Coupling between premixed flame propagation and swirl flow during boundary layer flashback

Flashback of premixed methane-air flames in the turbulent boundary layer of swirling flows is investigated experimentally. The premix section of the atmospheric model swirl combustor features an axial swirler with an attached center-body. Our previous work with this same configuration investigated the flame propagation during flashback using particle image velocimetry (PIV) with liquid droplets as seed particles that precluded making measurements in the burnt gases. The present study investigates the transient velocity field in the unburnt and burnt gas region by means of solid-particle seeding and high-speed stereoscopic PIV. The global axial and circumferential lab-frame flame propagation speed is obtained simultaneously based on high-speed chemiluminescence movies. By combining the PIV data with the global flame propagation speed, the quasi-instantaneous swirling motion of the velocity field is constructed on annular shells, which provides a more intuitive view on the complex three-dimensional flow-flame interaction. Previous works showed that flashback is led by flame tongues. We find that the important flow-flame interaction occurs on the far side of these flame tongues relative to the approach flow, which we henceforth refer to as the leading side. The leading side is found to propagate as a classical premixed flame front relative to the strongly modified approach flow field. The blockage imposed by flame tongues is not limited to the immediate vicinity of the flame base, but occurs along the entire leading side.