WATER POLLUTION CONTROL RESEARCH SERIES • 16080 DWP 11/70
Induced Air Mixing of
Large Bodies of Polluted Water
ENVIRONMENTAL PROTECTION AGENCY • WATER QUALITY OFFICE
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l6o80DRX10/69 Stratified Reservoir Currents; by Oregon State Univ.,
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Induced Air Mixing of Large Bodies
of Polluted Water
by
Stefan A. Zieminskl
and
Raymond C. Whittemore
Chemical Engineering Department
/*
University of Maine
Orono, Maine OM73
for the
ENVIRONMENTAL PROTECTION AGENCY
WATER QUALITY OFFICE
Program #16080 DWP
November 1970
For sate by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price 60 cents
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WQO Review Notice
This report has been reviewed by the Water Quality Office
and approved for publication. Approval does not signify
that the contents necessarily reflect the views and
policies of the Water Quality Office, nor does mention
of trade nanes or comnercial products constitute endorsement
or recomnendation for use.
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Abstract
The work discussed in this report constitutes the Phase I of the
investigation of induced air mixing of large bodies of water. The
objective of this work was to conduct a pilot scale study to estimate
the effects of variables such as the air flow rate, geometry of the
body of water, energy input, size of air bubbles, and the pumping
capacity of the air plume on the time of mixing. The latter was de-
fined as the time required to reach 90% of the equilibrium concentra-
tion of the KC1 tracer. In the study emphasis was put on the
direction and relative magnitudes of the variables in order to obtain
guidelines for large-scale investigation. Considerable time was spent
on the development of the various experimental techniques. The tests
were conducted in a plexiglas tank of 180 gallons capacity. It is
stressed that the induced air system was investigated only from the
viewpoint of its mixing performance. Its effect on aquatic life was
not considered in this work.
This report was submitted in fulfillment of project #16080 DWP
under the sponsorship of the Federal Water Pollution Control Adminis-
tration.
Key Words: Air Plume, Diffusion, Dispersion, Bubble Size, Energy
Input, Mixing, Mixing Time, Tracer.
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Contents
Page
Section
vi
Conclusions
viii
Recommendations
1
I Introduction
II Equipment and Operational Procedures
23
III Discussion of Results
45
IV References
ii
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List of Figures
Page
3
Figure 1 Test Equipment
Figure 2 Photograph of Test Equipment 5
Figure 3 Photograph of Restricted System 6
Figure 4 Photograph of Air Bubbles 8
Figure 5 Photograph of Air Plume 8
Figure 6 Photograph of the Rising Disturbance of the Plume . . 10
Figure 7 Photograph of Anemometer in Calibrator 12
13
Figure 8 Pumping Action of Plume
Figure 9 Photograph of Water Velocity Determination by
Stroboscopic photography
Figure 10 Photograph of Liquid Tracer Introduction 15
Figure 11 Conductivity Cell Locations l6
Figure 12 Conductivity vs. Time Chart (Solution Tracer
Introduction)
Figure 13 Photograph of Solid Tracer Introduction 18
Figure 14 Conductivity vs. Time (Solid Tracer Introduction. . . 20
Figure 15 Conductivity vs. Time (Continuous Tracer
Introduction)
Figure 16 Mixing Time vs. Air Rate (Solution Tracer) 24
Figure 17 Mixing Time vs. Air Flow Rate (Solid Tracer) 25
Figure 18 Photograph of Circulation Pattern (Solution Tracer,
10 seconds)
Figure 19 Photograph of Circulation Pattern (Solution Tracer,
30 seconds)
Figure 20 Photograph of Circulation Pattern (Solution Tracer, ^
1 minute)
iii
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List of Figures (continued)
Page
Figure 21 Photograph of Circulation Pattern (Solution Tracer,
2 minutes) 29
Figure 22 Photograph of Circulation Pattern (Solution Tracer,
3 minutes) 30
Figure 23 Stack Flow vs. Air Flow Rate 31
Figure 24 Photograph of Circulation Pattern (Solid Tracer,
10 seconds) 32
Figure 25 Photograph of Circulation Pattern (Solid Tracer,
30 seconds) • 32
Figure 26 Photograph of Circulation Pattern.(Solid Tracer,
1 minute) 33
Figure 27 Photograph of Circulation Pattern (Solid Tracer,
2 minutes) 33
Figure 28 Photograph of Circulation Pattern (Solid Tracer,
3 minutes) 34
Figure 29 Mixing Time vs. Power/Unit Volume (Liquid Tracer) . . 37
Figure 30 Flow in Stack vs. Bubble Size 40
Figure 31 Mixing Time vs. Bubble Size 41
Figure 32 eg)1/2 vs. (M-0'™ «*
iv
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Tables
Page
I Water Flow in Restricted System. 26
II Percentage of Water Pumped During Mixing Operation 27
III Efficiency of the Restricted System 35
IV Diffused Air Mixing of Large Bodies of Water 38
V Effect of Disperser Submergence on Mixing Time
39
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Conclusions
The conclusions given in this section refer only to the equipment and
operating conditions used in this work.
(1) The mixing time, as experimentally determined, depends to some
extent on the tracer technique used. Great care must be taken to ensure
uniformity of the introduction of the tracer. Because of the randomness
of the bubble motion, the circulation pattern undergoes random changes
except for the well defined horizontal and surface jets. This random-
ness is responsible for the comparatively poor reproducibility of mixing
time determinations.
(2) With air flow rates between 300 and 2000 cc/min, and volumes of
water ranging from 100 to 180 gallons, the mixing time decreased slowly
with increased air flow rate. However, below 300 cc/min a sharp increase
in mixing time could be observed. It appears that decreasing the rate
of flow a critical value is reached beyond which the currents generated
are not sufficiently strong to establish a circulation of an appreciable
magnitude in the whole body of water.
(3) The spread of experimental data increased with decrease of the
air flow rate. The probable error of the mixing time determination in-
creased from 8.6% at 2000 cc/min to 18% at 150 cc/min. Preliminary
anemometric measurements of surface velocities in the vicinity of the
plume indicated values ranging from 1 ft/sec at 2000 cc of air/min to
0.3 ft/sec at 150 cc/min. The velocity fluctuated between 25 to 50%
with a tendency to increase at lower air flow rates.
(4) A good correlation was obtained between the mixing time and
power input per unit volume of water. This correlation is of interest
in scaling up model studies to large scale installations.
(5) Except for very low air rates (<150 cc/min) there was no appar-
ent effect of water height on the mixing time. However, at the low air
flow rate the mixing time was shorter at lower water heights. For two
water depths the introduction of air at the side of the tank resulted
in longer mixing times as compared with the central location of the dis-
perser. The tests indicate that the central location of the plume gives
a better mixing performance.
