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• Using plasma to assist ammonia reactivity results in fewer NOx emissions than those in the flames assisted by hydrogen. • Plasma-assisted ammonia combustion is less prone to strain rate than that of hydrogen-assisted ammonia combustion. • High energy plasma discharges broaden the reaction zones by generating radical pools in the flame preheating zone, being more intense under fuel-rich conditions. • High energy plasma discharges activate the DeNOx mechanism in preheating and post-flame zones. • Increasing the mixture pressure deteriorates the impact of plasma on combustion. Such effects are weakly controlled by the reduced electric field. Synergistic effects of nanosecond plasma discharge and hydrogen on the combustion characteristics of ammonia/air are numerically studied under conditions relevant to gas turbine combustion chambers. It is shown that increasing the plasma contribution in assisting the flame results in lower NO X emissions by up to 27% than those in flames assisted by hydrogen for the range of operating conditions considered in this study. Plasma makes the consumption speed of the reactants less prone to the strain rate than that in flames assisted by hydrogen. It is found that discharging plasma with the pulse energy density of 9 mJ/cm3 alongside using 12% hydrogen by volume in the fuel increases the flame speed of ammonia/air to those of conventional fossil fuels such as methane—an improvement that is not achievable by just using hydrogen, even at a high concentration of 30%. Furthermore, raising the pulse energy density beyond a specific value broadens the reaction zones by generating radical pools in the flame preheating zone, which is expedited in fuel-rich conditions with high H 2 fuel fractions. Investigations show that the simultaneous utilization of high-energy plasma and hydrogen reduces the NO X emissions by activating the mechanisms of nitrogen oxide denitrification (DeNO X) in preheating and post-flame zones, being more significant under the lean condition as compared with rich and stoichiometric cases. It is shown that increasing mixture pressure significantly deteriorates the impacts of plasma on combustion. Such unfavorable effects are weakly controlled by changes in the reduced electric field caused by pressure augmentations. [ABSTRACT FROM AUTHOR]

In the present study, large eddy simulation is used to investigate combustion recession for the Engine Combustion Network Spray A flame at two ambient temperatures (850 K and 800 K) and two injection pressures (100 MPa and 50 MPa). The present numerical results are able to capture different combustion recession phenomena after the end-of-injection (AEOI). With an injection pressure of 100 MPa , the model predicts a "separated" combustion recession at the ambient temperature of 850 K and no combustion recession at the ambient temperature of 800 K , in which both predictions correspond to the measurements. The combustion recession is mainly controlled by the auto-ignition process at the ambient temperature of 850 K. At the ambient temperature of 800 K , the local temperature within the fuel-rich region is not high enough to promote the high-temperature ignition process. As time progresses, the mixture within the fuel-rich region rapidly transitions to become an overly fuel-lean mixture, which further hinders high-temperature ignition to occur. Nonetheless, it is shown that lowering the injection pressure to 50 MPa causes the combustion recession to occur at the ambient temperature of 800 K. This is likely attributed to the low injection case having a lower air entrainment rate AEOI, which causes the mixtures upstream of the lift-off position to transition slower from fuel-rich to fuel-lean mixtures. • LES model captures the evolution of flame structure during combustion recession. • At high P i n j of 100 MPa, combustion recession occurs at 850 K but not at 800 K. • Lowering P i n j to 50 MPa leads to combustion recession in the 800 K spray case. • Air entrainment after EOI affects the fuel–air mixing and combustion recession. • Lowering P i n j leads to lower entrainment rate after EOI and relatively richer mixture. [ABSTRACT FROM AUTHOR]

Numerical simulation of biomass gasification in fluidized bed reactors faces several challenges including proper modeling of the physical and chemical processes involved in the gasification and numerical stiffness of the two-phase dense particle flow problem. In this paper, the multi-phase particle-in-cell (MP-PIC) model coupled with a recently developed distribution kernel method (DKM) and a new one-step pyrolysis model are employed to investigate biomass gasification in two lab-scale fluidized bed gasifiers (FBGs). The results are evaluated by comparing with the results from the Particle Centroid Method (PCM) and experimental measurements. The performance of DKM is shown to improve the robustness of the model and the new pyrolysis model is shown to improve the sensitivity of the yields of gasification products to operating temperature. The simulation results using the new pyrolysis model agree well with the experimental data under different gasification conditions. The model is shown to be able to capture the trend of gas products with respect to variations of steam/biomass ratio (SR) and operating temperature (T r). The mechanisms of the formation of gas products are analyzed based on the numerical results. By increasing the SR and T r , the production of H 2 and CO 2 is shown to increase while the production of CO and CH 4 to decrease. It is shown that varying the steam/biomass ratio in the range of 0.8 ∼ 2.0 has a minor effect on the pyrolysis process and heterogeneous reactions, while homogeneous reactions are significantly affected, leading to changes in the final composition of the gas products. Varying the gasifier temperature T r has on the other hand a crucial effect on the pyrolysis process and as such a significant impact on the gasification products and carbon conversion. • An empirical one-step model was developed for biomass pyrolysis. • Prediction of gasification products at varying operating temperatures was improved. • Impact of steam/biomass ratio on gasification products was identified. • A distribution kernel model is shown to resolve local particle over-loading problem. [ABSTRACT FROM AUTHOR]

