Flame and flow dynamics during swirl flame flash-back

Abstract

Flash-back characteristics of lean-premixed syngas swirl flames were investigated using simultaneous OH planar laser-induced fluorescence and stereoscopic particle image velocimetry at a repetition rate of 10 kHz. The syngas consisted of carbon monoxide, carbon dioxide, and hydrogen. A stable burning condition was first reached. While keeping the flow rates of air and other fuel components fixed, the hydrogen flow rate was increased incrementally until the upstream-propagating flame suddenly flashed from the combustion chamber back into the plenum and quenched. There existed a condition at which appropriate changes in air/fuel flow rates could prevent the flame from irreversible flash-back; these conditions defined the recoverable operation limits. Spectral proper orthogonal decomposition results revealed a transition in flow characteristics from the precessing vortex core instability to Kelvin–Helmholtz (K–H) instability under recoverable conditions with increasing hydrogen, closely related to flow symmetry. A linear trend was observed between the bulk velocities under critical conditions and the corresponding laminar flame speeds, indicating a strong correlation between flow instability transition and flash-back limits.