[TRNSYS-users] Stability criteria.

Fri Jul 2 00:58:35 PDT 2010

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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Von: David Bradley [mailto:bradley at tess-inc.com] 
Gesendet: Donnerstag, 1. Juli 2010 17:21
An: gossard at cict.fr
Cc: trnsys-users at cae.wisc.edu
Betreff: Re: [TRNSYS-users] Stability criteria.

Didier,
  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 at 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
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