Convection

Convective mass transfer refers to the movement of mass through a medium as a result of the net motion of a material in the medium. It is an added effect of bulk flow on top of diffusion or dispersion, and is studied the same way as convective heat transfer.

Convection-Diffusion Over a Surface

Convection-diffusion mass transfer over a surface is analogous to convective heat transfer. The mass flow rate from 1 to 2 is:

Convective Mass Transfer

$$ N_{A_{1-2}} = h_{m} A (c_{1} - c_{2}) $$

where

$ N_{A_{1-2}} $ = mass flow rate from 1 to 2 $ \left[\frac{kg}{s}\right] $
$ h_{m} $ = convective mass transfer coefficient $ \left[\frac{m}{s}\right] $
$ A $ = area normal to flow $ [m^{2}] $
$ c_{1} $ = concentration of A in the fluid at the surface (not solid) $ \left[\frac{kg}{m^{3}}\right] $
$ c_{2} $ = concentration of A in the bulk fluid $ \left[\frac{kg}{m^{3}}\right] $

Convection-Dispersion in an Infinite Fluid

Convective Mass Transfer in Biological Systems

Boundary Layers

Concentration Boundary Layer

Dimensionless Flow Parameters

Convective Mass Transfer Coefficient

The convective mass transfer coefficient $ h_{m} $ includes the effect of both diffusion and bulk flow, and depends on geometry, fluid properties, flow, and concentrations. There are two types of convection:

Free convection: driven by a density difference in the fluid due to differences in concentration or temperature
Forced convection: driven by an external force such as a fan