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

[marcello.caciolo at cofelyaxima-gdfsuez.com]

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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