Plasma assisted combustion: Dynamics and chemistry

Plasma-assisted ignition and combustion

[1] Streamers are thin filamentary plasmas that can initiate spark discharges in relatively short (several centimeters) gaps at near ground pressures and are also known to act as the building blocks of streamer zones of lightning leaders. These streamers at ground pressure, after 1/N scaling with atmospheric air density N, appear to be fully analogous to those documented using telescopic imagers in transient luminous events (TLEs) termed sprites, which occur in the altitude range 40–90 km in the Earth's atmosphere above thunderstorms. It is also believed that the filamentary plasma structures observed in some other types of TLEs, which emanate from the tops of thunderclouds and are termed blue jets and gigantic jets, are directly linked to the processes in streamer zones of lightning leaders. Acceleration, expansion, and branching of streamers are commonly observed for a wide range of applied electric fields. Recent analysis of photoionization effects on the propagation of streamers indicates that very high electric field magnitudes ∼10 Ek, where Ek is the conventional breakdown threshold field defined by the equality of the ionization and dissociative attachment coefficients in air, are generated around the tips of streamers at the stage immediately preceding their branching. This paper describes the formulation of a Monte Carlo model, which is capable of describing electron dynamics in air, including the thermal runaway phenomena, under the influence of an external electric field of an arbitrary strength. Monte Carlo modeling results indicate that the ∼10 Ek fields are able to accelerate a fraction of low-energy (several eV) streamer tip electrons to energies of ∼2–8 keV. With total potential differences on the order of tens of MV available in streamer zones of lightning leaders, it is proposed that during a highly transient negative corona flash stage of the development of negative stepped leader, electrons with energies 2–8 keV ejected from streamer tips near the leader head can be further accelerated to energies of hundreds of keV and possibly to several tens of MeV, depending on the particular magnitude of the leader head potential. It is proposed that these energetic electrons may be responsible (through the “bremsstrahlung” process) for the generation of hard X rays observed from ground and satellites preceding lightning discharges or with no association with lightning discharges in cases when the leader process does not culminate in a return stroke. For a lightning leader carrying a current of 100 A, an initial flux of ∼2–8 keV thermal runaway electrons integrated over the cross-sectional area of the leader is estimated to be ∼1018 s−1, with the number of electrons accelerated to relativistic energies depending on the particular field magnitude and configuration in the leader streamer zone during the negative corona flash stage of the leader development. These thermal runaway electrons could provide an alternate source of relativistic seed electrons which were previously thought to require galactic cosmic rays. The duration of the negative corona flash and associated energetic radiation is estimated to be in the range from ∼1 μs to ∼1 ms depending mostly on the pressure-dependent size of the leader streamer zone.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JA011350

Monte Carlo model for analysis of thermal runaway electrons in streamer tips in transient luminous events and streamer zones of lightning leaders

This review covers the results obtained in the period 2006–2014 in the field of plasma-assisted combustion, and in particular the results on ignition and combustion triggered or sustained by pulsed nanosecond discharges in different geometries. Some benefits of pulsed high voltage discharges for kinetic study and for applications are demonstrated. The necessity of and the possibility of building a particular kinetic mechanism of plasma-assisted ignition and combustion are discussed. The most sensitive regions of parameters for plasma–combustion kinetic mechanisms are selected. A map of the pressure and temperature parameters (P–T diagram) is suggested, to unify the available data on ignition delay times, ignition lengths and densities of intermediate species reported by different authors.

https://iopscience.iop.org/article/10.1088/0022-3727/47/35/353001/pdf

Plasma-assisted ignition and combustion: nanosecond discharges and development of kinetic mechanisms

Observations of a shock wave propagating through a decaying plasma in the afterglow of an impulse high-voltage nanosecond discharge and of a surface dielectric barrier discharge in the nanosecond range were analysed to determine the electron power transferred into heat in air plasmas in high electric fields. It was shown that approximately half of the discharge power can go to heat for a short (~1 µs at atmospheric pressure) period of time when reduced electric fields are present at approximately 103 Td. A kinetic model was developed to describe the processes that contribute towards the fast transfer of electron energy into thermal energy under the conditions considered. This model takes into account previously suggested mechanisms to describe observations of fast heating in moderate (~102 Td) reduced electric fields and also considers the processes that become important in the presence of high electric fields. Calculations based on the developed model agree qualitatively with analyses of high-voltage nanosecond discharge observations.

