The influence of cooling air jets on the premixed flame structure and stability of air-cooled bluff-body flameholder

Abstract

An air jet cooling method is adopted to reduce the temperature of the trailing edge wall of a bluff-body flameholder. The added cooling air jets would rebuild the flow field and reduce the temperature and fuel–air ratio in the near-field wake region of the bluff-body, thereby affecting the flame structure and stability. The influence of cooling air jets on the flame structure and lean blowout limits is investigated in a rectangular premixed combustor using a high-speed image capturing system and a fuel supply system. Experimental results demonstrate that the cooling air jets injected into the wake region could form cooling air vortexes, reduce the fuel–air ratio leading to partial flame extinction, create a flame hollow-out zone at the near-field wake, finally achieve excellent cooling effectiveness. The increasing flow rate of cooling air would shrink the recirculation zone that disappeared until the blowing ratio of 2 and reduce the fuel–air ratio in the near-field wake region. As a result, the flame hollow-out zone gradually disappeared, and the flame base and flame anchoring location moved upstream to the near-wall region; meanwhile, the flame expansion boundary in the near-field wake region was narrowed, the flame intensity was weakened slowly, and the flame stability was reduced considerably. Relatively, the cooling air temperature has little impact on the flame structure and stability. Notably, when the blowing ratio was 1, the increased mainstream velocity and decreased mainstream temperature could enlarge the flame hollowed-out zone, migrate the flame anchoring location in the central symmetric axis downstream, and reduce the flame stability. In addition, as the equivalence ratio increased at blowing ratio of 1, more fuel was entrained into cooling air vortexes, which caused the mixed reactants ignition; therefore, the flame hollowed-out zone was shortened, and the flame base and anchoring location moved upstream.