Acoustic detection of flow anomalies in swirling flows

Swirl is commonly used in gas turbine engines to achieve stable, efficient, and clean combustion. Swirling flows are characterized by vortex breakdown and the emergence of a precessing vortex core (PVC). The PVC is a three-dimensional, time-dependent, hydrodynamic instability rotating about the central vortex core axis. The objective of the current experimental study was to detect the anomalies in swirling flows through acoustic measurements by capturing the effect of these anomalies on the PVC structure and its associated tonal frequency. A radial swirler with a converging-diverging nozzle was employed. The swirl number for this swirler was estimated to be 2.4. Two vane thicknesses were studied; 1.27 mm and 2.54 mm. This was intended as a test for detection of deposition on the vanes. Secondly, non-symmetric blockage was achieved by blocking three consecutive vanes. Narrow-band spectra of internal plenum pressure fluctuations and external sound were measured simultaneously. The swirler with 2.54 mm vanes produced higher PVC frequency at the same flowrate compared to 1.27 mm vanes swirler. The blockage of vanes led to further increase in PVC frequency. The rate of increase in PVC frequency with velocity was also affected by vane-thickness and vane-blockage as were the tones of plenum pressure fluctuations. A technique known as spectral decomposition was employed to successfully segregate the velocity dependent source features from the invariant system features. The Source Spectral Distribution Function clearly highlighted the effect of vane-thickness and vane-blockage. The study demonstrates successful acoustic detection of vane-depositions and vane-blockages in swirling flows by utilizing spectral decomposition and comparison.