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

[frodriguezpc@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@tecnalia.com,
www.tecnalia.com

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

27/09/2010 16:21

Para	Mariëlle Nuchelmans <M.Nuchelmans@chri.nl>
cc	"frodriguezpc@labein.es" <frodriguezpc@labein.es>, "trnsys-users@cae.wisc.edu" <trnsys-users@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

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David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

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d.bradley@tess-inc.com

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