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- Kärrholm Peng, Fabian, 1980, et al.
(författare)
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On Performance of Advection Schemes in the Prediction of Diesel Spray and Fuel Vapour Distributions
- 2008
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Ingår i: ILASS 2008, Como Lake.
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Konferensbidrag (refereegranskat)abstract
- We have investigated the performance of advection schemes (focusing mainlyon Total Variation Diminishing, TVD, schemes)applied in modelling diesel sprays, and assessed their influence on liquidspray penetration and fuel vapour distribution. Here,we compare sprays simulated using several types of TVD schemes (Superbee,MUSCL, limited Linear, and UMIST) – andstandard upwind and linear schemes as references – in conjunction withthree different turbulence models (standard, RNG andLaunder-Sharma k-ε models), to non-reacting diesel sprays observed inthe Sandia high-pressure, high-temperature constant-volume vessel. The OpenFOAM CFD code was used for all of the simulationsdescribed. In addition to comparing thesimulations to experimental data, we provide overall assessments of theperformance and utility of the TVD schemes in multi-dimensional diesel modelling.
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| 2. |
- Kärrholm Peng, Fabian, 1980, et al.
(författare)
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Three-Dimensional Simulation of Diesel Spray Ignition and Flame Lift-Off Using OpenFOAM and KIVA-3V CFD Codes
- 2008
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Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627.
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Tidskriftsartikel (refereegranskat)abstract
- Three-dimensional simulations of ignition and combustion of a dieselspray were conducted. The primary goal of the work was to compare twodifferent CFD codes: OpenFOAM, an object-oriented C++ based code, andKIVA-3V. The spray is modelled by the Eulerian-Lagrangian approach inboth codes, with several common submodels. Some important sub-modelsimplemented include \emph{inter alia} aKelvin-Helmholtz/Rayleigh-Taylor (KH/RT) model for spray break-up, animproved spray collision model, and a Partially Stirred Reactor (PaSR)model for turbulence-chemistry interaction. Both CFD codes solve thechemical reaction equations in a fully coupled manner. A cubic-shaped Cartesianmesh was used in the KIVA-3V simulations, while a polyhedral meshincluding a combination of hexagonal and prism-shaped cells wasconstructed for the OpenFOAM computations.The effects of high EGR and ambient temperature on the ignition and flamelift-off processes of a diesel spray were investigated. Sandia experimentsconducted in a high-pressure and high-temperature constant-volume vessel werechosen for the simulations and validations. A single spray was injected intothe vessel, and EGR was mimicked by reducing the oxygen concentration. Thediesel reference fuel (n-heptane) was considered. For the study, a medium-sizemechanism involving 83 species and 338 reactions was employed. The mechanismwas validated using the CHEMKIN II package and the reaction rate constantswere adjusted on the basis of measurements of auto-ignition delays ofn-heptane/air mixtures in shock-tube experiments (with equivalence ratiosranging from 0.2 to 0.4 at 50 bar, and from 0.5 to 2.0 at 13.5 bar and 41.0bar), laminar flame speeds (1 atm and 3 atm), and flame structure inburner-stabilized premixed flames (1 atm).The simulations demonstrate that both CFD codes are capable of spray ignitionand combustion studies, though both show stronggrid-dependence. The numerical results show that the ignition delay,flame lift-off and combustion temperature of the spray are stronglyinfluenced by EGR and ambient gas temperature. These predictions arein agreement with measurements. Nevertheless, differences are observedbetween the results predicted by OpenFOAM and those from KIVA-3V, forexample, the flame predicted by the former is thinner and longer than that by thelatter, which requires further investigation.
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| 5. |
- Tao, Feng, 1964, et al.
