Presentation on theme: "MAE /NCSU Unipolar Injection of Charge into Quiescent Gaseous Dielectrics Alexei V. Saveliev Department of Mechanical and Aerospace Engineering North."— Presentation transcript:
1MAE /NCSUUnipolar Injection of Charge into Quiescent Gaseous Dielectrics Alexei V. Saveliev Department of Mechanical and Aerospace Engineering North Carolina State University Presented at workshop on “ Electrostatic Atomization of Electrically Insulating Fluids: Principles and Applications” University of Southampton, March 2, 2009
2Outline 1. Introduction 2. Breakdown of Gaseous Dielectrics Plasma and ionized gases2. Breakdown of Gaseous DielectricsUniform field – Paschen’s lawNon-uniform field breakdownPositive and negative coronasCorona charge injection devicesPulsed streamer corona discharges3. Discharge initiation in Supercritical FluidsSCFs as a cluster fluidGeneration of SCF plasma in carbon dioxidePotential applications of SCF plasmas4. Breakdown and charging on liquid-gas interfaces
3Plasmas and Ionized Gases Plasma is an ionized gas consisting of charged and neutral particles and exhibiting collective behavior (Langmuir, 1929)Plasma may carry a currentPlasma is often considered to be electrically neutralPlasma is an ionised gasIonized gas is not always plasma
4E-field Breakdown Thermal Plasma Non-Thermal Electrical Discharges: Te = Ti = TgNon-Thermal PlasmaTe >> Ti>TgNon-Thermal Electrical Discharges:Electrons are accelerated in the electric field gaining energy sufficient for ionization of neutral moleculesSecondary electrons are produced in collisions forming electron avalanche propagating in the interelectrode gapDevelopment of electron avalanche1 eV K Ionization energy 10 eVE/N parameter me << mi,mn e >> iArE-fieldAr + e e + e + Ar+
5Kinetics of Ions and Electrons Elementary processes:Ionization e + Ar 2e + Ar+Electron attachment e + O2 O2Electrode processes - ion impact - thermionic emission - field emissionFowler – Nordheim, 1928
6E-field Breakdown – Uniform Field Townsend criteriondMultiplication of charges during avalanche propagation
7Paschen’s LawBreakdown voltage and breakdown reduced e-field between two parallel electrodes
8Breakdown in Non-uniform Field Modified Townsend criterionPeek’s formulaEcr –critical e-field in kV/cm; r – radius of inner sphere in cm; - relative air density
9Corona Discharge Corona configurations A – wire-in-cylinder; B – sphere-in-sphere; C – point-and-plane; D – parabola-to-plane; E – wire-to-planeE)
10Applications of Corona Discharge Charging and discharging of solids and fluidsManufacture of ozoneElectrostatic precipitatorsPollution control (VOCs, SOx, NOx)Processing of material surfacesPhotocopyIonic wind devicesGas ionizers for mass spectrometers and DMA
11DC Corona CurrentTypical voltage-current characteristics for negative point-to-plane coronaSolution for wire in cylinder coronaVcr – ignition voltage
12Streamer Propagation and Branching Streamer Model:Positive corona:Streamer propagation modes:Ionization in the negative corona is due to multiplication of avalanches. Continuity of electric current from the cathode into the plasma is provided by the secondary emission from the cathode (mostly induced by ion impact). Ignition of the negative corona actually has the same mechanism as the Townsend breakdown generalized taking into account non-uniformity and possible electron attachment.Ionization in the positive corona cannot be provided by the cathode, since the electric field there is low. Here the ionization processes are related to the formation of cathode directed streamers. Ignition criterion can be described using the criteria of cathode directed streamer formation. Generalization of the Meek criterion is a good approximation in this case.Veldhuizen and Rutgers, J. Phys. D. Appl. Phys. 35, 2002
13Streamer Propagation and Branching Morphology, length , and velocity of streamers are extremely important for discharge structure and ion generation efficiencyIonization in the negative corona is due to multiplication of avalanches. Continuity of electric current from the cathode into the plasma is provided by the secondary emission from the cathode (mostly induced by ion impact). Ignition of the negative corona actually has the same mechanism as the Townsend breakdown generalized taking into account non-uniformity and possible electron attachment.Ionization in the positive corona cannot be provided by the cathode, since the electric field there is low. Here the ionization processes are related to the formation of cathode directed streamers. Ignition criterion can be described using the criteria of cathode directed streamer formation. Generalization of the Meek criterion is a good approximation in this case.(a)(b)Fast intensified CCD imaging of the fast transient plasma generation with 5 ns gating: (a) pulsed corona discharge in air, (b) pulsed corona discharge in CO2.
