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Analysis of vaneless diffuser stall instability in a centrifugal compressor
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx).
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2017 (English)In: 12th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2017, KTH Royal Institute of Technology, 2017Conference paper (Refereed)
Abstract [en]

Numerical simulations based on the Large Eddy Simulation approach were conducted with the aim to explore vaneless diffuser rotating stall instability in a centrifugal compressor. The effect of the impeller blade passage was included as an inlet boundary condition with sufficiently low flow angle relative to the tangent to provoke the instability and cause circulation in the diffuser core flow. Flow quantities, velocity and pressure, were extracted to accumulate statistics for calculating mean velocity and mean Reynolds stresses in the wall-to-wall direction. The paper focuses on the assessment of the complex response of the system to the velocity perturbations imposed, the resulting pressure gradient and flow curvature effects.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017.
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-211880Scopus ID: 2-s2.0-85021790927OAI: oai:DiVA.org:kth-211880DiVA, id: diva2:1131612
Conference
12th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2017, Quality Hotel Globe Stockholm, Sweden, 3 April 2017 through 7 April 2017
Note

QC 20170815

Available from: 2017-08-15 Created: 2017-08-15 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. p. 230
Series
TRITA-MEK, ISSN 0348-467X ; ISRN KTH/MEK/TR-17/12-SE
Keywords
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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