Vortex dynamics and structures of methane/air jet diffusion flames with air coflow

In this paper, high speed direct/schlieren imaging system together with Particle Image Velocimetry (PIV) system is applied to investigate the vortex dynamics and structures of methane–air coflow diffusion flames. The schlieren and PIV images show that the dynamics of the vortices outside the visible flame are strongly affected by the coflow air velocity. The coflow air is observed to push the initiation point of toroidal vortices from the nozzle exit to downstream. When the vortices are pushed beyond the visible flame height, the typical flame flickering behavior disappears and the flame becomes stable. The critical air flow rate at which the flame stops oscillating is observed to increase with the fuel flow rate. The visible flame shows no obvious change when the coflow flow rate exceeds a critical value. However, the flow pattern and vortices outside the visible flame keep changing at different air flow rates. The velocity vectors and vorticity contours at different air flow rates are presented and analyzed. The shedding frequency of the toroidal vortex is in good agreement with the flame flickering frequency obtained from a photomultiplier.