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Sustainable and low-emission combustion is in need of novel schemes to enhance combustion efficiency and control to meet up with new emission standards and comply with varying quality of renewable fuels. Plasma actuation is a promising candidate to achieve this goal but few detailed experiments have been carried out that target how specific combustion and plasma related species are affected by the coupling of plasma and combustion chemistry. Atomic hydrogen is such a species that here is imaged by using the two-photon absorption laser induced fluorescence (TALIF) technique as an atmospheric pressure methane–air flame is actuated by nanosecond repetitively pulsed (NRP) discharges. Atomic hydrogen is observed both in the flame and in the discharge channel and plasma actuation results in a wide modification of the flame shape. A local 50% increase of fluorescence occurs at the flame front where it is crossed by the discharge. Atomic hydrogen in the discharge channel in the fresh-gases is found to decay with a time constant of about 2.4 μs. These results provide new insights on the plasma flame interaction at atmospheric pressure that can be further used for cross-validation of numerical calculations.

No abstract

In this Letter, we have investigated 845 nm lasing generation in atomic oxygen, present in a lean methane-air flame, using two-photon pumping with femtosecond 226 nm laser pulses, particularly focusing on the impact of nanosecond repetitively pulsed glow discharges forcing on the backward lasing signal. Characterizations of the backward lasing pulse, in terms of its spectrum, beam profile, pump pulse energy dependence, and divergence, were conducted to establish the presence of lasing. With plasma forcing of the flame, the backward lasing signal was observed to be enhanced significantly, ∼50%. The vertical concentration profile of atomic oxygen was revealed by measuring the backward lasing signal strength as a function of height in the flame. The results are qualitatively consistent with results obtained with two-dimensional femtosecond two-photon absorption laser-induced fluorescence, suggesting that the backward lasing technique can be a useful tool for studies of plasma-assisted combustion processes, particularly in geometries requiring single-ended standoff detection.

The temporal dynamics of the spatial distribution of atomic hydrogen and oxygen in a lean methane-air flame, forced by a nanosecond repetitively pulsed discharge-induced plasma, are investigated via femtosecond two-photon absorption laser-induced fluorescence technique. Plasma luminescence that interferes with the fluorescence from H and O atoms was observed to decay completely within 15 ns, which is the minimum delay required for imaging measurements with respect to the discharge occurrence. During discharge, H atoms in the excited state rather than the ground state, produced by electron-impact dissociation processes, are detected at the flame front. It was found that the temporal evolution of H and O fluorescence intensity during a cycle of 100 µs between two discharge pulses remains constant. Finally, the decay time of O-atoms produced by the discharge in the fresh methane-air mixture was about 2 µs, which suggests a faster reaction between O-atoms and methane than in air.