Structure of an acoustically forced turbulent spray flame

Abstract

This article investigates the effects of acoustic forcing on the structure of a confined turbulent spray flame. Combustion is stabilized by a standard six-slit circular flame holder inducing a weak swirl in the plate near field. The local effects of external forcing on the combustion are visualized by time-resolved spontaneous CH emission imaging and laser tomography. External forcing of the air flow reorganizes the large mixing layer structures. These structures control the development of the nonpremixed flame, modifying air entrainment and fuel distribution. The external forcing signal imposes a phase on the shedding of these structures, increasing their coherence. The flame exhibits a dual response: a nonpremixed flame develops periodically in the braids of the nascent structures for all forcing frequencies, while for certain frequencies, the fuel and air are entrained and mixed in the vortex core which ignites further downstream. The response of the spray flame is characterized in terms of three dimensionless parameters, the Strouhal number of the velocity perturbations, the Stokes number of the liquid droplets, and the ratio of droplet evaporation and acoustic times.