Study on Mechanism of Vortex Bursting

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Flame development along a straight vortex is experimentally studied to elucidate the effects of the maximum circumferential velocity and the density ratio of the flame and to compare with theories and numerical simulations. A pair of straight vortices is produced in nitrogen-diluted stoichiometric hydrogen-oxygen mixtures with density ratios of the flame ranging from 5.1 to 8.0 by a piston. The velocity field measured by the particle image velocimetry (PIV) shows that the vortex tube has a mean maximum circumferential velocity ranging from 18.0 m/s to 35.8 m/s and has a mean core diameter ranging from 5 mm to 6 mm. One of the vortices is ignited at the core by a focused laser light of 193 nm in order not to disturb the flow field. It is found that the flame propagates along the axis of the straight vortex with a speed much higher than that in the radial direction as well as that in the quiescent mixture. The axial propagation velocity increases in time and becomes nearly constant when … More the half-axial length of the flame becomes larger than the core diameter of the vortex tube. The axial propagation velocity at the steady state is roughly proportional to the maximum circumferential velocity and to the density ratio minus unity. The axial propagation velocity at the initial stage increases with the square root of the half-axial length of the flame as well as the maximum circumferential velocity and the density ratio minus unity. Numerical simulation of the same configuration as the experiment proves that the baroclinic effect produces azimuthal vorticity at the early stage of propagation, and that convection and stretch effects dominate the production at the later stage and as a result they provoke the flame to propagate along the vortex. Above results are presented at the IUTAM Symposium on Geometry and Statistics of Turbulence (1999), the First International Conference on Computational Fluid Dynamics (2000), the Third International Symposium on Scale Modeling, (2000), and the Thirty-Eighth Symposium (National) on Combustion (2000). Less