Thermoacoustic analysis of the dynamic pressure inside a model combustor during limit cycle oscillations

In this work comprehensive experimental and numerical studies incorporating the most relevant physical mechanisms causing limit cycle pressure and combustion rate oscillations (LCO) in a laboratory scale combustor will be discussed. The strong interaction between the aerodynamics-combustion-acoustic oscillations (ACA), and under specific conditions the aerodynamics-combustion-structural vibrations (ACS), is studied by a careful selection of experiments and numerical simulations performed using commercially available computational models. It is shown predominantly that the convective time scales due to the aerodynamics at the flame stabilizer and the time period related to acoustic propagation have to be of the same order in magnitude to be able to drive the system into LCO. The measurements indicated that the frequency spectrum of the oscillations of the LCO has a distinct peak close to the natural mode of the combustor along with higher order "harmonics" due to non-linear effects. Some non-harmonic higher order peaks are observed that are associated with the structural (liner) natural frequencies of vibration. A numerical simulation has been performed using the commercial code (ANSYS V13.0) that includes the effects of fluid-structure interaction by means of pressure load transfer on to the structure and vice-versa. The fluid domain is modeled using CFX and the structural domain is represented by ANSYS. The information is exchanged between the two domains dynamically at every time step computed. In order to reduce the computational effort and quickly gain insight into the problem only a 2 mm slice of the whole geometry has been considered making it essentially a 2D analysis. The good agreement between the model and measured instability frequencies shows a very promising approach in predicting the limit cycle oscillations in this kind of configurations.