Due to the increasing demand for higher efficiencies of centrifugal compressors, numerical optimization methods are becoming more and more relevant in the design process. To identify the beneficial features of a numerical optimized compressor design, this paper analyses the influence of arbitrary blade surfaces on the loss generation in a transonic centrifugal compressor. The paper therefore focuses on an analysis of the secondary flow development within the impeller blade passages. To do this, steady simulations were performed on both a baseline and an optimized blade design. Two distinct design features of the optimized compressor stage were identified, which lead to a more homogenous impeller exit flow and thus to an increase in total-to-static efficiency of 1.76% points: the positive lean in the near-tip region and the positive blade curvature in the rear part of the optimized impeller. Furthermore, through extensive experimental investigations conducted on a large scale test rig it has been possible to prove the particular impeller outflow characteristics of the baseline compressor stage.
This paper investigates the generation of rotor-alone tones and their contribution to the outflow noise in a transonic centrifugal compressor stage with vaneless diffuser and volute by means of unsteady full-annulus CFD-simulations. The aerodynamic field, as well as the generation and propagation of sound, were simulated simultaneously using the URANS-approach of the solver TRACE and a numerical grid consisting of 170M cells. To assess the accuracy of the predicted fluctuations, the investigation compares the simulated diffuser flow field to measured flow angles and pressure fluctuations obtained from experiments conducted on a large-scale test rig. The analysis explains the different sound generation mechanisms responsible for tonal components in the acoustic spectrum at the compressor outlet, based on the Fourier decomposition of the pressure fluctuations in diffuser and volute. Further, the paper analyzes the modal structure of the simulated sound field at the volute outlet by means of a radial mode analysis and discusses the influence of changing operating conditions on the sound power emitted. The analyses reveal that supersonic flow phenomena occurring at choked operating conditions cause a significant increase in noise emissions. Furthermore, the investigation shows that the sound field at the volute outlet is dominated by few low-order modes, a fact that justifies analysis using methods based on Compressed Sensing in future experimental investigations.
This paper investigates the generation of rotor-alone tones and their contribution to the outflow noise in a transonic centrifugal compressor stage with vaneless diffuser and volute by means of unsteady full-annulus CFD-simulations. The aerodynamic field, as well as the generation and propagation of sound, were simulated simultaneously using the URANS-approach of the solver TRACE and a numerical grid consisting of 170M cells. To assess the accuracy of the predicted fluctuations, the investigation compares the simulated diffuser flow field to measured flow angles and pressure fluctuations obtained from experiments conducted on a large-scale test rig. The analysis explains the different sound generation mechanisms responsible for tonal components in the acoustic spectrum at the compressor outlet, based on the Fourier decomposition of the pressure fluctuations in diffuser and volute. Further, the paper analyzes the modal structure of the simulated sound field at the volute outlet by means of a radial mode analysis and discusses the influence of changing operating conditions on the sound power emitted. The analyses reveal that supersonic flow phenomena occurring at choked operating conditions cause a significant increase in noise emissions. Furthermore, the investigation shows that the sound field at the volute outlet is dominated by few low-order modes, a fact that justifies analysis using methods based on Compressed Sensing in future experimental investigations.
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