Application of Low Order Thermo-Acoustic Network to DLE Staged Combustor

Lean premixed combustors have been developed to meet stringent NOx emission legislation. Operating close to the lean limit introduces a susceptibility to combustion oscillations, which have a damaging impact on hardware and affect combustor performance. The RB211 DLE aero derivative gas turbine has to be CO and NOx compliant from 60% power. Therefore the required turndown must be incorporated into the design. This is achieved by the addition of a second combustion stage, which provides the required CO burn out. Combustion instabilities can occur at different primary and secondary combustion temperatures in the form of noise islands. A 1/2 wave mode has been identified at high primary zonal temperatures and occurs over a certain range of secondary zonal temperatures. This observation suggests that the secondary flame can excite or dampen the instability depending on the phase relationship between the associated dynamic pressure and the fluctuating heat release rate. The mechanism is though to be fluctuations in temperature originating from the primary flame and convecting to the secondary flame. A second mode is observed at lower primary combustion temperatures, involving the axial combustor length and the circumference of the whole combustion system. A low order thermoacoustic network has been created by Stow et al [1] for the purpose of analysing eigen modes within aero annular combustors. This low order network is used to model the RB211 DLE combustion system. It uses linear perturbation theory to predict the eigen frequencies in the combustion system. Coupling between the unsteady flow and the fluctuating heat release rate is represented by a simple transfer function in both combustion zones. Results and discussion are given for both acoustic modes encountered on the RB211 DLE combustion system.