Comparative study of downwardly propagating CH4/C3H8—air flames along a closed-end tube

Abstract

Experimental work is reported for premixed flames propagating in tubes. The flames were ignited with a pilot flame and the flame propagation captured with high-speed cameras. Initial measurements were performed characterizing the rig. For downwardly propagating flames to a closed-end, methane and propane were studied. The flames initially propagated steadily, then at approximately a third of the way down the tube, the primary acoustic oscillation sets in, resulting to a change in the flame shape. This was then followed by a plateau of variable length before a more violent secondary acoustic oscillation. In some circumstances, flames were observed to rotate due to the primary acoustic instability. The flame front position growth rate for both methane and propane were similar despite the differences in the fuels. The total acoustic loss time for propane and methane increases from the lean limit with the equivalence ratio, peaks at φ = 1.1 and then decreases as the mixture becomes richer. There was also an increase in the total acoustic loss time as the angular speed of the flame increased. The results showed that the generation of acoustic energy for propane was smaller than that of methane due to the stronger natural damping effect of the former.