Hydrodynamic regimes induced by nanosecond pulsed discharges in air: mechanism of vorticity generation

Abstract

The mechanisms controlling the hydrodynamic effects induced by nanosecond pulsed discharges applied between two pin electrodes in air at atmospheric conditions are investigated experimentally and numerically. A cylindrical plasma kernel is formed between the electrodes during the discharge. After the discharge, schlieren images show that the cylindrical kernel evolves in different ways depending on the gap distance: (1) for short gap distances (), the cylindrical kernel collapses to form a torus-like structure. In this case, the flow field presents a significant source of vorticity; (2) for larger gaps (), the kernel retains its initial cylindrical shape and cools down primarily through heat diffusion. The objective of this work is to understand the mechanisms leading to the formation of these two hydrodynamic regimes. To this end, simulations were performed and compared with the experimental schlieren images. It is shown that the main source of vorticity is the baroclinic torque caused by the cylindrical blast wave that follows the fast energy addition during the discharge. Finally, a criterion is given to predict the occurrence of the two hydrodynamic regimes. This criterion is then validated against experimental results from the literature.

Actuation efficiency of nanosecond repetitively pulsed discharges for plasma-assisted swirl flames at pressures up to 3 bar

“…Depending on the applied voltage and PRF, two discharge regimes can be obtained, namely, NRP glow and the NRP spark discharges, as defined in [16,17]. The NRP glow discharges mainly have a chemical impact on the flame [18], while NRP spark discharges influence the flame hydrodynamically, thermally, and chemically [19][20][21]. First, it is interesting to note that for a given setting of the plasma actuation, both NRP glow discharges and NRP spark discharges may be observed, with a similar breakdown voltage of about 6 kV (peak voltage in figures 4a and 4b).…”

“…During an NRP spark discharge, the energy is deposited in a very short time and P d can be calculated considering heating at constant volume of the gas in the discharge channel. For the conditions of this study, the fraction of total energy deposited by the discharges that goes into ultra-fast heating, E thermal , is assumed to be 20% of the total deposited energy, as for example in [19]. Only NRP spark discharges are considered in this model.…”

Resonant dual-pulse laser ignition technique based on oxygen REMPI pre-ionization

“…In some cases, we also employ a computational fluid dynamics (CFD) model to analyze the temperature evolution of the laser plasmas. The CFD simulations are performed using a Riemann solver developed in-house based on the Navier–Stokes equations 16 , 45 , 46 . Previous work by the authors has demonstrated that the initial (end-of-the pulse) state of the NIR breakdown kernel can be obtained iteratively by fitting the experimental 1-D scattering profiles at early times as shown in Fig.…”

Improvement of lean blow out performance of spray and premixed swirled flames using nanosecond repetitively pulsed discharges

“…This energy is deposited during subsequent reflections at t = 90 ns and t = 180 ns. For detailed descriptions of the oscillograms, see [11,21].…”

“…Non-equilibrium NRP (Nanosecond Repetitively Pulsed) spark discharges last a few nanoseconds, with an overvoltage of a few kilovolts. Applied at a repetition rate in the 10 to 100 kHz range in a combustible mixture, they produce thermal, chemical, and hydrodynamic effects [11][12][13] that promote the combustion process [2,4,6,14]. These benefits have been extensively demonstrated at low and atmospheric pressures, and some recent studies [15,16] have also shown positive effects for the ignition of lean mixtures at pressures up to 16 bar.…”