Characteristics of flame propagation in a vortex core: Validity of a model for flame propagation

Characteristics of flame propagation for methane-, propane-, and hydrogen-air mixtures and the validity of a model for flame propagation in a vortex core were investigated experimentally using a vortex ring generated by pulsing a quantity of the mixture through a circular nozzle. The experimental results show that the ratio of the flame speed to the maximum tangential velocity of the vortex core decreases as the maximum tangential velocity is increased. This experimental observation agrees qualitatively with predictions of a model that considers the shape of the flame tip in the vortex core. The flame speeds calculated by the proposed model are proportional to the square root of the ratio of the density of the unburned gas to that of the burned gas (rho u/rho b)(1/2), which agree with the experimental results. The present predictions better match the experimental results than those calculated using Chomiak's model. The constant of proportionality between flame speed and (rho u/rho b)(1/2) depends on the type of fuel used. This experimental observation is not predicted by Chomiak's model. The flame speed in the vortex core can be predicted by a model of flame propagation taking into account the shape of the flame tip. The calculated flame speeds, however, are still higher than those obtained in experiments. In order to predict the flame speed exactly, not only the shape of the flame tip in the vortex core, but also the effects of baroclinic torque, curvature of the vortex core, unsteadiness of the propagation velocity, and structure of the flame in the vortex core should be considered. (C) 1997 by The Combustion Institute.