Combustion Instability in a Turbulent Liquid-Fueled Swirl-Stabilized LDI Combustor

Abstract

The simultaneous excitation of the half-wave mode and the one-wave mode is inherently unsteady and unstable. Disturbances, even in small amplitude, can destroy the subtle balance between the two modes, causing one mode to grow and the other to decay. The timevarying amplitude of the two modes is rooted in the nonlinear response of the reacting swirling shear layer to inlet disturbances. An increase in the time scales for convection, evaporation, and chemical kinetics favors the excitation of the half-wave mode, and a decrease in these time scales favors the excitation of the one-wave mode. Unsteadiness is an intrinsic feature of thermoacoustic oscillations in turbulent combustors. An external disturbance pushes the state off the equilibrium trajectory, and it may take a number of revolutions for the state to return to a small neighborhood of the equilibrium trajectory. Because of the ubiquity of external and background disturbances, both the amplitude and the frequency of thermoacoustic oscillations are constantly time-varying. With decreasing pressure amplitude or the positive feedback between pressure and heat release, the limitcycle thermoacoustic oscillations are increasingly vulnerable to external disturbances, as indicated by a larger neighborhood that the state trajectory roves over. The acoustic wave distribution along the combustion chamber deviates considerably from the natural one-wave mode. The natural acoustic modes refer to the acoustic eigenmodes in a combustion chamber which does not generate sound itself. Caution should be exercised when developing loworder models via Galerkin projection of the natural acoustic modes in gas turbine combustors.