Turbine rotor blades of gas turbines are known to comprise a platform having a root with typically a dovetail/fir tree shape to be connected to a corresponding seat of a blade carrier.
From the central portion of the platform an airfoil extends, shaped with a pressure side and a suction side arranged to cooperate with hot gases that pass through the turbine.
When assembled on the blade carrier, the turbine rotor blades are all arranged one adjacent to the other, such that their platforms define the inner surface of the annular hot gases path.
Nevertheless, these blades have a number of drawbacks, enumerated in detail in the following:
AERODYNAMICAL PROBLEMS—During operation a large amount of purge air must be injected into the hot gases path through the gaps between two adjacent platforms and additional purge air must be injected from the casing encircling the rotor turbine blades. This air injected into the hot gases path decreases the efficiency of the gas turbine.
In addition, the gaps between the tip of each airfoil and the casing let a leakage pass through; these leakages further decrease the efficiency of the gas turbine.
MANUFACTURING PROBLEMS—Blades have usually a number of internal cooling channels through which, during operation, cooling air is driven.
For this reason, blades are usually manufactured by casting them with an internal ceramic core forming the cooling channels. This casting technique is very expensive and time consuming; in addition the channels (formed in the ceramic core) usually are not provided with all ideal features from the cooling point of view, but they are optimised for making the manufacturing process easier and cheaper.
COOLING PROBLEMS—Because of the manufacturing constrains, the cooling channels could not provide an efficient cooling, such that during operation overheating and difficult cooling could become a problem.