Thermal and fluid dynamic analysis of partially premixed turbulent combustion driven by thermo acoustic effects

Thermo-acoustic instability can be caused by the feedback mechanism between unsteady heat release, acoustic oscillations and flow perturbations. In a gas turbine combustor limit cycles of pressure oscillations at elevated temperatures generated by the unstable combustion process enhance the structural vibration levels of the combustor. In this paper, the behavior of turbulent partially premixed flames in a laboratory-scale lean partially premixed combustor (called as LIMOUSINE combustor) operating on natural gas- methane fuel mixtures is studied by using CFD methods. Depending on the operating conditions, the flame shows a stable or an unstable behavior. In order to predict the frequency and magnitude of the thermo-acoustic instability, and also to capture the reacting flow physics within the combustor, the influence of operating conditions on combustion characteristics is examined by using unsteady three-dimensional RANS solution of the conservation equations. To understand the effects of operating conditions on the observed stability characteristics, the time averaged velocity fields were measured with Particle Image Velocimetry (PIV) for the thermoacoustically stable and unstable operating conditions of the combustor. The comparison of the CFD calculations with the mean velocity fields shows good agreement. The results of the present study demonstrate the relationship between the flame structure, the mean velocity filed and pressure fluctuations under different operating conditions.