Radial Mode Decomposition in the Outlet of a LP Turbine - Estimation of the Relative Importance of Broadband Noise

Abstract

The sound field in the outlet duct of a high speed low-pressure turbine with three stages was studied to deepen the understanding of its sound generating mechanisms. Special interest was given to the analysis of the two sound field constituents (tones and broadband noise). Three axial sensor arrays were mounted wall-flush downstream of the turbine stage in the annular duct section of the turbine exit. The arrays were positioned at three different azimuthal angles displaced by 120° and traversed azimuthally over 120 degrees in steps of 2 degrees to give a total of 4500 measurement points. Measurements were made at operating conditions from 68% to 93% rotor design speed. Special attention was given to the blade passing frequencies (BPF) of the three turbine rotors. The chosen experimental setup permitted their decomposition into azimuthal and radial modes. With this information, the tonal sound power transmitted upstream as well as downstream could be calculated. The mode analysis results provide a detailed view on the sound interaction processes between the turbine blade and vane rows. Finally, a novel broadband (BB) sound power determination method, previously validated against the ISO 5136 standard method for sound power determination 2 , was applied to the measurement data. The outcome of the BB sound power analysis permitted the comparison of the relative importance of LP turbine tonal and broadband noise.