Role of Transient-Plasma Generated Free Radicals in Hydrogen Combustion: Spectroscopic and Theoretical Insights

• TPI produces fast, non-equilibrium discharges generating abundant H· radicals. • Optical spectroscopy captures H α as direct evidence of radical formation. • Kinetic modeling shows H· accelerates OH· formation and H 2 oxidation. • TPI flames propagate faster than CSI beyond flame-wrinkling effects. • Lagrangian simulations reveal distinct TPI radical pathways. Transient plasma ignition is known to improve the combustion of various fuels. Here, we compare the combustion mechanisms of carbon-free fuels containing mixtures of H 2 and O 2 via transient plasma ignition and conventional spark ignition. The transient plasma ignition is produced by five 20 ns pulses at a frequency of 1 kHz with an amplitude of 20 kV. Compared to conventional spark ignition (CSI), TPI ignited four times faster at a lean equivalence ratio (ϕ = 0.4). Optical diagnostics confirmed the presence of H α emission during TPI, providing direct evidence of atomic hydrogen radicals generated by nanosecond plasma discharges. These radicals, coupled with electrostatic flow effects, reduced ignition delay and accelerated flame propagation beyond enhancements attributable to flame wrinkling alone. Complementary kinetic modeling with Cantera and one-dimensional Lagrangian simulations revealed that plasma-generated H· radicals bypass high-temperature initiation pathways. This shortens induction times and promotes earlier OH· formation, thereby accelerating H 2 oxidation and water production under both adiabatic and non-adiabatic conditions. Together, these findings demonstrate that TPI enhances hydrogen combustion by coupling radical-driven kinetics with fluid-dynamic effects, offering a promising strategy to improve ignition reliability and efficiency in carbon-free propulsion systems.