Introduction

Two-phase flows occur when two distinct thermodynamic phases—for instance liquid + gas or liquid + solid—share a common domain and continuously exchange mass, momentum, and energy. They are central to processes as diverse as cloud formation, boiling in heat exchangers, cavitation around ship propellers, and gas–liquid chemical reactors. Designing equipment that involves such flows therefore demands both a firm grasp of the physics and reliable numerical tools capable of capturing a moving interface.

Phase ≠ Component – A phase is a region with uniform intensive properties. Liquid water and its own vapour constitute two phases, one component. Adding air yields two phases, three components; the interface count remains two. Clear terminology prevents many rookie errors.

1 Fundamental Physics

On molecular scales diffusion smears the interface, but at continuum scales typical of engineering practice (≫ 1 μm) the interface can be treated as an infinitesimally thin surface that must be tracked numerically.

2 Why Modeling Two-Phase Flow Is Hard

1. Moving interface – Capturing sharp fronts without excessive numerical diffusion.
2. Large property jumps – Density and viscosity ratios of 1 : 1000 are common.
3. Multiscale coupling – Bubble breakup may depend on phenomena an order of magnitude smaller than equipment length-scales.
4. Phase transitions – Latent heat and rapid density change stiffen the equations.

3 CFD Workflow in OpenFOAM

3.1 Select an Appropriate Solver

3.2 Generate a Quality Mesh

1. Import geometry via CAD or analytic blocks.
2. Refine near the interface or high-curvature zones; consider adaptive refinement with dynamicRefineFvMesh.
3. Check metrics – non-orthogonality < 70°, skew < 4 for stability.

3.3 Assign Boundary & Initial Conditions

• Inlets – velocity, phase fractions, temperature, turbulent quantities.
• Outlets – zeroGradient pressure and wave-transmissive velocity to minimise reflections.
• Walls – noSlip (or slip) plus contactAngle and thermal BCs as required.
• Initial interface – specify via signed-distance or volume-fraction field (alpha.water).

3.4 Activate Physical Models

• Surface tension (sigma) – Continuum Surface Force (CSF) implementation.
• Turbulence – RANS (k-ε, k-ω SST) or LES (WALE, dynamicKEqn).
• Phase change – Schrage or Rayleigh–Plesset models with energy source terms.
• Drag & lift – Euler–Euler solvers require closure relations for interfacial momentum exchange.

4 Sample Applications & Benchmarks

5 Post-Processing Essentials

• Use paraFoam plus alpha.water isosurfaces to visualise the interface; enable surface LIC for velocity patterns.
• Plot phase volume fraction over time to quantify entrainment/separation.
• Monitor minimum pressure for cavitation risk; ensure values do not fall below vapour pressure in critical components.
• Calculate interfacial area density with the built-in functionObject for interFoam.

6 Common Pitfalls & Remedies

1. Interface smearing – refine mesh, use bounded advection (MULES, isoAdvector).
2. Spurious currents – employ balanced discretisation (Gauss linearCorrected for curvature).
3. Unstable time-step – satisfy both Courant and capillary numbers (Co, Co_capillary < 0.25).
4. α > 1 or < 0 – tighten solver tolerances and adopt bounded schemes.

7 Supplementary Materials

All case files, meshing scripts, and post-processing utilities are available on GitHub:

GitHub – SimuXAI/Two-Phase-Flows → https://github.com/simuxai/OpenFOAM-7/tree/main/SimuXAI/blog2

Follow the README.md to reproduce each example.

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