OPTICAL-DETECTION OF THE RESPONSE OF A DIFFUSION FLAME TO EXCITATION

The quantitative, dynamic laser schlieren sensing system has a spatial averaging characteristic response that makes it particularly sensitive to spatially coherent structures in a turbulent mixing region. This feature of the system forms the basis of its application in the present investigation to the detection of the response of a turbulent diffusion flame to broad band random excitation. The techniques of minimum mean square identification and homomorphic deconvolution are used to extract the system impulse response from measured correlation functions. It is found that a coaxial turbulent diffusion flame has maximum response at two distinct Strouhal numbers associated with mixing at inner and outer shear layers. The flame responds to pseudo random binary excitation applied acoustically from upstream by the formation of coherent mixing structures which form al the nozzle exit plane and correct downstream with the flow. The response is approximately linear up to an excitation level that is about 0.8% of the nozzle exit velocity. Higher levels of excitation produce saturated amplitude limited responses. It is inferred from the low level at which saturation occurs that the excitation acts by influencing the manner in which the shear layers break up, rather than by direct contributions of fluctuating vorticity in the shear layer. Although random excitation produces responses at more or less constant Strouhal number of all amplitudes, the application of pure tone excitation caused a substantial reduction in the lower Strouhal number of the outer flow structures for levels above the saturation level of excitation.