Time-resolved flow field investigation in an industrial centrifugal compressor application involving TR-PIV synchronized with unsteady pressure measurements

We report on combined velocity and unsteady pressure measurements obtained on an radial compressor with vaneless diffuser and asymmetric volute. Time-resolved PIV recordings were acquired at 26 kHz both upstream of the impeller as well as within the vaneless diffusor at several rotation speeds at clean conditions and prior to the onset of instabilities within the rotor. The velocity data was acquired with a high-repetition rate, double-pulse laser system consisting of two combined DPSS lasers and a high-speed CMOS camera that was synchronized with multi-point unsteady pressure measurements. Details on the facility, the utilized instrumentation and data processing are provided with particular focus on the spectral and coherence analysis. Power spectra obtained from time records of the inlet velocity and unsteady pressure reveal an increase of low-frequency fluctuations below the blade passing frequency and the occurrence of a mode-locked behaviour indicating the presence of rotating instabilities. High levels of correlation between velocity and unsteady pressure signals not only confirm the temporal coherence of the acquired data but also reveal a direct coupling between flow field and pressure signature that is more prominent upstream of the rotor rather than in the diffusor.