Critical Heat Flux 23
1
A uniformly heated rod bundle has operating characteristics given below.
The critical heat flux mechanism is
dryout, and can be correlated by the HenchLevy correlation as
′′
=
≤
−−
≤
≤
≥
⎧
⎨
⎪
⎩
⎪
q
xx
xG
x
x
x
x
x
c
cc
c
c
c
10
1
19 33
07
3
10
06 07
009
2
6
1
26
12
2
.(
)
. .
.
t
a
n
h
( / )
()
..
.
t
a
n
h
(
/)
(
)
where:
.
. t
a
n
h
(
)
.
.
tanh (
/
)
.
tanh (
/
)
G
c
c
1
2
0 273
0 212
3
10
05
0 269
3
10
0 0346
2
10
=−
+
Assuming a minimum Critical Heat Flux Ratio of 1.0, determine the maximum heat flux the channel can safely
operate.
Problem Data
Channel Mass Flux
1.33 x 10
6
lbm/hrft
2
Rod Pitch
0.640 inches
Rod Diameter
0.493 inches
Channel Pressure
1000 psia
Channel Height
146 inches
Inlet Temperature
532 F
SOLUTION
The critical heat flux decreases monotonically with quality and therefore position up the channel.
For a uniformly
heated channel, the minimum critical heat flux ratio will then occur at the channel exit.
The critical quality is given
by
fg
f
crit
crit
h
h
h
x
−
=
where
m
DH
q
h
h
in
crit
±
π
′
′
+
=
such that
⎟
⎠
⎞
⎜
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 Fall '08
 Doster
 Thermodynamics, Flux, Heat Transfer, critical heat flux, xcrit

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