The combined dynamics of swirler and turbulent premixed swirling flames

摘要

The dynamics of premixed confined swirling flames is investigated by examining their response to incident velocity perturbations. A generalized transfer function designated as the flame describing function (FDF) is determined by sweeping a frequency range extending from 0 to 400 Hz and by changing the root mean square fluctuation level between 0% and 72% of the bulk velocity. The unsteady heat release rate is deduced from the emission intensity of OH~* radicals. This global information is complemented by phase conditioned Abel transformed emission images. This processing yields the distribution of light emission. By assuming that the light intensity is proportional to the heat release rate, it is possible to deduce the distribution of unsteady heat release rate in W m~(-3) and see how it evolves with time during the modulation cycle and for different forcing frequencies. These data can be useful for the determination of regimes of instability but also give clues on the mechanisms which control the swirling flame dynamics. It is found from experiments and demonstrated analytically that a swirler submitted to axial acoustic waves originating from the upstream manifold generates a vorticity wave on its downstream side. The flame is then submitted to a transmitted axial acoustic perturbation which propagates at the speed of sound and to an azimuthal velocity perturbation which is convected at the flow velocity. The net result is that the dynamical response and unsteady heat release rate are determined by the combined effects of these axial and induced azimuthal velocity perturbations. The former disturbance induces a shedding of vortices from the injector lip which roll-up the flame extremity while the latter effectively perturbs the swirl number which results in an angular oscillation of the flame root. This motion is equivalent to that which would be induced by perturbations of the burning velocity. The phase between incident perturbations is controlled by the convective time delay between the swirler and the injector. The constructive or destructive interference between the different perturbations is shown to yield the low and high gains observed for certain frequencies.