Oscillating flames in open tubes

When a flame passes along a tube that is open at both ends a self induced fluctuating pressure/flow field is created which the flame has to traverse. Here fuel rich (1.1 < ϕ < 1.4) propane-air flames have been filmed travelling along a 20 mm internal diameter quartz tube. Fluctuations in the flame’s progression were observed to increase as the flame propagated, achieving a maximum oscillation amplitude of ±10 mm at 220 Hz that decayed as the flame progressed further towards the end of the tube. The impact of the periodic pressure gradients on the flame shape could be discerned with tongues of unburned reactants pushed into the products as well as the corresponding rapid acceleration of the flame into the unburned mixture. The impact of the fluctuations on flame chemistry was monitored by capturing the CH∗ and C2 chemiluminescence using a high speed colour camera. The CH∗/C2∗ ratio was observed to decrease as the flame was pulled back towards the burned mixture; and increased when the flame was pushed forwards. This was consistent throughout the flame progress even when small oscillations in the flame position were measured. This could be a significant feature of flames in this environment.