Page 132 - Modelling in Transport Phenomena A Conceptual Approach

112 CHAPTER 4. EVXLUATION OF TRANSFER COEFFICIENZS

s
Example 4.18 Water flows at an average velocity of 5 m/ through a duct of
equilateral triangular cross-section with one side, a, being equal to 2cm. Electric
wires are wrapped around the outer surface of the duct to provide a constant wall
heat flux of 100 W/ cm2. if the inlet water tempemture is 25 "C and the duct length
is 1.5m, calculate:
a) The power required to pump water through the duct,
b) The exit water temperature,
c) The average heat transfer weficient.
Solution

Physical properties
p = 997 kg/ m3
For water at 25 "C (298 K) : p = 892 x kg/ m. s
{ ep = 4180 J/ kg. K
Assumptions

1. Steady-state conditions prevail.
2. Changes in kinetic and potential energies are negligible.

3. Variations in p and cp with temperature are negligible.
Analysis

System: Water in the duct
a) The power required is calculated from Eq. (3.1-11)

The friction factor in Eg. (1) can be calculated from the modified fonn of the Chen
correlation, Eq. (4.5-16)

where
1.1098
(3)
The hydraulic equivalent diameter and the Reynolds number are

2
= - 1.155cm
=
4