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ORNL-TM-4122.txt
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| ORNL-TM-4122
DEVELOPMENT OF A
VENTURI TYPE BUBBLE GENERATOR FOR USE
IN THE MOLTEN-SALT REACTOR XENON
REMOVAL SYSTEM
C. H. Gabbard
MASTER
OPERATED BY UNION CARBIDE CORPORATION = FOR THE U.S. ATOMIC ENERGY COMMISSION
This report was prepared as an account of work sponsored by the United
States Government., Neither the United States nor the United States Atomic
Energy Commission, nor any of their employees, nor any of their contractors,
subcontractors, or their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness or
usefulness of any information, apparatus, product or process disclosed, or
represents that its use would not infringe privately owned rights,
ORNL-TM-4122
Contract No. W-Th0S5-eng-26
Reactor Division
DEVELOPMENT OF A VENTURI TYPE BUBBLE GENERATOR
FOR USE IN THE MOLTEN-SALT REACTOR XENON REMOVAL SYSTEM
C. H. Gabbard
Molten-Salt Reactor Program
NOTICE NOTICE
::tllfr:ealf;t wiznti?sa;f; rrxr-ljatmnfloffa preliminary This report was prepared as an account of . work
at the originatinp i:stallafioflali i 011; 1n_tfrna1 1se sponsored by the United States Government, Neither
vision or correc%ion and theré-f r]sdsu 1e°tto re- the United States nor the United States Atomic Energy
sent a final report. It is passf;é)tg thoes noi ;‘Epre- Commission, nor any of their employees, nor any of
confidence and should not be a-bstract?a;?)i}t? e[:;lin their contractors, subcontractors, or their employees,
disclosed without the a, roval‘ f th igri l:? r makes any warranty, express or implied, or assumes any
installation or DTI Extpps' 5 k e inating legal liability or responsibility for the accuracy, com-
ension, Oak Ridge. pleteness or usefulness of any information, apparatus,
product or process disclosed, or represents that its use
would not infringe privately owned rights.
December 1972
OAK RIDGE NATIONAL LABORATORY
Cak Ridge, Tennessee 37830
operated by
UNION CARRIDE CORPORATION .
for the ASTE R
U.S, ATOMIC ENERGY COMMISSION
SYSTOTRUTION OF THIS DOCUNENT IS LH&H&E%
it Y Snatenpk
15 sna MEG Guniraciors |
iii
TABLE OF CONTENTS
ABSTRACT
I. INTRODUCTION
ITI. BUBBLE GENERATOR DESIGN
IIT. OPERATING CHARACTERISTICS AND TEST RESULTS
IIT.~-1. Bubble Size
IIT.-2. Gas Injection Pressure Characteristics
IV. CONCLUSIONS AND RECOMMENDATIONS
V. ACKNOWLEDGEMENT
NOMENCLATURE
REFERENCES
APPENDIX
14
27
28
29
30
31
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
lo'
11.
12.
LIST OF FIGURES
Bubble Generator Design Configurations which were
Given Reduced Scale Evaluation Tests
Bubble Generator Design for the Gas Systems
Technology Facility
Bubble Size Produced by 2.1 in., Threoat Diameter
Bubble Generator as a Function of Liquid Flow Rate
Bubble Size Produced by GSTF Bubble Generator
as a Function of Surface Tension
Surface Tension as a Function of Sodium Oleate
Concentration for Laboratory Batch Samples and
for Loop Samples
Bubble Size Correlation for GSTF Design Bubble
Generator
Simplified Flow Diagram of the Gas Systems Technology
Facility
Gas Injection Pressure and Overall Pressure Drop
of Prototype Bubble Generator as a Function
of Gas Flow Reate
Geometry of Bubble Generator Used in Analysis of
Gas Injection Pressure and Overall Head Loss
Pressure Drop of Gas Feed Passages as a Function
of Gas Flow Reate
Pressure Drop Across the Gas Plume Interface
as a Function of Throat Void Fraction
Correlation of Plume AH (H6) to Throat Liguid
Velocity and Throat Void Fraction
10
11
16
16
17
19
22
2k
25
DEVELOPMENT OF A VENTURI TYPE BUEBLE GENERATOR
FOR USE IN THE MCLTEN-SALT REACTOR XENCN REMOVAL SYSTEM
C. H. Gabbard
ABSTRACT
A venturi type bubble generator was developed for appli-
cation in the xenon removal system proposed for a molten-salt
_breeder reactor. Gas injected into the high velocity liquid
‘at the“venturi throst is formed into bubbles by the fluid
turbulence in the diffuser cone. Tests were conducted using
aqueous solutions tc determine the various pressure drops of
the bubble generator as a function of liquid and gas flow
rates and to determine the bubble diameter produced.
