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Effects of compression on coherent structures in an enclosure
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. (Laszlo Fuchs)ORCID iD: 0000-0003-1511-2235
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.ORCID iD: 0000-0001-9976-8316
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effects of compression on turbulent swirling flows are studied using Large-Eddy Simulations (LES). In this study, the geometry investigated is a cylinder with swirling motions of dierent strengths with superimposed isotropic turbulence. During compression the evolution of turbulence and vorticity is investigated. During early compression, rapid diusion of turbulence is found. In the later part of the compression an increase of turbulence and vorticity is observed and linked to vorticity-dilatation interaction. It is shown that the swirling motion suppresses turbulence and turbulent anisotropy. The longitudinal integral length scale of the tangential fluctuations is found to be approximately twice the transverse length scale. The longitudinal length scale is largely unaffected by compression whereas an effect on the transverse length scale is observed.

National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-124235OAI: oai:DiVA.org:kth-124235DiVA: diva2:633839
Funder
Swedish Energy Agency
Note

QS 2013

Available from: 2013-06-27 Created: 2013-06-27 Last updated: 2015-04-20Bibliographically approved
In thesis
1. Numerical Investigation of Internal Combustion Engine Related Flows
Open this publication in new window or tab >>Numerical Investigation of Internal Combustion Engine Related Flows
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Internal combustion engines has been used for more than 100 years. The use of the abundant energy supply stored as hydrocarbon fueled unprecedented economic growth. The use of hydrocarbons increased the work output of human labor significantly, thus increasing the economy and prosperity. However, during the latter part of the twentieth century negative consequences of the internal combustion engine has been noticed. Initially the being emissions of local pollutants such as carbon monoxide, nitrogen oxides and unburnt hydrocarbons. These pollutants have to this day in the western world been reduced significantly and further reductions are under way. Thereafter, has the focus been shifted somewhat to global emissions such as carbon dioxide due to the effect on the climate. However, as the most accessible oil resources have been exhausted the price of oil has five folded since the turn of the century, straining the exponential economic growth enjoyed for two centuries.

Heavy duty diesel engine efficiency is still below 50\%, there is thus a need and a possibility to further increase engine efficiency. In this thesis, work has been done to increase the understanding of the flow prior to combustion. A better knowledge of pre-combustion in-cylinder flow would increase the possibility to reduce engine emissions and fuel consumption, through better mixing and lower heat transfer.

The work presented is ordered in such a way that the flow structures created during the intake is presented first. Thereafter, the effect of compression is investigated. Intake flow structures are studied using Large-Eddy Simulations (LES) and experiments on a steady swirl test rig. The effects of compression are studied using simulations of predefined flow structures undergoing compression.

It is found that the flow structures created during intake is qualitatively different depending of intake valve lift. And that a single Swirl Number (SN) is an insufficient quantity to characterize the flow created at low valve lifts, due to high fluctuations. During compression it is found that a high swirl number suppress small scale turbulence while the compression has an increasing effect of axial fluctuations due to vorticity-dilation interaction. Additionally, it is shown that turbulent kinetic energy is introduced in the flow field by the piston in the absence of tumble breakdown.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. viii, 68 p.
Series
Trita-MEK, ISSN 0348-467X ; 2013:15
Keyword
CFD, LES, Engine, Intake, Compression
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-124237 (URN)978-91-7501-824-9 (ISBN)
Presentation
2013-08-20, E2, Lindstedsvagen 3, Kungliga Tekniska Högskolan, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency
Note

QC 20130704

Available from: 2013-07-04 Created: 2013-06-27 Last updated: 2013-07-04Bibliographically approved
2. Creation and destruction of in-cylinder flows: Large eddy simulations of the intake and the compression strokes
Open this publication in new window or tab >>Creation and destruction of in-cylinder flows: Large eddy simulations of the intake and the compression strokes
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis is to increase engine efficiency by studying the flow structures created in an engine cylinder during the intake phase and the effect of the subsequent compression.

The invention of the combustion engine has enabled three centuries of economic growth fueled by energy stored as hydrocarbons. However, during the latter part of the twentieth century negative consequences on health and environment of the combustion engine were observed. In order to reduce emissions without increasing fuel consumption, improved knowledge of all physical processes occurring in the engine are necessary. The aim of this thesis is to increase the understanding of the flow prior to combustion, which can lead to reduced engine emissions and fuel consumption.

Intake flow structures are studied using large eddy simulations and experiments on a steady swirl test rig. Flow acceleration was observed to reduce the swirl coefficient, and higher swirl coefficient was found during valve closing as compared to during valve opening. This implies that the rotation is stronger during the later part of the intake then what has been previously assumed. In addition, the computations show that the volume above the valves has a profound effect on the swirl created during the intake. To take this into account a novel way of calculating the swirl number was suggested. This approach gives a lower swirl number as compared to the commonly used Thien methodology. The effects of compression are studied using simulations of predefined flow structures undergoing compression. The peak turbulence levels were found to be increasing with tumble number and decreasing with swirl. It was noted that compression increased the turbulent fluctuations in the cylinder axis leading to anisotropic turbulence and that a small tilt angle was observed to have a significant effect on swirl homogeneity at top dead center.  In this thesis, a new methodology was proposed and validated for calculation of in-cylinder turbulence for a flat piston.

The results of the thesis enhance the understanding of the dynamic effects encountered during intake as well recognizing that a small tumble component has a strong effect on the flow structures prior to combustion. These results can be used to improve the simplified computational methods used to optimize the engine.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. ix, 109 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2015:03
Keyword
Swirl, Tumble, Compression, Engine, LES, CFD, engine turbulence, engine simulations, intake flow structures
National Category
Fluid Mechanics and Acoustics Vehicle Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-164889 (URN)978-91-7595-471-4 (ISBN)
Public defence
2015-05-08, Sal D3, Lindstedtsvägen 3, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency
Note

QC 20150420

Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2015-04-20Bibliographically approved

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Söder, MartinPrahl Wittberg, Lisa

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