Sound emission from open turbulent premixed flames

Abstract

A study of the noise produced by turbulent premixed flames stabilized on open burners is described. It is shown that such flames may be represented acoustically as a collection of monopole sound sources in the combustion zone. The radiated sound pressure is dependent on the rate of change of the rate of increase of volume of the gas during combustion, which varies owing to the turbulence in the flow. The rate of volume increase is proportional to the rate of consumption of combustible gas mixture in the flame. To measure this quantity, an optical technique has been developed which relies on observations of changes in the intensity of emission from the free radicals C$_2$ or CH generated in the reaction zone. Good quantitative agreement is obtained between the radiated sound pressures calculated from these intensity measurements and the values recorded simultaneously with a microphone. A correlation observed between the mean emission intensities of C$_2$ and CH radicals and the total flow rate of combustible gas mixture both in the laminar and turbulent flames, and in their transition region, supports the wrinkled laminar flame concept of turbulent flame propagation.