Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
The Hurst exponent as a model-free precursor of centrifugal compressor surge
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.). Competence Center for Gas Exchange. (CCGEx / Compressor off-Design)ORCID iD: 0000-0002-4937-8915
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Competence Center for Gas Exchange.
Department of Aerospace Engineering, Indian Institute of Technology, Madras.
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Competence Center for Gas Exchange.ORCID iD: 0000-0001-7898-8643
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We investigate the generalized Hurst exponent, a measure of signal fractality, as an indicator of compression system stability. Tests were run on a centrifugal compressor of a light duty turbocharger on two test rigs: an acoustic test rig and a cold gas stand. While the compressor type is the same on both test benches, the other components of the compression system differ significantly, including the Greitzer B value and the presence of silencers. The Hurst exponent can be used to distinguish between stable and near-surge operation of the compressor independent of the compression system, with values larger than 0.5 at stable operation and values below 0.2 at the surge line. By extending the analysis towards a general Hurst exponent, we can identify a precursor to compressor surge that is also valid for both systems, namely the switch from monofractality to multifractality of the compressor outlet pressure signal. At low compressor speeds, this switch occurs at on the negative slope section of the compressor characteristic, where the system is theoretically fully stable. At higher compressor speeds, it coincides with the switch from negative to zero-slope, where theoretical models also predict compression system instability.

National Category
Fluid Mechanics and Acoustics
Research subject
Energy Technology; Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-206944OAI: oai:DiVA.org:kth-206944DiVA: diva2:1094420
Note

QC 20170510

Available from: 2017-05-09 Created: 2017-05-09 Last updated: 2017-05-10Bibliographically 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. 109 p.
Series
TRITA-MMK, ISSN 1400-1179
Keyword
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

Open Access in DiVA

No full text

Search in DiVA

By author/editor
Kerres, BertrandKabral, RaimoÅbom, Mats
By organisation
Machine Design (Dept.)Marcus Wallenberg Laboratory MWL
Fluid Mechanics and Acoustics

Search outside of DiVA

GoogleGoogle Scholar

Total: 60 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf