| 1. |
- Yu, Rixin, et al.
(författare)
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Effect of Turbulence on HCCI Combustion
- 2007
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Ingår i: Session: Homogeneous Charge Compression Ignition (HCCI) (Part 4 of 8) Combustion Modeling / Optical Diagnostics. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- This paper presents large eddy simulation (LES) and experimental studies of the combustion process of ethanol/air mixture in an experimental optical HCCI engine. The fuel is injected to the intake port manifolds to generate uniform fuel/air mixture in the cylinder. Two different piston shapes, one with a flat disc and one with a square bowl, were employed to generate different in-cylinder turbulence and temperature field prior to auto-ignition. The aim of this study was to scrutinize the effect of in-cylinder turbulence on the temperature field and on the combustion process. The fuel tracer, acetone, is measured using laser induced fluorescence (LIF) to characterize the reaction fronts, and chemiluminescence images were recorded using a high speed camera, with a 0.25 crank angle degree resolution, to further illustrate the combustion process. Pressure in the cylinder is recorded in the experiments. Spatial and temporal resolved LES was used to gain information on the turbulence mixing, heat transfer and combustion process. It was shown that gas temperature in the piston bowl is generally higher than that in the squish, leading to an earlier ignition in the bowl. Compared to the disc engine, the square bowl engine has a higher temperature inhomogeneity owing to the turbulence wall heat transfer. The experimentally observed higher combustion duration and slower pressure rise rate in the square bowl engine as compared to the disc engine can be explained by the higher temperature inhomogeneity in the square bowl engine.
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| 2. |
- Nygren, Jenny, et al.
(författare)
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Three-dimensional laser induced fluorescence of fuel distributions in an HCCI engine
- 2002
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Ingår i: Proceedings of the Combustion Institute. - 0082-0784 .- 1878-027X. ; 29, s. 679-685
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Konferensbidrag (refereegranskat)abstract
- Three-dimensional imaging of fuel tracer planar laser-induced fluorescence in a homogeneous charge compression ignition (HCCI) engine is presented. A high-speed multiple Nd:YAG laser and detection system, in combination, with a scanning mirror, are used to collect eight images, with an equidistant separation of 0.5 mm. Three-dimensional isoconcentration surfaces calculated from the data are visualized. Three-dimensional imaging offers new opportunities to study different combustion events, specifically the topology of flame structures. For example, it is possible to distinguish if separate islands in a fluorescence image really are separate or if it is an effect from wrinkling in and out of the laser sheet. The PLIF images were also analyzed by identifying five intensity ranges corresponding to increasing degrees of reaction progress. The gradual fuel consumption and thus combustion was then analyzed by calculating the volumetric fraction of these intensity ranges for different crank angle positions. The occurrence of multiple isolated ignition spots and the observed gradual decrease in fuel concentration indicates that HCCI combustion relies on distributed reactions and not flame propagation.
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| 3. |
- Olofsson, Jimmy, et al.
(författare)
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High-Speed Lif Imaging for Cycle-Resolved Formaldehyde Visualization in Hcci Combustion
- 2005
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Ingår i: SAE, Session: Combustion and Flow Diagnostics. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. ; 114:3, s. 645-652
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Konferensbidrag (refereegranskat)abstract
- High-speed laser diagnostics was utilized for single-cycle resolved studies of the formaldehyde distribution in the combustion chamber of an HCCI engine. A multi-YAG laser system consisting of four individual Q-switched, flash lamp-pumped Nd:YAG lasers has previously been developed in order to obtain laser pulses at 355 nm suitable for performing LIF measurements of the formaldehyde molecule. Bursts of up to eight pulses with very short time separation can be produced, allowing capturing of LIF image series with high temporal resolution. The system was used together with a high-speed framing camera employing eight intensified CCD modules, with a frame-rate matching the laser pulse repetition rate. The diagnostic system was used to study the combustion in a truck-size HCCI engine, running at 1200 rpm using n-heptane as fuel. By using laser pulses with time separations as short as 70 μs, cycle-resolved image sequences of the formaldehyde distribution were obtained. Thus, with this technique it is possible to follow the formaldehyde formation and consumption processes within a single cycle. The combustion evolution was studied in terms of the rate and spatial structure of formaldehyde formation and consumption for different engine operating conditions, e.g. different stoichiometries. Also, the impact on the rate of heat-release was investigated.
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| 4. |
- Seyfried, Hans, et al.
