Patent Application
20100183429
This invention is directed generally to turbine airfoils, and more particularly to cooling systems in hollow turbine airfoils.
Typically, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a turbine blade assembly for producing power. Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose turbine blade assemblies to these high temperatures. As a result, turbine blades must be made of materials capable of withstanding such high temperatures. In addition, turbine blades often contain cooling systems for prolonging the life of the blades and reducing the likelihood of failure as a result of excessive temperatures, particularly in concentrated areas of over temperature, sometimes referred to as hot spots.
Typically, turbine blades are formed from a root portion having a platform at one end and an elongated portion forming a blade that extends outwardly from the platform coupled to the root portion. The blade is ordinarily composed of a tip opposite the root section, a leading edge, and a trailing edge. The inner aspects of most turbine blades typically contain an intricate maze of cooling channels forming a cooling system. The cooling channels in a blade receive air from the compressor of the turbine engine and pass the air through the blade. The cooling channels often include multiple flow paths that are designed to maintain all aspects of the turbine blade at a relatively uniform temperature.
Size and space limitations make trailing edges one of the more challenging sections of a turbine blade to cool. Traditionally, a trailing edge camber line discharge together with pin fins or multiple impingements has been used airfoil trailing edge region cooling. Such design requires a thicker trailing edge that can induce higher aerodynamic blockage and reduce stage performance. Techniques for cooling a thinner trailing edge have been developed. For example, a first stage blade can utilize a pressure side bleed the exhausts on the pressure side adjacent to the tip of the trailing edge, rather than a camber line discharge at the center of the trailing edge. This cooling channel arrangement allows for a reduction in the effective thickness of the trailing edge when compared to the required thicknesses of both the suction side and pressure side regions of the trailing edge surrounding a camber line cooling discharge.
However, the pressure side bleed cooling approach causes a side flow and presents shear mixing between the cooling air and the mainstream flow as the cooling air exits the pressure side channel outlet. The shear mixing of the cooling air with the mainstream flow reduces cooling effectiveness of the trailing edge overhang, thus inducing over temperature or a hot spot at the trailing edge suction side location. Frequently, a hot spot can become the life limiting location for the entire airfoil. Thus, a need exists for a cooling system capable of providing sufficient cooling to a relatively thinner trailing edge of a turbine airfoil.
The invention relates to a turbine airfoil cooling system for a turbine airfoil used in turbine engines. In particular, a turbine airfoil cooling system can include a trailing edge cooling structure in which multiple submerged suction side cooling slots are spaced span wise along the blade and extend chord wise to provide an opening beginning at a front edge on the suction side and extending to a terminal location at the trailing edge. The terminal location can be around the curve or corner of the suction side region, at the center line of the trailing edge or even extend around the corner of the trailing edge into the pressure side region of the trailing edge. The cooling system further includes at least one trailing edge cooling channel positioned within the airfoil and extending toward the trailing edge, but terminating in a pressure side cooling slot that exhausts onto the pressure side upstream of the trailing edge. The pressure side cooling slot has a forward pressure side lip at its junction with the pressure side. The front edge of the suction side cooling slots is no further forward chord wise than the pressure side lip, i.e. the front edge is aligned with or rearward of the pressure side lip. The pressure side cooling slot can have a curving rearward side that curves to a pressure side outlet forward of the trailing edge. At least one boundary layer bleed hole connects the pressure side cooling slot to at least one of the suction side cooling slots. This bleed hole is rearward or downstream of the pressure side lip.
Each suction side cooling slot can have a depth profile with a variable depth into the outer wall of the airfoil on the suction side, and this slot depth can become deeper as the slot extends towards the trailing edge. The variable depth of each suction side cooling slot can transition from the suction side surface to a maximum depth at its opening at the trailing edge. The variable depth of each suction side cooling slot can curve towards the trailing edge. The depth of one of the suction side cooling slots can vary differently from the depth of another suction side cooling slot or they can be the same.
The interconnecting boundary layer bleed hole preferably extends from the curved rearward side of the pressure side cooling slot to at least one of the suction side cooling slots. The boundary layer bleed hole can be oriented relative to the convex suction side of the airfoil so that cooling flow in the suction side cooling slot is concurrent with the mainstream flow along the suction side, whereby shear mixing between the mainstream flow and the cooling flow is reduced. The profile shape of each suction side cooling slot can also be oriented relative to the convex suction side so that cooling flow in the suction side cooling slot is concurrent with the mainstream flow along the suction side to reduce this shear mixing.
The cooling system can provide multiple pressure side cooling slots, each paired to a separate one of the suction side cooling slots through a boundary layer bleed hole.
The pressure side cooling slot can be shaped to include a metering section sized to substantially match cooling flow conditions to hot gas flow conditions prior to discharge from the pressure side cooling slot. Additionally, the pressure side cooling slot can be shaped to include a diffuser section wherein the cross-sectional area of the pressure side cooling slot increases towards the pressure side outlet. These features can also be combined with a trailing edge cooling channel that provides impingement holes and cavities for double impingement cooling.
An advantage of this invention is that multiple suction side cooling slots reduce the airfoil trailing edge effective thickness, thus reducing base region heat load, resulting in reduced trailing edge metal temperature and improved airfoil life. A reduced trailing edge thickness can also reduce airfoil blockage and minimize stage pressure losses, translating into improved turbine stage performance.
