Mixing Tank: Side Entry, 3 Different Rotational Speeds

Side Entry Mixing Tank in 3 Different Rotational Speeds, ANSYS Fluent CFD Simulation Training

Description

This project simulates a side entry mixing tank at multiple rotational speeds using ANSYS Fluent. Mixing is a critical process across many industries — in oil and gas, for instance, water settling within storage tanks causes significant damage through corrosion, leakage, and eventual tank perforation. Side-entry mixing blades are commonly used to prevent this, since space limitations often prevent a blade from entering the tank from above. This simulation examines mixing performance at three rotational speeds — 400, 900, and 1400 rpm — using the mesh motion method.

The 3D geometry was designed in SpaceClaim, with a computational domain measuring 400 cm long, 400 cm wide, and 375 cm high. The domain was meshed in ANSYS Meshing using a relatively fine grid totaling 933,102 elements.

Methodology

Several assumptions were applied to the simulation: a pressure-based solver was used, the simulation was run as unsteady (transient), and gravity was included at -9.81 m/s² in the z-direction.

Key simulation settings included:

• Viscous model: Standard k-epsilon with standard wall functions

• Multiphase model: VOF, with three phases — air, oil, and water

• Boundary conditions: Stationary tank walls, with frame motion enabled on the fluid at rotational velocities of 400, 900, and 1400 rpm across the three separate cases

• Solution methods: SIMPLE pressure-velocity coupling, PRESTO! for pressure discretization, second-order upwind for momentum, and first-order upwind for turbulent kinetic energy and dissipation rate

• Initialization: Standard method

Conclusion

The mixing tank contains both water and oil, with water settling toward the tank bottom due to its higher density prior to mixing. The side mixer was used to blend the tank's contents across all three rotational speeds, with mixing completeness tracked using three sensors positioned at different locations within the tank, monitoring the oil and water volume fractions at each point over time.

These results showed clear differences in mixing time across the three speeds: 100 seconds at 400 rpm, 14 seconds at 900 rpm, and 8 seconds at 1400 rpm. This trend reveals that increasing rotational speed up to a certain point (around 900 rpm) delivers substantial mixing time reduction, but pushing speed higher still yields comparatively little further benefit — a finding directly relevant to selecting an appropriately sized mixer motor without over-specifying rotational speed and unnecessarily increasing equipment cost.