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Flame Driving of Longitudinal Instabilities in Liquid Fueled Dump Combustors.
Final rept. 1 Jul 84-30 Jun 87,
GEORGIA INST OF TECH ATLANTA SCHOOL OF AEROSPACE ENGINEERING
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This report describes the results of experimental and theoretical investigations of the mechanisms by which the core flow combustion process in coaxial, single inlet, dump type ramjet engines drives longitudinal combustion instabilities. To this end, the behavior of V-shaped flames, similar to those often occurring in ramjet combustors, stabilized in longitudinal acoustic fields has been studied. The presence of burning vortical structures is observed in the flame region. These structures appear at frequencies close to the first natural acoustic frequency of the combustor and are believed to be connected with a shear layer type of instability of the flame. Experiments conducted show that the unsteady combustion in these structures is capable of driving the acoustics at the fundamental acoustic mode frequency. With increase in fuel air ratio, a spontaneous instability involving the fundamental mode is observed and explained in terms of increased driving associated with the higher, unsteady heat release rates. The results of experiments conducted with external acoustic excitation of the flame at different frequencies are also reported and confirm the idea that the vortical structures arise due to a fluid mechanical instability of the flame. It is shown that the interactions between the vortical structures and the systems acoustic field affect the heat release rates from the flame and provide a mechanism for the driving of longitudinal mode instabilities. Keywords Coaxial dump type ramjet, Flame driving, Combustion, Acoustics longitudinal instability, Vortex shedding, Shear layer instability.
APPROVED FOR PUBLIC RELEASE