Page 276 - Modelling in Transport Phenomena A Conceptual Approach

256 CHAPTER 8. STEADY MICROSCOPIC BALANCES WITHOUT GEN.

{ Pr = 0.711
W/ m. K
k = 26.59 x
For air at 33.5"C (306.5K) : u = 16.33 x m2/s
v = 13.30 x m2/s
For air at 0 "C (273 K) k = 24.07 x W/ m. K
Pr = 0.717

Analysis
The rate of heat loss can be calculated from Eq. (8.2-19), i.e.,

WH(TA - TB)
Q=
1 L 1
(hA)+Ic+- (hB)

The average heat transfer weficients, (hA) and (hB), can be calculated from the
correlations given in Table 4.2. However, the use of these equations require physical
properties to be evaluated at the film temperature. Since the surface temperatures
of the wall cannot be determined a priori, as a first approximation, the physical
properties will be evaluated at the fluid temperatures.

Left-side of the wall

Note that the characteristic length in the calculation of the Reynolds number is
10m. The Reynolds number is

Re = -
Lchvco
v

Since this value is between 5 x lo5 and lo8, both laminar and turbulent conditions
exist on the wall. The use of Eq. (E) an Table 4.2 gives the Nusselt number as

(Nu) = (0.037 - 871) Pr'13
= [0.037(5.6 x 106)*15 - 8711 (0.708)'13 = 7480 (3)

Therefore, the average heat transfer coeficient is

= (7480) ( 27*80t (4)
= 20.8 W/ m2. K