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• Leaf-like Fe 3 O 4 nanosheet arrays with rich Fe2+ and oxygen defects are formed. • Ir-Fe 3 O 4 /IF-T-x is formed via galvanic replacement between IrCl 6 2− ions and Fe2+. • Ir-Fe 3 O 4 /IF-400-2 delivers a low overpotential of 316 mV at 500 mA cm−2 for OER. • The catalyst shows a good long-term stability after 36 h V-t test at 100 mA cm−2. The sluggish kinetics for oxygen evolution reaction (OER) significantly impede the practical applications of electrocatalytic water splitting technology for renewable energy source. The facile design of self-supporting electrocatalysts with high activity and stability for OER catalysis at industrial-grade current density is of paramount significance to mitigate the above bottlenecks. We report herein the construction of iron foam (IF) self-supported Ir-magnetite (Ir-Fe 3 O 4 /IF-T-x) electrode via galvanic replacement between IrCl 6 2− ions and Fe2+ in Fe 3 O 4 nanosheet arrays, which is simply synthesized by annealing IF in air and subsequent thermal treatment in reducing atmosphere. Such pre-treatment enables the growth of leaf-like Fe 3 O 4 nanosheet arrays with rich Fe2+ species and oxygen defects, which facilitate the depositing of Ir on the surface of nanosheet arrays and thereby promote the electrochemical OER performance in basic electrolyte. Impressively, the optimized Ir-Fe 3 O 4 /IF-400-2 delivers the highest electrocatalytic activity with a low overpotential of 316 mV to obtain industrial-grade current density of 500 mA cm−2 for alkaline OER. Meanwhile, the catalyst still shows good long-term stability after 36 h V-t test at 100 mA cm−2. This study offers a simple and efficient pathway to prepare self-supporting electrode for alkaline OER at industrial-grade current density. [ABSTRACT FROM AUTHOR]

The injection strategy is commonly manipulated to control the stratification level to simultaneously achieve a low pollutant emission and a high efficiency in internal combustion engines. This paper reports on a joint experimental and numerical study of the emission characteristics and engine performance in a heavy-duty direct-injection compression ignition (DICI) engine operating in the HCCI and PPC regimes, with a primary reference fuel made up of iso-octane (81% by volume) and n-heptane (19%) as a model gasoline fuel. The injection timing was varied to achieve different levels of stratification of the charge in the cylinder while the intake air temperature was adjusted accordingly to keep the same combustion phasing at 3° crank angle (CA) after top dead centre (ATDC). The main results of the present study are: (1) In gasoline DICI engines, the fuel is injected into the cylinder at an earlier SOI. The combustion process can be divided into different regimes, HCCI, PPC, and transition from HCCI to PPC, depending on SOI. Two distinctive classes of in-cylinder combustion temperature distributions could be found from the simulation results for the studied engine: one was for the SOI range from - 100 to - 48 °CA ATDC, which was the HCCI regime and/or the transition from HCCI to PPC regime, where the mean effective in-cylinder temperature was lower than 1700 K. The second class was for the SOI range from - 44 to - 20 °CA ATDC, where the combustion temperature was higher than 1850 K. This corresponded to the PPC regime. (2) The NO x emission was not only affected by the mean temperature but also the distribution of temperature in the cylinder. (3) The main source of unburned hydrocarbon (UHC) emission in the HCCI and the transition regimes was the fuel trapped in the crevice region where the oxidation process could not function properly. (4) Carbon monoxide (CO) emissions showed a non-monotonic variation with the injection timing, with one peak forming in the bowl region and one forming in the squish region during the transition from HCCI to PPC. The main source of CO emission was in the low temperature and fuel-lean region in the cylinder. (5) In the present engine operation, the pressure rise rate (PRR) in the PPC regime was higher than that in the HCCI regime, which is contrary to most results reported in the literature. This is a combined effect of low equivalence ratio in the piston bowl and in the squish region, and the stratification of the ignition delay time in the mixture. (6) Highest thermodynamic efficiency was achieved in the PPC regime with SOI from - 44 to - 31 °CA ATDC. In the transition from HCCI to PPC regime, the thermodynamic efficiency reached its lowest due to the poor combustion efficiency. [ABSTRACT FROM AUTHOR]

