PREMIXED COMBUSTION IN A PERIODIC-FLOW FIELD .1. EXPERIMENTAL INVESTIGATION

An experimental investigation into the detailed mixing, combustion and ignition processes occurring in a how field under the influence of re sonant acoustic perturbations has been performed. These acoustic pertu rbations were created by a pulse combustor of the ''Helmholtz'' type. Cycle-resolved ensemble-averaged measurements of the velocity field, c ombined with instantaneous images of chemiluminescence and of schliere n, are presented. During the first two-thirds of the resonant cycle th e inlet jet forms a well-defined toroidal vortex that is phase-locked with the combustor cycle. This toroidal vortex is responsible for the convection and mixing of the reactants with the residual hot products. Although chemiluminescence is always present in quiescent regions of the combustor, none exists in the region of the reactants during injec tion. Moreover, the combustion of the fresh reactants begins late in t he cycle along the outer edges of the rolled up toroidal vortex, and n ot in the quiescent regions of the flow. The combustion of the reactan ts moves into the center of this toroidal vortex where rapid uniform i gnition and subsequent combustion of the fresh reactants occurs. This flow field is dominated by a strong toroidal vortex which entrains on a large scale and mixes on a fine scale the fresh reactants with the r esidual hot products, preparing them for a rapid, almost volumetric, c ombustion process initiated by thermal and/or radical ignition.