Experimental and kinetic studies of laminar burning velocities of ammonia with high Lewis number at elevated pressures

•The laminar flame speeds of NH3/O2/He/Ar mixtures at elevated pressures (up to 1.5 MPa) were firstly (to our knowledge) measured.•The measured data expanded the available database of ammonia kinetic model validation targets.•Physical and chemical effects of N2 addition on the production of N2 and NOx were discussed.•The main kinetic effect of N2 addition on NO production: N2 addition shifts the reaction N + NON2 + O to the left-hand side. The present study aims to expand the available database of laminar burning velocities to a higher initial pressure for ammonia kinetic model validation and to investigate the ammonia flame chemistry especially for the effect of N2 addition on N2 and NOx production. In the present study, a high-pressure constant-volume combustion vessel with the pressure release tank was developed and validated. The variations of Lewis number with diluent (He/N2, He/Ar) ratio and the relationships of Lewis number and adiabatic flame temperature with initial pressures were investigated to prevent the flame instability effects and create safe experiments at high pressures. The laminar burning velocities of NH3/O2/He/Ar mixtures with Lewis number around 1.5 and the fixed adiabatic flame temperature of 2250 K at elevated pressures (up to 1.5 MPa) were measured and the overall reaction order of about 1.6 was determined, besides, the effects of pressure on laminar burning velocities and NOx formations were investigated. Moreover, to provide more useful validation data at high pressure, the laminar burning velocities of NH3/O2/He/Ar mixtures at different adiabatic flame temperatures and initial pressure of 1.0 MPa were presented. Additionally, the predictions of four kinetic models on the experimental data were compared and the discrepancies between the calculated and measured results were found. Since ammonia contains nitrogen element, the effects of N2 addition on the laminar burning velocities and the production of N2 and NOx in ammonia combustion were investigated and the physical and chemical effects of N2 addition were separated. Results showed that the laminar burning velocities and NOx concentration for NH3/O2/N2 flames decreased with the increase of N2 dilution and a slight non-linear tendency was observed. Both physical and chemical effects inhibit the N2 production in the NH3/O2/N2 mixtures. The chemical effects play a dominant role in the inhibition of N2 production at the N2 dilution rates below 0.3. The main kinetic effect of N2