Study of the effect of bluff body size on stability limits of a premixed natural gas swirl burner

Flame stabilization is highly important in lean premixed combustion which is considered a desirable technology for low-emission gas turbines. Swirl stabilization is one of the most common methods used for this purpose. Simple generation of swirl, however, is not enough to reach an acceptable operating range. A method of improving stability is using a bluff body in order to resist flame flashback caused by combustion-induced vortex breakdown at moderate to high swirl numbers. The present study aims to investigate the effect of bluff body size on the stability boundaries of flashback and blowout and also on flame shape. A premixed swirl burner is designed and built to operate with natural gas at atmospheric condition. In addition to gas and air flow rates at stability limits, normal and chemiluminescence flame images are documented. The non-reacting flow is also simulated to investigate the flow pattern out of the burner nozzle. It is found that bluff body size has a significant influence on flashback prevention and to a lesser extent on lean blowout limit while having minor effect on the shape of the flame. Just before blowout, low-frequency fluctuations are observed in the chemiluminescence radiations that are associated with the periodic changes in flame zone. Nonetheless, no warning signs are observed before flashback. Looking into the velocity profiles at the nozzle exit also shows an increase in axial velocity near the center of the nozzle with larger bluff bodies, revealing the mechanism of improvement of burner resistance to flame flashback.