Diffusion Fluxes and the Sherwood Number

A local (or differential) diffusion flux of a solute to the surface is determined by 

The total (or integral) diffusion flux can be obtained by integrating (3.1.25) over the entire surface S  

The total diffusion flux J is the amount of substance that reacts on the entire surface per unit time. 

In mass transfer problems with the boundary conditions (3.1.2) and (3.1.3), one often replaces (3.1.25) and (3.1.26) by the dimensionless diffusion fluxes 

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