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Centrifugal Compressor: The Sound of Surge
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.ORCID iD: 0000-0001-7715-863X
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.ORCID iD: 0000-0001-7330-6965
2015 (English)In: 21st AIAA/CEAS Aeroacoustics Conference, 2015, 1-17 p.Conference paper, Published paper (Refereed)
Abstract [en]

When the centrifugal compressor operates at low mass flow rates (close to the unstable operating condition called surge), flow instabilities may develop and severe flow reversal may occur in the wheel passage. Under such conditions, noise generation has been reported resulting in a notable discomfort induced to the passengers in the cabin.

The aim with this study is to predict the flow field associated with a centrifugal compressor and characterize the acoustic near-field generation and propagation under stable and off-design (near-surge) operating conditions. The Large Eddy Simulation (LES) approach is employed. The unsteady features in the flow field leading to acoustic noise generation are quantified by means of statistical averaging, Fourier data analysis and flow mode decomposition techniques. The decomposition method is performed inside the rotating impeller region for several stable and off-design (including surge and near-surge) operating condi- tions. The acoustic near-field data are presented in terms of noise directivity maps and sound pressure level spectra.

For the near-surge condition an amplified broadband feature at two times the frequency of the rotating order of the shaft (possible whoosh noise) was captured. However, an amplified feature around 50% of the rotating order was captured as well. These features are present also during the investigated surge operating conditions, but occur at lower amplitudes as compared with the captured low surge frequency of 43 Hz. 

Place, publisher, year, edition, pages
2015. 1-17 p.
Keyword [en]
Acoustic waves, Aeroacoustics, Centrifugal compressors, Centrifugation, Flow fields, Fourier series, Large eddy simulation, Decomposition methods, Flow instabilities, Low mass flow rates, Noise generation, Operating condition, Rotating impellers, Sound pressure level, Statistical Averaging, Acoustic noise
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
URN: urn:nbn:se:kth:diva-169882DOI: 10.2514/6.2015-2674Scopus ID: 2-s2.0-84962514109ISBN: 978-1-62410-367-4 (print)OAI: oai:DiVA.org:kth-169882DiVA: diva2:825712
Conference
21st AIAA/CEAS Aeroacoustics Conference, Meeting Location: Dallas, TX, 22-25 June 2015
Note

QC 20160616

Available from: 2015-06-24 Created: 2015-06-24 Last updated: 2017-11-17Bibliographically approved
In thesis
1. Flow instabilities in centrifugal compressors at low mass flow rate
Open this publication in new window or tab >>Flow instabilities in centrifugal compressors at low mass flow rate
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A centrifugal compressor is a mechanical machine with purpose to convert kineticenergy from a rotating impeller wheel into the fluid medium by compressingit. One application involves supplying boost air pressure to downsized internalcombustion engines (ICE). This allows, for a given combustion chamber volume,more oxygen to the combustion process, which is key for an elevated energeticefficiency and reducing emissions. However, the centrifugal compressor is limitedat off-design operating conditions by the inception of flow instabilities causingrotating stall and/or surge. These instabilities appear at low flow rates andtypically leads to large vibrations and stress levels. Such instabilities affectthe operating life-time of the machine and are associated with significant noiselevels.The flow in centrifugal compressors is complex due to the presence of a widerange of temporal- and spatial-scales and flow instabilities. The success fromconverting basic technology into a working design depends on understandingthe flow instabilities at off-design operating conditions, which limit significantlythe performance of the compressor. Therefore, the thesis aims to elucidate theunderlying flow mechanisms leading to rotating stall and/or surge by means ofnumerical analysis. Such knowledge may allow improved centrifugal compressordesigns enabling them to operate more silent over a broader operating range.Centrifugal compressors may have complex shapes with a rotating partthat generate turbulent flow separation, shear-layers and wakes. These flowfeatures must be assessed if one wants to understand the interactions among theflow structures at different locations within the compressor. For high fidelityprediction of the complex flow field, the Large Eddy Simulation (LES) approachis employed, which enables capturing relevant flow-driven instabilities underoff-design conditions. The LES solution sensitivity to the grid resolution usedand to the time-step employed has been assessed. Available experimentaldata in terms of compressor performance parameters, time-averaged velocity,pressure data (time-averaged and spectra) were used for validation purposes.LES produces a substantial amount of temporal and spatial flow data. Thisnecessitates efficient post-processing and introduction of statistical averagingin order to extract useful information from the instantaneous chaotic data. Inthe thesis, flow mode decomposition techniques and statistical methods, suchas Fourier spectra analysis, Dynamic Mode Decomposition (DMD), ProperOrthogonal Decomposition (POD) and two-point correlations, respectively, areemployed. These methods allow quantifying large coherent flow structures atvfrequencies of interest. Among the main findings a dominant mode was foundassociated with surge, which is categorized as a filling and emptying processof the system as a whole. The computed LES data suggest that it is causedby substantial periodic oscillation of the impeller blade incidence flow angleleading to complete system flow reversal. The rotating stall flow mode occurringprior to surge and co-existing with it, was also captured. It shows rotating flowfeatures upstream of the impeller as well as in the diffuser.

Place, publisher, year, edition, pages
Kungliga Tekniska högskolan: Kungliga Tekniska högskolan, 2017. 230 p.
Series
TRITA-MEK, ISSN 0348-467X ; ISRN KTH/MEK/TR-17/12-SE
Keyword
Centrifugal compressor, flow instabilities, rotational flows, rotating stall, surge, compressible Large Eddy Simulation
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-217821 (URN)978-91-7729-555-6 (ISBN)
Public defence
2017-12-13, D3, Lindstedtsvägen 5, Stockholm, 10:15 (English)
Supervisors
Note

QC 20171117

Available from: 2017-11-17 Created: 2017-11-16 Last updated: 2017-11-24Bibliographically approved

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Semlitsch, BernhardMihaescu, Mihai

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