Mitigation of combustion instabilities by nitrogen micro-injectors in a pressurized premixed swirl-stabilized combustor

Inbar Greenberg
Work towards MSc degree under the supervision of Asst. Prof. Dan Michaels (Technion)
Department of Aerospace Engineering
Technion – Israel Institute of Technology

In this study, we examined the impact of injecting diluent gas into the outer recirculation zone of a swirl stabilized premixed methane/air flame on the flame shape and combustion dynamics. This study extended previous work on the effect of the micro-injectors by considering the influence of a converging exit nozzle and elevated pressures, up to 3.5 atm. Two modes of instability were observed under different operating conditions. The first was a low-frequency mode at 5-15 Hz which corresponded to periodic flame propagation and extinction in the outer recirculation zone and was related to the transition of the flame from a V shape to an M shape. The second thermo-acoustic combustion instability mode was at 50-100 Hz, and was related to non-uniform mixing at the converging nozzle in the combustor exit that generated acoustic waves, commonly referred to as a mixed acoustic-entropy instability mode. Hence, the addition of a converging nozzle at the exit significantly influenced the flame mode shapes and coupling mechanisms in the combustor. The overall flame macrostructures and flame dynamics were similar to the ones seen at atmospheric pressure. Increasing the pressure shifted the V-to-M flame shape transition to a higher equivalence ratio, shortened the flame, and widened its angle. Operating at various combustion pressures, nitrogen was injected into the outer recirculation zone, at mass flow rates of 3% and 6% compared to the bulk flow of the methane/air mixture. The dilution promoted a shift of both instability modes to higher equivalence ratios. There appeared to be a saturation of the effect when the dilution percentage was increased. For the lower nitrogen flow rate, the dilution effect on the flame shape was found to scale according to the extinction strain rate, highlighting the chemical kinetic effect of the diluent gas. However, for the higher flow rates similar conclusions could not be made, likely due to a more significant fluid dynamic effect as the flow rate and momentum of nitrogen microjets increased.

The talk will be given in English

Wed, 14-09-2022, 13:30-14:30 (Gathering at 13:30)

Classroom 165, ground floor, Library, Aerospace Eng. &

Light refreshments will be served before the lecture

Mitigation of combustion instabilities by nitrogen micro-injectors in a pressurized premixed swirl-stabilized combustor