Measurements of flame speeds in combustible vortex rings: Validity of the back-pressure drive flame propagation mechanism

Abstract

Using a vortex ring with a large diameter core, the flame speed V f along the vortex axis has been measured for lean, stoichiometric, and rich methane/air mixtures. The maximum tangential velocity V θmax has been measured by hot-wire anemometry and the flame/core diameter ratio d f / d has been roughly measured by schlieren photography. The results show that the flame speed V f increases almost linearly with the maximum tangential velocity V θmax , and its slope is about unity for all three mixtures. With an increase in V θmax , the d f / d ratio is decreased, and it becomes less than 0.5 in the lean mixture, takes a value between 0.5 and 1.0 in the stoichiometric mixture, but remains around unity in the rich mixture. Accordingly, the flame speed is lowered down in the lean mixture for V θmax ≥5 m/s, saturated in the stoichiometric mixture for V θmax ≥10 m/s, but still increased in the rich mixture up to V θmax =14 m/s. Except when the flame speed is lowered down in the lean mixture or saturated in the stoichiometric mixture, the measured flame speeds are in good quantitative agreement with the values predicted by the back-pressure drive flame propagation mechanism, if the mixture is assumed to expand radially. This elucidates the validity of this new mechanism and also indicates the lateral expansion nature of the flame propagation in the vortex ring in contrast to the axial expansion nature of flame propagation in the vortex flows in a tube. A baroclinic torque mechanism has been briefly discussed on the basis of the vorticity equation.