How can I fix the "internal energy 'too low' " error in Simscape?
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In the picture you can see the subsystem that contains the Receiver Accumulator involved with this problem. The Accumulator is connected to a thermal network of course. At some point in the simulation, Liquid Helium is able to fill the container without evaporating. When the filling level reaches a little more than 50%, the simulation stops along with the error message "Specific internal energy of liquid volume must be greater than or equal to Minimum valid specific internal energy."
However, the set mininum specific internal energy is negative (-6.3 kJ/kg).
So what is the (general) reason for this error? How can the specific internal energy become negative? What modifications could I make to the Receiver Accumulator to fix the problem?
A few FYI's:
1) Shortly before the error occurs, the specific internal energy of the gaseous helium shows strong oscillations.
2) The higher the helium flow into the accumulator, the earlier this error occurs.
3) You can ignore all black parts
Yifeng Tang on 21 Jun 2021
I'll have to make some guess ... could you please check how the heat flows into or out of the H port of the accumulator? The error message seems to be invoked by the liquid part of the accumulator. My suspicion is that somehow H port is pulling heat from the fluid (gas+liquid) volume. There are parameters of the accumulator specifying the heat transfer coefficients for the gas and liquid portion of the volume. There is a chance that these values somehow leads to the liquid being too cold. If you don't mind sharing the model (or part of it), I can help take a look.