A good way to think about PVT is with these two main effects, which as engineers you are already used to:

• in aero, we talk about compressible vs. incompressible air flows. typically assume incompressible below a certain Mach #, and compressible above, etc.
• in GUNNS, we need to think more generally.
• as in the real world, in GUNNS, ALL FLUIDS ARE ALWAYS COMPRESSIBLE.
  • there’s no such thing as incompressible, there’s only varying amounts of compressibility
  • liquids, which are usually assumed to be incompressible, are NOT. It is better to describe them as “slightly compressible”, as compared to gasses, which are “very compressible”.
  • You can still assume incompressibility, when appropriate, as a way to approximate & simplify your analysis, but don’t forget that GUNNS is always modeling the actual compressibility effects, even if they’re small.
• “compressibility” relates pressure and density.
  • high compressibility (gas) means a given change in pressure causes a large change in density. Conversely, a given change in density causes small change in pressure.
  • low compressibility (liquid) means a given change in pressure causes a small change in density. Conversely, a given change in density causes large change in pressure.

Density of Water @ 70° F

• fluids contract or expand when their temperature changes.
  • Usually heating causes expansion:
    • density decreases at constant pressure, or pressure increases at constant density
  • However some fluids in some conditions expand when they cool
    • for instance, liquid water begins to expand as it cools within a few degrees of freezing

Density of Water @ 1 atm

• The whole PVT ties thermal expansion & compressibility together.
  • for liquids in GUNNS, this is especially important. If you change temperature (adding/removing heat), then in a fixed volume (fixed density), this causes a very large change in pressure. This is the same reason why your car engine block cracks in the winter if your water freezes. The GUNNS equivalent is: if you have a liquid system with a fixed mass and no flexible volume, then density is fixed and then very small changes in temperature cause very large changes in pressure. This can easily “blow up” your network.
  • for this reason, self-contained liquid systems (fixed mass) should have some flexible volume to accommodate density changes. This is just as true in the real-world as it is in GUNNS. This is accomplished with the use of bellows, accumulators, and flexible pipes.
  • For large liquid volumes, NEVER use a fixed-volume GUNNS link like GunnsFluidCapacitor or GunnsFluidTank. Instead, always use one of the accumulator links.

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