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From: Walter Driesen on 23 Jun 2010 04:23
Hi Edoardo,

I am facing similar problems while modeling a heat exchanger in simscape (see http://www.mathworks.com/matlabcentral/newsreader/view_thread/284135#756748 ).

I would agree with you to add only the temperature as an across variable, without adding any corresponding trough variable. As far as my understanding of physics goes, the heat flow in such a hydro-thermal system does not follow the direction of the volumetric flow of the fluid. So, for me, heat flow can't be a conserving variable.

I implemented such a domain and for the moment it seems to work fine. Maybe I will run into problems later. I don't know.

Best,
Walter
From: Arnaud Miege on 1 Jul 2010 11:49
"Walter Driesen" <walter.driesen(a)REMOVETHIS.fmtc.be> wrote in message <hvsg9p$dr8$1(a)fred.mathworks.com>...
> Hi Edoardo,
>
> I am facing similar problems while modeling a heat exchanger in simscape (see http://www.mathworks.com/matlabcentral/newsreader/view_thread/284135#756748 ).
>
> I would agree with you to add only the temperature as an across variable, without adding any corresponding trough variable. As far as my understanding of physics goes, the heat flow in such a hydro-thermal system does not follow the direction of the volumetric flow of the fluid. So, for me, heat flow can't be a conserving variable.
>
> I implemented such a domain and for the moment it seems to work fine. Maybe I will run into problems later. I don't know.
>
> Best,
> Walter

Simscape applies generalized Kirchoff's laws: the algebraic sum of the through variables (e.g. current) at one node is zero, while two ports connected together have the same across variable (e.g. voltage). That is why it is generally recommended to have through and across variables in pairs. Even if the heat flow doesn't follow the volumetric flow of the fluid, you still need to do an energy balance on the heat exchanged somewhere, which is why you would use the heat flow as a through variable. I agree though that it isn't an easy problem.

Arnaud
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