exp 4-fuid mixing

7/30/2019 Exp 4-Fuid Mixing

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1

ABSTRACT/SUMMARY

In this experiment, students were asked to do an experiment with two different

objectives. For the first objective (Experiment 1), students need to determine the flow pattern

when using the different type of impeller with and without the use of baffles. For this objective,

three types of impellers which are turbine, flat blade and propeller are used. Students need to

observe the flow patterns when using the impeller with and without baffles. For the second

objective (Experiment 2), students need to determine the power consumed by a mixer that varies

with speed and the inclusion of baffles. Students need to use hydraulic oil as the fluid to be filled

in the tank. Then, it is needed to determine its power and its rotation speed with and without

baffle. At the end of the Experiment 1, it can be observed that different impellers produce

different type of flow patterns in a baffled tank. In a baffled tank, flat paddle and turbine impeller

produce radial flow pattern whereas screw propeller produces axial flow pattern. In an unbaffled

tank, three types of impellers produced the same flow pattern which is tangential flow pattern.

For Experiment 2, it can be seen that power consumed increases as the speed increases. It also

can be seen that the power consumed in a baffled tank is higher than an unbaffled tank.



2

INTRODUCTION

Mixing of liquid-liquid or solid-liquid system is a complex operation to analyze and

subject to many variables. The choice of mixer for a particular application depends on the degree

of bulk movement or shear mixing required by the process. In order to predict full-scale

requirements, it is usual to model the system and apply dimensional analysis.

Before the dimensional analysis can be used, three conditions must apply:

1. Geometric similarity – This will define the boundary conditions, corresponding

dimensions will have the same ratio.

2. Kinematics similarity – This requires that velocities at corresponding points must have

the same ratio ac those at other corresponding points.

3. Dynamic similarity – This requires that the ratio of forces at corresponding points is

equal to that at other corresponding points.

The modes of flow behavior exist in a mixer laminar and turbulent flow. Both these flow

conditions may be described dimensionally but for turbulent flow its behavior is less significant.

In particular, the power number becomes independent of Reynold’s number beyond a certain

turbulence range. A further factor to consider is surface waves, which are, described by the

Froude number group. In a mixer this phenomena is usually function of the height of the vortex,

which forms. Arm field have developed a model mixer, which can be used to predict the power

consumption of a full-sized mixer by equating Reynold’s number and Froude number. The effect

of placing baffles in the mixer vessel is also investigated. In this experiment, we used liquid

mixing apparatus (MB 23) as our operating machine.



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Figure 1 : Liquid Mixing Apparatus (MB 23)



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AIMS/OBJECTIVE

Experiment 1 :

The objective of this experiment is to observe the flow patterns that can be achieved by the use

of different impellers with and without the use of baffles.

Experiment 2 :

The objective of this experiment is to show how the power consumed by a mixer varies with

speed, types of impeller and with the inclusion of baffles.



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THEORY

An impeller is a rotating component of a centrifugal pump which transfer energy from the

motor that drives the pump to the fluid being pumped by accelerating the fluid outwards from the

center of rotation. The velocity achieved by the impeller transfers into pressure when the

outward movement of the fluid is confined by the pump casing. Impeller are usually short

cylinders with an open inlet (called an eye) to accept incoming fluid, vanes to push the fluid

radially, and splined center to accept a driveshaft. There are three types of mixing flow patterns

that are markedly different. The so-called axial-flow turbines actually give a flow coming off the

impeller of approximately 45 degree and therefore have a recirculation pattern coming back into

the impeller at the hub region of the blades.

Axial flow impellers include all impellers in which the blade makes an angle of less than

90 degree with the plane of rotation. They run at the highest efficiency and they have the lowest

NPSH requirement. They require the highest power requirement at shut off, so they are normally

started with the discharge valve open. Axial flow impellers may also be mounted near the bottom

of the cylindrical wall of the vessel.

Radial flow impellers have blades which are parallel to the axis of the drive shaft. The

smaller multiblade ones are known as turbines; larger, slower-speed impeller with two or four

blades are often called paddles. The diameter f a turbines is normally between 0.3 and 0.6 of the

tank diameter. They should be specified for high head and low flow conditions.

As we know, baffles are needed to stop the swirl in a mixing tank. Almost all the impeller

rotate in the clockwise or counter clockwise direction. Without baffles, the tangential velocity

coming from any impeller causes the entire fluid mass to spin. Most common baffles are straight

flat plate of metal that run along the straight side of vertically oriented cylindrical tank or vessel.

For unbaffles tank, there is a tendency for a swirling flow pattern to develop regardless of the type of impeller. A vortex is produced owing to centrifugal force acting on the rotating

liquid. However, there is a limit to the rotational speed that may be used, since one the vortex

reaches the impeller, severe air entrainment may occur. In addition, the swirling mass of liquid

often generates an oscilating surge in the tank, which coupled with the deep vortex may create a

large fluctuating force acting on the mixer shaft.



