UNIT OPERATION LABORATORY 2(CCB 2092)EXPERIMENT 8:

FLUID MIXINGGROUP MEMBERS : (GROUP 11)1) SEAN SURAJ JEREMIAH (14286)

2) SUHAIQAH SYARFINAZ BINTI MAT SAID (15508)

3) SITI AISHAH BINTI KHALED (14306)

4) SYAZANA FITRI BINTI ROZIAEDY (14568)

5) TENGKU SYAZA AMALINA BINTI TENGKU AZAM (14356)DATE OF SUBMISSION : 25 JUNE 2012Title :Determining the fluid flow patterns in mixing tanks.Objectives :

1) To determine the fluid flow pattern in a mixing tank a. with and without baffles.b. with different impeller positionsc. with different types of impellers.Theory :

Large liquid-circulation loops developed in stirred vessels make mixing performance poor. For mixing to be effective, fluid circulated by the impeller must sweep the entire vessel in a reasonable time. In addition, the velocity of fluid leaving the impeller must be sufficient to carry material into the most remote parts of the tank. Turbulence must also be developed in the fluid; mixing is certain to be poor unless flow in the tank is turbulent. All these factors are important in mixing, which can be described as a combination of three physical processes: distribution, dispersion and diffusion.The process whereby materials are transported to all regions of the vessel by bulk circulation currents is called distribution. Distribution is an important process in mixing, but can be relatively slow. In large tank, the size of the circulation paths is also large and the time taken to traverse them is long; this, together with the regularity of liquid pumping at the impeller, inhibits rapid mixing. Accordingly, distribution is often the slowest step in the mixing process. However, if the rotational speed of the impeller is sufficiently high, superimposed on the distribution process is turbulence. Turbulence flow occurs when fluid no longer travels along streamlines but moves erratically in the form of cross-currents.

The kinetic energy of turbulent fluid is directed into regions of rotational flow called eddies; masses of eddies of various size coexist during turbulent flow. Large eddies are continuously formed by action of the stirrer; these break down into small eddies which produce even smaller eddies. Eddies, like spinning tops, posses kinetic energy. When the eddies become so small that they can no longer sustain rotational motion, their kinetic energy is dissipated as heat.

The process of breaking up bulk flow into smaller and smaller eddies is called dispersion; dispersion facilitates rapid transfer of material throughout the vessel. The degree of homogeneity as a result of dispersion is limited by the size of the smallest eddies which may be formed in a particular fluid.

This size is given approximately as the Kolmogorov scale of mixing, or scale of turbulence, .

Within eddies there is little mixing because rotating flow occurs in streamlines. Therefore, to achieve mixing on a scale smaller than the Kolmogorov scale, we must rely on diffusion. Molecular diffusion is generally regarded as a slow process, however, over small distances it can be accomplished quite rapidly. Within eddies of 30~100 m diameter, homogeneity is achieved in about 1 s for low-viscosity fluids. Consequently, if power input to a stirred vessel produces eddies of this dimension, mixing on a molecular scale is accomplished virtually simultaneously.

The figure shows the types of impellers used in the experiment, a propeller (left) and a flat blade disk turbine (right).

The picture shows the flow pattern in a unbaffled tank. Circular flow is formed around the shaft and a vortex is formed at the centre.

Pictured above is flow in a baffled tanks. Left axial flow, flow is parallel to the axis. Right radial flow, flow is mainly around the axis of rotation.

The picture illustrates the fluid flow around a baffle. Cavitation occurs around the region around the wall of the tank obstructed by the baffle.Results :

YELLOW BEADS - SHROUDED FLAT BLADE

REFER TO APPENDIX I

RED BEADS - DISC MOUNTED FLAT BLADE REFER TO APPENDIX II

Discussion

Determination of various flow patterns exhibited by different types of impellers (Mixing tank without baffle) In this experiment, two types of impeller (shrouded flat blade and disc mounted flat blade) are used to study the pattern of fluid flow without the use of baffle.

As the speed of the rotating impeller increases, the movement of the yellow beads increases in speed while circular pattern is maintained. This applies for both of the impellers

From this experiment, it is observed that the diameter of the circular motion of the sand particle is smaller when shrouded flat blade is used, as compared to that of disc mounted flat blade

Determination of various flow patterns exhibited by different types of impeller (Mixing tank without baffle) In this experiment, two types of impeller (shrouded flat blade and disc mounted flat blade) are used to study the pattern of fluid flow with the use of baffle.