Another advantage is that the suction side submerged cooling slots provide additional convective cooling for the trailing edge corner, thus minimizing hot spots in this region of the airfoil. The relatively increased depth of the submerged cooling slots can lower the cooling air velocity and yield good downstream film effectiveness. This slot design can also minimize shear mixing, thus lowering aerodynamic loss and maintain high film cooling effectiveness for the suction side surface of the airfoil. This design also reduces pressure side cut back, thus minimizing shear mixing and increasing film effectiveness on the pressure side.
These and other embodiments are described in more detail below.
The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the presently disclosed invention and, together with the description, disclose the principles of the invention.
As shown in
The turbine blade into which the cooling system is integrated can have a general overall construction similar to existing turbine blades, and made from conventional alloys or similar materials. The turbine blade can have application, for example, in a first stage of a turbine engine. As shown in
The outer wall 16 can have a concave pressure side 38 and a convex suction side 36 separated rearwardly by the trailing edge 22. The trailing edge 22 can be formed by the junction of the concave pressure side 38 and the convex suction side 36 and be considered to have an imaginary central line 40 that can be aligned with the camber line of the airfoil 12. The trailing edge 22 can have a pressure side region on one side of the center line 40 and a suction side region on the other side of the center line 40.
Trailing edge cooling channels such as trailing edge cooling channel 18 may be positioned proximate to the trailing edge 22 and terminate in a pressure side cooling slot 28 curved to exhaust onto the pressure side 38 forward of the trailing edge 22. A pressure side lip 32 is defined at the forward junction of the pressure side cooling slot 28 with the pressure side 38. In at least one embodiment, a series of suction side cooling slots, such as suction side cooling slot 26, are formed on the suction side 36 near and extending to the region of the trailing edge 22 from a front edge 48. The front edge 48 of the suction side cooling slot is no further forward chord wise than the pressure side lip 32, and is preferably aligned chord wise, but can be positioned rearward. Cooling air flow is supplied to the suction side cooling slot 26 from the pressure side cooling slot 28 through a boundary layer bleed hole 42, which is downstream of the pressure side lip 32. The suction side cooling slot 26 can be recessed or submerged in the suction side 36 of the outer wall 16 near the trailing edge 22 and the depth of the suction side cooling slot recess 26 can extend into the thickness of the outer wall 16 “around the corner,” e.g. past the center line 40 into the pressure side region of the trailing edge 22.
The trailing edge cooling channel 18 can be provided with an impingement cooling system, such as the double impingement cooling system as shown. A series of impingement holes 50 and impingement cavities 24 can be provided along the cooling flow path from the first up-pass 14 of a serpentine cooling flow circuit towards the trailing edge 22. The double impingement cools the upper portion of the airfoil trailing edge in conjunction with the multiple bleed slots at the trailing edge exit.
According to an aspect of the invention, the suction side cooling slots 26 can be sized and positioned so that the openings of the slots 26 extend to the trailing edge region. The suction side cooling slots 26 can terminate on the suction side near at the center line 40 of the trailing edge 22 but through the curve or corner of the suction side region of the trailing edge. Alternatively, the suction side cooling slot opening 26 can terminate rearwardly at the center line 40 of the trailing edge 22. Even further, the suction side cooling slot opening 26 can extend past the center line 40 of the trailing edge 22 and terminate in the pressure side region of the trailing edge 22 as shown in
The depth of the suction side cooling slots 26 can be varied along the chord wise length of each slot. For example, the depth profile can begin at the outer wall surface 16 on the suction side 36 at the forward edge of the slot 26 and then become deeper towards the rearward terminal end in the trailing edge region 22. This variable depth can take on different profiles, including linear and curved. The depth profile of the suction side cooling slots 26 can be different from each other, or can be the same.
In order to reduce shear mixing between the cooling exit flow and the mainstream hot gas on the pressure side 38, the submerged suction side cooling slots 26 on the suction side 36 are combined with pressure side cooling slots 28 whose rear sides 44 are curved to provide an outlet on the pressure side 38, forward of the trailing edge 22 and forward of the rearward terminal end of the suction side cooling slots 26. This combination can provide cooling to the pressure side trailing edge region on both the pressure side and the suction side while minimizing shear mixing. Because of the curved rear surface 44 of the pressure side cooling slots 28, the cut back distance for these pressure side cooling slots 28 is reduced and the film cooling effectiveness is improved for the cooling of the pressure side region of the trailing edge 22.
According to another aspect of the invention, the front edge 48 of the suction side cooling slots is aligned chord wise or rearward of the front pressure side lip 32 of the pressure side cooling slot 28. The bleed hole 42 is rearward or downstream of the pressure side lip 32. With this arrangement, hot spots in the suction side region rearward of the pressure side lip 32.
During use, cooling fluids may flow into the cooling system 10 from a cooling fluid supply source. A portion of the cooling fluids may flow into the leading edge supply channel 66, through the supply orifices 68 and into the leading edge cooling channel 64. The cooling fluids may then flow from the leading edge supply channel 66 through film cooling holes 70 forming a showerhead in the leading edge 33. The remaining portion of cooling fluids may flow from the cooling fluid supply source into the serpentine cooling channel 54. The cooling fluids may flow back and forth span wise between the root 30 to the tip section 34 in the serpentine cooling channel 54. A portion of the cooling fluids in the serpentine cooling channel 54 may be exhausted through the film cooling holes 70. The remaining portion of the cooling fluids may be passed through the one or more exhaust orifices 58 into the central trailing edge cooling channel 18. The cooling fluids may then flow past the impingement holes 50. The cooling fluids may then be exhausted through the pressure side cooling slots 28 and the suction side cooling slots 26 to cool the trailing edge region 22.
The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention.
