Theoretical Formulation for the Investigation of Acoustic and Entropy-Driven Combustion Instabilities in Gas Turbine Engines

Abstract

In gas turbine combustors, the unsteady flame is a source of acoustic and entropy waves, leading to combustion noise. When these fluctuations couple with the flame and establish a positive feedback mechanism, they grow in amplitude resulting in combustion instability. Combustion instability can be driven either by the acoustic waves or acceleration of entropy waves. Entropy-driven instability is one of the dominant cause of low frequency combustion instability in industrial gas turbines, where the flow exiting the combustor is accelerated by the turbine nozzle guide vanes. This chapter presents the theoretical framework to model the generation, convection, acceleration and reflection of acoustic and entropy waves in gas turbine combustors with variable area geometry. We also discuss in detail the procedure to solve the equations and present a comparison between acoustic-driven instability and entropy-driven instability.