Empirical relaticnsktips were developed which could be used in
combination with the more conventional fluid flow equations
to predict the overell head loss and the gas injection pres-
sure of the bubble generator. A dimensionless correlation
for predicting the tubble diameter was developed for bubble
generators of similsr geometry.
Keywords: Bubble Generator, Bubbles, Bubble Size, Gas
Injection, Fused Salts, MSBE, MSBR, Performance, Xenon, Fluid
Plow. o —
I, INTRODUCTION
In a nuclear reactor operating in the thermal energy range, the con-
tinuous removal of the geseous fission product poison xenon-135 is neces-
sary to obtain a breeding ratio greater than 1.0. In a molten-salt
breeder reactor (MSBR), the xenon-135 circulates in solution with the
molten flucride fuel salt. A proposed method of removing this xenon is
to continuously inject helium bubbles into the salt stream to a gas
volume fraction of 0.2 to 1.0 percent at the reactor core midplane. The
xenon-135 would transfer by turbulent diffusion and would be stripped
from the salt when the bubbles were removed. Calculations indicate that,
even with this low gas volume fraction, adequate xenon-135 removal would
be cbtained by stripping the bubbles from a bypass stream which is about
10 percent of the main salt flow. Little advantage would be gained by
stripping larger flows. A more complete discussion of xenon removal from
a MSBR by this method is presented in Reference (1).
This report describes the design, development, and operating char~
acteristics of the bubble generator proposed for use in a 150 MW(t)
molten-salt breeder experiment (MSBE).(z) A full scale Plexiglas model
of this bubble generator was stfiaied in a test facility using water,
glycerin-water mixtures, and CaCl2 aqueous solutions. A prototype model
of Hastelloy "N" will be further evaluated with molten salt as part of
the test program of the Gas System Technology Facility (GSTF).(B)
ITI. BUBBLE GENERATOR DESIGN
The uwltimate goal of the development program was to obtain informa-
tion which could be used to design a full scale bubble generator which
could be tested in the GSTF using molten salt., Several design criteria
that have evclved during the development of the bubble generator are
listed in Table I. Devices requiring auxiliary power or having moving
parts were considered originally but were rejected as being unnecessarily
complex for a high-temperature molten-salt system. Fluid powered devices
basically resembling flow venturi appeared to satisfy the criteria and
three configurations were selected for continued development. The dif-
ferent configurations, shown in Figure 1, are variations in the method of
forming the high velocity throat region. Helium injected into the high
velocity salt stream at the throat forms small bubbles as a result cf the
fluid turbulence in the diffuser section.
Reduced scale tests were performed on these three configurations and
each performed satisfactorily. Initial testing of the "teardrop" design
indicated that the resulting bubble size was about one-fourth of the salt
flow gap over the range of liquid flows tested.(u) Consequently, a flow
passage of 0,080 in. would be required to produce 0.020 in. diameter
bubbles. The "multivane' design was an extension of this principle to
provide a more uniform bubble distributicon over larger pipe sizes and to
avoid the large diameter that would have been required in a full scale
teardrop design with a 0.080 in. annulus. Tests of a single vane prototype
revealed a flow oscillation around the trailing edge of the vane. In
addition, the gas distribution along the width of the vane and btetween
the flow passages on either side of the vane was difficult to control.
Reduced scale tests on the "venturi" design were performed using 3/k in.
and 1 1/2 in. pipe size commercial jet pumps that were modified to more
Table I
Bubble Generator Criteria
The bubble generator shculd be sized for application in the
MSBE.
Nominal salt flow rate = 500 gpm,
Gas flow rate = ¢ - 0,65 scfin helium.
The generated bubble diameter should be 0.020 in. or less.
The gas bubbles should be uniformly dispersed in the flowing
salt stream.
The bubble generator should be simple, reliable, and
maintenance-~-free.
The bubble generator should operate from pressure drop inherent
in the overall system design and should not require a gas com-
pressure for the injection cof gas.
ORNL-DWG 71-10220
GAS FEED-EH
VANES |
H
O
RECTANGULAR .
CROSS SECTION —g[
PN
FEEDE;}}
e
FLow) |
MULTIVANE VENTURI " TEAR DROP
FIGURE 1
BUBBLE GENERATOR DESIGN CONFIGURATIONS WHICH WERE
GIVEN REDUCED SCALE EVALUATION TESTS
closely provide a venturi geometry. These tests showed that well dis-
tributed bubbles of abcut the desired size could be produced.