(författare)
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Effect of ultra-high injection pressure on diesel ignition and flame under high-boost conditions
- 2008
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Ingår i: SAE Technical Papers: 2008 SAE International Powertrains, Fuels and Lubricants Congress; Shanghai; China; 23 June 2008 through 25 June 2008. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- In this work, we conducted three-dimensional numerical simulations to investigate the effect of ultra-high injection pressure on diesel ignition and flame under high-boost and medium-load conditions. Three injection cases employed in experiments with a multi-cylinder Volvo D12 engine were applied for validation. The simulations were performed using the KIVA-3V code, with a Kelvin-Helmholz/Rayleigh-Taylor (KH/RT) spray breakup model and a diesel surrogate mechanism involving 83 species and 445 reactions. A range of higher injection pressure levels were projected and the injection rates estimated for the current study. Three different rate shapes of injection were projected and investigated as well. All the projected injection events start at top dead center (TDC). Computations demonstrate that high-pressure injection strongly affects engine ignition and combustion. An increase in injection pressure leads to reduced ignition delay time, higher in-cylinder pressure peak, advanced combustion phasing, and faster flame propagation. The study found that the ultra-high pressure injection does not cause the flame lift-off length in the engine to increase, the trend of which seems to be contradictory to the observations obtained from the studies in high-pressure, high-temperature constant-volume vessels. While the burn durations reduced with an increase in injection pressure, the simulations of three different injection rate shapes suggest that the rate-falling injection leads to a shorter, early (10-30%) burn duration angle but a longer, late (70-90%) burn angle. The prediction indicates that the engine has a relatively larger flame area of higher temperature in the late cycle for the rate-rising injection than for the rate-falling one. The existence of higher temperature in the late engine cycle may be beneficial to soot oxidation. On the other hand, the simulations show that higher injection pressure results in a faster NO production rate in the early phase of combustion but leads to a lower NO peak level. The rate-rising injection lowers NO production compared with the other two injection strategies.
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| 6. |
- Tao, Feng, 1964
(författare)
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Numerical Modeling of Soot and NOx Formation in Non-Stationary Diesel Flames with Complex Chemistry
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A complex chemistry model of reduced size (65 species and 268 reactions) derived on the basis of n-heptane auto-ignition kinetics, small hydrocarbon oxidation chemistry, polyaromatic hydrocarbon (PAH) and NOx formation kinetics together with a phenomenological soot model has been implemented in the KIVA code for multidimensional Diesel spray combustion simulations. An EDC (Eddy Dissipation Concept) based partially stirred reactor model is used to handle the turbulence-chemistry interaction. The results obtained from numerical simulations for direct-injection (DI) Diesel sprays, injected into a high-pressure combustion vessel at engine-like conditions or a real engine geometry, show that the approach is able to reproduce the transient Diesel auto-ignition and combustion processes as observed in optical imaging studies. The simulated results (for the cases tested) indicate that the auto-ignition of DI Diesel spray occurs at a site close to the mean stoichiometric surface. The ignition spot grows on the lean side, crosses over the mean stoichiometric surface, enters into the rich zone and develops further in a very short time. The prediction demonstrates that the post-ignition, fully developed combustion process occurs in a lifted diffusion flame stabilized at a large distance from nozzle exit. The spatial distributions of soot and NOx in the predicted lifted flame are similar to those described in Dec's conceptual Diesel combustion model. Further numerical investigations performed show that, the lower the ambient gas pressure and temperature, the longer the auto-ignition delay times of the sprays and vice versa. Increase in ambient gas pressure or temperature causes a reduction in the flame liftoff length. The results demonstrate also that the flame liftoff length is more sensitive to the change in the ambient temperature. The liftoff has a strong influence on the soot and NOx formation. The farther the flame is stabilized, the lower the emissions. Studies of air dilution effects were also performed to investigate the EGR effects on ignition delay, soot and NOx emissions of Diesel flames. The simulations suggest that Diesel auto-ignition delays are controlled by the oxygen concentrations not by the nature of diluents. The soot and NOx formation is suppressed by the dilution. Moreover, studies demonstrate that the initial temperature has a strong effect on soot formation whereas the pressure effect is much weaker. For the same amount of fuel injected, the longer the fuel injection duration time, the higher the maximum value of the averaged soot mass concentration produced. It is expected that the present numerical study combined with experimental studies may provide a better insight into Diesel spray combustion and pollutant formation in Diesel flames.
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