Nanosecond pulsed discharges for reliable ignition of ultra-lean hydrogen-air mixtures

The safe ignition and stabilization of ultra-lean hydrogen-air mixtures remain a critical challenge for enabling low-emission hydrogen combustion in gas turbines. This study investigates nanosecond repetitively pulsed discharges for reliable ignition under conditions relevant to lean-premixed hydrogen operation. Experiments were conducted in a modular combustion rig, where the influence of plasma parameters – including pulse energy, pulse repetition frequency, and pulse number – on ignition and flame kernel development was systematically explored. High-speed OH${}^{\ast }$ chemiluminescence imaging tracked ignition kernel formation and propagation, while optical emission spectroscopy provided characterization of the plasma properties. For equivalence ratios of $\varphi =0.15-0.2$, successful ignition is mostly a function of pulse energy, more than pulse number, with a clear transition from non-ignition to reliable ignition observed above a critical energy threshold. This transition coincides with the change from glow to spark regime. Rather than showing a probabilistic ignition behavior, ignition occurs reliably in the spark regime, and fails at the transition to glow. Optical emission spectroscopy measurements of gas and vibrational temperatures indicate that this shift coincides with an increased production of radicals rather than vibrational excitation, which are more effective in enhancing ignition. For equivalence ratios of $\varphi =0.086$ and 0.1, kernels were created but did not expand in the flowing mixture, and the interaction between pulses — pulse number and repetition frequency became critical. At these ultra-lean conditions, these interactions enable the formation of larger ignition kernels that are less prone to extinction. These findings demonstrate that nanosecond repetitively pulsed based plasma-assisted ignition can significantly extend the ignition limits of lean hydrogen mixtures, offering a promising pathway for stabilizing ultra-lean hydrogen combustion with minimized NO${}_x$ emissions. Moreover, the ability to reliably ignite ultra-lean mixtures is highly relevant for hydrogen internal combustion engines, where consistent ignition at low equivalence ratios is crucial to reducing cycle-to-cycle variability and improving efficiency.