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.

Further experimental results on the structure and acoustics of turbulent jet flames

“…The visible flame region (high luminosity flame surface) is seen in fig. 8 to expand with increases in the mean flow rate, consistent with previous observations (Hurle, et al 1968;Strahle and Shivashankara 1973;Shivashankara, et al 1973). All of the above observations conform to the general idea that noise generation in premixed flames is due to In non-premixed combustion, the highly distorted waveforms (sharp gradients) suggest some form of abrupt volume expansion, in the source region, randomly distributed in space and time.…”

“…2, 6) is at variance with the earlier findings of Smith and Kilham (1963), Strahle andShivashankara (1973), Hurle, et al (1968), all of whom reported broad peaks. (On a dB scale, the peaks seen in fig.…”

“…There appears to be a general agreement that the local volumetric reaction rate in the combusting field is directly related to the noise production. In the premixed situation, a one-to-one correlation between the fluctuations in the (optically measured) volumetric reaction rate and the noise output was demonstrated by Hurle, et al (1968). Their results on this important aspect were extended over a much wider range by Shivashankara, et al (1974).…”

“…Here, we restrict ourselves to the noise problem, where many aspects of the problem have been clarified in recent times (Strahle and coworkers 1971,,;, Smith and Kilham 1963, Kotake and Hatta 1965, Knott 1971, Hurle, et al 1968, Giammar and Putnam 1970. An examination of the literature indicates that the following points are fairly clear.…”

Autoignitions and detonations in engines and ducts

“…At a sufficient distance, d, from the autoigniting hot spot, the pressure pulse approximates that from a monopole sound source. Simple acoustic theory gives the associated instantaneous sound pressure, P(t), above the ambient at time t as [12] …”