[TRNSYS-users] radiation

[Jeroen Van der Veken]

Dear collegues,
I want to model the dynamics of the combination of an hydronic radiator 
and the wall behind it more in detail. To simulate the behaviour of the 
wall behind the radiator, I added an extra zone. This seems to work 
fine, but I ran into another problem.

Usually when modelling a radiator in TRNSYS, the radiation sent out by 
the radiator is regarded as a radiative gain. But regarding comfort, 
this is not correct, since this implies that the radiation is leaving 
from the zone point in the centre and only the part that is reflected by 
the walls is then felt by the person standing in the same zone point. In 
reality a part of the radiation will reach the person directly.

So I built a wall with the same surface and temperature as the radiator 
and a very low convective heat transfer so that only radiation would 
occur and not convection.
This is working fine at the backside of the radiator, but doesn't at the 
front; the average surface temperature and operative temperature of the 
zone seem OK, but the Qcomi of the hot surface is much too high. Qcomi 
only gives a reasonable value (close to the calculated value for 
radiation) when I reduce the size of the other walls in the room.

Is this strange behaviour caused by the star network ? or another 
simplification in TRNSYS? or did I do something wrong ?
I guess I can simply avoid the problem by recalculating the operative 
temperature myself, but I wanted to use the same strategy to model 
stratification of air above the radiator. Or is there another method to 
model stratification; to build an imaginary wall between two zones that 
allows to have two zone temperatures but can still be transparant for 
long wave radiation ?

Is it possible to change the long wave emissivity of inner wall surfaces ?

And is it still possible to change the time base of the transfer 
coefficient calculations ? I seem to remember that I could change it 
manually, but I can't find it anymore...

I guess TRNSYS16 is not the ideal building simulator for this kind of 
intrazonal problems, but all my other heating system models are working 
fine, so it would be nice to stick to it !

Thanks for reading and hopefully answering some of my questions,
best regards,
Jeroen Van der Veken


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