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Effect of swirl/tumble (Tilt) angle on flow homogeneity, turbulence and mixing properties
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. Scania CV, Sweden. (Laszlo Fuchs)ORCID iD: 0000-0003-1511-2235
Scania CV, Sweden.
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0001-9976-8316
KTH, School of Engineering Sciences (SCI), Mechanics.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this work, the effect of swirl to tumble ratio on homogeneity, turbulence and mixing in a generic heavy duty Diesel engine during compression, is investigated using Large-Eddy Simulations. The main conclusion is that the relative importance of dilatation (relative volume change) increases whereas the effect of tumble breakdown decreases with the swirl to tumble ratio.In detail, we show that an increase in tumble raises the peak turbulence level and shifts the peak to earlier crank angles, which in turn leads to higher dissipation. Moreover, maximum turbulence level at top dead center is obtained for a combination of swirl and tumble rather than for pure tumble. Furthermore, it is observed that the peak turbulent kinetic energy displays levels three times greater than the initial kinetic energy of the tumble motion. Thus, energy is added to the flow (turbulence) by the piston through generation of vorticity by vorticity-dilatation interaction. Also, the intermediate swirl/tumble ratios are found to introduce large non-uniformity in the flow field, leading to a non-solid body like rotation. Swirl/tumble (tilt) angles larger than 19 deg are necessary for complete mixing of the gas within the engine cylinder. Taken together, the combined effect of a combination of swirl and tumble turbulence during compression is investigated. This knowledge is important both for engine development as well as more theoretical aspects regarding the breakdown of large scale structures in an engine.

National Category
Fluid Mechanics and Acoustics
URN: urn:nbn:se:kth:diva-164883OAI: diva2:806421

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.
TRITA-MEK, ISSN 0348-467X ; 2015:03
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
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)
Swedish Energy Agency

QC 20150420

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

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