Comparative effects of plasma and preheating in assisting premixed ammonia/air flames: A DNS study

• Plasma proved to be a more effective method for enhancing ammonia flame speed compared to preheating. • However, preheating increases the flame surface area more significantly than plasma-assisted ammonia flames. • Detailed kinetic pathways are presented and the role of plasma in altering the oxidation and NO emissions are explored. • Plasma-assisted ammonia flames led to relatively lower NO emissions compared to preheating. • It is revealed that, in plasma-assisted flames, NO emissions are less dependent on the local heat release rate when contrasted with preheated flames. In this study, Direct Numerical Simulations are utilized to investigate turbulent premixed NH 3 /air flames assisted by two distinct methods: non-equilibrium nanosecond plasma discharges and preheating, while maintaining equal input energy levels for both methods. The results show that plasma is more effective than preheating in increasing the turbulent burning velocity, namely by up to 31% under lean and by 26% in rich conditions. Furthermore, the flame structure is less affected by turbulence when using plasma. A negative correlation between flame displacement speed and local flame curvature is observed for all cases. Furthermore, negatively curved parts of the flame front are dominated by the reaction mode of combustion. In contrast, the positively curved parts are controlled by flame propagation mode in both preheated and plasma-assisted cases. It is shown that, when plasma is utilized, NO emissions are less sensitive to local heat release rate, and the amount of NO emissions is found to be 19% lower in comparison to the preheated case.