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STATES ENT OF ERCE

IATION NBS TECHNICAL NOTE 771

Some Experiments on

the Stirring of

Viscous Liquids
NATIONAL BUREAU OF STANDARDS

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- -i

77/

73

Some Experiments

on The Stirring of Viscous Liquids

Albert Napolitano

Inorganic Materials Division

Institute for Materials Research

-l/.J National Bureau of Standards

*** Washington, D.C. 20234

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NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of

transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

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U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued May 1973
National Bureau of Standards Technical Note 771

Nat. Bur. Stand. (U.S.), Tech. Note 771, 60 pages (May 1973) CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:771).

Price $0. 85 domestic postpaid or $0.60 G.P.O. Bookstore
Some Experiments on the Stirring of Viscous Liquids*

by

Albert Napolitano

Inorganic Materials Division

The stirring actions of several designs of small stirrers were

observed and photographed in transparent viscous model liquids. The

fluidity of the liquids was comparable to that of a molten borosilicate

glass at fining temperatures. The most efficient features of one or

more of these stirrers were incorporated into larger model stirrers

and observed under actual stirring conditions. As a result of these

performance tests, a final design of stirrer was selected and fabricated

of platinum-rhodium metal and used in an experimental glass tank. The

design of one of the pots in the experimental glass tank was modified

to obtain the maximum stirring action with the selected stirrer.

Key words: Agitators; borosilicate glass; glass melt; glass tank;

liquids; stirrer; viscosity.

*This study (see ref. 1) was performed under Contract AF (33-038)

49-4393 sponsored by the Air Material Command, United States Air Force

under Project 0902-33-4405 (Continuous Process for Melting Optical

Glass) at the National Bureau of Standards in 1951.
I. Introduction

One of the major problems encountered in the production of good

quality optical glass in small pots (0.25 to 20 liters) is the proper

homogenizing of the melt. To accomplish this, stirrers have to be

designed which give an effective stirring or mixing action in the melt.

The design of the stirrers and their efficient use in glass melts has,

2 3 4 therefore, been the object of numerous investigations. '

'

In considering the type of stirrer that will produce the desired

mixing action, various modifications of basic designs of stirrers can

5 6 7 be made. ' ' These changes may be in the size and the shape of the

blades, the number of blades employed, and the location and pitch of

blades on the shaft of the stirrer. The selection of an efficient

stirrer also depends on the size and shape of the pot, the density

and viscosity of the glass to be stirred, the depth of glass in the

pot, the location of the stirrer in the pot, and the speed of rotation

8 9 of the stirrer in the glass. '

In order to produce good quality optical glass precautions must

be taken not to contaminate the glass during its melting and fining

stages. When glass is melted and stirred in ceramic pots there usually

occurs some solution of the refractory which not only changes the composition of the glass but also, invariably, lowers the quality of glass.

With the use of platinum-lined pots and platinum stirrers this corrosion i
eliminated and the homogeneity of the glass is much improved. In addition,

the design and configuration of the platinum stirrer are very important

because the stirrer requires rugged construction to overcome the loss in

strength of the platinum metal at high temperatures (1100° to 1450 °C)

During the years 1946 through 1956, an experimental glass tank

furnace was developed at the National Bureau of Standards for the continuous production of optical glass. In order to produce glass of

optical quality, the melt was stirred in the fining stage with two

propeller-shaped platinum stirrers. Early in the development of this

tank it became apparent that the stirring was inadequate for producing

striae-free glass. An investigation on the design and the performance

of stirring rods was therefore conducted. It was expected that the

experience gained from these tests would also be applicable to platinum

stirrers used in the making of experimental laboratory melts.

II. Experimental Procedure and Results

All the performance tests in this study were carried out at room

temperature. This meant that the properties of the molten glass had

to be simulated at room temperature with a transparent viscous liquid

or oil. The liquid used in these tests was a standard viscous oil

(Oronite Polybutene, No. 24)* which had a viscosity at room temperature

*Dow Corning 200 fluids with viscosities ranging from .01 to

1000 poises (1 poise = 1 g/cm s) can also be used in such tests as performed here.
equivalent to that of borosilicate glass at 1200°C. The viscosity of

this oil at 20°C was 350 poises, at 25°C was 220 poises, and at 40°C

was 61 poises. The density of this oil at room temperature was approxi3 mately 0.88 g/cm . This was not the same as that of molten borosilicate

3

glass which has a value of approximately 2.20 g/cm . In order to check

what effect the density of the liquid played in the circulation paths

from different stirrers, some effort was made to make a liquid whose

density and viscosity were the same as molten glass at room temperature.

