(142g) Bubble Growth and Expulsions in Natural Circulation Boiling Loop

Bubble Growth and Expulsions in
Natural Circulation Boiling Loop

Aranab
Karmakar and Swapan Paruya

Chemical
Engineering Dept., NIT Durgapur, India ? 713209

Introduction

Natural circulation boiling loop
(NCBL) containing steam-water mixture has extensive applications in boiling
water reactors (BWRs) and all the thermosyphon reboiler units of chemical,
refining and petrochemical plants, etc. due to its excellent heat removal
properties without the application of pump. During startup operation, NCBL
experiences various nonlinear flow instabilities¹. One of the major
instabilities is geysering phenomenon induced by wall-superheating at boiling
incipience. The mechanism and dynamic characteristics of the geysering have
been studied by many investigators^2-7. However, a cycle of geyseing
is completed in three consecutive stages - (1) refilling through condensation
of vapor to stop boiling, (2) incubation to initiate bubble formation, and (3)
expulsion of liquid and vapor as a result of self-evaporation induced by
decrease in static head.  The expulsion
is usually recognized by surge of loop flow rate in NCBL.

Efforts have been made in the recent
past to identify geysering to be periodic or chaotic.  As described above, the phenomenon is highly
complex. It is therefore always challenging to investigate the same. Here we
present an experiment based on picture motion browser (PMB) with the help of
Sony Handy Cam (HRD-XR160, 24 frames/sec, 3.3 mega
pixels) to visualize the bubble growth and the associated expulsion
phenomenon in geysering.

The present experimental study
reports bubble growth, expulsion time and expulsion period (cycle time) under
the varied conditions of heater power and inlet subcooling in a natural
circulation boiling loop. The analysis of high-speed video reveals that expulsions
coupled with the bubble growth phenomenon and the associated flow oscillations
are highly complex depending on inlet subcooling and heater power. 

Bubble Growth and Flow Pattern
Transition

Our NCBL
mainly consists of six parts ― heated section, adiabatic riser,
condenser, down-comer, upper and lower plenum. The rectangular loop was
fabricated of SS 316. RTDs, pressure sensors and DPT, and electromagnetic flow
meter (EFM) have been employed to measure temperature, local pressure,
differential pressure and loop flow rate respectively.. An accurate control of
heater power has been made by a thyristor power controller in PID mode. All
signals from sensors come to SCADA panel. The data-logger software (DAQ) in the
computer records the data from all sensors via SCADA panel @ 0.5 Hz.

At the lower heater power Q and high
inlet subcooling DT_sub
(=T_sat-T_in), expulsions occur at irregular and
low frequency. During rapid expulsion, both bubbles and liquid flow upward with
very high velocity. However, the vapor-jet pulls the liquid up in the channel
during the expulsions. The developing bubbly flow is shown in Figure 1(a). Just
after 0.2 sec the developing spherical bubbles coalesce to form the developed
bubbly flow, which is shown by Figures 1 (b-c). With the initiation of fully
developed bubbly flow, the violent vapor expulsion starts, and the flow regime
quickly changes to the slug one as a consequence of the coalescence of the
developed spherical bubbles shown in Figures 1 (d-e).  During violent expulsion of vapor and liquid,
big slug-bubbles form due to the coalescence of small ones and then get
condensed to small ones in the riser section (refer Figures 1 (f-p)). After
some time, the expulsions stop.  At the
time instants of 1.28, 2.40, 2.60 and 3.0 sec, the slug-bubbles are observed to
be big and glazy ones. In the intermediate time, the less glazy and smaller
slug bubbles form. At 2.64 sec and 3.04 sec, the bubble collapse due to
condensation is observed.

