It is known to attach turbine blades to a rotor shaft by means of fir tree blade roots. Such blade roots comprise, starting from a root base, a number of alternating ridges and grooves. Said roots are slidingly received in counterpart slots within the shaft. When loaded e.g. through centrifugal forces, the roots bear on upward pointing, that is, pointing towards the airfoil, bearing surfaces of the ridges.
The airfoils of blades are typically arranged in a center region of the blade in a lengthwise direction of the blade foot. Thus, the centrifugal load of the airfoil acts more fiercely in said center region and consequently resulting in bending strains bending the foot along its lengthwise direction. Differential thermal expansion between the airfoil and the blade root further contributes to said bending.
Typically, blades in gas turbines are cooled. A coolant flow is introduced into hollow blade airfoils through openings and channels in the blade roots at a lengthwise position of the airfoil. However, due to the presence of these channels the elastic deformation of a blade root upon loading is further enlarged in the region where the channels are arranged. That is, in the region where a bending displacement is induced, the bearing surfaces are more firmly pressed against their counterparts in the shaft. This results in enhanced stresses locally induced in the blade roots as well as in the counterpart features of the shaft in the region of the coolant channels. In turn, early fatigue may occur and parts may be needed to be replaced more frequently.
One possibility known in the art to account for this problem is to cool the shaft in the attachment areas to improve mechanical properties of the material. However, this consumes cooling air, which reduces efficiency and power, and may not be readily possible due to other constraints, like space, complexity, cost and lifetime.
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