[TRNSYS-users] air flow between adjacent zones

[David BRADLEY]

Pu,

  The definition of a zone (or more precisely in Trnsys an "airnode") is 
a volume of a building that can be characterized as having a single 
temperature because the air is well mixed. While we are able to 
physically place airnodes next to each other, TRNSYS is not designed to 
automatically determine the convection between them. There are some 
tools (TRNFlow, CONTAM, and COMIS) that can determine the airflow 
between adjacent airnodes. All three tools are called "bulk air flow 
models" and all make the assumption that the resistance to air flow 
between airnodes is high in comparison to the resistance to airflow 
within the airnode's volume and that flow is driven by pressure and 
temperature differences.

   If you have a single large volume that you want to break up into 
smaller volumes in order to model temperature distribution within the 
larger volume then there are basically two options. One is that you can 
use a bulk air flow model to try and get some idea of the air movement 
even though the situation is one in which there is as little resistance 
to air flow between nodes as there is within nodes. This is particularly 
difficult because bulk air flow models only allow for unidirectional 
airflow through large horizontal openings, a situation that often 
results in numerical instabilities in the model not to mention modeled 
behavior that is pretty clearly unreasonable.

   The other option that I have seen used is to model the full space 
using a CFD tool under a wide variety of boundary conditions so that you 
can essentially map the internodal air flows that result from the 
boundary conditions that will develop over the course of the simulation. 
I have seen some couplings between Trnsys and CFD tools but typically 
the CFD engine takes a very long time to compute the flow rates and it 
is not practical to allow Trnsys to call it every time step.

kind regards,

  David


On 11/14/2021 03:42, Huijie Pu via TRNSYS-users wrote:
>
> Hello,
>
>       I am using TRNSYS to simulate a zonal model, that is, a large 
> space is divided into five subzones. You know, in practice, there is 
> no physical partition between adjacent areas. In the manual of TRNSYS, 
> I don't see the relevant description of the zonal model, but in the 
> relevant literature on implementing the zonal model with TRNSYS, it is 
> pointed out that there is a virtual wall between the two adjacent 
> thermal zones of TRNSYS, that is, the air in the two adjacent zones is 
> not circulating, and the air flow in the adjacent zones needs to be 
> realized with air flow software such as contam or comis, So as to 
> truly reflect the large circulation of air flow in the actual large 
> space. I don't know if this is correct, so I want to ask you.
>
> kind regards,
> Pu
>
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-- 
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
3 North Pinckney Street - suite 202
Madison, WI  53703 USA

P:+1.608.274.2577
d.bradley at tess-inc.com

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