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Assessment of the 3D Flow in a Centrifugal compressor using Steady-State and Unsteady Flow Solvers
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes. (CCGEx – Competence Center for Gas Exchange)
KTH, School of Engineering Sciences (SCI). (FLOW, KTH Mekanik)ORCID iD: 0000-0001-7715-863X
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Mechanics of Industrial Processes. (CCGEx – Competence Center for Gas Exchange)ORCID iD: 0000-0001-7330-6965
2014 (English)In: SAE Technical Paper, 2014-01-2856, 2014, SAE International , 2014Conference paper, Published paper (Refereed)
Abstract [en]

Numerical analysis methods are used to investigate the flow in a ported-shroud centrifugal compressor under different operating conditions, i.e. several mass flow rates at two different speed lines. A production turbocharger compressor is considered, which is widely used in the heavy automotive sector. Flow solutions obtained under steady-state and transient flow assumptions are compared with available experimental data.

The steady-state Reynolds Averaged Navier-Stokes method is used to assess the overall time averaged flow and the global performance parameters. Additionally, the Large Eddy Simulation (LES) approach is employed to capture the transient flow features and the developed flow instabilities at low mass flow rates near the surge line.

The aim of this study is to provide new insights on the flow instability phenomena in the compressor flow near surge. Comparison of flow solutions obtained for near-optimal efficiency and near-surge conditions are carried out. The unsteady features of the flow field are quantified by means of Fourier transformation analysis, Proper Orthogonal Decomposition and Dynamic Mode Decomposition. For a near optimal efficiency set-up the frequency spectra are broad- banded with no distinct instabilities. Close to the surge line, the spectra show a distinct surge cycle frequency, which is due to flow pulsation in the compressor.

The modal flow decomposition elucidates a mode occurring at the surge frequency. The mode explains the oscillating pumping effect occurring during surge. The surface spectra contours reveal the shape of the pressure pulsation during surge and support that a pressure gradient occurs with the oscillating modes found with the modal decomposition. 

Place, publisher, year, edition, pages
SAE International , 2014.
Series
SAE Technical Paper, ISSN 0148-7191
Keyword [en]
Compressor Flow, Turbocharger, Internal Combustion Engine
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-152344DOI: 10.4271/2014-01-2856OAI: oai:DiVA.org:kth-152344DiVA: diva2:749723
Conference
SAE 2014 International Powertrain, Fuels & Lubricants Meeting,October 20-23, 2014,Birmingham, UK
Funder
Swedish Energy Agency
Note

QC 20141106

Available from: 2014-09-25 Created: 2014-09-25 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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