CFD Analysis of Mixing in a

Digester Comparing the cases with modified angles of mixers

1

Background and purpose

The study is done for a digester equipped with 5 flygt mixers.

The purpose of the CFD analysis is to study the effect of the

mixers orientation on the size of the low velocity region in the

middle of the tank.

2

Fluid Properties

3

• Setup 5 – compact

• 3 x 4680

• 0,77 m diameter

• Thrust = 4300 N

• Torque = 400 Nm

• Setup 5 – banana blade

• 2 x 4460

• 2,5 m diameter

• Thrust = 3600 N

• Torque = 1200 Nm

Mixer Selection

• Rheology – Herschel-Bulkley

• Viscosity evaluation: s = 5 + 5*g.0.5, g.0 = 5

Thrust and torque values are modified according to the fluid

properties.

Simulated geometry

4

Banana Blade Mixer

4460

Banana Blade Mixer

4460

Compact Mixer

4680

Compact Mixer

4680

Compact Mixer

4680

Outlet

Inlet

Original Configuration

5

72 ̊

30 ̊ 6

.5 m

1.6

75

m

3.7

5 m

30 ̊

Both Compact and Banana mixers oriented 30 degrees off from

the center.

30 ̊

30 ̊

30 ̊

Top View Side view

Case 1 Configuration

6

72 ̊

30 ̊ 6

.5 m

1.6

75

m

3.7

5 m

10 ̊

10 ̊

30 ̊

30 ̊

Two of the Compact mixers are 10 degrees off from the center.

Top View Side view

Case 2 Configuration

7

72 ̊

10 ̊ 6

.5 m

1.6

75

m

3.7

5 m

30 ̊

30 ̊

10 ̊

30 ̊

Banana blade mixers are 10 degrees off from the center.

Top View Side view

8

Comparison of the new cases with the original one.

Results

In the whole tank Original Case Case 1 Case 2

Velocity magnitude < 0.1 m/s 15.5 % 18.1 % 14.5 %

Velocity magnitude < 0.05 m/s 7.0 % 8.1 % 6.0 %

In the center part* Original Case Case 1 Case 2

Velocity magnitude < 0.1 m/s 27.1 % 27.3 % 21.6 %

Velocity magnitude < 0.05 m/s 11.3 % 11.8 % 7.4 %

* Within the radius of 10 meters from the center of the tank.

Results of the Modified Cases

9

Streamlines colored by velocity magnitude [m/s]

Case 1 Case 2

10

Surfaces for three levels of velocity

1, 0.2 and 0.1 m/s

Results

Case 1 Case 2

11

Results

Velocity profile on surface [m/s]

Case 1 Case 2

12

Results

Velocity profile [m/s] at h=6.5 m

Case 1 Case 2

13

Results

Velocity profile [m/s] at h=3.75 m

Case 1 Case 2

14

Results

Velocity profile [m/s] at h=1.675 m

Case 1 Case 2

15

Conclusions

• As it is reported in this presentation, reorienting the Banana Blade

Mixers is helping to decrease the extent of the region with low velocity.

• Reorientation of the compact mixers is not improving the situation.