Continuum Transport Processes
Evaporation and condensation phenomena in clouds determine the aerosol size and rate of production of rain, and in industrial applications condensation following nucleation of molecular clusters can produce undesirable fogs such as sulfuric acid mists or
- PDF / 6,230,615 Bytes
- 73 Pages / 439 x 666 pts Page_size
- 26 Downloads / 270 Views
This attempt, while not successf ul in the f orm in which it had been planned, led to a modification of the cloud method which seemed at the time, and which has actually proved since, to be of far-rea ching importance. It made it for the first time possible to make all the measurements on individual droplets.
R. A. Millikan (1935)
5.1
Transport Regimes Evaporation and condensation phenomena in clouds determine the aerosol size and rate of production of rain, and in industrial applications condensation following nucleation of molecular clusters can produce undesirabl e fogs such as sulfuric acid mists or very desirable nanophase particle s such as ceramic precursors . Evaporation and condensation processes are also responsible for the formation of hazes such as those produced by terpene s and other volatile organic compounds which evaporate from pine trees and condense in the atmosphere over forested areas or become oxygenated organic acids due to photochemical reaction in the atmosphere. Combustion processes generate soot, flyash and other particulate matter that is an environmental hazard. In addition, combustion processes that generate SOz, HzS or NO x lead to the formation of acid rain. The rate at which particles settle out of the atmosphere or are transported to a surface by convective motion of the surrounding fluid is a fluid mechanical issue, and heat and mass transfer rates can be affected by such convection. The mechanics of aerosols have been thoroughly examined in the treatise by Fuchs (1964), and more recent work on the motion of particles in gases and on the dynamics of aerocolloidal systems, including coagul ation, deposition and re-suspension , have been reviewed by Williams and Loyalka (1991) in their monograph. We shall confine our attention to phenomena and properties associated with the single particle. Heat, mass and momentum transfer rates to or from particles in dense gases and liquids are usually described by the equations of continuum mechanics. In this theoretical framework only macroscopic properties such as density, mass concentration, viscosity, thermal conductivity, diffusivities, thermodynamic properties and the like are considered. In the upper atmosphere and in vacuum systems the laws of continuum mechanics break down, and it is necessary to treat
E. J. Davis et al., The Airborne Microparticle © Springer-Verlag Berlin Heidelberg New York 2002
302
5 Continuum Transport Processes
transport phenomena from a molecular or statistical mechanical point of view. For small particles both points of view are encountered, and the criterion for the appropriate theoretical approach is important. Continuum theory applies when the mean free path, £, of the molecules in the surrounding medium is small compared with the characteristic dimen sion of the particle. For a sphere of radius a, continuum theory applies when Kn = f/a « 1, where Kn is called the Knudsen number in honor of Knudsen's pioneering work on fluid flow and heat transfer with rarefied gases. In this continuum regime a molecule
Data Loading...