(6) Limited data was taken for two different depths of submergence
of the disperser at the same air flow rate and water depth. The results
showed that the smaller the energy input (smaller submergence) the
longer was the mixing. It is emphasized that the greater is the distance
between the disperser and the bottom of the tank the thicker is the com-
paratively stagnant bottom layer which hinders efficient mixing.
vi
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(7) Tests conducted with bubbles of different sizes showed that for
bubble radii above the range of 0.06 to 0.12 cm the mixing time did not
depend much on bubble sizes. However, below this range the velocity of
rise of the bubbles, and therefore the density of the plume, dropped
rapidly resulting in a shorter mixing time.
(8) The mixing time, the depth of water, and the principal dimen-
sion of the tank were successfully correlated by dimensional analysis
with the densimetric Froude number. It is expected that still better
correlation could be obtained with the use of a larger tank where the wall
effect would be of smaller magnitude.
vii
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Recommendations
It is strongly recommended that the present research be continued
in a tank of an approximate capacity of 5,000 gallons and at air flow
rates of about 5 to 50 liters/min. The larger dimensions of the tank
would allow a more accurate correlation of the variables. The experi-
mental techniques as developed in the present work could be success-
fully applied without major modifications. The use of a larger tank
would also allow a study of multiple air introductions which was im-
possible in the present work because of the small dimensions of the
tank. The recommended investigation would thus constitute an inter-
mediate step between the pilot scale studies as discussed in this
report and the actual large scale investigation defined as Phase II
in the original research proposal (WP-01376-01, Nov. 28, 1967, page 8).
viii
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SECTION I
Introduction
The storage of water in large lakes or reservoirs gives rise to pro-
blems of physical, chemical, and biological nature. Organic matter of
industrial and natural origins tends to settle on the bottom where it
decays, depleting the water of dissolved oxygen necessary to insure nor-
mal aquatic life. Although the upper surface of water which is in
direct contact with the atmosphere may be saturated with oxygen, the
mixing resulting from the density differences or the action of wind is
usually too small to replenish the oxygen deficiency in the lower layers.
In order to alleviate these adverse conditions and improve the qual-
ity of water, attempts were made to destratify large bodies of water by
artificial means of which the use of induced air seems to be one of the
more promising.
A considerable amount of work has been conducted in the general
field of circulation and destratification of large bodies of water by
means of induced air (21). Little, however, has been published of a
nature basic enough so that the results obtained in one case could be
used to predict the results in another.
The objective of this work was to conduct a pilot scale study to
estimate the effects of variables such as the air flow rate, geometry of
the body of water, energy input, size of air bubbles, and the pumping
capacity of the air plume on the time of mixing. The latter was defined
as the time required to reach 90% of the equilibrium concentration of
the KC1 tracer. In studying the effects of the various variables, empha-
sis was put on their direction and relative magnitude in order to obtain
better guidelines for large-scale investigation. It is expected that
the rather successful correlation derived by dimensional analysis could
be used as a starting point in correlating the results of large-scale
tests.
The tests were conducted in a comparatively small tank (180 gallons)
and therefore the obtained results should not be directly extrapolated
to large bodies of water.
One of the main difficulties in conducting this kind of research is
the pronounced randomness of the system caused by the random motion of
bubbles. A great number of tests had to be conducted in order to obtain
reliable averages.
Another difficulty was the unavailability of some of the measuring
techniques. E.g. the determination of the velocity of rise of the air
plume, required for the estimation of the density of the plume and
therefore for estimation of the densimetric Froude number. Some of the
techniques which have proved successful in one set of experimental
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conditions gave erratic results when the conditions were changed.
A considerable part of this report will therefore be devoted to the
description of the various experimental techniques and measurements
even though some of them were later discarded as less reliable under
conditions used in this work. They may, however, prove valuable under
a different set of experimental conditions and therefore of interest to
a future investigator.
It must be stressed that the induced air system was investigated
only from the viewpoint of its mixing performance. Its effect on aquat-
ic life was not considered in this work.
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ROTAMETERS
ION EXCHANGER
WATER IN
MANOMETER
FLUORESCENT LIGHT
—*-
tV«»•^""*'!"!!" ".- !
BDLEX CAMERA
CONDUCTIVITY
CELL
SOLU-
METER
ANEMOME
MODULE
TANK; isoo** H2o
DISPERSER
RECORDER
QUICK OPENING VALUE
AIR CYLINDER
FIGURE
TEST EQUIPMENT
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SECTION II
Equipment and Operational Procedures
General
The tests were conducted in a plexiglas tank (Figures 1 & 2)
having a maximum capacity of 1500 Ib. of water (180 gallons). The tank
was rectangular in shape and was 48" long, 32" wide, and 36 high.
was mounted on a steel support with facilities for mounting auxi
equipment and instruments. The plexiglas construction enabled phot
graphic and motion picture investigation of the interior of the
In addition to the drain, the bottom of the tank had two openxngs
insertion of the air disperser. One of them was located in the cente:
and the other 12" from the side and 15" from the front wall.
The water used in the tests was first passed through a porous
filter to remove suspended matter and then purified in an ion exchange
demineralizer. The specific resistance of this water was about
cm. The high resistivity water was necessary in the determination of
the mixing time which involved the measurement of the changer: of conduc-
tivity after introduction of the KC1 tracer. The air to the disperser
Photograph, of Test Equipment
Figure 2
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was supplied from a breathing air cylinder and metered by two rota-
meters. The air disperser consisted of an Aloxite plate, one inch in
diameter, cemented in a brass holder. The height of the holder could
be adjusted to vary the depth of submergence of the disperser.
The Restricted System (Figure 3)
The air plume issuing from the disperser undergoes a random
swaying motion during its rise to the surface. This lack of stability
affects the dimensions of the plume, the water flow pattern, and makes
difficult the anemometric measurement of the water flow produced by
the plume.
Photograph of Restricted System
Figure 3
In order to decrease this difficulty, and to improve the repro-
ducibility of the water flow measurements, some tests were conducted in
a system in which a plexiglas cylinder was mounted around the plume. In
a later part of the report this cylinder is referred to as stack. The
cylinder was 4 inches in diameter, 18 inches long, and its lower end
was at a distance of 4 inches from the bottom of the tank. The upper
end was 2 inches below the level of water. Although the presence of
this cylinder increased somewhat the flow resistance of the system,
enabled at the same time to obtain valuable data on the pumping perfor-
mance of the plume. It is of interest to note that under identical
operating conditions the mixing timesobtained with and without the
cylinder were practically the same. The above indicates that the
6
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presence of the cylinder did not exert any major effect on the
operating efficiency of the system despite the fact that the flow pat-
tern in the vicinity of the plume was changed. In the freely rising
plume the surrounding water entered the plume through its sides up to
the level just below the surfact jet. In the restricted system this
side inflow was eliminated by the presence of the cylinder and the
water was forced to flow at a uniform rate from the bottom to the top
of the cylinder.
