Formation and development of a leader channel in air

A self-consistent model is constructed that makes it possible to investigate the formation of a leader channel in air and the evolution of the channel parameters in the developed stage, when the leader is as long as several meters or more. The initial stage of the formation of the channel is characterized by a rapid increase in the electron density and gas temperature, which is a consequence of the onset of thermal-ionizational instability. The radius of a fully developed plasma column at the current I=1 A in air at atmospheric pressure is R _h?10^?2 cm. Then, because of the gas-dynamic and thermal expansion, the plasma radius R _h increases considerably; as a result, the electric field and the reduced field E/N in the corresponding parts of the channel decrease. In the case under consideration, the field in the “oldest” parts of the leader drops to 200 V/cm and even lower and the reduced field becomes as weak as E/N≤10 Td. In this case, the densities of the main species of neutral and charged particles at the center of the channel remain close to their thermodynamically equilibrium values. The results of calculations are compared with the available experimental data.     

Discharge dynamics and the production of active particles in a cathode-directed streamer

The emission spectroscopy technique is used to analyze a cathode-directed streamer discharge in air at atmospheric pressure in point-plane geometry at interelectrode distances of up to 100 mm and a high-voltage pulse amplitude of 18 kV. The densities of molecules in the N₂(C ³Π_u, v=0), N ₂ ⁺ (B ²Σ _u ⁺ , v=0) and NO(A ²Σ⁺, v=0) states are determined, and the reduced electric field in the streamer head is estimated. It is shown that the increase in the average electric field in the discharge gap substantially intensifies the production of active particles in the discharge plasma and makes the plasma more homogeneous. This effect is only related to the increase in the fraction of regions with a high electric field in the discharge gap and, as a result, the reduction of the discharge energy losses via rapidly thermalized degrees of freedom. The active particles are only produced in the streamer head, including the case in which the interelectrode gap is bridged by the streamer channel.     

Charging of Macroparticles in a Corona Discharge in an Air Flow

The charging of Al₂O₃ macroparticles with dimensions ranging from 20 to 40 μm in a gas flow passing through a multielectrode corona discharge is investigated. The corona discharge is produced by a system of wire electrodes arranged across the gas flow. The particle charge and mass are measured using a linear electrodynamic trap. For a corona voltage of 18 kV, the average charge-to-mass ratio is found to be 1.69 × 10¹³e/g for particles charged in the positive corona and 1.35 × 10¹³e/g for particles charged in the negative corona.     

Simulation of ethylene conversion initiated by a streamer corona in an air flow

The conversion of ethylene (C₂H₄) at concentrations of 400 and 930 ppm in an air flow at a temperature of 295 K is simulated. Ethylene is added to air either upstream of the discharge chamber or in the reaction tube, downstream of a pulsed corona discharge. It is taken into account that the distribution of the gas components in the discharge zone is nonuniform due to the streamer nature of the discharge. In the reaction tube, all of the components are assumed to be uniform. Simulation results agree with the experiments carried out at voltage pulse amplitudes of 30 and 40 kV, a gas flow rate of 2–10 l/min, and a specific energy deposition of up to 0.15 J/cm³. It is shown that the ozone produced plays a governing role in the C₂H₄ conversion. It is found that it is possible to minimize the energy spent on conversion by choosing the optimum pulse repetition rate and the specific energy deposited per pulse. The presence of water vapor impedes the ethylene conversion and increases the concentration of formaldehyde and methane.     

Glow discharge in singlet oxygen

Currently, there is no experimental data on the plasma balance in gas mixtures with a high content of singlet delta oxygen O₂(¹Δ_g). These data can be obtained by studying the parameters of an electric discharge in singlet oxygen produced by a chemical generator. The O₂(¹Δ_g) molecules significantly change the kinetics of electrons and negative ions in plasma. Hence, the discharge conditions at low and high. O₂(¹Δ_g) concentrations are very different. Here, the parameters of the positive column of a glow discharge in a gas flow from a chemical singlet-oxygen generator are studied. It is experimentally shown that, at an O₂(¹Δ_g) concentration of 50% and at pressures of 1.5 and 2 torr, the electric field required to sustain the discharge is considerably lower than in the case when all of the oxygen molecules are in the ground state. A theoretical model of the glow discharge is proposed whose predictions are in good agreement with the experimental data.     

