Periodic partial extinction in acoustically coupled fuel droplet combustion

Abstract

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., Combustion and Flame, 161(6):1604–1619, 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.

A Review of Environmentally Friendly Approaches in Fire Extinguishing: From Chemical Sciences to Innovations in Electrical Engineering

“…Since it is difficult to extinguish flames using low-power sound sources, researchers note the need to use higher power. Thus, in practical applications, it is possible to increase the distance between the flame source and the extinguisher output by using high and very high acoustic power [ 3 , 13 , 14 , 32 , 45 , 46 ]. At the design stage of the acoustic system, it is worth considering the multipoint placement of sound sources so that the acoustic waves are directed towards the flame source.…”

“…They could be installed at several locations, while one or more outputs may have resonant horn tips. This allows the acoustic flux to be directed from multiple outputs to the flame source and, by using constructive interference, the extinguishing efficiency is improved [ 45 ]. Multiple smart components, such as cameras, mobile terminals, and sensors, can be deployed in the environment, as shown in [ 116 ], among others.…”

“…In turn, Bennewitz et al [ 45 ] included extinction criteria for single drops of three different fuels.…”

Dynamics of acoustically coupled single- and multi-port jet diffusion flames

“…The present work builds upon prior studies of the acoustically coupled droplet combustion of neat fuels (Sevilla-Esparza et al 2014;Karagozian 2016;Bennewitz et al 2018) and liquid nanofuel mixtures (Sim et al 2020a), and more recently gaseous methane laminar jets (Sim et al 2020b). The present focus is on laminar gaseous jets emanating from both single-and multiple-port burner geometries, with emphasis on the analysis of the flame dynamics and transitions in the combustion process.…”

“…2014; Karagozian 2016; Bennewitz et al. 2018; Sim et al. 2020 a ) using optical diagnostics, and recently the focus has been on the combustion of gaseous jets emanating from single microjets (Sim et al.…”