Premixed combustion in a periodic flow field. Part I: Experimental investigation

Abstract

An experimental investigation into the detailed mixing, combustion and ignition processes occurring in a flow field under the influence of resonant acoustic perturbations has been performed. These acoustic perturbations were created by a pulse combustor of the "Helmholtz" type. Cycle-resolved ensemble-averaged measurements of the velocity field, combined with instantaneous images of chemiluminescence and of schlieren, are presented. During the first two-thirds of the resonant cycle the 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 injection. Moreover, the combustion of the fresh reactants begins late in the cycle along the outer edges of the rolled up toroidal vortex, and not in the quiescent regions of the flow. The combustion of the reactants moves into the center of this toroidal vortex where rapid uniform ignition 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 residual hot products, preparing them for a rapid, almost volumetric, combustion process initiated by thermal and/or radical ignition.