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Three axisymmetric 2D numerical model including a spherical and two cylindrical constant volume combustion vessel with different diameter to length ratio are constructed to explore the geometric effects and possibility of adopting cylindrical vessel in measurement of laminar burning velocity (LBV) by constant volume method (CVM). Two methods are adopted. First, the exact combustion properties from simulation are used to extract the LBV. Second, the estimated combustion properties based on the experimental techniques are applied. The results show that the combustion pressure and pressure rise rate are almost same in three different chambers before flame touching the wall, but in cylindrical vessel, flame surface area is changed by cylindrical confinement. The violation of spherical flame assumption applied in CVM results in the obvious overestimation of LBV. However, the discrepancy of LBV is not shown when the estimated combustion properties commonly adopted in practice are used, because the simplified properties do not capture the true burned mass fraction. Therefore, the pressure data before flame hitting the chamber wall in cylindrical vessel could perform same accuracy with that of spherical vessel in experiment. The flame impacting timing could be easily identified by the sudden drop of pressure rise rate. The only concern is that the final pressure used in calculating the burned mass fraction may be reduced by heat loss in cylinder vessel. It is suggested that before the pressure date in cylindrical vessel is utilized in determination of LBV, the final combustion pressure obtained in cylindrical vessel should be compared with the data in spherical vessel, at least compared with ideal constant volume equilibrium pressure to evaluate the deviation. [ABSTRACT FROM AUTHOR]

In order to obtain the precise predicted values of methane/hydrogen/air burning velocities from simulations, the performances of GRI mech 3.0, Aramco mech 1.3, USC mech 2.0 and San diego mech mechanisms were systematically studied under various conditions by PREMIX code and compared with experimental data from literature. The conditions where each mechanism gave their good performance are obtained and concluded. The flowrate sensitivity and rate constants of key elementary reactions were analyzed to insight the different behavior of each mechanism. The results showed that all these widely used small hydrocarbon mechanisms could gave reasonable predictions for pure methane and methane hydrogen blends. Nevertheless, they lack sensitivity for rich hydrogen at elevated pressures due to their complex reactions competitions controlled by hydrogen sub model. USC mech 2.0 was found more suitable for being used at low hydrogen contents while San diego mech gained better results at high hydrogen contents. GRI 3.0 gave good predictions for methane hydrogen blends except for high initial pressures. Generally, Aramco mech 1.3 showed the best performance for all testing conditions. Moreover, there was relatively large deviation from the predicted results and experimental data in the transition regime where the hydrogen fractions were between 60% and 80%, it may could be optimized by tuning the rate constants of reactions. [ABSTRACT FROM AUTHOR]

Recently, ammonia is considered to be an energy medium for hydrogen or energy storage and ammonia/hydrogen can be a potential fuel blend for the spark-ignition engine. Nevertheless, the combustion properties of ammonia/hydrogen mixtures could be distinctive under different initial conditions. In this study, several essential combustion properties, including laminar burning velocity, minimum ignition energy, NOx and ammonia emissions, combustion efficiency, and mixture heating values of ammonia/hydrogen/air premixed combustion were extensively studied under a wide range of equivalence ratios (ϕ), hydrogen fractions (α) and different compression ratio using one-dimensional laminar planar flame and compared with stoichiometric methane, methanol, and ethanol combustion. The behavior of NOx generation changing with ϕ and α was analyzed in detail. Results showed that most properties of ammonia/hydrogen combustion could be comparable to that of hydrocarbon fuels under engine-relevant conditions, except for the mixture heating value is slightly lower in all conditions. Besides, the NO mole fraction non-monotonically changes with ϕ and α due to the competition between the effects caused by the reduction of nitrogen-atom species and the enrichment of H/O radicals. The NO mole fraction of stoichiometric ammonia/hydrogen could be even lower than that of hydrocarbons. Considering the properties of hydrocarbon fuels as a reference, promising working conditions for ammonia/hydrogen mixtures are ϕ from 1.0 to 1.05 and α from 40% to 60%. Besides, a high compression ratio is more suitable for ammonia/hydrogen mixtures due to the excellent knock resistance ability of NH 3 /H 2 and better improvement than hydrocarbon fuels under high compression ratio. • Combustion properties of premixed NH 3 /H 2 /air were assessed under engine conditions. • One dimensional steady flame was simulated under different initial conditions. • NOx generation of NH 3 /H 2 /air combustion is analyzed in detail. • Most combustion properties, including NOx are comparable to light hydrocarbon fuels. • Potential working conditions are suggested based on the analysis. [ABSTRACT FROM AUTHOR]

Ammonia is considered as a carbon-free alternative fuel and hydrogen carrier to realize green and renewable energy conversion, but the oxidation mechanism of NH 3 still needs to be improved and validated by more experimental data. In this study, the laminar burning velocity (LBV) was measured in a constant volume vessel at various equivalence ratios, initial temperatures, and oxygen fractions up to undiluted condition. Thirteen latest NH 3 kinetics mechanisms were collected and compared with each other and the experiments. Results showed that large discrepancies of predictions presented between different mechanisms. The maximum LBV of undiluted NH 3 /O 2 mixtures could reach 125.05 cm/s. The maximum predicted LBV of NH 3 /O 2 is nearly two times higher than the minimum one for NH 3 /O 2 mixtures. The reduced Okafor (2019) and Zhang (2017) mechanism gave the best agreement with experiments for lean and rich mixtures, respectively. Sensitivity analysis of rate constant, species thermal, and transport parameters were adopted to identify the critical reactions and species in the premixed flame. It was found that because of the coupled nature of the properties mentioned above in the premixed flame, the species involved in the essential reactions also exhibit substantial thermal and transport sensitivities. [ABSTRACT FROM AUTHOR]

In this study, the laminar premixed CH 4 /H 2 /air combustion was investigated by the global reaction pathway (GP) analysis and sensitivity analysis of physical parameters for individual species under various hydrogen fractions (α) and pressures to further understand the blended fuel combustion and explore two questions: (1) whether extra GPs exist in the blended fuel combustion beyond the pure fuel reacting system; (2) whether there are insignificant species in pure CH 4 and H 2 combustion showing large influence in binary fuel combustion. Results show that different GPs dominate fuel oxidation under different α. The increasing radicals are responsible for the linearly increasing laminar burning velocity (LBV) at small α , and the transition from CH 4 chemistry to H 2 chemistry is responsible for the nonlinearly increasing LBV at large α. Extra GPs are shown and play a role in the binary fuel combustion, but they are still within the CH 4 chemistry. For most species, the sensitivity of transport and thermal parameters to laminar burning velocity were found has a positive relationship with their maximum mole fraction at different α. No species were found only important to the binary fuel, however, some species such as H 2 O show higher sensitivity for binary fuel combustion than that of pure fuel combustion. The changes of GPs with increasing pressure verifies that CH 4 chemistry is dominant for small α (20%) combustion while H 2 chemistry dominant is for large α (80%) combustion. Most physical parameter sensitivity will increase with increasing combustion pressure, which should be paid more attention in high-pressure combustion. [ABSTRACT FROM AUTHOR]