• 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]

Highlights • Chemical kinetic modelling of combustion of ammonia/hydrogen/air mixtures. • Addition of H 2 to NH 3 flames increases exponentially flame speed and NO x emissions. • Existing mechanisms predict scattered ignition delay times, flame speeds, and NO x. • NO formation is mainly produced through the NH 3 /O 2 chemical process. • Mechanisms' differences due to relative importance of NNH and HNO sub-mechanisms. Abstract This work reports on a study of chemical kinetic modelling of ammonia/hydrogen/air ignition, premixed flame propagation and NO emission. A survey of chemical mechanisms available in the literature was first conducted, and the performance of 10 mechanisms was analysed in terms of the prediction of shock tube ignition delay times, laminar flame speeds and NO x concentrations for NH 3 /air and NH 3 /H 2 /air flames for a wide range of operating conditions. Then, three of these mechanisms were reduced and their performance compared against the behaviour of the original mechanisms. The results confirm that pure NH 3 flames have high ignition delay times and rather low flame speeds, and that the addition of H 2 to NH 3 flames increases exponentially the flame speed, and significantly the NO x emission. The currently available chemical kinetic mechanisms predict rather scattered ignition delay times, laminar flame speeds, and NO x concentrations in NH 3 flames, indicating that improvements in the sub-mechanisms of NH 3 and NH 3 /H 2 oxidation are still needed. Sensitivity analysis for NO formation indicates that NO formation in NH 3 flames is mainly produced through the NH 3 /O 2 chemical process, and sensitivity analysis for flame speed reveals that the differences among mechanisms are due to the relative importance of the reactions of the NNH and HNO sub-mechanisms. The reduced mechanisms show high fidelity when compared with the original ones, despite some discrepancies at high pressures. [ABSTRACT FROM AUTHOR]

Highlights • Effects of ambient pressure (P am) on spray flames are characterized. • Mixture fraction during the main ignition does not vary monotonically with P am. • The main ignition shifts to the fuel-lean side in the case with the highest P am. • Factors that lead to higher soot formation rates at high P am are identified. Abstract This work reports on numerical investigation of effects of ambient pressure (P am) on spray combustion under engine-like conditions. Three cases with different P am of 42, 85 and 170 bar at a fixed ambient temperature of 1000 K are considered. Zero-dimensional calculations are first performed for autoignition of stagnant adiabatic homogenous mixtures to evaluate performance of the selected diesel surrogate fuel models and to identify the P am effects on the most reactive mixture. An Eulerian-based transported probability density function model is then chosen for the three-dimensional computational fluid dynamics study. The results show the predicted ignition delay times and flame lift-off lengths are in reasonably good agreement with experiment, with the relative difference below 28%. The current work reveals that low-temperature reactions occur across a wide range of mixture fraction but a noticeable rise of temperature (>100 K above ambient temperature) is detected first on the fuel-lean side of the stoichiometric line in all three cases. The high-temperature ignition occurs first on the fuel-rich side in the 42 and 85 bar cases, where the igniting mixture appears to be more fuel-rich in the latter case. As P am is further increased to 170 bar, the igniting mixture becomes more fuel-lean and the high-temperature ignition occurs on the fuel-lean side. The ignition behavior is found to depend on both physical and chemical processes. At 170 bar, the reaction rate increases and the associated transition from low- to high-temperature ignition is relatively fast, as compared to the transport of warmer products from the lean zone into the fuel-rich mixture. Also, within the fuel-rich region, the local temperature is low due to liquid fuel vaporization and the condition is not appropriate for ignition. These collectively cause the high-temperature ignition to occur on the fuel-lean side. Analyses on the quasi-steady spray flame structures reveal that, apart from poorer air entrainment due to reduced lift-off length, the higher rich-zone temperature and lower scalar dissipation rate also lead to a higher peak soot volume fraction at higher P am. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel catalytic CO 2 desorption processes using a jacket vessel heat exchanger were developed. • Experimental investigations on promising catalytic heat and mass transfer performance. • High turbulence flow led to more favorable thermal distributions was observed. • Positive effect on catalytic enhancement and catalyst demand. • New insights into the practicability of the catalytic desorption. This was the first work that pioneered the development a novel catalytic CO 2 desorption process using an agitated jacket vessel with a coil heat exchanger (JVC) into a bench-scale pilot plant for post-combustion carbon capture technology. The key parameters of heat and mass transport e.g., overall heat transfer coefficient, overall volumetric liquid-side mass transfer coefficient and the enhancement factor for catalyst effect, were experimentally investigated by varying the operating conditions, including temperature, stirring speed, and heating methods. The proposed investigations revealed that the utilization of the JVC was a promising technology for improving heat transfer and mass transfer performance in the desorption process due to the high turbulence flow and more favorable thermal distributions achieved by the JVC. In comparison to the conventional process with a fixed catalyst bed, the JVC demonstrated a remarkable approximately 30 % catalytic enhancement and a substantial 50 % reduction in catalyst demand. Additionally, other potential benefits and challenges of further practical operations were discussed in this paper. This research carried significant potential to contribute to the advancement of knowledge and offer valuable insights into the practicability of the catalytic desorption process for further commercial application in carbon capture. The integration of catalyst and JVC could potentially motivate the utilization of catalyst towards commercial-scale applications. [ABSTRACT FROM AUTHOR]

