Instability of diffusion flames

The constant frequency oscillation of buoyant jet diffusion flames is a phenomenon that has been studied extensively during the past three decades. In recent experimental and theoretical works, the coherent flow held and flame structures were analyzed successfully. However, the inception of the wave-like flow structures, which result in coherent vortices further downstream, remained not fully understood. In a methane jet diffusion flame, we studied the flow field instability with multipoint laser Doppler velocimetry (LDV) and a linear stability analysis. The LDV experiments yielded information on the frequency, phase Velocity, and location of inception of the instability waves. The influence of exit velocities of both methane and the coflowing oxidizer (air or pure oxygen) was examined. ii linear stability analysis was conducted to study the role of density variations across the flame front. It is evident that absolute instability of the flow field close to the burner rim causes the construct frequency oscillation of the buoyant diffusion flame. Self-excited, axisymmetric wave-like structures propagate up- and downstream of this location.