Numerical simulation of stable/unstable combustion in a bluff-body stabilized com-bustor

In the present study the impact of two different combustion models on prediction of limit cycle frequencies in a model combustor are analysed. CFD analysis is carried out on a partially premixed bluff body stabilized methane-air combustor, which is self-excited. Cost-effective URANS simulations are performed with SAS-SST turbulence model and two combustion models - (1) BVM model (also known as Turbulent Flamespeed Closure model) and (2) CFI model developed at the University of Twente. The results show that the BVM model predicts unstable combustion whereas the CFI model is stable at the same operating condition. The pressure spectrum of the BVM simulation shows two distinct peaks, the base frequency corresponds to the longitudinal acoustic mode of the downstream duct of the combustor and the second frequency is assumed to be due to non-linear effects in the coupling between combustion and acoustics of the system. To validate results the numerical source terms are compared to OH^* chemiluminescence data from experiments. It is seen that the compactness of the BVM source leads to instability. Also acoustic pressure and velocity fields are reconstructed to get impedances at the burner plane and outlet. For unstable combustion the values of impedances matched with experimental findings. The analysis further suggests that the upstream and downstream sections resonate at different frequencies. The reflection coefficient calculations at the burner plane show the decoupled behaviour of the combustor.