Abstract
Centrifugal turbomachines of smaller sizes operating at higher speeds have become pervasive due to the increased specific power and reliability achieved by improvements in manufacturing, materials and computational methods. The presence of these small turbomachines, specifically compressors, in helicopters, unmanned aerial vehicles (UAVs), auxiliary power units (APUs), turbochargers and micro gas turbines necessitates superior aerodynamic performance over a broad operational range which is widely achieved by ported shroud casing designs. In addition to aerodynamic performance, acoustic emissions have become a critical aspect of design for these small centrifugal compressors due to high operational speeds. Therefore, in this thesis, a high-speed turbocharger compressor with the ported shroud (PS) casing treatment is used as a subject to understand the flow-induced noise in high-speed centrifugal machines using high-fidelity numerical (CFD) methods. Furthermore, the impact of PS design and operating speed on the acoustic emission of the compressor is also established by comparing the operation of PS open and PS blocked compressor configurations at 99 krpm and 130 krpm speedlines.The numerical model to predict the acoustic characteristics of the compressor is developed and validated by comparing the acoustic and performance results with the experimental values. The impact of various critical parameters on the performance and acoustic predictions is quantified by exploring a range of statistical and scale resolving methods of turbulence formulations along with their sensitivity to spatial and temporal resolution. The results demonstrate the need for higher spatial resolution for scale resolving models to yield credible acoustic predictions.
The results from the selected numerical configuration are analysed to establish the relationship between the flow field and the acoustic characteristics of the compressor. The acoustic spectra for the design point are seen to be dominated by a characteristic ‘buzz-saw’ or Rotating Order (RO) tonal noise. These ‘buzz-saw’ or RO tones are confirmed to be caused by the sonic conditions on the leading edges of the impeller blades. For the near surge operation, the low-frequency broadband features associated with near surge operation are alleviated by the PS casing treatment and are not observed in the corresponding spectra. Furthermore, the characteristic ‘whoosh’ noise is also not observed in the spectra of either design or near surge points.
| Date of Award | 1 Oct 2019 |
|---|---|
| Original language | English |
| Supervisor | John Allport (Main Supervisor) & Simon Barrans (Co-Supervisor) |