• A novel n-heptane reform gas combustion is investigated using LES and t-PDF model. • The n-heptane reform gas is shown to suppress the ignition and increase the liftoff. • The reforming gas weakens the NTC ignition behavior of n-heptane. • The combustion process involves inner diffusion flame and outer lean premixed flame. • The premixed flame in the surrounding reforming gas/air mixture enhances soot and suppresses NOx emission. This paper presents a large eddy simulation study of n-heptane spray combustion in an n-heptane low temperature reform (LTR) gas environment in a constant volume combustion chamber, under conditions relevant to single-fuel reactivity controlled compression ignition (RCCI) combustion engines. The LTR gas is made up of partially oxidized intermediate species from rich n-heptane/air mixture in an external constant temperature reformer. It is found that a higher reform temperature results in a longer ignition delay time of the n-heptane spray and a higher liftoff length, due to the chemical effect of the LTR gas and the difference in the reaction zone structures. A significantly different spray flame structure is identified in the RCCI case from that of single-fuel spray combustion. After the onset of high temperature ignition, a double-layer flame structure is established in the RCCI case, with a diffusion flame layer and a lean premixed flame layer. The lean premixed flame affects the flow field, which significantly suppresses the mixing around the spray tip. As a result, the RCCI case exhibits a lower NOx formation but a higher soot formation than the single-fuel case. [ABSTRACT FROM AUTHOR]

• CFD simulation of a lab-scale biomass boiler is presented with detailed chemistry. • Multiple tar species are considered to avoid using the global reaction schemes. • Application of selective non-catalytic reduction with ammonia is studied. • Effects of ammonia injection height and flow rate on NO reduction is investigated. In this paper, a numerical study of flow, reactions and NO x emissions from a biomass boiler is presented. Detailed reaction mechanisms for the decomposition of tar species, combustion of hydrocarbons and formation of NO x are employed by adopting appropriate reaction sets from literature. The proposed mechanism is used to perform CFD simulations of a laboratory-scale biomass boiler, and temperature and species concentrations are compared with the experimental data. NO x formation and emissions are studied in detail and the contribution of different pathways for NO x formation are identified. Furthermore, the CFD model is used to study the application of Selective Non-Catalytic Reduction (SNCR) method in this boiler. The effects of height and flow rate of ammonia injection on the performance of SNCR method are studied. It is observed that the SNCR, can reduce up to 63% of NO x emissions. [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]