14Positive CoronaExistence regions for positive sphere-to-plane corona forms in atmospheric air, r =10 mm
15Negative CoronaExistence regions for negative sphere-to-plane corona forms in atmospheric air
16DC Corona Chargers Cathode The need for more efficient methods to charge gas and aerosol streams has resulted in a renewed interest in corona discharge ionizers.+++++++Anode++++++++Space charge buildup in the positive wire-to-cylinder corona.EIonization regionr
17DC Corona Diffusion Chargers Several types of such chargers have been proposed in recent years, most of them based on the original design of Hewitt (1957)Biskos et al., Journal of Electrostatics 63(2005)Distribution of ion concentration at various pressures
18DC Corona Diffusion Chargers Sharp-point electrode ionizerAlguacil and Alonso, Aerosol Science 37 (2006)Total ion number concentration at ionizer outlet
19Pulsed Streamer Corona Discharge Pulsed corona discharge can be readily generated at atmospheric pressureVoltage is applied to the sharp electrode as a series of fast rising pulses:100 ns duration10 ns rise time20 kV peak voltageElectron energies up to 20 eV (~230,000 K )Generates and efficiently separates the charges
20Pulsed Electrostatic Precipitators The first use of corona to remove particles from an aerosol was by Hohlfeld in 1824Electrostatic precipitators were first used in industry to remove of sulfuric acid mist from exhaust fumes in the beginning of the 20th centuryModern controls minimize sparking and prevent arcing by applying pulsed voltage and thus avoiding damage to the components.
21Pulsed Corona Discharge for VOC Removal 10-1010210-610-410-2110-8ElectronsRadicalsBy-ProductsTime, sHigh energyelectronsRadical-moleculeIon-moleculeClusteringHeterogeneousExcitationIonizationDissociationReactionsONN4+N2+OHCH3HH2ONO2COCO2O3
22Elevated Pressure Plasmas Breakdown in Supercritical FluidsThe area of non-thermal plasmas is expanding to elevated pressure gases, liquids and microplasmasFor a given voltage VTe = f(E/n)=F(V/nd)n - neutral density d - characteristic system sizeLiquid PlasmasSCF plasma1 atmElevated Pressure PlasmasMicroplasmaNanoplasmaTraditional area1 m
23SCF Plasma - Problem Statement and Motivation The generation of non-thermal plasma in supercritical fluid (SCF) media is interesting both from fundamental and applied viewpoints. Sustaining plasmas in SCF media bridges the gap between gaseous and liquid processing since SCFs have properties that are intermediate between the gas and liquid statesApplications of supercritical plasmas:- pollutant removal- bacterial deactivation- plasma polymerization- material synthesis
24Supercritical Carbon Dioxide Critical point : 304 K, 73 barEnvironmentally friendly “green” solventThe properties of supercritical fluids are intermediate to corresponding gas and liquid statesSCF combines heterogeneous chemistry with efficient mass transfer realized by the low viscosity, high diffusivity and zero surface tension
25Clusters in Supercritical CO2 SCF is often referred as a cluster fluidClusters are effectively formed near the critical pointClusters affect reaction and transport properties of SCFClusters are crucially important for plasma breakdown mechanism
26SCF Plasma Reactor Technical Approach Non-thermal plasma is initiated in supercritical carbon dioxide at pressures above 75 bar using high-voltage pulses of nanosecond durationThe pulses are applied to the system of microelectrodes arranged in point-to-plane and wire-to-plane geometriesThe discharge and plasma parameters are studied using optical and electrical diagnostics
27Microbreakdown in Supercritical CO2 d = 80 mIEEE Trans. Plasma Sci., 33: 850, 2005Ito and Terashima, Appl. Phys. Lett., 80: , 2002.
