Gas & Petrochemical: Tank Charge, 2-Phase Flow

Tank Charge Between Two Reservoirs (Two-Phase) — ANSYS Fluent CFD Simulation

This project models the filling, or "charge," of a tank between two equal-height reservoirs using ANSYS Fluent. As water advances from one reservoir into the air-filled one, the two fluids exchange places — water flows in while air rises out — until the connected system settles into balance. A two-phase VOF approach captures the water–air interaction, reflecting the kind of phase separation and transfer operations that are common in chemical and petrochemical processing.

The geometry consists of two 2-D reservoirs, each 1.25 × 2.5 m, built in Design Modeler and meshed in ANSYS Meshing with a structured grid of 32,510 cells.

The simulation runs as a pressure-based, transient case with gravity enabled (−9.81 m/s² along Y). The water and air are tracked with the VOF model using two phases (air as primary, water as secondary), a sharp interface, and implicit formulation. Turbulence is handled with the realizable k-ε model and standard wall functions. Both the inlet and outlet vents are set to 0 Pa gauge pressure, so the flow is driven purely by gravity and the pressure imbalance between the reservoirs rather than by a forced inlet velocity — which is what makes this a natural transfer problem rather than a pumped one. Pressure–velocity coupling uses the Coupled scheme, with PRESTO! for pressure and a Compressive scheme for the volume fraction to keep the interface crisp. The case is initialized with the water region patched to a volume fraction of 1, then advanced with a 0.001 s time step over 10,000 steps.

At the end of the solution, you generate 2-D contours of volume fraction, pressure, velocity, and turbulent kinetic energy, along with an animation of the transfer. The animation shows air rising as the water advances toward the air-filled tank. After several seconds, the system approaches hydrostatic balance — equal pressure at equal elevations across the two connected reservoirs — confirming the transfer reaches equilibrium as expected. By the end of this project, you'll be able to set up a transient gravity-driven VOF simulation, configure pressure-vent boundaries for a natural transfer process, patch initial phase distributions, and interpret how a two-phase system evolves toward hydrostatic equilibrium.