Investigation of the effect of impeller rotation on the mixing of iron powder particles in a two-phase flow of water and iron powder, ANSYS Fluent Training

A mixing tank typically consists of a cylindrical vessel and one or more impellers. The impellers are driven by an external motor that rotates them. When the impellers rotate, they create a flow of fluid in the tank. This flow can be axial (up and down) or radial (outwards towards the wall of the tank), depending on the type of impeller used. The fluid flow causes the substances in the tank to move and collide with each other leading to their mixing. The closed mixing tank contains water liquid while the impeller starts rotating with 120 Revolutions per minute angular velocity, creating a massive vortex in the middle of the tank.

The geometry of the present model is three-dimensional and has been designed using ANSYS SpaceClaim software. We do the meshing of the present model with ANSYS Meshing software. The mesh type is unstructured, and the element number is about 1,260,000. Around the shaft and impeller, a boundary layer mesh with 5 layers is applied so that y+ does not get too high in these sensitive areas.

Methodology

A pressure-based, transient solver was employed to capture the interaction between the water and iron powder phases over time in the mixing tank. Iron powder particles with a density of 7800 (kg/m^3) and diameter of 10 micrometers were used. The realizable k-epsilon turbulence model was used because it provides improved predictions for flows with strong streamline curvature, vortices, and rotation. A suitable wall function was implemented to accurately resolve the near-wall region, which is essential for capturing detailed interactions between fluid flow and particle dynamics near the impeller walls. Additionally, appropriate forces and models were considered to enhance the accuracy of the Eulerian multiphase model settings used in the analysis. The Moving Reference Frame (MRF) method was used to simulate the rotation of the impeller.

Results and Conclusion

From the simulation, the following results are obtained:

• In the case of pressure, it is visible that the amount of water pressure in front of the impeller is much more than the amount behind it.
• water axial and radial velocity contours from above and the water velocity magnitude from front are shown in the figures. As was expected, the speed of the flow front of the impeller is more than in other areas in the domain.
• From the y+ contours, it is evident that the y+ values of the rotating components (shaft and impeller) are approximately in the range of 0 to 7.3. This indicates that the chosen wall function is functioning correctly, with the y+ values falling within the acceptable range for its application.
• The radial velocity of water versus position on a line along the y-axis near the impeller is shown in the figures. As can be seen from the graph, the maximum radial velocity of the water occurs approximately halfway through the impeller blade and is about 0.048 (m/s). It should be noted that the negative values ​​of the velocity in this graph indicate the direction of the radial velocity vector, meaning that at those locations the radial velocity vector is pointing toward the center of the impeller, not outwards.
• The moment on the impeller versus time graph is shown in the figures. We see that the moment on the impeller increases significantly (up to about 0.00215 N.m) in the initial moments of the solution (up to about 0.08 s). After that, until about 1.7 (s), the moment on the impeller decreases and again takes a constant value of 0.0002 (N.m). This initial jump in moment is because the impeller starts rotating from rest.