Because of its simplicity and ability to meet the other requirements,
the venturi design was selected for full scale development. Figure 2 shows
the final design chosen for further testing with molten salts at high tem-
perature in the GSTF, This design is a modified venturi with the 2.10 in.
diameter throat stepped to 2.18 in. at the gas feed holes. The gas is
injected through 18 - 1/8 in. diameter radial holes into the high velocity
region at the venturi throat. An annular gas cavity forms between the wall
of the bubble generator and the flowing liquid in the 2.18 in. diameter
cylindrical mixing chamber. The length of this cavity depends on the gas
flow rate, and at full gas flow the cavity extends into the 15° diffuser
section. The actual bubble formation occurs in the fluid turbulence in
the entry of the diffuser cone.
ITI. OPERATING CHARACTERISTICS AND TEST RESULTS
A full scale model of the proposed bubble generator with a 2.1 in.
diameter throat and with 4 in. diameter inlet and outlet piping connec-
tions was fabricated of Plexiglas for complete testing and evaluation,
Tests on this bubble generator were conducted to determine the bubble
size produced, various pressure drops, and general operating characteris-
tics. The tests were run with demineralized water, L41.5 wt percent
glycerin in water, and 31 wt percent CaCl,. agueous solution. The glycerin-
2
water mixture and the Ca012 solutipn have the same kinematic viscosity as
fuel §alt and provided dynamic similarity. Tests were also conducted with
up to about 200 ppm n-butyl alcochol or sodium oleate added to demineralized
water, The n-butyl alcohol, a surfactant; stabilized small bubbles and
inhibited coalescence but had little effect on the density, viscosity,
or surface tension of the bulk fluid. The sodium cleate, also a surfactant,
decreased the surface tension by about a factor of two and inhibited bub-
ble coalescence, but did not alter the density or viscosity of the bulk
fluid.
ORNL-DWG 72-9609
_-GAS INJECTION LINE
" Y~in, OD x 0072-1in.
WALL TUBING
18 Y-in-diam
GAS INJECTION HOLES —._
5047 in.
diam
ANNULAR GAS D|STR|BUT|ON/‘/ : |
CHANNEL 3/4 in. x 5/8 in, — n
- 4'%g in—
I
-
— 9% in. —
FIGURE 2
BUBBLE GENERATOR DESIGN FOR THE
GAS SYSTEMS TECHNOLOGY FACILITY
I1IT.,-1. Bubble Size
I1T1.-1.1 Test Condition
In the proposed xenon removal system, helium bubbles are to be
injected and removed in a 10 percent bypass loop. The bubbles on the
average are expected to circulate several times around the primary cir-
cuit of the reactor before being processed in the side stream. During
this circulation, the bubbles will be affected by solution and dissclution
as they pass through different regions of pressure and temperature, and
by breakup and ccalescence as they pass through high and low shear regions
(e.g., the pump). Consequently, the circulating bubble size is likely to
be controlled by the system dynamics rather than by the bubble generator
itself. However the size generation characteristics of the bubble gen-
erator should be of general interest for other systems and for possible
unanticipated modes of operation, such as full flow gas injection and
removal, In addition, the size produced may serve as an "initializing"
condition for monitoring changes as the bubbles pass through the system.
Consequently, some analysis and some limited tests were made to obtain
an indication of the bubble size produced by the bubble generator as it
is affected by flow and fluid properties. Flow rate was varied from
200 gpm to 550 gpm and surface tension was varied from 72 dynes/cm to
30 dynes/em by adding different amounts of sodium oleate. An antifcaming
agent, G.E. Silicone Emulsion AF-T2, was also added at concentrations of
10 percent of the sodium oleate.
III1.-1.2 DBubble Size Measurements
The bubble size distributions produced by the bubble generator were
determined by taking still photographs at the discharge of the diffuser
cone. A conventional studio camera with a 12 in., focal length lens was
used to take the photographs on 4x5 Polaroid film. A strobe light with a
1/30,000 second duration was used to "stop" the bubble motion and to pro-
vide back lighting.
The photographs, which were about actual size, were enlarged to
obtain a total magnification of 8. Enlargements to greater magnification
resulted in a loss of resolution. The bubble size distributions for each
condition were determined by scaling bubble sizes directly from the
enlargements. The diameters were measured by comparison with a plastic
template having drilled holes ranging from 1/32 to 3/4 in. in increments
of 1/32 in. A volume averaged bubble diameter as defined below was cal-
culated for each distribution:
1/3
L(n, 4,
where: n. is the number of bubbles of a given diameter,
di,per unit area of the photograph.
