Re: [TRNSYS-users] digging into trnflow convergence problems

[David BRADLEY <d.bradley@tess-inc.com>]

Francisco,
  I believe that TRNFlow's "horizontal opening" models allow flow only in one direction at a given time step. I would guess that what happens in your simulation is that when the zone is hotter than ambient, the building wants to exhaust air and that when the building is colder than ambient it doesn't (maybe it even backflows). Since air and go only one direction at a timestep, the model tries to exhaust air but enough goes out during a timestep that the building decides that no, it should intake air and tries that. But it intakes enough air that the building controls decide no, it should exhaust...and round and round, never converging.

  I believe that TRNFlow has something called a "relaxation solver" built in (controlled from the TRNSYS control cards) that tries to cope with this problem by first relaxing, then tightening the solution tolerances to come up with a stable solution. A trick that is possible when using CONTAM (type97) is to use a short timestep (1 minute max) and delay the zone temperatures that are being passed to the airflow model but I don't think that's possible with TRNFlow. There is some research being done on how best to provide a path up and a path down for airflow in a bulk airflow model such as CONTAM or TRNFlow (COMIS) but as it is a cooperative agreement and not funded, there isn't a timeline on it.
Regards,
 David


On 9/27/2010 02:59, Mariëlle Nuchelmans wrote:

Are you using a thermostate or an equation to control the cooling? You should use an thermostate. An equation gives a zero or an one, so it can go off/on/off/on and your program crashes (on because it reaches setpoint and then off because wanted temperature is reached) A thermostate has a dead band and it chooses on/off after 5 iterations. It is less accurate but your program runs.

 

Good luck

 

Mariëlle

 

Van: frodriguezpc@labein.es [mailto:frodriguezpc@labein.es]
Verzonden: maandag 27 september 2010 9:42
Aan: Mariëlle Nuchelmans
CC: trnsys-users@cae.wisc.edu
Onderwerp: RE: [TRNSYS-users] digging into trnflow convergence problems

 

It crashes even at n=7 timestep... I am pretty sure it has to do with this temparature thing. A check to my lst file shows this clear pattern with lack of convergence in al the external temperature peaks, so I turned off cooling and the result is I get convergence thanks to interior temperatures going above exterior at most of the previous conflict timesteps... still there are a couple times of year where I still get higher ext. temps, and convergence problems show

Francisco Rodríguez Pérez-Curiel

TECNALIA-Construcción
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Mariëlle Nuchelmans <M.Nuchelmans@chri.nl>

27/09/2010 09:18

Para	"frodriguezpc@labein.es" <frodriguezpc@labein.es>
cc	"trnsys-users@cae.wisc.edu" <trnsys-users@cae.wisc.edu>
Asunto	RE: [TRNSYS-users] digging into trnflow convergence problems

 

You could try even smaller time-steps, I’ve models with timestep: 0,03125 (n=5), 0,015625 (n=6) and even 0.0078125 (n=7). It just takes a while before it’s finished calculating.
 
Kind regards,
 
Mariëlle
 

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