(författare)
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High-Speed PLIF Imaging for Investigation of Turbulence Effects on Heat Release Rates in HCCI Combustion
- 2007
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Ingår i: Session: Homogeneous Charge Compression Ignition (HCCI) (Part 4 of 8) Combustion Modeling / Optical Diagnostics. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- High-speed laser diagnostics was utilized for single-cycle resolved studies of the fuel distribution in the combustion chamber of a truck-size HCCI engine. A multi-YAG laser system consisting of four individual Nd:YAG lasers was used for planar laser-induced fluorescence (PLIF) imaging of the fuel distribution. The fundamental beam from the lasers at 1064 nm was frequency quadrupled in order to obtain laser pulses at 266 nm suitable for excitation of acetone that was used as fuel tracer. Bursts of up to eight pulses with very short time separation were produced, allowing PLIF images with high temporal resolution to be captured within one single cycle event. The system was used together with a high-speed framing camera employing eight ICCD modules, with a frame-rate matching the laser pulse repetition rate. The combustion evolution was studied in terms of spatial distribution and rate of fuel consumption for different engine hardware configurations as well as operating conditions e.g. different stoichiometries and combustion phasing. Two different piston crown geometry were used for altering the degree of turbulence in the combustion chamber. In addition to the optical investigations, the impact of turbulence effects was also studied by calculating the rate of heat-release and combustion phasing from the pressure trace.
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| 5. |
- Särner, Gustaf, et al.
(författare)
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Simultaneous PLIF Measurements for Visualization of Formaldehyde- and Fuel- Distributions in a DI HCCI Engine
- 2005
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Ingår i: SAE technical paper series. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Tidskriftsartikel (refereegranskat)abstract
- Simultaneous laser-induced fluorescence (LIF) imaging of formaldehyde and a fuel-tracer have been performed in a direct-injection HCCI engine. A mix of N-heptane and iso-octane was used as fuel and Toluene as fluorescent tracer. The experimental setup involves two pulsed Nd:YAG lasers and two ICCD cameras. Frequency-quadrupled laser radiation at 266 nm from one of the Nd:YAG lasers was used for excitation of the fuel tracer. The resulting fluorescence was detected with one of the ICCD cameras in the spectral region 270-320 nm. The second laser system provided frequency-tripled radiation at 355 nm for excitation of formaldehyde. Detection in the range 395-500 nm was achieved with the second ICCD. The aim of the presented work is to investigate the applicability of utilizing formaldehyde as a naturally occurring fuel marker. Formaldehyde is formed in the low-temperature reactions (LTR) prior to the main combustion and should thus be present were fuel is located until it is consumed. Measurements were performed when injecting fuel early and late in the compression stroke. Early injection timing results in a homogeneous charge at the time of auto-ignition, while late timing gives a more stratified charge. The crank angle position at which measurements were performed was altered to cover the entire combustion cycle. The measurement images show instantaneous distributions of toluene and formaldehyde respectively. Images from both early and late injection and at all crank angle degrees show good spatial resemblance between toluene signal area and formaldehyde signal area. The work presented in this paper shows that formaldehyde is a feasible alternative to traditional fuel tracers for visualizing fuels featuring low-temperature reactions in HCCI combustion
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| 6. |
- Weinrotter, Martin, et al.
(författare)
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Optical Diagnostics of Laser-Induced and Spark Plug-Assisted Hcci
- 2005
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Ingår i: SAE, Session: Homogeneous Charge Compression Ignition (HCCI) Combustion (Part 6 & 7). - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. ; 114:3, s. 284-295
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Konferensbidrag (refereegranskat)abstract
- HCCI (Homogeneous Charge Compression Ignition), laser-assisted HCCI and spark plug-assisted HCCI combustion was studied experimentally in a modified single cylinder truck-size Scania D12 engine equipped with a quartz liner and quartz piston crown for optical access. The aim of this study was to find out how and to what extent the spark, generated to influence or even trigger the onset of ignition, influences the auto-ignition process or whether primarily normal compression-induced ignition remains prevailing. The beam of a Q-switched Nd:YAG laser (5 ns pulse duration, 25 mJ pulse energy) was focused into the centre of the cylinder to generate a plasma. For comparison, a conventional spark plug located centrally in the cylinder head was alternatively used to obtain sparks at a comparable location. No clear difference in the heat releases during combustion between the three different cases of ignition start could be seen for the fuel of 80/20 iso-octane/n-heptane used. However, with optical diagnostic methods, namely PLIF (Planar Laser-Induced Fluorescence), Schlieren photography and chemiluminescence imaging, differences in the combustion process could be evaluated.
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| 7. |
- Yu, Rixin, et al.
