Patent Application
20050191182
This invention relates to shrouded turbine blades and more specifically to an improved cutter tip design that reduces the bending stresses in the shroud to airfoil interface region of a turbine blade.
Gas turbine engines have compressor and turbine blades of varying length in order to compress and expand the fluid flow passing through the engine. For the turbine section, as energy is extracted from the hot combustion gases, the fluid expands and the turbine section expands accordingly, including the stages of turbine blades. As turbine blade length increases, the blades become more susceptible to vibration and require dampening. In order to dampen the vibrations, a shroud is added to the blade, most often at the blade tip. The shroud serves to reduce blade vibrations by interlocking adjacent turbine blade tips, as well as to seal the blade tip region to prevent hot combustion gases from leaking around the blade tip and bypassing the turbine.
While this sealing and dampening design is effective, the use of a shroud causes additional load and stress on the turbine blade due to its shape, weight, and position. Specifically the shroud has a radial stress component on the blade attachment due to its weight and radial position. Furthermore, the shroud exhibits a bending moment at the interface region between the shroud and airfoil due to the large mass cantilevered along the edges of the shroud. This bending moment is further complicated by the mass due to a cutter tooth located along at the edge of the shroud knife edge. As the operating temperature of the turbine blade increases, it stretches radially outward and approaches an outer compliant rub strip that surrounds the row of turbine blades. The rub strip is typically fabricated from segments of honeycomb. The cutter tooth is designed to cut a groove in the honeycomb of the surrounding rub strip to allow the shroud sufficient area under all operating conditions to seal and not adversely contact the rub strip. Depending on the size and position of the cutter tooth, the bending moment between the shroud and airfoil increases, and the associated shroud bending stresses will increase by as much as 20%, thereby reducing the durability of the shroud.
An example of this type of shroud design is shown in
A shrouded turbine blade having reduced bending stresses at the blade tip region is disclosed. In general, the turbine blade comprises an attachment, neck, platform, airfoil, and shroud. More specifically, the shroud comprises a first surface fixed to an end of the airfoil, a second surface in spaced relation and generally parallel to the first surface, with a plurality of radially extending sidewalls connecting the first surface and second surface to give the shroud a thickness. Extending outward from the shroud second surface and across the second surface is at least one knife edge having knife ends at the shroud sidewalls. Positioned immediately adjacent the at least one knife edge yet a distance substantially away from the knife ends is at least one tooth used for cutting a groove in a compliant rub strip that surrounds the turbine blade tip.
The tooth, which in prior art shroud designs, has been known to be a significant factor in shroud bending stresses, is repositioned to reduce its bending moment on the airfoil to shroud region and associated shroud bending stresses. It has been determined that the tooth can be repositioned without compromising cutting performance, while at the same time reducing shroud bending stresses for the preferred embodiment by approximately 18% over the prior art configuration.
It is an object of the present invention to provide a shrouded turbine blade having lower shroud bending stresses.
It is another object of the present invention to provide a shrouded turbine blade with smaller clearances between the blade tip and surrounding seal.
In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.
Referring to
Positioned immediately adjacent knife edge 30, but a distance substantially away from knife end 31, is at least one tooth 32, having a second height H2, that is substantially equal to knife edge first height H1. At least one tooth is positioned adjacent the knife edge of a turbine blade shroud in order to cut a groove in a surrounding compliant rub strip for the relatively thin knife edge of the shroud such that a seal between the turbine blade and surrounding rub strip is provided. Cutting a slot wider than the width of the knife edge ensures the thinner knife edge will not contact the rub strip and adversely wear. Cutting a wider slot with margin on either side of the knife edge to compensate for shroud movement can be accomplished by multiple cutter teeth as shown in
Depending on the operating temperatures of the turbine, often times turbine blades require cooling in order to reduce the overall blade temperature to an acceptable level for the blade material. An example of blade cooling is shown in
Depending on the size of the turbine blade shroud, more than one knife edge may be necessary in order to provide an effective seal between the turbine blade and surrounding compliant rub strip. An example of this alternate shroud configuration is shown in
One skilled in the art of turbine blade design will understand that the use of this type of shroud configuration is independent of the turbine blade geometry. Therefore, the shroud and knife edge geometry disclosed herein could be used in combination with other airfoil, platform, neck, and attachment configurations.
While the invention has been described in what is known as presently the 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 within the scope of the following claims.