As the speed of the rotating impeller increases, the movement of the yellow beads increases in speed. Circular pattern is only maintained at speed 100 rpm. At 300 rpm, the sand particles begin to swirl in turbulent motion, and this effect is more pronounced at 500 rpm. This applies for both of the impellers

From this experiment, it is observed the motion of the sand particle in the disc mounted flat blade is much more disordered at the speed increases, as compared to that of shrouded flat blade.

Determination of the influence of impeller position to rate of dispersion of solid in liquid (tank without baffle)

In this experiment, two types of impellers (shrouded flat blade and disc mounted flat blade) are used to study the flow pattern of the liquid in the tank without the baffle through the beads using various positions and speedShrouded flat blade When the impeller is set up to its initial position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The motion of sand particle remains in circular or ordered pattern regardless of the speed. No dead spot is observed. When the impeller is set up to be lower than its original position, the lifting capacity drastically increases as the speed of the rotating impeller is increased. The motion of the sand particle remains in a circular motion regardless of the speed. No dead spot is observed. When the impeller is set up to be higher than its original position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The motion of sand particle remains in circular or ordered pattern regardless of the speed. No dead spot is observed. When the impeller is positioned on the side of the tank, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The motion of the sand particle is circular regardless of the speed. No dead spot is observed. When the impeller is set to an inclined position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The motion of sand particle remains in circular or ordered pattern regardless of the speed though as the speed increases the sand particle move nearer to the bottom of the impeller. No dead spot is observed.Disc mounted flat blade When the impeller is set up to its initial position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The motion of the sand particle remains in an orderly pattern at speed 100 rpm, but the speed reaches 500 rpm, the sand particle begins to swirl around in bigger diameter. No dead spot is observed. When the impeller is set up to be lower than its original position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The motion of the sand particle remains in an orderly pattern at speed 100 rpm, but the speed reaches 500 rpm, the sand particle begins to swirl around in bigger diameter. No dead spot is observed. When the impeller is set up to be higher than its original position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The motion of the sand particle remains in an orderly pattern at speed 100 rpm, but the speed reaches 500 rpm, the sand particle begins to swirl around in bigger diameter. No dead spot is observed. When the impeller is positioned on the side of the tank, the lifting capacity fluctuates as the as the speed of the rotating impeller is increased. The motion of sand particle remains in circular or ordered pattern regardless of the speed. No dead spot is observed. When the impeller is set to an inclined position, the lifting capacity fluctuates as the speed of the rotating impeller is increased. The motion of sand particle remains in circular or ordered pattern regardless of the speed but as the speed reaches 500 rpm, the sand particle begin to swirl in turbulent motion. No dead spot is observed. From this experiment, it is observed that for this application, the best configuration is disc mounted flat blade, in an upright position, 100mm apart from the bottom of the tank, at speed 500 rpm, for it can induce turbulent flow and even mixing.Determination of the influence of impeller position to rate of dispersion of solid in liquid (tank with baffle) In this experiment, two types of impellers (shrouded flat blade and disc mounted flat blade) are used to study the flow pattern of the liquid in the tank with the baffle through the beads using various positions and speedShrouded flat blade When the impeller is set up to its initial position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The sand particles move in turbulent motion regardless of the speed. Dead spots are more noticeable as the speed increases. When the impeller is set up to be lower than its original position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The sand particles move in turbulent motion regardless of the speed. Dead spots are more noticeable as the speed increases. When the impeller is set up to be higher than its original position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The sand particles move in turbulent motion regardless of the speed. Dead spots are more noticeable as the speed increases. When the impeller is positioned at the side of tank, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The sand particles move in turbulent motion regardless of the speed. Dead spots are more noticeable as the speed increases. When the impeller is positioned at an angle, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The sand particles move in turbulent motion regardless of the speed. Dead spots are more noticeable as the speed increasesDisc mounted flat blade When the impeller is set up to at its original position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. At 100 rpm, the sand particles move in circular while at 300 rpm and 500 rpm, the sand particles move in turbulent motion regardless of the speed. Dead spots are more noticeable as the speed increases. When the impeller is set up to be lower than its original position, the lifting capacity progressively increases as the speed of the rotating impeller is increased. At 100 rpm, the sand particles move in circular while at 300 rpm and 500 rpm, the sand particles move in turbulent motion regardless of the speed. Dead spots are more noticeable as the speed increases When the impeller is set up to be higher than its original position, the lifting capacity drastically increases as the speed of the rotating impeller is increased. At 100 rpm, the sand particles move in circular while at 300 rpm and 500 rpm, the sand particles move in turbulent motion Dead spots are more noticeable as the speed increases. When the impeller is positioned at the side of the tank, the lifting capacity progressively increases as the speed of the rotating impeller is increased. The motion of sand particle remains in circular or ordered pattern regardless of the speed. Dead spot starts to appear when speed reaches 300 rpm and increases in size as the speed increases. When the impeller is positioned at an angle, the lifting capacity progressively increases as the speed of the rotating impeller is increased. At 100 rpm, the sand particles move in circular while at 300 rpm and 500 rpm, the sand particles move in turbulent motion regardless of the speed. Dead spots are more noticeable as the speed increases From this experiment, the best configuration for this application is either disc mounted flat blade or shrouded flat blade impeller, at any position as long as the speed is between 300 rpm and 500 rpm, in order to minimize dead spots, while inducing turbulence for an effective mixing.Conclusion