(författare)
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Effect of Turbulence and Initial Temperature Inhomogeneity on Homogeneous Charge Compression Ignition Combustion
- 2006
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Ingår i: SAE technical paper series. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191. ; :2006-01-3318
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Tidskriftsartikel (refereegranskat)abstract
- A 0.5-liter optical HCCI engine firing a mixture of n-heptane (50%) and iso-octane (50%) with air/fuel ratio of 3 is studied using large eddy simulation (LES) and laser diagnostics. Formaldehyde and OH LIF and in-cylinder pressure were measured in the experiments to characterize the ignition process. The LES made use of a detailed chemical kinetic mechanism that consists of 233 species and 2019 reactions. The auto-ignition simulation is coupled with LES by the use of a renormalized reaction progress variable. Systematic LES study on the effect of initial temperature inhomogeneity and turbulence intensity has been carried out to delineate their effect on the ignition process. It was shown that the charge under the present experimental condition would not be ignited without initial temperature inhomogeneity. Increasing temperature inhomogeneity leads to earlier ignition whereas increasing turbulence intensity would retard the ignition. This is mostly due to the effect of turbulence on the bulk flow that turbulence tends to decrease the temperature inhomogeneity by enhanced eddy heat transfer. The LES results suggest that desirable ignition timing could be achieved by controlling the turbulence intensity and temperature inhomogeneity.
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| 8. |
- Liu, Xin, et al.
(författare)
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Investigation of turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor
- 2021
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 46:24, s. 13377-13388
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Tidskriftsartikel (refereegranskat)abstract
- Methane and hydrogen-enriched (25 vol% and 50 vol% H2-enriched CH4) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re), showing the dependence on Re and H2 concentrations. The effects of equivalence ratio (Φ), Re, and H2 enrichment on flame structure were examined employing OH-PLIF measurement. For CH4/air cases, the flame was stabilized with an M shape; while for H2-enriched cases, the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H2 enrichment, occurring due to the increase in flame speed, higher resistance of the flame to the strain rate, and change in the inner recirculation zone. Flow fields of CH4/air flames were compared between low and high Re cases employing high-speed PIV. The flashback events, led by two mechanisms (combustion-induced vortex breakdown, CIVB, and boundary-layer flashback, BLF), were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH4 flames with M shape, whereas the BLF occurs for all H2-enriched flames with П shape.
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| 9. |
- Petersson, Per, et al.
(författare)
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Simultaneous PIV/PH-PLIF, Rayleigh thermometry/OH-PLIF and stereo PIV measurements in a low-swirl-flame
- 2007
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Ingår i: Applied Optics. - 2155-3165. ; 46:19, s. 3928-3936
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Tidskriftsartikel (refereegranskat)abstract
- The diagnostic techniques for simultaneous velocity and relative OH distribution, simultaneous temperature and relative OH distribution, and three component velocity mapping are described. The data extracted from the measurements include statistical moments for inflow fluid dynamics, temperature, conditional velocities, and scalar flux. The work is a first step in the development of a detailed large eddy simulation (LES) validation database for a turbulent, premixed flame. The low-swirl burner used in this investigation has many of the necessary attributes for LES model validation, including a simplified interior geometry; it operates well into the thin reaction zone for turbulent premixed flames, and flame stabilization is based entirely on the flow field and not on hardware or pilot flames.
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| 10. |
- Wang, Haiou, et al.
(författare)
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Direct numerical simulations of a high Karlovitz number laboratory premixed jet flame - An analysis of flame stretch and flame thickening
- 2017
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 815, s. 511-536
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Tidskriftsartikel (refereegranskat)abstract
- This article reports an analysis of the first detailed chemistry direct numerical simulation (DNS) of a high Karlovitz number laboratory premixed flame. The DNS results are first compared with those from laser-based diagnostics with good agreement. The subsequent analysis focuses on a detailed investigation of the flame area, its local thickness and their rates of change in isosurface following reference frames, quantities that are intimately connected. The net flame stretch is demonstrated to be a small residual of large competing terms: The positive tangential strain term and the negative curvature stretch term. The latter is found to be driven by flame speed-curvature correlations and dominated in net by low probability highly curved regions. Flame thickening is demonstrated to be substantial on average, while local regions of flame thinning are also observed. The rate of change of the flame thickness (as measured by the scalar gradient magnitude) is demonstrated, analogously to flame stretch, to be a competition between straining tending to increase gradients and flame speed variations in the normal direction tending to decrease them. The flame stretch and flame thickness analyses are connected by the observation that high positive tangential strain rate regions generally correspond with low curvature regions; these regions tend to be positively stretched in net and are relatively thinner compared with other regions. High curvature magnitude regions (both positive and negative) generally correspond with lower tangential strain; these regions are in net negatively stretched and thickened substantially.
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