Determination of the Bubble Size
Since the pumping action of the plume depends on its buoyancy and
therefore on the density of the air-water dispersion, it was decided to
investigate the effect of the size .of the bubbles. It was expected that
the decrease in the size of the bubbles and the resulting drop in the
velocity of rise would increase the air-holdup, decrease the density of
the plume and thus improve the pumping effect. It appears from our
literature survey that very little information is available in this
field.
The bubble size was determined in this work by photographing the
bubbles rising in the air plume. A Bolex 16 mm movie camera with a
telephoto lens and Kodak 4X negative film were used in this determina-
tion. The lighting was accomplished by a 1531-A Strobotac and 1539-A
Stroboslave (G.R.C.). The lamp of the Stroboslave was placed in the
back of the tank and the flash was directed through the tank into the
lens of the camera. A white paper backing was used at the back of the
tank to reduce intensity of the flash. A metal rod of known diameter
was used as a standard of reference. It was mounted at an angle to the
camera so that a part of it was always in focus. Pictures obtained by
this back lighting technique were of excellent contrast (See Figure 4).
After developing, the film was projected and the major and minor axes of
at least 40 bubbles were measured and the equivalent spherical radius
was calculated for each bubble. The average equivalent spherical radius
was then used in evaluation of results. The accuracy of this method was
checked by means of glass spheres of known diameters and was found to be
+ 1.5%.
Since the photographs taken at the bottom of the plume and at its
top showed practically the same bubble sizes and distribution, the
bubbles were photographed only at the top of the plume.
The uniformity of bubble sizes was very satisfactory at air flows
of 500 cc/min S.T.P. and higher. At lower flows, however, the unifor-
mity of sizes decreased resulting in a greater error of measurement. To
obtain a conservative estimate of the reproducibility of the measurement,
the probable error was estimated at the low flow rate of 300 cc/min
S.T.P. and was found to be 6.2%.
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Photograph" of Air Bubbles
Figure 4
Photograph of Air Plume
Figure 5
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Determination of the Shape of the Air Plume
The shape of the air plume is necessary for the determination of |he
plume density required in the densimetric Froude number. The measure-
ments of the plume were taken by photographing the plume and a refer-
ence scale by means of a Polaroid MP-3 camera. The black and white
photographs were projected and the diameters were measured at various
heights of the plume (See Figure 5). The impression that the bubbles
did not reach the surface of the water was caused by the tilt of the
camera when taking this particular photograph. At first the pictures
were taken from both sides of the tank to estimate the possible effect
of the different plume to wall distances. Since no difference was ob-
served, only one side of the plume was photographed and in the calcu-
lations the plume was assumed to be a perfect cone.
For rates of air flow below 2000 cc/min S.T.P. the plume was very
stable. However, at higher flow rates the plume became unstable due to
horizontal swaying. On two occasions the bubbles in the plume formed a
vortex. The volume of the plume was calculated by the equation:
V=(ff)(rl2 + rir2 +r22)
where V = volume, cc.
H = depth of immersion of disperser, cm.
ri = radius at plume top, cm.
ro = radius at plume bottom, cm.
From the known volume of the plume and the instantaneous volume of air
(air holdup), the plume density can be easily calculated from the rela-
tionship:
pw = v - v Pw + vpa
V
where: v = instantaneous volume of air, cc.
pw = density of water, g/cc.
pa = " of air, g/cc.
The Velocity of Rise of the Plume
The velocity of rise of the plume is an important quantity for the
determination of the instantaneous volume of air (air holdup) necessary
for estimation of the density of the plume. The principle of this
measurement is based on creating a disturbance in the plume and follow-
ing this disturbance by means of motion picture photography.
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The disturbance was produced by an abrupt discontinuation of the
air flow by means of a quick release valve which vented the air into the
atmosphere (See Figure 6).
The vertical displacement was determined with the help of a refer-
ence scale mounted parallel to the plume. The corresponding time was
obtained from the number of frames and the known framing rate of the
Bolex movie camera.
Photograph of the Rising Disturbance of the Plume
Figure 6
For each run 5 or 6. determinations were made and the results aver-
aged. The framing rate of the camera was checked by photographing the
dial of a milli-second timer and counting the number of frames for a set
time. This method of velocity determination was subject to two errors.
At low air velocities (below 600 cc/min) the line of disturbance started
to become irregular because of increasing irregularity in the size of
the bubbles and, therefore in their velocity of rise. This problem did
not occur at higher air flow rates. The other difficulty was caused by
the effect of the disturbance on the velocity of circulation in the
tank. In order to decrease this effect, the velocity of rise was
measured up to a distance of 8 to 10 inches above the disperser. In
this way, the air flow was interrupted for only about one second.
check this technique a hot film anemometer was mounted within the plume
and the water velocity was measured after the interruption of the air
flow. The measurement showed that the water velocity remained constant
for approximately 1 second within the region used in our measurements.
10
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The instantaneous air volume "v" (air holdup) in the tank was cal-
culated from the equation:
v = (H/vr)Q
where: H = depth of immersion of disperser
Q = air flow rate
v = velocity of rise of plume
Measurement of the Water Flow in the Tank
The rate of water flow produced by the air plume was measured by
means of hot film anemometry. A cylindrical hot film sensor (TSI Model
1210 60W) and the anemometer module (TSI Model 1053-B) were used in
this work. The constant temperature anemometer measured the fluid
velocity by sensing the rate of heat transfer from the heated sensor to
the surrounding water which was kept at constant temperature. As the
water velocity increased, the sensor tended to cool, changing the resis-
tance. The amplifier then adjusted the current to the sensor to main-
tain constant temperature. These changes in the output of the amplifier
were then related to changes in water velocity by means of calibration.
The use of anemometer in this work was not without errors. Lint or
other suspended impurities could have collected on the sensor, producing
change of heat transfer unrelated to the change in water velocity. For-
mation of minute air bubbles on the wire of the anemometer was another
potential source of error. In order to detect bubble formation, the
sensor was illuminated by a beam of light and observed with a magnifying
glass. Usually a light knock on the sensor holder would displace the
bubble. Decrease of the sensor temperature by lowering the current was
of help in decreasing bubble formation but at the same time adversely
affected the sensitivity of the instrument.
The sensors were calibrated in a TSI Model 1125 Calibrator. A
cross-section through the calibrator nozzle and the sensor is shown in
Figure 7. The sensor wire (invisible on the photograph) is in the
center of the nozzle supported by the V-shaped arms of the probe. In
order to avoid breaking of the delicate and expensive sensor, a spe-
cial protecting device was constructed (see upper part of the Figure)
which prevented damage to the sensor while inserting and removing it
from the calibrator. The flow of water through the instrument was
measured by a calibrated rotameter. Addition of dye showed a flat
velocity profile at the plane of the sensor. To insure against changes
in calibration each probe was calibrated before and after each test.
