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. 

 Kraven på effektivitet och stabilitet för centrifugalkompressorer i tunga lastbilar ökar på grund av stränga, vetenskapligt baserade utsläppsmål för skadliga emissioner och den stigande kostnaden för alternativa bränslen som grön vätgas. Dessutom innebär de senaste faserna av EU:s akustiklagstiftning ytterligare utmaningar för konstruktörer som vill hantera tonbrus, särskilt bladfrekvenssignaturer (BPF), i centrifugalkompressorer. Nuvarande metoder för att karakterisera prestanda och BPF-ljud utförs huvudsakligen på stegnivå (fläns-till-fläns), vilket ger begränsad insikt för turbomaskinkonstruktörer och aeroakustiker som vill förbättra eller förnya kompressordesigner. Följaktligen finns det ett behov av att förstå aerodynamisk prestanda och aeroakustiskt beteende på komponentnivå för att underlätta optimal komponentmatchning i linje med tvärvetenskapliga krav. 

Även om prestanda på komponentnivå för centrifugalkompressorer har utforskats i litteraturen, är sådana studier vanligtvis begränsade till en enda designpunkt eller några få punkter utanför design på kompressorkartan. Medan aeroakustisk forskning har undersökt överfört brus i stor utsträckning, finns det jämförelsevis lite fokus på källkarakterisering. Kvantifiering av tonalt (BPF) brus kräver generellt kostsam prototypframställning, omfattande experimentella mätningar och arbetsintensiv dataanalys. Högpresterande simuleringar, såsom Unsteady Reynolds-Averaged Navier-Stokes (URANS) eller Large Eddy Simulations (LES), kan användas för att kvantifiera BPF-brusnivåer, men dessa metoder är opraktiska under kompressorns designfas. Syftet med denna forskning är att identifiera metoder och parametrar, baserade på fysiken bakom tonalbrusgenerering, för att rangordna kompressorkonstruktioner utifrån BPF-brus under bladdesignfasen. 

Den experimentella uppställningen som används för detta arbete omfattar en aero-termodynamisk layout, inklusive en serie tryck- och temperaturmätningar på turboladdarkompressorn vid hetgasanläggningen i Traton AB. Detaljerade tryckmätningar erhölls med hjälp av Kiel-sonder, som korsades för att kvantifiera aero-termodynamisk prestanda på komponentnivå. Den akustiska experimentella metoden utvecklades i gasanläggningen, vilken inkluderar instabila trycksensorer och mikrofoner för att kvantifiera ljudtrycksnivåer (SPL) och ljudintensitetsnivåer (SWL) för både splitter- och fullbladskompressorkonfigurationer.   Detta arbete inkluderar även numeriska simuleringar med RANS-metoder (Reynolds-Averaged Navier-Stokes) för att karakterisera aeroakustiska signaturer. 

Resultat från aerotermodynamiska tester och simuleringar möjliggör riktade designinsatser på specifika komponenter, såsom impeller, diffusor eller volut, och etablerar en robust grund för att undersöka de akustiska signaturerna hos kompressorkomponenter. Acoustic Crest Factor (ACF) visade en korrelation med SPL i experimentella mätningar, eftersom den kvantifierar signalimpulsivitet, den primära tvångsfunktionen för BPF-ljud vid impellers utgång. ACF kan användas för att akustiskt rangordna design för de flesta driftspunkter, förutom den positiva incidensregionen för Splitter-blade-pumphjulet, redan under designfasen, med hjälp av data från RANS-simuleringar. För att hantera begränsningarna med ACF, särskilt i den positiva incidensregionen av kompressorkartan för splitterbladsarrangemang, utvecklades nya faktorer: OSAI och QuIET. Dessa faktorer uppvisar förbättrade korrelationstrender för både splitter- och fullbladskonfigurationer. OSAI och QuIET kan användas för att rangordna nya och befintliga konstruktioner för akustisk prestanda och kan också fungera som kostnadsfunktioner i optimeringsstudier inom aerodynamik och akustik i turbomaskiner som används för olika applikationer. 

 The centrifugal compressor design involves trade-offs involving several geometric parameters. The goals of the design are generally multi-disciplinary and multi-dimensional. Recent legislative demands on the heavy-duty truck industry push the limits for efficiency and noise. Acoustic measurements indicate that the exit noise generated from the rotor is the major contributor to the tonal BPF noise levels. Advanced CFD techniques such as URANS and LES are employed to quantify noise levels. However, these methods demand more resources and time. The objective of this study is to identify a simple parameter that can indicate the impeller exit tonal Blade Pass Frequency (BPF) noise of a centrifugal compressor using RANS simulations and validate the same with experimental measurements. This work introduces a new parameter called acoustic crest factor that can indicate the tonal BPF noise from the impeller exit using the circumferential variation of static pressure at the impeller exit. Acoustic crest factor can predict the trends observed in normalized sound pressure level values measured at the impeller exit for both full and splitter-blade wheels. This is further validated by using acoustic crest factor to rank four different compressor impellers tested in an anechoic chamber with sound power level measurements. This shows that the identified parameter ‘Acoustic Crest Factor’ could be used as a design tool to rank designs based on acoustic signatures and also as an objective function for optimization. 

