Acoustic modification of sooting combustion

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Martin, Karl Matthew

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Acoustic-combustion interactions have interesting potential as a pollution control technology. Studies by a number of authors have shown it to be effective in reducing emissions of NOx and CO from flames, and suppression of soot in flames has been indicated qualitatively. In order to understand the interaction of high intensity acoustics with sooting flames, a laminar co-flow acetylene burner apparatus was developed which allowed a gaseous jet flame to be acoustically driven at sound pressure levels above 140 dB. It was found that the acoustic field could somewhat increase soot emissions from the flame, and could also completely suppress soot emissions from the flame, depending on the frequency and intensity of the acoustic driving. Acoustic driving changed the shape of the flame, creating a pulsed shape. High speed video of the pulsations showed that the acoustic driving was imposing a velocity on the flame that was proportional to, and of the same order as, the first-order RMS acoustic particle velocity. This velocity is comparable to, or greater than, the fuel jet velocity. Spectrometry measurements performed on the flame showed that the temperature of the flame increased with increasing acoustic forcing. Extinction measurements in the flame were used to reconstruct the radial distribution of soot in the flame. The measurements show quantitatively that, just as with the emitted soot, low power acoustic forcing increased the in-flame soot concentration somewhat, but high power forcing suppressed soot formation almost completely. The in-flame soot profile and the spectrometry measurements were also performed on partially premixed flames, where air was mixed with the fuel to create equivalence ratios from infinity (no premixing) to 3. The results from the two flames were quite similar, demonstrating that the primary effect of the acoustic driving on the flame is to premix air into the fuel just before it burns. While a detailed analysis of the flow patterns in the burner was not performed, the preponderance of the available data indicates that the acoustic driving is causing a synthetic jet flow pattern, which draws air from the burn zone into the fuel tube and premixes it with the fuel before the mixture burns.