Patent Application
20050220618
This invention relates to the configuration of film-cooling holes utilized as part of the cooling circuit in the airfoil portion of a turbine blade or bucket.
Film-cooling has been a major aspect of gas turbine cooling for many years. The application of effective film-cooling techniques provides the first and best line of defense for hot gas path surfaces against the onslaught of extreme heat fluxes, serving to directly reduce the incident convective heat flux on the surface. As film-cooling holes first go into service, they are typically cleaned of all obstructions or unwanted debris. Film holes in this condition may also include certain protective coatings, either diffusion or thermal barrier coatings (TBC), for such purposes as oxidation protection. In operation, film holes and hot gas path surfaces see a multitude of conditions and environments which can result in the sudden or gradual blockage of holes to various degrees, thereby influencing the film-cooling performance to lesser or greater extents.
It has been discovered, however, that as a TBC coating is sprayed on the airfoil of the bucket, some of the coating material enters the exit of the film-cooling holes. Thus, the TBC adheres to the inside surface of the film-cooling holes, decreasing the effective exit area of the holes and reducing the film-cooling effect from the design intent.
The present invention solves the partial obstruction of film-cooling holes due to TBC's sprayed on the airfoils of the buckets by changing the configuration of the film-cooling holes to include a counter-bore at the outlet or exit ends of the film-cooling holes. It is contemplated that the counter bores would be applicable to the holes along the leading edge shower head, gill holes and the holes around the bucket tip region. The counter-bore diameter and depth are specific to the design, and have been optimized for performance. For example, in one exemplary embodiment, a counter-bore of 0.053 inches on a 0.033 inch diameter through-hole extends 0.03 inches from the outlet surface of the airfoil on the minimal dimension.
The general concept of incorporating a counter-bore or flared shape can be applied to all film-cooling holes for various gas turbine buckets, nozzles and shrouds with TBC application on those parts.
Accordingly, in one aspect, the invention relates to a turbine component having a plurality of film-cooling holes formed in a region of the component to be cooled, the cooling holes having specified diameter, each hole at an exit thereof formed with a counter-bore of predetermined depth; the component having a coating applied thereto at least in the region, wherein the counter-bore provides an area for excess coating material to accumulate without reducing the specified diameter.
In another aspect, the invention relates to a gas turbine bucket having an airfoil portion and a shank portion, the airfoil portion having a plurality of film-cooling holes therein, each hole at an exit thereof formed with a counter-bore of predetermined depth; the component having a coating applied thereto at least in the region, wherein the counter-bore provides an area for excess coating material to accumulate without reducing the specified diameter; and wherein the coating comprises a first bondcoat layer and a second thermal barrier coating layer.
In still another aspect, the invention relates to a method of maintaining cooling efficiency of film-cooling holes in a turbine component where the film-cooling holes have specified diameters and the turbine component has a protective coating therein comprising: a) before coating, forming each film-cooling hole with a counter-bore and an exit end of the film-cooling hole; and b) spraying the coating onto the turbine component at least in areas surrounding the film-cooling holes such that excess coating material accumulates in the counter-bore without reducing the specified diameter of the cooling holes.
The invention will now be described in detail in connection with the drawings identified below.
Referring now to
It is known to apply a thermal barrier coating (TBC) to various regions of the bucket including adjacent the leading edge 24. Typical TBC's include a first bondcoat layer and a second ceramic coating layer. The bondcoat layer may be an NiAl-based bondcoat, and the thermal barrier coating layer may be a yttria-stabilized zirconium layer. It is also known to provide film-cooling holes 28 in various regions of the bucket including but not limited to the leading edge 24.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
