Large-volume excitation of air, argon, nitrogen and combustible mixtures by thermal jets produced by nanosecond spark discharges

Abstract

This work presents experimental observations of strong expanding thermal jets following the application of nanosecond spark discharges. These jets propagate in a toroidal shape perpendicular to the interelectrode axis, with high velocities of up to 30 m/s and over distances of centimeter scale. Their propagation length is much larger than the thermal expansion region produced by the conventional millisecond sparks used in car engine ignition, thus greatly improving the volumetric excitation of gas mixtures. The shape and velocity of the jets is found to be fairly insensitive to the shape of the electrodes. In addition, their spatial extent is found to increase with the number of nanosecond sparks and with the discharge voltage, and to decrease slightly with the pressure between 1 and 7 atm at constant applied voltage. Finally, this thermal jet phenomenon is observed in experiments conducted with many types of gas mixtures, including air, nitrogen, argon, and combustible CH 4 /air mixtures. This makes Nanosecond Repetitively Pulsed (NRP) discharges particularly attractive for aerodynamic flow control or plasma-assisted combustion because of their ability to excite very large volumes of gas.

Fully ionized nanosecond discharges in air: the thermal spark

“…However, this recirculation does not occur before 1 µs, as can be clearly seen from Figure 5 of Ref. [45], obtained for discharge under similar conditions. Therefore, recirculation does not affect the decay of ne shown in Figure 13.…”

“…At longer timescales (t > 1 μs), hydrodynamic effects lead to the formation of a torus and the mixing of the plasma column with fresh gas [45][46][47][48][49]. In the numerical study of Ref.…”

Hydrodynamic effects induced by nanosecond repetitive pulsed discharges

“…It should be noted that for different pin-electrodes (i.e. for various pin-tip radii of curvature) no difference was observed, neither in the energy deposition nor in the hydrodynamic effects induced by the plasma [37]. So the results presented here for pin-to-pin geometry can be considered to be quite general.…”

“…The same dependence of the energy deposition on the burst frequency is observed for pure argon and oxygen (see figure 2g. As the result the discharges initiated in these gases induce hydrodynamic affects at similar temporal and spatial scales, as demonstrated in [37].…”

“…Therefore, the average density was 0.2 mJ/mm 3 for the 2-mm gap and 0.1 mJ/mm 3 for the 7-mm gap. It should be noted that both cases correspond to volumes that are significantly larger that the ignition kernel produced by a conventional spark ignition system (for more details see [37]), whereas the volumetric energy density is still sufficient for ignition of combustible mixtures. Thus NRP can be considered as an efficient tool to redistribute spatially the energy deposited in the plasma during a period shorter than a typical ignition delay time.…”