Engineering context

Liquid-gas two-phase flow is modelled inside a steady-state pipe network rather than judged pipe-by-pipe, because pressure drop, vapour quality, and the flow regime are coupled and change along the line and across the connected system. As pressure drops along a pipe, quality and velocity can rise and the flow regime can shift, so results are read across the whole network.

This page is the primary workflow for liquid-gas two-phase flow and slug-flow modelling. Where slug flow cannot be avoided through sizing or routing, the response shifts from prevention to mitigation — FluidFlow detects the regime so the engineer can decide the next step.

Engineering workflow

1. Define the modelling objective and acceptance criteria — state what you are evaluating (pressure drop across the two-phase section, vapour quality, flow regime, velocity limits, and any flow regimes to avoid, in particular slug flow).
2. Set the calculation basis — select the supported two-phase flow correlation the model depends on, together with the fluid’s property data.
3. Build and connect the network — place and define the boundary conditions (the fluid is selected from the fluid database, which carries its properties, and applied at the boundary), then add pipes, fittings, elevations, and equipment. Elevation changes matter, since they influence regime and pressure drop in two-phase flow.
4. Set up the operating cases — inlet quality or phase split, throughput range, and hot versus cold, so the regime and pressure-drop behaviour are checked across the operating envelope.
5. Solve and run a visual sweep — confirm solution health on the status bar, check flow-direction arrows on all open paths, and look for red component highlights.
6. Work through the Messages tab — review two-phase, high-velocity, and flow-regime messages to understand where and why behaviour of concern arises.
7. Review pressure drop, quality, velocity, and flow regime — read pressure drop and velocity across the two-phase section, vapour quality along the pipe, and confirm mass balance at junctions.
8. Use charts to locate and characterise behaviour — two-phase pipe charts (Flow Path Length vs Vapour Quality; the Static Two-Phase Flow Pattern Map to identify the flow regime and locate slug flow), with EGL/HGL composite plots for the pressure and elevation profile.

Why the full system matters

Pressure drop, vapour quality, and flow regime are coupled in two-phase flow — a change in one affects the others along the pipe and through the connected network. The flow regime (stratified, annular, slug, mist) determines how pressure drop should be evaluated and what operating risks are present. Slug flow in particular carries loads that can affect pipework integrity, so identifying the regime in the network model is the first step in deciding whether to resize, reroute, or design for mitigation.

How FluidFlow helps

FluidFlow models liquid-gas two-phase flow within its steady-state pipe network solver using supported two-phase correlations, reporting pressure drop, vapour quality, velocity, and flow regime across the connected network. Two-phase pipe charts and the Static Two-Phase Flow Pattern Map locate and characterise behaviour and identify slug flow, with EGL/HGL plots showing the pressure and elevation profile.

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