https://iopscience.iop.org/article/10.1088/0022-3727/43/25/255201/pdf

Mechanism of ultra-fast heating in a non-equilibrium weakly ionized air discharge plasma in high electric fields

Kinetics of ignition of saturated hydrocarbons by nonequilibrium plasma: C2H6- to C5H12-containing mixtures

The kinetics of ignition in stoichiometric C n H 2 n +2 :O 2 :Ar mixtures with 90% dilution for n  = 1–5 has been studied experimentally and numerically under the action of a high-voltage nanosecond discharge. It was shown that the initiation of the discharge by a high-voltage pulse 115 kV in amplitude with a specific deposited energy of 10–30 mJ/cm 3 leads to more than an order of magnitude decrease in the ignition delay time. The generation of atoms, radicals and excited and charged particles by the discharge was numerically described. The role of different atoms and radicals (O, H and C n H 2 n +1 ) was analyzed. The temporal evolution of the densities of intermediate components in the plasma assisted ignition was discussed.

Mechanism of ignition by non-equilibrium plasma

Plasma decay after a high-voltage nanosecond discharge was studied experimentally and numerically in room temperature N2, CO2 and H2O for pressures between 1 and 10 Torr. The time-resolved electron density was measured by a microwave interferometer for initial electron densities in the range 8 × 1011–3 × 1012 cm−3 and the effective electron–ion recombination coefficient was determined. It was shown that this coefficient varies in time and depends on pressure. A numerical simulation was carried out to describe the temporal evolution of the densities of charged particles under the conditions considered. A good agreement was obtained between the calculated and the measured electron density histories. It was shown that the loss of electrons is governed by dissociative recombination with complex ions, their density being dependent on pressure. In N2 at low pressures, a hindered electron thermalization in collisions with molecules led to a delay in the plasma decay. This effect was observed both experimentally and theoretically.

https://iopscience.iop.org/article/10.1088/0022-3727/40/15/019/pdf

Plasma decay in N2, CO2 and H2O excited by high-voltage nanosecond discharge

Experiments are described in which spark discharges in long non-uniform gaps are studied in Ar with a small addition (1-5%) of O2 under standard conditions. The results obtained show that even the addition of 1% O2 drastically reduces the conductivity of the streamer channel and increases the average electric field required to bridge the discharge gap by a streamer. Breakdown, producing short circuiting, is developed through leader propagation in Ar:O2 mixtures, in contrast to a non-thermal streamer mechanism of breakdown that was previously observed in pure Ar. A 1.5-dimensional simulation was carried out which supports the general observed properties of streamers in long gaps in Ar:O2. An analysis of the simulation results shows that the effect of small O2 additions on the streamer properties in Ar is explained by the quenching of excited Ar atoms by O2 molecules, which leads to the deceleration of two-step ionization in the streamer channel.

The effect of small O2 addition on the properties of a long positive streamer in Ar

This paper presents the results of experimental and theoretical studies of an afterglow in room temperature air and O2 excited by a high-voltage nanosecond discharge for pressures between 1 and 10?Torr. We measured time-resolved electron density by a microwave interferometer for initial electron densities in the range (2?3)???1012?cm?3. Discharge uniformity was investigated by optical methods. The balance equations for charged particles and electron temperature were numerically solved to describe the temporal evolution of the densities of electrons and ions in the discharge afterglow. It was shown that the loss of electrons is governed by dissociative and three-body electron recombination with ions under the conditions considered. Good agreement between the calculated and measured electron density histories could be obtained only when the rate of three-body recombination was increased by an order of magnitude and when the dependence of the recombination rate on electron temperature was changed. This could testify that the well-understood mechanism of three-body electron recombination with atomic ions could be noticeably modified in the case of molecular ions.

https://iopscience.iop.org/article/10.1088/0022-3727/45/25/255202/pdf

Nikolay Aleksandrov

Papers

Kinetics of ignition of saturated hydrocarbons by nonequilibrium plasma: C2H6- to C5H12-containing mixtures

Mechanism of ignition by non-equilibrium plasma

Plasma decay in N2, CO2 and H2O excited by high-voltage nanosecond discharge

The effect of small O2 addition on the properties of a long positive streamer in Ar

Plasma decay in air and O2 after a high-voltage nanosecond discharge