Effects of Flame Development and Structure on Thermo-Acoustic Oscillations of Premixed Turbulent Flames

Abstract: Non-stationary confined premixed turbulent flames stabilized behind a bluff body are studied. A simple kinematic model of such flames was developed by Dowling who reduced the combustion process to the propagation of an infinitely thin flame with a constant speed. The goal of this work is to extend the model by taking into account the real structure of premixed turbulent flames and the development of turbulent flame speed and thickness. For these purposes, so-called Flame Speed Closure model for multi-dimensional simulations of premixed turbulent flames is adapted and combined with the aforementioned Dowling model. Simulations of the heat release rate dynamics for ducted flames due to oncoming flow oscillations have been performed. Typical results show that the oscillations in the integrated heat release rate follow the oncoming flow velocity oscillations with certain time delay, which controls the sign of the well-known Rayleigh integral and, hence, the global instability or stability of the combustor. The delays computed using the Dowling and the above approach are different, thus indicating the importance of resolving the flame structure when modeling ducted flame oscillations