Profiles of vapour concentration over surfaces

Assuming K(z) is the same for vapour and momentum, the equation corresponding to (6.9) is 

The term on the left of (6.14) is the resistance to mass transfer multiplied by u, and is denoted r+(z). If (6.14) holds, r+(z) should plot as a straight line against ln z, with the same slope as the plot of/+(z) against ln z. 

The points in the r+ profiles diverge from the straight line at somewhat lower values of z — d than the points diverge from the/+ line. The resistance to vapour transport is greater than the resistance to momentum transport (as shown by the horizontal separation of the profiles), and hence the heights to which the r+ profiles are fully developed are less. 

Owen Thomson (1963) analysed heat and mass transfer from rough surfaces in terms of a parameter B, termed the sublayer Stanton number, defined by the equation 

Thus B 1 also is a measure of the ratio of the roughness lengths for momentum and vapour. 

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