Mode Shapes and Dominant Frequency Predictions in a Swirl Stabilized Premixed Air-Methane Combustor Using Modal Analysis and Large Eddy Simulations (LES)

Abstract

Stringent emission regulations force the gas turbine combustor community to come up with new designs. Lean Premixed (LPM) combustion is gaining popularity to meet the emission regulations. However, lean combustion process is prone to other issues like combustion instabilities and noise. Self-excited combustion instabilities in a gas turbine play vital role in the life cycle of combustor, noise generation and pollutant formation. If the instabilities in combustor dominate at natural modes, there are risks of resonance which can lead to bursting damage to the combustors. In the present work, modal analysis is carried out to predict the longitudinal and the transverse modes in a swirl-stabilized premixed methane-air combustor. The geometrical details and the boundary conditions used in this work are described in Broda et al. [1]. In addition to the modal analysis, Large Eddy Simulations (LES) with Flamelet Generation Manifold (FGM) combustion model are carried out to find out the instabilities and their sources. In the large eddy simulation, at the inlet of the combustor, a broadband impedance boundary condition is used. This will consider the effect of upstream travelling acoustic waves at the inlet. The outlet of combustor is specified with non-reflecting boundary condition. The inlet mass flow rate and the temperature conditions are consistent with Broda et al. [1]. The longitudinal and transverse modes predicted by the modal analysis and the dominant frequency predicted in the LES case are compared with the experimentally observed values. The predicted first longitudinal mode at ∼1760 Hz compares well with the experimental value of 1760 Hz. Predicted values of first and second tangential modes at 10459 Hz and 17344 Hz are also in good agreement with the experimental measurement. The dominant frequency predicted by the LES simulation is 1940 Hz. After applying the appropriate correction to this value for the wall heat transfer effect, it is in-line with that obtained from the modal analysis and the experiments. The spectral analysis at different probe location in LES simulation shows higher thermo-acoustic coupling at natural frequencies. In this work, the effect of variation in inlet swirl number and the temperature is also studies. The predicted trends in the change in dominant frequency with the increase in inlet swirl number and inlet temperature are captured accurately. For each condition, calculations were performed for about four flow-through times (around 12 ms) after the flow field had reached to its limit cycle to obtain statistically steady condition.