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Towards understanding the effect of compression on swirl and tumble in the context of turbulence and mixing
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. Scania CV, Sweden. (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 rotational motions of different strength and tilt angles in a generic heavy duty Diesel engine during compression are investigated using Large Eddy Simulations. The main conclusion is that the relative importance of dilatation (relative volume change) decreases whereas the effect of tumble breakdown increases with the tumble number. For rotational motions with similar BDC tumble numbers the presence of swirl has a dampening effect on peak turbulence during compression but not on turbulence level at TDC.In detail, we show that peak turbulence levels are strongly affected by BDC tumble number, while a local maximum turbulence level at TDC was found for a tilted rotational motion with a tilt angle of 61 deg and BDC tumble number of 0.9. The effect of tilt and BDC total kinetic energy on TDC total kinetic energy and mixing is presented. It is observed that a small tilt angle is necessary in order to obtain a radially stratified mixture, e.g. stratified EGR.The combined effect of a rotational strength and tilt on turbulence and mixing during compression is investigated. This knowledge is important both for a better theoretical understanding of the compression process, and for engine development.

National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-164886OAI: oai:DiVA.org:kth-164886DiVA: diva2:806422
Note

QS 2015

Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2015-04-20Bibliographically approved
In thesis
1. 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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