Flutter-free design of aeroderivative gas turbine nozzles with simplified aero-mechanical models

This work presents a simplified procedure to design a flutter free airfoil geometry for aeroderivative gas turbine stator blade rows. The common workflow for a fully 3D flutter numerical assessment with an uncoupled method firstly involves a FE modal analysis to obtain the airfoil mode shapes which will be used for unsteady aeroelastic computations with moving airfoil. Nozzle geometries are usually very complex: airfoils in a packet configuration and different mechanical features needed to attach the packet itself to the turbine casing make the structural meshing a not trivial task. The aim of this paper is to demonstrate how, in the early aero-mechanical design phases simplified models including only the airfoil geometry and the endwalls chunks can be efficiently used to design flutter-free components. First of all, detailed comparisons between modal results from full and simplified mechanical models have been performed. Then, flutter computations have been carried out using modal results both from the full mechanical model (the sector or packet model) and two different simplified models (single airfoil models). Finally, flutter results in terms of logarithmic decrement curves will be presented, showing the capability of this approach to guarantee flutter free geometries even from the early design stages. This aspect is essential to avoid bladerow redesign due to flutter issues which may occur during the final design of the airfoil.