Flame Instabilities in Open Ended Tubes

Premixed flame propagation in an open-ended horizontal tube has been investigated. Methane-air flames with equivalence ratio ranging between ϕ = 0.8 – 1.5 and hydrogen addition, RH = 0 – 0.8, were filmed with a high-speed camera and the pressure captured. Two-thirds along the tube, the flames encountered an acoustic field and were subjected to longitudinal oscillations.

Depending on the hydrogen content, significant differences were observed in the behaviour of the flames, which were categorized into 3 different categories, a steady flame, a pulsating flame and an oscillating flame. A steady flame propagated steadily, while a pulsating flame was shortened and elongated due to the oscillating pressure.

For oscillated flames, the flame speed initially decreased as it entered the field, and was then subjected to violent oscillations, resulting in a high acceleration and speed. The accelerations were attributed by the formation of Rayleigh-Taylor instabilities, which existed in the form of spikes of unburned mixture into the burned gasses that coupled with the pressure oscillations.

This satisfied the Rayleigh criteria that the acoustically driven instabilities require the heat release and pressure waves to be in phase. When the hydrogen content was increased, as the flame passed through the acoustic field, the flame speed decreased, and no Rayleigh-Taylor instabilities were observed. This difference in behaviour with hydrogen content was attributed to the higher flame speeds with higher hydrogen concentrations.