Axisymmetric, time-dependent simulations are performed for a binary, single-step chemically reacting flow in a two-stream, coaxial-jet bluff-body combustor using the Lagrangian vortex-scalar element method in order to study the effect of the large-scale eddies on the mixing and the rate of product formation in the recirculation region behind the bluff body. The method solves the unaveraged time-dependent governing equations by discretizing the vorticity and the scalar field using a number of finite-area vortext and scalar ring elements, respectively. Simulations have been performed at three flow conditions: the annular-flow dominated regime, U_j/U_a chemical bounds 0.62; the transition regime, U+j/U_a chemical bounds 1.04; and the jet dominated regime, U_j/U_a chemical bounds 2.08. In agreement with experimental studies, our simulations show that when the annular flow dominates the field, a short, compact flame is confined to the recirculation rgion with quasi periodic shedding of large packetws of reacting fluid from the recirculation zone. In the transition regime where the velocity of the jet and the annular flow are approximately equal, an intermittient pulsating flame with a reaction zone attached to the bluff body is observed. In the jet dominated regime, the flow experimences much less flutuation and entrainment, and most of the jet fluid is confined to a narrow region close to the centerline where mixing and reaction are most intense. The simulations also reveal that regardless of the velocity ratio, products are found mostly in regions associated with the large-scale vortical structures located on the outskirts of the shear region defined by these structures. The time-averaged product distribution, consistent with experimental results, shows lower overall concentration and is free of local hot spots due to the cyclical nature of the flow field. However, the unsteady field exhibits local hot s pots where the product mass fraction is unity.
展开▼
