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Creation and destruction of in-cylinder flows: Large eddy simulations of the intake and the compression strokes
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. Scania CV, Sweden. (Laszlo Fuchs)ORCID iD: 0000-0003-1511-2235
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 [en]
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: urn:nbn:se:kth:diva-164889ISBN: 978-91-7595-471-4 (print)OAI: oai:DiVA.org:kth-164889DiVA: diva2:806426
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
List of papers
1. A Coupled PIV-LES Approach to Understand PortGenerated Structures
Open this publication in new window or tab >>A Coupled PIV-LES Approach to Understand PortGenerated Structures
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Inside an engine cylinder the flow field is very complex due to high gas velocities, flowseparation and pressure pulses at the ports. Historically, simple characteristic numbers such as swirland tumble have been used in order to quantify the flow in the cylinder. These integral quantities are likely to be insucient for optimization of the mixing process and combustion in IC engines. Instead,there is a need for detailed data with adequate temporal and spatial resolution. We simulate the flowpast the valves and validate these simulations using PIV measurements. By using LES data upstreamof the measurement plane an explanation to the structures seen in the PIV measurement plane can begiven.Here we show that at low valve lifts, the flow was blocked in such a way that the inertia created at theports were counteracted leading to the formation of two unstable counter rotating vortices. We alsodetected that the fluctuations in swirl number was one order of magnitude larger at the lower valvelifts. Furthermore, the small scale turbulence created at lower lifts was more anisotropic than for thehigher valve lifts. This work has thus increased the confidence in the CFD simulations in addition toproviding an explanation to the structures identified in PIV data.

Keyword
CFD, Engine
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-124233 (URN)
Funder
Swedish Energy Agency
Note

QS 2013

Available from: 2013-06-27 Created: 2013-06-27 Last updated: 2015-04-20Bibliographically approved
2. Compression of a swirling and tumbling flow
Open this publication in new window or tab >>Compression of a swirling and tumbling flow
2013 (English)In: ASME 2013 Internal Combustion Engine Division Fall Technical Conference, ICEF 2013: Fuels; Numerical Simulation; Engine Design, Lubrication, and Applications, 2013Conference paper, Published paper (Refereed)
Abstract [en]

The effect of compression on a swirling/tumbling flow is studied using Large-Eddy Simulations (LES). In this study the geometry investigated is a cylinder with an artificially created swirling/tumbling motion. During compression the evolution of turbulence and vorticity are investigated. An increase of turbulence and vorticity is observed and linked to vorticity-dilatation interaction. It is shown that for swirling/tumbling flows turbulent kinetic energy available at Top Dead Center (TDC) is introduced by the piston through the vorticity-dilatation interaction and that turbulence increases independently of the presence of instability of the large scale flow structures.

Keyword
LES, Engine
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-124236 (URN)10.1115/ICEF2013-19128 (DOI)000359026600034 ()2-s2.0-84902359148 (Scopus ID)978-079185610-9 (ISBN)
Conference
ASME 2013 Internal Combustion Engine Division Fall Technical Conference, ICEF 2013; Dearborn, MI; United States; 13 October 2013 through 16 October 2013
Funder
Swedish Energy Agency
Note

QC 20140912. Updated from manuscript to conference paper.

Available from: 2013-06-27 Created: 2013-06-27 Last updated: 2015-09-10Bibliographically approved
3. Effects of compression on coherent structures in an enclosure
Open this publication in new window or tab >>Effects of compression on coherent structures in an enclosure
(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:nbn:se:kth:diva-124235 (URN)
Funder
Swedish Energy Agency
Note

QS 2013

Available from: 2013-06-27 Created: 2013-06-27 Last updated: 2015-04-20Bibliographically approved
4. Study of flow generated by the port in a heavy-duty diesel engine at different valve lifts using PIV
Open this publication in new window or tab >>Study of flow generated by the port in a heavy-duty diesel engine at different valve lifts using PIV
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In-cylinder flow structures, also prior to ignition, have a large effect on combustion efficiency and emissions. Therefore, understanding the mechanism of formation and changes in such structures is of great importance in the work of reducing fuel consumption and emissions. Here, the flow entering the cylinder was studied using stereoscopic Particle Image Velocimetry (PIV). The measurements were carried out on a steady swirl test rig, commonly used to measure engine characteristics such as the swirl number. In this study, fluctuations in swirl coefficient were found to be greater than the mean swirl at low valve lifts, the flow was found to be Reynolds number independent and the turbulent fluctuations were observed to be axisymmetric.

Keyword
PIV, Swirl
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-164882 (URN)
Note

QS 2015

Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2015-04-20Bibliographically approved
5. Effect of swirl/tumble (Tilt) angle on flow homogeneity, turbulence and mixing properties
Open this publication in new window or tab >>Effect of swirl/tumble (Tilt) angle on flow homogeneity, turbulence and mixing properties
(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
Identifiers
urn:nbn:se:kth:diva-164883 (URN)
Note

QS 2015

Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2015-04-20Bibliographically approved
6. Towards understanding the effect of compression on swirl and tumble in the context of turbulence and mixing
Open this publication in new window or tab >>Towards understanding the effect of compression on swirl and tumble in the context of turbulence and mixing
(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:nbn:se:kth:diva-164886 (URN)
Note

QS 2015

Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2015-04-20Bibliographically approved
7. Investigating the dynamic effects on flow structures generated during the intake stroke in heavy-duty diesel engines using Large Eddy Simulations
Open this publication in new window or tab >>Investigating the dynamic effects on flow structures generated during the intake stroke in heavy-duty diesel engines using Large Eddy Simulations
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The aim of this work is to investigate whether a quasi-steady assumption is applicable for the flow entering a heavy-duty diesel engine. That is, what errors can be expected when a steady swirl test rig is used to characterize the flow structures entering the cylinder. The main conclusion is that the dynamic effects have an effect on the flow generated during intake.In detail, we show that the swirl coefficient is higher during fluid deceleration as compared to during fluid acceleration. The swirl coefficient was also found to be around 40~\% higher during valve closing as compared to similar lifts during valve opening. The flow of angular momentum into the cylinder is found to be delayed by the time it takes to empty the volume above the valves. A new ratio, port delay ratio (Rp), is proposed to account for this phenomena. Minimizing the port delay ratio during engine design is likely to lead to a stronger and more homogeneously distributed swirling motion.Taken together, the dynamic effects on swirl and mass flow expected during the intake have been investigated. This knowledge is important when measured mean swirl numbers are used as initial condition in sector model simulations of combustion. Sector model simulations are very common during the optimization process of diesel combustion and a small difference between real and simulated swirl numbers may have a significant effect on engine performance.

National Category
Fluid Mechanics and Acoustics Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-164888 (URN)
Note

QS 2015

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

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