Analytical study of low-frequency Helmholtz mode oscillation in a model combustor

The "growl" mode combustion instability is one of the most destructive phenomenon occurring in the lean burn aeroengine combustor at low power condition. This topic is widely investigated these years focusing on the mechanism of flame structures, oscillation modes and the development of prediction methods. Recently, an analytical prediction method which based on the linear solution of Helmholtz equation was successfully used to predict the inception of the growl instability. In this model, the flame tube and the inflow duct are modeled as individual cavities and connected through a swirler acting as the neck of Helmholtz resonators. However, in many applications, the inflow duct has more complex geometry and thus complex acoustic boundary condition. So there is the need to extend this approach in a more universal form. This is the motivation of the current work. This paper firstly presents the measured dynamic pressure and the flame motions in a single sector combustor. By changing the cross section area of the inflow duct, self-excited combustion instability was observed. This combustion instability mainly occurred at the frequency of 350HZ∼400Hz. The test results showed that flame front moves with the bulk flow in the axial direction while the pressure fluctuations in the inflow duct show a harmonic feature. It confirms that the combustion instability corresponds to the Helmholtz mode. This phenomenon was next analyzed with a new low-order linear acoustic method developed in this paper. The method integrates the model of Helmholtz resonator within harmonic oscillation (also called IM2H), and is validated by comparing with experimental data. By considering Helmholtz oscillation in the flame tube, the predicted frequency and mode agree well with experiments. The results show that the stability of the system is more sensitive to the geometry design parameter than the aerodynamics parameters.