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h i g h l i g h t sLaminar burning velocities of ammonia/air flames at high pressures are evaluated. Maximum value of laminar burning velocity of ammonia/air flame is about 7 cm/s. Laminar burning velocity decreases with the increase in the pressure. Markstein length increases with the increase in equivalence ratio. Markstein lengths at high pressure are lower than those at 0.1 MPa. a b s t r a c tAmmonia is expected to be useful not only as a hydrogen-energy carrier but also as a carbon-free fuel. In order to design an ammonia fueled combustor, fundamental flame characteristics of ammonia must be understood. However, knowledge of the characteristics of ammonia/air flames, especially at the high pressures, has been insufficient. In this study, the unstretched laminar burning velocity and the Markstein length of ammonia/air premixed flames at various pressures up to 0.5 MPa were experimentally clarified for the first time. Spherically propagating premixed flames, which propagate in a constant volume combustion chamber, were observed using high-speed schlieren photography. Results indicate that the maximum value of unstretched laminar burning velocities is less than 7 cm/s within the examined conditions and is lower than those of hydrocarbon flames. The unstretched laminar burning velocity decreases with the increase in the initial mixture pressure, tendency being the same as that of hydrocarbon flames. The burned gas Markstein length increases with the increase in the equivalence ratio, the tendency being the same as that of hydrogen/air flames and methane/air flames. The burned gas Markstein lengths at 0.1 MPa are higher than those at 0.3 MPa and 0.5 MPa. However, the values of burned gas Markstein length at 0.3 MPa and 0.5 MPa are almost the same. In addition, numerical simulations using CHEMKIN-PRO with five detailed reaction mechanisms which are presently applicable for the ammonia/air combustion were also conducted. However, qualitative predictions of unstretched laminar burning velocity using those reaction mechanisms are inaccurate. Thus, further improvements of reaction mechanisms are essential for application of ammonia/air premixed flames.

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Ammonia is a carbon-free fuel and its application to internal combustion engines is expected. However, few studies on ammonia flames, especially at high pressures, have been carried out because ammonia has not been considered to be a fuel owing to its lower combustion intensity. Most of NOx, which is formed by ammonia combustion, is considered to be the fuel NOx. The objectives of this study were to investigate the fundamental characteristics of NOx experimentally, such as NO emission and chemiluminescence of ammonia/air flames not only at the atmospheric pressure but also under high pressures and to explore NO formation/reduction mechanisms using numerical simulation. Experiments were carried out using a nozzle-type burner. NH2 ammonia  band spectra were observed, and it was clarified that the color of ammonia flame is mainly determined by the NH2 ammonia  band and H2O spectra. Burned gas was sampled from ammonia flame stabilized at the burner. The mole fraction of NO decreased with the increase in equivalence ratio at atmospheric pressure. Reaction flow analysis was performed, and it was clarified that the decrease in the mole fraction of NO for rich mixtures was caused by NHi (i = 2, 1, 0). High pressure experiments were performed using a high pressure combustion test facility for stoichiometric ammonia flame. Consequently, the decrease in the mole fraction of NO was experimentally observed and its tendency was found to qualitatively agree with the results of the numerical simulation. It was clarified that the third body reaction of OH + H + M  H2O + M plays an important role in the reduction of the mole fraction of NO at high pressure.