Experimental study of a low-pressure glow discharge in air in large-diameter discharge tubes: I. conditions for the normal regime of a glow discharge

The initiation and characteristics of a low-pressure glow discharge in air in large-diameter discharge tubes are studied. A deviation from the Paschen law is observed: the breakdown curves U _dc(pL) shift toward the higher values of U _dc and pL as the interelectrode distance L increases. It is shown that the normal regime of a glow discharge is accompanied by gas ionization in the anode sheath. This takes place only for pL values lying to the right of the inflection point in the breakdown curve. The cathode-sheath characteristics in the normal and abnormal regimes of an air discharge for a duralumin cathode are determined. The axial profiles of the ion density, electron temperature, and plasma potential, as well as the anode voltage drop, are measured at various air pressures.     

Experimental Study of Heating of a Liquid Cathode and Transfer of Its Components into the Gas Phase under the Action of a DC Discharge

An atmospheric-pressure dc discharge in air (i = 10–50 mA) with metal and liquid electrolyte electrodes was studied experimentally. An aqueous solution of sodium chloride (0.5 mol/L) was used as the cathode or anode. The electric field strength in the plasma and the cathode (anode) voltage drops were obtained from the measured dependences of the discharge voltage on the electrode gap length. The gas temperature was deduced from the spectral distribution of nitrogen emission in the band N₂(C³Π _u → B³Π _g , 0–2). The time dependences of the temperatures of the liquid electrolyte electrodes during the discharge and in its afterglow, as well as the evaporation rate of the solution, were determined experimentally. The contributions of ion bombardment and heat flux from the plasma to the heating of the liquid electrode and transfer of solvent (water) into the gas phase are discussed using the experimental data obtained.     

Plasma parameters in the channel of a long leader in air

The time evolution of the electric field in the leader channel and other characteristics of the leader plasma in long air gaps are simulated. Calculations are performed in the one-dimensional time-dependent model with allowance for the time-varying energy deposition in the channel, the channel expansion, and the nonequilibrium ionization kinetics in the leader plasma. The calculations show that, at a gas temperature of 4500–6000 K, associative ionization becomes a dominant ionization mechanism in the leader channel; as a result, the electric field decreases to 100–200 V/cm in 10^?4–10^?3 s under the conditions typical of the leader discharge. The calculated electric field agrees well with the data from the experimental modeling of long leaders by a spark discharge in short gaps.     

Mixing characteristics and liquid circulation in a new multi-environment bioreactor

The theoretical and experimental aspects of the hydrodynamics and mixing in a new multi-environment bioreactor that uses the air-lift design were investigated. This study focused on the mixing characteristics, residence time distribution, liquid circulation between three zones of aerobic, microaerophilic and anoxic, and liquid displacement in the bioreactor at influent flow rates of 720–1,450 L/day and air flow rates of 15–45 L/min. The theoretical analysis of liquid displacement led to the estimation of the specific rate of liquid discharge from the bioreactor at any given influent flow rate, and the number of liquid circulations between various bioreactor zones before the discharge of a given quantity of wastewater. The ratio of mean residence time to the overall hydraulic retention time (t _m/HRT) decreased with the increase of air flow rate at any given influent flow rate, and approached unity at higher air flow rates. Mixing was characterized in terms of the axial dispersion coefficient and Bodenstein number, demonstrating a linear relationship with the superficial gas velocity. A correlation was developed between the Bodenstein number and the Froude number. The study of liquid circulation between the zones showed that less than 1.5 % of the circulating liquid escapes circulation at each cycle and flows towards the outer clarifier, while the percentage of escaped liquid decreases with increasing air flow rate at a given influent flow rate. The specific rate of liquid discharge from the bioreactor increased from 0.19 to 0.69 h^?1 with the increase of air and influent flow rates from 15 to 45 L/min and 500 to 1,450 L/day, respectively. Under the examined operating conditions, mixed liquor circulates between 364 and 1,698 times between the aerobic, microaerophilic and anoxic zones before 99 % of its original volume is replaced by the influent wastewater.     

Diagnostics of a nonequilibrium nitrogen plasma from the emission spectra of the second positive system of N2

A method is proposed for determining the electron density N _e and the electric field E in the non-equilibrium nitrogen plasma of a low-pressure discharge from the spectra of the second positive system of N₂. The method is based on measuring the specific energy deposition in the plasma and the distribution of nitrogen molecules over the vibrational levels of the C ³Π _u state, as well as on modeling this distribution for a given energy deposition. The fitting parameters of the model are the values of N _e and E. A kinetic model of the processes governing the steady-state density of the C ³Π _u nitrogen molecules is developed. The testing of this method showed it to be quite reliable. The method is of particular interest for diagnosing electrodeless discharges and provides detailed information on the processes occurring in the discharge plasma. Preliminary data are obtained on the plasma parameters in a cavity microwave discharge and an electrode microwave discharge. In particular, it is found that the electric field in an electrode microwave discharge in nitrogen is lower than that in a hydrogen discharge. This effect is shown to be produced by stepwise and associative processes with the participation of excited particles in nitrogen.     

