Active Control and Noise Reduction of Turbulent Diffusion Flames with Coherent Structure by Acoustic Excitation

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In this investigation two types of diffusion flames, which are established behind a rear facing semi-circular cylinder in a 2-D combustion tunnel and are characterized by the coherent structure inherent in shear flows, are acoustically excited under the same geometrical and flow conditions. Two kinds of excitation techniques are used ; one is the flat speaker method, in which the sound pressure fluctuation applied by the speaker stimulates directly the Kelvin-Helmholtz instability in the shear layr and promotes the formation of a series of coherent structure, and the other is the fuel supply velocity fluctuation method, in which fluctuation having a prescribed amplitude of fuel flow rate is applied to the fuelflow supply line upstream of the nozzle. In order to elucidate effects of the acoustic excitation on the combustion characteristics, mean concentration profiles and temperature fluctuations are measured. Also an attempt is made to construct algorithm using the PIV-technique, that … More enables optical and planar measurements of the velocity vector profiles with a recirculation zone behind a bluffbody. The results obtained are summarized as follows.(1) addition of the bias-velocity using a rotating mirror method makes discrimination of the velocity vector direction possible. Based on various kinds of observations and considerations, the PIV-algorithm is constructed, which consists of the pattern discrimination, the vector calculation, the error vector elimination and the vector interpolation. It is shown that the proposed PIV-algorithm makes possible picture processing of the reactive turbulent flow field with reverse flows.(2) Although two types of excitation techniques equally enhance the periodicity of the coherent structure, the fuel supply velocity fluctuation method is found to be more effective than the flat speaker method.(3) With increasing the excitation intensity, the scale of organized eddies becomes uniform, the flame region is concentrated into the central zone behind the semicircular nozzle, and the flame length becomes short, indicating the promotion of reaction and combustion intensity. As a result, in the central flame region NOx and CO_2 increase, and CO and HC decrease, whereas in the organized eddy-flame region the very reverse holds, due to the severe cooling effect by the intense engulfing motion of the organized eddies. Less