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Computation and Analysis of EGR Mixing in Internal Combustion Engine Manifolds
KTH, School of Engineering Sciences (SCI), Mechanics.
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 [en]
Turbulent Mixing, Large Eddy Simulation, URANS, Internal Combustion Engines, Intake Manifolds, T-junction, Exhaust Gas Recirculation
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-117911ISBN: 978-91-7501-639-9 (print)OAI: oai:DiVA.org:kth-117911DiVA: diva2:603864
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
List of papers
1. Turbulent flow mechanisms in mixing T-junctions by Large Eddy Simulations
Open this publication in new window or tab >>Turbulent flow mechanisms in mixing T-junctions by Large Eddy Simulations
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
Keyword
T-junction, Mixing, Turbulence, URANS, LES
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-124159 (URN)10.1016/j.ijheatfluidflow.2013.06.014 (DOI)000331349700012 ()2-s2.0-84892504695 (Scopus ID)
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
2. Modeling of EGR Mixing in an engine intake manifold using LES
Open this publication in new window or tab >>Modeling of EGR Mixing in an engine intake manifold using LES
2012 (English)Conference paper, Published paper (Refereed)
Abstract [en]

We investigate the mixing process of exhaust gases with fresh air in Internal Combustion Engines (ICE). For this purpose, the flow in an inlet manifold of a six-cylinder heavy-duty Diesel engine is computed using compressible Large Eddy Simulations (LES). The Exhaust Gas Recirculation (EGR) concentration is modeled as a passive scalar. The results are validated by on-engine measurements of the EGR concentration using CO2-probes. The boundary conditions for the highly pulsating flow are taken partly from one-dimensional simulations, partly from pressure measurements on the engine. In order to assess the sensitivity to the boundary conditions, changes are applied to the base-line case. The mixing quality is evaluated in terms of cylinder-to-cylinder distribution and the spatial RMS over the outlet cross-sections. Different averaging techniques are applied. It was found that the temporal and spatial EGR distribution is different among the cylinders. The EGR distribution within the cylinder inlet is non-uniform. These factors imply that one should not use a time-averaged EGR value as indicator for the EGR content. Furthermore, it was found that the flow pulsations at the EGR inlet have a large influence on the EGR distribution. By comparing the LES results with measurements, it was shown that LES gives a better and deeper insight into the mixing in such turbulent, pulsating flow situations.

Place, publisher, year, edition, pages
IFPEN Energies nouvelles, 2012
Series
Oil & Gas Science and Technology - Revue d'IFP Energies Nouvelles, ISSN 1953-8189
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-117924 (URN)
Conference
International Conference on LES for Internal Combustion Engine Flows, Paris, 28-29 november 2012
Note

QC 20130207

Available from: 2013-02-07 Created: 2013-02-07 Last updated: 2013-02-08Bibliographically approved
3. Flow decomposition methods applied to the flow in an IC engine manifold
Open this publication in new window or tab >>Flow decomposition methods applied to the flow in an IC engine manifold
2014 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 65, no 1-2, 57-65 p.Article in journal (Refereed) Published
Abstract [en]

Large Eddy Simulation (LES) of the flow and mixing in an engine manifold of a six cylinder Diesel engine is carried out. Proper Orthogonal Decomposition (POD) and Dynamical Mode Decomposition (DMD) are applied to sets of LES data from these computations. The engine manifold under consideration includes a pipe-junction used for Exhaust Gas Recirculation (EGR). The methods of analysis, POD and DMD, are applied to the velocity field and the exhaust concentration field. It is found that POD facilitates the analysis of the pulsating, complex and turbulent flow field as compared to instantaneous fields. The flow field is dominated by the EGR pulses and their interaction with other pulses originating from the cylinder valves. The maldistribution of EGR concentration among the first cylinder ports can clearly be linked to the EGR pulsations. DMD is able to extract flow structures at certain frequencies. Using these properties of DMD, the motion of the EGR pulses can be extracted and visualized. The combination of LES and flow decomposition is found to give a rational for the interpretation of the flow phenomena, which might facilitate the optimization of engine manifolds in terms of EGR non-uniformity.

Keyword
Engine Flow, IC engine, flow decomposition, POD, DMD
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-117926 (URN)10.1016/j.applthermaleng.2013.12.082 (DOI)000335099100007 ()2-s2.0-84893039564 (Scopus ID)
Note

QC 20140305. Updated from manuscript to article in journal.

Available from: 2013-02-07 Created: 2013-02-07 Last updated: 2017-12-06Bibliographically approved

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