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Evaluation of centrifrugal compressor performance models using large eddy simulation data
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.).ORCID iD: 0000-0002-4937-8915
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0001-7330-6965
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: urn:nbn:se:kth:diva-199014DOI: 10.1115/GT2016-57169ISI: 000387297500040Scopus ID: 2-s2.0-84991738228OAI: oai:DiVA.org:kth-199014DiVA, id: diva2:1066749
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
In thesis
1. On Stability and Surge in Turbocharger Compressors
Open this publication in new window or tab >>On Stability and Surge in Turbocharger Compressors
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Turbochargers are used on many automotive internal combustion engines to increase power density. The broad operating range of the engine also requires a wide range of the turbocharger compressor. At low mass flows, however, turbo compressor operation becomes unstable and eventually enters surge. Surge is characterized by large oscillations in mass flow and pressure. Due to the associated noise, control problems, and possibility of mechanical component damage, this has to be avoided.

Different indicators exist to classify compressor operation as stable or unstable on a gas stand. They are based on pressure oscillations, speed oscillations, or inlet temperature increase. In this thesis, a new stability indicator is proposed based on the Hurst exponent of the pressure signal. The Hurst exponent is a number between zero and one that describes what kind of long-term correlations are present in a time series.

Data from three cold gas stand experiments are analyzed using this criterion. Results show that the Hurst exponent of the compressor outlet pressure signal has good characteristics. Stable operation is being indicated by values larger than 0.5. As compressor operation moves towards the surge line, the Hurst exponent decreases towards zero. An additional distinction between the long-term correlations of small and large amplitude fluctuations by means of higher order Hurst exponents can be used as an early warning indicator.

Further tests using compressor housing accelerometers show that the Hurst exponent is not a good choice for real-time surge detection on the engine. Reasons are the long required sampling time compared to competing methods, and the fact that other periodically repeating oscillations lead to Hurst exponents close to zero independent of compressor operation.

Abstract [sv]

Turboladdare används ofta på förbränningsmotorer för att öka motorns effekttäthet. Motorns breda driftområde ställer krav på ett brett driftområde för turboladdarens kompressor. Vid låga massflöden blir kompressordriften dock mindre stabil, och surge kan uppträda. Surge innebär stora oscillationer i tryck och massflöde genom kompressorn. På grund av oljud, reglerproblem och risken för mekaniska skador vill man undvika surge.

Det finns indikatorer för att bedöma kompressorns stabilitet på ett gas stand. Indikatorerna är baserade på tryckoscillationer, varvtalsoscillationer, eller temperaturökning i gasen i kompressorinloppet. I denna avhandling presenteras en ny indikator baserad på Hurst-exponenten, beräknad på trycksignalen. Hurst-exponenten är ett tal mellan noll och ett som beskriver vilka typer av långtidskorrelationer det finns i signalen.

Mätningar från tre gas-stand-experiment har analyserats på detta sätt. Analyserna visar att Hurst-exponenten baserad på kompressorutloppstrycket fungerar bra som som surgeindikator. Stabil drift av kompressorn indikeras av att Hurst-exponenten är större än 0.5. När kompressordriftpunkten närmar sig surgelinjen faller Hurst-exponenten mot noll. En distinktion mellan oscillationer med små och stora amplituder kan används för att få en tidig varning.

Analyser av vibrationsmätningar på kompressorhuset vid motorapplikation visar att Hurst-exponenten inte är lämplig som realtidsindikator på en motor. Detta kommer sig dels av att data behöver samlas in under en längre tid än med andra tänkbara indikatorer, dels av att andra periodiska oscillationer i signalen kopplade till motorns naturliga beteende leder till Hurst-exponenter nära noll även vid stabil kompressordrift.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2017. p. 109
Series
TRITA-MMK, ISSN 1400-1179
Keywords
Turbocharger, Radial Compressor, Stability, Surge, Hurst exponent, Fractals
National Category
Mechanical Engineering
Research subject
Machine Design
Identifiers
urn:nbn:se:kth:diva-206737 (URN)978-91-7729-378-1 (ISBN)
Public defence
2017-06-02, D1, Lindstedtsvägen 17, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
CCGEx - Compressor off-Design
Note

QC 20170510

Available from: 2017-05-10 Created: 2017-05-08 Last updated: 2017-05-10Bibliographically approved
2. 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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Sundström, EliasKerres, BertrandMihaescu, Mihai

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