Transfer Data for Simple Situations

Experimental data are usually obtained by blowing gases over various shapes wet with evaporating liquids or causing liquids to flow past solids which dissolve. Average, rather than local, mass-transfer coefficients are usually obtained. In most cases, the data are reported in terms of K, and the like, 

Particularly when fluids flow past immersed objects, the local mass-transfer coefficient varies with position on the object, due especially to the separation of the boundary layer from the downstream surfaces to form a wake. This phenomenon has been studied in great detail for some shapes, e.g., cylinders [8]. The average mass-transfer coefficient in these cases can sometimes best be correlated by adding the contributions of the laminar boundary layer and the wake. This is true for the second entry of item 5, Table 3.3, for example, where these contributions correspond respectively to the two Reynolds-number terms. 

For immersed objects the turhulence level of theJncident fluid also has an important effect [8]. Thus, for example, a bere.falling-thiQUgh a,qiiifit.fluidwaall result in a mass-transfet-coefficient different from that where theiluid-flows-past a stationary—sphere. In most cases the. distinction has not yet been fully established. Surface roughness, which usually increases the coefficient, has also been incompletely studied, and the data of Table 3.3 are for smooth surfaces, for the most part. 

When mass or heat transfer occurs, it necessarily follows that the physical properties of the fluid vary along the transfer path. This problem has been dealt with theoretically (see, for example. Refs, 25 and 47), but in most cases empirical correlations use properties which are the average of those at the ends of the transfer path. The problem is most serious for natural convection, less so for forced. 

For situations not covered by the available data, it seems reasonable to assume that the mass-transfer coefficients can be estimated as functions of Re from the corresponding heat-transfer data, if available. If necessary, in the absence of a known effect of Pr, the heat-transfer data can be put in the jp form and the analog completed by equatingy to jjf at the same Re. This should serve reasonably well if the range of extrapolation of Sc (or Pr) is not too large, 

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