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Smoke Getting Logged Inside Volume in Smoke Ventilation Simulations – Seeking Suggestions #13950

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Darwishjahan opened this issue Dec 26, 2024 · 8 comments

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@Darwishjahan
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Hi everyone,

I’m a learner working on multiple simulations related to smoke ventilation, and I’ve been consistently facing an issue where the smoke gets logged inside the volume. Despite following the standard procedures, the problem persists.

Here are the details I’ve worked with:

Extract fans are functioning properly, and make-up air is 85% of the extract.
Fire load, HRR, and heat of combustion are all set correctly.
Open boundary conditions are properly defined and have been verified by a couple of CFD engineers working with PyroSim, who confirmed that the values and model are correct.
Soot yield is set to 0.1 and CO yield to 0.042.
The material used is Polyurethane GM 27, and I have also tried GM 37 with its default values.
Despite these efforts, I still face the issue of smoke being logged inside the volume. I’ve attempted this in multiple projects, and the issue remains.

Has anyone encountered a similar problem or can suggest any reasons why this might be happening? Any help or insight would be greatly appreciated.

Thanks in advance!

@drjfloyd
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If makeup is 85 % of extract, hopefully you are somehow supplying the other 15 % so that you are not drawing a vacuum inside your structures.

In real fires smoke will basically go everywhere at some concentration. Not all concentrations of smoke pose a hazard for occupants trying to egress. It is not clear what you mean by "smoke being logged inside the volume." Just looking at smoke3d results in smokeview does not constitute a quantitative analysis of smoke hazard. You should be looking at quantitative measures of tenability with requirements developed prior to the start of simulaitons. If you are looking at quantitative measures and you are violating your tenability requirements possible factors could be:

  1. Insufficient extract flow. (Have you done an NFPA 92 or similar calculation to ballpark the required exhaust?)
  2. Makeup air interactions with the smoke layer (Does makeup air velocity near the fire meet the requirements of your local codes and standards?).
  3. General flow field resulting from the fire, make up air, and exhaust causes smoke to mix into the lower layer.

@Er9y714
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Er9y714 commented Dec 26, 2024

There was a smoke output visualising issue after restaring the simulation in the earlier versions. It is a slim chance that you are encountering the same bug and using the earlier versions, but wanted to double check. Are you using the latest FDS version?

@Darwishjahan
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Thank you for your detailed feedback. Yes, I am specifically looking at visibility and temperature at a height of 1.8m, as per the local standards, which is a design requirement. This design parameter, along with the values provided for supply and extract, was obtained from a friend working in the industry. He mentioned that this setup has already passed CFD analysis when they used it. It seems that something may have gone wrong in the way I modeled it, as you suggested. He has reconfirmed that this design aligns with the NFPA 92 calculations.

I will take a closer look at points 2 and 3 that you mentioned (makeup air interactions with the smoke layer and general flow field) and will let you know the updates after further investigation.

Regarding the issue with the velocity and temperature measurements, I have placed two velocity and temperature devices at the start and end points of the duct-based extraction system. The velocity is appropriate at both ends, but the temperature coming out of the extract system is ambient, rather than matching the inlet temperature. I have attached the system screenshots and plots for your review. Is this behavior as expected, or could there be something wrong with the system setup?

image
image
image

"Device" is placed at outlet of extract, and "Device 02" is placed at inlet of extract.

Please look into it.

If makeup is 85 % of extract, hopefully you are somehow supplying the other 15 % so that you are not drawing a vacuum inside your structures.

In real fires smoke will basically go everywhere at some concentration. Not all concentrations of smoke pose a hazard for occupants trying to egress. It is not clear what you mean by "smoke being logged inside the volume." Just looking at smoke3d results in smokeview does not constitute a quantitative analysis of smoke hazard. You should be looking at quantitative measures of tenability with requirements developed prior to the start of simulaitons. If you are looking at quantitative measures and you are violating your tenability requirements possible factors could be:

  1. Insufficient extract flow. (Have you done an NFPA 92 or similar calculation to ballpark the required exhaust?)
  2. Makeup air interactions with the smoke layer (Does makeup air velocity near the fire meet the requirements of your local codes and standards?).
  3. General flow field resulting from the fire, make up air, and exhaust causes smoke to mix into the lower layer.

@drjfloyd
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A picture from Pyrosim with all obstructions hidden tells me nothing about what your actual FDS inputs are. You need to provide the actual FDS (not Pyrosim) input file. You can change the .fds to .txt or append .txt and attach to this discussion.

@Darwishjahan
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untitled_(1).txt

Sorry for attaching it in the previous way. I am new into this. A similar file has been attached as txt file. Please look into it.

Thanks in advance.

A picture from Pyrosim with all obstructions hidden tells me nothing about what your actual FDS inputs are. You need to provide the actual FDS (not Pyrosim) input file. You can change the .fds to .txt or append .txt and attach to this discussion.

@Darwishjahan
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There was a smoke output visualising issue after restaring the simulation in the earlier versions. It is a slim chance that you are encountering the same bug and using the earlier versions, but wanted to double check. Are you using the latest FDS version?

Thank you for the response. I have updated to the latest version but it still persists. Also

@drjfloyd
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Look carefully the smokeview rendering of your domain. Do you see any Exhaust ? Your exhaust is 0.2 m in y and 0.3 m in z but your grid is 0.4 m. These OBST are too small to be resolved on your grid. So you actually have no exhaust in your model. You should also have seen this looking at your velocity slice. One of the first things you should do when results do not seem as expected is check very carefully that your inputs have resulted in the geometry model that you want. Things like checking that or your boundary conditions are as expected should be part of this

You have a 40 cm grid. Why have you set SIMULATION_MODE='LES'? Unless you are very familiar with FDS I would suggest not changing default values. The defaut value of VLES was chosen with typical fire protection applications and grids in mind.

You have a 4 m tall room and a 40 cm grid. This is only 10 cells vertically. This is not a lot of resolution. Also for a 2500 kW fire this is D*/dx of < 4 which is not well resolved for the fire either.

SURF_ID='INERT' is a wall that is always TMPA. Does this reflect the actual construction of your building?

You have a sealed domain with a fire and unbalanced supply and exhaust. You are either going to pressurized or depressurize your building to non-physical values both of which are going to have negative effects on the validity of your results.

I would suggest starting with simpler problems with smaller domains that run quickly where you test input and learn to use FDS.

@drjfloyd
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Also should note that for a smoke control system supplying air into layer is counter productive and violates the requirements of both the IBC and NFPA codes.

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