This liquid consisted of a mixture of light corn syrup which had been

evaporated down to 70-75% of its original volume and a concentrated

solution of zinc bromide. The prepared liquid had a density of 2.08 g/cm"

at room temperature, but by varying the ratio of corn syrup to zinc

bromide, within narrow limits, the viscosity could be changed as well

as the density. This liquid was only used to test the final design of

the stirrer.

An oil soluble red dye was used as a coloring agent to observe the

circulation paths by the striae method during the stirring cycle. Carbon

black was also used and proved to be more satisfactory for photographing.

All of the preliminary performance tests were carried out with

1/3 scale model reproduction of the stirrers to be used in the experimental glass tank. A circular container, a 3000 ml beaker, was used to

run the tests on each of the small stirrers. The diameter and height
of the beaker were 14 and 21 cms, respectively. During the tests the

liquid depth in the beaker was kept approximately equal to the diameter

of the beaker. The diameter of the beaker was a little over twice the

diameter of the stirrer.

The small stirrers, Fig. 1, were constructed of brass and copper

with an overall diameter of 6 cm and a 1.25 cm diameter shaft. The blades

were soldered to the grooved shaft and were 0.16 to 0.32 cm thick, depending

on the design of the stirrer. The basic designs of small stirrers

used in this study were (a) the propeller, (b) radial propeller,

(c) a combination of regular and radial propeller, (d) the helix, and

(e) the spiral. Other combinations or modifications of the pitch,

shape, and location of the blades of the above types of stirrers were

tested. Those that gave the best stirring actions, Figs. 2-12, were

made into full scale models to be tried out in the larger pots of the

experimental tank.

Four of the large stirrers tried in the tank-size pots are shown

in Fig. 13. The single two-bladed propeller which was originally used

in the tank is not shown in Fig. 13 (see Figs. 15 and 16)

The shapes of the pots used in the experimental glass tank were reproduced with plaster-of-paris models for the room temperature tests. Each

pot had glazed portholes on each side for illumination and observation

of the stirring action in the middle and bottom of the pots. The

usual technique of creating visible circulation paths with the large
stirrers was used; that is, a small quantity of carbon black mixed in with

the parent oil was placed into the pot before stirring was started. The

different shapes of pots used in these performance tests with the large

stirrers are shown in Fig. 14. Shapes 'a' and 'b' were the original

container shapes used in the experimental tank and in which the stirring

of the glass was carried out before delivery from the tank. After a

few tests in container 'b 1 with the stirrers in the larger end of the

container, it was apparent that it was too long for efficient mixing

and it was shortened to the length of container 'c' . Of course, the

shape of containers shown in 'a 1 and 'd' of Fig. 14 are ideal shapes

for maximum stirring actions.

The performance tests for the large stirrers are shown in Figures

15-19. Figure 20 shows a schematic drawing of the stirring action of

the final design of the stirrer, No. 2 in Fig. 13, in container 'd' of

Fig. 14. No pictures were taken of stirrers 3 and 4 in Fig. 13, even

though performance tests were made, because they were not considered

in the final design of the stirrer. The large combined radial and

regular propeller stirrer gave the same divided mixing in container "a 1

and 'b' as it did in the small beaker. The mixing action in the small

end of container 'b* was excellent with this stirrer but, again, to

only half of the depth. The bottom half was mixed separately and thoroughly

also and with its own circulation paths.
The double three-bladed propeller gave very effective stirring

action in container '

1 at low speeds. At higher speeds the mixing

became turbulent and because of the concentric shafting and opposite

rotations of each set of blades, this stirrer could not be considered,

as previously stated, for high temperature application. It was also

noticed that when the distance between each set of blades was increased

from 2.5 cm to 5 and 7.5 cm the mixing was much more efficient.

