ANALOGIES BETWEEN THE PHENOMENA OF MOLECULAR TRANSPORT

The laws which govern the phenomena of molecular transport of quantities of matter, of heat and of momentum, seen in the paragraphs above, can be presented in the following general form  

Law (21) can be written in another form involving the quantity concentration defined by the relationship  

The diffusivities D, a, v characterize the ease of the molecular transports of the three quantities matter, heat and momentum, respectively. These diffusivities can be expressed in the same units, i.e. m s in basic SI units. 

The ratio of two diffusivities is therefore a non-dimensional number which represents the relative efficiency of the two modes of transport. Three non-dimensional numbers have been defined, the Schmidt numbers (Sc), the Prandtl numbers (Pr) and the Lewis numbers (Le)  

For gases, where the molecular collisions are less numerous than for liquids, the transports of the three quantities (mass, heat, momentum) occur with efficiencies which are very close to one another. It is concluded that the Schmidt numbers, the Prandtl 

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Table2 Analogies between the phenomena of molecular transport. ...

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