Role of inertial forces in flame-flow interaction during premixed swirl flame flashback

This study investigates the flame-flow interaction during a fully-premixed swirl flame flashback from flame-frame-of-reference. To capture the flame front movement during upstream propagation, high-speed chemiluminescence imaging and simultaneous three-component PIV measurements are taken at 4 kHz. The upstream propagation of the flame occurs along a helical path around the center-body. For low-turbulence and high-swirl conditions (Re-h = 4000, Swirl number similar to 0.9), the lab-frame speed of the flame structure remains nearly constant during the period of investigation. Simultaneously, the leading side of the flame tongue retains its topology during propagation. The steady-state propagation behavior of the flame structure and stationarity of the flame topology allows us to make a frozen-flame-surface assumption. Applying space-time equivalence, the three-dimensional flame surface and flow field are reconstructed by shifting and stacking the time-series of the planar flame front profiles and the three-component planar velocity data. Further, the steady flow in the flame frame-of-reference provides a powerful means of investigating the flame-flow interaction. Quasi-pathlines are constructed in the unburnt and burnt regions of the flow field. The motion of the approach flow along a quasi-pathline is analyzed to understand the role of centrifugal and Coriolis forces. It is shown that the tug-of-war situation between Coriolis and centrifugal forces gets disrupted by the dilatation-driven blockage effect from the flame surface. It leads to a radial deflection of the approach flow, which results in reduction in the flame-normal approach flow speed, thereby assisting in the flame propagation. In the burnt gas, the Coriolis Effect bends the pathlines towards the center-body. We show - for the first time - that the azimuthal motion of the flame assists in the upstream propagation of the flame structure. Error assessment shows that the approximations made to construct the flame-surface and the flow-field retains the physics of flame-flow interactions. (C) 2018 by The Combustion Institute. Published by Elsevier Inc.