Thermo-acoustic frequency selection by swirled premixed flames

The problem of combustion-induced vibrations in industrial gas turbines has been studied by investigating the response of a model swirl burner in a laboratory combustion system. The acoustic boundary conditions for the flame were deduced from the transfer function between two microphones, and die response of the flame, from the chemiluminescent emission of UV.;Axial distributions of heat release and of its phase relative to the burner exit velocity were determined from the local UV emission. Changes were made to the location of the swirler, to the acoustic response of the premixture supply system, and to the fuel, from propane to methane. Simple empirical models are developed to describe the acoustic response of the different flames in one particular mode of excitation. From these models, the frequencies are identified at which the acoustic boundary conditions can be satisfied. These are the eigen values of the system, but by checking each component of the calculation in this way, a surer foundation for further applications has been laid than if a complete solution had been attempted initially, ii consistent picture emerges of the variation in the frequency of excitation. The delay in response associated with retracting the swirler and with the slower burning fuel (methane) at the equivalence ratio of 0.6 leads to a reduction in the frequency of self-excitation.