7th International
Conference
on Nuclear Engineering
Tokyo, Japan, April 19-23,1999
ICONE-
Two-phase Flow Instability in a Double-Channel Natural Circulation Loop
with Equal or Unequal Heating Powers
W.L. Chen, S.B. Wang, C.R. Chung, S.S. Twu and Chin Pan
Department of Engineering and System Science
National fiing Hua University, Hsinchu, Taiwan 30043, ROC
Fax: 886-3-5720724
Tel : 886-3-5725363
E-mail: cf,an~~fucuty.I-l fhu. edu. tw
Abstract
This study investigated
experimentally
double channel natural circulation
the glass channel
the two-phase
flow instability
phenomena
loop with equal or unequal heating powers. The results of
heated with a power of 5kW, and with power ratios between
channel and glass channel of 1, 0.822, 0.676, 0.524 and 0.448, respectively,
discussed. Two-phase
is varied.
in a
flows with periodic or aperiodic oscillations
the steel
are reported and
appear as the power ratio
The mean flow rate in the glass channel, which is heated with a constant power of
5 kW for all the cases in this study, decreases with higher heating power in the steel channel.
The ratio of mean flow rate between steel and glass channel increases linearly with increase
the power ratio.
Keyword: two-phase flow, instability, multiple channel, natural circulation loop
l.INTRODUCTION
Two-phase
natural circulation
process thermosiphon
loops have many industrial
reboiler, waste heat recovery
applications,
and next generation
e.g., chemical
nuclear
reactors,
because of its simplicity, high heat transfer capability and inherently passive nature. Because
of safety considerations,
operation
of the industrial
the unstable
thermal-hydraulic
conditions
are not permissible
in
system. A designer or operator needs to know the conditions
at
which stability can be loose and what the behavior of the loop system will be.
known that several different kinds of instability can arise in a two-phase
1
It is well
natural circulation
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01999
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loop under certain conditions(
Aritomi et al., 1992; Furuya et al., 1997; Hsieh et al., 1997; Jiang
et al.,1 997; Wang and Pan, 1998). The instability in a multiple channel natural circulation
could be more complicated
due to channel-to-channel
interaction.
of unequal heating powers on the unstable thermal-hydraulic
significant importance
into the effect
behavior is of great interest and
for the design, operation and control of a system. The heating power is
one of the key parameter influencing
experimental
Investigation
loop
the stability of a two-phase flow system. Most previous
studies on double-channel
natural circulation
loops(eg.
Aritomi
et al., 1992)
were conducted with equal power dissipated in both channels. It is not clear what the effect of
unequal power dissipation
The primary objective
phenomena
in the two channels on two-phase natural circulation phenomena
of the present study is to investigate
the two-phase
is.
flow instability
in a double channel natural circulation loop with equal or unequal heating powers.
2. EXPERIMENTAL
The experimental
our previous
loop employed
study. The experimental
APPARATUS AND PROCEDURE
in this study, shown in Fig. 1, is a modified
loop consists of two parallel channels
heated flow passage and adiabatic riser, condenser,
downcomer,
version of
with annular
upper and lower horizontal
sections (Chen et a1.1998) . One of the two parallel channels is a stainless steel tube and the
other is a transparent
Pyrex glass tube that allows direct flow visualization.
double channel is designed with an annular passageway
electric heated
respectively.
rods in the stainless
for fluid.
The heater of the
The outer diameters of the
steel and Pyrex tubes are 10.6 mm and 11.2 mm,
The outer diameters
of the annular for both channels are 20.2 mm. The total
length of the heated region is l.lm
but only the top 0.92m is actually heated. The length of
riser is 3.65m. The two channels have common
upper plenum and lower mixing tanks. The
working fluid used was distilled water. A single DC power supply device controls the heating
power. A special circuit was designed such that a DC power supply may provide two channels
with equal or unequal heating powers. The glass channel was heated with a power of 5kW,
and the power ratios between the steel channel and glass channel were 1, 0.822, 0.676, 0.524
and 0.448, respectively.
