Page 68 - Modelling in Transport Phenomena A Conceptual Approach

3.3. MASS T'SFER COEFFICIENT 49

Substitution of Eqs. (2) and (3) into Eq. (1) gives the surface temperaturn as

3.2.2 Radiation Heat nansfer Coefficient

The heat flux due to radiation, qR, from a small object to the surroundings wall is
given as
qR = E u (Tt - Ti) (3.2-12)
where E is the emissivity of the small object, c is the Stefan-Boltzmann constant
(5.67051 x W/ m2. K4), TI and T2 are the temperatures of the small object
and the wall in degrees Kelvin, respectively.
In engineering practice, Q. (3.2-12) is written in an analogous fashion to Q.
(3.2-4) as
qR = hR (Ti - T2) (3.2-13)

where hR is the radiation heat transfer weficient. Comparison of Eqs. (3.2-12)
and (3.2-13) gives
EO (Tf - Ti)
hR = 21 4~ u(T)~ (3.2-14)
Tl - T2
provided that (T) >> (TI - T2)/2, where (T) = (TI + T2)/2.

3.3 MASS TRANSFER COEFFICIENT

Let us consider a flat plate suspended in a uniform stream of fluid (species S)
having a velocity 21, and species A concentration CA, as shown in Figure 3.5. The
surface of the plate is also coated with species A with concentration CA, .

Figure 3.5 Flow over a flat plate.

As an engineer we are interested in the total number of moles of species A
transferred from the plate to the flowing stream. This can be calculated by inte-
grating the total molar flux at the wall over the surface area. The total molar flux