Low-frequency pressure oscillations in a model ramjet dump combustor

An experimental investigation has been undertaken to understand the nature of low frequency, high amplitude pressure oscillations that occur in ramjet dump combustors. These oscillations, which are commonly called low frequency ramjet instabilities, often cause significant practical problems such as excessive vibration, excessive heat transfer, and inlet unstart. A two-dimensional, variable geometry, dump combustor model has been used to obtain experimental data. To determine the sources of pressure disturbances in the flow field, a cold flow study, utilizing a hot film, was carried out. A reacting flow study included pressure, velocity, and radiation measurements as well as flow visualizations which employed a phase sensitive schlieren technique.; Pressure and velocity measurements were made at various locations in the inlet duct and combustor to determine the pressure and velocity field during an unstable operation. Global C2 and CH radical emission intensities in the combustor were measured to determine the phase relation between heat release rate and pressure in the combustor. The phase sensitive schlieren technique allowed tracking of the flame front during an entire pressure cycle. Holding the equivalence ratio fixed, the combustor geometry and inlet velocity were varied under two different inlet configurations. The parametric variations were used to study the effect of mean fluid residence time on frequency of the oscillation.; The experimental results were interpreted with the aide of acoustic and residence time analyses. The results show that the low frequency instability does not always occur at the acoustic resonance frequencies. Instead, the frequency appears to be controlled by both the acoustics in the inlet duct and the fluid convection inside the combustor. Specifically, the instability period appears to be the sum of the fluid convection time inside the combustor and the corresponding feedback time from the inlet acoustic resonance characteristic. The sustenance of the oscillations during instability seems to be the result of oscillating heat release which occurs at the proper phase which satisfies Rayleigh's criterion.