Ramjet Rumble: The Low-Frequency Instability Mechanism in Coaxial Dump Combustors

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Abstract
The problem of low-frequency instability in ramjet dump combustors is examined. An instability model is presented which asserts that a coupling occurs between two processes, one located in the combustion zone and the other at the choked nozzle. Pressure disturbances are produced as entropy waves convect through tlie nozzle, and entropy waves are generated as the pressure disturbances perturb the combustion zone. An approximate linearized stability theory is developed for the case of near blow-off (which corresponds to maximum rumble ). Both oscillation frequencies and amplification rates are obtained. The theory is used in conjunction with available experimental data to analyze the effect oi combustor-to-inlct area ratio, nozzle-to-combustor area ratio, inlet stagnation temperature, presence of a flameholder, mode of fuel injection, fuel-air ratio, and combustor size on stability. Both frequency calculations and stability analysis agree well with the experimental measurements and observations.

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