Blade Resonant Forced Response Excited by Combustor Acoustic Eigenmodes

Abstract

In accordance with common engineering practice, the main source for forced response and subsequent high cycle fatigue problems in axial turbines is classical rotor-stator interaction. This criterion determines the excitability of a blade row for its relative circumferential motion with respect to upstream and downstream blade rows that generate time dependent pressure fields due to potential and viscous flow phenomena as well as possible shock waves. This publication focuses on a less common source of blade vibration excitation induced by acoustic eigenmodes of a combustion chamber. The article gives an overview of a historical example where acoustic pulsations had the potential for exciting harmful vibration on the adjacent first rotating turbine blade row. A 3D acoustic FEM analysis is performed to predict acoustic eigenmodes of the combustion cavity that could potentially excite vibration of the first turbine stage. Acoustic modes in the range of the critical frequencies from the point of view of resonance with structural eigenfrequencies of neighbouring components are identified and compared to engine measurements. The knowledge gained of the critical frequencies allows for mitigation of the excitation sources with Helmholtz dampers. This paper delivers an additional excitability criterion of rotating blades by acoustic pressure fluctuations in the combustor. Turbine blade excitation is currently assessed only by considering the number of burners in the combustor chamber.