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The generation of sound by flames

The conditions under which laminar flames in tubes can generate sound are investigated. It is shown that gaseous oscillations in a tube are amplified by the presence of a flame, and that the amplification arises through the dependence of the laminar burning velocity on the density and temperature of the combustible gas mixture into which the flame propagates. The theory is compared with observations made by Markstein and good agreement is achieved without the use of adjustable parameters. Some of the observed features of the noise of turbulent flames are interpreted on the basis of the same physical process as occur in tube-flames. In this case amplification is possible in the quadrupole or higher w-pole modes because of the well established property that such modes radiate very little energy if the dimensions of the source, in this case the flame, are small compared to the wavelength of the sound. Comparisons are made with the experimental results of Smith & Kilham (1963).