Why do I get the wrong values for enthalpy on the P-h diagram block in Simulink/Simscape?

Hello, I am using the P-h diagram block from the (Simscape > Fluids > Two-Phase Fluid > Utilities) menu for analyzing R-134a thermodynamic properties.

The following is an example of the wrong value where the enthalpy value is not the same with theoretical value (textbook, website, etc). In the diagram, the X-value represents the enthalpy, the Y-value represents pressure (in MPa), and the colored lines represent isothermic lines which indicate constant temperatures (in K).

As you can see in the P-h Diagram, the temperature is 300 K = 27 C. The corresponding pressure is approx. 0.7 MPa or 7 bar which agrees with the properties table. But the enthalpy (X-value) in the diagram is 412.7 kJ/kg which is significantly different from the theoretical value provided in the properties table (around 262 kJ/kg).

Please help me figure this out. Thank you!

Answer 1

[Torsten]

The reference enthalpy for R-134a is often given for the saturated liquid at -40°C, with a value of 148.4 kJ/kg. If you look at your table, it seems that the enthalpy of the saturated liquid is set to 0 kJ/kg at T = -40°C. Maybe you have to add this value to your table data to get the Simscape property values, but I'm not sure. Did you check the predefined reference enthalpy used in Simscape ? My guess is: it is set to 0.

[Harish]

Ah yes, the enthalpy of the saturated liquid at T= -40 C is set to 0 kJ/kg in the table data I am using as the theoretical basis.

How do I fix the values of enthalpy of the diagram so that they agree to the thermodynamic table I am using? How to check reference enthalpy used in Simscape, and is there a way to modify this value?

Thank you!

[Torsten]

How do I fix the values of enthalpy of the diagram so that they agree to the thermodynamic table I am using?

It's not necessary. All computations involving enthalpies work with enthalpy differences, and they are the same independent of the reference enthalpy chosen (h1 + h_ref) - (h2 + h_ref) = h1 - h2 independent of h_ref.

But I think the values from Simscape are the values that are usually taken in the engineering society (e.g. in the VDI Wärmeatlas).

How to check reference enthalpy used in Simscape, and is there a way to modify this value?

I don't know. And as said: it's not necessary. Is there no documentation for the tool ?

That's what I found in Chapter 6-4 of the User's Guide:

Consistent Specific Enthalpy Reference Temperature

Specific enthalpy is a thermodynamic quantity that is measured with respect to a reference state. To ensure consistency in a moist air mixture, all the specific enthalpies must have the same reference temperature. The moist air domain uses a consistent reference temperature of 0 degC. When you enter specific enthalpy vectors in the Moist Air Properties (MA) block, the block internally shifts these vectors to be 0 kJ/kg at 0 degC. The library blocks then use these adjusted values in the internal calculations. The simulation data log also displays the adjusted specific enthalpy values.

You can use the Enthalpy reference state parameter of the Thermodynamic Properties Sensor (MA) block to view the specific enthalpy values adjusted with respect to different reference states.

[Harish]

Okay, I'll just use other table that set the enthalpy as 148.4 kJ/kg at T= -40 C then. Thanks!