Coupled droplet dynamics: Investigation of acoustic-spray interactions in a reacting flow field

Abstract

Droplet dynamics in a self-excited thermoacoustic instability are not well understood due to the tight coupling of competing effects and difficulty in taking high-quality data. In this investigation, droplet data from a combustion instability is used to illustrate how a spray couples with a thermoacoustic instability. Commonly accepted ensemble averaging techniques such as the Sauter mean diameter (D32) is shown to be inadequate in describing the coupling phenomena, and instead, a new approach based on droplet surface area is used to characterize the spray. The present study extends literature concerning nonreacting acoustic-spray interactions and considers these in a fully self-excited instability. A lean direct injection (LDI) geometry is used at atmospheric pressure and phased averaged Phase Doppler Particle Analyzer (PDPA) measurements, acoustic pressure measurements, and OH* chemiluminescence images are taken to characterize the flow field. It is shown that while some of the trends found in nonreacting experiments apply to reacting systems, the mechanisms are notably different due to the presence of a flame. Flame response is shown to be important due to its influence on the evaporation of droplets.