1 / 28

Condensation

Condensation. Reading: Chap 13. In the continuum regime, diffusion theory is used. At steady state. Q: Is n d equal to n s ?. Rate of diffusional condensation. Q: How much is it?. Volume growth. Size growth. Q: How can the condensation rate be increased?

aradia
Download Presentation

Condensation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Condensation Reading: Chap 13 • In the continuum regime, diffusion theory is used. At steady state Q: Is nd equal to ns? Rate of diffusional condensation Q: How much is it? Volume growth Size growth Aerosol & Particulate Research Laboratory

  2. Q: How can the condensation rate be increased? Q: How does the growth rate depend on size? Implication? Ex: For dp = 0. 1 and 1 mm, calculate the new dp after 1 sec of water vapor condensation at 20 oC. • In the free molecular regime, the gas kinetic theory is applied Volume growth Size growth Aerosol & Particulate Research Laboratory

  3. Q: How does the growth rate depend on particle volume? Particle size? Implication? For the entire range (Sutugen, 1971) (Pratsinis, 1988) Kelvin effect Aerosol & Particulate Research Laboratory

  4. Log-Normal Model Free Molecular Continuum Pratsinis, S. J. Colloid & Interface Science, 124(2), 416-427. Aerosol & Particulate Research Laboratory

  5. Non-dimensionalize the equations Q: N(t)? dp(t)? sg(t)? S(t)? Assuming condensation only. Aerosol & Particulate Research Laboratory

  6. Asymptotic Behavior: after a long time For the Continuum regime Eliminating dt Integrating and noting that the initial values are very small Important!!!  s = ? Note Aerosol & Particulate Research Laboratory

  7. Condensation Characteristic Time Q: When? Constant Particle Size: Limited vapor with a high particle loading Q: Physical meaning? Constant Vapor Concentration: a small particle loading with unlimited vapor Q: When? Aerosol & Particulate Research Laboratory

  8. Characteristic time for simultaneous particle growth and vapor consumption Free Molecular regime (same for C+) Q: Physical meaning? Wu, C. Y. and Biswas, P., Aerosol Sci. Technol., 28:1-20, 1998 Aerosol & Particulate Research Laboratory

  9. Continuum regime (same for C+) Continuum Free Molecular Aerosol & Particulate Research Laboratory

  10. Free Molecular Regime Aerosol & Particulate Research Laboratory

  11. Comparison of the characteristic times Aerosol & Particulate Research Laboratory

  12. Condensation Nuclei Counter Q: How to measure N by light scattering intensity if dp is different? Q: Can we use CNC to measure dp? Q: What if we combine a DMA and a CNC? Aerosol & Particulate Research Laboratory

  13. Aerosol & Particulate Research Laboratory

  14. Effects of Temperature • Temperature at the surface may be different from that away from the surface • Droplet heating may take place due to the release of latent heat Q: What’s the resultant impact of latent heat on condensation? H: latent heat kv: thermal conductivity of the gas Dv: diffusion coefficient of the molecule Aerosol & Particulate Research Laboratory

  15. Empirical equation for temperature difference T1 in oC. Fuchs correction factor (necessary for small particles) Q: Resultant effect of Φon condensation? Aerosol & Particulate Research Laboratory

  16. Aerosol & Particulate Research Laboratory

  17. Condensation with Soluble Nuclei & Ions Saturation ratio for droplets containing dissolved materials m: mass of the dissolved salt i: number of ions each molecule of salt forms Mw: molecular weight of solvent (water) Ms: molecular weight of salt Aerosol & Particulate Research Laboratory

  18. Saturation ratio vs droplet size for 10-16 g NaCl (Solid sphere of 0.045 mm in diameter) Aerosol & Particulate Research Laboratory

  19. Relative humidify vs droplet size (a larger scale) Droplet formation & crystalization 10-14g Q: Implication? Aerosol & Particulate Research Laboratory

  20. Evaporation • The reverse process of particle growth though there is no ciritical size. • Droplet lifetime/drying time Q: Where is evaporation important? • For volatile materials, cooling of the droplet due to the latenet heat of evaporation needs to be considered. Droplet lifetime for dp > 1.0 mm Aerosol & Particulate Research Laboratory

  21. Aerosol & Particulate Research Laboratory

  22. Q: How can we control evaporation time? Aerosol & Particulate Research Laboratory

  23. Q: For a given aerosol system, what’s the effect of heating followed by cooling? Condensation Aerosol Generator Aerosol & Particulate Research Laboratory

  24. Reflection Aerosol & Particulate Research Laboratory

  25. Condensation Characteristic Time – Constant dp (continuum regime) (pressure-concentration conversion) Volume growth is Vapor concentration change is Aerosol & Particulate Research Laboratory

  26. Integrate both sides Initial condition The final solution is then Aerosol & Particulate Research Laboratory

  27. Condensation Characteristic Time – Constant C (continuum regime) (pressure-concentration conversion) Size growth Integrate both sides Aerosol & Particulate Research Laboratory

  28. Initial condition The final solution is then Aerosol & Particulate Research Laboratory

More Related