Estimation of acoustic oscillations frequencies in gas turbine outlet diffuser

Calculated estimation of acoustic resonances in the flow part of gas turbines in the outlet diffusers is a current problem. Its solution will improve vibration reliability of essential elements of gas turbine plant (GTP) – the rear bearing housing support and the blade apparatus. At present there are only a limited number of calculated and experimental works devoted to the study of acoustic resonance effect in the output devices of gas turbines. The objective of this paper is to estimate frequencies of acoustic oscillations in the GTP output diffuser. It allows to avoid coincidence of the indicated frequencies with the frequencies of natural oscillations of GTP structural elements. The results of the analysis of frequencies of acoustic oscillations should be confirmed in further experimental studies of the specified output device, i.e., the output diffuser. The method to solve the problem is computational. The results of numerical calculations of acoustic resonances in the GTP outlet diffuser are used for the analysis. The calculations have been performed in the Modal Acoustics ANSYS Workbench program. The model is described and the computational program to calculate the acoustic modes in the output diffuser is specified. The boundary conditions to perform the specified numerical acoustic calculations are proposed. The modes and corresponding frequencies of acoustic oscillations that are possible in the GTP outlet diffuser are considered. Justification of validity of the results of numerical calculations is their partial comparison with analytical solution for the simplest acoustic model, as well as with the preliminary result of the experimental research. The results of the acoustic calculations of the model of the GTP outlet diffuser allow to determine the modes of acoustic oscillations, and to estimate the frequency range of these modes. To substantiate the calculated estimates of frequencies of acoustic pressure pulsations, as well as to clarify the amplitudes of these pulsations, experimental aerodynamic studies of the model of the outlet diffuser with a rotating turbine stage at the inlet are necessary. Refined calculation model of acoustic resonances in the GTP outlet diffuser will contribute to the offset of frequencies of acoustic gas vibrations from the natural frequencies of vibrations of the GTP structural elements, adjacent to the diffuser. The indicated frequency offset will improve the GTP vibration reliability at the design stage.