III. Summary

The results of these tests with small stirrers show that the triple

two-bladed propeller, Fig. 1 - No. 2, gave the fastest mixing. This

type of stirrer has considerable lifting action and, consequently, it

circulates the liquid in a round container very rapidly. As the liquid

is being lifted by the propeller, some of it tumbles off the tips of

the blades (see Fig. 20) and not only travels through a minor circulation

path but enters the main circulation path. The tumbling action gives a

separate circulation path around the tips of the blades on other stirrers,

especially the helical and spiral types. As some photographs show, there

is a region of little mixing here, and showed that this type of stirrer

has no throwing-out action and, therefore, could not be considered as

effective stirrers. The helical and spiral type of stirrer appear to

have more lifting power than the propeller, but the swirling action of

these stirrers tends to increase the region of little mixing.
As a direct result of these performance tests, some conclusions

can be made for the stirrers and containers that can be used in the

mixing of liquids or molten glass. These are as follow:

1. The modified double two-bladed propeller gives the best

mixing because of its lifting and throwing out action.

2. A circular container with a near-hemispherical or 'dished 1

bottom is the ideal shape for a container.

3. The diameter of the stirrer should be approximately half the

diameter of the container. When the stirrer was placed a little off

center of the container, better mixing resulted because of the

destruction of the symmetry of the circulation paths. The distance

off the bottom of the container was approximately half the width of

the blade.

4. The speed of rotation of the smaller stirrers could be varied

between 100 to 200 RPM depending on the viscosity of the liquid. For

the larger stirrers a maximum speed of rotation was 75 RPM.

5. The propeller-type stirrer is rugged in construction and can

be fabricated of 90% PT-10% Rh for high temperature use.

The writer wishes to recognize the contributions of C. A. Faick

(deceased) who was project leader at the time that this work was performed and to Max Kahn as a co-worker on the project.
References

1. A. Napolitano, M. Kahn, and C. A. Faick, "Some Experiments on the

Stirring of Viscous Liquids," National Bureau of Standards Report

No. 1083, August 6, 1951.

2. Ordnance Department Document 2037, "The Manufacture of Optical

GLass and of Optical Systems," pp. 120-128, 1921, Government

Printing Office, Washington, D.C.

3. F. W. Glaze and C. H. Hahner, "The Manufacture of Optical Glass

at the National Bureau of Standards," National Bureau of Standards

Circular 469, November 1, 1948.

4. F. W. Glaze, "History of Optical Glass Production in the

United States," Amer. Ceram. Soc. Bull., 3_2, July 242-247,

August 281-284, September 313-317 (1953).

5. C. Tyler, "Mixing and Agitators," Chem. and Met. Eng. 29_, 441 (1923)

6. D. H. Killefer, "Agitators," Ind. and Eng. Chem. 15_, 144 (1923).

7. I. S. Brumigan, "Some Fundamental Factors in Choice of Liquid

Agitators," Chem. and Met. Eng. 5_3, 110 (1946J .

8. E. S. Bissell, "Propeller-type Mixers," Ind. and Eng. Chem. 30 , 493 (1938).

9. E. S. Bissell, H. J. Everett, and J. H. Rushton, "Mixing Liquids

in Shallow Tanks Aided by Design Analysis," Chem. and Met. Eng. 53,

118 (1946)

10. R. E. Chaddock, "Practical Aspects in Design of Liquid Agitators,"

Chem. and Met. Eng. 53_, 151 (1946) .

11. E. J. Lyons, "Practical Mixer Technology," Chem. Eng. Prog. 44 , 341 (1948)
Figure 1. Small Stirrers.

1. Single two-bladed propeller - pitch of blades 15°. This design

of stirrer was in use in small melts as well as in the experimental tank

in the larger size.

2. Triple two-bladed propeller - pitch of blades 15° with each pair

of blades 60° apart around the shaft and 3.75 cm along the shaft.

3. Radial propeller - pitch of horizontal blade 15° and vertical

vanes at each end 3.75 cm long.

4. Combined radial and regular propeller - top and bottom pair of

blades at a 15° pitch but opposite to each other. Vertical blades were

still 3.75 cm long. This design of stirrer was also being used in small

melts because of its rugged construction.

5. Helical stirrer with three turns and a pitch of 2.5 cm.

6. Helical stirrer with two turns and a pitch of 3.75 cm.

7. Helical stirrer with two turns and a pitch of 3.75 cm and

tapered blade from bottom to top.

