These are some state properties of fluids in GUNNS:

• Pressure (P). Standard GUNNS Trick units is kilopascals (kPa).
• Temperature (T). Standard GUNNS Trick units is kelvins (K).
• Density (ρ). Standard GUNNS Trick units is kilograms per cubic meter (kg/m3).

Related to density, but not tracked as fluid state properties, are:

• Mass. Standard GUNNS Trick units is kilograms (kg). Strictly speaking, mass is not a fluid property but is a measure of an amount of fluid.
• Moles. Standard GUNNS Trick units is kilogram-moles (kg*mol). Similar to mass, this is a measure of an amount of fluid. We use (kg*mol) instead of the standard gram-mole because we track mass in (kg). Keeping moles & mass consistent makes it easy to convert between them by normal values of molecular weight: mass (kg) = moles (kg*mol) * MW (1/mol)
• Volume. Standard GUNNS Trick units is cubic meters (m3). This is, by definition, mass divided by density. Strictly speaking, volume is not a fluid property but rather a property of a space or container that some fluid is inside of.
• Specific Volume (V). Standard GUNNS Trick units is cubic meters per kilogram (m3/kg). This is, by definition, the inverse of density.

All fluids have a relationship between the pressure, density and temperature states that is always true. For a given pressure and temperature, there is always exactly one density that it must have, and so on. This relationship is approximated with an equation of state. There are varied equations of state that are better at modeling different fluid types and phases or regions of the PVT space. We call these “PVT equations of state” to distinguish from other state sets. But usually when you hear “equation of state” we’re talking about PVT.

The important thing to take away is that in GUNNS, the equation of state is always satisfied. Changing one or two of the P, ρ, T parameters must cause the fluid’s other parameters to change so that its equation of state stays satisfied. This creates a dynamic interaction between these 3 states that is always happening.

All real-world fluids have a complex & non-linear interaction between PVT. All equations of state are only approximations to the real-world and always have some error.

• Ideal Gas Law is a linear PVT, but is only valid in regions where gasses are near-linear:
  • low pressure or high temperature
  • basically, far away from the saturation curve
• A different PVT is implemented in the fluid properties for different fluid types:
  • ideal gas law for ideal gasses (GUNNS_O2)
  • a polynomial curve fit for most liquids (GUNNS_WATER)
  • higher-fidelity table lookups for a limited set of fluid types:
    • hi pressure gasses near saturation curve or supercritical (GUNNS_O2_REAL)
    • water (GUNNS_WATER_PVT) for the reversal of expansion direction with temperature near freezing
    • these have limited range of P & T
  • More on the different equations of state that we use here: Fluid Properties: Pressure & Density.

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