flow radially from the tip trailing edge slot  225  without obstruction from the tip outer wall  222 . In this configuration, the tip trailing edge slot  225  is defined by a slot inner wall  266  that is generally parallel to the outer wall  212  and formed radially inward from the perimeter edge  256 , a slot lower wall  260  (with an edge or a curve feature  262  connecting the slot inner wall  266  to the slot lower wall  260 ), a slot trailing edge  258 , and a slot front edge  264 . The relative proportions and configuration of the slot inner wall  266 , the slot lower wall  260 , the slot trailing edge  258 , and a slot front edge  264  can be designed and configured to accommodate the desired airflow of cooling air out of the tip trailing edge slot  225 . Analytical tools known in the art, such as conjugate heat transfer (CHT) analysis tools, can be employed by a person having ordinary skill in the art to select a suitable “open” tip trailing edge slot design for any given turbine blade implementation, in accordance with the teachings of the present disclosure. 
     In operation, cooling air exits the tip trailing edge slot  225  through opening  250 , which is defined by the slot front edge  264 , the tip outer wall perimeter edge  252 , the slot inner wall  266 , and the slot lower wall  260 . The cooling air exits the tip trailing edge slot  225  in a direction that is both chordwise and radial with respect to the rotation of the blade. As such, the cooling air exits through the opening  250  and proceeds toward the slot trailing edge  258  and also toward the tip outer wall perimeter edge  256  to provide cooling air flow to the trailing edge  216 . 
     In  FIGS. 4 and 5 , a cross-section of a portion of the blade  200  is shown ( FIG. 5  being an expanded view of  FIG. 4  in the area of the tip trailing edge slot  225 ), illustrating the internal cooling circuit  275  formed therein. Cooling air (indicated by arrow  270 ) flowing to the tip trailing edge slot  225  proceeds through a channel portion  272   a  that runs generally parallel to tip outer wall  222  until it reaches bend  273 , located near the tip trailing edge slot  225 . Bend  273  guides the cooling air flow radially into channel portion  272   b , such that when it enters the tip trailing edge slot  225 , it has a radial velocity enabling it to flow radially outward toward the tip outer wall perimeter edge  256 . The bend  273  and the radially outer wall  274  of flow channel portion  272   b  are designed to impart a radial component of velocity to the cooling air  270  without causing flow separation. In some embodiments, an expansion transition or fillet  280  may be provided near the opening  250  that utilizes the Coandra effect (the tendency of a fluid stream to be attracted to a nearby surface), in conjunction with rotational body forces, to diffuse the cooling air flow with minimal separation before the flow travels over the tip outer wall perimeter edge  256  (and also over the tip outer wall portion  222   a ) to provide cooling to the trailing edge  216 . The length of the channel  272   b  over which the radial angle acts is sufficiently long so as to provide direction and metering to the cooling air flow. CHT analysis tools, for example, can be employed by a person having ordinary skill in the art to select a suitable bend  273  angle and position with respect to the tip trailing edge slot  225  for any given turbine blade implementation to give the cooling air the desired radial flow characteristics over the outer wall portion  222   a  to provide sufficient cooling to the trailing edge  216 . In alternate embodiments cooling air  270  may be supplied from cooling circuit  275  prior to reaching bend  273 . 
     As best shown in  FIG. 5 , the slot lower wall  260  in the tip trailing edge slot  225  is tapered to diffuse the cooling flow in the trailing edge slot  225 . The angle between channel  272   b  and slot lower wall  260  is used to draw cooling flow over the slot trailing edge  258 . Thus, with cooling air being directed over the outer wall portion  222   a  and the slot trailing edge  258 , substantially the entire trailing edge  216  is provided with sufficient cooling air flow to maintain the trailing edge  216  within acceptable temperature limits. 
     In a further aspect of the present disclosure, it has been found to be desirable to move the tip plenum  226  forward (with respect to the rotation of the blade, and relative to prior art designs) to allow for the radial bend  273  and the channel portion  272   b , which directs cooling air flow radially over the outer wall portion  222   a . Holes  228  in the tip plenum  226  as well as holes  228  along the pressure side  218  are used to provide cooling to the tip plenum  226  and to the outer wall  222  in the region between the tip plenum  226  and the tip trailing edge slot  225 . In some embodiments, one or more film holes  230 , as best illustrated in  FIG. 3 , may be added behind the tip plenum  226  for additional cooling in this region. 
     As discussed above, for any turbine blade design, the optimized configuration for the present invention is determined from a computational fluid mechanics CHT analysis on a configuration that includes the features described herein. As such, it is expected that the size, shape, position, angle, or dimensions of any feature described herein can be optimized by a person having ordinary skill in the art for any given turbine design using CHT analysis. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiments of the heat exchange system are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the inventive heat exchange system. It is understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.