The resolution of the photographs was adequate to measure bubble diameters
in the 0.008 in. range (1/16 in. on the enlargement), but no bubbles
could be identified in the 0.004 in. diameter range. The results of these
tests are shown on Figures 3 and 4.
Figure 3 shows the volume average bubble diameter produced by two
bubble generator designs plotted as a function of liquid flow rate at
several values of surface tension. The data are compared with a slcpe of
-0.8 power dependence discussed in greater detail later in this report.
There was a high degree of scatter in some of the sets of data at constant
surface tension. Consequently, only selected data sets having low scatter
are shown on fihe plot. Although there were differences in the slope of
the various lines, the data tend to support a -0.8 power dependence.
Similar data taken previously alsc support a -0.8 power, and none of the
data have suggested a slope significantly different from -0.8.
Figure L is a plot of the bubble diameter as a function of surface
tension at three flow rates. The measured surface tension data from
locp samples taken during the course of this experiment were scattered
and did not agree with the data from previous laboratory scale samples
which were in general agreement with the sodium cleate supplier's litera-
ture. The values of surface tension used in Figure L were obtained from
the calculated concentrations in the test loop and the surface tension
vs concentration data from the laboratory semples as shown on Figure 5.
The measured surface tension data from the loop samples are also shown
on Figure 5. The discrepancy between these is not fully understocd.
However, the actual circulating concentration of sodium cleate could change
BUBBLE DIAMETER , VOLUME AVERAGED (in.)
ORNL-DWG 73—1541
GSTF STEPPED BORE DESIGN
DEMINERALIZED WATER
10 ppm SODIUM OLEATE
O
®
A 20 ppm SODIUM OLEATE
A 75 ppm SODIUM OLEATE
SMOOTH BORE DESIGN
V DEMINERALIZED WATER
v
0.050 |
o
v
> @1+— 0
N
v
0.020 \g
. TN
v
0.010
>
FLOW RATE (gpm)
FIGURE 3
BUBBLE SIZE PRODUCED BY 2.1 IN. THROAT DIAMETER BUBBLE GENERATOR
AS A FUNCTION OF LIQUID FLOW RATE
BUBBLE DIAMETER, VOLUME AVERAGED (in.)
0.060
0.050
0.040
0.030
0.020
0.010
0.009
ORNL—DWG 731542
A LIQUID FLOWRATE = 200 gpm
@ LiQUID FLOWRATE = 350 gpm
O LIQUID FLOWRATE = 500 gpm
L’
/ °
/ : |
@
(d,) =c (0/p)0-B A @
O
ei O
A A ®
O
)
A
A O
®
O
O
10 20 30 40 50 60 70 80 90
SURFACE TENSION {dynes/cm)
FIGURE 4
BUBBLE SIZE PRODUCED BY GSTF BUBBLE GENERATOR
AS A FUNCTION OF SURFACE TENSION
100
0T
SURFACE TENSION (dynes/cm)
80
70
60
50
40
30
20
ORNL—-DWG 73-1539
® LABORATORY SAMPLES
O O LOOPSAMPLES
o .
\
.\ 5
o O
° L
© q
-\.
\\
e ————
\’
20 | 40 60 80 100 120 140 160 180 200
CONCENTRATION OF SODIUM OLEATE (ppm)
FIGURE 5
SURFACE TENSION AS A FUNCTION OF SODIUM OLEATE CONCENTRATION
FOR LABORATORY BATCH SAMPLES AND FOR LOOP SAMPLES
1T
12
during a given test run because the sodium oleate, being a surfactant,
would be stripped from the circulating loop along with the bubbles. The
concentration in the loop samples could then be less than the calculated
average concentraticn for the entire loop depending on the time the samples
were taken. The bubble photographs were taken immediately after gas flow
was started following -an hour's circulastion without gas flow. This proce-
dure should have provided a concentration of sodium cleate essentially
equal to the.calculated average 8t the time the photographs were taken.
The bubble diameter data of Figure 4 are too scattered to accurately
determine the actual power dependence. However, the data tend to support
a value cf 0.6 as predicted by the theoretical considerations discussed
below and as illustrated on Figure L.
IIT.-1.3 Analysis of Bubble Size Data
The bubbles produced by the bubble generator are apparently formed
in the entrance region of the conical diffuser as a result of fluid tur-
bulence. The following equation has been proposed to predict the size of
gas bubbles produced by fluid turbulence.
