Aerosol processes and stability

Such processes are most evident for what are sometimes referred to as high dispersed aerosols (HD As). HD As have particle sizes less than 100 nm and Knudsen numbers oiKn 1. 

Aerosol particles move by diffusion when there is a particle concentration gradient or when there is a concentration gradient in the continuous gas phase (diffusio-phoresis). Diffusion is described further in Section 5.5. A specific kind of diffusion flow - called Stephan flow - occurs when the continuous phase gas molecules are themselves diffusing either towards a condensing surface or away from an evaporating surface. In either case, the suspended particles are carried along with the gas and they move perpendicular to the surface. This can occur in any wet scrubber and can influence the particle collection efficiency. 

For the two main processes determining aerosol stability, sedimentation was discussed already in Section 2.5.1, and aggregation is discussed further in Section 5.5. For quite large particles, and Kn , Stokes law describes the sedimentation process. Thermophoretic velocity is not strongly influenced by interactions between the aerosol particles and molecules of the gas. 

Separate from particle/droplet size and the Knudsen number, there is another reason that aerosol sedimentation does not always follow Stokes law. As the flow regime goes from laminar flow (viscous dominated Reynolds number, Af u 1) to turbulent flow (inertia dominated Reynolds number, Nr 1000), things change (see Section 6.1 and Equation 6.6 for more on the Reynolds number). 

In the laminar flow regime Stokes law (Equation 2.20) does hold, and the drag coefficient, Cp is given by 

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