Page 60 - Modelling in Transport Phenomena A Conceptual Approach

Chapter 3

Interphase Transport and

Transfer Coefficients

In engineering calculations, we are interested in the determination of the rate of
momentum, heat and mass transfer from one phase to another across the phase in-
terface. This can be achieved by integrating the flux expression over the interfacial
area. Equation (2.42) gives the value of the flux at the interface as
( Interphase ) = [ Gradient of

flux Quantity/Volume
+ ( volume ) ( Characteristic interface
Quantity
velocity

Note that the determination of the interphase flux requires the values of the quan-
tity/volume and its gradient to be known at the interface. Therefore, equations
of change must be solved to obtain the distribution of quantity/volume as a func-
tion of position. These analytical solutions, however, are not possible most of the
time. In that case we resort to experimental data and correlate the results by the
transfer coefficients, namely, the friction factor, the heat transfer coefficient, and
the mass transfer coefficient. The resulting correlations are then used in designing
equipment.
This chapter deals with the physical significance of these three transfer coef-
ficients. In addition, the relationships between these transfer coefficients will be
explained by using dimensionless numbers and analogies.

3.1 FRICTION FACTOR

Let us consider a flat plate of length L and width W suspended in a uniform stream
having an approach velocity v, as shown in Figure 3.1.

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