[TRNSYS-users] Internal wall modelling

[Marion Hiller]

Dear Loïc

I have to confess that this is a little confusing.
I would have loved to remove the surface category “internal” completely. However, due to backwards compatibility it remained.
I recommend to use category 'boundary' and the boundary condition 'identical' because there it is more clear what is calculated.

Please find my comments below:

I noticed that the first option implies to consider the total exposed surface (both sides) for the area parameter, unlike the second one. I guess that in (a) the middle of the wall is assumed adiabatic in order to model only a half wall but with an area 2 times higher. If this is correct, it is implicitly assumed that the internal wall composition is symetric. By the way, I found that the wall balance is not null otherwise.

Is the (a) modelling really only concern symetric composition?
=> Yes, for the option “internal” the composition has to be internal

Nonetheless, when I compare (a) with a 2*S area and (b) with a S area, for a symetric composition, the results are not exactly the same. I succeed having same results by setting the composition of (a) with only the half of it (from a face to the middle).

=> For having (a) and (b) giving similar results you have to use for (b) the same exposed area to the airnode/zone as for (a) (area =2*S).
     Otherwise the airnode/zone doesn’t have “access” the same amount of thermal mass. This results in different heat fluxes to the inner surface
    node.
So, the (a) modelling need half composition?
=> No,  (b) requires double area (see above).
    Have in mind that “Internal” means that both sides of the construction are facing towards the zone. You have to define the total construction
   for having the correct mass. The program divides the construction itself.


Regards,

Marion


-----------------------------------------------------------------------------------------------------------------------------------------------------------
Dipl.-Ing. Marion Hiller
TRANSSOLAR Energietechnik GmbH
Stuttgart - Munich - New York - Paris
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Von: TRNSYS-users <trnsys-users-bounces at lists.onebuilding.org> Im Auftrag von Loïc Frayssinet via TRNSYS-users
Gesendet: Donnerstag, 21. Februar 2019 15:35
An: trnsys-users at lists.onebuilding.org
Cc: Loïc Frayssinet <frayssinet at enertech.fr>
Betreff: [TRNSYS-users] Internal wall modelling


Dear all,

I was wondering if it is similar to model an internal wall as a wall :

(a) with the category 'internal' or
(b) with the category 'boundary' and the boundary condition 'identical'

I noticed that the first option implies to consider the total exposed surface (both sides) for the area parameter, unlike the second one. I guess that in (a) the middle of the wall is assumed adiabatic in order to model only a half wall but with an area 2 times higher. If this is correct, it is implicitly assumed that the internal wall composition is symetric. By the way, I found that the wall balance is not null otherwise.

Is the (a) modelling really only concern symetric composition?

Nonetheless, when I compare (a) with a 2*S area and (b) with a S area, for a symetric composition, the results are not exactly the same. I succeed having same results by setting the composition of (a) with only the half of it (from a face to the middle).

So, the (a) modelling need half composition? If yes, why non-symetric compositions causes balance divergences?

Regards,

--
Loïc FRAYSSINET
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