High-frequency instability driving potential of premixed jet flames in a tubular combustor due to dynamic compression and deflection

Constructive interference of acoustic oscillations and combustion heat release can result in high-frequency thermoacoustic instabilities in gas turbine combustion chambers with strong pressure pulsations. They may cause component wear and limit the safe operating range of the engine. During the development of stable combustors the influence of design variables on the driving mechanisms of these instabilities is of particular interest. This paper studies the influence of design parameters on the linear growth rates of high-frequency thermoacoustic transverse modes in a tubular combustor of hexagonal cross-section equipped with 12 turbulent premixed jet burners. Two flame dynamics models are used, i. e. the dynamic compression and the deflection mechanisms, which have in the past been validated for turbulent swirl burners. To demonstrate the applicability as well as the shortfalls of these flame dynamics models the impact of different geometrical and flow parameters on the driving potential of high-frequency thermoacoustic modes are considered. A parameter variation study of thermal power, air excess ratio, diameter of the combustor and the radial position of jet burners was performed. The first transversal eigenmodes and eigenfrequencies were computed by solving the inhomogeneous Helmholtz equation with the flame driving source terms in the frequency domain using the finite element method. The required mean fields of temperature and heat release rate were obtained using a generic flame distribution scaled with respect to the experimental OH^? chemiluminescence measurements. The resulting growth rates give a measure for the thermoacoustic driving potential.