I think that the simplest way to understand this is by analogy to heat transfer (and yes: Heat and mass transfer are analogous processes). If you have a constant temperature driving force (bulk-to-wall) then increasing the fluids velocity will decrease the temperature boundary layer thickness (or the 'laminar sublayer' in the case of turbulent pipe flow) and increase mixing in the turbulent part of the boundary layer. This in turn reduces the resistance to heat transfer in the near-wall region, and consequently increases the heat transfer coefficient. The exact same process is taking place in mass transfer; without changing the concentration driving force, the mass transfer coefficient (and mass flux) increases as the fluid velocity increases due to the reduced resistance in the boundary layer.

Suggested reading: Any advanced textbook on convective (heat and) mass transfer, e.g. Kays/Crawford/Weigand: Convective Heat andf Mass Transfer or Mills: Mass Transfer

I would suggest to read this book Chapter.

I am open to any discussion on this topic.

Regards

Carlo Gualtieri

Dear all,

Thanks for replying and giving a nice picture.

I am thinking in the same way only. If velocity increases, concentration boundary layer (CBL) thickness decreases, and D/delta increases. (where D, diffusivity and  delta is the concentration boundary layer thickness). So the flux= D/delta(Cs-Cb), increases. where, Cs is the concentration at the interface and Cb is the concentration outside the CBL. if the velocity increase, will not there be any increase in driving force? 

Thanks

Vivek

The mass transfer coefficient is defined by the concentration difference between the wall and the bulk as driving force. These may of course be kept constant while changing the fluid velocity. However, the amount of mass transfer (i.e. the mass flux) will affect the mass transfer coefficient due to the added mass introduced into or removed from the boundary layer at the interface (so-called 'blowing' or 'suction'). Both will influence the concentration profile in the boundary layer. Simplistically, 'blowing' will thicken the boundary layer and reduce the mass transfer coefficient, and 'suction' will have the opposite effect. Consequently, the mass transfer coefficient depends on the direction and magnitude of the mass flux. The driving force may remain unaffected for these situations.

Many mass transfer problems involves phase change (e.g. evaporation, condensation or sublimation) at the interface. This means that there is heat transfer in addition to the mass transfer. This may involve heat transfer between the interface and the flowing fluid (e.g. air), and the heat transfer coefficient is of course affected by air velocity change.  For such cases the interface condition (temperature) can be influenced by the presence of heat transfer, meaning that the equilibrium  mass concentration concentration at the interface changes. Again, this is covered by the readings I suggested.

Thanks a lot

Essentially you mean that difference between the concentrations at two locations remains constant. Individual concentrations may may change. Right?

Thanks 

Vivek

I would add this :

In multicomponent mixture, a species may have different  diffusion velocity than its neighbors due to four driving forces: (1) species gradient; (2) Temperature gradient ( Soret effect); (3) pressure gradient; (4) external non uniformly distributed forces over mixture species. 

Thank you

Younis