Page 77 - Modelling in Transport Phenomena A Conceptual Approach

58 CHAPTER 3. INTERPHASE TRANSPORT

the j-factors. The validity of the Chilton-Colburn analogy for flow in different
geometries is given in Table 3.3.

Pressure force
A I

Direction of mean flow Pressure force
I

L plat surface
Figure 3.8 Pressure force acting on curved and flat surfaces.

Table 3.3 Validity of the Chilton-Colburn analogy for various geometries.
Flow Geometry Chilton-Colburn Analogy

f .
Flow over a flat plate - = JH = jM
2
Flow over a cylinder jH = jM

Flow over a sphere

f
Flow in a pipe - = j~ = jM if Re > 10,000 (Smooth pipe)
2

Example 3.4 Water evaporates from a wetted surface of rectangular shape when
air at 1 atm and 35 "C is blown over the surface at a velocity of 15 m/ s. Heat
transfer measurements indicate that for air at 1 atm and 35 "C the average heat
transfer weficient is given by the following empirical relation
(h) = 21 vk6

where (h) is in W/ m2. K and v,, air velocity, is in m/ s. Estimate the mass
transfer weficient and the rate of evaporation of water from the surface if the area
is 1.5m2.