Double period thermoacoustic oscillation in swirl stabilised flame*

A partially-premixed swirl-stabilised CH₄-Air flame is studied using Large Eddy Simulation (LES). LES is performed at a thermal load of P_th = 25kW and a global equivalence ratio of Φ = 0.9 using an unstrained flamelet model for sub-grid reaction rate. Simulation results show good agreement with PIV and Raman measurements for velocity, temperature and mixture fraction statistics. A self-excited thermoacoustic instability undergoing a period-2 limit cycle oscillation (LCO) at 314 Hz (f₁) and 628 Hz (f₂) is observed. The pressure amplitude of the LCO has a time-varying behaviour at f₂. Time-evolution of the flame, flow fields and coherent structures are studied during the thermoacoustic cycle to reveal the feedback mechanism. The three-dimensional phase space representation of pressure shows a double-loop attractor. The heat release rate strongly couples with the first mode compared to the second mode resulting in a single-loop attractor. Rayleigh index based on the Rayleigh Criterion [2] is analysed to understand the stronger coupling of the heat release rate and pressure fluctuations at f₁ compared to f₂.