Structures induced by periodic acoustic excitation of a diffusion flame

Signal recovery techniques have been used to measure the periodic structures formed in a coannular diffusion flame due to acoustic excitation of the flow from upstream. The flow response was sensed using the quantitative schlieren technique, and consistent results were obtained on the basis of both axial and transverse optical beam deflections and appropriate analysis of signal records. The two dominant modes of response were identified as a series of alternating disturbances along the flow axis at the higher frequency, and a series of alternating ring disturbances containing on axis disturbances of opposite sign at the lower frequency. The former mode is essentially associated with the fuel jet shear layer, while the latter is associated with the outer annular shear layer surrounding the air flow from the outer nozzle. The strength of disturbances was consistent with mixing fluctuations between cold fuel gas and the products of combustion. Structures induced in the outer shear layer weakened rapidly with distance from the nozzle, indicating a relatively sudden breakup of coherent structures caused by excitation near to the nozzle.