Geometrical Modification of the Unsteady Pressure to Reduce Low-Pressure Turbine Flutter

Abstract

In this work, the impact of the airfoil shape on flutter is investigated. Flutter occurs when the blade structure is absorbing energy from its surrounding fluid leading to hazardous amplification of vibrations. The key for a more stable design is the local modification of the blade motion induces unsteady pressure, which is responsible for local stability. Especially for free-standing blades, where most exciting aerodynamic work transfer is found at the upper tip sections, a reshaping of the airfoil is expected to beneficially influence stability. Two approaches are pursued in this work. This first approach is based on flow physics considerations. The unsteady pressure field is decomposed into four physical mechanisms or effects and each effect investigated. The second approach is used to validate the conclusions made in the theoretical part by numerical optimizing the geometry of a representative turbine blade. Selected optimized designs are picked and compared with each other in terms of local stability, aerodynamics, and robustness with respect to the boundary conditions. Both approaches are applied for a freestanding and interlocked turbine blade section. The found design potential is discussed and the link to the differences mechanisms, introduced in the first part, established. Based on the observations made, design recommendations are made for a flutter-reduced turbine design.