Search code examples
modelica

Problems with the dymola runtime ln function

I was writing a dynamic model recently, but I ran into some problems that I couldn't solve. The following is part of the code, the function I want to implement is that when Tb2 is less than the saturation temperature Tbaohe, the logarithmic average temperature difference T1 is used to solve, and when Tb2 is equal to the saturation temperature, continue to use the arithmetic average temperature difference T2 on the basis of the above solution results to continue the solution, where Tc1 is equal to Tbaohe.

  Kc*Ac*T3 = (Wnjs + Gna)*(hc1 - hb2);
  T1 = ((Te1 - Tbaohe) - (Tc1 - Tb2))/(ln((Te1 - Tbaohe)/(Tc1 - Tb2)));
  T2 = (Te1 + Tbaohe)/2 - (Tc1 + Tb2)/2;
  if Tb2 < Tbaohe then
    T3 = T1;
  else
    T3 = T2;
  end if;

But when I solve, I get the following error. Apparently there is a problem with the ln function here, it seems that when Tb2 is infinitely close to the saturation temperature Tbaohe, the run stops automatically, I can't fix this.

Error: The following error was detected at time: 10.86044238659505

Model error - division by zero: (Te1-Tbaohe) / (Tc1-Tb2) = (104.936) / (0)

First evaluation failed for non-linear solver.

First evaluation failed for non-linear solver.

enter image description here

Thanks in advance for your reply!

Solution

  • If the two temperatures become close, you are dividing by zero. Tb2 should not become larger than Tc1, otherwise you would get a negative value for the ln function. You could use max(1e-4, (Tc1 - Tb2)) instead of (Tc1 - Tb2). If Tb2 is intended to become larger than Tc1, than there might be an error in your equation. The complete source code would be helpful to identify variables and parameters.