Why does the Moist Air Local Restriction create vastly different mass flow results when compared to the Gas Local Restriction?

I have set up 2 identical models in Simulink/Simscape Moist Air and Gas in R2022a.

For Moist Air: Controlled Reservoir to Local Restriction to Controlled Reservoir.

For Gas: Reservoir to Local Restriction to Reservoir.

I set the upstream conditions to 14.7 psi, 70 F for both. For Moist Air, i set Trace Gas and Humidity to 0. Down stream is set to 14.65488601 psi and 70F. The area of each Local Restriction is set to 7 in^2. The Cd is set to 1. Min / Max constrictioins are set to 1e-10 and 20 in^2 respectively. The Cross-sectional area at ports A and B is set to 21 in^2. The Upstream / Downstream Resevoir Cross-sectional areas are port A are set to 100 in^2.

I am using PS-Simulink Converters to get lbm/min as the unit from each downstream mass flow sensor.

I am using PS-Simulink Converters to go from a text box with 7 to in^2 to go into the AR port of the local restrictions for both models.

For Moist Air, i get 52.5 lbm/min, for Gas i get 15.04 lbm/min. I have an Excel based hand calc for orifices using basic isentropic relations and get 16.29 lbm/min, much closer to the Gas result.

What is going on with the Moist Air results? I am using the default perfect gas values in the Gas properties block and Air in the Moist Air block. The parameters don't seeem too different, no where near enough to result in the differences seen below.

[dpb]

I don't have Simulink so "know nothink!" about the details,but...is there any chance that one of the components in the moist air model is based on a different set of units than assumed so it's a scaling problem? If you return the density of the air, what is it? It would seem it likely must still think there's moisture entrained even after having set the RH to 0%. Maybe the model doesn't handle zero RH and that gets interpreted as 100%?

If you change parameters in the two models, do they track together as expected; that behavior might be a clue as to what is going on.

Answer 1

[Erin McGarrity]

Hi,

As a general rule, it's a good idea to attach a model like this so it can get another pair of eyes on it. It's hard to determine based on the schematic itself.

Things to check in a situation like this are:

Units -- Are all the units set to what they should be?

Port areas -- Are all the port areas set correctly? (These should be approximately the size of the actual thing being modeled.)

Parameter values -- are the parameters set to the correct values?

Specific to this case, we determined in our tech support call, the above model had an incorrect unit set for one of the parameters. Fixing this, we achieved results accurate to within a couple percent, as we expect.

Thanks,

Erin

[dpb]

"... the above model had an incorrect unit set for one of the parameters."

Precisely one of my initial suggestions/questions that a units mismatch would be a likely suspect cause...

Nice to have the feedback.

Answer 2

[Jakub]

Hi, looking at the model, you have no pipe in there. The only armature you have modeled is the local restriction. Now, your pipe has 0 meters long. Consider to add even small pipe both sides of your local restriction. It may reduce the stiffness of the model.

https://www.mathworks.com/help/hydro/ug/troubleshooting-fluids-models.html#mw_64116b8e-6095-452e-9d94-d977d1832874