The present invention relates generally to gas turbine buckets or blades and particularly relates to cooling a so-called platform portion interposed between the bucket airfoil and the bucket shank.
Over the years, gas turbines have trended toward increased inlet firing temperatures to improve output and engine efficiencies. As hot gas path temperatures have increased, however, bucket platforms have increasingly exhibited distress including oxidation, creep and low-cycle fatigue cracking, spallation and in some cases, platform liberation. With the advent of closed circuit steam cooling in, for example, the buckets and nozzles in the first two stages of industrial gas turbines, inlet profiles have become such that the bucket platforms are exposed to temperatures close to peak inlet temperatures for the blade row. The problem is particularly acute at the leading edge fillet where the airfoil joins the platform at the forward portion of the pressure side of the airfoil.
Accordingly, it would be beneficial if more effective cooling arrangements can be designed to cool the platform areas of buckets used particularly in the first and second stages of the turbine.
In a first exemplary but nonlimiting embodiment, the present invention relates to a cooling circuit for a turbine bucket having a shank portion, a platform portion and an airfoil portion, the cooling circuit comprising a first cooling passage extending from a cooling air inlet located at a radially inward end of said shank portion so as to communicate with a turbine wheelspace when in use, said first cooling passage connecting to a second cooling passage extending within and across at least one region of said platform, said second cooling passage connecting with a third cooling passage extending radially outwardly in said airfoil portion, said third cooling passage terminating at one or more cooling air outlets located at a radially outward end of said airfoil portion.
In another exemplary but nonlimiting embodiment, the invention relates to a cooling circuit for a turbine bucket having a shank, a platform and an airfoil, the cooling circuit comprising: a first cooling passage extending from an inlet located at a radially inward end of the shank and adapted to communicate with a turbine wheel-space, the first cooling passage, in use, supplying cooling air to a serpentine cooling circuit extending within and across at least one region of the platform, said serpentine cooling circuit connecting with a separate internal cooling circuit passage proximate a trailing edge of the airfoil, such that the cooling air used to cool the platform is re-used in the airfoil cooling circuit; wherein the platform includes a first region on a pressure side of the airfoil portion and a second region on a suction side of the airfoil portion, the at least one region comprising the first region on the pressure side of the airfoil.
In still another exemplary but nonlimiting embodiment, the invention provides a method of cooling a gas turbine bucket platform comprising: extracting compressor cooling air from a wheel space area between blade wheels mounted on a turbine rotor; feeding extracted compressor cooling air from a radially oriented passage along a leading edge of a shank portion of the bucket to a serpentine cooling passage formed in the platform; dumping the extracted compressor cooling air into an internal cooling circuit in the bucket airfoil; and exhausting the extracted compressor cooling air along a trailing edge of the bucket airfoil.
The invention will now be described in detail in connection with the drawings identified below.
In general terms, the present invention relates to a turbine bucket platform cooling arrangement where a portion of the compressor-extracted air that is used to cool the wheel space areas between the rotor wheels is fed to the bucket platform through a passage on the lower outlet side of the bucket shank portion. This passage will feed the extracted air radially outwardly to the platform where it will turn substantially 90 degrees and follow a serpentine passage along and around the “inner portion” of the platform, i.e., that portion on the pressure side of the bucket airfoil. The extracted cooling air will then dump into one of the radially-extending internal core cooling passages of the bucket airfoil to be used for airfoil cooling.
More specifically, and with reference to
Turning now to
In each of the above-described embodiments, the serpentine cooling circuit 24, 124 and 224 formed in the bucket platform 18 is fed from compressor-extraction air taken in at the lower, leading side of the bucket shank. The cooling air is then routed along the serpentine platform cooling circuit before being dumped into the internal airfoil cooling circuit where the platform cooling air is re-used for cooling the airfoil. The cooling air is then exhausted through the trailing edge of the bucket along with the airfoil cooling circuit air. This arrangement effectively film cools both the forward face of the shank and the platform, while providing additional cooling air to the airfoil. In addition, pulling compressor extraction air directly into the bucket provides air at higher pressure to the problematic platform area which helps reduce the platform temperature and prolong the life of the bucket. This, in turn, results in reduced repair costs over the service life of the bucket.
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.