The outputs
recorded
of thermocouples,
and analyzed
flowmeters
by two data acquisition
and differential
systems:
pressure
transducer
HP3852 and ARllOOA.
are
Both are
connected with a personal computer. The former provides 96 channels for recording the data
with a sampling rate of 0,5Hz, while the later has four channels only and can have a much
higher sampling rate. The AR1 1OOA was used during parts of transient and quasi-steady
intervals which are of significant
interest for the measurements
of the two channels, riser fluid temperature
The signals of LEDs are used to synchronize
state
of the mass flow rate in each
in the glass tube and voltage outputs of the LED.
the thermal hydraulic data with the risers(Hsieh
et a1.,1997)
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To enable independent
flow measurement,
section, in which an orifice flowmeter
both channels have their own lower horizontal
is installed. Both flowmeters
forward and reversed flows. The uncertainties
in measurement
were calibrated for both
in the low flow rate region, at
which the uncertainly
is the highest, for forward and reversed
4.9kg/hr, respectively,
in the glass channel and &4.5kg/hr
flow are rfr8.4kg/hr and+
and &7.3kg/hr,
respectively,
in
the stainless steel channel.
The purpose of this study is to investigate the effect of unequal heating power in the two
channels on natural circulation behavior. Thus, the loop gage pressure is maintained
m Hz0 in the joint
temperature
station
of the condenser
is kept
at the exit on the primary side of the condenser
the flow rate and inlet temperature
of the secondary
referred to as the inlet fluid temperature
at the inlets is also oscillating
cooling
The working
fluid
at 60 + I “c by adjusting
water. This temperature
is
as it is not much different from that at both channel
inlets for the cases with steady circulation.
controllable
and the downcomer.
at 6 +0.5
Under oscillation conditions,
due to possible
reversed
the fluid temperature
flow and cannot be treated as a
variable. The heating powers in both channels are first gradually increased to 5
kW. Care was exercised to maneuver the operation such that the steady state is obtained with
the inlet temperature
and loop pressure
pressurizer is disconnected
of gas in the pressurizer
state has maintained
Thereafter,
at the desired
levels.
At that stage, the lower
with the loop, which might evil effect of the compressible
on the loop thermal
- hydraulic
volume
instability. After the quasi-steady
for about 1 hour, AR1 1OOA was turned on to record the data for 52 min.
the power in the steel channel
is decreased
to the next desired
level and the
process as for the case with 5kW in each channel is repeated.
3. EXPERIMENTAL
Two-phase
RESULTS AND DISCUSSION
flow prevails when both channels
are operated with equal power of 5kW.
This value is about the power for the incipience of two-phase flow in the loop under the given
conditions
(Chen et a1.,1997). Moreover early out-of-phase
channels as shown in Fig.2. The oscillation
flows. The oscillation
periodic flows persist in the two
is also characterized
by the presence of reversed
frequency is about 0.06Hz. The peak forward flow rate, in the range of
500 to 600 kg/hr, is usually accompanied
with slug or churn flows in the riser. The very
existence of such flow patterns indicates that the void fraction in the channel is high and the
buoyancy force is significant
Chen et al.( 1997) provides
occurrence
an explanation
for the out-of-phase
oscillation
and the
of reversed flows. The reversed flow is initially caused by the hot fluid entering
from top pushed by the large flow in the other channel. The significant reversed flow ‘peak’ at
the end of reversed flow period results from the rapid evaporation
heated anuulus of the channel. Such a rapid vapor expansion
3
and vapor expansion in the
expels the liquid both upward
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and downward
and causes the reversed flow peak. On the other hand, this expanded
vapor blanket provides
a very large buoyancy
large
force and makes the fluid flow forward again
and rises sharply to its positivepeak. At this stage, the flow patterns in the riser is typically
slug on churn flow. Therefore,
temperature
the thermocouple
in the lower riser will sense the vapor
and a spike is recorded, as show in Fig.2(a) (Chen et al.,l997).
Such a out-of-
phase periodic flow persists when the heating power in the steel channel is reduced to 4.1 lkW,
as show in Fig.3. The glass channel is heated with a power higher than that in the steel
channel and should have a potential to oscillate more frequently.
However, the power in the
steel channel is not much lower than in the glass channel and the close interaction
between
two channels makes them oscillate together with a single frequency.