8. Helical stirrer with two turns and a pitch of 3.75 cm. The

blade was tilted downward 15° so that it was cone-shaped.

9. Spiral stirrer with a 180° twist in the blade. The blade was

12 cm long.

10. Spiral stirrer with a 180° twist in the blade. The blade was tapered from bottom to top and 11.5 cm long.

Shaft diameter - 1.25 cm Diameter of stirrer - 3.75 cm

10
11
Figure 2. Two-bladed Propeller.

This group of photographs* shows the conventional stirring action of

a propeller for clockwise rotation. At the end of one minute in photograph *b' the dye is in thenain circulation path along the side of the

container and has formed an envelope two-thirds of the way down the outside path. After two minutes of stirring as shown in 'c', the dye has

reached the bottom of the container, has been lifted through the middle

of the stirrer, and is spreading out on the top surface of the oil ready

for the second round. This picture clearly shows the lift of the propeller with each revolution because the successive layers of dye may be seen above the blades and also the upward movement of the dye in the

cyclonic cone below the blades. The pushing forward or the propelling

action of the stirrer was also noticeable in this photograph because

the cone below the blades waved back and forth as the stirrer rotated.

At the end of six minutes photograph 'd' shows the successive revolutions

of the propeller as streaks of dye throughout the container of oil and

with each journey through the circulation path the dye gradually disperses. After twenty-six minutes of stirring, the dye is completely

dispersed as shown in photograph 'e 1 . Photograph ' rotated 90° more than shown in photographs a - d.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

*A11 photographs in this figure as well as those that follow were

taken with the stirrer stopped. The viscosity of the oil was high enough

to prevent the circulation patterns from dispersing in the time required

for the pictures to be taken.

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Figure 3. Triple Two-bladed Fropeller.

This group of photographs shows the stirring action of the triple

two-bladed propeller to be the same as that of the single two-bladed

propeller but completely disperses the dye in one-fifth the time. This

stirrer was placed about 2.5 cm off the bottom of the container, and it

is believed that this position gave a more vigorous stirring action.

Speed of rotation - 120 KPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

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Figure 4. Radial Propeller.

This set of photographs shows the top circulation path of a radial

propeller. The carbon black used here photographs much better than the

dye. Adding the vanes to a modified propeller blade resulted into two

circulation paths for this stirrer. The liquid which is lifted by the

horizontal blade from the bottom of the container is caught in the circulation path set up by the side vanes and thrown out to the sides of the

container and returns to the bottom. As the liquid is thrown out to the

sides of the container, the liquid to replace it is not only drawn from

the bottom of the container but also drawn down from the top along the

shaft. In photographs 'b' and 'c' after 30 seconds and one minute,

respectively, of stirring this action is clearly shown. After four

minutes of stirring, photograph 'd', the maximum mixing with this type

of stirrer has been reached. Longer periods of stirring and different

positions of the stirrer in the liquid were tried, but no radical changes

occurred in the mixing.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

16
a. 15 sec. b. 30 sec.

c. I min. d. 4 min.

17
Figure 5. Combined Radial and Regular Propeller.

This stirrer gives the two main circulation paths as discussed

under Fig. 4 but to a more marked degree. It gives a perfect divided

field of mixing after 28 minutes. Photograph ' f

' clearly shows the

top half with the dye dispersed and the lower half without any dye.

The bottom half gave the identical result except that the circulation

path was opposite to that of the top. This was tested when a quantity of

dye was allowed to sink to the bottom and the stirring started. When

the stirrer was either raised or lowered, the line between the two

fields of mixing followed the change in position of the stirrer.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

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Figure 6. Helical Stirrer - Three Turns.

These three photographs show the typical circulation path of the

helical stirrer in the first minute. As shown, the carbon black follows

the movement of the liquid in the outside path and up through the middle

of the stirrer.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

20
a, 15 sec, b. 30 sec.

c. I min.

21
Figure 7. Helical Stirrer - Two Turns.

The straight helical stirrer with two complete turns gave

essentially the same results as the helical stirrer with three

turns, except that it took a little longer to give the same amount of stirring.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

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Figure 8. Tapered Helical Stirrer - Two Turns.