Initially it was proposed to determine the pumping effect of the
air plume from the velocity profile of the surface jet (Figure 8). Un-
fortunately, the randomness and turbulence of flow in the surface jet
11
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Photograph of Anemometer in Calibrator
Figure 7
gave erratic results. Consequently, no attempt was made to measure the
velocity profiles in this region. The measurement of the water velocity
was therefore limited to the restricted system where the velocity was
measured in the cylinder at a level below the disperser.
A photographic technique for measuring the flow of water was also
briefly investigated. Selected polystyrene beads (Comak Chemicals Ltd.,
London, K.E. England) of density close to that of water were allowed to
circulate in the tank. The beads were photographed by means of the
Polaroid MP-3 camera and stroboscopic illumination. Every time the
stroboscope flashed an image of the bead appeared on the film (Figure 9).
From the distance between the images and the known rate of flashing, the
velocity component in the vertical direction was calculated. Usually a
few pictures were taken and the average velocity was calculated. The
difference between the anemometer and photographic measurements was in
the range of 10 to 15%.
The anemometric technique was selected in this work because of its
simplicity and the fact that it can be used for measurement of point
velocities.
Determination of the Mixing Time
The mixing time is one of the most important quantities studied in
12
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i,,
SURFACE JET
WATER
WATER INFLOW
VERTICAL JET
WATER INFLOW
WATER
AIR SOURCE
FIGURE Q
PUMPING ACTION OF PLUME
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Photograph of Water Velocity Determination by Stroboscopic Photography
Figure 9
this work. Considerable time was spent on developing of various tech-
niques and on estimation of their relative merits. The principal
difficulty of this measurement lies in the randomness and poor repro-
ducibility of the system itself. The randomness of bubble motion is
mostly responsible for it. It is therefore practically impossible to
find out to what extent this poor reproducibility is due to the method
of determination.
Gay and Hagedorn (5) in their study of mixing of stratified bodies
of water measured changes in salinity. Similarly Harrell (7) used
changes in conductivity in'his flow of fluid system. Zlokarnik (24)
estimated the mixing time by adding acid to a solution of a base in the
presence of phenolphthalein and following the change of color. Nicker-
son (14) introduced a fluorescent dye, Rhodamine, into a water reservoir
and monitored the changes with a fluorometer. In other investigations
(16) changes in temperature and oxygen concentration profiles were used
to advantage.
14
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In the methods applied, in this work the mixing time was estimated
by introducing a KC1 tracer and following the changes in conductivity.
A Beckman Solu-Meter (Model RA-5) with temperature compensation and
Leeds & Northrup Speedomax recorder were used in the investigation. The
cells were Epoxy Dip Type fabricated from 5/16" diameter epoxy rod 2 ft.
long and having unshielded platinized nickel electrodes of 0.2/cm cell
constant.
Three different tracer techniques were investigated. In the first
method 200 cc of KC1 solution was introduced at the surface through a
1/8 inch opening. The concentration of the solution was so selected as
to give a conductivity change of 20 micromhos. At this conductivity
change one full tank of water could have been used for 8 tests.
Photograph of Liquid Tracer Introduction
Figure 10
Figure 10 shows a black dye being introduced by this technique. The
tube was located 2 inches from the right side wall. The conductivity
cells were placed on the opposite side of the tank in several locations
(See Figure 11).
Before each test the air flow was adjusted to the required value
and allowed to flow through the tank for at least 30 minutes before
introduction of the tracer. This was necessary to establish a uniform
flow pattern and avoid temperature gradients in the tank which would
affect the conductivity measurements. At the end of the test when the
conductivity approached equilibrium value, the contents of the tank
15
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INJECTION
TUBE
CELLS
w
H
h
2 From Wall
i.
FRONT VIEW
TOP VIEW
INJECTION \
TUBE 2"FROM WALL
FIGURE ]|
CONDUCTIVITY CELL LOCATIONS
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60
8 50
c
o
q 40
i
2
O
O
2
I
O
30
20
10
90% —J
EQUILIBRIUM
I
I
EQUILIBRIUM
VALUE
\
012345678
TIME - MINUTES
FIGURE 12
CONDUCTIVITY VS TIME CHART ( SOLUTION TRACER INTRODUCTION )
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were vigorously mixed with a paddle and the terminal (equilibrium) con-
ductivity determined. In the analysis of the charts, the time corres-
ponding to 90% of the terminal conductivity was arbitrarily selected as
the mixing time. A representative sample of a recorder chart is shown
in Figure 12 where the horizontal axis was reduced by a factor of two
and the vertical axis was expanded by the same factor.
For each set of experimental conditions a total of eight tests
were conducted (2 tests for each cell location) and the results aver-
aged. A series of photographs showing the progress of tracer distri-
bution will be shown in the section dealing with the interpretation of
results.
A number of tests conducted under analogous experimental conditions
showed a probable error of a single determination to be 8.6% and 18% at
the air flow of 2000 and 150 cc of air/min respectively. As was men-
tioned before, this value reflects also on the effect of the randomness
of the system.
In the second tracer technique, ground KC1 crystals were spread
over the top of the air plume. The crystals were ground to a fine
powder to ensure their dissolution before reaching the bottom of the
tank. They were spread uniformly by means of a perforated dish over a
circular surface, 5 inches in diameter. A metal ring of this diameter
was placed above the top of the plume to insure a uniform and more re-
producible addition of the tracer. Figure 13 shows a dye being intro-
duced by this technique. The conductivity cells were placed in
Photograph of Solid Tracer Introduction
Figure 13
18
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geometric centers of each half of the tank. The conductivity versus
time data were obtained by switching from one cell to the other every
10 seconds. In the analysis of the chart the two conductivity readings
were averaged and plotted against time (See Sigure 14). The mixing
time was defined as that point at which the average conductivity was
_+ 5% of the terminal (equilibrium) value. The average value of the
2 cells was used because there was always some tendency of the crystals
to be carried more to one side than to the other. The probable error
of this measurement was found to be 9.2%. As in the first method the
effect of the randomness of the system is included in this figure.
The third method investigated in this work consisted of adding a
dilute KC1 solution at a constant rate over a period of 30 minutes. It
was expected that in this way the effect of the randomness of the flow
pattern on the mixing time would be decreased. Again as in previous
methods enough salt was added to produce in the mass of water a change
in conductivity of 20 micromhos. If the mixing time were zero (instan-
taneous mixing), the conductivity vs. time curve (line A Figure 15)
would form a straight line starting from the origin. However, since
the mixing is never instantaneous, the actual curve is displaced by the
amount corresponding to the time required for mixing. This method has
an unquestionable advantage over the previous techniques in the fact
that the time of tracer introduction was much longer and therefore the
effect of the randomness of the system considerably decreased. This
method was not used in our work because of the lengthy analysis of the
charts.
