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MAE /NCSU Unipolar Injection of Charge into Quiescent Gaseous Dielectrics Alexei V. Saveliev Department of Mechanical and Aerospace Engineering North Carolina.

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Presentation on theme: "MAE /NCSU Unipolar Injection of Charge into Quiescent Gaseous Dielectrics Alexei V. Saveliev Department of Mechanical and Aerospace Engineering North Carolina."— Presentation transcript:

1 MAE /NCSU Unipolar 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

2 MAE /NCSU Outline 1. Introduction Plasma and ionized gases 2. Breakdown of Gaseous Dielectrics Uniform field – Paschen’s law Non-uniform field breakdown Positive and negative coronas Corona charge injection devices Pulsed streamer corona discharges 3. Discharge initiation in Supercritical Fluids SCFs as a cluster fluid Generation of SCF plasma in carbon dioxide Potential applications of SCF plasmas 4. Breakdown and charging on liquid-gas interfaces

3 MAE /NCSU Plasmas and Ionized Gases Plasma is an ionized gas consisting of charged and neutral particles and exhibiting collective behavior (Langmuir, 1929) Plasma may carry a current Plasma is often considered to be electrically neutral Plasma is an ionised gas Ionized gas is not always plasma

4 MAE /NCSU Non-Thermal Electrical Discharges: 1.Electrons are accelerated in the electric field gaining energy sufficient for ionization of neutral molecules 2.Secondary electrons are produced in collisions forming electron avalanche propagating in the interelectrode gap Ar + e  e + e + Ar + Ar E-field Development of electron avalanche 1 eV  10 000 K Ionization energy  10 eV E/N parameter m e >  i Thermal Plasma T e = T i = T g Non-Thermal Plasma T e >> T i >T g E-field Breakdown

5 MAE /NCSU Elementary processes: 1.Ionization e + Ar  2e + Ar + 2.Electron attachment e + O 2  O 2  3.Electrode processes - ion impact - thermionic emission - field emission Kinetics of Ions and Electrons Fowler – Nordheim, 1928

6 MAE /NCSU E-field Breakdown – Uniform Field Townsend criterion Multiplication of charges during avalanche propagation d

7 MAE /NCSU Breakdown voltage and breakdown reduced e-field between two parallel electrodes Paschen’s Law

8 MAE /NCSU Breakdown in Non-uniform Field Peek’s formula E cr –critical e-field in kV/cm; r – radius of inner sphere in cm;  - relative air density Modified Townsend criterion

9 MAE /NCSU Corona Discharge Corona configurations A – wire-in-cylinder; B – sphere-in-sphere; C – point-and-plane; D – parabola-to-plane; E – wire-to-plane E)

10 MAE /NCSU Applications of Corona Discharge Charging and discharging of solids and fluids Manufacture of ozone Electrostatic precipitators Pollution control (VOCs, SO x, NO x ) Processing of material surfaces Photocopy Ionic wind devices Gas ionizers for mass spectrometers and DMA

11 MAE /NCSU DC Corona Current Typical voltage-current characteristics for negative point-to-plane corona Solution for wire in cylinder corona V cr – ignition voltage

12 MAE /NCSU Positive corona: Streamer Propagation and Branching Streamer propagation modes: Veldhuizen and Rutgers, J. Phys. D. Appl. Phys. 35, 2002 Streamer Model:

13 MAE /NCSU Streamer Propagation and Branching 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 CO 2. (a) (b) Morphology, length, and velocity of streamers are extremely important for discharge structure and ion generation efficiency

14 MAE /NCSU Positive Corona Existence regions for positive sphere-to-plane corona forms in atmospheric air, r =10 mm

15 MAE /NCSU Negative Corona Existence regions for negative sphere-to-plane corona forms in atmospheric air

16 MAE /NCSU DC Corona Chargers The need for more efficient methods to charge gas and aerosol streams has resulted in a renewed interest in corona discharge ionizers. Space charge buildup in the positive wire-to-cylinder corona. r E Cathode Anode Ionization region + + + + + + + + + + + + + + +

17 MAE /NCSU DC 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) Distribution of ion concentration at various pressures Biskos et al., Journal of Electrostatics 63(2005)

18 MAE /NCSU DC Corona Diffusion Chargers Total ion number concentration at ionizer outlet Alguacil and Alonso, Aerosol Science 37 (2006) Sharp-point electrode ionizer

19 MAE /NCSU Pulsed Streamer Corona Discharge Pulsed corona discharge can be readily generated at atmospheric pressure Voltage is applied to the sharp electrode as a series of fast rising pulses:  100 ns duration  10 ns rise time  20 kV peak voltage Electron energies up to 20 eV (~230,000 K ) Generates and efficiently separates the charges

20 MAE /NCSU Pulsed Electrostatic Precipitators The first use of corona to remove particles from an aerosol was by Hohlfeld in 1824 Electrostatic precipitators were first used in industry to remove of sulfuric acid mist from exhaust fumes in the beginning of the 20 th century Modern controls minimize sparking and prevent arcing by applying pulsed voltage and thus avoiding damage to the components.

