Quenching mechanism study of oscillating flame in micro channels using phase-locked OH-PLIF

Abstract

This paper presents the development of phase-locked OH-PLIF imaging system and OH 2-line flame temperature measurement for the investigation of quenching mechanism of oscillating flame in external heated micro quartz channels. The oscillating flame is a periodic process of flame auto-ignition at the channel exit, propagation and quenching upstream. The periodic change of CH∗ chemiluminescence intensity of the oscillating flame is captured by a photomultiplier, and the signal is used to produce output trigger with delays for synchronized OH-PLIF imaging of the oscillating flame. The present phase-locked OH-PLIF images of an oscillating flame reveal that the flame front has a similar concave shape as that of the steady flame in the micro channel. Flame initializes at the center of the exit, and then the flame front expands in both the streamwise and spanwise directions during the first stage of the upstream propagation. It is found that the flame tails quenches first, and then the flame head; after the tails of the flame front with local maximum temperature expands to the sidewalls, they are ‘bounced’ backwards to the centerline, which makes the head of the flame front with local minimum temperature hotter and expand faster in the streamwise direction. Therefore, during the final stage of the propagation, the flame front is stretched by the shrinkage of the flame tail to the center and the acceleration of the flame head. Flame temperature measured by the OH 2-line method also shows that there is a temperature rise after the ignition and a decrease during upstream propagation by wall heat losses, followed by another small temperature rise, and finally a decrease leading to flame quenching. Flame velocity measured with time-resolved chemiluminescence imaging also demonstrated the deceleration of flame propagation to quenching of the oscillating flame.