The flame dynamics in a novel, ultra-lean non-premixed model gas turbine (GT) burner flame was numerically studied using large eddy simulation (LES) coupled with probability density function (PDF) based on the Euler stochastic field (ESF) method. One non-reacting case and three reacting cases with global equivalence ratio ϕ glob = 1.0 , 0.6 , 0.3 were simulated. Comparison of mean flow fields and OH distributions between numerical and experimental results was conducted. Flame dynamics including flame stabilization, structures and transitions of combustion modes were investigated. The simulation results were in close agreements with the experimental measurements showing the capability of PDF-ESF LES model in predicting the flame behaviors. At higher ϕ glob , the fuel jet velocity was higher, which yielded higher scalar dissipation rate, χ , near the burner exit, leading to local extinction and flame lifted-off. In the extinction region, a series of relatively low to medium temperature reactions were active, providing favorable conditions for re-ignition downstream where χ is lower than the critical scalar dissipation rate for flame extinction, χ crt . In addition, with ϕ glob decreasing, the flame height decreased due to a smaller jet velocity and χ , and thus the mechanism of flame stabilization changed from the swirl-stabilized to the bluff-body stabilized. [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]

Lean premixed turbulent methane-air flames have been investigated using direct numerical simulations (DNS) for different Karlovitz numbers (Ka), ranging from 65 to 3350. The flames are imposed to a high intensity small-scale turbulent environment, corresponding to high Ka conditions, and the effect on the flame structure is investigated during the transition from initial laminar flame to highly distorted turbulent flame. The focus is on the internal structure of different sub-layers of these flames. The preheat layer, fuel consumption layer and oxidation layer are characterized by the distribution of formaldehyde, the fuel consumption rate and the CO consumption rate, respectively. Different measures that quantify sub-layer thickness for turbulent flames have been defined and analyzed. The flame brush is broadened with time while the local thickness (excluding large scale wrinkling) of all three layers initially show thinning due to the interaction of the flame with the turbulent flow field. As time passes, the local thickness of the preheat layer and fuel consumption layer are restored while the oxidation layer remains thinned due to suppression of CO consuming reactions. As Ka increases there is an increasing probability of finding thinned, large gradient regions in each of these sub-layers. The contribution to the evolution of flame thickness from normal strain rate, chemical reaction and normal and tangential diffusion is analyzed in terms of a gradient transport equation. The relative size of the terms changes as Ka increase and, in particular, the term due to chemical reactions loses its relative significance. The observed thinning of the local flame structure is attributed to the preferential alignment of the flame normal with the compressive strain rate eigenvectors. Such alignment provides a mechanism for the flame thinning, consistent with the behavior of non-reacting scalars. A preferred angle of about 20 degrees is observed between the flame normal and the compressive strain rate eigenvector. [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]

• Transient evolution of soot mass is captured using LES. • Underlying mechanisms behind the formation of soot spike are proposed. • Before QSS, LES predicts a larger soot formation region as compared the URANS model. Numerical simulations using large eddy simulation (LES) and Unsteady Reynolds Averaged Navier–Stokes (URANS) are carried out to identify the underlying mechanisms that govern the early soot evolution process in an n -dodecane spray flame at 21% O 2 by molar concentration. A two-equation phenomenological soot model is used here to simulate soot formation and oxidation. Both ignition delay time (IDT) and lift-off length (LOL) are found to agree with experimental measurements. The transient evolution of soot mass, in particularly the soot spike phenomenon, is captured in the present LES cases, but not in the URANS cases. Hence, a comparison of numerical results from LES and URANS simulations is conducted to provide a better insight of this phenomenon. LES is able to predict the rapid increasing soot mass during the early stage of soot formation due to having a large favorable region of equivalence ratio (ϕ > 1.5) and temperature (T > 1800 K) for soot formation. This favorable region increases and then decreases to reach a quasi-steady state in the LES case, while it continues to increase in the URANS simulation during the early time. In addition, the soot spike is a consequence of the competition between soot formation and oxidation rates. The time instance when the total soot mass reaches peak value coincides with the time instance when the total mass of soot precursor reaches a plateau. The soot spike is formed due to the continuous increase of oxidizing species in the LES case which leads to a more dominant oxidation process than the formation process. [ABSTRACT FROM AUTHOR]

