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Re: [TRNSYS-users] User-defined CHTC: pane temperature calculation diverging

Dear David,
Thank you for your answer. I know about Type93, in fact I have tried with one or four time steps, did not make a difference. About the correlation, I have manually tried several variable or constant values, which I would expect would challenge convergence, but as long as I did not use the input value recall, the simulation would run. In the meantime, I was experimenting a bit, and found the following strange solution. The first equation does work, the second one doesn't.. 
(1) max(0.01,2*abs(surftemp-airtemp)**0.31 + 0*time)
(2) max(0.01,2*abs(surftemp-airtemp)**0.31)
I can accept this as a workaround, but would like to understand why it is needed.. 

Thanks,

Sarah
ir. arch. Sarah Leenknegt - KU Leuven - Building Physics Section - Department of Civil Engineering - Kasteelpark Arenberg 40, bus 2447 - B-3001 Heverlee, Belgium - tel. +32(0)16321347 

Op 2/25/2013 6:49 PM, David BRADLEY schreef:

Sarah,
If you are using Type93 for the input value recall please be aware that its first output is not delayed; it is the value of the input at the current iteration. You need to use the second output in order to get the input from the last time step.
Whether or not the above fixes the problem, I don't think that Trnsys3D has anything to do with the issue. Trnsys3D is really just a way of getting data into TRNBuild in a more visual manner. You might also look at your convection coefficient correlation. The correlations that are built in to Type56 raise the surface to air temperature difference to a power less than 1 (dampening the effect of the temperature difference). It may be that your correlation is too sensitive to the temperature difference, leading to convergence problems. 
Kind regards,
 David


On 2/25/2013 04:49, Sarah Leenknegt wrote:

Dear TRNSYS-user,
I have the following problem. I am trying to input a CHTC-correlation in the building model. I am using the input value recall to get the surface and air temperature of one timestep before. I can calculate the CHTC and plot it with an online plotter, but whenever I try to use it as an input, a get the "pane temperature calculation is diverging" warnings from the start of the simulation and the simulation stops. The CHTC is non-zero and positive, and for now, the equation used is simply max(1,2*abs(surftemp-airtemp)). If I calculate a variable CHTC without using the surface and air temperature, the simulation runs without error.
I have used TRNSYS3D to make my one-zone model, including one window facing south-west (TURN = -45°) and 4 air nodes. I am using the detailed radiation model, so all air nodes are combined into one zone. Viewfactors and insolation matrix are generated without problem. I have gone back to the most basic simulation, using only the idf imported in a 3D building project standard template, with a standard Meteonorm Uccle climate.
Following recommendations in previous posts, I have set the CHTC at the window to the standard fixed values for inside and outside. I have also checked if something went wrong when drawing the window in sketch-up, but I do not see any lines that shouldn't be there. Changing to a different glazing doesn't help either.
Can anyone give me some help as to what else I can try? I have done this before without problems, but it is the first time I try it with trnsys3D. Please answer to me privately as well, as I am not subscribed to the mailing list. 

Kind regards,

Sarah

--
ir. - arch. Sarah Leenknegt,
Afdeling Bouwfysica, Dept. Burgerlijke Bouwkunde,
Faculteit Ingenieurswetenschappen, KULeuven,
Kasteelpark Arenberg 40 - bus 2447, 3001 Heverlee, België

sarah.leenknegt@bwk.kuleuven.be
tel. 016 321347
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