Characterization of an Acoustically Self-Excited Combustor for Spray Evaporation

Abstract

A combustor with a divergent inlet passage and a coaxially movable bluff-body flame holder has been developed to enable self-excitation of high-amplitude acoustic oscillations, for the purpose of enhancing the rate of evaporation of water, sprays. Commercially available liquid petroleum gas is used as fuel. The acoustic pressure levels and modal contents are controlled by varying the location of the bluff body along the axis of the combustor relative to its inlet. Locating the bluff body closer to the inlet yields higher acoustic pressure amplitudes over a wider range of fuel-air ratios. The excitation of high amplitudes is accompanied by a shift from the fundamental to the first harmonic at different fuel-air ratios for different air mass flow rates in the fuel-lean range. When high sound pressure levels are excited in the combustor, the heat transfer due to water cooling of the combustor walls and across the walls of the tailpipe increase considerably, up to 40% for an increase in sound pressure levels from 145 to 160 dB. Water-spray evaporation experiments are performed with low and high sound pressure levels of acoustic excitation for different water injection rates over a range of mass flow rates of fuel and air. For a given mass flow rate of air and fuel, it is found that the water evaporation rate increases significantly above a certain threshold acoustic pressure, up to 107% for a sound pressure level of 156 dB, despite the considerable increase in heat loss due to the acoustic oscillations.