Numerical investigation on the flame propagation process of ammonia/ hydrogen blends under engine-related conditions

Fundamental experiments under high-temperature and high-pressure conditions are challenging, leading to a gap in research on the flame propagation process of ammonia/hydrogen fuel in fields such as internal combustion engines. To compensate for the lack of flame speed under engine-relevant conditions, the premixed flame propagation for ammonia/hydrogen fuel was numerically studied through a one-dimensional simulation model based on Cantera. The temperature and pressure at the end of the ideal compression process were calculated in an isentropic process. Sensitivity analysis of flame speed and calculations of typical radical concentrations and pollutant emissions in the flame were performed. The promoting effects of initial temperature (500 K-900 K), hydrogen ratio (0-50%), and equivalence ratio (0.8-1.5) on ammonia/hydrogen flame propagation and the inhibitory effects of initial pressure (2 MPa similar to 6 MPa) were quantified. Calculation results indicated that the initial pressure increase in the high-temperature atmosphere causes the flame speed to decrease by more than 60%. In contrast, the temperature increase in the high-pressure environment provides the flame speed increase with a factor of 5. Under high temperature and high pressure, the flame speed of ammonia/hydrogen increases by more than 3.4 times when the hydrogen ratio is 50% and the flame speed reaches its peak when the equivalence ratio is 1.1-1.2, with an improvement of 65.8%. Key elementary reactions R5, R22, R24, and R30 that significantly influenced flame propagation were identified. In addition, the research also revealed emission characteristics during ammonia/hydrogen premixed flame propagation. NO emissions in the flame could be reduced by 26.7% through an increment in initial pressure from 2 MPa to 6 MPa. As the equivalence ratio ranges from 0.8 to 1.5, the minimum NO emissions occur at an equivalence ratio of 1.4, reduced by 98.9%. However, an increase in NO emissions was observed by elevating the initial temperature from 500 K to 900 K, by 54.6%, and a gradual rise in the hydrogen ratio from 0 to 50% was linked to a significant increase in NO emissions within the flame, by 79.8%. In the practical application of ammonia/hydrogen fuel in premixed combustion mode, an equivalence ratio below 1.4 is advisable.