Many lean-burn combustors are prone to high levels of pressure oscillations resulting in early structural failure. These oscillations have their origins with the natural acoustic characteristics of the combustor flow/geometry and amplification and excitations factors associated with well-mixed flames. The coincidence of the frequency of these excitations with the mechanical vibration modes of the combustor may result in resonance and high cycle fatigue failure. Often with high levels of pressure oscillations the fuel system itself can become coupled driving the dynamics to higher levels. Thus detailed acoustic and mechanical vibration analysis of the combustor becomes important. This paper describes the numerically predicted transient flow characteristics of two configurations of DLN combustor double swirler in contra- and co-rotating arrangements with the sole difference being in the orientation of rotation of the inner nozzle airflow. Although much useful information has been obtained from the previous steady-state analysis, there remain many unresolved issues such as discrepancies of vortex breakdown and acoustic instabilities, which is also important for a final design selection. The transient analysis was performed for each configuration to compare flow instability and acoustic characteristics where the model includes the inlet air annulus, double swirler, main reaction zone and dilution duct. The studies indicate that there is a significant discrepancy in flow structures when the vortex breaks down between the two configurations. And there exists a strong interaction for the remaining swirl with the dilution jets, resulting in the hot core penetrating far downstream inside the transient duct in the co-rotating case. An FFT analysis indicates a significant discrepancy on main low acoustic frequencies and the magnitudes of oscillatory pressure.

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