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.
Dipl.-Ing.
Transsolar Energietechnik GmbH
Curiestraße 2
70563
Telefon: +49 711 67976-0
http://www.trnsys.de
Transsolar: KlimaEngineering- Technologien für energieeffizientes Bauen und Nutzerkomfort in Gebäuden
Geschäftsführer :Diplom-Ingenieur
Eingetragen beim Amtsgericht
Steuernummer: 99073/00911,USt-IdNr.: DE 152272639
Von:
Gesendet: Freitag, 21. Februar 2014 15:54
An:
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
Cellule Efficacité Energétique et Environnementale - Pôle Projets et Réalisations
46 Boulevard de la Prairie au Duc
BP 40119 - 44201 Nantes Cedex 02
Tél.: +33 02 40 41 06 57
marcello.caciolo@cofelyaxima-gdfsuez.com
P
Pensez à l'environnement avant d'imprimer ce message