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heat convection coeff if-else

What is the problem?

  • When using calculation method acc. to EN ISO 6946 the model AixLib.Utilities.HeatTransfer.HeatConv_inside switches between to heat convection coefficients alpha depending on temperature at wall and in the room.

Equation is:

      if port_b.T >= port_a.T then
        alpha = 5;
        // downward heat flow
      else
        alpha = 0.7;
      end if;
  • This leads to a hard state event forcing the solver to solve a totally changed equation system (e.g. heat flows turn around and value of heat transfer changes its magnitude).

How do we want to solve it? Describe the solution you'd like

  • Maybe just use the smooth-function. Or any other kind of make the zero-crossing differentiable.

Describe alternatives you've considered

An alternative would be to emphasize this issue in the documentation. Since the EN ISO approach is correct and directly taken from the standard, I defnitly vote for let this calculation method in the model.

The approach acc. to Bernd Glueck is a differentiable function, so it should be recommended.

Additional context

The approach acc. to Bernd Glueck follows 2 functions (depending if the wall is a ceiling or a floor (or a vertical wall)). The two functions have the following behavior: image