21 MAE /NCSU 10 -10 10 2 10 -6 10 -4 10 -2 110 -8 Electrons Radicals By-Products Time, s High energy electrons Radical-molecule Ion-molecule Clustering Heterogeneous Excitation Ionization Dissociation Reactions O N N4+N4+ N2+N2+ OH CH 3 H H2OH2O NO 2 CO CO 2 O3O3 Pulsed Corona Discharge for VOC Removal

22 MAE /NCSU Breakdown in Supercritical Fluids Traditional area Elevated Pressure Plasmas Nanoplasma Microplasma Liquid Plasmas SCF plasma 1  m 1 atm The area of non- thermal plasmas is expanding to elevated pressure gases, liquids and microplasmas For a given voltage V T e = f(E/n)=F(V/nd) n - neutral density d - characteristic system size

23 MAE /NCSU 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 states Applications of supercritical plasmas: - pollutant removal - bacterial deactivation - plasma polymerization - material synthesis SCF Plasma - Problem Statement and Motivation

24 MAE /NCSU Supercritical Carbon Dioxide Critical point : 304 K, 73 bar Environmentally friendly “green” solvent The properties of supercritical fluids are intermediate to corresponding gas and liquid states SCF combines heterogeneous chemistry with efficient mass transfer realized by the low viscosity, high diffusivity and zero surface tension

25 MAE /NCSU Clusters in Supercritical CO 2 SCF is often referred as a cluster fluid Clusters are effectively formed near the critical point Clusters affect reaction and transport properties of SCF Clusters are crucially important for plasma breakdown mechanism

26 MAE /NCSU SCF Plasma Reactor Technical Approach Non-thermal plasma is initiated in supercritical carbon dioxide at pressures above 75 bar using high-voltage pulses of nanosecond duration The pulses are applied to the system of microelectrodes arranged in point-to-plane and wire-to-plane geometries The discharge and plasma parameters are studied using optical and electrical diagnostics

27 MAE /NCSU Microbreakdown in Supercritical CO 2 IEEE Trans. Plasma Sci., 33: 850, 2005 d = 80  m Ito and Terashima, Appl. Phys. Lett., 80: 2854-56, 2002.

28 MAE /NCSU Electron Kinetics in Supercritical CO 2 Ionization Recombination Attachment Dissociation

29 MAE /NCSU Wire-in-cylinder Corona Discharge Discharge inception voltages in gaseous and supercritical CO 2. The original Paschen’s data are shown for reference. IEEE Trans. Plasma Sci., 33: 850-3, 2005

30 MAE /NCSU Wire-in-cylinder Corona Discharge Dependence of critical E/p on pd in gaseous air and CO 2 for uniform and non-uniform fields. IEEE Trans. Plasma Sci., 34: 2467, 2006

31 MAE /NCSU Atomic Force Microscopy Electrode assembly Electrode gap Point-to-plane Corona Discharge

32 MAE /NCSU Periodic Pulsed Corona Discharge Distance 200  m Needle tip 5  m T= 60  C P= 83 bar Voltage 13 kV

33 MAE /NCSU Sliding Liquid-Jet Discharge The high voltage nanosecond pulses are applied to the micro-jet Overcharged micro-jet generates corona discharge with sharp liquid electrode Application for material processing and sensors Plasma as an ignition source for engines and combustors using liquid fuels 100  m

34 MAE /NCSU Experimental Setup Surface discharge operation

35 MAE /NCSU Experimental Setup

36 MAE /NCSU Discharge 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 U 0 =10 kV, (b) – conversion of the glow discharge to the single sliding surface discharge, (c, d) – developed surface discharge at U 0 =23 kV; (e) - degenerated arc discharge at U 0 =30 kV

37 MAE /NCSU 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. Discharge Modes – Distilled Water

38 MAE /NCSU 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. Discharge Regimes - Electrolyte Jet

39 MAE /NCSU Water jet atomization at Q = 0.5 mL/s, U = 2 (a), 10 kV (b). Appearing the surface discharge at U = 13 kV (c), I = 8 mA and its development at U = 16 kV, I = 16 mA (d). Jet Atomization

40 MAE /NCSU Unipolar injection of charge into quiescent gaseous dielectrics is well established scientific area. Understanding of ionization and charge separation processes is developed Corona discharges are widely applied for charging of fluids and solid surfaces Pulsed discharges represent modern trend in plasma generation for various charge injection devices Discharges in supercritical fluids, liquids, at material interfaces and discharges in micro and nanoscale are current areas of research interest THANK YOU! Summary

41 MAE /NCSU Acknowledgements Collaborators: Dr. Lawrence A. Kennedy Dr. Alexander A. Fridman Dr. Vladimir ShmelevDr. Evgenyia Lock Dr. Ozlem Yardimci Dr. Wilson Merchan-Merchan Emiliano Giacchetti Mario Sobacchi Research support: NSF grants #9812905, #0522578 DOE grant FWP 49885 CRDF grant # RUC2-2824 Argonne National Laboratory Texaco, Inc. Air Liquide, Inc. Innovative Energy Solutions, Inc. Energy Systems Laboratory 2003

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