Flame and Spray Dynamics During the Light-Round Process in an Annular System Equipped With Multiple Swirl Spray Injectors

The ignition process of an annular combustor can be divided in several steps that end with the light-round. This corresponds to the sequence from the ignition of the first injector to the merging of the two flame fronts spreading in the annular system. The present article focuses on this important step, where two arch-like flame branches propagate in the chamber. These two turbulent travelling flames, nearly perpendicular to the combustor backplane, successively ignite the injection units and finally collide head-on and merge. In the present study, light-round of spray flames fed by liquid n-heptane is investigated. A high-speed camera operating at a frame rate of 6000 Hz and equipped with a filter centered on CH* emission is positioned on the side of the annular combustor, at the chamber backplane level and records images of one half of the chamber annulus. Acoustic pressure fluctuations are recorded through waveguide microphones plugged on the chamber backplane and microphones flush mounted in the annular plenum. The behavior of one injector ignited by the passing flame front is examined. One finds that the swirling flame structure formed by each injection unit evolves in time and that the anchoring location changes just after the passage of the travelling flame and during a period of a few milliseconds. This behavior can eventually lead to a flashback of the flame in the injector with possible severe damages. This dynamical phenomenon is described in detail. The propagation of the arch-like flame branch is then investigated. Flame images are used to determine the direction and velocity of the flame front by making use of a PIV like processing. One may distinguish two regions for the flame propagation. One is near the backplane, moving in a purely azimuthal direction, while the other corresponds to the remaining flame motion in the azimuthal and axial directions due to the volumetric expansion of the burnt gases. Filtered light images give some indications on the complex flame structures and on the typical length scales characterizing the moving front. Information is also obtained on the dynamics of the spray by shining a continuous laser sheet passing through one injector central axis and recording the light scattered by the n-heptane spray of droplets. These images are used to determine the influence of the incoming flame front on the evaporating n-heptane liquid droplets. A major result is that the flame modifies the spray much before its leading point reaches the injector unit and that its passage through the spray drastically changes the local droplet concentration and thus the local mixture composition.