Thermoacoustic instabilities are a limitation on the operability of any combustion system, especially under lean fuel conditions. The ability to avoid or eliminate rumble, especially at an early stage of the development of a combustion system, is a useful capability. The occurrence of thermoacoustic modes can be predicted by an acoustic network of the combustion system, where the aerodynamic/flame interaction is described by a flame transfer function. In the EC funded KIAI project a series of tasks were performed to measured flame transfer functions of lean fuel injectors with the aim of predicting thermoacoustic Eigen modes in a full annular combustion rig.
The linear flame transfer functions, of the injector, were measured at Cambridge University on a single intermediate pressure sector siren rig via OH* and CH* chemiluminescence and unsteady pressure measurements. These flame transfer functions were implemented in a low order acoustic network of the rig in the form of an inverse FFT method. The acoustic network uses linear perturbation theory, where the Eigen modes are solved by satisfying the acoustic boundary conditions. Alternatively, the acoustic network can be used to predict the forced pressure response of the system. Finally, the network was used to predict the frequencies, stability and spectra in a full annular combustor rig, utilising the same injector.