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.

A practical workflow starts by defining the modelling objective and acceptance criteria (pressure drop, vapour quality, flow regime, velocity limits, and any regimes to avoid — in particular slug flow), then setting the calculation basis: the supported two-phase flow correlation and the fluid’s property data. Engineers build and connect the network — placing and defining the boundary conditions, with the fluid selected from the fluid database (which carries its properties) and applied at the boundary, and paying attention to elevation changes that influence regime and pressure drop — then set up the operating cases (inlet quality or phase split, throughput range, hot versus cold). After solving, behaviour is read from the visual sweep and the Messages tab, from pressure drop, vapour quality, and velocity across the two-phase section, and from the two-phase pipe charts (Flow Path Length vs Vapour Quality and the Static Two-Phase Flow Pattern Map, which identifies the flow regime and locates slug flow) and EGL/HGL plots for the pressure and elevation profile. Where the slug-flow regime cannot be avoided through sizing or routing, the response shifts from prevention to mitigation — such as additional pipe supports to handle slugging-induced vibration — and the key value is that 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) and the criteria you will judge against.
2. Set the calculation basis — select the supported two-phase flow correlation the model depends on, together with the fluid’s property data, so pressure drop and flow regime are evaluated on a consistent basis.
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 and connect them with pipes to establish topology. Elevation changes matter, since they influence regime and pressure drop in two-phase flow.
4. Set up the operating cases — as combinations of component settings, model the conditions of interest: 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 that flow-direction arrows appear on all open paths, and look for red component highlights; a missing arrow or a highlight is itself a diagnostic signal.
6. Work through the Messages tab — review the enunciated showstoppers, warnings, and hints; two-phase, high-velocity, and flow-regime messages flag where and why behaviour of concern arises. Resolve root causes and apply engineering judgement on which warnings are acceptable for the service.
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, against the acceptance criteria set in step 1.
8. Use charts to locate and characterise behaviour, then document and compare — two-phase pipe charts (Flow Path Length vs Vapour Quality to see where quality develops; 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. Document the correlation, basis, and assumptions, and compare design or operating alternatives.

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. It is not CFD and does not perform full transient slug-flow simulation or CFD-style local flow prediction.

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