| 1. |
- Hadadpour, Ahmad, et al.
(författare)
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An extended FGM model with transported PDF for LES of spray combustion
- 2023
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 39:4, s. 4889-4898
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Tidskriftsartikel (refereegranskat)abstract
- An enhanced flamelet generated manifold (FGM) model for large eddy simulation (LES) of turbulent spray combustion is presented. In the enhanced FGM model, a transported probability density function (TPDF) description of the FGM variables is employed. The TPDF is represented using the Eulerian stochastic fields (ESF) approach, and the method is applied to LES of spray combustion under conditions relevant to internal combustion engines. The new ESF/FGM method achieves an improved accuracy of predictions due to the ESF modelling of the subgrid-scale turbulence-chemistry interaction. It also achieves high computational efficiency due to the FGM tabulation of the chemical kinetic mechanism. The performance of the new ESF/FGM model is assessed by simulation of the Spray-A flames from Engine Combustion Network (ECN) and comparison of the results, firstly, with experimental measurements, and secondly, with conventional FGM model simulation results. It is shown that the ESF/FGM method is capable of predicting both global and local combustion characteristics, i.e., pressure rise, ignition delay time, flame lift-off length and the thermo-chemical structure of the spray flames with improved accuracy compared to the conventional FGM model that is based on the presumed PDF description of FGM variables. The sensitivity of the predictions using ESF/FGM to the number of stochastic fields is examined by varying the number of these fields in the range of 4-128. Furthermore, the influence of different FGM reaction progress variables on the simulations is investigated, and a new reaction progress variable based on the local consumption of oxygen is proposed. The results show that the new progress variable improves predictions of spray combustion, including the prediction of the start of injection, the quasi-steady state liftoff length, the post-injection oxidation, and the pressure evolution.
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| 2. |
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| 3. |
- Hadadpour, Ahmad, et al.
(författare)
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Effects of pre-injection on ignition, combustion and emissions of spray under engine-like conditions
- 2022
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 241
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Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulation (LES) of multiple-injection spray combustion under engine-like conditions is performed using a coupled model of an Eulerian stochastic field transported probability density function (ESF T-PDF) and a flamelet generated manifold (FGM). This coupled model allows the use of the T-PDF methods in modeling the interaction of turbulence and chemistry at affordable computational costs for engine applications. Simulation results are compared with the available experimental data for spray flames with multiple-injection and at a high level of exhaust gas recirculation (EGR) under conditions relevant to internal combustion engines. It is shown that the T-PDF/FGM LES model is capable of reproducing not only global combustion characteristics, such as the pressure rise and ignition delay time but also replicating the evolution of liftoff length and the spray flame structure. The effects of pre-injection strategies are then investigated, by systematically varying the pre-injection duration and the dwell time between two injections, while keeping the total injected fuel mass constant. The LES results reveal different mechanisms by which a pre-injection can change the ignition delay time, the combustion mode and the emissions in spray flames, depending on the injection timing. It is shown that even an extremely short non-igniting pre-injection can substantially change the ignition and emissions characteristics of the main-injection. It is shown that the combustion mode of a single-injection can be altered by splitting the injection to pre- and main-injections. The current study also demonstrates that decreasing the dwell time, within the range that is examined here, at a given pre-injection, will potentially increase the soot oxidation rates while it does not significantly change the rate of soot formation.
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| 4. |
- Hadadpour, Ahmad, et al.
(författare)
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Jet-jet interaction in multiple injections: A large-eddy simulation study
- 2018
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 234, s. 286-295
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports on studies of multiple-injection strategies of gaseous fuel in a model combustion chamber and the role of jet-jet interactions on the mixing processes in the chamber using large-eddy simulation (LES). A high-pressure non-reacting gas flow injected through a jet with a nozzle diameter of 1.35 mm into a quiescent inert air environment is considered. First, we validate the method and our computational setup by comparing the simulation results of a single injection case with available experimental data. It is shown that the transient ensemble-averaged LES results agree well with the experimental measurements. Second, we simulate and compare fourteen injection strategies in order to understand the effect of the main and the post-injections duration, the dwell time and the mass flow rate of post-injection on the mixing, jet penetration, and near-nozzle mixture. The contribution of each injection in the local mixture composition is quantified by solving transport equations for the mixture fraction of each injection.The results show that the turbulence generated in the main injection is enhanced when the post-injection flow into the main injection flow. The increase of the local turbulence intensity is in favor of increasing the scalar dissipation rate and enhancing the mixing rate. However, the penetration of the post-injection flow into the main injection flow and the level of the gas flow from the interaction of two injections depend on the dwell time and the momentum of the post-injection.The results also show that the post-injection modifies the near-nozzle mixture. The comparison of cases with different mass flow rates in the post-injection indicates that the momentum of the post-injection can be optimized either to push away the near-nozzle remaining gas from the main injection and reduce the near-nozzle residue by more than or enrich this fuel-lean region and increase the near-nozzle gasses by more than . These results are very interesting for optimization of the post-injection to reduce engine-out emissions.
