Hi all,
in order to avoid these problems with thick and
lightweighted walls, there are another methodes to calculate the heat transfer
through walls, so called finite difference methods.Especially the crank-nicholson method is a very
good possibility to to that. Our first
experiences with a CN-Type are positive. Moreover this method seems to
allow the simulation of pcm-materials.
best
Hans-Jürgen
GN
Bauphysik Ingenieurgesellschaft mbH
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70372 Stuttgart
Tel.: 0711-9548800
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Von: David Bradley [mailto:bradley@tess-inc.com]
Gesendet: Donnerstag, 1. Juli 2010 17:21
An: gossard@cict.fr
Cc: trnsys-users@cae.wisc.edu
Betreff: Re: [TRNSYS-users] Stability criteria.
Type56 uses something called the conduction transfer function (CTF) method for determining the energy transfer through walls. The (admittedly oversimplified) idea of the CTF method is that a series of weighted averages going back in time is developed to tell how important the temperature difference across the wall is now, how important it was an hour ago, how important it was two hours ago, etc. The size of the step back in time is called a time base and it doesn't have to be one hour; you can set it in the TRNBuild "outputs" window . There are also a number of other factors (such as the history of the energy transfer through the wall) for which there are also weighted averages determined. Type56 and TRNBuild are limited to using the first 20 coefficients determined.
If you have a wall that is too massive or not massive enough then TRNBuild has a hard time determining the CTF coefficients. For example, if you have a thick stone wall, the conditions of that wall more than 20 hours ago may matter quite a lot to the energy transfer now (in other words, there isn't enough there aren't enough CTF coefficiencts developed). If you have a very lightweight wall (like a metal door) then all the conditions that matter to the energy transfer across that wall occur right now (in other words the time base is too big). TRNBuild generates the "failed to fulfill stability criteria" error. You can tell which wall caused the problem by looking in the *.inf file. Look first near the bottom and the inf file will show you the coefficients that it generated for each wall. Note the name of the last wall for which it generated coefficients then scroll up to the top of the file. The wall AFTER the one that successfully generated in the list of walls is the one that caused the problem.
If it is a lightweight wall that caused the problem, simply treat it as a "resistance only" in other words, that wall should be made up only of a massless layer.
If it is a very massive wall that caused the problem you can increase the time base.
Best,
david
On 6/30/2010 08:48, gossard@cict.fr wrote:
Hi all,
I'm working with TRNSYS 16 with Type56. TRNBuild suddenly stops with the following error message :
"Error creating the wall transfer function coefficients :
Stability criteria not fullfilled
Check layer definition (thickness, ratio of conductivity/density)
Please, check the INF-file for further information."
What is this "stability criteria" ?
Please find enclosed the INF and BUI-files.
Best regards,
Didier.
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-- *************************** David BRADLEY Principal Thermal Energy Systems Specialists, LLC 22 North Carroll Street - suite 370 Madison, WI 53703 USA P:+1.608.274.2577 F:+1.608.278.1475 bradley@tess-inc.com http://www.tess-inc.com http://www.trnsys.com