Combustion oscillations in a ducted burner

Abstract

Unsteady combustion, confined within a duct, is a poorly understood and incompletely documented phenomenon. This is particularly true of the violent, low frequency combustion oscillation encountered in gas turbine reheat systems, commonly referred to as reheat buzz. This thesis describes an investigation, both experimental and theoretical, of oscillatory premixed propane-air combustion in a ducted burner. A wide range of experimental techniques is employed to provide time-resolved information on flame motion and the pressure field within the combustor.

A detailed parametric study is reported of both high frequency acoustic resonances and the large amplitude, low frequency buzz oscillation in which the effects of burner configuration, equivalence ratio and flow velocity are assessed. Important differences are observed in the dependence on these parameters of clearly identified longitudinal modes of oscillation (^150 Hz) and the low frequency buzz (~100 Hz). These suggest important differences in the basic mechanisms which sustain these oscillations, a feature confirmed by frame-by-frame analysis of high speed cine Schlieren photographs.

A simple control volume model of low frequency oscillatory burning is developed which permits important features of the parametric study to be reproduced. Two columns of unburnt mixture and fully burnt gas are linked by a simplified flame zone. Conservation of mass and momentum, supplemented by an empirical phase relationship between pressures at the chamber inlet and exit, provide a plausible simulation without incorporating a detailed model of the unsteady heat release. Possible refinements to this model in respect of turbulent burning are suggested in the light of the insight into the flow/heat release interaction provided by the experiments.

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