Flame Driving of Longitudinal Instabilities in Dump Type Ramjet Combustors

Abstract

Coaxial, dump type ramjet combustors are often prone to combustion instability problems that can seriously affect their performance. This study is concerned with the highly detrimental low frequency instabilities which often occur in such engines and investigates the coupling between wedge shaped flames and longitudinal acoustic fields in an experimental apparatus specifically developed for this purpose. 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 apparatus 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 mode frequency. with increase in fuel-air ratio, a spontaneous instability involving the fundamental mode is observed and explained in terms of increased driv ing 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. The measured data shed new light on flame/acoustic mode interactions in a ramjet combustor-like environment and the limitations of current state of the art theoretical model s of this phenomenon.