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 sensitiv e to spatially coherent structures in a turbulent mixing region. This feature of the system forms the basis of its application in the presen t investigation to the detection of the response of a turbulent diffus ion flame to broad band random excitation. The techniques of minimum m ean square identification and homomorphic deconvolution are used to ex tract the system impulse response from measured correlation functions. It is found that a coaxial turbulent diffusion flame has maximum resp onse at two distinct Strouhal numbers associated with mixing at inner and outer shear layers. The flame responds to pseudo random binary exc itation applied acoustically from upstream by the formation of coheren t mixing structures which form al the nozzle exit plane and correct do wnstream with the flow. The response is approximately linear up to an excitation level that is about 0.8% of the nozzle exit velocity. Highe r 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 br eak up, rather than by direct contributions of fluctuating vorticity i n the shear layer. Although random excitation produces responses at mo re or less constant Strouhal number of all amplitudes, the application of pure tone excitation caused a substantial reduction in the lower S trouhal number of the outer flow structures for levels above the satur ation level of excitation.