Page 108 - Modelling in Transport Phenomena A Conceptual Approach

88 CHAPTER 4. EV'ALUATION OF TRANSFER COEFFICIENTS

Tosun and Akqahin (1992) proposed the following single equation for the friction
factor that covers the entire range of the Reynolds number in the form

Once the friction factor is determined, the drag force is calculated from

(4.45)

Example 4.9 A distillation column has an outside diameter of 8Ocm and a
height of 10m. Calculate the drag force exerted by air on the column if the wind
speed is 2.5 m/ s.
Solution
Physical properties

Assumption
1. Air temperature is 25°C.
Analysis

From Eq. (4.4-3) the Reynolds number is
DVWP
hD=-
P

The we of Eq. (4.4-4) gives the friction factor as
f=-( l8 1 + 0.36 Re?) 8/5
h8/9
D
- 6.18 [1+ 0.36 (1.29 x 10~)~/~]~/~
-
1.2
=
(1.29 x 105)8/9
Therefore, the drag force is calculatedfrom Eq. (4.4-5) as

1
(l.l845)(2.5)' (1.2) = 35.5N