Transport along a thermal gradient

Thus the addition of an inert gas which does not intervene chemically in the uansport reaction but adds to the density of die gas, reduces the segregation due to thermal diffusion. An example of this is the reduction of tlrermal separation in a mixture of H2 and H2O by the addition of Hg vapour (Dastur and Chipman, 1948). 

Transport across the direction of flow can be determined using Fick s first law of diffusion for the flux of material in particles/cm2 sec-1 in a steady, time-independent state 

For a gas in laminar flow over a condensed phase sample of length L, the mass transport across the boundary layer, in terms of the flux of molecules from the sample to the gas phase, is therefore 

The dimensionless numbers in this equation are the Reynolds, Schmidt and the Sherwood number, Nsh, which is defined by this equation. Dv/g is the diffusion coefficient of the metal-transporting vapour species in the flowing gas. The Reynolds and Schmidt numbers are defined by the equations 

The advantage of the use of dimensionless numbers in calculations such as these is that they may be applied independent of the physical size of the system being described. 

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