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For baffles tank, for vigorous agitation of thin suspensions, the tank is provided with

baffles which flat vertical strips set radially along the tanks wall as shown in figure 1. Four

baffles are almost always adequate. A common baffle width is 1:10 to 1:12 of the tanks diameter.

For Reynolds number greater than 10,000, baffles are commonly used with turbine impellers and

with on-centerline axial-flow impellers.

In the transition region (Reynolds number, from 10 to 10,000), the width of the baffles

may be reduced, often to ½ of standard width. If the circulation pattern is satisfactory when the

tank is unbaffled but a vortex creates a problem, partial length baffles may be used. These are

standard width and extend downward from the surface into about1/3 of the liquid volume.

In the region of laminar flow (NRe< 10), the same power is consumed by the impeller

whether baffles are present or not, and they are seldom required. The flow pattern may be

affected by the baffles but not always advantageously. When they are need, the baffles are

usually placed one or two widths radially, to allow fluid to circulate behind them and at same

time produce some axial deflection of flow.

Figure 2 : Flow pattern of agitation by flat paddle



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Figure 3 : (a) Flow pattern of agitation by propeller

(b) Flow pattern of agitation by turbine



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APPARATUS & MATERIALS

Liquid Mixing Apparatus (Model: MB 23)

Water

Plastic pellets

Flat paddle

Tank

Baffle

Screw propeller

Force indicator

Turbine

impeller

Speed

controller

Hydraulic Oil



9

PROCEDURE

General Start-up procedure

1. The power outlet is switched on.

2. All the tightening screws is fastened.

3. The working surrounding area is ensured to be dry and clean.

4. The shaft is lifted up using lifting chain attached to the shaft.

5. The experiment is carried out.

General shut-down procedure

1. Any liquid inside the tank is removed by opening the outlet valve

2.

The tank is washed and rinsed to make sure no oil residue after the experiment.3. The paddle/impeller inside the tank is removed and washed after use.

4. The power outlet is shut down.

Experiment 1

1. The tank is filled with water up to a depth of 30L.

2. Flat paddle is attached with the end of the shaft.

3. A small quantity of plastic pellet is added to the tank.

4. The speed of the impeller is turned up in small increments: 50 rpm, 100 rpm, 150m rpm

and 200 rpm. The pellets are seen to swirl around in the water showing flow patterns.

5. The movement of the pellets and the flow pattern is observed and drawn.

6. The procedures are repeated by replacing the flat paddle with other impellers : turbine

impeller and screw propeller.

7. The procedures are also repeated with the baffles fitted in the tank with each flat paddle,

turbine impeller and screw propeller.



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Experiment 2

1. The tank filled with hydraulic oil up to a depth of 30 L

2. Flat paddle is attached with the end of the shaft.

3. The speed of the impeller is turned up to 50 rpm and the reading of force is recorded.

4. The speed is then turned up to 100 rpm, 150 rpm and 200 rpm with the force recorded at

the respective speed.

5. Step 3-4 is repeated with the baffles fitted in the tank.

6. The power consumed for each of the speed is calculated.



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RESULTS

EXPERIMENT 1: Observation of the flow patterns

WITH BAFFLES

Flat Paddle Screw Propeller Turbine Blade

WITHOUT BAFFLES

Flat Paddle Screw Propeller Turbine Blade

Notes : All of the flow patterns drawn are viewed from bottom view.



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EXPERIMENT 2

WITHOUT BAFFLES

Flat paddles blade

Angular speed(rpm)

Angular speed,

(rads-

Force, F(N)

Torque, T(Nm)

Power, W(watts)

50 5.236 0.00 0.0000 0.0000

100 10.472 0.02 0.0022 0.0230

150 15.708 0.19 0.0209 0.3283

200 20.944 0.42 0.0462 0.9676

WITH BAFFLES

Flat paddles blade

Angular speed

(rpm)Angular speed,

(rads-

Force, F

(N)

Torque, T

(Nm)

Power, W

(watts)

50 5.236 0.00 0.0000 0.0000

100 10.472 0.22 0.0242 0.2534

150 15.708 0.60 0.0660 1.0367

200 20.944 1.10 0.1210 2.5342



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Graph of power consumption (W) vs speed (rad/s)

-0.5

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25

P o w e r ( w a t t s )

Speed (rads/s)

Graph of Power (W) vs Speed (rads/s)

without baffles

with baffles



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SAMPLE OF CALCULATIONS

(Flat paddles without baffle at 200 rpm)

Angular seed, = rpm

= 200 rpm

= 20.944 rads-1

Torque arm, r = 0.11m

Torque, T = F r

= 0.42 0.11 m

= 0.0462 Nm

Power, W = T

= 0.0462 20.944

= 0.9676 Watts.



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DISCUSSIONS

This experiment is divided into two parts: Experiment 1 and Experiment 2 with two

different objectives. Experiment 1 is conducted to observe the flow patterns that be achieved by

the use of different impellers with and without the use of baffles. Whereas Experiment 2 is

conducted to show how the power consumed by a mixer varies with speed and the inclusion of

baffle. To achieve these objectives, liquid mixing apparatus (model:MB23) is used.

