Anwendung der Infinite-Tube-Technique zur instationären Druckmessung in der Radialturbine eines Abgasturboladers (English shop)

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This work addresses the qualification of unsteady pressure measurement technology for use under the extreme conditions prevailing in a radial turbine with variable turbine geometry (VTG) of an exhaust gas turbocharger. For this purpose, probes based on the principle of infinite-tube measurement technology were designed, calibrated and successfully applied.

Calibration was carried out both in the shock tube and on the turbocharger under cold operating conditions. Whereas only acoustic signals are present in the shock tube, in the turbocharger the signal is composed of hydrodynamic and acoustic components. It was shown that the ITT measurement technique completely damps friction-induced hydrodynamic fluctuations and isolates acoustic effects from them. Exploiting this particular property of the ITT probe, the use of both calibration methods made it possible, in a first step, to develop a so-called “indirect method”. As a result, it was possible for the first time to separate the acoustic and the hydrodynamic signal components using the signals of only two sensors, compared with the three sensors previously required for this purpose (known as the 3-microphone method). With a view to using the measurement technology under hot operating conditions, and against the background that the characteristic determined under cold operating conditions of the turbocharger is limited to a restricted frequency range and thus to a part of the performance map, the “direct method” was additionally developed. By introducing the friction force and linking the experimentally determined characteristics with the velocity triangle, this work was able for the first time to provide a mathematical description in the form of an exponential function, which permits extrapolation of the behaviour as a function of frequency and thus the description of the entire performance map range.

Subsequently, the measurement technology was successfully employed on the turbocharger test bench under hot test conditions and evaluated by means of the transfer behaviour determined in the shock tube. The tests comprised steady operating points as well as transient operating conditions, which were set on the one hand by throttling with the VTG open and on the other hand by varying the rotational speed via VTG adjustment. In the measured spectra, two subsynchronous effects could be identified – a vibration-induced rotor instability due to journal bearing anisotropy and one due to flow-induced turbulence downstream of the turbine rotor. In addition to these effects, the unbalance could be captured, which behaves like a vibration relative to the probe. Forced blade vibrations, excited by the wakes of the VTG and by the potential-theoretical interaction, which were already indicated during calibration, could be tendentially confirmed in the hot measurements. Definitive confirmation, however, can only be provided by measurements carried out in parallel with blade vibration measurements using conventional methods such as BSSM (non-contact blade vibration measurement).