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| 5. |
- Hadadpour, Ahmad
(författare)
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Spray combustion with multiple-injection in modern engine conditions
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Combustion of fuel in diesel engines emits substances harmful to the environment such as soot. These emissions can be reduced by either in-cylinder treatments or after-treatments. One of the common in-cylinder treatments is multiple-injection, which divides a single fuel injection to multiple smaller injections. There are many open questions on the physical processes of the ignition, combustion and emissions of diesel spray flame with multiple injections. The current PhD project aims at studying these processes using large-eddy simulations (LES) and strives to answer some of the open questions. To develop a fast and robust LES tool for this study, a new method is formulated for spray combustion simulation. This method is developed based on the flamelet-generated manifold (FGM) method and the Eulerian stochastic fields (ESF) method. The new ESF/FGM method relaxes some of the substantial assumptions in conventional FGM, while it still keeps the computational costs at a reasonable level for engineering applications. Additionally in this work, a new reaction progress variable for FGM models is proposed by using local oxygen consumption, and the advantages and limitations of this progress variable are explored. Spray-A from Engine Combustion Network (ECN) which is designed to mimic modern engine conditions is chosen as the baseline case for simulations. In this case, liquid n-dodecane, which is a diesel surrogate, is injected into a high-pressure constant-volume vessel. The comparison of simulation results with experimental measurements shows that the ESF/FGM method with the new progress variable can predict the spray combustion characteristics such as ignition delay time, ignition location, lift-off length, pressure rise and thermochemical structure of the spray flame, accurately. After validation of simulation results against experimental measurements, the new ESF/FGM and other available turbulence-combustion simulation tools are applied to simulate multiple-injection spray combustion. Different multiple-injection strategies are investigated by systematically changing the injection timing. The effects of applying each strategy on the ignition, combustion, mixing and emissions are investigated. The results show that in split-injection and post-injection strategies the major physical reason for reduction of soot is better air entrainment and lower local equivalence ratio. It is shown that increasing the dwell time and retarding it toward the end of injection can enhance this effect. On the contrary, for the pre-injection strategies, shortening the ignition delay time of the main injection reduces its pre-mixing and increases its soot formation. In these strategies, the high-temperature region from the pre-injection combustion can increase soot oxidation of the main injection fuel, only if this region is not cooled down as a result of air entrainment during dwell time. Therefore, in such cases shortening the dwell time decreases net soot emissions.
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| 6. |
- Hadadpour, Ahmad, et al.
(författare)
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The Effect of Splitting Timing on Mixing in a Jet with Double Injections : A Large-Eddy Simulation Study
- 2018
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Ingår i: Flow, Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1573-1987 .- 1386-6184. ; 101:4, s. 1157-1171
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Tidskriftsartikel (refereegranskat)abstract
- We present large-eddy simulation (LES) of a high-pressure gas jet that is injecting into a quiescent inert environment. The injection is through a nozzle with a diameter of 1.35 mm. Four injection strategies are considered in which the results of a single continuous injection case are compared with those of double injection cases with different injection splitting timing. In all double injection cases, the injection pulsing interval is kept the same, and the total injected mass is equal to that of the single injection case. On the other hand,the splitting timing is varied to investigate the effects of various injection splitting strategies on the mixture formation and the penetration length of the jet. Results show that the jet penetration length is not so sensitive to the splitting timing whereas the mixing quality can significantly change as a result of shifting the onset of injection splitting toward the end of injection. Especially, it is found that by adopting a post-injection strategy where a single injection splits into the main injection and late small injection near the end of injection period the mixing between the injected gas and ambient air is significantly improved. This trend is not as obvious when the injection splitting timing shifts toward the beginning or even in the middle of injection period. The increase of entrainment in the tail of each injection is one of the underlying physics in the mixing improvement in double injection cases. In addition to that, splitting a single injection into two smaller injections increases the surrounding area of the jet and also stretches it along the axial direction. It can potentially increase the mixing of injected gas with the ambient air.