In Experiment 1, the flow patterns of water with the aid of the plastic pellets are

observed. Six set of flow patterns are drawn with the use of 3 different impellers with and

without the baffle. These impellers are actually the agitator for mixing the substances. Agitation

and swirling are the method for combining the compounds. Therefore, flow patterns of the water

depend on the type of impeller/agitator used. There are three different types of flow patterns

which are axial, radial and tangential.

Based on the flow patterns drawn, it can be observed that all three types of impellers

without the inclusion baffles produced the same flow pattern. That is, a circular and rotary

motion. Another observation seen during the experiment is the production of the deep vortex in

an unbaffled tank. These characteristics can be related to the tangential flow pattern. Thus,

tangential flow patterns are produced in an unbaffled tank. Tangential flow patterns offer very

little mixing because of the small velocity gradients.

With the inclusion of baffle, the use of flat paddle and turbine blade as the impeller

produced the same flow pattern. At the corners of the tank near the baffle, it can be seen that the

pellets are flowing parallel to the impeller and then towards the tank wall. The flow is observed

to have actually split into two streams which are then continuing flowing along the tank wall and

back to the impeller. The flow pattern produced is radial. This makes the flat paddle and turbine

the radial-flow impellers.



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CONCLUSIONS

From the observation in experiment 1, it can be concluded that different impellers

produce different type of flow patterns in a baffled tank. In a baffled tank, Flat paddle and

turbine impeller produced radial flow pattern whereas screw propeller produced axial flow

pattern. For a tank without the inclusion of baffle, it can be concluded that all three type of

impellers produced the same flow pattern which is tangential flow pattern. Based on the results

and the graph obtained for Experiment 2, it can be seen that power consumed increases as the

speed increases. It also can be seen that the power consumed in a baffled tank is higher than an

unbaffled tank.

Even though the objective of the experiment is reached, a lot of improvements must be

considered to overcome the experimental problems and obtain the best results throughout this

experiment. For example, ensure that the equipment is in a proper operating good condition as

the unit has been used at many times and the tank should be cleaned thoroughly especially after

conducting the experiment to prevent the oil and water from mixing. It is also necessary to make

sure the balance is operating in a good condition to avoid experiencing problems with the

reading of the force balance. Moreover, ensure that the eyes are perpendicular to the scale of the

tank during refill the tank with water to avoid parallax error and different color plastic pellet

must be used to detect flow patterns easily. Also, two tanks should be provided by the laboratory

so that the water and oil can be prevented from mixing.



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RECOMMENDATIONS

Due to the experiment that has been done, there are few recommendations that should be

considered to get the best results needed throughout in this experiment. First and the foremost,

ensure that the equipment is in a proper operating good condition. It is suggested to repair and

always do some maintenance for this equipment when the unit has been used at many times.

Since this experiment should be done with water source and coagulant or oil as a fluid,

the tank should be cleaned thoroughly especially after conducting the experiment with oil. This

is because to prevent the high concentration of coagulant or oil to be attached the wall of the

tank. This is also to prevent the oil and water from mixing. Therefore, the tank can be used again

when conducting the next experiment.

Other than that, it is necessary to make sure the balance is operating in a good condition.

If the balance is not operating very well, it will affect the reading for the force balance regarding

to each speed of impellers. It is not the right way to measure the force by operating the tension

spring manually, it is much better to use the tension spring that operated by computer itself.

Indirectly, the accurate readings can be obtained as the best results needed.

Furthermore, ensure that the eyes must be perpendicular to the scale of the tank during

refill the tank with water to avoid parallax error. Besides that, always make sure that the water

does not spill over the side of the tank by increasing the speed too much. In addition, several

different colors of plastic pallets can be used, so the flow pattern can be detected easily. Because

of that, the movement of the plastic pellets can be seen clearly.

Last but not least, it is much better to suggest that two tanks should be provided by the

laboratory. One of them is for water and the remaining one for coagulant or oil. This is to ensure

that the water and oil can be prevented from mixing. The mixture between water and oil can be

happened if only one tank provided and used the same tank to conduct the experiment. This willaffect the data as well as inaccurate readings when conducting this experiment.



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REFERENCES

[1] Douglas J.F Fluid mechanics fourth edition, Prentice Hall 2004

[] Perry’s Chemical Engineer Handbook, 8th

Edition, Don W. Green, Robert H. Perry,18-6 –

18-13.

[3] Frank M.W, Fluid Mechanics ninth edition, McGraw-Hill.2005

[4] www.sciencedirect.com



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APPENDICES

1. Liquid Mixing Apparatus

Liquid Mixing Equipment (Model : MB23)

Control Panel



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2. Type of Impellers

(Flat Paddle)

(Screw Propeller)

(Turbine Impeller)



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Baffle Hydraulic Oil



23



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exp 4-fuid mixing

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