At higher DT_sub
and lower Q vapor-mushrooms are observed to form. The vapor-mushrooms are trailed by vapor stems
at their bottom. In the subcooled boiling flow, the vapor-mushrooms and vapor
stems are also reported in the literature^8-9. Figures 2(a-h) display
flow-patterns transition captured at Q=4 kW and DT_sub
= 30 ^oC. Figures 2(a-d) show the gradual growth of the developed
(large) slugs from small slugs or bubbles. If the heater power is lower, the
big slug bubbles do not form frequently. 
The smaller slug bubbles form just after the rapid expulsion of the larger
slug bubbles. The smaller ones coalesce with the larger ones to form a
mushroom-like shape shown in figure 2(e). Figures 2(f-h) show the different
size and shape of the vapor-mushrooms at different time of the expulsion. The
different size distribution of bubbles in turn produces shear stress
fluctuation in the vapor-liquid interface and creates the distortion of the
interface. As a result, the vapor-liquid
interface becomes more complex in case of lower heater power.

Figure 1. Flow-patterns transition captured at Q=5
kW and DT_sub=
30 ^oC

Figures
2. Flow-patterns transition captured at Q=4 kW and DT_sub=
30 ^oC.
Figure 3 shows the expulsion time
and cycle time in consecutives 24 cycles at different Q and DT_sub.
At Q=5 kW and DT_sub = 30^oC,
the expulsion time varied in the range of 3.92-5.8
sec, whereas the cycle time in the range of 11.68-15.42
sec. The points are close to each other compared to lower heater power. At Q=4
kW and DT_sub
= 30^oC, the expulsion time and cycle time vary widely in the range
of 3.48-11.8 sec and
9.44-31.84 sec,
respectively. Increase of DT_sub to 50 ^oC
at Q=4 kW, variations of both expulsion time and cycle time are in the range of
5.24-31.44 sec and
23.04-73.52 sec,
respectively. One may easily conclude that at high DT_sub and low Q, the expulsion time and cycle time are
larger and have wider distribution, and both expulsion time and frequency are
highly irregular.

Figure 3.
Expulsion time vs. cycle time at different Q and DT_sub

Flow expulsion

Interestingly, the expulsions have
been indicated by high loop flow rate W_loop. The time series of W_loop
at different Q and DT_sub
are shown in Figure 4. The peak flow rates are the signature of the expulsions
at different time intervals. In Figure 4(a), the expulsion stage and the
refilling stage are indicated by vertical dotted line a, b, c and d. In the
region ab, developing bubbly flow observed, where loop flow rates start to
increase. The region bc corresponds to the violent jet-flow in the heater section
(expulsion), where the maximum flow rates were observed. In the region cd, the
loop flow rates decrease suddenly due to the collapse of bubbles in the riser
of NCBL. The cold liquid the riser enters the heater section just after violent
expulsion. Reverse flow or flow termination occurs and non boiling condition
exists in the heated section. After the region cd incubation starts, where
initial stage of nucleate boiling observed in the heated channel. This stage is
noticed by the flat line in the time series just after the region cd. At the
low DT_sub
and high Q, the frequency of flow oscillations increases and becomes more
regular as evident in Figures 4(b-c).

Figure 4. Time
series of loop flow rate, 4 (a) at Q=4 kW and DT_sub=50 ^oC, 4 (b) at Q=4 kW
and DT_sub=30
^oC, 4 (c) at Q=5 kW and DT_sub=30 ^oC.

Conclusions

The present experimental investigations suggest that the expulsion
phenomenon is highly complex and chaotic and influenced by bubble growth and
condensation.  Mainly two major shapes of bubbles have been identified based on
PMB. They are vapor-slugs at high Q and low DT_sub, and
vapor-mushrooms at low Q and high DT_sub.
The chaotic phenomenon depends on Q and DT_sub. At lower Q and higher DT_sub, the
expulsion time and cycle time have been established to be larger and have wider
distribution and both expulsion time and frequency are highly irregular. At the
identical conditions, flow oscillations become more chaotic and encounter
reverse flow.

Acknowledgement

The authors also
thankfully acknowledge the financial support of Department of Science and
Technology, New Delhi, India under SERC scheme (Sanction #
SR/S3/CE/089/2009).

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