Flame propagation along a nonpremixed vortex ring

Characteristics of flame propagation along a nonpremixed vortex ring, which is generated by the ejection of propane by a single stroke motion of a speaker, are investigated experimentally. A Nd:YAG laser is used as the ignition source to prevent it from disturbing the flow fields. A schlieren technique is adopted for flow visualization, through which the dimension and translational velocity of the vortex ring are measured and the ignition location and timing are determined. Flame shapes and flame displacement speeds are measured from ICCD images and high speed photographs. The flame displacement speed along a vortex ring accelerates during the initial stage of flame kernel development and, subsequently, reaches a near constant value. Near the completion of propagation, the flame decelerates and finally extinguishes. Ignition location affects the displacement speed minimally. Steady displacement speed is found to be linearly proportional to the circulation of a vortex ring in much the same way as the flame propagation speed is along a premixed vortex ring. When the circulation of a vortex ring is large, the flame near the central region of the vortex ring is concave in the direction of flame propagation. With low circulation, a convex flame front with nearly uniform intensity is observed. The results suggest that the flame inside a nonpremixed vortex ring propagates along the maximum burning velocity contour, that is, along the stoichiometric contour.