Study of Aero-Thermodynamic and Aero-Acoustic Aspects of Centrifugal Compressors

Abstract

 The efficiency requirements for centrifugal compressors in heavy-duty truck powertrains are increasing due to stringent, science-based tailpipe emission targets and the rising cost of alternative fuels such as green hydrogen. Furthermore, recent phases of EU acoustic legislation pose additional challenges for designers seeking to address tonal noise, specifically blade passing frequency (BPF) content, in centrifugal compressors. Current methods for characterizing performance and BPF noise are predominantly conducted at the stage level (flange-to-flange), which provides limited insight for turbomachinery designers and aero-acousticians seeking to improve or innovate compressor designs. Consequently, there is a need to understand aerodynamic performance and aero-acoustic behavior at the component level to facilitate optimal component matching in line with multidisciplinary requirements. 

Although component-level performance of centrifugal compressors has been explored in the literature, such studies are typically limited to a single design point or a few off-design points on the compressor map. This research aims to quantify the component-level performance of a centrifugal compressor using detailed measurements at varied operating conditions. While aero-acoustic research has extensively examined transmitted noise, there is comparatively little focus on source characterization. Quantifying tonal noise at BPF generally requires costly prototyping, comprehensive experimental measurements and labor-intensive data analysis. In the open literature, high-fidelity simulations, such as Unsteady Reynolds-Averaged Navier-Stokes (URANS) or Large eddy simulation (LES), have been used to quantify BPF noise levels, but these approaches are impractical during the compressor design phase. The objective of this research is to identify methods and parameters, grounded in the physics of tonal noise generation, for ranking compressor designs according to BPF noise during the blade design phase. 

 The experimental setup consisted of an aero-thermodynamic configuration incorporating a series of pressure and temperature measurements on the turbocharger compressor at the hot gas stand facility in Traton AB. Detailed pressure data were collected using Kiel probes, which were traversed to quantify component-level aero-thermodynamic performance. The acoustic experimental methodology was developed in the gas stand facility, which includes unsteady pressure sensors and microphones to quantify sound pressure levels (SPL) for both splitter and full-blade compressor configurations. This work also includes numerical simulations using compressible RANS (Reynolds-Averaged Navier-Stokes) methods to characterize aero-acoustic signatures. A method for ranking designs based on impeller exit BPF noise has been introduced and validated using sound power level (SWL) measurements in an anechoic test chamber. 

Results from aero-thermodynamic testing and simulations enable targeted design efforts on specific components, such as the impeller, diffuser or volute, and establish a robust basis for investigating the acoustic signatures of compressor components. In addition, the results were plotted on a compressor map using iso-contours to show variations in component-level performance parameters. Two new parameters, namely the fictive area ratio and the uniformity index, can aid in the effective matching of an overhung volute to an impeller diffuser arrangement. For the aero-acoustic part, three new parameters, namely, ACF, OSAI and QuIET, have been introduced. The Acoustic Crest Factor (ACF) showed a correlation with SPL in experimental measurements, as it quantifies signal impulsiveness, the primary forcing function for BPF noise at the impeller exit. ACF can be used to acoustically rank designs for most operating points, except the positive incidence region of the Splitter-blade impeller, already during the design phase, using data from RANS simulations. To address the limitations of ACF, particularly in the positive-incidence region of the compressor map for splitter-blade arrangements, new factors were developed: OSAI and QuIET. These factors exhibit improved correlation trends for both splitter and full-blade configurations. OSAI and QuIET can be used to rank new and existing designs for acoustic performance and can also serve as a cost function in optimization studies in aerodynamics and acoustics in turbomachinery used for various applications. 

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