From this experiment, we can conclude that the pattern flow of water is affected by type, positions of the impellers as well as the presence of baffle. As for the speed, at 100 rpm, there is not much of movement of the beads is observed. The beads accumulate at the circumference of the mixing tanks bottom. This is because the radially swirling flow pushes away the beads far to the circumference of the tank. The movement of the agitator does not produce significant mixing of the fluid. There is also no significant lifting capacity at lower RPM of the impeller. However, as the RPM is increased, the beads concentrate at the bottom middle of the tank. This is because the current is most intensified at the center of the tank. At 300 to 500 rpm, the flow of the water and movement of beads start to increase. It shows the relation when the speed is increase, the flow also will increase. The patterns of the beads movements starting to be change slowly from radial pattern flows to axial pattern flows.

APPENDIX IYELLOW BEADS - SHROUDED FLAT BLADE

(Higher = 150 mm ; Normal = 100 mm ; Lower = 50 mm)

1) 150mm without baffle

0 rpm

100 rpm

300 rpm

500 rpm

2) 150mm with baffle

0 rpm

100 rpm

300 rpm

500 rpm

3) 50 mm with baffle

0 rpm 100 rpm

300 rpm 500 rpm

4) 100 mm with baffle

0 rpm 100 rpm

300 rpm 500 rpm

5) 100 mm with baffle (right/left hand side)

0 rpm 100 rpm

300 rpm 500 rpm

6) 100 mm with baffle (inclined)

0 rpm 100 rpm

300 rpm 500 rpm

7) 50 mm without baffle

0 rpm 100 rpm

300 rpm 500 rpm

8) 150 mm without baffle

0 rpm 100 rpm

300 rpm 500 rpm

9) 100 mm without baffle

0 rpm 100 rpm

300 rpm 500 rpm

10) 100 mm without baffle (right/left hand side)

0 rpm 100 rpm

300 rpm 500 rpm

11) 100 mm without baffle (inclined)

0 rpm 100 rpm

300 rpm 500 rpm

APPENDIX 2RED BEADS - DISC MOUNTED FLAT BLADE (Higher = 100 mm ; Normal = 75 mm ; Lower = 50 mm)1) 100mm without baffle0 rpm

100rpm

300rpm

500 rpm

2) 75mm without baffle

0 rpm

100 rpm

300 rpm

500 rpm

3) 50mm without baffle

0 rpm

100 rpm

300 rpm

500 rpm

4) 75mm without baffle

0 rpm

100 rpm

300 rpm

500 rpm

5) 75mm without baffle (inclined)0 rpm

100 rpm

300 rpm

500 rpm

6) 100mm with baffle0 rpm

100 rpm

300 rpm

500 rpm

7) 75mm with baffle

0 rpm

100 rpm

300 rpm

500 rpm

8) 50mm with baffle

0 rpm

100 rpm

300 rpm

500 rpm

9) 75mm with baffle (right/left hand side)

0 rpm

100 rpm

300 rpm

500 rpm

10) 75mm with baffle (inclined)

0 rpm

100 rpm

300 rpm

500 rpm

3

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