VIBRATION CHARACTERISTICS OF PRETWISTED AEROFOIL CROSS-SECTION BLADE PACKETS UNDER ROTATING CONDITIONS

Turbine blades are usually pretwisted and have asymmetric cross sections. Furthermore, turbine blades are formed into packets by joining them at critical locations with shrouds or lacing. Such a procedure alters the vibration characteristics of the blade. In this paper, the free vibration characteristics of rotating, shrouded, pretwisted aerofoil cross-section blade packets are investigated using a finite element model. One end of the pretwisted blade is assumed to be fixed at the periphery of a disk rotating about its center, whereas the other end of the blade is connected by a curved shroud. Expressions for kinetic and strain energies of a pretwisted blade packet subjected to centrifugal force are derived. Shear deformation and rotary inertia effects are neglected in the analysis. The effects of pretwist, stagger angle, rotational speed, shroud length, shroud thickness, shroud width, and the distance of shear center from the centroid on the vibration characteristics of blade packets are investigated. Comparisons made between theoretical and experimental results show very good agreement.