The role of strain rate, local extinction, and hydrodynamic instability on transition between attached and lifted swirl flames

Abstract

The coupling between fluid strain rate, local flame extinction, hydrodynamic instability, and flame lift-off was studied in premixed swirl flames using multi-kHz repetition-rate OH* chemiluminescence (CL), OH planar laser induced fluorescence (PLIF), and stereoscopic particle image velocimetry (S-PIV). Over 50 different combinations of fuel composition (CH4/H2 ratio), equivalence ratio, and reactant preheat temperature were studied, allowing systematic variation of the reactant-to-product density ratio, laminar flame speed, and Lewis number. Depending on the test conditions, the flame could either be stably attached to the nozzle, stably lifted, or intermittently transitioning between attached and lifted states. Transition between stabilization states was linked with the transition between convective instability and absolute instability at the flame base; formation of an m=1 (m denotes the azimuthal wavenumber) globally unstable wave was associated with the lifted flame, which was manifested by a helical precessing vortex core (PVC). The minimum bulk velocity at which the flame was stably lifted was linearly correlated with the laminar flame extinction strain rate, while none of the other commonly reported key parameters governing hydrodynamic instability was able to collapse the data alone. Hence, lift-off was associated with a relatively constant Damköhler number based on the bulk fluid strain rate and extinction strain rate. The roles of local strain and extinction on the transition process were further elucidated by the cases with intermittent lift-off/reattachment. The probability of the flame being in the lifted state was roughly linearly correlated with the degree of local extinction at the flame base while the flame was still in the attached state. Moreover, this probability also was linearly related to the ratio of fluid-dynamic strain rate to extinction strain rate, but not the fluid-dynamic strain rate itself. These results demonstrate the importance of predicting extinction and hydrodynamic stability for predicting the attachment state of swirl flames.