On the whole it may be said that the estimation of mixing times
from conductivity measurements was experimentally simple. It had,
however, the disadvantage of the use of large quantities of low conduc-
tivity water and therefore the necessity of frequent (each 6 to 8 tests)
exchange of the contents of the tank.
19
-------
20
o
o
z 18
o
c
o
'6
o
X
o
w
14
12
10
I I I
o o
00°
\
95% VALUE
o o
o
o o
o o
> o
EQUILIBRIUM
VALUE
36 9 12 15 18 21 24
TIME - MINUTES
FIGURE 14
CONDUCTIVITY VS TIME ( SOLID TRACER INTRODUCTION )
-------
20
o
O 18
o
c
o
H l6
i
2
O
:o
o
I
O
14
12
10
INSTANTANEOUS MIXING
LINE A
ACTUAL MIXING
O O
o ?
AVERAGE LAG = 3.1 MINUTES
I
8
24 28 32 36
12 16 20
TIME - MINUTES
FIGURE 15
CONDUCTIVITY VS TIME (CONTINUOUS TRACER INTRODUCTION)
-------
Section III
Discussion of Results
The Effect of Air Flow Rate on the Mixing Time.
The results of this group of tests are summarized in Figures 16 and
17. In Figure 16 the height of the water in the tank was used as a
parameter while Figure 17 also shows the effect of the location of the
air disperser. It must be noted that in Figure 16 the branch of the
curve between air flows of 300 to 2000 cc/min is an average of the three
water heights (15, 24, 30 inches) investigated in this work. This was
done because in this region the water heights studied did not show a
definite effect on the mixing time. In the region of air flows below
300 cc/min. curve A represents the average of the data taken at 24 and
30 inches of water while curve B represents the average for 15 inches.
In Figure 16 the liquid tracer was used while in the tests presented in
Figure 17 solid tracer was introduced over the plume for estimation of
mixing times. The curves, therefore, are not directly comparable.
In both figures the bubble sizes (equivalent spherical radius)
varied from 0.07 cm at 150 cc/min to 0.18 cm at 2000 cc/min of air. It
will be shown later that the effect of this difference in bubble sizes
on the mixing time is negligible. The effect of geometry (water height
and disperser location) will be discussed in a separate section of the
report.
It can be seen from both figures that the mixing time becomes
shorter with increase of the air flow rate. This dependence, however,
becomes less pronounced at higher rates of flow. As was mentioned before,
the mixing time, as measured in this work, is influenced by the ran-
domness of the system, as well as by the details of the tracer technique.
It seems therefore advisable to discuss first the circulation currents
established in the tank as a result of the pumping action of the air
plume.
The water entering the plume is pumped upwards because the potential
energy of the rising bubbles is transformed into the kinetic energy of
the water (1, 11, 13). In the forthcoming discussion this water current
will be referred to as the vertical jet (Figure 8). Upon reaching the
surface the bubbles escape into the atmosphere but the momentum of the
vertical jet is converted into a thin horizontal surface jet (1, 10, 18).
According to Baines (1) the thickness of this current is less than 5%
of the depth of the submergence of the disperser. It can be seen from
Figure 13 that this surface jet is very thin.
23
-------
26
,'4
. V.
X
z 16
d
ni '*
| 12
c
w 10
e
JL
o WATER HEIGHT • 30 '
WATER HEIGHT • 24'
. WATER HEIGHT -15
A AVERAGE OF 30* AND 24"
8 AVERAGE OF IS'
J.
i
300
600
900 1200 1500 1800
AIR FLOW RATE - CC/ MINUTE
2100
24OO
2700
FIGURE 16
MIXING TIME VS AIR RATE ( LIQUID TRACER INTRODUCTION )
3000
-------
I
>-
6)
-i
i
m
12
to
8
6
H
fr,
DISPERSER IN CENTER
1
o 24" Water
• 15 "Water
a 24 " Water
• 15 "Water
DISPERSER ON SIDE
J
1
1
0
100 200 300 400 500 600 700
AIR FLOW RATE - CC / MINUTE
FIGURE I?
MIXING TIME VS AIR FLOW RATE (SOLID TRACER INTRODUCTION)
-------
This surface current produced by the vertical jet has been studied
by a number of investigators (1, 4, 10, 18, 19). Most of them (4, 10,
18, 19) were interested in its damping action on ocean waves (pneumatic
breakwaters). They found that the water velocity of the surface jet
was inversely proportional to the distance from the plume. Hence, the
momentum of the surface jet was used to generate currents in the layers
below.
Gay and Hagedorn (5) studying induced air mixing of a stratified
body of water found in addition to the surface jet a strong bottom
current toward the disperser and a stagnant zone in the vicinity of the
disperser. The presence of this stagnant layer was also noticed by
Straub et al. (18) in their work on pneumatic breakwaters. It is also
of interest to note that the presence of this zone may constitute a
potential source of error in mixing time determination. Figures 24 to
28 give a good idea of the water circulation pattern encountered in the
present work.
The effect of the air flow rate on the water flow (and therefore
on the mixing time) can best be seen in Figure 23 where the restricted
system was used and the water velocity was measured directly by hot
film anemometry. The results shown in this figure were taken at a con-
stant water height. The form of this curve is in agreement with the
data of Kurihara (10) obtained in an unrestricted system. He found that
the flow of water is proportional to the air-flow rate to the 1/3 power.
Table I gives the values of Q1'3 calculated from Figure 23. It can
be seen that except for the lowest rate of air the above relationship
between the flow of water (F) and that of air (Q) applies well to the
system investigated in this work.
Table I
Water Flow in Restricted System
Flow of Water
(F) liters/min
10
20
30
40
50
Air Flow
(Q) cc/min
50
110
225
500
1050
F
J73
2.7
4.2
4.9
4.9
4.9
26
-------
This interdependence was also confirmed by Cyr (3) for higher air flow
rates. Taylor (19), in fact, showed theoretically that the maximum ver-
tical current of water produced by a flow of gas is proportional to the
1/3 power of the gas flow rate. Table II shows the dependence of the
mixing time on the water pumped in the restricted system. The mixing
times were determined by the solid tracer technique.
Table II
Percentage of Water Pumped During Mixing Operation
Air Flow Mixing Time Total Water % Tank
cc/min Minutes Pumped in Capacity
Mixing Time, Pumped
Liters
150 8.34 200 33
300 4.92 170 28
600 4.11 175 29
Water volume = 610 liters
The table shows a close dependence of the mixing time on the amount of
the water pumped. It is of interest to note that only about 30% of the
tank capacity had to be pumped in order to approach closely the equilib-
rium concentration.
Bryan (25) states that in the diffused air mixing of the Blellam
Tarn reservoir only 26% of its volume was pumped during the time required
for mixing. Although the reservoir may not have been as well mixed as
the tank used in this research, this information does serve to illustrate
the importance of the currents generated by air dispersion.
