Numerical investigation of thermal and chemical effects of nanosecond repetitively pulsed discharges on a laminar premixed counterflow flame

摘要

This paper focuses on modelling the influence of nanosecond repetitively pulsed (NRP) corona discharges on a lean premixed methane-air flame at atmospheric pressure and ambient temperature. The discharges are produced by high voltage pulses of 3.5 kV amplitude and 10 ns duration, with a repetition frequency of 30 kHz. Taking into account the geometry of the electrodes, at breakdown, the reduced electric field is then comprised between 40 and 500 Td. The flame has an equivalence ratio of 0.7. A numerical model is developed to determine the effect of the discharges on the unburnt gas. In a first scenario, all the plasma energy goes to chemical reactions, eventually resulting in ozone production. A second scenario considers that all the plasma energy is thermal, thus leading to a temperature increase of the fresh gases. The third scenario considers that half the energy goes to ozone production and the other half to fresh gases heating. The ozone concentrations or gas heating resulting from each of these scenarios are then implemented into the simulation of a freely propagating flame, allowing the comparison of thermal and chemical effect of plasma discharges, as well as their combined effect, on the laminar flame speed and adiabatic flame temperature. The high ozone concentrations (up to 10,000 ppm) resulting from the NRP corona discharges enhance the laminar flame speed up to 40% of its initial value, while the temperature increase (up to 10 K) results into a maximum flame speed enhancement of 25%. The effect on the adiabatic flame temperatures follow a similar trend.