A THEORETICAL STUDY ON NEAR-BLOWOFF CHARACTERISTICS OF RECIRCULATION-ZONE STABILIZED COMBUSTION

A theoretical framework has been developed for recirculation-zone stabilized combustion near lean blowoff, based on the assumption that the flame is stabilized in the adjacent shear layer. Once the flame is stabilized in the shear layer, the fluid approaching the flame is actually a mixture of the free stream and the recirculating flow since the shear layer simultaneously entrains fluids from both sides. If the flame is lean, the recirculating flow would essentially consist of products and excess oxygen but no fuel, the mixture approaching the flame should thus be leaner than the free stream. Accordingly, even if the free stream is flammable, the equivalence ratio of the mixture in the shear layer may be outside of the flammability limits. Analyses show that, for a recirculation-zone stabilized lean flame, the effective equivalence ratio of the mixture approaching the flame stabilized in the shear layer is lower than that of the free stream due to the existence of the recirculation zone. A diagram of regimes for the recirculation-zone stabilized combustion near lean blowoff is then constructed according to the theoretical analyses, which involves four parameters: the freestream equivalence ratio, the shear-layer entrainment ratio, the blowoff limit and the reignition limit. In the diagram, four regimes have been identified: a super-stable flame, a sub-stable flame, an oscillating flame and blowoff. In particular, the identification of an oscillation region in the parameter space is impressive, which introduces an intrinsical instability mechanism that has not previously been noted for near-blowoff flames. In this mechanism, the equivalence ratio oscillations in the shear layer are indeed driven by the combustion process since the position/oscillation of the flame affects the fluid composition entering into the recirculation zone and thus the equivalence ratio in the shear layer. Therefore, the feedback loop between the equivalence ratio oscillations and the combustion process is closed accompanied with periodic blowoff and flashback/re-stabilization of the flame.