Combustion and Flame, Vol.189, 46-61, 2018
Periodic partial extinction in acoustically coupled fuel droplet combustion
This experimental study explored the response of burning liquid fuel droplets to one-dimensional acoustic standing waves created within a closed, atmospheric waveguide. Building upon prior droplet combustion studies quantifying mean and temporal flame response of several alternative fuels to moderate acoustic excitation (Sevilla-Esparza et al., 2014), the present work focused on higher amplitude acoustic forcing observed to create periodic partial extinction and reignition (PPER) of flames enveloping the droplet. Detailed examination of ethanol droplets exposed to a range of acoustic forcing conditions (frequencies and amplitudes in the vicinity of a pressure node) yielded several different combustion regimes: one with sustained oscillatory flames, one with PPER, and then full extinction at very high excitation amplitudes. Phase-locked OH* chemiluminescence imaging and local temporal pressure measurements allowed quantification of the combustion-acoustic coupling through the local Rayleigh index. Similar behavior was observed for JP-8 and liquid synthetic fuel derived via the Fischer-Tropsch process, but with quantitative differences based on different reaction time scales. Estimates of the mean and oscillatory strain rates experienced by the flames during excitation assisted with interpreting specific relationships among acoustic, chemical, and fluid mechanical/straining time scales that can lead to a greater understanding of PPER. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Keywords:Acoustically-coupled combustion;Droplet combustion;Periodic partial extinction and reignition