[TRNSYS-users] Long wave radiation heat exchange and number of surfaces

[Marion Hiller]

Dear Marcello,

yes, the star-node model work has same limitations especially for extreme geometries and temperature deviations .
For those cases the star-node work shouldn't  be applied because the results are wrong.
The "detailed" model is able to handle these extreme cases correctly.

For ordinary space shapes the star-rnode model does a good job as shown in several comparisons.
If you aren't sure if the star-node model performs well for your geometry/temperatures you can check the
results with the detailed model based on Gebhart factors.

Personally, I simulate 4 surfaces at least in a zone.

Marion



Dipl.-Ing. Marion Hiller

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________________________________
Von: trnsys-users-bounces at cae.wisc.edu [mailto:trnsys-users-bounces at cae.wisc.edu] Im Auftrag von marcello.caciolo at cofelyaxima-gdfsuez.com
Gesendet: Freitag, 21. Februar 2014 15:54
An: trnsys-users at cae.wisc.edu
Betreff: [TRNSYS-users] Long wave radiation heat exchange and number of surfaces

Dear TRNSYS users,

I have a very simple non-geometrical type 56 model with one zone with two surfaces: one boundary surface of 40 m2 and an external surface of 400 m2. External condition and boundary temperature are held constants.

When I plott surface temperatures and heat exchange between the two surfaces, I found a very large long wave radiation exchange between the two surfaces.

By hand calculation (Q=sigma S (T1^4 - T2^4), whit S = area of the little surface), I found that the value given of heat exchange by radiation by TRNSYS is about ten times larger than what I expected. Indeed, it seems that value given by TRNSYS is near to the value given by the same equation but considering the largest surface area.

However, if I split the external surface in two surfaces of half the area, the value calculated by TRNSYS is reduced of about 7 times. If I split again these surfaces, radiation exchange continue to drop but more slightly, tending to a value near the one calculated by hand.

My question is : have you already found the same behavior? Is there a recommendation to define at least three surfaces in a zone in order to calculate properly heat exchange by radiation?

Thank you for your help!

Best regards,
Marcello Caciolo
Responsable Outils et Méthodes d'Analyse Energétique
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marcello.caciolo at cofelyaxima-gdfsuez.com<mailto:prenom.nom at aximaseitha-gdfsuez.com>
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