Mixing Tank Containing Iron Powder: Transient Solver

Mixing Tank Containing Iron Powder, Transient CFD Simulation by ANSYS Fluent

Description

A mixing tank typically consists of a cylindrical vessel fitted with one or more impellers, driven by an external motor. As the impellers rotate, they generate fluid flow within the tank — either axial (moving up and down) or radial (moving outward toward the tank wall), depending on the impeller type used. This flow causes the substances within the tank to move and collide, driving the overall mixing process.

This project investigates the effect of impeller rotation on the mixing of iron powder particles within a two-phase flow of water and iron powder. The closed mixing tank contains liquid water, with the impeller rotating at 120 rpm, generating a substantial vortex at the center of the tank.

The 3D geometry was designed in SpaceClaim and meshed in ANSYS Meshing using an unstructured grid totaling approximately 1,260,000 elements. A boundary layer mesh with 5 layers was applied around the shaft and impeller to keep y+ values within an acceptable range in these sensitive, high-gradient regions.

Methodology

A pressure-based, transient solver was used to capture the evolving interaction between the water and iron powder phases over time. The iron powder particles were defined with a density of 7800 kg/m³ and a diameter of 10 micrometers. Turbulence was resolved using the Realizable k-epsilon model, chosen for its improved accuracy in flows involving strong streamline curvature, vortices, and rotation, paired with an appropriate wall function to accurately resolve near-wall behavior around the impeller. The Eulerian multiphase model was applied with suitable interphase forces to capture the coupled fluid-particle dynamics, while impeller rotation itself was modeled using the Moving Reference Frame (MRF) method.

Conclusion

The results reveal several key behaviors. Pressure contours show substantially higher water pressure ahead of the impeller compared to behind it, while axial and radial velocity contours confirm that flow speed directly in front of the impeller exceeds that found elsewhere in the domain.

The y+ contours on the rotating components (shaft and impeller) fall within approximately 0 to 7.3 — confirming the selected wall function is operating correctly within its intended range. Radial velocity plotted along a line near the impeller shows a peak value of roughly 0.048 m/s occurring approximately halfway along the impeller blade, with negative values elsewhere indicating flow directed inward toward the impeller's center rather than outward.

The moment on the impeller over time shows a sharp initial rise, peaking at approximately 0.00215 N·m within the first 0.08 seconds — a direct consequence of the impeller accelerating from rest — before settling into a decline that stabilizes at a constant value of approximately 0.0002 N·m by around 1.7 seconds, reflecting the system's transition from startup transient behavior to steady rotational mixing.