GUNNS has models of the fluid properties of a limited set of fluid types. Most of these types represent “pure fluids”. A pure fluid is made up of one chemical compound. So for instance water (H2O) is a different pure fluid than hydrogen (H2) or oxygen (O2). Some of our fluid types represent fixed mixtures of two or more pure fluids. For instance, GUNNS_PG30 is a 30% mixture of propylene glycol and water. Our fluid types are distinguished by phase (gas vs. liquid), so we have separate fluid types for gas and liquid for a given compound. We name these types with a naming convention that implies the phase:

• Gasses are named with the chemical compound formula, so water vapor is GUNNS_H2O.
• Liquids are named with the spelled-out name, so liquid water is GUNNS_WATER.

These types and their properties are defined in the FluidProperties class in the ms-utils/ submodule at: ms-utils/properties/FluidProperties.hh. A list of all of our current fluid types here.

GUNNS can model mixtures of fluid types, but for best results we only recommend mixing ideal gas types (more on that later). An important mixture is air, which mixes pure ideal gas N2, O2, CO2, H2O and other trace gasses.

This is a generalized fluid phase diagram, showing the solid, liquid, gas & supercritical phases, the boundaries between them and a couple of points of interest:

Here’s what we do and don’t model from that diagram:

• Our liquid fluid types model the liquid and compressible liquid regions.
• Our gas fluid types model the vapour and gaseous phase regions. Some gas types extend below the triple point.
• We model the critical & triple points and the liquid/vapour phase boundary between them (called the “saturation curve”).
• We don’t generally model the supercritical region, but we’ll use the gas type as an approximation.
• We don’t model the solid phase or freezing/melting. Liquid types will remain liquid at temperatures below their freezing point.

GUNNS currently can’t model mixed phases in a single location. So for instance we can’t model gas bubbles in a liquid stream. A network can, however, model both gas and liquid phases in different locations. So we have some tricks to model phase mixing and transitions while still keeping them in separate locations in the network. More on that later.

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