Plasma assisted NH3/H2/air ignition in nanosecond discharges with non-equilibrium energy transfer

Abstract

Ammonia (NH₃), with its high energy density and easiness to store and transport as a hydrogen carrier, has become a promising alternative green fuel. However, its adoption in power generation is hindered by challenges such as low burning velocity, slow low-temperature oxidation, high NO_x emissions, and ignition difficulty. This work computationally investigates the effects of non-equilibrium energy transfer by nanosecond discharges on NH₃ ignition and flame propagation in an NH₃/H₂/air flow at 700 K and 1 atm. The simulation results demonstrate that NH₃/air mixtures require a large ignition energy due to their large critical ignition radius. It is shown that adding 30 % hydrogen significantly reduces the critical ignition radius and minimum ignition energy. Two-dimensional modeling further shows a non-monotonic dependence of ignition kernel volume on the applied voltage and reduced electric field. The optimum ignition enhancement occurs at 200 Td where the generation of electronically excited species and radicals including N₂(B), O(¹D) and OH becomes most efficient. Higher voltages divert electron energy toward ionization, which makes it less effective for NH₃ ignition. The study also identifies an optimal electrode gap size for a given pulse energy. Smaller gap sizes increase deposited energy density, raising temperature and radical concentrations. However, excessive reduction of the gap distance reduces flame propagation speed due to the flame stretch effect in rich mixtures with the effective Lewis number greater than unity. A nonlinear relationship between pulse repetition frequency and ignition kernel volume is observed in a nanosecond pulsed high frequency discharge (NPHFD). An optimal frequency range of 200 kHz to 2 MHz is found when two pulses are used. In addition, an optimal number of pulses exists for each pulse repetition frequency, with higher frequencies requiring more pulses to maximize the overlap region. These findings provide critical insights on developing controlled plasma discharge techniques for efficient NH₃ ignition in reactive flows within internal combustion engines and gas turbines.