# Problems with the "System-Level Heat Exchanger (TL-MA)" Simscape block

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Elia Bontempelli am 13 Okt. 2023
Kommentiert: Yifeng Tang am 20 Okt. 2023
Dear all,
I'm doing a thesis about the heating system in a vehicle. I was trying to design the model, where I get MA from the environment (at 0/5°C) and goes through an heat exchanger where it has to be heated by a flow of hot water (90°C) and then goes into the cabin. I can regulate also the mass flow rate of the water.
I'm trying to use the Simscape block "System-Level Heat Exchanger (TL-MA)" but unfortunately I have some problems with it, understanding how are the explicit equation that computes the outlet MA temperature.
I don't get what is the direct impact on the Moist air outlet temperature of the initial conditions, Initial TL temperature and Initial MA temperature; morever how the "nominal temperatures and rate of heat transfer" parameters influence the computation of the MA temperature at the outlet of the heat exchanger. What is the relationship between these variables?
Most of the time I get the error: "Moist air outlet temperature at nominal operating condition must be less than or equal to Thermal liquid temperature at nominal operating condition." For example when I set the following parameters:
• Rate of heat transfer: 9kW
• Nominal intlet TL temperature: 90°C
• Initial TL temperature: 35°C
• Nominal intlet MA temperature: 15°C (I put this because before the heat exchanger has about this value)
• Initial MA temperature: 0°C
Another questionable beahviour happened when I was regulating the mass flow rate of the inlet TL, setting it at almost 0, so I would expect that MA won't be heated, but instead I can see like an overshoot of the outlet MA temperature at the beginning of the simulation.
Any suggestions with these problems?
Thanks to all!
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### Antworten (1)

Yifeng Tang am 18 Okt. 2023
Starting from "nominal temperatures and rate of heat transfer" means at time of 0, the fluids inside the volume will have the pressure, temperature, etc. at the nominal condition. This will change the initial values of the states, as well as the initial heat transfer. The fluid volumes on two sides of the heat exchanger means that there is some thermal inertia from the working fluids, and if the nominal conditions includes a temperature difference (it probably does!), you'll have some heat transfer and temperature change starting t=0. This is probably why you see that overshoot in MA temperature when TL flow is zero, in your 3rd question.
The error of "Moist air outlet temperature at nominal operating condition must be less than or equal to Thermal liquid temperature at nominal operating condition." has to do with the consistency of the nominal conditions you have set. As the MA flow is heated up in the heat exchanger, its temperature cannot be raised to a value higher than the temperature of the liquid, as otherwise it'll violate the 2nd law of thermodynamics. Roughly speaking Q=cp*mdot*dT, so if your heat flow (Q) is too large, or mdot is too small, dT will become too large, and the outlet temperature will be higher than what's possible (liquid temp.). The Simscape block will evaluate this relationship using the nominal conditions you provided and give you this error if it sees such violation. It's basically telling you that the conditions you provided are physically impossible. Check your data, or math. Usually it's too much nominal power or too little mass flow.
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Elia Bontempelli am 20 Okt. 2023
Bearbeitet: Elia Bontempelli am 20 Okt. 2023
You are saying that the nominal conditions set the beahviour at time of 0, but if I specify the "initial conditions"? If I set the initial temperatures for thermal liquid and moist air, at starting t = 0 the characteristics of the fluids are the ones set in nominal conditions or the ones specified in initial conditions? Otherwise what is the purpose of the "Initial condition specifications"?
Thank you
Yifeng Tang am 20 Okt. 2023
If you check that "Initialize thermal liquid to nominal operating conditions" box, the initial condition will be what you set for the nominal conditions. If it's unchecked, you can specify a set of initial conditions.
https://www.mathworks.com/help/hydro/ref/systemlevelheatexchangertlma.html#mw_bea1654e-d2cb-48ce-85f4-85309174c266

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