Progressively stricter pollutant emission targets in international agreements have shifted the focus of combustion research to low carbon fuels. Ammonia is recognized as one of the promising energy vectors for next-generation power production. Due to the low flame speed and high auto-ignition temperature, ammonia is often burned with a high reactivity pilot fuel (e.g. diesel). However, chemical kinetic mechanisms describing the combustion of ammonia and large hydrocarbon fuels (such as n-heptane, a surrogate of diesel) are less developed. In this work, a skeletal chemical kinetic mechanism for n-heptane/ammonia blend fuels is proposed using a joint decoupling methodology and optimization algorithm. A sensitivity analysis of the ignition delay times of the ammonia/n-heptane mixture is performed to identify the dominant reactions. A genetic algorithm is used to optimize the mechanism further. The final skeletal mechanism is made up of 69 species and 389 reactions. The skeletal ammonia/n-heptane mechanism is validated against the experimental data for combustion of pure ammonia, ammonia/hydrogen and ammonia/n-heptane mixtures, including the global combustion characteristic parameters such as ignition delay times measured in shock tubes or rapid compression machines, laminar burning velocities measured in heat flux burners or spherical flame vessels, and species data measured in jet-stirred reactors. Comparing the results from the skeletal mechanism with those from other mechanisms from the literature is conducted to evaluate the mechanism further. The present skeletal mechanism can well predict the combustion processes for a wide range of conditions, and the mechanism is computationally efficient, showing good potential to model ammonia/n-heptane combustion with good accuracy and efficiency. • A skeletal chemical kinetic mechanism for n-heptane/ammonia blend fuels is proposed. • The skeletal mechanism is widely validated against various experimental datasets. • The mechanism shows a satisfactory prediction for a wide range of conditions. • The final skeletal mechanism is made up of 69 species and 389 reactions. [ABSTRACT FROM AUTHOR]

• Fuel-tracer PLIF is used to quantify the equivalence ratio and temperature. • Misfire of PPC is due to synergistic effect of equivalent ratio and temperature. • In high direct injection pressure, the misfire is due to excessive premixing. • In late direct injection timing, the misfire is due to thermodynamic environment. • Misfire region most likely appears when the equivalence ratio is lower than 0.49. To clarify the misfire mechanism is important for stabilizing combustion in partially premixed combustion (PPC) under low load. Fuel-tracer planar laser-induced fluorescence (PLIF), formaldehyde PLIF, flame and OH* natural luminosity imaging were utilized to qualify the local equivalence ratio, low-temperature reaction and the high-temperature flame features in an optical engine. Results show that in high direct injection (DI) pressure (1000 bar), due to excessive premixing, the local equivalence ratio in the initial timing of the high temperature heat release (HTHR) is low. Although the auto-ignition flame kernels are formed in high DI pressure, they cannot stably develop, resulting in misfire during the flame development process. In late DI timing (-5 crank angle degree after top dead center, °CA ADTC), since the whole heat release process occurs in the expansion stroke, the in-cylinder temperature and pressure continue decreasing. Although the local equivalence ratio in some regions is high enough, the in-cylinder thermodynamic environment does not support the generation of more auto-ignition flame kernels, thus a small amount of auto-ignition flame kernels can only develop through flame propagation. In short, the misfire of PPC occurs in regions where the equivalence ratio is low or the in-cylinder thermodynamic environment does not further support flame development. Therefore, the trade-off relationship between equivalence ratio and temperature determines the formation of auto-ignition kernels. The local equivalence ratio and temperature distribution near the initial timing of HTHR is the key factor to ensure the subsequent stable combustion. Taking the ambient pressure of 18 bar as an example, the boundary condition where the autoignition kernels are most likely formed or the charge is most likely ignited by the nearby flame kernels is in the range of 0.53–0.62 for equivalence ratio and 740–757 K for temperature. The misfire region most likely appears when the equivalence ratio is lower than 0.49. It can be concluded that the misfire of PPC results from the synergistic effect of local equivalent ratio and temperature. The controlling parameters of injection pressure and injection timing are actually optimizing the suitable combinations of equivalence ratio and temperature to stabilize combustion. [ABSTRACT FROM AUTHOR]