Gas-dynamics of the propagation of a microwave discharge excited by the H 10 waveguide mode

A two-dimensional gas-dynamic model is applied to calculate the characteristics of the steady-state propagation of a microwave discharge excited by the H ₁₀ waveguide mode. The stream pattern is found on the basis of gas dynamics of a slowly propagating discharge, taking into account the non-one-dimensional character of the gas flow ahead of the discharge front. The calculated values of the propagation velocity agree with the experimental results.     

Effect of the Air Flow Velocity on the Characteristics of a Pulsating Discharge Produced by a DC Power Source

Results are presented from experimental studies of a pulsating discharge produced by a dc power source in subsonic and supersonic cold (T = 150–300 K) air flows at static air pressures in the flow of 40–760 Torr. Two modes of pulsating discharge were implemented experimentally: without and with (from one to five) intermediate breakdowns. The discharge pulsation frequency, the maximum attainable voltage across the discharge gap, the length of the plasma channel, and the electric field in the discharge plasma were studied as functions of the air flow velocity and discharge current.     

Alternating nonsteady gas-discharge modes in an atmospheric-pressure air flow blown through a point-plane gap

The electric and spectral characteristics of a nonsteady discharge in an atmospheric air flow blown through a point-plane interelectrode gap were investigated experimentally. The discharge was produced by applying a constant positive voltage to the point electrode, the amplitude of the applied voltage being much higher than the corona ignition voltage. The nonsteady character of the discharge is due to the spontaneously repeating streamer-spark breakdown, followed by the formation of either a diffuse ultracorona or a filamentary glow discharge. In the latter case, the length of the plasma column increases progressively, being blown off by the gas flow from the discharge gap. The extinction of a filamentary discharge is unrelated to the break of the current channel: the discharge decays abruptly when the filament length reaches its critical value. The distribution of active particles (O, OH, and N*₂) carried out from the discharge gap is determined from the data of spectral measurements.     

Production of C 2 * radicals in a transverse volume discharge

Conditions for producing stable transverse volume discharges in freon-containing media (CCl₄/air mixtures at a pressure of P=0.1?2 kPa) are studied. It is shown that a transverse discharge produced in the CCl₄/air=(1–2)/0.03 kPa mixture at a moderate discharge voltage (U _ch=8–15 kV) and an interelectrode distance of d=2.2 cm is a selective source of C₂(d ³Π_g-a ³Π_u) 468.0-and 516.5-nm radiation and C(2p-3s) 247.9-nm radiation. The brightness of the C₂(d-a) band is comparable with that of the N₂(C-B) 337.1-and 357-nm bands. The transverse discharge in CCl₄ is of interest for generating pulsed emission via the d-a transitions of C₂ molecules in the blue-green region of the visible spectrum.     

Concentric-tube airlift bioreactors

Gas holdup investigations were performed in three concentric-tube airlift reactors of different scales of operation (RIMP: 0.070 m³; RIS-1: 2.5 m³; RIS-2: 5.2 m³; nominal volumes). The influences of the top and bottom clearances and the flow resistances at the downcomer entrance were studied using tap water as liquid phase and air as gaseous phase, at atmospheric pressure. It was found that the gas holdup in the individual zone of the reactor: riser, downcomer and gas-separator, as well as that in the overall reactor is affected by the analyzed geometrical parameters in different ways, depending on their effects on liquid circulation velocity. Gas holdup was satisfactorily correlated with Fr, Ga, bottom spatial ratio (B), top spatial ratio (T), gas separation ratio (Y) and downcomer flow resistance ratio (A _d /A _R ). Correlations are presented for gas holdup in riser, downcomer, gas separator and for the total gas holdup in the reactor. All the above stressed the importance of the geometry in dynamic behaviour of airlift reactors.     

On the theory of microwave discharges excited on the surface of a dielectric antenna

Microwave gas discharges excited near a dielectric surface are investigated. Such discharges can exist over a broad range of gas pressures and thereby can be used to solve a wide variety of applied problems. The wave dispersion properties favorable for discharge excitation are analyzed, and a kinetic discharge model is considered that can be used to calculate the discharge parameters. A model of a steady discharge at gas pressures of 10²–10⁴ Pa is constructed.     

