Mass-Transfer Coefficients for a Single Sphere

Investigators have studied the mass transfer from single spheres and have correlated the Sherwood number by direct addition of terms representing transfer by purely molecular diffusion and transfer by forced convection in the form 

For transfer into liquid streams, the equation of Brian and Hales (1969), 

Equations (2-34), (2-35), and (2-36) can be used only when the effects of natural convection are negligible, that is, when 

The following equations by Steinberger and Treybal (1960) are recommended when the transfer occurs in the presence of natural convection 

Estimate the distance a spherical drop of water, originally 1.0 mm in diameter, must fall in quiet, dry air at 323 K and 1 atm to reduce its volume by evaporation by 50%. Assume that the velocity of the drop is its terminal velocity evaluated at its mean diameter during the process, and that the water temperature remains at 293 K. 

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Figure 9-26 Calculated mass-transfer coefficient for a single sphere settling at its terminal velocity in a liquid.8...

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