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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: Smoke ventilation from parking lots

12-17-2013 06:06 AM in reply to: sahar.rubin

Hi Sahur,

 

Is this the same CFZ as last time or did it not save correctly? 

There are a few things I was expecting to see that I cannot:

 

A gap under the fire - this needs to be connected to the main air domain. We could just make that solid block air in the meantime as that would be closer to what we need but it would be better to change the CAD.

Fan mesh refinement (you can use groups here to speed this up and assign them all at once)

Air jet regions.

 

Kind regards,

Jon



Jon Wilde
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Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-17-2013 06:35 AM in reply to: wildej

it's the same .CFZ because there is a gap under the fire, and in my other computer i did the adjustments and arrange the mesh as you said, 

the results are after the changes.

 

so, what's wrong ?
why doesn't the space heat??? (now the other computer continues and after 250 iterations the max temp. is 32.071)

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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: Smoke ventilation from parking lots

12-17-2013 07:05 AM in reply to: sahar.rubin

Hi Sahur,

 

There do need to be some changes made, I can do some of this but some needs to be done in CAD.

 

1. The underside of the fire must be connected to the main air volume - so that the clean/semi-clean air can enter, flow upwards through the 'fire' and exit out as 100% smoke. Trim off the bottom section of that solid part you have here to allow for this.

 

Airflow.png

 

2. Add some refinement regions for the fans - I recommend doing this in CAD, although as your fans are in-line with the x and y axes you could use meshing regions

 

Example Mesh Region.png

 

 

3. Another point I noticed is that we should also assign an Initial Temperature to the air volumes.

 

Does that all make sense? I am not sure if I am making myself clear.

 

Kind regards,

Jon

 

 



Jon Wilde
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Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-19-2013 12:35 AM in reply to: wildej

Hi again :smileyhappy:

ok, i did everything you said and "surprisingly" it worked great!

the fire burns, and the smoke commensing, some problems though:

 

1. I can't ventilate the smoke ! i extract 60000 cfm for 16 air changes and i can't really understand the results, (2 attached)

 

2. how can i know how much wattage to assign for the fire? 5000W doesn't really affect the volume and 4MW is really too much, now i tried 0.5MW but it's just a speculation...

another thing, i know fire changes during time, don't i need to make the total heat generation transient?

 

3. it it correct to assume that scalar is smoke (thats why inlet gets 0 and fire gets 1), if so, why isn't there a similarity between smoke visibility and scalar?

4. how can i see the results as is in the guidelines

 

thanks for everything! :smileyvery-happy:

 

Sahar

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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: Smoke ventilation from parking lots

12-19-2013 01:51 AM in reply to: sahar.rubin

Hi Sahur,

 

I just ran this for 20 iterations (timestep 0.5 - I think 1s is too large, 0.5 is pushing it). I did spot one other point which I am not sure I mentioned. The fan mesh refinement also needs to be applied to the fire - at least 5 elements from bottom to top (inlet to outlet).

 

To answer your questions

 

1. I can't ventilate the smoke ! i extract 60000 cfm for 16 air changes and i can't really understand the results, (2 attached)

You have a constant smoke production, should this be the case?

Have you checked that the flow is behaving as you expect? Use global vectors to check inlets and outlets and vectors on cut-planes to help visualise it

 

Global Vectors.png

cut-plane Vectors.png

 

Looking at a vertical cut - we may need more mesh - there is still not additional volume to capture the jet from the fans.

Try a 0.7 refinement to the air and see if the results change - ideally they will not as we already had enough mesh.

 

Jet fan mesh.png

 

2. how can i know how much wattage to assign for the fire? 5000W doesn't really affect the volume and 4MW is really too much, now i tried 0.5MW but it's just a speculation...

another thing, i know fire changes during time, don't i need to make the total heat generation transient?

There are so many different specifications here that it is hard to comment. I have seen a wide range of heat loads assigned... Do you have any sort of specification to meet that might provide some guidance?

 

3. it it correct to assume that scalar is smoke (thats why inlet gets 0 and fire gets 1), if so, why isn't there a similarity between smoke visibility and scalar?

Scalar 1 is smoke. Scalar 0 is air. The diffusion coeff tells CFD how well they mix.

There shoud be a rough similarity between the two but they are not the same values  - please see the attached

4. how can i see the results as is in the guidelines

Use an ISO surface - I believe a few were used here. Clearly, they only show a single value for scalar so you may need a few to cover the full spectrum.

 

Quick example (you can right click on the legend to tweak colours):

ISO.png



Jon Wilde
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Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-19-2013 02:18 AM in reply to: wildej

Thanks for the fast reply (Once again :smileyhappy: )

 

- I changed it to timestamp 0.5 and the 5 elements to the fire as you said.

 

1. The case supposed to simulate a car catching fire, something like:

 1.JPG

 

the smoke coming out of the car is pretty constant, it can be an assumption.

The flow is behaving exactly as I expected.

Still, why doesn't the smoke visibility clears near the exhausts???

 

2. Don't have any guidelines for the wattage, it changes as I said.. I thought you could recommend on something from your experience.

 

3. OK

 

4. So I use the ISO surface for scalar (and just paint it - from black to white ?

 

Thanks again! 

I'm summarizing all of our comments to a document, when it'll finally be as we want and have all the conclusions I'll upload it so everyone can learn and you won't need to deal with people like me :smileyvery-happy:

 

Sahar

 

 

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Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-19-2013 02:20 AM in reply to: sahar.rubin

The enlarged picture:

 

1.JPG

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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: Smoke ventilation from parking lots

12-19-2013 02:44 AM in reply to: sahar.rubin

You can assign a transient load to the fire, no problem. The only tripping point CFD seems to have is a transient scalar value, that needs to stay at 1.

 

I will check with my colleagues on this one, right now you have 162C so can certainly use a higher power output.

Have you tried assigning the first curve you have shown?

 

As for the outlet, I need to run it out a little. I would have thought the smoke would only fully clear once the fire had stopped but at least you can control it's path of evacuation?

 

Kind regards,

Jon

 

 



Jon Wilde
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Active Contributor
sahar.rubin
Posts: 26
Registered: ‎12-03-2013

Re: Smoke ventilation from parking lots

12-19-2013 02:53 AM in reply to: wildej

i can set the fire evacuation route and that's ok, but i need to extract the smoke to enable people to escape and make the air breathable,

 

now i tried to do the same without fans and extraction, the temp was raised but there was less smoke to deal with, it just clears out for me that the extraction or smoke doesn't work, can you try it also and see the differences?

 

how did you make this beautiful ISO plane? could you send pics of the preferences? or write them down?

 

thanks !!!

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Product Support
wildej
Posts: 755
Registered: ‎08-25-2011

Re: Smoke ventilation from parking lots

12-19-2013 05:46 AM in reply to: sahar.rubin

Regarding the ISO surface, I only did 3 things:

 

  1. Add an ISO surface
  2. Change the quantity to Scalar (I think I had Colour By as velocity)
  3. Go to Edit and drag the slider to the desired value

You can then right click on the Scalar and Velocity legends and change the scale/colours.

 

As an additional and important note:

 

You also still need to add mesh refinement regions for the jets.

 

Kind regards,

Jon



Jon Wilde
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