Abstract
Characteristics of lifted flames in coflow jets with highly diluted propane fuel have been analyzed numerically accounting for the buoyancy effect. In a certain range of fuel jet velocity periodically oscillating lifted flames were predicted within the frequency range of 2.5-4.0 Hz. Stationary lifted flames were predicted when the fuel jet velocity is either relatively small or large. The flame oscillation was maintained by the interaction of the propagation characteristics of a tribrachial flame at the base of a lifted flame and local flow velocity variation by the buoyancy. Visualization using OH and acetone fluorescence techniques substantiated the phenomena. To elucidate the reason for the limited velocity range of oscillation, the responses of representative reaction rate, buoyancy, and convection were analyzed. The results showed that the responses of reaction and convection had out-of-phase interaction in the jet velocity range of lifted flame oscillation. Since the reaction and convection are the indications of flame base propagation to and from the nozzle, respectively, destabilizing interaction maintained the oscillation. Outside this velocity range, the responses have nearly in-phase interaction, thus the stabilizing interaction led the flame to be stationary. Transition behaviors between stationary and oscillating lifted flames were also analyzed, and the transition histories explained in the phase diagram of the reaction and convection.