[TRNSYS-users] digging into trnflow convergence problems

[frodriguezpc at labein.es]

David,

As I mentioned in an earlier post, I was thinking about backflows or 
something similar, except that  am using large vertical openings for my 
windows and interior doors. AFAIK, and  I could easily be wrong, they are 
supposed to allow air in and out at the same timestep. But the rest of 
your explanation makes a lot of sense to me. When I get some time for 
that, I will look at some detailed flow outputs and see what is exactly 
happening in the stack and windows...

Thanks for your help.

Francisco Rodríguez Pérez-Curiel

TECNALIA-Construcción
Parque Tecnológico, Edificio 700. E-48160 Derio (Bizkaia) (SPAIN)
Tel. +34 94 607 33 00,  Fax. +34 94 607 33 49
francisco.rodriguez at tecnalia.com,
www.tecnalia.com

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David BRADLEY <d.bradley at tess-inc.com> 
27/09/2010 16:21

Para
Mariëlle Nuchelmans <M.Nuchelmans at chri.nl>
cc
"frodriguezpc at labein.es" <frodriguezpc at labein.es>, 
"trnsys-users at cae.wisc.edu" <trnsys-users at cae.wisc.edu>
Asunto
Re: [TRNSYS-users] digging into trnflow convergence problems






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

-- 
***************************
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
d.bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com




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