Visualization of vortex formation in jet diffusion flames

Schlieren-image visualization of the slowly flowing diffusion flames influenced strongly by the effects of vortex formation in the initial flow regime is discussed. It is noted that for initial velocities of up to 1.7 m/s (10-mm nozzle diameter) the inner and the outer vortices of the concentric vortex system, observed in propane flames, rotate in the opposite directions. For velocities greater than 1.7 m/s the boundary layer of a nozzle flow becomes unstable and rolls up to a system of inner vortices which rotate initially in the same direction as the outer ones. Furthermore, in the velocity range of 1.7-3 m/s the rotation direction of the inner vortices is noted to change downstream, reportedly due to the outer vortex motion.