This paper presents the numerical results of combustion and soot recession in the Engine Combustion Network (ECN) Spray A and D flames using large eddy simulations (LES). The nominal injector nozzle diameters for the ECN Spray A and D are 90 μ m and 186 μ m. A two-equation soot model is implemented to model the soot formation and oxidation processes. The numerical model is validated by comparing the simulated and measured data in terms of ignition delay time (IDT), lift-off-length (LOL), and temporal evolution of soot mass. The combustion and soot recession processes are analyzed after the end-of-injection (AEOI). The combustion recession processes are first driven by auto-ignition wave propagation followed also by the convective flow in both the Spray A and D flames. A separated high-temperature flame structure is observed in the Spray A flame due to having small favorable mixture regions for the auto-ignition to occur upstream of the quasi-steady lift-off position AEOI. In contrast, a spatially-continuous high-temperature flame structure is formed in the Spray D flame due to having more ignitable mixture regions upstream of the quasi-steady lift-off position and a higher heat release. Soot recession is observed in the Spray D flame but not in the Spray A flame. This is attributed to the mixture upstream of the quasi-steady state soot region becoming favorable to promote soot formation in the Spray D flame, but it becomes fuel-lean AEOI in the Spray A flame. • Effects of nozzle diameters on combustion and soot recession are studied. • ECN Spray A shows separated high-T flame structure after end of injection. • ECN Spray D shows spatially-continuous high-T flame structure after end of injection. • Soot recession is present in the ECN Spray D flame, but not in the ECN Spray A flame. [ABSTRACT FROM AUTHOR]

With the increase of computing power, large eddy simulation (LES) within the Lagrangian–Eulerian two-phase flow framework has evolved as an useful numerical tool to gain insights to the spray combustion processes in advanced internal combustion engines. However, the inherent effect of grid/filter size (G/FS) on the physics revealed by LES is not fully understood. In this paper, we present several new viewpoints on this by analyzing the results from a Lagrangian–Eulerian LES study of Engine Combustion Networks Spray A with a newly developed multi-region adaptive mesh refinement method in OpenFOAM. It is found that G/FS affects the predicted spray characteristics, the modes of spray combustion, the ignition delay time and the liftoff length in different ways. First, owing to the nature of Lagrangian particle-in-cell approach, the spray liquid penetration length converges as the G/FS approaches to the nozzle diameter. The convergence behavior is rather independent of the spray model parameters. Second, it is critical to have a well resolved spray liquid region; the main spray combustion characteristics, e.g., the mean pressure rise profile, the onsets of the cool flame and hot flame, and the main flame structure, are shown to be similar and independent of G/FS once the liquid region is properly resolved. However, the detailed flame structures at the liftoff position are sensitive to G/FS; the upstream auto-ignition events, the low temperature ignition assisted flame propagation and the hot flame propagation, are shown to partially rely on the adopted G/FS. Finally, it is found that a finer G/FS predicts a slower fuel/oxidizer mixing and a shorter flame liftoff length, yielding a higher soot mass. • We proposed a novel multi-region adaptive mesh refinement (AMR) method for ECN Spray A flames. • Using the multi-region AMR method can obviously improve the grid size convergence of flame characteristics. • The spray ignition position and timing are insensitive to the grid size employed (from 250 μ m to 62. 5 μ m). • The flame liftoff and soot formation are closely related to the grid size employed. • More fruitful combustion modes at flame liftoff position are captured with finer grid size. [ABSTRACT FROM AUTHOR]

In this study, large eddy simulations, coupled a two-equation soot model, are performed to investigate the effects of ambient carbon dioxide (CO 2) and water (H 2 O) additions on the soot formation and oxidation processes in an n -dodecane spray flame. In the soot model, acetylene (C 2 H 2) is soot precursor and surface growth species, while hydroxyl radical (OH) and oxygen (O 2) are soot oxidizers. The effect of ambient CO 2 and H 2 O additions on soot formation/oxidation can be separated into thermal and chemical effects. For the thermal effects, the ambient CO 2 and H 2 O additions increase C 2 H 2 but reduce OH formation by lowering the flame temperature. This leads to a higher soot mass formed. On the contrary to the thermal effects, the ambient CO 2 and H 2 O additions reduce the soot formation due to their chemical effects. The reaction CH 2 ∗ + CO 2 ↔ CH 2 O + CO is found to be responsible for reducing C 2 H 2 formation. The ambient H 2 O addition results in a higher OH but lower the C 2 H 2 mass formed owing to the reverse reactions H 2 + OH ↔ H 2 O + H and OH + OH ↔ H 2 O + O. Furthermore, the chemical effects is more significant than the thermal effects under the tested conditions. This leads to a lower soot mass formed when adding ambient CO 2 and H 2 O. • Soot suppression effect due to ambient CO 2 and H 2 O additions is captured using LES. • Thermal effects of CO 2 and H 2 O promote soot, but their chemical effects inhibit soot. • The mechanisms of soot suppression due to ambient CO 2 and H 2 O additions are revealed. [ABSTRACT FROM AUTHOR]

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/m 3) to form a homogeneous mixture (equivalence ratio ϕ m 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. • LES of methanol/n-heptane dual-fuel spray ignition/flame interaction is studied. • Three-stage heat release is found under high temperature and pressure conditions. • A delayed injection leads to a shorter IDT of n-heptane and longer IDT of methanol. • The injection timing affects the cool flame and high temperature flame structure. [ABSTRACT FROM AUTHOR]