Date of Award
Mechanical, Automotive, and Materials Engineering
Combustion, Flame, Image Processing, Propagation, Stratification, Tulip Flame
CC BY-NC-ND 4.0
This work reports on an experimental investigation of a flame propagating through a propane-air mixture in a rectangular duct where the ignition end is kept shut. Flame propagation through a homogeneous charge with equivalence ratios 0.8, 0.9, 1.0 and 1.1 was investigated initially. The flames were tested with the exit end fully open and fully closed. In all the cases, propagation of the flame occurs through a series of acceleration and deceleration periods. This movement was termed the “Leap Frog” phenomenon. The flame develops a shape called the “tulip flame” at the first period and “flame inversions” during the subsequent periods. The formation of the tulip flame and inversions occur right after the acceleration period in a sequence, suggesting that the Leap Frog phenomenon is influenced by the Rayleigh–Tayler (R-T) instability. Flame images of the tulip flame occurrence are qualitatively similar to the interface evolution of two fluids during the R-T instability. The pressure variation at the ignition end of the duct correlates well with the tulip and inversion formations; with a peak pressure at the inversions and tulip flame formation positions. The pressure was filtered with a low pass filter of 25Hz. This frequency is less than the first harmonic of the longitudinal acoustic frequency of the duct, suggesting that acoustic pressure oscillations do not heavily influence the Leap Frog phenomenon. Next, the flame propagating through a stratified medium, was examined (initial equivalence at 1.1) for the open–exit end condition. Stratification was achieved by air and propane injections to the duct. Also, a mixture the same as the initial charge was injected to the propagating flame to identify effects of pure flow perturbation. Large quantities of air, fuel and mixture were injected when the flame front was within 200 mm from the injector. The flames extinguished at the first inversion for fuel injections of more than 34.6 mg, while the tulip flame position was unchanged. However, for air injections of more than 37 mg, the flames did not extinguish and the tulip flame position was displaced downstream. The flame behavior was similar to large air injections with mixture injections. However, a threshold mass of mixture could not be determined. The tulip flame was displaced when the flames were within 100 mm from the injector at the start of injection. When less than 34.6 mg of fuel and less than 37 mg of air were injected, the tulip flame was not displaced nor did the flames extinguish; but the position of the first inversion shows a good correlation with the injection timing. These experiments suggest that the flame propagation reverts to the “Leap Frog” phenomenon irrespective of extreme stratification and heavy perturbations, suggesting a single dominating factor influencing the formation of the tulip flame and inversions in all of these extreme conditions. It is postulated that the R-T instability is this factor.
Gallage, Indika, "The Study of a Flame Propagation in a Rectangular Duct with a Locally Stratified Flow-Field" (2016). Electronic Theses and Dissertations. 5906.