Page 150 - Modelling in Transport Phenomena A Conceptual Approach

130 CHAPTER 4. EVALUATION OF TRANSFER COEFFICIENTS

surface temperature of the pipe at 82 "C. If the length of the pipe is 5 m, determine
the outlet temperature of water.
(Answer: 51 "C)

4.16 Dry air at 1 atm pressure and 50 "C enters a circular pipe of 12 cm internal
diameter with an average velocity of 10cm/s. The inner surface of the pipe is
coated with a thin absorbent material which is soaked with water at 2OOC. If the
length of the pipe is 6m, calculate the amount of water vapor carried out of the
pipe per hour.
(Answer: 0.067kg/ h)

4.17 A column with an internal diameter of 50 cm and a height of 2 m is packed
with spherical particles of 3mm in diameter so as to form a packed bed with
E = 0.45. Estimate the power required to pump a Newtonian liquid (p = 70 x
10-3kg/m.s; p = 1200kg/m3) through the packed bed at a mass flow rate of
1.2 kg/ s.
(Answer: 39.6 W)

4.18 The drag force, FD, is defined as the interfacial transfer of momentum from
the fluid to the solid. In Chapter 3, power, W, is given by Eq. (3.1-11) as

w = FD vch (1)
For flow in conduits, power is also expressed by Eq. (4.5-2) in the form

73/ = Q 1API (2)
a) For flow in a circular pipe, the characteristic velocity is taken as the average
velocity. For this case, use Eqs. (1) and (2) to show that

FD = A (AP( (3)
where A is the cross-sectional area of the pipe.

b) For flow through packed beds, the characteristic velocity is taken as the actual
average velocity or, interstitial velocity, i.e.,

in which vo is the superficial velocity and E is the porosity of the bed. Show that

where A is the cross-sectional area of the packed bed.