Combustion instability in axisymmetric dump combustors with uniform ducts on either side of the dump plane and with fully open as well as constricted exits is examined, and flammability and stability limits established as a function of upstream and downstream duct lengths. With the exit fully open, the oscillations depend mainly on the downstream length, while the upstream length affects the strength of the oscillations in combustors with a large downstream to upstream diameter ratio. For small diameter ratios, the upstream length has a decisive influence on the occurrence of instability, The use of a strategically located orifice in the upstream duct and of a coaxial annular quarter-wave tube to suppress oscillations in combustors with acoustically closed and acoustically open upstream ends is demonstrated. Combustion instability is not observed with long downstream ducts constricted at the exit and the possible reasons are discussed. Oscillations observed in short ducts are associated with the acoustic quarter-wave frequency of the upstream duct and occur only when the period of the frequency matches the residence time in the downstream duct, and can be suppressed by the use of an orifice located in the upstream duct.
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