An Experimental Study of Acoustically Excited, Vortex Driven, Combustion Instability within a Rearward Facing Step Combustor

Citation

Smith, Duane A. (1985) An Experimental Study of Acoustically Excited, Vortex Driven, Combustion Instability within a Rearward Facing Step Combustor. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xn67-7h10. https://resolver.caltech.edu/CaltechETD:etd-03262008-160540

Abstract

An internal feedback mechanism, capable of sustaining combustion instabilities, is investigated inside a small laboratory combustor in which the flame is stabilized behind a rearward facing step. Pressure and optical measurements are employed to define the acoustic field and heat release rate within the combustor, while shadowgraph records are used to visualize the reacting, kinematic flow field.

The acoustic flow field creates an unsteady flow inside the combustor, which produces an unsteady heat addition. When this fluctuating heat release is in the appropriate phase relationship with the pressure oscillation, energy is supplied to the acoustic field, and the strong acoustic oscillations are sustained.

As a result of the strong acoustic oscillations present during combustion instability, the flow surges into the combustor periodically, and large vortices are formed at the acoustic frequency which produce the large velocity fluctuation in the vicinity of the flameholder. The magnitude of the velocity fluctuation, relative to the mean flow speed, determines whether or not the vortex will form. The frequency of the combustion instability appears to be directly proportional to the amplitude of the velocity fluctuation so that the actual magnitude of the velocity fluctuation determines the instability frequency. The dependence of the instability frequency upon the velocity fluctuation creates the possibility of exciting a large range of frequencies.

Continued existence of the combustion instability depends upon the mean flow speed, fuel type, and fuel-air ratio.

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