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boundary conditions for HVAC grille analysis

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Old   September 20, 2015, 04:57
Thumbs down boundary conditions for HVAC grille analysis
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Hi everybody,

We are working in a system with an HVAC duct with two outlets, something like in this sketch][/url]

My question is: doesi it makes sense using a model applying the following BC?:
- inlet flow fixed, and P=0 at every hole in the front surface of the plate and the slots (almost aligned with the duct wall).

I donīt feel too comfortable setting P=0 at the exact holes of the perforated plate or at the slots, but in this phase the main objective is
just to model the pressure loss needed. is that a good practice?

Another way around that makes me more comfortable with the physics, is to make a "free stream" space rounding the duct and assign P=0 to the walls, then inspect pressure jump from inlet to diffusers outlets.
I feel it like it makes more sense, but the department that makes our CFD doesnīt think like that and I donīt feel with enough background to told them how to do it.

Regards and thanks for reading.
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Old   September 21, 2015, 03:29
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I found this, as an example, in a CFD soft documentation:

Outlet conditions should be positioned far enough downstream from sudden turns or contractions to allow the flow to reach a fully developed state, which is the condition assumed by AutodeskŪ CFD. Furthermore, if the outlet is too close to a sudden expansion, flow will come back in through the outlet. This may cause convergence difficulties:

So I would say that it is not OK to set the outlet P=0 at a fluid accelerating zone, like in the slots of a diffuser.

I would appreciate any feedback to know if this is the correct direction.

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