ACOUSTIC EXCITATION OF DUAL DIFFUSION FLAMES WITH COHERENT STRUCTURE BEHIND A REAR-FACING SEMICIRCULAR CYLINDER

In this paper two types of dual diffusion flames formed behind a rear- facing semicircular fuel nozzle are experimentally investigated; one i s the plane diffusion flame consisting of an ordinary laminar diffusio n flame on the lower-velocity side of the fuel nozzle and a lifted fla me on the higher-velocity side, the other the bluffbody stabilized wak e flame composed of two lifted flames on both sides of the fuel nozzle . The CAPLA(computer-aided phase-locked averaging) method is combined with the acoustic excitation to examine not only effects of the acoust ic excitation on the flame structure, but also phase characteristics o f the formation processes of organized eddies and eddy-fames. The acou stic excitation at a constant frequency of 300 Hz is applied by a flat loudspeaker flush-mounted on the higher-velocity side wall of the com bustion tunnel. According to flash shadowgraphs of the flames the acou stic excitation is found to make the outline of eddy-flames clear with increasing the excitation intensity due to the rapid and clear format ion of organized eddies, leaving general features of the flames almost unvaried. Frequency spectra of fluctuating temperature for two kinds of flames show that, at the maximum excitation intensity applied, only a fundamental peak at 300 Hz and some of its harmonics can be observe d without exhibiting any white noise character. Therefore, it is confi rmed that the acoustic excitation is very useful in the phase-locked-a veraging measurements of turbulent flames. The phase-averaged two-dime nsional temperature profiles at eight consecutive phases obtained for two kinds of flames demonstrate clearly the entire process from the fo rmation of an initial organized eddy to the establishment of an eddy-d ame. As a result, the organized eddy-fame in the plane diffusion flame is found to have an average lifetime of about three times the period of the organized eddy formation of 1/300 sec, while the lifetime of th e organized eddy-flame in the bluffbody stabilized wake flame is about twice the period of the organized eddy formation.