Numerical Study of NH₃/H₂-Air Ignition in Nanosecond Plasma Discharges with Non-Equilibrium Energy Transfer

Ammonia (NH3), with its higher energy density and easer storage and transportation as a hydrogen carrier, is emerging as a promising green alternative fuel. However, its application in power generation is limited by challenges such as a low burning velocity, slow oxidation at low temperatures, ignition difficulties, and nitrous oxide formation. This study computationally examines the role of non-equilibrium energy transfer with nanosecond discharges on NH3 ignition and flame propagation in NH3/H2-air flows at 700 K and 1 atm using two-dimensional simulations. A non-monotonic relationship between ignition kernel volume and applied voltage is observed, with optimal ignition enhancement occurring at 200 Td. At this reduced electric field, the production of radicals and electronically excited species is maximized. The study identifies an optimal electrode gap size for a given pulse energy. While smaller gap sizes increase energy density, enhancing temperature and radical concentrations, overly small gaps restrict the ignition kernel size. A nonlinear dependency between pulse repetition frequency and ignition kernel volume is observed in nanosecond pulsed high-frequency discharges (NPHFD). There is an optimal frequency of plasma discharge for a given plasma energy and the number of discharge pulses. These findings provide valuable insights for designing controlled plasma discharge strategies to improve NH3 ignition in reactive flows, with potential applications in internal combustion engines and gas turbines.