To increase the tumbling action of this type of stirrer, a tapered blade (i.e. width of blade narrows from bottom to top) was

used. The tumbling action is clearly evident in photograph 'c' in

which the carbon black seems to fall off in successive revolutions

of the stirrer. The last photograph in this set clearly shows a region of light or no mixing around the stirrer. This concentric

'doughnut' or region of no mixing was characteristic of this type

of stirrer.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

24
a. 15 sec. b. 30 sec

c. i mm. d. 4 min.

25
Figure 9. Cone-shaped Helical Stirrer - Two Turns.

To increase the tumbling action still further, a cone-shaped

helical blade stirrer was used. After four minutes of stirring, the

carbon black was' well dispersed in the oil with the exception of the

light region which still remained. This stirrer gave the same results

in less time than the other types of helical stirrers and gave evidence

that the downward slope of the blade increased the tumbling action

which in turn increased the mixing.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

26
a. 15 sec. b. 30 sec.

c. » mm. d. 4 min.

27
Figure 10. Spiral Stirrer - 180° Twist.

A particular pattern of mixing is set up by this stirrer, as shown in photograph 'c 1 after one minute of stirring. Photograph

'd' shows the maximum mixing that this type of stirrer can accomplish. After fifteen minutes of stirring, there was no change in

the mixing pattern. The carbon black never reaches the top, the

outer edges, or the bottom of the main body of oil. The light

region of little mixing, the concentric 'doughnut', around the edge

of the blade is very pronounced with this type of stirrer.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

28
a. 15 sec. b. 30 sec.

c. I min. d. 4 min.

29
Figure 11. Tapered Spiral Stirrer - 180° Twist.

This stirrer has a blade which is wide at the bottom and decreases

in width towards the top. As the photographs show, this stirrer does

much better than the previous stirrer, Figure 10, in that it disperses

the carbon black to the walls of the container and also to the top and

bottom surfaces. After fifteen minutes of stirring, this stirrer also

gave the light region or region of little mixing at the edge of the

blade, but lower down, at the widest section of the blade. This was evidently caused by the increase in tumbling action along the upper

portion of the stirrer.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

30
a. 15 sec. b. 30 sec.

c. I mm.

31
Figure 12. Perforated Disc Stirrer.

A stirrer not shown in Fig. 1 consisting of a perforated disc

mounted on the end of a rod at a tilt of 15 Q is shown in this group

of photographs. The circulation paths are the same as those obtained

with the combined radial and regular propeller, Fig. 5, and similar

to those obtained by the radial propeller, Fig. 4. Even though a divided mixing also occurs with this stirrer, a comparison with the

others shows that it is much slower in its mixing and that the carbon

black actually never reaches the container walls as shown in the last

photograph. Closer examination of the last picture reveals that some of the carbon black has reached the bottom half of the beaker and is

being dispersed in the circulation paths opposite to those in the top

half. Once again, raising or lowering the stirrer had no radical

effect on the resulting circulation paths.

Speed of rotation - 120 RPM

Container - beaker 14 cm diameter, 21 cm high

Depth of oil - approximately 15 cm

32
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Figure 13. Large Stirrers.

1. Tapered helical stirrer.

a. Blade 5 cm pitch - width of blade 5 cm at the bottom

tapering to 2.5 cm at top.

b. Three turns of blade.

c. Shaft diameter - 3.18 cm O.D.

2. Modified double two-bladed propeller.

a. Bottom blades - 15° pitch, 7.5 cm wide.

b. Top blades - 45° angle downward with axis of shaft, 19 cm from the tip of shaft; blades are about 9 cm long and

7.5 cm wide, and 90° around the shaft from the bottom

blades.

c. Shaft diameter - 3.18 cm O.D.

3. Combined radial and regular propeller.

a. Top and bottom propeller blades - 15° pitch and opposite

to each other and 12.5 cm apart. Each blade 5 cm long

and about 3 cm wide.

b. Side vanes about 3 cm wide.

c. Counter-clockwise rotation.

d. Shaft diameter - 3.18 cm O.D.

4. Double three-bladed marine propeller.

a. Blades - 15° pitch and 120° apart. Each set of blades of

opposite pitch; therefore, opposite rotation - bottom

clockwise, top counter-clockwise. Width of blade - 5 cm.

b. Shafts - concentric - outer shaft 3.18 cm O.D. - inner

shaft 2.54 cm O.D.

c. Distance between each set of blades - variable up to about

20 cm.