It was already stressed before that Figures 16 and 17 differ in
the method of tracer introduction. In the former, liquid tracer was
introduced close to the smaller wall of the tank, while in the latter
the solid tracer was added over the top of the plume. On the whole it
may be said that the liquid tracer introduction (Figure 16) showed great-
er mixing time than the corresponding curve for solid tracer (Figure 17,
disperaer in center). This difference in results was probably due to
the fact that in the case of liquid tracer, the whole volume of the
solution was introduced in a pulse which spread into the region of
comparatively stagnant liquid at a considerable distance from the plume.
On the other hand, the solid tracer was added in the center of the surface
jet where the chance of its rapid distribution was much better.
27
-------
Photograph of Circulation Pattern; 10 seconds. Solution Tracer
Figure 18
Photograph of Circulation Pattern; 30 seconds. Solution Tracer
Figure 19
28
-------
Photograph of Circulation Pattern; 1 minute Photograph of Solution Tracer
Figure 20
Photograph of Circulation Pattern. 2 minutes Solution Tracer
Figure 21
:
-------
Figures 18 to 22 show addition of dye introduced by the liquid tracer
method. The time after introduction of the dye is noted in each Figure.
It can be seen from the figures that the tracer introduced in the
form of a pulse forms a highly concentrated zone in the right-hand side
of the tank which has to be "pulled" by the slowly circulating liquid to
the left-hand side (cell location) before any changes in the conductivity
can be detected. It may therefore be expected that because of this initial
lag the liquid tracer method should give longer mixing times as shown in
Figure 16 and 17 (lower curve).
However, in the case of solid tracer introcuced above the plume,
that is in the center of the surface jet, the situation was different.
In order to illustrate the distribution of the tracer, a small quantity
of a black dye was mixed with the KC1 crystals and photographs were taken
at time intervals between 10 sec. and 3 minutes. The results are shown
in Figures 24 to 28. The tiny crystals dissolved rapidly and the
resulting solution was spread uniformly over the whole surface of the
tank. This, already dilute solution, was then distributed throughout
the mass of water by the circulation currents produced by the plume.
Photograph of Circulation Pattern. 3 minutes Solution Tracer
Figure 22
30
-------
60
u>
200
400
600
80O
1000
1200
1400
1600 1800
AIR FLOW RATE - CC/ MINUTE
FIGURE 23
STACK FLOW VS AIR FLOW RATE
-------
Photograph of Circulation Pattern.Solid Tracer: 10 sec,
Figure 24
Photograph of Circulation Pattern.Solid Tracer: 30 sec,
Figure 25
32
-------
Photograph of Circulation Pattern. Solid Tracer: 1 minute
Figure 26
Photograph of Circulation Pattern. Solid Tracer: 2 minutes
Figure 27
-------
The greater degree of the initial dispersion of the tracer seems
to be the main reason for the shorter mixing times obtained by this
tracer method. It must therefore be emphasized that the mixing time is
not an absolute value but depends on the method of determination and
only the results obtained by the same method can be subject to general
correlation.
Photograph of Circulation Pattern.
Figure 28
Solid Tracer: 3 minutes
It is of interest to note the sharp increase in the mixing time
which can be seen in Figure 16 at an air flow rate between 150 and 300
cc/min. This sudden break in Figure 16 can be attributed to the
magnitude of the currents generated in the mass of water. It can
be expected that decreasing the rate of air flow some critical value
would be reached at which the currents generated would not be suffi-
ciently strong to establish a circulation of an appreciable magnitude
in the whole body of water. Apparently at 150 cc/min. the system was
handicapped in that the surface jet was unable to completely overcome
the inertia of the stagnant body of water. Figure 17 (lower curve)
which was obtained with the solid tracer technique, shows a similar
break but of much smaller magnitude. This apparent discrepancy can be
readily explained by the differences in the two tracer techniques which
were discussed before. It may be expected a sharp transition of this
34
-------
kind would occur independently of the tracer method used, although at
different air flow rates.
Another important observation which can be made from Figures 16
and 17 is the spread of data which decreases with increased air flow
rate. This is especially noticeable in Figure 16. In the case of
the liquid tracer the probable error of a single determination was
8.6% at 2000 cc/min while at 150 cc/min it increased to 18%. This
relatively poor reproducibility at low air flow rates is attributed to
the smaller intensity and greater randomness of the generated water
currents.
Preliminary studies of surface velocities conducted by hot film
anemometers indicated values ranging from 1 ft/sec at 2000 cc/min of air
to 0.3 ft/sec at 150 cc/min. It was further noted that the fluctuations
in these velocities ranged from 25 to 50% with a tendency to increase
at lower flow rates of air. In order to obtain some idea of the circula-
tion pattern in the body of water, polystyrene beads were dropped in the
tank and allowed to circulate freely. Except for the surface and the
vertical jets, it was rather difficult to discern any definite flow pattern.
Very little circulation was observed near the bottom of the tank, especial-
ly at the corners. Gay and Hagedorn (5) found in their work that a
bottom layer approximately 1" thick was relatively unaffected by the
mixing.
Efficiency of the Pumping Action of the Air Plume
The efficiency E of the air plume as a pump is defined, in this
work, as the fraction of the net energy of the entering air En which is
transformed into the kinetic energy Ek of water. The net energy of the
air is expressed as the difference between the work of isothermal
compression and the sum of the entrance, exit and frictional losses in
the cylinder of the restricted system. Calculations showed that the energy
losses caused by the presence of the cylinder were practically negligible
(10% or less) . The results are summarized in Table III.
En
Table III
Efficiency of the Restricted System
Air Flow Rate Efficiency
cc/min. Ex 100
150 5.42
300 8.25
600 7.05
35
-------
Although a direct comparison of the restricted system with that of
a free rising plume is not fully justified, some interesting observations
can be made from these figures. Kurihara (1, 10) in his studies
(open plume) found a marked decrease in efficiency with decrease of the
depth of submergence. The comparatively small efficiency in the present
system might have been due to the very small height of the plume
(1 1/4 - 2 1/2 ft as compared with 5 1/2 - 25 ft). Another point of
interest is that under the same operating conditions (depth of water and
air flow) the mixing times for both the restricted and unrestricted
systems were practically identical. This was rather surprising because
in the restricted system the water inflow from the side of the plume
was absent whereas in the open plume it formed an essential part of the
circulation pattern. There was therefore a considerable difference in the
flow characteristics of the systems.
Average Mixing Time vs. Power Per Unit Volume
Figure 29 presents the mixing time as a function of the power in-
put per unit volume. The power input was calculated by the equation:
H - Q,P, In It
where: H = Power
Q = Air flow rate
P - Absolute pressure
Subscripts 1 & 2 refer to initial and
final conditions respectively.
Because for a given depth the power input IS proportional to the air flow
rate, the curve resembles closely Figure 16 where the mixing time ^ was plot-
ted against the air flow. The power per unit volume vs. mixing time
correlation is of interest in scaling up model studies to large scale
installations (see also Parker Ref . 15).
