Acoustic Excitation of Organized Eddy Structure and Dual diffusion Flames Behind a Rear-Facing Simicircular Cylinder

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Two types of dual diffusion flames established behind a rear-facing semicircular fuel nozzle were experimentally investigated ; one is the plane diffusion flame consisiting of an ordinary laminar diffusion flame on the lower-velocity side of the fuel nozzle and a lifted flame on the higher-velocity side, the other the bluffbody stabilized wake flame composed of two lifted flames on both sides of the fuel nozzle. The computer-aided phase-locked averaging method (the CAPLA method for short) was combined with the acoustic excitation to examine not only effects of the acoustic excitation on the flam estructure, but also phase characteristics of the formation processes of organized eddies and eddy-flames. The acoustic excitation was apploed by a flat loud-speaker flush-mounted on the higher-velocity side wall of the combustion tunnel at a constant frequency of 300Hz.According to flash shadowgraphs of flames the acoustic excitation was found to make the outline of eddy-flames clear with incr … More easing the excitation intensity due to rapid and clear formation of organized eddies, leaving general features of flames almost unvaried. Frequency spectra of fluctuating temperature and ion current intensity for two kinds of flames showed that, at the maximum excitation intensity applied, only the fundamental peak at 300Hz and some of its harmonics were observed without exhibiting any white noise character. Therefore, it was proved that the acoustic excitation is very usefulin the phase-locked-averaging measurements of turbulent flames.Two dimensional phase-averaged temperature and ion current intensity profiles at eight consecutive phases obtained for two kinds of dual flames demonstrated clearly the entire process from the formation of an organized eddy to the establishment of an eddy-flame. As a result, an organized eddy-flame in the plane diffusion flame was found to have an mean lifetime of about three times the period of organized eddy formation of 1/300 sec, while the lifetime of an organized eddy-flame in the wake flame was about twice the period of the organized eddy formation. Less