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A Coupled PIV-LES Approach to Understand PortGenerated Structures
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.
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
(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 [en]
CFD, Engine
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-124233OAI: oai:DiVA.org:kth-124233DiVA: diva2:633837
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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