Investigating the laminar burning velocity of NH3/H2/air using the constant volume method: Experimental and numerical analysis

The fundamental combustion field of ammonia-hydrogen fuel is being extensively researched. However, studies on the laminar burning velocity (LBV) under high pressure with varying hydrogen concentration conditions are relatively scarce. This study innovatively used the constant volume method (CVM) to measure LBV of ammonia-hydrogen fuels with various hydrogen concentrations X-H2 = 15%-30%, initial temperatures T-i = 298-428 K, initial pressures P-i = 1-3 bar, and equivalence ratios phi = 0.8-1.4. The LBV of the fuel mixture up to the temperature and pressure of 540 K and 7 bar under isentropic adiabatic assumption was determined with the CVM method. The results illustrate that the enhancing effect of hydrogen concentration on LBV is suppressed under high pressures, supporting the argument that the equivalence ratio associated with the maximum LBV is solely determined by the initial fuel composition. Chemical kinetics simulations were also analyzed including flame structure, sensitivity analysis, and reaction pathways. It indicated that the elementary reaction H + O-2<=>O + OH predominantly governs the LBV of ammonia-hydrogen. An increase in hydrogen concentration intensifies this process by boosting the concentration of free radical hydrogen, albeit with a concomitant rise in NO emissions due to nitrogen-containing radical oxidation. Conversely, an increase in initial pressure diminishes NO emissions by strengthening the pathway from NO to NNH. The findings of this study enhance the LBV database of ammonia-hydrogen fuel at high pressures and high temperatures, and can further act as a reference for other experiments.