Such a nice cooperation
between two channels no longer exists when the heating power
in the steel channel is reduced
channel.
Figure
4 illustrates
oscillates more frequently
conceivable
to 3.38kW, i.e., 0.676 times of heating power in the glass
the chaotic-like
oscillations
in the two channels.
in the glass channel though the oscillation
The flow
is not periodic. This is
in that the heating power in the glass channel is higher than that in the steel one.
In addition to the high temperature
peaks”, low temperature
time. This indicates that part of the low temperature
glass channel, at which the riser fluid temperature
spikes also appear form time to
fluid may be able to reach the riser of the
is measured.
Such an irregular oscillation
continues to display when the heating power in the steel channel is further decreased to 2.62
kW, i.e., the power
ratio between
steel and glass channel
Although the time plots look like periodic oscillations,
is 0.524, as shown
in Fig.5.
the power spectrum of the flow rate in
the glass channel presents no distinct peaks. The intensity of the one distinct peak at 0.04Hz
for the flow rate in the steel channel is relatively
low compared
to those in Fig.2 or Fig.3.
Therefore, the oscillation is considered irregular.
The flow becomes periodic again when the heating power in the steel channel is further
reduced to 2.24kW, i.e., 0.448 times of that in the glass channel, which is apparent in Fig.6
The flow in glass channel is of double frequencies,
i.e., 0.035 and 0.071H2, while the flow in
the steel channel is of single frequency of 0.035 Hz. The low frequency oscillation
channel due to its relatively low heating power is able to match the fundamental
the glass channel. Consequently,
the periodic oscillation
in the steel
frequency of
in the system may be resumed. It is
also worth noticing that the flow in the steel channel is characterized
by the presence
of a
relatively long-last reversed flow. During the reversed flow interval in the steel channel there
are two forward flow excursions in the glass channel.
The power ratio between two channels also has a significant
effect on the mean flow
rate, as shown in Fig.7. The mean flow rate in the glass channel, which is heated with a
heating power of 5 kW for all the cases in this study, decreases with increase in the heating
power in the steel channel. This is because the reversed flow interval in the glass channel is
4
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increased with increase in the heating power of the steel channel The mean flow rate in the
steel channel increases with increase in heating power as expected resulting in the elevation of
total loop flow rate with increase in total heating power and the mean flow rate ratio between
two channels increases approximately
As shown in
linearly with increase in power ratios.
Fig7c, the ratio of flow rate in the two channels can be expressed by the following equation:
QC-- 0.91
-.--L-
E=2.31
QA
where FC / FA is mass flow rate ratio between steel and glass channel; Qc /Q, is heating
power ratio between steel and glass channel.
4. CONCLUSION
This study explores
experimentally
double channel natural circulation
demonstrate
the significant
circulation phenomena.
the two-phase
flow instability
of power
ratio between
If a power ratio results in oscillation
frequency
power of 5 kW, periodic
oscillation
in a
loop with equal and unequal heating powers. The results
effect
matches the fundamental
phenomena
two chartnel
frequency
on the natural
in the steel channel
in the glass channel which is heated with a constant
will appear in the system. If not, aperiodic
prevails in the two channel. Such a significant
interest for the operation of a muti-channel
oscillations
effect of heating power ratio is of significant
natural circulation loop.
ACKNOWLEDGEMENTS
This work was supported
by the National Science Council of Republic of China under
Contract No. Nsc84-2212-E-007-068.
REFERENCES
1.
M. Aritomi,
J.H. Chang and M. Mori, 1992,
Fundamental
Studies on Safety-Related
Thermo-Hydraulics
of Natural Circulation Boiling Parallel Channel Flow Systems Under
Startup Conditions
(Mechanisms
of Geysering
in Parallel Channels),”
Nuclear
Safety,
Vol.33, pp. 170-182.
2.
M. Furuya, F. Inada and A. Yasuo, 1997, “Thermohydraulic
Circulation
Loop with a Chimney
(Part 2. Experimental
Instability in Detail),” Heat Transfer-Japanese
3.
C.C. Hsieh, S.B. Wang and C. Pan, 1997,
Instability of Boiling Natural
Approach
to Clarify the Flow
Research, Vol. 24, pp. 577-588.
