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Flow instabilities in centrifugal compressors at low mass flow rate
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). (Competence Center for Gas Exchange)ORCID iD: 0000-0002-0543-5148
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
Keyword [en]
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: urn:nbn:se:kth:diva-217821ISBN: 978-91-7729-555-6 (print)OAI: oai:DiVA.org:kth-217821DiVA, id: diva2:1157882
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
List of papers
1. Assessment of the 3D Flow in a Centrifugal compressor using Steady-State and Unsteady Flow Solvers
Open this publication in new window or tab >>Assessment of the 3D Flow in a Centrifugal compressor using Steady-State and Unsteady Flow Solvers
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
Compressor Flow, Turbocharger, Internal Combustion Engine
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-152344 (URN)10.4271/2014-01-2856 (DOI)
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
2. Generation Mechanisms of Rotating Stall and Surge in Centrifugal Compressors
Open this publication in new window or tab >>Generation Mechanisms of Rotating Stall and Surge in Centrifugal Compressors
(English)Manuscript (preprint) (Other academic)
Keyword
Centrifugal Compressor, flow instabilities, rotational flows, rotating stall, surge, compressible Large Eddy Simulation
National Category
Fluid Mechanics and Acoustics
Research subject
Applied and Computational Mathematics
Identifiers
urn:nbn:se:kth:diva-217874 (URN)
Note

QC 20171122

Available from: 2017-11-17 Created: 2017-11-17 Last updated: 2017-11-22Bibliographically approved
3. Centrifugal Compressor: The Sound of Surge
Open this publication in new window or tab >>Centrifugal Compressor: The Sound of Surge
2015 (English)In: 21st AIAA/CEAS Aeroacoustics Conference, 2015, p. 1-17Conference 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. 

Keyword
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:nbn:se:kth:diva-169882 (URN)10.2514/6.2015-2674 (DOI)2-s2.0-84962514109 (Scopus ID)978-1-62410-367-4 (ISBN)
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
4. Evaluation of centrifrugal compressor performance models using large eddy simulation data
Open this publication in new window or tab >>Evaluation of centrifrugal compressor performance models using large eddy simulation data
2016 (English)In: PROCEEDINGS OF THE ASME TURBO EXPO: TURBINE TECHNICAL CONFERENCE AND EXPOSITION, 2016, VOL 2C, ASME Press, 2016Conference paper, Published paper (Refereed)
Abstract [en]

Zero-dimensional (OD) compressor performance models, which consist of several sub-models for different loss terms, are useful tools in early design stages. In this paper, one typical model for centrifugal compressors is evaluated by comparing the loss-terms predicted by the model to data extracted from experimentally validated Large-Eddy-Simulation. The simulations were run on a truck-sized turbocharger compressor with a ported shroud and a vaneless diffuser. Four operating points are considered: One mass flow at design conditions and one mass flow close to surge, on two speedlines. The performance prediction models evaluated are impeller incidence loss, impeller skin friction loss, diffuser skin friction loss, and the tip clearance loss. Results show that the total losses are well-predicted by the model at design conditions. Friction losses are approximately independent of mass flow in the LES data, while the OD model assumes a quadratic increase. The assumption of constant tip clearance loss is validated by the LES data, and the impeller incidence loss model also fits the data well. Due to the ported shroud, most of the losses as calculated by entropy increase occur through isobaric mixing at the impeller inlet.

Place, publisher, year, edition, pages
ASME Press, 2016
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-199014 (URN)10.1115/GT2016-57169 (DOI)000387297500040 ()2-s2.0-84991738228 (Scopus ID)
Conference
ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition, GT 2016; Seoul; South Korea; 13 June 2016 through 17 June 2016
Note

QC 20170119

Available from: 2017-01-19 Created: 2016-12-22 Last updated: 2017-11-17Bibliographically approved
5. Acoustic signature of flow instabilities in radial compressors
Open this publication in new window or tab >>Acoustic signature of flow instabilities in radial compressors
(English)Manuscript (preprint) (Other academic)
Keyword
Centrifugal Compressor, flow instabilities, rotational flows, rotating stall, surge, compressible Large Eddy Simulation
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-217875 (URN)
Note

QC 20171122

Available from: 2017-11-17 Created: 2017-11-17 Last updated: 2017-11-22Bibliographically approved
6. Analysis of vaneless diffuser stall instability in a centrifugal compressor
Open this publication in new window or tab >>Analysis of vaneless diffuser stall instability in a centrifugal compressor
Show others...
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:nbn:se:kth:diva-211880 (URN)2-s2.0-85021790927 (Scopus ID)
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

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