Fluorescence lifetime imaging of nitric oxide in nanosecond pulsed discharge-assisted NH3/air flames

Laser-induced fluorescence is a widely used non-invasive method for characterizing NOx emission, mostly in combustion applications, but also in many plasma facilities. Under the carbon-free prerequisite, non-thermal plasma-assisted combustion is a promising technology to address the low flammability issues of ammonia (NH3) flames, but nitric oxide (NO) emission remains unknown. NO quantification in such plasma-flame environments is a daunting task due to largely unknown fluorescence quenching, which urgently drives this study. In this work, we map the NO fluorescence lifetime (τ) in an NH3/air flame sustained in a nanosecond pulsed discharge (NPD) at various time delays. Firstly, in both burnt and unburnt zones, τ increases slightly in the first 2 μs after the discharge, and then almost remains constant. Secondly, the impact of NPD on τ differs between the burnt and unburnt zones. In the burnt zone, τ of NO exhibits a modest increase (less than 10%) compared to that without NPD pulses, which can be theoretically explained by the temperature rise (i.e. decreased number density) due to the NPD pulse. Besides, a shock front originates from the anode in the burnt zone and exhibits a dip in τ. This further supports the decisive role of number density in quenching of laser-excited NO(A). However, in the unburnt zone, where plasma-induced NO is primarily generated, within the measured 1-100 μs delay after the discharge, τ is unexpectedly long, twice that of the theoretical calculation. It might be attributed to the plasma-induced NH3 decomposition and other excited radicals at low temperatures. These experimental findings clarify, for the first time, the impact of non-thermal NPD on NO(A) quenching, providing a foundation for quantitative analysis of NO in plasma applications.