Dynamic
Visualization
of Two-Phase
Flow
Patterns in a Natural Circulation Loop,” Int. J. Multiphase Flow, Vol.98 pp. 616-622.
4.
S.Y. Jiang, X.X. Wu, S.R. Wu, H.H. Bo, Y.J. Zhang and P, Han, 1997,
5
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Experimental
01999
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Study on Flashing
Concerned
S.B. Wang and C. Pan, 1998,
Circulation
in a Natural
Kerntechnek,
Heating Reactor Conditions,”
5.
Instability
Circulation
system
at Nuclear
Vol. 62, pp. 148-153.
Two-phase
Flow Instability
Loop Using the Taguchi Method”, Experimental
Experiment
in a Natural
Thermal and Fluid Science,
Vol. 17, pp. 189-201.
6.
W.L. Chen, S.B. Wang, S.S. Twu, C.R. Chung and C. Pan, 1998,
Double
Channel
Engineering,
Natural
Circulation
Loop,”
Proc.
of @ Int.
Hysteresis
Conf.
Effect in a
On Nuclear
(in CD-ROM), San Diego, CA. May 10-14, 1998.
Fig. 1
The double channel natural circulation loop
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0
50
100
150
TIME (see)
125
,~~~
0.000.05o.*o0.15 0.20
Frequency
250
200
_
0.25
(Hz)
300
-
~.
0.35
0.30
0.40
Fiq. 2 Time evolution of thermal hydraulics for both channels operated at
5.0 kW. (a) riser fluid temperature
in the glass channel (b) flow
rate in the glass channel(FA) (c) flow rate in the steel channel(FC)
(d) power spectrum of FA (e) power spectrum of FC.
Q078TS02
0
50
100
3,s
0.00
0.05
Fiq. 3 Time evolution
l6m
0.10
60’1:
150
TIM f (see]
0.15
0.20
Frequency
of thermal
4.113
250
200
0.25
(Hz)
kW)
0.35
0.30
hydraulics
300
0.40
for the power ratio (steel /
glass) of 0.822. (a) riser fluid temperature
in the glass channel (b)
flow rate in the glass channel (FA) (c) flow rate in the steel channel
(FC) (d) power spectrum of FA (e) power spectrum of FC.
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Ii6
fi,
ii
;
~.
0.00
0.05
Fiq. 4 Time evolution
0.10
0.15
0.20
0.25
Frequency(Hr)
0.30
of thermal hydraulics
0.35
0.40
for the power ratio (steel /
glass) of 0.676. (a) riser fluid temperature
in the glass channel (b)
flow rate in the glass channel (FA) (c) flow rate in the steel channel
(IX) (d) power spectrum of FA (e) power spectrum of FC.
1078SSO2
0.00
Fiq. 5 Time evolution
0.05
0.90
(6m
60%
0.15
0.20
Frequency
2.62:6
0.25
(Hz)
of thermal hydraulics
kW)
0.30
0.35
0.40
for the power ratio (steel /
glass) of 0.524. (a) riser fluid temperature
in the glass channel (b)
flow rate in the glass channel (FA) (c) flow rate in the steel channel
(FC) (d) power spectrum of FA (e) power spectrum of FC.
8
Copyright 01999 by JSME
1000
$
500
01
5.
Y
O
0
9
-q
Qk
2
9
g
Q
100
150
100
50
250
200
300
TlME (WC)
-
75
50
25
100
80
60 ~~
40
0.00
0.05
0.10
0.15
0.25
0.20
Freq*ncy 0-w
0.30
0.35
0.40
Fiq. 6 Time evolution of thermal hydraulics for the power ratio (steel / glass) of 0.448.
(a) riser fluid temperature in the glass channel (b) flow rate in the glass channel
(FA) (c) flow rate in the steel channel (FC) (d) power spectrum of FA (e) power
spectrum of FC.
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0
+
FA
- - - - FC
9.0
8.0
Total Power (kW)
0.40
0.50
0.60
0.70
0.80
0.90
1 .oo
Power Ratio (QJQA)
Fiq. 7 Effect of power ratio (steel / glass) on mean flow rate.
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