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

[Sarah Leenknegt]

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 at bwk.kuleuven.be
>> tel. 016 321347
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>