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May the force be with the particles!

Electrical Migration. Reading: Chap15. May the force be with the particles!. Q: Difference between impaction and electrostatic collection in terms of forces acting on the system?. Millikan Experiment. (Robert Millikan, US, 1868-1953) Nobel Laureate, 1923. Electrostatic Force.

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May the force be with the particles!

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  1. Electrical Migration Reading: Chap15 May the force be with the particles! Q: Difference between impaction and electrostatic collection in terms of forces acting on the system?

  2. Millikan Experiment (Robert Millikan, US, 1868-1953) Nobel Laureate, 1923

  3. Electrostatic Force • Statcoulomb (stC): the charge that causes a repulsive force of 1 dyne when 2 equal charges are separated by 1 cm (3.3310-10C) • Unit charge: 4.8 10-10stC (1.610-19C) • Coulomb’s law • Units (esu) • Electric Field q = ne (e: unit charge)

  4. Electrical Mobility • Terminal velocity in an electrical field (force balance) Q: physical meaning? (for Re < 1) Charging Mechanisms Flame Charging Static Electrification: Electrolytic charging, Spray electrification, Contact charging Diffusion Charging Field Charging

  5. Diffusion Charging • Random collisions between ions and particles • No external electrical field needed; independent of materials Total charge Ni: ion concentration Q: Does q depend on time? on composition? on size?

  6. Saturation charge Charges by field charging Zi: ion mobility (450 cm2/stVs) e: dielectric constant Field Charging • Bombardment of ions in the presence of a strong field Q: Is the charging rate dependent on particle size? On field strength?

  7. - + + - + Step 2 + - + + - + Step 3 - - + + - Step 4 + - Positive Corona Negative Corona - + - + Step 1 Electrode Collection Plate Electrode Collection Plate particle molecule electron

  8. 2 3 1 Electrostatic Precipitator Turbulent Flow with Lateral Mixing Model Three assumptions: 2 1 3 (28) (24) (12)

  9. Electrostatic Precipitator • Deutsch-Anderson Equation

  10. Comparison of Diffusion & Field Charging • Nit = 107 s/cm3 • = 5.1 E = 5 KV/cm T = 298 K Q: How does n vary wrt dp? Which mechanism is more important? Q: Does collection efficiency increase as particle size increase (because of a higher number of charges)?

  11. Typical fly ash size distribution Q: If the ESP is used to collect the fly ash, how will the particle size distribution at ESP outlet look like?

  12. Number of charges on a particle after it has been exposed to a bipolar ion concentration Equilibrium (Boltzmann) Charge Distribution (dpin µm) Average equilibrium charge (dp > 0.05 mm)

  13. Bipolar Charging Soft X-ray (< 9.5 keV) Q: How to produce bipolar charges? • Radioactive source: Kr-85, PO-210 • Alternating corona Kr discharge tube http://en.wikipedia.org/wiki/File:Krypton_discharge_tube.jpg

  14. Electrical Measurement: Electrical Mobility Analyzer U V: potential difference between plates U: mean flow velocity h: half the inter-plate distance L: inlet to exit distance Q: If monodisperse aerosols are introduced, what information can we obtain using this instrument?

  15. Electrical Measurement EAA (Electrical Aerosol Analyzer) DMA (Differential Mobility Analyzer)

  16. Smoke Detector – Ionization type Ionization Chamber http://en.wikipedia.org/wiki/Smoke_detector http://www.cna.ca/curriculum/cna_nuc_tech/images/smoke1.gif • 241Am emitting  particles as the radiation source. •  particles permit a small, constant current between two electrodes • If  particles are absorbed by smoke aerosol absorbs, the current is interrupted which sets off the alarm

  17. Reflections

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