Overall diameter of stirrer - 15.2 cm.

34
35
Figure 14. Container Shapes Used with Large Stirrers.

a. Cylindrical with a dished bottom.

b. Semi-cylindrical with one end or side elongated and half the

radius of the other end or side - also with a dished bottom.

c. Modified version of *b' with a dished bottom.

d. Cylindrical glass container with a flat bottom.

36
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Figure 15. Stirring Action of Two-bladed Propeller in Container 'a',

Fig. 14.

The carbon black was allowed to sink to the bottom of the

container before the start of stirring. In photograph *b* after

1.5 minutes of stirring, the carbon black has been mostly dispersed

but the main circulation path is till evident as the liquid is being

lifted up from the bottom of the container. In the last photograph

the color has been completely dispersed but has not reached the sides or the top.

This set of photographs, as well as those that follow, are taken

looking down into the top of the container.

Speed of rotation - 75 RPM

Depth of oil - 25.5 cm

38
(a) START

(b) I min 30 sec (c) 3 min

39
Figure 16. Stirring Action of Two-bladed Propeller in Container 'b*,

Fig. 14.

The stirring action with the single two-bladed propeller was incomplete in the small end of this container as shown in photograph 'c' No definite circulation paths could be observed. When the carbon black

was put into the small end of container 'c' and stirring started, it

curved downward parallel to the wall and in the direction of rotation

of the stirrer. As the carbon black came to the top again, it was not

pushed out completely to the small end of the container but pulled down

about ten centimeters from ihe wall

Speed of rotation - 75 RPM

Depth of oil - 25.5 cm

40
a. Start

b. 20 sec. c. 2 min. 5 sec.

41
Figure 17. Stirring Action of the Tapered Helical Stirrer, Fig. 13-1,

in Container 'a 1 , Fig. 14.

The tapered helical stirrer gave a faster stirring action in the

round container than the single two-bladed propeller. At the end of

nine minutes, this stirrer had accomplished the same results as the

two-bladed propeller in twenty minutes.

Speed of rotation - 75 RPM

Depth of oil - 25.5 cm

42
(a) START (b) 40 sec

(c) 1 min 30 sec (d) 3min

43
Figure 18. Stirring Action of the Tapered Helical Stirrer in

Container ' b * , Fig . 14

The performance of the helical stirrer in container 'b' was not

as efficient as the two-bladed propeller in that it gave a very slow

action in the small end of the container. Here there was a definite

line between the mixed and unmixed portions of the liquid which gave

considerable evidence that no stirring action occurred in the small

end of the container. When the carbon black was added to the top

surface of the liquid in the small end of the container as in the case of two-bladed propeller, it followed the same circulation path

except for the appearance of a reverse flow or counter-current in the

bottom and middle of the container. In this test, when the stirrer

Was placed off center, that is, about 5 cm towards the small end, the

stirring action was similar to that of the propeller and gave better

stirring.

Speed of rotation - 75 KPM

Depth of oil - 25.5 cm

44
<

co

00

ro

"O

o

CD

(/) O

45
Figure 19. Stirring Action of the Modified Double Two-bladed

Propeller, Fig. 13-2, in Container 'c'.

The double two-bladed propeller gave the best stirring action

in container 'b'. The continued presence of an eddy or countercurrent flow in the small end of the container was the same as that

with the single two-bladed propeller and the helical stirrer. This

indicated that the container was too long and, with this in mind,

container • c' was designed to correct this condition in the small

end of the container. This set of photographs shows the modified

two-bladed propeller in container 'c'. The stirring action was adequate throughout the container including the eventual disappearance of the eddy current flow in the small end of the container.

The same degree of mixing was accomplished in half the time in this

container as in container 'b' with this stirring rod.

Speed of rotation - 75 RPM

Depth of oil - 25.5 cm

46
a. 20 sec. b. 40 sec.

c. I min. 30 sec. d. 3 min.

47
Figure 20. Circulation Paths in Container 'd', Fig. 14, with the

Modified Double Two-bladed Propeller.