It is of interest to compare the magnitudes of the power per unit
volume values obtained in this work with those of large-scale studies
(Table IV). The comparison is only approximate because the degree of mixing
was in this work considerably greater than in the large-scale tests where
the only goal was to break thermal stratification. The data obtained in
this research ranged from 1CT6 to 1.3 x 1(T7. The difference in the order
of magnitude can be due to the fact that in this research the contents of
the tank were mixed close to the equilibrium concentration whereas in the
large-scale tests this was not required. It is of interest to note that
despite the wide differences in size and geometry of the model as compared
with the reservoirs the difference in power input was not that great.
36
-------
ni
o
m
2
c
H
CO
17
16
14
\2
S
X
Z 10
G>
8
LIQUID TRACER
INTRODUCTION
o WATER HEIGHT = 30"
• WATER HEIGHT = 24"
• WATER HEIGHT =15"
2 4 6 8 10
POWER/UN fT VOLUME - ( HP/ft3 ) X 10s
FIGURE 29
MIXING TIME VS POWER/UNIT VOLUME
12 13
37
-------
Table IV
Diffused Air Mixing of Large Bodies of Water (21)
Reservoir
Indian Brook
Wohlford
Blelham Tarn
Volume
13.8 x 106
10.9 x 106
26.0 x 106
Power Input
hp/ft3
5.8
4.4
3.5
x 10"7
x 10"7
x 10~8
Effect of Geometry on the Mixing Time
Figures 16 and 17, in addition to the effects of air flow rate,
indicate also the effect of the geometry of the system on the mixing time.
In this work the geometry is expressed in terms of the height of water
level and location of the disperser.
It can be seen from Figure 16 that except for very low air flows
(< 150 cc/min.) there was no apparent effect of water height on the
mixing time. However, at 150 cc/min. and at a water height of 15 inches
(curve B), the mixing time was about 20% lower than those for the other
two heights (curve A). The lower curve of Figure 17 (solid tracer) shows
a similar behavior indicating practically no effect of depth (15 and 24
inches) on the time of mixing. The effect of introducing the air at the
side of the tank is shown by the two upper curves of Figure 17. For
both the water depths used in this work, the side location of the air
disperser resulted in longer mixing times. A similar behavior was also
observed by Gay and Hagedorn (5). It can also be seen from Figure 17 that
the effect of the side location of the disperser was more pronounced for
the greater depth, resulting in longer mixing times for 24 inches as
compared with 15 inches of water.
The adverse effect of the side location can probably be explained
by the proximity of the wall and therefore its greater damping effect on
the generated water currents as compared with the central location.
Limited data was also taken for two different disperser submergences
at the same air flow rate and water depth. The results are shown in
Table V.
It can be seen that the smaller the energy input (smaller submergence)
the longer was the mixing time. In addition to the smaller energy input, the
system with a disperser raised 13 cm off the tank bottom, was further
handicapped in that the water level below the disperser was relatively
stagnant and thus more resistant to the currents produced by the plume.
38
-------
Table V
Effect of Disperser Submergence on Mixing Time
Air Flow
cc/min
150
300
600
150
300
600
Depth of
Disperser, cm
43
43
43
56
56
56
Water Height
cm
61
61
61
61
61
61
Mixing Time
min.
7.34
5.00
3.78
5.13
3.67
2.50
The above is in agreement with the observation of Gay and Hagedorn
(5) who found that for a fixed flow rate of air, mixing was achieved
more rapidly when the disperser was placed at the bottom of the tank
rather than at an elevated position.
Effect of Bubble Size on the Pumping Capacity and the Mixing Time
Since the density of the plume depends on the air holdup and there-
fore on the velocity of rise of bubbles, it was expected that the small
slowly rising bubbles should give a better pumping effect and therefore
a shorter time of mixing.
Figures 30 and 31 show the effect of the bubble size (equivalent
radius) on the pumping capacity and mixing time respectively.
The tests were conducted in the restricted system. The size of
the bubbles was adjusted by introduction of a small amount of n-heptanol
to reduce bubble coalescence (22). The largest bubble size was obtained
by mounting over the disperser a small cap with a hole 1/32" in diameter.
The mixing times were determined by means of the solid tracer.
It can be seen from Figures 30 and 31 that a 6-fold decrease in the
bubble radius resulted in a 23% increase in the water flow and a 19%
decrease in the mixing time.
Baines and Hamilton (1), studying the effect of orifice size on the
surface jet velocity, found that a change in the orifice size from 0.10
to 0.30 cm had no effect. Similarly, Evans (4) in a study of pneumatic
breakwaters noticed only a small change in the velocity of the surface jet
when the bubble diameter was changed from 0.45 to 0.65 cm. Cyr (3),
39
-------
Tl
o
O)
>
o
z
c
H
50
40
30
20
10
AIR FLOW RATE
= 600 CC/MINUTE
0.00
0.050 0.100
0.150
0.200 0.250 0.300 0.350 0.400 0.450
AVERAGE BUBBLE SPHERICAL RADIUS - CM
FIGURE 30
FLOW IN STACK VS BUBBLE SIZE
-------
m
m
4.75 -
4.50
x
o 4.25
m
1 4.00
2
z
c
m
375
3.50
0.00
AIR FLOW RATE
= 600 CC/MINUTE
-L
_L
_L
J.
0.050
0.100 0.150 0.200 0.250 0.300 0.350
AVERAGE BUBBLE SPHERICAL RADIUS - CM
FIGURE 3|
MIXING TIME VS BUBBLE SIZE
0.400 0.450
-------
investigating the effect of process variables on the performance of a gas
lift circulator found that a 25% increase in the water flow was obtained
by decreasing the bubble radius from 0.33 to 0.04 cm.
Inspection of charts relating bubble velocities to their sizes
(17, 6) shows that for bubble radii above the range of 0.06 to 0.08 cm
the velocity of rise (and therefore the holdup) does not depend much on
the bubble size. However, below this range the bubble velocity drops
rapidly with the size of the bubbles resulting in a greater air holdup.
It is noted that for both the work of Baines and Hamilton (1) and
Evans (4) the bubble sizes were in the region where the velocity of rise
was relatively independent of size. Hence, no significant change in the
pumping effect would be expected.
In the present research the bubble size was decreased to a radius
less than 0.05 cm. Bubbles of this size have velocity of rise about
10 cm/sec while those having a radius of 0.3 cm; rise with a velocity almost
three times as great (28 cm/sec). The smaller velocity of the bubbles used
in this work, and the resulting increase in the residence time must have
been the main factor responsible for the increased pumping effect.
Although the effect of bubble size on both the pumping performance
and mixing time was not very great, it was, however, sufficiently pronounced
to be considered as a design variable.
