Dynamics and structure of interacting nonpremixed flames

Interaction between neighboring parallel and nearly parallel flames occurs more frequently as the turbulent length scales become smaller, and when a flame trapped in a large vortical structure spins further inside its core. In this paper, a model to simulate the interaction between neighboring, strained thin flames is proposed, and is used to investigate the mechanism and impact of the interaction for the case of a methane-air flame described by a four-step reduced kinetics mechanism. In the model, the interaction between neighboring strained flames is simplified by combining them into a single stagnation-point flow structure with one of the major reactants trapped in the vicinity of the stagnation plane. We find that in both cases of flames converging toward the oxidizer or the fuel, the interaction does not occur until the reaction zones overlap. The difference between the two cases is caused by the deep penetration of oxygen into the fuel side in the former case as opposed to the rapid decomposition of the fuel as soon as the temperature rise is sufficiently high in the latter case. In both configurations, during the interaction phase, the burning rate increases as the hydrogen radical concentration rises due to the overlap of the two reaction zones, leading to a sudden termination of combustion as the trapped reactant is depleted, followed by an overshoot in the peak temperature. Thus, the interaction between neighboring flames is governed by chemical rather than thermal exchange. The burning enhancement in the first case is connected with the abundance of H in the last stages of oxygen consumption, while in the second case, it is due more to the burning of CO. Results also show that the after-extinction evolution of CO is quite different in two the cases. In the case of flames interacting across the oxidizer, the after-extinction dynamics of CO is similar to that of other major species, while for flames converging toward the fuel, CO after reaching levels higher than encountered in isolated flames, is rapidly consumed. (C) 1998 by The Combustion Institute.