Patent Application
20050135922
This application relates to an airfoil for a turbine blade, wherein pedestals connecting opposed walls in a trailing edge cooling chamber have cross-sections designed to accommodate thermal stress.
Turbine blades are utilized in gas turbine engines. As known, a turbine blade typically includes a platform, with an airfoil shape extending above the platform. The airfoil is curved, extending from a leading edge to a trailing edge. Moreover, there is a pressure side and a suction side to the airfoil. The pressure side becomes much hotter than the suction side during operation.
Cooling channels are formed within the airfoil body to circulate cooling air. One type of cooling channel which is used particularly adjacent the trailing edge is an open chamber having cylindrical pedestals connecting opposed suction and pressure side walls. Cooling air flows around these pedestals, and through the open chamber. Typically, the pedestals have had a generally equal diameter.
In this prior art, the cylindrical pedestals have sometimes been subject to concentrated heat-induced stress. In particular, since the pressure side is much hotter than the suction side, there is more thermal expansion on the pressure side. This is particularly true adjacent the platform. Since the pressure side of the airfoil expands for a greater extent than the suction side, concentrated stresses are applied to the pedestals. This is undesirable.
In a disclosed embodiment of this invention, non-cylindrical pedestals connect the opposed walls of a trailing edge cooling chamber. In particular, the pedestals are formed to have a greater dimension in a direction generally parallel to a plane of the platform. This greater dimension spreads the stress concentration over a greater length. Most preferably, the shape is elliptical, with the major diameter being measured generally parallel to the platform. This shape creates a larger radius that lowers the stress concentrations.
In other features of this invention, the elliptical pedestals are utilized only in selected areas. In particular, the elliptical pedestals are preferably utilized adjacent the platform, wherein the stress concentrations are of greater concerns.
Having cylindrical pedestals at other areas reduces the manufacturing complexity of forming the inventive airfoil. As will be explained below, it is somewhat easier to form a cylindrical pedestal than to form an elliptic pedestal.
In another broad aspect, the pedestals could be said to have differing cross-sections across the array of pedestals. In one preferred embodiment, there are cylindrical pedestals of a first smaller diameter adjacent a leading edge end of the cooling chamber, and cylindrical pedestals of a larger diameter adjacent the trailing edge. The elliptical pedestals are most preferably arranged intermediate the two sizes of cylindrical pedestals. The purpose of this arrangement will be described in greater detail below.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.
As shown in
An open cooling chamber 30 is formed adjacent the trailing edge 23. A wall 33 separates passage 35 from chamber 30. As shown in chamber 30, pedestals 34 and 36 connect the opposed pressure 26 and suction 28 walls. Other than the discrete pedestals 34 and 36 (and pedestals 32, see
As shown in
Generally, it is somewhat easier to form cylindrical pedestals than to form elliptic pedestals. Thus limiting the elliptic pedestals 34 to the region 38 where they are most needed does somewhat simplify manufacturing. In particular, the blade 20 is typically cast from a lost core casting technique. In such a technique, the core will initially have openings that form the pedestals 32, 34 and 36. Such openings have a flashing that is to be removed. A cylindrical opening is easiest to clean, as a simple cylindrical tool might be inserted into the opening. The elliptic openings require more work to clean.
As can be appreciated from
In a most preferred embodiment, the cylindrical pedestal 32, and the remaining pedestals generally above the area 38 all have a first smaller diameter than the cylindrical pedestals 36 that are adjacent the trailing edge. The pedestals 32 that are adjacent the leading edge can better withstand the thermal stresses, even adjacent platform 22, in that they tend to be longer than the pedestals spaced closer to the trailing edge. As can be appreciated from
A length to diameter (or L/D) measurement could be defined as the length of the pedestals or distance between the chamber walls, and the diameter of the pedestal. This L/D ratio can help define when the smaller diameter pedestals 34 can withstand thermal stresses. If the L/D ratio is greater than 1.5, then the pedestal is more flexible and accommodates thermal gradients rather than creating a high stress. For this calculation, the nominal diameter of the smaller diameter pedestals 32 is used for D. When the L/D ratio is less than 1.5, then the elliptical shape, or larger diameter pedestal concept might be considered.
For this reason, larger diameter pedestals 36 are arranged adjacent the trailing edge. The elliptic pedestals 34 intermediate the pedestals 32 and 36 preferably have a major diameter that roughly approximates the diameter of the cylindrical pedestals 36, while the elliptic pedestals 34 have a minor diameter that roughly approximates the diameter of the cylindrical pedestals 32. The range of the larger diameter pedestals 36 to the diameter of the smaller diameter pedestals 34 is also set by the preferred ratio range of 1.25 to 1.75 as described above.
The cylindrical pedestals 36 adjacent the trailing edge will be among the shortest, and thus the most susceptible to damage from the thermal stresses. A worker of ordinary skill in the art may recognize that making the pedestals 34 cylindrical, but of a larger diameter, rather than elliptical, might provide benefits. It is also true, however, that if the pedestals 34 were made larger and cylindrical, it would be difficult to form an appropriate loss core for forming the pedestals. There would be less space between the pedestals, and it could be difficult to form a functioning core. For this reason, it is not desirable to simply make the pedestals 34 cylindrical, but larger.
The present invention thus presents a unique shape for a pedestal that lowers stress concentrations, and improves the ability of the rotor blade to withstand thermal stresses.
Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
