How can I avoid temperature blow-up when mass-flow goes to zero in thermal circuits?

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Ubaldo on 17 Jan 2022
Answered: Yifeng Tang on 1 Jul 2022
I have the following circuit.
When I fully close Valve1 (Valve 2 is kept fully open) I have a twofold effect:
1) The pressure difference between the ports A and B of the Mass Flow Rate Source (TL) block blows-up;
2) The temperature between the ports A and B of the radiatior block blows-up;
I solved the problem 1) by adding the DifferentialByPassValve that you see above, but I have no idea on how to solve the problem 2).
Any hints?
I report below the logs of temperatures at port A and port B of the Radiator block and the mass-flow at port A. Valve 1 is closed around t = 100s. The actuator time constant is 1.5s.
EDIT: I think I troubleshot the problem but still, any hint on how to solve it is appreciated. :)
When Valve1 is closed the fluid enter a loop where it is heated at constant rate. Given that such heat is not released anywhere, then the fluid temperature would increase monotonically. Such very hot fluid flows back towards Valve2 from Port B to port A, thus reaching port B of the radiatior at very high temp.
An easy solution would be to turn off the heater when the fluid temperature got too high, or, more elegantly, to use a temperature control valve that opens when the fluid temperature crosses a certain threshold (but I don't know how to fit this mechanism in the overall scheme at the moment).

Answers (1)

Yifeng Tang
Yifeng Tang on 1 Jul 2022
One idea you can try: instead of using a heat flow rate source at the radiator, using a temperature source intead. This would represent the working fluid exchanging heat with the ambient, which can be reasonably approximated as a constant temperature. You would then need to parametrize the radiator pipe so that it reject the correct amount of heat for a certain flow rate, which you'll need some test data to calibrate.
The model should then be more physically sound this way. As the inlet flow to the radiator is turned off, the fluid inside should gradually cool down to ambient temperature.
You may still see the B port temperature shoots up, because the pipe next to the pump is still heating up the fluid with a fixed heat flow. Since there is no volume between the B port of that pipe and the B port of the radiator (valves have no volumes), the temperature will still be the same.

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