Combustion instability of a premixed swirl flame with/without central bluff body

This study investigated the effects of equivalence ratio and central bluff body on the instability of premixed swirl propane/air flame. First, a low-order thermoacoustic network model based on the n−τ model was employed for a premixed swirl combustor, and several potential longitudinal modes were predicted within the system. Subsequently, self-excited oscillation experiments revealed mode switching phenomena between two longitudinal modes (f1 and f2). Moreover, further analysis demonstrated that this mode transition was related to both the equivalence ratio of premixed gas and the characteristics of the swirler. The central bluff body in the swirler was found to modify the gain and time delay of flame transfer function, thereby influencing the stability of thermoacoustic modes of the combustor. Finally, thermoacoustic mode analysis incorporating experimentally measured flame transfer functions showed a good agreement between the model predictions as well as the thermoacoustic oscillation experimental results.