Flashback limits for combustion induced vortex breakdown in a swirl burner

Flame flashback from the combustion chamber into the mixing zone limits the reliability of swirl stabilized lean premixed combustion in gas turbines. In a former study, the combustion induced vortex breakdown (CIVB) has been identified as a prevailing flashback mechanism of swirl burners. The present study has been performed to determine the flashback limits of a swirl burner with cylindrical premixing tube without centerbody at atmospheric conditions. The flashback limits, herein defined as the upstream flame propagation through the entire mixing tube, have been detected by a special optical flame sensor with a high temporal resolution. In order to study the effect of the relevant parameters on the flashback limits, the burning velocity of the fuel has been varied using four different natural gas-hydrogen-mixtures with a volume fraction of up to 60% hydrogen. A simple approach for the calculation of the laminar flame speeds of these mixtures is proposed which is used in the next step to correlate the experimental results. In the study, the preheat temperature of the fuel mixture was varied from 100°C to 450°C in order to investigate influence of the burning velocity as well as the density ratio over the flame front. Moreover, the mass flow rate has been modified in a wide range as an additional parameter of technical importance. It was found that the quenching of the chemical reaction is the governing factor for the flashback limit. A Peclet number model was successfully applied to correlate the flashback limits as a function of the mixing tube diameter, the flow rate and the laminar burning velocity. Using this model, a quench factor can be determined for the burner, which is a criterion for the flashback resistance of the swirler and which allows to calculate the flashback limit for all operating conditions on the basis of a limited number of flashback tests.