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Author Topic: Problem with open vent  (Read 7163 times)

Swissi

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Problem with open vent
« on: February 15, 2011, 08:12:35 am »

Hi everyone,
I''ve a problem in setting an open vent at the top of a room (to allow the smoke to go out).
When I set the vent (Open surface), the smoke is not going out.
I suppose I''ve a problem with the mesh boundaries but I don''t really understand what to do.
I''ve just set a normal mesh.

What do I have to do? Thanks a lot!!
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Charlie Thornton

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Re: Problem with open vent
« Reply #1 on: February 15, 2011, 08:50:37 am »

It would be best if you could post your FDS input file - then we can probably tell you exactly what is going on.

Otherwise:

- When you run the simulation, is FDS printing any errors or warnings about VENTS?
- How did you make your open VENT? The way I like to open the top is by right-clicking the MESH (in the tree), then selecting the Open Mesh Boundaries option. That will add hidden open vents to all six sides and you can delete any open vents you don''t want.
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Swissi

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Re: Problem with open vent
« Reply #2 on: February 15, 2011, 08:58:56 am »

Hello Charlie,
Thanks for your answer!

I think I resolved the problem. Indeed, I didn''t well understand the principle of open mesh boundaries options. I read it in the FDS user guide but I didn''t understand the concept. Could someone please help me?  ???
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Charlie Thornton

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Re: Problem with open vent
« Reply #3 on: February 15, 2011, 09:31:44 am »

I don''t think I can say it any better than the FDS users guide. From section 7.4.2 (Special Vents):

Quote
...it denotes a passive opening to the outside. By default, FDS assumes that the exterior boundary of the computational domain (the XBs on the MESH line) is a solid wall. To change this, use an OPEN vent as if it were an open door or window.

That section goes on to describe what parameters control the environment beyond the open vent.

What part is giving you trouble? If you aren''t comfortable with how FDS is doing the solver calculations across the VENT, you might want to check out the technical reference. If it''s something else, maybe we can help.
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mdangelmaier

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Re: Problem with open vent
« Reply #4 on: November 17, 2015, 09:37:11 am »

I was wondering how did you doing it eventually? I'm kind of having the same trouble thus I tried making a very simple model to understand how it works.

Still the smoke stays inside the mesh though. I have attached the .fds file maybe I am doing something wrong?
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Charlie Thornton

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Re: Problem with open vent
« Reply #5 on: November 17, 2015, 11:00:07 am »

From what I see, the OPEN vent at the top is working. The burner is so big (~3400 kW) that it supplies adequate smoke and fire to keep the compartment full despite the top vent. If you plot velocity or pressure and run with and without the top vent you'll see the difference.
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mdangelmaier

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Re: Problem with open vent
« Reply #6 on: November 17, 2015, 11:53:12 am »

You are right Charlie it is actually working. I put a way smaller fire and it is working. I am wondering though how you would
plot velocity in z-direction?
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Charlie Thornton

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Re: Problem with open vent
« Reply #7 on: November 17, 2015, 02:04:34 pm »

I should have said to add a slice to plot velocity or pressure. For example, if you add an y=0.0 slice and set vectors to true,  you can see the vectors pointing out of the top when it is open and the circulation when the open vent is missing.
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