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Turbulent flow mechanisms in mixing T-junctions by Large Eddy Simulations
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-7330-6965
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2014 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 45, no 1, 135-146 p.Article in journal (Refereed) Published
Abstract [en]

We consider the turbulent mixing process in two T-junction geometries as simplified models for mixing in the intake manifolds of Internal Combustion (IC) engines. These junctions have square and circular cross-sections, respectively. The turbulent flow structures and modes are analyzed by Large Eddy Simulations (LES). A grid sensitivity study is performed and the velocity field and the mixing scalar are compared to experimental data. The agreement is good for high enough mesh resolutions. Furthermore, the LES results are compared to unsteady Reynolds averaged Navier-Stokes (URANS) results, in order to gain an understanding of the shortcomings associated with URANS. The secondary structures found in both geometries include Dean-like vortices due to flow curvature in the region of the junction. Further downstream of the junction, these vortices are dissipated and due to an upward motion of the bulk flow, new vortical structures are generated. These downstream vortical structures rotate in the opposite direction relative to the upstream ones and govern the mean scalar distribution far downstream of the junction. We find also that the URANS results show qualitatively different flow structures leading to different scalar distributions as compared to experimental and LES results. The mixing quality is studied using a uniformity index showing a more uniform and faster mixing in the circular cross-section case. Spectral analysis of the LES data show for both geometries a shear layer instability with a dimensionless frequency in the order of unity. Additionally to that, vortex-shedding phenomena are observed in the circular case at St approximate to 0.5.

Place, publisher, year, edition, pages
Elsevier, 2014. Vol. 45, no 1, 135-146 p.
Keyword [en]
T-junction, Mixing, Turbulence, URANS, LES
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-124159DOI: 10.1016/j.ijheatfluidflow.2013.06.014ISI: 000331349700012Scopus ID: 2-s2.0-84892504695OAI: oai:DiVA.org:kth-124159DiVA: diva2:633240
Note

Updated from "Manuscript" to Accepted for publication International Journal of Heat and Fluid Flow, 2013.

QC 20140313. Updated from accepted to published.

Available from: 2013-06-26 Created: 2013-06-26 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Computation and Analysis of EGR Mixing in Internal Combustion Engine Manifolds
Open this publication in new window or tab >>Computation and Analysis of EGR Mixing in Internal Combustion Engine Manifolds
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with turbulent mixing processes occurring in internal combustion engines, when applying exhaust gas recirculation (EGR). EGR is a very efficient way to reduce emissions of nitrogen oxides (NOx) in internal combustion engines. Exhaust gases are recirculated and mixed with the fresh intake air, reducing the oxygen con- centration of the combustion gas and thus the peak combustion temperatures. This temperature decrease results in a reduction of NOx emissions. When applying EGR, one is often faced with non-uniform distribution of exhaust among and inside the cylinders, deteriorating the emission performance. The mixing of exhaust gases and air is governed by the flow in the engine intake manifold, which is characterized by unsteadiness due to turbulence and engine pulsations. Moreover, the density cannot be assumed to be constant due to the presence of large temperature variations.Different flow cases having these characteristics are computed by compressible Large Eddy Simulations (LES). First, the stationary flows in two T-junction type geometries are investigated. The method is validated by comparison with experimental data and the accuracy of the simulations is confirmed by grid sensitivity studies. The flow structures and the unsteady flow modes are described for a range of mass flow ratios between the main and the branch inlet. A comparison to RANS computations showed qualitatively different flow fields.Thereafter, pulsating inflow conditions are prescribed on the branch inlet in or- der to mimic the large pulsations occurring in the EGR loop. The flow modes are investigated using Dynamical Mode Decomposition (DMD).After having established the simulation tool, the flow in a six-cylinder engine is simulated. The flow is studied by Proper Orthogonal Decomposition (POD) and DMD. The mixing quality is studied in terms of cylinder-to-cylinder non-uniformity and temporal and spatial variances. It was found that cycle-averaging of the concentration may give misleading results. A sensitivity study with respect to changes in the boundary conditions showed that the EGR pulsations, have large influence on the results. This could also be shown by POD of the concentration field showing the significance of the pulses for the maldistribution of exhaust gases.Finally, the flow in an intake manifold of a four-cylinder engine is investigated in terms of EGR distribution. For this geometry, pipe bends upstream of the EGR inlet were found to be responsible for the maldistribution.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. vi, 62 p.
Series
Trita-MEK, ISSN 0348-467X ; 2013:02
Keyword
Turbulent Mixing, Large Eddy Simulation, URANS, Internal Combustion Engines, Intake Manifolds, T-junction, Exhaust Gas Recirculation
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-117911 (URN)978-91-7501-639-9 (ISBN)
Public defence
2013-02-22, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20130207

Available from: 2013-02-07 Created: 2013-02-07 Last updated: 2013-07-03Bibliographically approved

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Mihaescu, Mihai

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