• 3-D URANS simulation of diesel spray using Eulerian Stochastic Field method. • Ignition characteristics at different ambient densities and nozzle diameters. • Ignition site shifts from spray head to periphery as nozzle size increases. • Ignition occurs at fuel-rich mixture in high ambient density, large nozzle case. • Higher local scalar dissipation rate in large nozzle cases lead to longer IDT. Numerical simulations are performed to investigate the effects of ambient density ( ρ am ) and nozzle diameter ( D noz ) on the ignition characteristic of diesel spray combustion under engine-like conditions. A total of nine cases which consist of different ρ am of 14.8, 30.0, and 58.5 kg/m3 and different D noz of 100, 180, and 363 μ m are considered. The results show that the predicted ignition delay times are in good agreement with measurements. The current results show that the mixture at the spray central region becomes more fuel-rich as D noz increases. This leads to a shift in the high-temperature ignition location from the spray tip towards the spray periphery as D noz increases at ρ am of 14.8 kg/m3. At higher ρ am of 30.0 and 58.5 kg/m3, the ignition locations for all D noz cases occur at the spray periphery due to shorter ignition timing and the overly fuel-rich spray central region. The numerical results show that the first ignition location during the high-temperature ignition occurs at the fuel-rich region at ρ am ⩽ 30.0 kg/m3 across different D noz . At ρ am = 58.5 kg/m3, the ignition occurs at the fuel-lean region for the 100 and 180 μ m cases, but at the fuel-rich region for the 363 μ m nozzle case. This distinctive difference in the result at 58.5 kg/m3 is likely due to the relatively longer ignition delay time in the 363 μ m nozzle case. Furthermore, the longer ignition delay time as D noz increases can be related to the higher local scalar dissipation rate in the large nozzle case. [ABSTRACT FROM AUTHOR]

• LES with state-of-the-art approach (TPDF) is used to study a dual-fuel spray flame. • A negative correlation between Fm and the value of the first stage HRR is observed. • It is found that ambient methanol affects the n-heptane LTC and cool flame structure. 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 ( ϕ m ) 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 ϕ m 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. [ABSTRACT FROM AUTHOR]

Experimental and numerical studies were performed to understand the stabilization of lean premixed natural gas/air flames in a gas turbine model combustor which was equipped with a swirl burner, known as the CECOST burner, designed to replicate the flow and flame structures in an industrial gas turbine engine. The operability range, flame stabilization, and flashback were investigated employing simultaneous OH– and CH 2 O-PLIF, and high-speed chemiluminescence imaging. Large eddy simulation (LES) was carried out for analysis of the vortex breakdown structures under non-reacting conditions. It was found that the vortex breakdown structures under isothermal conditions were insensitive to the Reynolds number (Re) for Re ≥ 10000; however, the stability of the flames and operability range of the burner were highly sensitive to Re as well as to equivalence ratio (ϕ). The equivalence ratio was varied at various Reynolds numbers to observe different regimes of the flame ranging from the lean blowout (LBO) limit to the flashback limit. The LBO limit was found to be mainly a function of equivalence ratio while being nearly independent of the Reynolds number, whereas the occurrence of flashback showed distinct characteristics for different ranges of the Reynolds number. At low and moderate Reynolds numbers, (Re ≤ 17000), flashback occurred when increasing ϕ from lean towards stoichiometric conditions. The coupling between the flow field and heat release induces vortex breakdown in the mixing tube and initiates flashback. In contrast, at higher Reynolds numbers (Re > 17000) no flashback was observed even when ϕ was increased to stoichiometric conditions. At these conditions with high Re, the increase in the bulk flow velocity affects the vortex breakdown structure, pushing the vortex breakdown downstream, which in turn prevents the flame from flashing back into the mixing tube. [ABSTRACT FROM AUTHOR]

