Characterization of Ionic Winds from Flames and Corona Discharges

Transcription

1 Characterization of Ionic Winds from Flames and Corona Discharges Derek Dunn-Rankin Mechanical and Aerospace Engineering University of California, Irvine California Institute of Technology, Pasadena, CA February 25, 2005

2 Creating Ionic Wind via Corona Negative DC Corona Discharge Chattock wind Ion wind Ionic wind EHD flow Electric wind 1 Td = V m 2 E = electric field strength (V/m) N = neutral density (1/m 3 ) Drawing not to scale from Chen and Davidson, Plasma Chemistry and Plasma Processing, Vol. 23, No. 1, March 2003.

3 Ion Winds and The Electric Fly Wilson (1750) built spinning pinwheel Cavallo (1777) claims similar-charged air repels points does not work in vacuum Electric fly used to discuss ion wind theory: Michael Faraday (1838), James Clerk Maxwell (1873), among others Most recently discussed by Leonard Loeb in Electrical Coronas (1965) Electric fly motion can be observed before any wind motion spins at voltages well below breakdown

4 Rotational Speed Linear with Voltage 2000 Electric Fly Speeds 1500 RPM ABS Voltage (kv)

5 Applications of Ionic Winds Coronas Ionic Breeze fan and particle removal Air delivery without moving parts Electrostatic precipitator enhancement Heat transfer and evaporation Plasma actuators Flames Dynamic flame control Heat direction in microgravity Earth-based microbuoyancy

6 Ionic Wind Applications HV Sharperimage.com

7 PIV Experiment o Air vent HV 1 Laser 4 Ion wind generator 7 Mist-seeded airflow 2 Sheet optics 5 Mist canister 3 Camera 6 Rubber tubing Copper Vapor laser sync d to 2kHz camera captured high-speed video Camera shifted 45º forward of perpendicular to maximize intensity Personal Air Purifier electrodes powered by variable -HV power supply Water/glycerin mist traced air flow (seeded downstream of electrodes)

8 High-speed video HV=-10.3 kv (max before breakdown) shown for subtle nozzle ( ID = 9.5 mm ) Actual orientation: directly upwards Capture rate: 1,825 frames per second Real time elapsed during 500 images: 1/4 seconds Viewing rate: 25 frames per second Total time to view 500 images: 20 seconds

9 PIV results -- Velocity profile at exit kv Velocity [ m/s ] kv 8.9 kv 8.1 kv 7.4 kv 5.9 kv 4.4 kv r/r o

10 Point-to-Cylinder Ionic Wind X o Y o Charged needle -HV 2R Gas Flow V High Voltage Power Supply i Earthed ring Acrylic tube Nozzle (optional)

11 Optimizing the Ion Wind Stagnation Pressure [Pa] Xo = 0 [in] Xo = 0.25 Xo = Yo, Length of cylindrical ground electrode [ in ]

12 Flow Limit of Single Stage 8 8 Mean Velocity [ m/s ] Ideal Ideal is velocity needed to maintain 6.4 l/min flow rate as nozzle exit area decreases Volumetric Flow Rate [ SLPM ] Exit Area / Inlet Area 0

13 Stacking Stages to Increase Pressure

14 Multi-Stage Ion Blower

15 Pressure Delivered 5.5 Pa per stage

16 Staged System Performance Ionic Wind Fan Curves

17 Future Activities Swirl Generator ESP enhancement Microscale Gas Delivery Point of application flows All Electric Burner Modeling

18 Applications of Ionic Winds Coronas Ionic Breeze fan and particle removal Electrostatic precipitator enhancement Heat transfer and evaporation Plasma actuators Flames Dynamic flame control Heat direction in microgravity Earth-based microbuoyancy

19 Flame Derived Ion Wind Ions travel from the flame towards ground Electrons conduct through the flame to the capillary e - e - V Neutralization by electrons at the cathode

20 Force and Velocity from Ion Wind K is mobility of the charge carrier -- 1 cm 2 /s/v g is gravitational acceleration; ρ is the density x is the distance over which the field acts j is current density flame: 1 µa/ cm 2 F F e b = K j ρ g v e = ρ j K x Order of 1-2 Order of 1 m/s over 1 cm distance

21 Applied Potential (Volts) Effect on Small Diffusion Flame Methane fuel flowrate: 9 cc/min Capillary diameter: 1.7 mm Electrode Spacing: 7 cm Conditions 1. Naturally Buoyant 0 Volts 2. Microbuoyant 2880 Volts 3. Negatively buoyant 4300 Volts

22 Double Exposure Holographic Interferometry Images (setup #1) 0 Volts 2242 Volts 2765 Volts

23 Temperature (K) Degrees 5 Degrees 10 Degrees Diffusion Profile Temperature Profiles Radial Position (mm) Y X

24 Flame Current saturation plateau Current (microamps) Relative CH 430 nm Emission Zone 1 Zone 1 Zone 2 Zone Applied Potential (Volts) Zone 3 Zone 3 Saturation current can be characteristic of the flame Current provides a detection and/or decision opportunity (e.g., ion probe)

25 Toward Feedback Control y r Controller Σ e D(s) - Saturation Plant H(s) y what do we measure for feedback? CH* HV Amplifier earthed Computer lens 430nm filter iris Amplifier PMT commonly accepted that CH* is a good measure of the global heat release in a flame CH* emission is linearly related to the fuel flow rate the electronically excited states are confined to a thin region of the primary flame surface