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| 7. |
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| 8. |
- Hadadpour, Ahmad, et al.
(författare)
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The role of a split injection strategy in the mixture formation and combustion of diesel spray: A large-eddy simulation
- 2019
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 37:4, s. 4709-4716
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Tidskriftsartikel (refereegranskat)abstract
- The role of a split injection in the mixture formation and combustion characteristics of a diesel spray in an engine-like condition is investigated. We use large-eddy simulations with finite rate chemistry in order to identify the main controlling mechanism that can potentially improve the mixture quality and reduces the combustion emissions. It is shown that the primary effect of the split injection is the reduction of the mass of the fuel-rich region where soot precursors can form.Furthermore, we investigate the interaction between different injections and explain the effects of the first injection on the mixing and combustion of the second injection. Results show that the penetration of the second injection is faster than that of the first injection. More importantly, it is shown that the ignition delay time of the second injection is much shorter than that of the first injection. This is due to the residual effects of the ignition of the first injection which increases the local temperature and maintains a certain level of combustion some intermediates or radical which in turn boosts the ignition of the second injection.
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| 9. |
- Xu, Shijie, et al.
(författare)
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Large-eddy simulation of the injection timing effects on the dual-fuel spray flame
- 2022
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 310
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Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulations (LES) coupled with a partially-stirred reactor model and a finite-rate chemistry are carried out to investigate the effects of n-heptane injection timing on the methanol fueled dual-fuel (DF) combustion. Methanol is premixed with air in a constant volume chamber (T=1000 K, ρ=14.8 kg/m3) to form a homogeneous mixture (equivalence ratio ϕm of 0.3). Liquid fuel n-heptane is provided from a high pressure injector to mimic the pilot fuel injection in DF engines. First, mesh sensitivity analysis and LES model validation are conducted. The experimental data of Spray-H (n-heptane fueled) from the Engine Combustion Network is used for model validation. It is shown that the present mesh and LES model are capable of replicating the liquid and vapor penetration length, mixture fraction, temperature distribution, pressure rise profile and ignition delay time (IDT). Second, the effects of n-heptane injection timing are investigated, by varying the start of injection (SOI) time. The LES results reveal that there are three stage heat releases in the DF combustion. With the delay of SOI, the mass fraction of hydrogen peroxide in the ambient mixture increases, leading to an early formation of hydroxyl. Therefore, the two-stage IDTs of n-heptane decrease, while the ambient methanol IDT increases. Results also show the cool flame and high-temperature flame evolution after methanol auto-ignition. The cool flame disappears while the high-temperature flame is found near the injector nozzle, which leads to a relatively high heat release rate.
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| 10. |
- Xu, Shijie, et al.
(författare)
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LES/TPDF investigation of the effects of ambient methanol concentration on pilot fuel ignition characteristics and reaction front structures
- 2021
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 287
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Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulations with a transported probability density function model coupled with a finite-rate chemistry is applied to study the ignition process of an n-heptane spray in a constant volume chamber with a premixed methanol-air atmosphere under conditions relevant to reactivity controlled compression ignition (RCCI) engines. Three reacting spray cases with initial methanol-air equivalence ratio (ϕm) ranging from 0 to 0.3 are investigated at an initial temperature of 900 K. The case setup is based on the Engine Combustion Network Spray-H configuration, where n-heptane fuel is used. The effects of the ambient methanol-air equivalence ratio on the ignition characteristics and the reaction front structures in n-heptane/methanol RCCI combustion are studied in detail. It is found that the ambient methanol affects the low temperature chemistry of n-heptane, which results in a change of spatial distribution of key species such as heptyl-peroxide, and therefore the cool flame structure. With the presence of methanol in the ambient mixture cool flame is found in the entire fuel-rich region of the n-heptane jet, while when methanol is absent in the ambient mixture, the cool flame is established only around the stoichiometric mixture close to the n-heptane injector nozzle. In general, both low- and high-temperature ignition stages of n-heptane ignition are retarded by the methanol chemistry. An increase in ϕm leads to a decrease of the peak heat release rate of the n-heptane first-stage ignition. The chemistry of methanol inhibits the n-heptane ignition by decreasing the overall hydroxyl radicals (OH) formation rate and reducing the OH concentration during the transition period from the first-stage ignition to the second-stage ignition. As a result, the transition time between the two ignition stages is prolonged. Under the present lean methanol/air ambient mixture conditions, the impact of methanol on n-heptane ignition has a tendency of reducing the high-temperature, fuel-rich region, which is in favor of soot reduction.
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