Non-equilibrium plasma discharges for combustion applications: experiments and diagnostics*

With the urgency of mitigating climate change, the combustion community is working on innovative and environmentally friendly solutions to reduce the amount of carbon dioxide released into the atmosphere by energy conversion systems such as piston engines, gas turbines, burners, or furnaces. One of the promising strategies is to use non-equilibrium plasmas to enhance combustion, also called plasma-assisted combustion (PAC). Over the last two decades, PAC using non-equilibrium plasmas has demonstrated good abilities in flame stabilization, control of thermoacoustic instabilities, or ignition. However, lab results did not translate to industrial actuators, yet. Several questions regarding plasma efficiency under industrial-relevant conditions remain open. More work is needed on both a better understanding of the combustion-plasma coupling, and the optimization of the plasma source. To answer these questions, lab-scale experiments and quantitative diagnostics of highly reactive media are required. In this presentation, after reviewing some of the recent results obtained for PAC in industrially relevant conditions, a canonical experiment is presented, and some important data measured by optical diagnostics are detailed. The selected configuration is a 2-D axisymmetric wall-stabilized laminar flame of methane and air at ambient conditions. The non-equilibrium plasma produced by nanosecond repetitively pulsed discharges is applied across the flame front, on the symmetry axis of the flame. Discharges are generated partly in the methane-air mixture, partly in the flame front, and partly in the burned gases. Measurements by femtosecond two-photon absorption laser-induced fluorescence, as well as planar laser-induced fluorescence, and optical emission spectroscopy were performed to determine the effect of the discharges on the concentration of O, H, CH, OH, and the gas temperature.