26 Sine Tracking Desired Actual CH Chemiluminescence Time (sec) Applied Potential (kv)

27 Refined Apparatus Acrylic chamber Ion current measured at the cathode (ground plane) Temperature, pressure, and relative humidity are monitored Continuous running via air flushing THERMOCOUPLE & HYGROMETER CAMERA (VIDEO OR STILL) MICROSCOPE FUEL CAPILLARY HV FLAME EXHAUST SCRUBBER MANOMETER CERAMIC STANDOFFS R ref V probe DMM ELECTRODES INLET DIFFUSER Earthed AIR INLET (optional)

28 Schlieren Imaging Illustrates Complexity Identify buoyancy regimes IRIS LIGHT SOURCE Determine relative electrical characteristics between fuels Z-type schlieren apparatus MIRROR (f = 48 in) 6 in MIRROR (f = 48 in) Conventional and high speed video VIDEO CAMERA VELLUM SCREEN LENS LENS KNIFE EDGE OR IRIS Z TYPE SCHLIEREN APPARATUS Ethane Fuel [12.7 mg/min] 1000 V Buoyant (negligible current) 2000 V under balanced (subsaturated) 2400 V Balanced condition (saturation) 3000 V Partially neg-buoyant (some 2 nd ary ionization) 5300 V Negatively buoyant (much 2 nd ary ionization)

29 Ion Current Profiles Point-to to-plane distribution (Warburg Law) = cos 5 Θ Integrated current provides ion density boundary conditon Current distribution for model validation new data set 2 new data set 3 Jan 21 data Warburg Law HV 0.4 INSULATED ION PROBE 0.2 TRAVERSE V probe R probe DMM Radius [mm] Earthed R ground plane

30 j( θ) Warburg curve Electric force Point to Plane Does not Produce Uniform Balance θ 800 Temp( θ) Temperature Inverse of buoyant force θ 0.5 Balance condition F ratio ( θ) Force ratio F e /F b θ

31 Numerical Simulation Gauss s Law 2 V = ρ e = ε o ce ε o Momentum Mass Energy Ion concentration u T ρ ρ 1 t T ρ t ρ c t ( c T ) p t ( u ) u = P o ρ g ( µ u) cee ( ρu) = 0 ρu o ( c T ) = u P ( k T ) p ( u ) c = ( D c cek V )

32 Computational Model FEMLAB simulation Finite element method General equation solver Staged solution 1. Electrokinetic (V, c) 2. Electrodynamic (V, c) 3. Hot flow (V, u, v, P, T, c) 2-D axisymmetric Assumptions Balance condition Isotropic Fickian diffusion Single ion species (positive ions) Temperature is constant within the flame (hot flow) Uniform ion concentration at the flame Ion properties are of H 3 O AXIAL DIRECTION Symmetry boundary R cap (0.5 mm) R flame (3.0 mm) h cap Anode boundary 63.5 mm Cathode boundary Gravity direction Infinite condition RADIAL DIRECTION

33 Electrokinetic Potential Field Ion motion is decoupled from the field (electrokinetic assumption) Flux pathlines illustrate no flux beyond 20 mm away from the centerline

34 Normalized ion current h=60mm 13.8mg/min h=35mm 13.8mg/min norm uniform ion concentration ekf D/K = 1000 ekf D/K = 100 ekf D/K = 10 ekf D/K = Radius [mm] Role of Ion Diffusion Diffusivity is varied to match experimental profile D(T) varies by a factor of approx 20 from K Results True D/K approximately 1 V Ion drift speed more than 1000 times diffusion velocity Ion diffusion cannot account for measured profile Space charge dispersion is significant Diffusivity (cm 2 /s) Diffusivity [m 2 /s] D i, air = 10 T Temperature [K]

35 Including Space Charge Effects Gauss s law is solved including space charge Space charge drives ions to travel further from the centerline

36 Effect of Ion Concentration 1 Normalized ion current h=60mm 13.8mg/min h=35mm 13.8mg/min 1e8 ions/cc K D(T) 3kV 1e9 ions/cc K= D(T) 3kV 1e10 ions/cc K= D(T) 3kV 1e12 ions/cc K= D(T) 3kV Radius [mm] Overall ion concentration in flame is varied Lower concentration (10 8 ions/cc) profile narrows Higher concentration ( ions/cc) profile broadens

37 Buoyant Flow Preliminary Results

38 Hot Flow at Balance Preliminary Results c = 10 9 ions/cc V = 2800 Volts

39 Flame Derived Ion Wind V Ions travel from the flame towards ground Electrons conduct through the flame to the capillary High ion concentration at the center decreasing radially outward Induced electric field (causes horiz. migration) e - e - Neutralization by electrons at the cathode Re-circulation driven by neutralized buoyant gas redirection Source of unstable T and O2 Observed in shadowgraphs

40 Final Thoughts Ionic winds produce modest flows (a few m/s) with no moving parts Positive and negative polarity produces flow in the same direction (allows AC approaches) Efficiencies (flow power out/electrical power in) are low but not far worse than standard fans at small sizes Flame source ion winds require far less electrical energy (100 times less) than corona winds Optimizing ionic wind systems should be possible by modeling systems with various ground plane geometries

41 Acknowledgments Matt Rickard (ionic wind) and Mike Papac (electric flame) current UCI graduate student researchers Ben Strayer (flame control) and Jonathan Posner (holographic interferometry) past UCI graduate students Prof. Felix Weinberg and Dr. Fred Carleton Imperial College NASA Microgravity Combustion Program