This point may be of special importance in the case where bubble coal-
escence occurs at the surface of the air disperser. Minute quantities of
surfactants or other coalescence preventing agents would then have a
marked effect on the size of the bubbles. A similar effect could be
expected in solutions of inorganic salts (e.g. sea water) but at a higher
concentration (23).
Correlation of Data
The experimental variables were correlated by means of Dimensional
Analysis assuming isothermal operating condition. This last assumption
was in agreement with the conditions of the work and at the same time
gave the advantage of reducing the number of variables to those of
essential importance in the system. The analysis resulted in the
following relationship:
"(fR
where: 0 * Mixing time, min.
2
g = Acceleration of gravity, cm/sec
D = Principal dimension of tank, cm
42
-------
H = Height of water in tank, cm
Fr = Froude number, dimensionless.
The appearance of Froude number in the above equation is understandable
since both inertia and gravitational forces are of primary importance in
this system. The above correlation is shown graphically in Figure 32 in
which the Froude number was defined as the densimetric Froude number
expressed as (26):
Fr .
where: Q = Air flow rate, cc STP/min
A = Cross section of tank, cm
Ap = Density difference between water and plume,Sr/cc
pw » Density of water Sr/cc
Although there is a considerable spread of data, this correlation shows
a satisfactory relationship, especially when the small size of the tank
is taken into consideration.
43
-------
5000
4000
3OOO
2000 -
1000
^ 900
•se eoo
° 700
5OO
4OO
300
2OO
100
SOLUTION TRACER
INTRODUCTION
o 30 WATER
• 24* WATER
« 15" WATER
I ' I—LJ_L
5 6 7 8 9 10
30
0,5 -050 3
(FR) IH/D) x 10
FIGURE 32
e
vs
-0.20
-------
SECTION IV
References
1. Baines, W. D. and Hamilton, G. F., "On the Flow of Water Induced
by a Rising Column of Air Bubbles," 8th Congress. Int. Assoc. of
Hydraulic Res., Aug. 1959.
2. Cook, M. W. and Water, E. D., "Operational Characteristics of Sub-
merged Gas Lift Circulators," U.S. Atomic Energy Commission, Dec.
1955.
3. Cyr, Steven, "Induced Air Circulation of Water," Unpublished
M.S. Thesis, U. Maine, Orono, 1970.
4. Evans, J. T., "Pneumatic and Similar Breakwaters," Proc. Royal
Soc., A231, 1955, p. 456.
5. Gay, F. and Hagedorn, Z., "Forced Convection in a Stratified Fluid
by Air Injection," M.I.T. Thesis, Jan. 1962.
6. Haberman, W. and Morton, R. K., "An Experimental Investigation of
the Drag and Shape of Air Bubbles Rising in Various Liquids," David
Taylor Model Basin, U.S. Navy, Sept., 1953.
7. Harrel, J. E., "Mixing of Fluids in Small Diameter Tanks by Circu-
lation," AEC Y-1531, April, 1966.
8. Hinze, J. 0., "Turbulence," McGraw-Hill Book Co., New Ydrk, 1959,
pp. 119-122.
9. Johnstone, R. E. and Thring, M. W., "Pilot Plant Models and Scale-
up," McGraw Hill Book Co., New York, 1957, pp. 33-35.
10. Kurihara, M., "Pneumatic Breakwaters," Parts I, II, and III, Proc.
of 1st Conferences on Coastal Eng. in Japan, Nov., 1954.
11. Lament, A. G. W., "Air Agitation and Pachuca Tanks," Can. J. of
ChE., Aug. 1958, pp. 153-160.
12. Morton, B. R., et al, "Turbulent Gravitational Convection from
Maintained and Instantaneous Sources," Proc. Royal Soc., 234A,
Jan. 1956, pp. 1-23.
13. Nevers, Noel De, "Bubble Driven Fluid Circulations," AIChE J.,
March, 1968, pp. 222-226.
14. Nickerson, H., "Gloucester-Forced Circulations of Babson Reser-
voir," Div. of San. Eng., Mass. Dept. of Health, Boston, 1960.
15. Parker, N. H., "Mixing," ChE, June 8, 1964, pp. 165-220.
45
-------
16. Riddick, T. M., "Forced Circulation of Reservoir Waters," Water
and Sewage Works, Vol. 104, No. 6, June, 1957, pp. 231-237.
17. Rosenberg, B., "The Shape and Drag of Air Bubbles Moving in
Liquids," Report 727 Navy Dept., Sept., 1950.
18. Straub, L. G., and others, "Experimental Studies of Pneumatic and
Hydraulic Breakwaters," U. Minn., St. Anthony Falls Hydraulic Lab.,
Tech. Paper No. 25, Series B, Aug., 1959.
19. Taylor, G., "The Action of a Surface Current Used as a Breakwater,"
Proc. Royal Soc., A231, 1955, pp. 467-478.
20. Thermal Systems, Inc. Bulletin TBS, St. Paul, Minn.
21. U. S. Dept. of H.E.W., Public Health Service, "Water Qualities
Behavior in Reservoirs," compiled by J. Symons. (1969)
22. Zieminski, S. A., et al, "Behavior of Air Bubbles in Dilute Aqueous
Solutions," Ind. Eng. Chem., May, 1967, pp. 233-242.
23. Zieminaki, S. A. and Whittemore, R.C., "Behavior of Gas Bubbles in
Aqueous Electrolyte Solutions," ChE. Sci., Publication Pending.
24. Zlokarink, M., "Homogenisieren von Flussigkeiten durch Aufsteigende
Gasblasen," Chem. Ing.-Techn., 4_0, 1968, pp. 765-769.
25. Bryan, J. G., "Improvement in the Quality of Reservoir Discharges
Through Reservoir Mixing and Aeration," R. A. Taft San. Eng. Center,
Cinn., Ohio, April 3-5, 1963, PHS Publ. No. 999-WP-30, June 1965,
pp. 317-34.
26. King, D.L., "Hydraulics of Stratified Flow," Report No. Hyd-563,
Hydraulics Branch, Division of Research, U. S. Dept. of Interior,
June 3, 1966.
46
-------
1
5
Accession Number
n Subject Fii-ld & Group
05G
SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Organization
University of Maine, Orono, Maine 04473
INDUCED AIR MIXING OF LARGE BODIES OF POLLUTED WATER
10
Auttt9t
-------
WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Reports describe the
results and progress In the control and abatement of pol-
lution in our Nation's waters. They provide a central
source of information on the research, development, and
demonstration activities in the Water Quality Office,
Environmental Protection Agency, through inhouse research
and grants and contracts with Federal, State, and local
agencies, research institutions, and industrial organizations.
Inquiries pertaining to Water Pollution Control Research
Reports should be directed to the Head, Project Reports
System, Planning and Resources Office, Office of Research
and Development, Water Quality Office, Environmental
Protection Agency, Room 1108, Washington, D. C. 20242.
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