Fritz Kennepohl

Turbine blade/vane interaction noise: acoustic mode analysis using in-duct sensor rakes

The sound field in the outlet duct of a high speed low-pressure turbine was studied to deepen the understanding of the sound generating mechanisms in a three-stage turbine. Special interest was given to the influence of the exit guide vanes (EGV) downstream of the turbine on the noise generation. Six radial rakes carrying ten Kulite-sensor probes each were mounted downstream of the EGVs in the cylindrical duct section of the turbine exit. The rakes were positioned at different azimuthal angles and staggered axially in pairs to avoid probe wake interference. The rakes were traversed azimuthally over 180 degrees in steps of 1.5 degrees to give a total of 240x30 measurement points. All sensor signals were acquired simultaneously with a sampling frequency of 22 kHz and stored digitally for later analysis in the frequency range 0-8.5 kHz. Measurements were made at operating conditions from 63% to 99% rotor design speed. Special attention was given to the blade passing frequencies (BPF) of the three turbine rotors. The chosen experimental setup permits decomposition of the sound field into azimuthal and radial modes. With this information, the sound power transmitted upstream as well as downstream can be calculated for frequencies up to 6 kHz. The results of the mode analysis provide a detailed view on the sound generation mechanisms and interaction processes between the various blade and vane rows. According to Tyler & Sofrin, the noise sources can be separated in rotor/statorand rotor/stator/EGV-interactions with associated azimuthal modes.

Acoustic Mode Decomposition of Compressor Noise under Consideration of Radial Flow Profiles

§To explore the sound generating mechanisms of turbomachinery like turbines or compressors in rig tests, the sound field radiated in the inlet or outlet duct is acquired by means of a sensor array and fitted to a theoretical model of the modal sound field. For this purpose, an analytical model with a uniform radial flow profile is usually deployed. This paper presents the derivation of an alternative sound field model under consideration of a realistic radial flow profile. The new model, which has to be solved numerically, was fitted against experimental data acquired in the inlet of a three-stage low pressure compressor, yielding the amplitudes of all radiated modes. The results are compared with the outcome of the classical radial mode decomposition approach basing on the analytical model.

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

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.

Sound emission of a turbine stage due to an azimuthally periodic mean temperature

It is shown in a simulation that the tonal sound emission of a high-pressure turbine stage may increase considerably when the mean temperature of the inflow is spatially not uniform. The tonal sound power emitted in the downstream direction is increased by 5.9 dB at the blade passing frequency f b and by 11.7 dB at 2 f b . The mean total temperature of the incoming flow is assumed to vary periodically in the azimuthal direction with a temperature amplitude of ±9% as a result of 14 discrete burners inside the annular combustion chamber. The temperature also varies radially with minima at the inner and outer walls of the annular channel. The turbine stage consists of a stator with 70 vanes and a rotor with 73 blades, while the temperature field in the simulation has 14 periods in the circumference.

Sound generation of a turbine stage due to non-uniform mean flow temperatures

The tonal sound emission of a high-pressure turbine stage is investigated numerically. The mean total temperature of the incoming flow is assumed to vary sinusoidally in the azimuthal direction with a temperature amplitude of 9 %. The results show that the sound power emitted in the downstream direction is increased slightly by 0.2 dB at the blade-passing frequency and by a remarkable 2.7 dB at the first harmonic of the blade-passing frequency. The turbine stage consists of a stator with 70 vanes and a rotor with 73 blades while the temperature field in the simulation has 35 periods in the circumference. A non-uniform mean temperature may be a result of the discrete burners inside the annular combustion chamber, a typical situation for an aero-engine.