GUNNS fluid properties has many simplifications and limitations and doesn’t implement all the behavior of real-world fluids. In order for you to use GUNNS most effectively you need to understand these limitations. First we’ll explain some things about how real fluids behave, and then present GUNNS’ limitations in the context of that understanding.

The Temperature-Entropy (T-s) diagram is a good one-stop shop for understanding real fluids. It plots pressure, enthalpy, and density in coordinates of temperature and entropy for gas, liquid & supercritical phases and gas-liquid mixtures. Here is the T-s diagram for water:

• The black lines are lines of constant pressure
• The red lines are lines of constant specific enthalpy
• The green lines are lines of constant density
• The thick blue line is the gas-liquid phase boundary (saturation curve)
• The area under the blue line is the mixed gas-liquid region
  • 100% liquid towards the left, and 100% gas towards the right
  • The dashed blue lines delineate the phase mixture by mass, in increments of 10%.
  • The blue circle at the top of this region is the critical point

This next image highlight the areas of different fluid phases:

Let’s note a few important things that the T-s diagram shows:

• The pressure lines in the liquid region are squeezed close together. As a result, there’s not much difference in the other parameters from one pressure line to another. This shows the relative incompressibility of the liquid phase.
• Towards the right-hand side, in the gas phase at low pressures, there are a couple of things to note about the ideal gas:
  • Density lines closely parallel the pressure lines. This follows from the directly proportional relationship between density and pressure in the ideal gas law.
  • Specific enthalpy is only a function of temperature and is directly proportional to temperature
• In the mixed-phase region, lines of constant pressure also have constant temperature. This means there’s a one-to-one correlation between pressure and temperature in the mixed-phase region. This defines the pressure-temperature saturation curve, and the mixed phase region always occurs on that curve.

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