flame structure
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Ammonia combustion is a promising energy source as a carbon free fuel without greenhouse gas emissions. However, since the auto-ignition temperature is 651 degrees Celsius and the range of flammability limit is not wide compared to other fuels, fundamental studies on ammonia fires have rarely been conducted so far. Therefore, this study aims to numerically estimate fire spread characteristics when ammonia fuel in a high-pressure state leaks to the outside, especially focusing on the flammability limit according to oxygen concentration. Three kinds of reaction mechanism for numerical analysis were adopted to compare the flame structure, flammability limit, and combustion characteristics. Plank-mean absorption coefficients of nitrogen species were taken for the radiation model, in addition to the optically thin model. The effect of radiation heat loss could be identified from the maximum flame temperature trend at a low strain rate. It was confirmed that the pyrolysis of ammonia in the preheated zone results in hydrogen production, and the generated hydrogen contributes to heat release rate in the flame zone. It is found that the contribution of hydrogen would be an important role in the flammability limit of ammonia combustion. Finally, Karlovitz and Peclet numbers showed well the extinction behaviors of ammonia combustion as a result of LOC (Limit Oxygen Concentration) analysis as a function of global strain rate.

Large-Eddy Simulations (LES) of a gasoline spray, where the mixture was ignited rapidly during or after injection, were performed in comparison to a previous experimental study with quantitative flame motion and soot formation data [SAE 2020-01-0291] and an accompanying Reynolds-Averaged Navier–Stokes (RANS) simulation at the same conditions. The present study reveals major shortcomings in common RANS combustion modeling practices that are significantly improved using LES at the conditions of the study, specifically for the phenomenon of rapid ignition in the highly turbulent, stratified mixture. At different ignition timings, benchmarks for the study include spray mixing and evaporation, flame propagation after ignition, and soot formation in rich mixtures. A comparison of the simulations and the experiments showed that the LES with Dynamic Structure turbulence were able to capture correctly the liquid penetration length, and to some extent, spray collapse demonstrated in the experiments. For early and intermediate ignition timings, the LES showed excellent agreement to the measurements in terms of flame structure, extent of flame penetration, and heat-release rate. However, RANS simulations (employing the common G-equation or well-stirred reactor) showed much too rapid flame spread and heat release, with connections to the predicted turbulent kinetic energy. With confidence in the LES for predicted mixture and flame motion, the predicted soot formation/oxidation was also compared to the experiments. The soot location was well captured in the LES, but the soot mass was largely underestimated using the empirical Hiroyasu model. An analysis of the predicted fuel–air mixture was used to explain different flame propagation speeds and soot production tendencies when varying ignition timing.

HVAF (High Velocity Air Flame) flame spraying can generate supersonic high-temperature gas jets, enabling thermal spraying at unprecedented speeds. However, there is a problem with the energy cost of this device. This study focused on combustors that used cheap liquid fuel (kerosene) as the fuel for HVAF. In this research, we have developed a compact combustor with a narrow channel as a heat source for the HVAF heat atomizer. Using this combustor, the stability of the flame formed in the combustor, the morphology of the flame, and the temperature behavior in the combustion chamber were investigated in detail. As a result, the magnitude of the swirling airflow had a great influence on the structure of the flame formed in the combustor, and the stable combustion range of the combustor could be determined. As the swirling air flow rate changes, the equivalent ratio of the entire combustor changes significantly, and the flame structure also transition from the premixed flame to the diffusion flame. From this study, it was confirmed that the temperature inside the combustor has great influence on the flame structure.

Since both ethanol and acetone are the main components in many alternative fuels, research on the burning characteristics of ethanol-acetone blends is important to understand the combustion phenomena of these alternative fuels. In the present study, the burning characteristics of ethanol-acetone fuel blends are investigated at a temperature of 358 K and pressure of 0.1 MPa with equivalence ratios ranging from 0.7 to 1.4. Ethanol at 100% vol., 25% vol. ethanol/75% vol. acetone, 50% vol. ethanol/50% vol. acetone, 75% vol. ethanol/25% vol. acetone, and 100% vol. acetone are studied by the constant volume combustion chamber (CVCC) method. The results show that the laminar burning velocities of the fuel blends are between that of 100% vol. acetone and 100% vol. ethanol. As the ethanol content increases, the laminar burning velocities of the mixed fuels increase. Furthermore, a detailed chemical kinetic mechanism (AramcoMech 3.0) is used for simulating the burning characteristics of the mixtures. The directed relation graph (DRG), DRG with error propagation (DRGEP), sensitivity analysis (SA), and full species sensitivity analysis (FSSA) are used for mechanism reduction. The flame structure of the skeletal mechanism does not change significantly, and the concentration of each species remains basically the same value after the reaction. The numbers of reactions and species are reduced by 90% compared to the detailed mechanism. Sensitivity and reaction pathway analyses of the burning characteristics of the mixtures indicate that the reaction C2H2+H(+M)<=>C2H3(+M) is the key reaction.

Abstract Poly-Oxymethylene Dimethyl Ethers (OMEx) are being intensively investigated because of their potentially renewable synthesis path, which make them suitable as liquid fuels for low-carbon transport applications. In the present contribution, a computational study on the difference in combustion characteristics between dodecane and OMEx-type fuels under Engine Combustion Network (ECN) Spray A conditions is reported. In particular, a blend of different OMEx fuels have been investigated and compared to dodecane, which is a more conventional diesellike fuel. The modelling framework consists of a high-fidelity LES approach together with a Eulerian-Lagrangian spray model and flamelet-based turbulent combustion model. Results indicate ignition delay time and lift-off length according to the fuel reactivity properties, with the OMEx fuel performing similarly to dodecane. Flamelet calculations show that ignition of the oxygenated fuels is in general similar to that of dodecane, but it occurs at higher mixture fraction values due to the differences in stoichiometry. One of the most relevant outcomes of the study is the important effect that the oxygenated characteristics of OMEx has on the flame structure. Results show that for OMEx the reaction front is stabilized at distances closer to the nozzle than for dodecane, and that the flame shape as well as its internal structure is clearly affected.