Why the dry air enthalpy in simscape perfect gas model is 420 ? It should be 293

Hi, recently I try to build a air transportation system with simscape gas library, using the default gas properties,which is dry air mentioned in the matlab help. when I checked the default (dry air) enthalpy in perfect gas, the value is 420kJ/kg. However, I checked the value in text book and other software(like REFPROP), the enthalpy should be 293kJ/kg. Similar problem can also be found in semiperfect and real gas models. Did I make a mistake or the default value is not right? Does anybody notice this problem?

[AreMah]

Hi Xi,

Have you found out why this happens and how to fix it?

Answer 1

[Yifeng Tang]

I believe it depends on where 0 kJ/kg specific enthalpy is defined.

In Simscape, looks like it's defined such that at 0K, h=0 kJ/kg. In newer releases, you can visualize it by plotting the gas property (right click on Gas Properties (G) block - Foundation Library - Plot Gas Properties). When using semi-perfect and real gas options, the specific enthalpy surface seems to extrapolate to 0K at 0kJ/kg.

REFPROP apparently has a different reference point. For example, T = 56K and P = 0.005MPa will give you specific enthalpy of -170.4kJ/kg for dry air, so its 0kJ/kg is definitely not at 0K.

In terms of modeling, one rarely cares about the absolute values of the specific enthalpy and it's usually the change in h that matters. So as long as the difference in h between states are accurate, you should be OK. That said, if you run into a situation where you need to write customized Simscape block using some correlation or formula which involves specific enthalpy, do pause and ask what the reference point (0 enthalpy) is defined.

[Xi Chen]

Thank you!

[ouss_zh]

Hello Yifeng,

I am currently having trouble finding the different properties of Hydrogen H2 that I need to set in the Gas Properties (G) block. I was wondering if you could provide me with some guidance on this matter.

Any help you can offer would be greatly appreciated.

Best regards,

[Yifeng Tang]

@ouss_zh, check out this example:

https://www.mathworks.com/help/simscape/ug/pem-fuel-cell-system.html

It's available starting R2021a or b I think.

On the anode side, there is a Moist Air property block, where the trace gas is H2. Maybe you can use some information from that.

Similarly, this more sophisticated example in Simscape Fluids: https://www.mathworks.com/help/hydro/ug/fuel-cell.html, available starting R2022a, also contains a property block that include H2.