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Swirl and Injection Pressure Effect on Post-Oxidation Flow Pattern Evaluated with Combustion Image Velocimetry, CIV, and CFD Simulation
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
2013 (English)Conference paper, Published paper (Refereed)
Abstract [en]

In-cylinder flow pattern has been examined experimentally in a heavy duty optical diesel engine and simulated with CFD code during the combustion and the post-oxidation phase. Mean swirling velocity field and its evolution were extracted from optical tests with combustion image velocimetry (CIV). It is known that the post-oxidation period has great impact on the soot emissions. Lately it has been shown in swirling combustion systems with high injection pressures, that the remaining swirling vortex in the post-oxidation phase deviates strongly from solid body rotation. Solid body rotation can only be assumed to be the case before fuel injection. In the studied cases the tangential velocity is higher in the centre of the piston bowl compared to the outer region of the bowl. The used CIV method is closely related to the PIV technique, but makes it possible to extract flow pattern during combustion at full load in an optical diesel engine. Injection pressure was varied from 200 up to 2500 bar at 1000 rpm without EGR. Swirl was varied between 1.2 and 6.4 at BDC. The CFD simulation was a sector simulation on the same in-cylinder geometry and boundary conditions as in the optical engine.

The main findings show that with increased injection pressure, together with swirl, the angular velocity increases in the centre of the piston bowl meanwhile the angular velocity decreases slightly in the outer region. The total angular momentum decreases slightly when injection starts and the total rotational kinetic energy increases significantly. The redistribution of the angular velocity is caused by the driving force from the injection. When the swirling bulk flow acts on the injected spray/flame, its orbit is slightly directed to the leeward side of the swirl. When the flame is directed back to the cylinder centre, by the bowl, it has thereby an offset from where it is injected. This offset together with the high flow velocity from the flame increases the angular velocity in the central region of the combustion chamber. The angular velocity in the outer part of the bowl decreases slightly when angular momentum is moved into the centre of the bowl were the velocity increases. This deviation in angular velocity has been observed in both the CFD results and in the CIV results were it survives into the post-oxidation phase with slow dissipation during the expansion stroke. The dissipation is a source for late cycle turbulence generation that affects the soot oxidation.

Place, publisher, year, edition, pages
2013.
Series
SAE technical paper
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-136977DOI: 10.4271/2013-01-2577Scopus ID: 2-s2.0-84890339703OAI: oai:DiVA.org:kth-136977DiVA: diva2:677621
Conference
SAE/KSAE 2013 International Powertrains,Fuels & Lubricants Meeting
Note

QC 20131210

Available from: 2013-12-10 Created: 2013-12-10 Last updated: 2013-12-10Bibliographically approved
In thesis
1. In-cylinder Flow Characterisation of Heavy Duty Diesel Engines Using Combustion Image Velocimetry
Open this publication in new window or tab >>In-cylinder Flow Characterisation of Heavy Duty Diesel Engines Using Combustion Image Velocimetry
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In-cylinder flow in diesel engines has a large impact on combustion and emission formation. In this work, the flow is characterised with a new measurement method called combustion image velocimetry (CIV). This technique is used to explain how airflow introduced during induction affects soot emissions and interacts with injection pressures up to 2500 bar. The CIV measurements enable flow analysis during the combustion and post-oxidation phases. The flow velocities inside the cylinder of a heavy duty optical engine, was measured with a crank angle (CA) resolution of 0.17° at injection pressures of 200–2500 bar and up to nearly full load (20 bar indicated mean effective pressure (IMEP)), were investigated with this method. The flow field results were combined with optical flame temperature and soot measurements, calculated according to Planck’s black body radiation theory.

At the high injection pressures typical of today’s production standard engines and with rotational in-cylinder flow about the cylinder axis, large deviations from solid-body rotational flow were observed during combustion and post-oxidation. The rotational flow, called swirl, was varied between swirl number (SN) 0.4 and 6.7. The deviation from solid-body rotational flow, which normally is an assumption made in swirling combustion systems, formed much higher angular rotational velocities of the air in the central region of the piston bowl than in the outer part of the bowl. This deviation has been shown to be a source for turbulent kinetic energy production, which has the possibility to influence soot burn-out during the post-oxidation period.

The measured CIV data was compared to Reynolds-averaged Navier–Stokes (RANS) CFD simulations, and the two methods produced similar results for the flow behaviour. This thesis describes the CIV method, which is closely related to particle image velocimetry (PIV). It was found in this work that the spatial plane in the cylinder evaluated with CIV corresponds to a mean depth of 3 mm from the piston bowl surface into the combustion chamber during combustion. During the post-oxidation phase of combustion, the measured spatial plane corresponds to a mean value of the total depth of the cylinder. The large bulk flow that contributes to the soot oxidation is thereby captured with the method and can successfully be analysed. The link between changes in in-cylinder flow and emissions is examined in this work.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. vi, 97 p.
Series
TRITA-MMK, ISSN 1400-1179 ; 2013:17
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-136978 (URN)978-91-7501-963-5 (ISBN)
Public defence
2014-01-15, Q1, Osquldasväg 4, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131210

Available from: 2013-12-10 Created: 2013-12-10 Last updated: 2014-01-20Bibliographically approved

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