Flame noise: sound emission from spark-ignited bubbles of combustible gas

Abstract

A study has been made of the noise produced by a flame developing in a spherical volume of homogeneous premixed combustible gas confined within a soap bubble and ignited by a spark. Such a spherically expanding flame was shown to act as a monopole source of sound, the pressure waveform being dependent on the rate of change of the rate of increase of volume of the gas during combustion. The rate of volume increase was proportional to the burning velocity of the combustible mixture multiplied by the flame area, and was estimated from optical measurements of flame movement which were made with a schlieren system and a rotating drum camera. Simultaneously the pressure in the sound wave emitted by the flame was recorded as a function of time. For a bubble ignited centrally, good quantitative agreement is obtained over a wide range of conditions between the measured pressure waveforms of the sound and those calculated from the optical records. The pressure amplitude in the sound wave increases markedly with burning velocity. It is shown that the sound pressure level in the radiated sound wave is substantially reduced by eccentric ignition compared with the centrally ignited case. The qualitative features of the pressure waveform obtained when a bubble is ignited eccentrically are adequately accounted for. It is further shown that the maximum fraction of energy of combustion that can be radiated as sound from a hydrocarbon-air flame of burning velocity 60 cm/s is of the order of 10 -6 .