• First study of methanol/diesel DDFS engine at low loads is conducted. • Two-stage combustion with tri-modal heat release is identified in DDFS engine. • A novel solution is developed to improve performance of LTC engine at low loads. • A guideline for optimal parameters in different combustion modes is summarized. • A co-optimization strategy is proposed for further improving engine performance. Reactivity controlled compression ignition (RCCI) engines suffer from low thermal efficiency at low loads due to the high hydrocarbon and carbon monoxide emissions. Correspondingly, a direct dual fuel stratification (DDFS) combustion mode is investigated by directly injecting methanol and diesel into cylinder. Multi-objective optimization and detailed comparison are first conducted for the two engine strategies. Compared to RCCI, the optimized DDFS case shows higher thermal efficiency, lower emissions, lower demand for the in-cylinder initial temperature, and higher potential of energy recovery. Different from the single-stage combustion in RCCI, DDFS shows a two-stage combustion, the second stage of which is owing to its near-top dead center injection of methanol. Compared to RCCI, DDFS requires a lower initial temperature to retard combustion phasing, and a larger amount of exhaust gas recirculation rate to control nitrogen oxide and ringing intensity. The optimized methanol fraction and injection timings of diesel are similar for RCCI and DDFS, and they are determined in compromise of combustion efficiency and heat transfer loss. A large spray-included angle of diesel injector is preferable for RCCI to target diesel spray to the piston lip. In DDFS, a small spray-included angle of diesel injector is needed for more complete fuel oxidation, and a medium spray-included angle of methanol injector is required to avoid excessive heat transfer loss. Due to the non-sooting nature of methanol, DDFS produces as low soot emissions as RCCI. The present study shows that the co-optimization of operating parameters and fuel properties offers a promising approach to meet the more stringent regulation on efficiency and emission. [ABSTRACT FROM AUTHOR]

• Investigation on NO, CO and NH 3 emissions from NH 3 /CH 4 /air premixed flames. • NO x emissions increase with fraction of NH 3 up to 0.5, beyond which they decrease. • NO x emissions decrease as equivalence ratio reduces towards fuel-lean conditions. • CO and NH 3 emissions are rather low regardless of the operating conditions. • Kinetic simulations showed fair agreement with the experimental data. Ammonia (NH 3) is regarded as one of the most viable alternatives to produce carbon-free energy. However, its low flammability and high NO x emissions associated with its combustion inhibit the implementation of pure NH 3 as a fuel. Dual-fuel approaches with NH 3 and more reactive fuels have been proposed; however, information on pollutant emissions is still scarce. The present work focuses on quantifying the gaseous pollutant emissions from the combustion of mixtures of NH 3 and CH 4 as a function of the equivalence ratio and amount of NH 3 in the fuel mixture. A premixed laminar burner was fired with various mixtures of NH 3 /CH 4 in air with equivalence ratios of 0.8, 0.9 and 1, and a fixed thermal input of 300 W. The NH 3 molar fraction in the fuel mixture was varied from 0 (pure CH 4) up to 0.7. Gas temperatures along the burner axis and emissions of NO x , CO and NH 3 were measured for all conditions. The results were compared with kinetic simulations using recent chemical kinetic mechanism models. The experimental results showed that the NO x concentration initially increases as the fraction of NH 3 in the fuel mixture rises up to 0.5, decreasing afterwards. Furthermore, NO x emissions decrease as the equivalence ratio is reduced towards fuel-lean conditions. CO and NH 3 emissions are fairly low indicating complete combustion of CH 4 and NH 3. The chemical kinetic mechanisms considered showed fair agreement with the experimental trends, with the NO x and CO emissions being over-predicted for most conditions. [ABSTRACT FROM AUTHOR]