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Heat Exchanger (TL-TL) breaking thermodynamics?!

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In Simscape, the Heat Exchanger (TL-TL) block is allowing "negative" heat transfer, as it were. I.e., the hot liquid (in port A1 and out port B1) somehow continues to cool to below the temperature of the cool fluid (in port A2 and out port B2) at port B2 by the time the hot fluid reaches the output port B1. (See plot below - I believe the red line should not go below the yellow line).
Does anyone have an explanation of how this happens? - Especially since the basic laws of thermodynamics mean that the hot fluid (Red line, B1) should only cool down to equilibium with the cold fluid (Yellow line,B2) in the heat exchanger?
Could it perhaps be one of my inputs causing this, or, could it be that the Heat Exchanger (TL-TL) block is simply following the maths and has nothing to prevent this issue from occuring?
Legend for plot:
  • Green = A1 Temp (IN)
  • Red = B1 Temp (OUT)
  • Orange = A2 Temp (IN)
  • Yellow = B2 Temp (OUT)

Accepted Answer

Yifeng Tang
Yifeng Tang on 3 Apr 2024
This shouldn't happen for parallel-flow heat exchangers, because of 2nd law of thermo-dynamics.
But this may happen for counter-flow and cross-flow heat exchangers. For counter-flow heat exchanger, the lower limit of the temperature at the outlet of the hot stream is the inlet temperature of the cold stream. Cross-flow heat exchanger usually won't go that far, but it's also possible to have it colder than the outlet of the cold stream I think.
If what you have observed is happening for a heat exchanger block in Simscape with "parallel flow" configuration, please attached the model and I can take a look. I'd be really surprised though if that's the case.
BTW: I saw the Release info is given as "R2021a" (6 releases back). I'd like to let you know that there has been quite a few heat exchanger blocks, for various domains, added to Simscape Fluids since then. Check them out in case some can be useful for you.
Willis on 4 Apr 2024
Edited: Willis on 4 Apr 2024
Also, I forgot to mention that I've used the Simple Heat Exchanger model variant (dynamic compresibility turned off) as opposed to the E-NTU variant.
Could this potentially explain why it's behaving how it is?
Willis on 4 Apr 2024
Update - I've found that the issue was related to flow rate and that at this portion of my data the flow rate of the cold fluid was much greater than that of the hot fluid, therefore the hot fluid was cooling down to below the temperature of the cold fluid at the exit of the heat exchanger.
Hope this helps anyone with the same question.

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