Acoustic analysis of gas turbine combustors

Combustion instability has become a major issue for gas turbine manufacturers. Stricter emission regulations, particularly on nitrogen oxides, have led to the development of new combustion methods, such as lean premixed prevaporized (LPP) combustion, to replace the traditional diffusion flame. However, LPP combustion is much more liable to generate strong oscillations, which can damage equipment and limit operating conditions. As a tutorial, methods to investigate combustion instabilities are reviewed. The emphasis is on gas turbine applications and LPP combustion. The flow is modeled as a one-dimensional mean with linear perturbations. Calculations are typically done in the frequency domain. The techniques described lead to predictions for the frequencies of oscillations and the susceptibility to instabilities for which linear disturbances grow expotentially in time. Appropriate boundary conditions are discussed, as is the change in the linearized flow across zones of heat addition and/or area change. Many of the key concepts are first introduced by considering one-dimensional perturbations. Later higher-order modes, particularly circumferential waves, are introduced, and modal coupling is discussed. The modeling of a simplified combustion system, from compressor outlet to turbine inlet, is described. The approaches are simple and fast enough to be used at the design stage.