Page 278 - Modelling in Transport Phenomena A Conceptual Approach

258 CHAPTER 8. STEADY MICROSCOPIC BALANCES WITHOUT GEN.
Therefore, the film temperatures at the left- and right-sides of the wall are (42 +
50)/2 = 46 "C and (- 10 - 5)/2 = - 7.5 "C, respectively. Since these temperatures
are not very much digerent from the fluid temperatures, the heat transfer rate can
be considered equal to 4956 W.

b) For k = 20 W/ m. K, the use of Eq. (1) gives

- +-+-
20.8 20 34
The surface temperatures TI and TZ can be calculated as

Q
T2 = TB + -
A(hB)
20,574
= -lo+- 10°C
(30x34) -
In this case, the film temperatures at the left- and right-sides are (17 + 50)/2 =
33.5 "C and (- 10 + 10)/2 = O"C, respectively. Since these values are diflerent
from the fluid temperatums, it is necessary to recalculate the average heat transfer
coeficients.
Left-side of the wall

Using the physical properties evaluated at 33.5 "C, the Reynolds number becomes
Lchvoo
Re = -
U

The Nusselt number is
(Nu) = (0.037Re4,/5 - 871) Pr1l3

= b.037 (6.1 x 106)4/5 - 8711 (0.711)1/3 = 8076 (15)
Therefore, the average heat transfer coeficient is

= (8076) ( 26.59G lo-.> = 21.5 W/ m2. K