Eddy Diffusion/Forced Diffusion

Eddy diffusion in mass transfer refers to the turbulent transport of mass within a fluid due to the chaotic and random motion of eddies in the flow. For Mass Transfer this type of diffusion is significantly more effective than molecular diffusion, especially in turbulent flows where it dominates the MT aspect leading to enhanced mixing and transport of mass within fluids.

While molecular diffusion occurs due to random molecular motion, eddy diffusion is driven by the larger-scale turbulent motions of the fluid. In turbulent flows, eddy-diffusion dominates over molecular diffusion. It is characterized by rigorous mixing.

Note: N(B)=0 means Diffusing A Non-Diffusing B

N(A)=-N(B) = Both A and B are diffusing in opposite direction

Key Aspects of Eddy/Forced Diffusion in Mass Transfer:

1. Turbulent Transport: In turbulent flows, the chaotic motion of fluid parcels (eddies) leads to the mixing and transport of mass more rapidly and over larger distances than molecular diffusion. This process is termed eddy-diffusion or turbulent diffusion.
2. Eddy Diffusivity (or Eddy Diffusion Coefficient): The effectiveness of eddy-diffusion is quantified by the eddy-diffusivity, often denoted by Dt (for mass transfer). This coefficient is analogous to the molecular diffusivity Dm, but it is typically much larger in turbulent flows.
3. Modeling Eddy-Diffusion: In mass transfer, the eddy-diffusivity is used in models to predict the rate and pattern of mixing of substances in turbulent flows. For example, in the turbulent diffusion equation

Eddy Diffusivity Estimation: Eddy diffusivity is not a constant but depends on the flow conditions, such as the intensity of turbulence and the scale of the eddies. It can be estimated using empirical correlations or turbulence models that account for factors like the Reynolds number or the structure of the turbulence.

Note: Here terms are same as used in Treybal Book

Molecular Diffusion & Fick’s Law-Mass Transfer Operation

Image Compressor