Page 424 - Design and Operation of Heat Exchangers and their Networks

Experimental methods for thermal performance of heat exchangers 407

Fig. 8.5 Schematic of the test rig for condensation and evaporation on horizontal tubes.
(From Institute of Thermodynamics, Gottfried Wilhelm Leibniz University Hannover,
Germany.)

The correlation for tube-side flow is

0:14
λ i m 1=3 μ i
α i ¼ C i Re Pr (8.63)
i i μ
d i
w,i
and for pool boiling on the tube outside surface, Eq. (8.54) is used. The local
heat flux can be expressed as
t i t s
ð
q o ¼ kt i t s Þ ¼ 0:14
ð
A o d i μ w,i ln d o,r =d i,r Þ 1
+ A o +
m
A i C i λ i Re Pr 1=3 μ i 2πLλ t C o q n o
i i
(8.64)
where t s is the saturation temperature of the fluid on the tube outside.
Substituting Eq. (8.64) into Eq. (8.62), we obtain
dt i t s t i
¼ " #
dx μ 0:14
ð
d i w,i ln d o,r =d i,r Þ 1
_ m i c p,i m 1=3 μ + + n
A i C i λ i Re Pr i 2πLλ t A o C o q o
i i
(8.65)
The initial condition is given as
(8.66)
x ¼ 0 : t i ¼ t i,in
This ordinary differential equation can be integrated numerically, for
example, with the modified Euler method, and the calculated temperature