Two-dimensional flashback simulation in strongly swirling flows

In lean premised swirl stabilized gas turbine combustors a common burner design goal is to obtain lowest NO_x-emissions by increasing the mixing length without deterioration of the burner reliability. In these burners flame flashback from the combustion zone into the mixing zone leading to thermal overload and subsequent rapid destruction of the burners must be avoided under all operating conditions. In previous experimental studies (Fritz et al. 2001, Kröner et al. 2002) a new phenomenon, the combustion induced vortex breakdown (CIVB), has been identified as the predominant cause for flashback in swirl stabilized burners. The current work aims at providing a theoretical analysis of this phenomenon as well as tools for the investigation of arbitrary geometries on the basis of a URANS-CFD model. A two-dimensional axialsymmetric model has been developed, which allows to include the computation of the upstream flow path as well as the swirl generator. Following this strategy, the specification of predetermined velocities at the swirler inlet which become unphysical during flashback can be avoided. As the benefit, a much better simulation of the transient flame propagation process is obtained. Comparisons of the numerical results obtained so far with the experiments from the previous studies reveal that the two-dimensional model accurately captures the essential features of the process although the reacting flow exhibits a three-dimensional structure during flashback in reality.