Periodic Partial Extinction in Acoustically Coupled Fuel Droplet Combustion

Abstract eng:
This experimental study investigates the response of burning liquid fuel droplets exposed to one-dimensional standing waves created within an acoustic waveguide. Building upon prior droplet combustion studies1 quantifying mean and temporal flame response of several alternative fuels to moderate acoustic excitation, the present work extends recent studies2 pertaining to the discovery of periodic partial extinction of flames during exposure to higher amplitude acoustic forcing. This investigation explores several alternative liquid fuels exposed to a range of acoustic forcing conditions (frequencies and amplitudes) creating sustained oscillatory combustion, periodic partial extinction, and full extinction. Phase-locked OH* chemiluminescence imaging and local temporal pressure measurements allow quantification of the combustion-acoustic coupling through the local Rayleigh index as well as temporal oscillations in the flame standoff distance. An estimation of the oscillatory strain rates experienced by the flames during excitation helps to explain specific relationships among acoustic, chemical, and fluid mechanical/straining time scales that can lead to periodic flame extinction.

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