Decomposition of toluene in a steady-state atmospheric-pressure glow discharge

Results are presented from experimental studies of decomposition of toluene (C₆H₅CH₃) in a polluted air flow by means of a steady-state atmospheric pressure glow discharge at different water vapor contents in the working gas. The experimental results on the degree of C₆H₅CH₃ removal are compared with the results of computer simulations conducted in the framework of the developed kinetic model of plasma chemical decomposition of toluene in the N₂: O₂: H₂O gas mixture. A substantial influence of the gas flow humidity on toluene decomposition in the atmospheric pressure glow discharge is demonstrated. The main mechanisms of the influence of humidity on C₆H₅CH₃ decomposition are determined. The existence of two stages in the process of toluene removal, which differ in their duration and the intensity of plasma chemical decomposition of C₆H₅CH₃ is established. Based on the results of computer simulations, the composition of the products of plasma chemical reactions at the output of the reactor is analyzed as a function of the specific energy deposition and gas flow humidity. The existence of a catalytic cycle in which hydroxyl radical OH acts a catalyst and which substantially accelerates the recombination of oxygen atoms and suppression of ozone generation when the plasma-forming gas contains water vapor is established.     

Investigation of the mechanism for rapid heating of nitrogen and air in gas discharges

A rapid heating of nitrogen-oxygen mixtures excited by gas discharges is investigated numerically with allowance for the following main processes: the reactions of predissociation of highly excited electronic states of oxygen molecules (which are populated via electron impact or via the quenching of the excited states of N₂ molecules), the reactions of quenching of the excited atoms O(¹ D) by nitrogen molecules, the VT relaxation reactions, etc. The calculated results adequately describe available experimental data on the dynamics of air heating in gas-discharge plasmas. It is shown that, over a broad range of values of the reduced electric field E/N, gas heating is maintained by a fixed fraction of the discharge power that is expended on the excitation of the electronic degrees of freedom of molecules (for discharges in air, η_E?28%). The lower the oxygen content of the mixture, the smaller the quantity η_E. The question of a rapid heating of nitrogen with a small admixture of oxygen is discussed.     

Spatial distribution of the RF power absorbed in a helicon plasma source

The spatial distributions of the RF power absorbed by plasma electrons in an ion source operating in the helicon mode (ω _ci < ω < ω _ce < ω _pe ) are studied numerically by using a simplified model of an RF plasma source in an external uniform magnetic field. The parameters of the source used in numerical simulations are determined by the necessity of the simultaneous excitation of two types of waves, helicons and Trivelpiece-Gould modes, for which the corresponding transparency diagrams are used. The numerical simulations are carried out for two values of the working gas (helium) pressure and two values of the discharge chamber length under the assumption that symmetric modes are excited. The parameters of the source correspond to those of the injector of the nuclear scanning microprobe operating at the Institute of Applied Physics, National Academy of Sciences of Ukraine. It is assumed that the mechanism of RF power absorption is based on the acceleration of plasma electrons in the field of a Trivelpiece-Gould mode, which is interrupted by pair collisions of plasma electrons with neutral atoms and ions of the working gas. The simulation results show that the total absorbed RF power at a fixed plasma density depends in a resonant manner on the magnetic field. The resonance is found to become smoother with increasing working gas pressure. The distributions of the absorbed RF power in the discharge chamber are presented. The achievable density of the extracted current is estimated using the Bohm criterion.     

Concentric-tube airlift bioreactors

Liquid circulation velocity was investigated in three concentric-tube airlift reactors of different scales (RIMP, V _L =0.07 m³; RIS-1, V _L =2.5 m³; RIS-2, V _L =5.20 m³). The effects of top and bottom clearance and resistance in flow pathway at downcomer entrance on the riser liquid superficial velocity, the circulation time, the friction coefficient and flow radial profiles of the gas holdup and the liquid superficial velocity in riser, using water-air as a biphasic system, were studied. It was found that the riser liquid superficial velocity is affected by the analyzed geometrical parameters in different ways, depending on their effects on the pressure loss. The riser liquid superficial velocity, the friction coefficient and the parameters of the drift-flux model were satisfactorily correlated with the bottom spatial ratio (B), gas separation ratio (Y) and downcomer flow resistance ratio (A _d /A _D ), resulting empirical models, with correlation coefficients greater than 0.85.