Centrifugal compressors are widely used in turbocharged powertrains for Heavy-Duty applications. The volute of the centrifugal compressors is generally of the overhung type. The design of volutes is often neglected, and the shape of the volute is dictated by the packaging requirements of the engine. However, with the advent of Hydrogen-based propulsion systems, the efficiency of all the components of the centrifugal compressor stage is important in order to achieve a higher efficiency at the stage level and also to understand the contribution of the volute towards stage performance. Measuring static pressure and total pressure inside and at the exit of the volute is a challenging task due to the complex nature of the flow inside an overhung volute. This study involves an experimental campaign to quantify the volute performance using static and total pressure measurements at different locations on the volute. The total pressure measurements were carried out using calibrated rotating Kiel probes to quantify the flow angles and the corresponding total pressure values. This enabled the measurement of the maximum total pressure value at a location for a given operating condition. These total pressures and static pressures were measured for a variety of operating conditions across the entire compressor map at the inlet and exit of the volute. In addition, two new parameters were introduced, a fictive area ratio and uniformity index that could be used to quantify volute performance and also help match the volute to a given diffuser and impeller configuration.

This paper deals with the component level breakdown of performance characteristics of a centrifugal compressor using detailed measurements. The compressor stage used for Heavy-Duty applications consisting of the impeller, vaneless diffuser, and volute has been experimentally tested in a gas stand. Several static pressure measurements combined with total pressure and temperature measurements on the compressor stage have been carried out. Based on the detailed measurements, the component level aerothermal performance parameters are calculated on the entire compressor map that includes tip speeds from subsonic to supersonic speeds. The effect of individual component performance characteristics and their impact on the design of the entire stage is discussed both at design and off-design conditions. A better understanding of the component level performance aids to design of a compressor stage for increased efficiency, range, and better surge margin. However, this component-level detailed breakdown of the compressor stage for HD truck application is unavailable in open literature. In addition to the detailed measurements, a 1D model of the impeller is created to correlate measurements with predictions iteratively to identify the relevant 1D modeling parameters for varied tip speeds on the compressor map. 

A compressor map with variations in component level characteristics is introduced. This novel approach to plot component level characteristics identify and give context to the critical design parameters such as flow angles, pressure coefficients, and loss coefficients across the entire compressor map along with the conventional performance parameters. Compressor design involves a significant amount of CFD at a stage level, but the decisive importance of component performance, design and analysis are many times overshadowed by 3D CFD, both steady RANS and unsteady RANS/LES/DES. Although the authors support the development of high-fidelity computational tools, validating these computational results with experimental measurements is essential. A detailed analysis of the Scania's in-house designed compressor stage tested in the Scania Gas stand shows that in order to create an optimal design, component level performance assessment is necessary for the impeller, diffuser and volute. The variation of performance parameters along a speed line and across varied tip speeds are shown along with indications for the onset of instabilities at a component level. This is a crucial input for the design of a compressor stage. In addition to detailed analysis of experimental results, this work includes predictions from two different CFD solvers and discussion of deviations between them. Both the CFD solvers can be used to predict performance parameters at a stage level for most parts of the map. For operating conditions close to surge and higher tip speeds (565 m/s), there are major deviations in the CFD solvers' predictions. The impeller pressure rise and efficiency could be predicted well with the CFD solvers for most of the tip speeds. However, deviations between measurements and predictions increase with increasing tip speeds. The diffuser performance is predicted well by one of the solvers. The other solver underpredicts the diffuser and overpredicts the volute performance. These are also important inputs for 1D design and also for positioning of the best characteristics on the compressor map for the optimal operation of the machine. The goal of this work is to quantify performance predictions on a component level accurately, so that stage level optimization is worth executing. Further, such validated models could aid reliable design predictions and an optimization of the entire compressor stage.

This study introduces two new factors for ranking centrifugal compressor impeller designs, encompassing both Full and Splitter types, based on their aeroacoustic blade-passing frequency (BPF) noise signatures across flow regimes from subsonic to supersonic. Building on previous research by the authors, which introduced the Acoustic Crest Factor (ACF) and demonstrated its correlation with BPF noise across most regions of the compressor map, the present work addresses the lack of correlation observed in the positive incidence region for Splitter impellers, attributed to tip vortex effects. The newly proposed parameters, OSAI and QuIET, more accurately capture acoustic trends than ACF, as demonstrated by improved Dynamic Time Warping (DTW) scores. Additionally, a review of the underlying physics of tonal noise generation using the Lagrangian approach indicates that blade loading, traditionally regarded as the primary mechanism of noise generation, constitutes a linear approximation of an inherently non-linear phenomenon. The circumferential variation of static pressure, which reflects this non-linearity, acts as a body force potential across the impeller and contributes to the inherent unsteadiness responsible for tonal noise.