28Electron Kinetics in Supercritical CO2 Highenergyelectrons(CO2)nCO+Recombination+ eAttachmentO-1IonizationDissociationIonizationRecombinationAttachmentDissociation
29Wire-in-cylinder Corona Discharge Discharge inception voltages in gaseous and supercritical CO2. The original Paschen’s data are shown for reference.IEEE Trans. Plasma Sci., 33: 850-3, 2005
30Wire-in-cylinder Corona Discharge The ionization potential for solid CO2 clusters is well known. It can be estimated that the ionization potential of the supercritical fluid near the critical point is 10 % lower than in the gaseous phase. Then for electron temperature of 1 eV, the required breakdown voltage at supercritical state can be estimated from comparison of ionization rates suggesting value 3.5 times lower than that in the corresponding gaseous phase.Dependence of critical E/p on pd in gaseous air and CO2 for uniform and non-uniform fields.IEEE Trans. Plasma Sci. , 34: 2467, 2006
31Point-to-plane Corona Discharge Atomic Force MicroscopyElectrode assemblyElectrode gap
32Periodic Pulsed Corona Discharge Distance 200 mNeedle tip 5 mT= 60 CP= 83 barVoltage 13 kVPulsed corona discharge is generated with frequency of 1 Hz. When the duration between pulses is long enough transition from corona to spark discharge is observed. The video is made with normal video camera. Further ICCD diagnostics of the streamer propagation will be conducted.
33Sliding Liquid-Jet Discharge The high voltage nanosecond pulses are applied to the micro-jetOvercharged micro-jet generates corona discharge with sharp liquid electrodeApplication for material processing and sensorsPlasma as an ignition source for engines and combustors using liquid fuelsTaylor Cone: cone-shaped pendant drop that emits a fine jet from its tip when is exposed to a strong electrical fieldElectric forces exceed surface tension forcesFluid accelerates into apex regionMicro and nano jet brakes into charged droplets creating “electrospray”100 m
36Discharge Propagation - Electrolyte Jet a) b) c) d) e)Discharge propagation modes. The lower electrode is anode, R = 840 kΩ, L = 18 mm, δ = 2 mm; (a) – low current degenerated glow discharge at U0 =10 kV, (b) – conversion of the glow discharge to the single sliding surface discharge, (c, d) – developed surface discharge at U0 =23 kV; (e) - degenerated arc discharge at U0 =30 kV
37Discharge Modes – Distilled Water Voltage-current characteristics of the discharge for distilled water: L = 18 mm, Q = 0.3 ml/s, d = 0.4 mm, С = 0.47 nF, δ = 1 (1) , 2 (2), 3 (3), 4 mm (4). I – the area of a continued degenerated glow discharge; II – pulsed surface discharge.
38Discharge Regimes - Electrolyte Jet Voltage-current characteristics at С = 0.73 nF, L = 20 mm, d = 0.5 mm, Q = 0.5 mL/s.Ring electrode of 4 mm diameter; σ = 6·10-2 (1), 7·10-2 (2), 2·10-1 (3), 1.2 S/m (4);I – the area of a continued degenerated glow discharge;II –the pulse surface discharge, III – continued degenerated arc discharge.
39Jet AtomizationWater jet atomization at Q = 0.5 mL/s, U = 2 (a), 10 kV (b). Appearing the surfacedischarge at U = 13 kV (c), I = 8 mA and its development at U = 16 kV, I = 16 mA (d).
40SummaryUnipolar injection of charge into quiescent gaseous dielectrics is well established scientific area.Understanding of ionization and charge separation processes is developedCorona discharges are widely applied for charging of fluids and solid surfacesPulsed discharges represent modern trend in plasma generation for various charge injection devicesDischarges in supercritical fluids, liquids, at material interfaces and discharges in micro and nanoscale are current areas of research interestTHANK YOU!
41Energy Systems Laboratory 2003 AcknowledgementsCollaborators:Dr. Lawrence A. Kennedy Dr. Alexander A. FridmanDr. Vladimir Shmelev Dr. Evgenyia LockDr. Ozlem YardimciDr. Wilson Merchan-MerchanEmiliano GiacchettiMario SobacchiResearch support:NSF grants # , #DOE grant FWP 49885CRDF grant # RUC2-2824Argonne National Laboratory Texaco, Inc.Air Liquide, Inc.Innovative Energy Solutions, Inc.Energy Systems Laboratory