The double two-bladed propeller was tested in container 'd' with a liquid of proper density and viscosity (corn syrup and zinc bromide)

Rather than mix carbon black in the body of liquid, solid objects (pieces

of glass) were used to study the mixing action of the stirrer. The

stirring action was adequate, as shown in the schematic drawing, and

showed much improvement in this liquid of higher density. This removed

the doubt that the higher density of the liquid would be a detriment to

good mixing with this stirrer. Several speeds were tried with this

stirrer in this liquid, and it was found that at the lower speeds, the

mixing was considerably slowed down.

Speed of rotation - 75 RPM

Depth of oil - 25.5 cm

48
49
Figure 21. Drawing of Modified Double Two-bladed Propeller.

Stirrer 90% Pt 10% Rh

For

Large Crucible

15" (38.1 cm)

12-14" (30.5-35.6 cm)

12" (30.5 cm)

7-1/2" (19 cm)

27-1/2" (70 cm)

1-1/4" (3.2 cm)

1/2" (1,3 cm)

2" (5.1 cm)

3-1/2" (8.9 cm)

1/16" (0.15 cm)

6" (15.2 cm)

3-1/4" (8.3 cm)

1/4" (0.6 cm)

1/10" (0.25 cm)

1/4" (0.6 cm)

1/6" (0.4 cm)

2-3/8" (6.0 cm)

For

Laboratory Crucible

Depth 3-1/2" (8.9 cm)

Diameter 2-1/2" (6.4 cm)

Depth of Glass 3" (7.5 cm)

Stirrer Dimensions

K 1-3/4" (4.5 cm)

L* 14" (35.6 cm)

M 3/16" (0.48 cm)

N 3/16" (0.48 cm)

1/2" (1.3 cm)

P 7/8" (2.22 cm)

Q Does not apply-solid shaf

R 1-1/2" (3.8 cm)

S 5/8" (1.6 cm)

T 3/32" (0.24 cm)

U 1/16" (0.15 cm)

V 3/32" (0.24 cm)

W 1/16" (0.15 cm)

Y 21/32 (1.67 cm)

Approximate Weight

of

Stirrer

4200 g 150 g

* L - depends on furnace design.

50
oo -a

U-j CO

CO

1 rv

<=>

1 LU pfe

1

^ - ' CO

1— c=i

C=>

. . <t LLJ «=E

) CO U_l CC

rr7

51
Figure 22. Platinum-lined Tank and Stirring Rods.

This photograph shows the original Pt-lined tank with the three

compartments. The pots are interconnected and continuous: the first,

at the top, being the melting area; the second, the refiner; and the

third being the conditioning area where the glass is ready for discharge.

The stirring is conducted in pots two and three. The third pot has been

shortened as a direct result of these stirring experiments as previously

stated. The final design of the stirring rod, 90% Pt, 10% Rh, is shown

in the photograph.

52
53
Figure 23. Platinum Liner for Larger Crucible and Stirring Rods.

This size of liner was not only used for a pot in the experimental

glass tank but, also, for a pot used in unit type melting. Here about

20 liters of glass was usually melted, stirred, and cast into slab form.

This method of glass making has been used extensively to make special

types of glass. Again, stirring of the melt is very important and

requires a special stirrer. The stirrer design resulting from this

study satisfied these conditions and homogenous glass could be produced.

54
55
form NBS-114A <i-7i) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-771

2. Gov't Accession No.3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Some Experiments on the Stirring of Viscous Liquids

5. Publication Date May 1973

6. Performing Organization Code

7. AUTHOR(S)

Albert Napolitano

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. Project/Task/Work Unit No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Same as No. 9,

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The stirring actions of several designs of small stirrers were observed and photographed in transparent viscous model liquids. The

fluidity of the liquids was comparable to that of a molten borosilicate

glass at fining temperatures. The most efficient features of one or more

of these stirrers were incorporated into larger model stirrers and observed under actual stirring conditions. As a result of these performance

tests, a final design of stirrer was selected and fabricated of platinumrhodium metal and used in an experimental glass tank. The design of one

of the pots in the experimental glass tank was modified to obtain the

maximum stirring action with the selected stirrer.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Agitators; borosilicate glass; glass melt; glass tank;

liquids; stirrer; viscosity.

18. AVAILABILITY STATEMENT

fxl UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

60

22. Price

$.85 Domestic Postpaid

$.60 GP0 Bookstore

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