- 2
e, 3/5 /5
1{ o €g
(5) to calculate droplet diameters produced
Equation (1) was used by Hinze
by emulsification of one ligquid in another in an isotropic-turbulent
flow field. Assuming turbulent flow in a conduit with conditions such
that the friction factor would be constant, the power dissipation per
(6)
TR
e =k Re (2)
2 2 L *
p D2 Ea
unit volume (e) can be expressed as:
Substituting this relationship for the power dissipation, Equation (1)
gives:
gp D, g "1V, D5 o
2
—2 ¢ 2 (3)
2
2 u H
13
The bubble size data presented in Figures 3 and 4 generally confirm a
3/5 power dependence for the surface tension term, but indicate an expon-
ent of -0.8 for the Reynolds Number term rather than -1.2 as indicated by
Equation (3). This would tend to confirm the form of Equation (1), but
suggests a relation different from Equation (2) for the power dissipation
rate in the bubble generation region of our device. Equation (3) might
apply when power is added to the fluid continuously as in an agitated tank
or in pipeline flow where the friction losses represent a continuous energy
dissipation within the fluid. In the present bubble generator, the fluid
may receive an "energy impulse' as some of the kinetic energy of the high
velocity fluid in the thrcat is converted to fluid turbulence in the dif-
fuser, and the above equations may not apply specifically for this mechan-
ism.,
An alternate expression for the power dissipation rate based on the
wall shear stress has been proposed by Kress* for the GSTF bubble genera-
tor design. Using his prcpesed relation for power dissipation, Equaticn
(1) gives the following relationship predicting a 3/5 power dependence
cn surface tension and a -4/5 power dependence on the Reynoclds number
as observed.
3/5 -L/5
gp D 1/3 68/3 g V, D. p
g_}z C 2 C t 72 (L)
D, L/3 2/3
Ue Ug Ue
At the present time, there are insufficient data to verify Equation (L)
because only the liquid velocity and liquid surface tension have been
varied. Therefore, we have elected to empirically correlate the data
using the dimensionless groups that appear in Equation (3). These same
dimensionless groups have been obtained independently by dimensionsal
analysis.
The recommended form of the equation for the
GSTY bubble generator is then:
-
D, 8J3/5 v ol
_ 272
<%}-—KD2 5 J " (5)
=2 H
where K = L.54 x 10 ~.
*Personal communication, T. Kress to C. H. Gabbard, Dec. 4, 1972.
1k
The comparison of this correlation with the data is shown in Figure 6.
Several data points which were not used in determining the wvalue of K are
- indicated on the plot. These were the points on Figure L4 that did not
fall on the lines representing the 3/5 power of surface tension. Based
on this correlation, the bubble diameter produced by the GSTF bubble
generator operating with fuel salt flowing at 500 gpm should be about
0.01, The value of "K" given above is believed applicable only to
bubble generators that are geometrically similar to the GSTF design. This
is shown by the data on Figure 3 for the smooth bore design which had the
same throat diameter, but had a 7° diffuser cone instead of the 15° cone
in the GSTF design. A larger value of "K" would be required for the smooth
bore design.
I1TI.-2. Gas Injection Pressure Characteristics
To appreciate the importance of the gas injection pressure, anlunder—
standing is needed of the relationship of the bubble generator to other
portions of a reactor system. Figure 7 is a simplified flow diagram of
the GSTF which is representative of a reactor system in regard to the oper-
ation of the bubble generator. The gas injected into the fecllowing salt at
the bubble generator is removed by the bubble separator and is recycled
back to the bubble generator via the bulk salt separator, the drain tank,
and the gas holdup tank. The gas holdup tank including the throttle valves
on either end simulates the delay time and flow restriction of a L8-hr
charcecal trap which in a reactor system, would allow radioactive decay of
the Xe-135 concentration to an acceptable level prior to reinjection of
the helium sweep gas back into the salt system. If the pressure required
to inject the gas into the bubble generator were sufficlently below the
pump tank (or drain tank) pressure to provide the pressure drops for the
48-hr charcoal bed and for the gas flow control valve, a compressor for
highly radiocactive gas would not be required. This concept has been shown
to be feasible and the necessary design features have been incorporated
into the final GSTF bubble generator and system designs for continued
evaluation with hot fuel salt.
The measured pressure differences vs gas flow rate of the final GSTF
prototype bubble generator are shown in Figure 8 for a liquid flow rate of
500 gpm. These pressure differences are expressed as zero-void liquid head
GSTF BUBBLE GENERATOR «d, (in.)
0.050
0.040
_Cl
<
)
o
0.020
0.010
ORNL—-DWG 73—1546