High-amplitude fuel jet forcing has been found to result in dramatic changes to a transitional nonpremixed methane flame shape: over a range of excitation frequencies the flame can be driven to split into a central jet and one or two side jets. The split is accompanied by a partial detachment of the flame from the nozzle exit, a shortening of the flame by a factor of two, and a change in flame color from yellow to blue. The forcing frequencies required to drive the flame to split correspond to the acoustic resonances of the combustor plenum. Under some conditions, the flame bifurcates between a split state and a typical transitional nonpremixed flame.
Flow visualization has revealed that the flame splits in response to side jet formation in the fuel jet. A nonreacting fuel jet was observed to split along both the major and minor axes under strong axial velocity perturbation. At less than one nozzle diameter downstream of the exit, side jets form along the major axis of the elliptic cross-section nozzle and continue to develop until approximately five diameters downstream. Pairs of streamwise vortex structures are observed in the side jets adjacent to the roller. Additional structure is seen in the side jets further from the roller, suggesting that fluid there had been ejected in streamwise structures from previous cycles.
We propose that side jets are the result of a reconnection event involving pairs of streamwise braid structures. The resulting loops then propagate perpendicular to the jet due to self induction. Self induction thus provides the mechanism for convection of fluid far from the jet. The evidence of streamwise vortex structures in the side jets and the position of braid structures relative to the rollers support this hypothesis.