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

In this study, Direct Numerical Simulations are utilized to investigate turbulent premixed NH3/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.