Page 112 - Modelling in Transport Phenomena A Conceptual Approach

92 CHAPTER 4. EVfiUATION OF TRANSFER COEFFICIENTS

Therefore, the average heat transfer coeficient and the rate of heat loss from the
body are

- = 17.5 W/ m2. K
- (226) (23.28 x
0.3
Q = (T x 0.3 x 1.8) (17.5) [30 - (- lo)] = 1188 W

ChurchilEBernstein correlation
The use of Eq. (4.4-8) gives

NU = 0.3 + 0.62 hr Pr'l3 [1+ ( ReD )"'"] 4/5
28,200
[I+ (04 pr)2/31 'j4

x
= 0.3 + 0.62 (8.46 x 104)1/2(0.714)1/3 [1+ (8.46 28,200 IO^,"'^] 4'5 = 340
[1+ (0.4/0.714)2/3] 'I4
The average heat tmnsfer coefficient and the rate of heat loss from the body are
(h) = Nu (a)

-
- (340) (24.86 x lo-.> = 28.2 W/ m2. K
0.3
Q = (T x 0.3 x 1.8) (28.2) [30 - (- lo)] = 1914W
Comment: The heat tmnsfer coeficient predicted by the Churchill-Bemtein cor-
relation is 70% greater than the one calculated wing the Whitaker correlation. It
is important to note that no two correlations will exactly give the same result.

4.4.3 Mass Transfer Correlations

Bedingfield and Drew (1950) proposed the following correlation for cross- and
parallel-flow of gases to the cylinder in which mass transfer to or from the ends of
the cylinder is not considered:
I Sh = 0.281 Re2 Sco*" I (4.410)

Equation (4.410) is valid for
400 5 ReD 5 25,000

0.6 I Sc 5 2.6