Page 41 - Modelling in Transport Phenomena A Conceptual Approach

22 CHAPTER 2. MOLECULAR AND CONVECTNE TRANSPORT

Diffusion coefficients for liquids are usually in the order of lo-' m2/ s. On the other
hand, DAB values for solids vary from lo-'' to m2/ s.

Example 2.3 Air at atmospheric pressuw and 95 "C flows at 20 m/ s over a flat
plate of naphthalene 80cm long in the direction of flow and 60cm wide. Experi-
mental measurements report the molar concentration of naphthalene in the air, CA,
as a function of distance x from the plate as follows:

X CA
( cm) ( mol/ m3)
0 0.117
10 0.093
20 0.076
30 0.063
40 0.051
50 0.043

Determine the molar flux of naphthalene from the plate surface under steady con-
ditions.
Solution

Physical properties
Diflusion weficient of naphthalene (d) in air (a) at 95°C (368K) :

= 0.84 x m2/ s
Assumptions

1. The total molar concentration, c, is constant.
2. Naphthalene plate is also at a temperature of 95OC.

Analysis
The molar flux of naphthalene transferred from the plate surface to the Bowing
stream is determined from

It is possible to calculate the concentration gradient on the surface of the plate by
using one of the several methods explained in Section A.5 in Appendix A.