Development and application of a new prediction method for thermo-acoustic instability based on acoustic mode coupled with incompressible flow

Abstract

Combustion instability, also known as “humming”, “screeching”, “buzzing”, and by various other names, has been a serious problem in rocket engines since the 1930’s in solid, liquid, and gaseous propellant engines. Generally, it refers to sustained pressure fluctuations of an acoustic nature in a chamber where unsteady combustion takes place. It is essentially a self-excited oscillation, involving a complex interplay between unsteady heat release, the acoustic fluctuation, and the vorticity field, according to experimental observations. In modern propulsion energy systems, combustion instabilities are often established as large amplitude pressure fluctuations, which can result in serious problems. These high pressure fluctuations are often associated with heat release as well as oscillations of the flow in the combustor. The various oscillatory phenomena when coupled often have been analyzed as an acoustic resonance of a system.

This study concerns thermo-acoustic characterization in Rijke tube and cavity afterburners. The objective is to develop a prediction method for thermo-acoustic instability coupled with acoustics and incompressible flow. Rijke tube oscillation is a classic example of thermo-acoustic oscillations. The proposed method is verified with a simple entropy induced acoustic in a Rijke tube and is extended to flow and entropy induced acoustic resonant cases. First, a prediction method was achieved using an acoustic analogy in terms of vorticity and entropy as sources of sound. A well-validated numerical tool has been used for the numerical simulations. Second, a numerical method using Eigen modes has been employed to study the flow and associated sound generation in the Rijke tube. This study is an initial survey of many aeroacoustic problems. The validation of the method was consistent with the computational results. With respect to the compressible method used in the studies, the present numerical method performed well in capturing the essence of the flow visualization and the characteristics. With the numerical method, the two main sources of sound, which are the lamb vector and entropy, can be observed and it is possible to perceive how both sources interplay with each other. In addition, the present numerical method was faster compared to the conventional compressible solver.

Finally, this study was extended to two areas: 1) checking the flow characteristics when a known plate in a wind-tunnel problem has been replaced with a heater. The flow characteristics were compared with that of a Rijke tube with similar conditions in a wind-tunnel problem. Interesting observations have been made regarding the flow characteristics. 2) The noise characteristics of the cavity afterburner, a trapped vortex combustor (TVC), were also studied. Here, the pressure history and the frequency signals at different inlet velocities show that single cavity TVC has high pressures and a high frequency range. The results show that the proposed thermo-acoustic prediction method can predict the instabilities in cavities and ducts. The significance of this research it that it shows the coupled phenomena of acoustics and entropy can be captured through the present numerical method.