Boundary layer flashback of swirl flames
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Flame flashback in the boundary layer of swirling flows is investigated experimentally in a model swirl combustor. The model combustor features a mixing tube with an axial swirler and an attached center body. The findings provide novel insight into the mechanism facilitating boundary layer flashback of swirl flames. Turbulent, lean-premixed flames of methane and hydrogen are studied at atmospheric pressure and bulk flow velocities up to 5 m/s. Hydrogen contents range from 0% to 95% and equivalence ratios range from 0.4 to 1. The focus in the present work is on the upstream flame propagation inside the mixing tube. Stereoscopic particle image velocimetry (PIV) is applied at kilohertz-rate to provide the time-resolved, three-component velocity field. The flame front is detected simultaneously based on the acquired Mie scattering images. Simultaneous high-speed chemiluminescence imaging provides the overall flame shape and global propagation direction. In addition to the planar measurements, a technique capable of detecting the instantaneous, time-resolved, 3D flame front topography is developed and applied successfully. Oil droplets, which vaporize in the preheat zone of the flame, serve as the marker for the flame front. The droplets are illuminated with a laser and imaged from four different views followed by a tomographic reconstruction to obtain the volumetric particle field. The velocity field in the unburnt gas is measured using tomographic PIV. The resulting data include the simultaneous 3D flame front and volumetric velocity field at 5 kHz. Flashback is found to occur in the form of large-scale, convex-shaped flame tongues, which swirl in the bulk flow direction as they propagate in the negative axial direction along the center body wall. Gas dilatation associated with the heat release imposes a blockage effect on the approach flow, which causes a 3D deflection of streamlines. As a result, a region of negative axial velocity forms along the leading side of the flame tongues, which facilitates flashback. These regions of negative axial velocity, already observed in previous studies, are shown to be the result of a predominantly swirling fluid motion as opposed to boundary layer separation or flow recirculation. The effect of hydrogen addition on flashback is investigated. Flashback occurs at significantly leaner conditions for hydrogen-rich flames, but the mechanism driving flashback is found to be independent of the hydrogen content for the conditions investigated in the present work. Quantitative differences in the flame-flow interaction between methane and hydrogen-rich flashbacks are discussed in detail.