[TRNSYS-users] (no subject)

Thu Feb 11 00:18:01 PST 2010

Hello David,

Thanks for your answers. I still have some questions.

You said that I can use geosurf for the longwave radiation, but I thought geosurf is only useable for shortwave (sun) direct radiation.

I still don't understand how trnsys determines the resistance to long wave-radiation (Ri-j,rad = 1/(e * A * Gij * sigma * Tmean3) [Seem]

 1.  Gij is the absorption factor (fraction of energy emitted by surface I which is absorped by surface j,[Gebhart].
 2.  Tmean is the average temperature of surfaces in room.

Ad 1: Gebhart calculates the absorption fraction by using view factors. Because trnsys 16 doesn't have view factors, I wonder how this factor is determined.

Ad 2: Does trnsys calculate the average temperature every timestep? Because in the manual (pg. 6-79) they refer to a RTEMP which is used to linearize the longwave radiation exchange. So I suspect this temperature is the mean temperature. So Tmean = RTEMP (RTEMP is a constant)

Best,

Mariëlle Nuchelmans
________________________________
________________________________
Van: David Bradley [mailto:bradley at tess-inc.com]
Verzonden: woensdag 10 februari 2010 22:44
Aan: Mariëlle Nuchelmans
CC: trnsys-users at cae.wisc.edu
Onderwerp: Re: [TRNSYS-users] (no subject)

Mariëlle,
  In TRNSYS 16, the absorption for solar radiation can be set using the wall type manager. The emissivity for the wall surface, however is hardcoded inside Type56. You are correct that Type56 uses a star network for radiation exchange. All surfaces radiate to the airnode and the airnode radiates to all surfaces. There are no view factors. The one control you have is the geosurf factor with which you can specify the fraction of longwave radiation entering a zone that falls on a particualr surface.

 In TRNSYS 17 you can define "radiation zones" which DO have view factors provided that you have used the Trnsys3D plugin for Google SketchUp to generate those view factors for your building's geometry. You can also adjust the emissivity of the walls within TRNBuild.

  I am not as sure what to suggest for setting the temperature of the wall. Inside (and outside) surface temperatures are calculated as part of the zone energy balance. You might be able to use a thermostat and the "wall gain" feature to control the inside surface temperature of your warm wall.
Best,
 David

Mariëlle Nuchelmans wrote:
Hello,

I'm trying to understand how trnsys deals with long-wave radiation. I know that trnsys calculates it with the star network method from Seem  (adjusted by Koschenz in 1997). I looked at the method and I wonder how the absorption factor between surfaces is determined. Seem refers to Gebhart, but this method uses view-factors and trnsys doesn't know them. So how does trnsys determine this absorption factor?

I also wonder if it is possible to adjust (or make variable) the mean average wall temperature (used to linearize the long-wave radiation exchange between surfaces between zones). In my project I have one wall which probably is a lot warmer than the other walls.

Met vriendelijke groet,

Mariëlle Nuchelmans
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