Simulation of limit cycle pressure oscillations with coupled fluid-structure interactions in a model combustor

Large amplitude pressure oscillations, leading to limit cycle behaviour, do appear in many of the modern lean premixed (LP) combustors. This behaviour is classically known as thermoacoustic instabilities and is mainly due to formation of a strong feedback loop between the aerodynamics, flame and acoustics inside the combustor. However, in the combustion systems with relatively thin combustor walls (e.g., gas turbine combustor liner), this feedback loop is strongly influenced by the interaction of the flame with the structure. Therefore, an attempt has been made (1) to numerically simulate the limit cycle behaviour under these circumstances. The information is exchanged between the fluid domain and the structure domain transiently. In order to reduce the computational effort and yet to quickly gain insight into the coupled fluid structure interaction (2-way FSI) a quasi 2D simulation of a Rijke tube kind of model combustor (LIMOUSINE) has been performed. Here only a slice of 2 mm wide is considered for simulation instead of the entire width (150 mm) of the combustor. This removes all the 3D flow patterns and on the structural side does not include the end walls of the duct that otherwise would have added additional stiffness. This means in the present simulation, significant higher levels of unsteadiness would occur compared to the full 3D case. The simulations are performed corresponding to the operating points of 40 and 60 kW with equivalence ratio of 0.625. The pressure field shows the presence of strong peaks corresponding to the first and third quarter wave mode of the whole combustor (harmonic). On the other hand it also indicated a nonlinear frequency doubling phenomenon (non-harmonic) due to presence of high non-linearities during the limit cycle oscillations. The simulations were compared with the experimental data obtained on the LIMOUSINE setup. The preliminary analysis of the data looks promising.