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Author Topic: Smoke Layer Not Descending Past 3 Meters  (Read 2279 times)

2sBlind

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Smoke Layer Not Descending Past 3 Meters
« on: August 24, 2015, 10:16:11 am »

Hello,

I've created a model that, if possible, I would like to get some feedback on.  I've got a fairly large room with a concrete slab, steel walls/roof, and some concrete shielding (thermal properties for the surfaces have been applied).  The fire source is a ~4MW fire and the model has been set up for 900 seconds.  The issue I'm having is that at around 300 seconds the smoke layer (per the layer height output) seems to stay constant at about 3 meters and stays this way for the rest of the model run time.  A door has been added to the model, so it shouldn't represent a sealed box, but the 3m height is above the highest elevation of the door.

With the HRR staying constant at 4MW for the entire run time after the ramp time (so always generating smoke), and with the boundaries and concrete shielding absorbing energy (reducing smoke temperature and buoyancy), I would have expected the smoke layer to descend to the floor or to at least reach equilibrium some where within the height of the door.  Although the smokeview model seems to show smoke reaching the floor, no smoke is observed flowing out of the door and the layer height output (see the attached pdf on page 23) stays constant at about 3m.  I've talked over the results with some other people and the consensus seems to be that where the layer height is being measured, the concrete shielding is trapping a pocket of ambient temperature air that is stopping the smoke from descending (ie 'bouncing' or 'deflecting') due to temperature differences. 

I'm going to rerun the model with some more outputs inserted (vector velocity slice at the door to see if cold air is being drawn in, visibility slices near the floor, layer height somewhere out in the open, etc) but I wanted to see in the meantime if anything with the model stood out as being wrong or if the output could be correct.

Additionally, why are the displayed textures in smokeview different from when smokeview is opened through the simulation run dialog box and the smv file in the output?  Also, how is the smoke layer height output measured?  Is it by temperature difference or by a soot density threshold?

Let me know what you all think.

Thanks.   
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2sBlind

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Re: Smoke Layer Not Descending Past 3 Meters
« Reply #1 on: August 26, 2015, 11:32:41 am »

Hi All,

Is anyone able to take a look at my input file and see if everything looks normal or if something jumps out as incorrect?  I would appreciate it!

Thanks.
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Dave McGill

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Re: Smoke Layer Not Descending Past 3 Meters
« Reply #2 on: August 27, 2015, 08:53:32 am »

Hi,

There are a couple of issues at play here. This is reflecting a change in the default settings for Smokeview.

In PyroSim, you can create all sorts of fancy geometry, such as the angled walls in your model. It is also possible to import very complex geometry from REVIT or 3D CAD files.  What you see in PyroSim may not have a very tenuous connection with what is built and modelled in FDS. When a file is exported from PyroSim, the details of the complex geometry are contained in the file jobname.ge1. In the the past, this information was not shown in Smokeview unless it was explicitly loaded (Show/HIde -> Geometry -> Obstacles -> CAD). It is now loaded by default, and can be turned off with the "q" key.

When you load your file the geometry looks like what you saw in PyroSim, but which is not what was modelled. Load up your model in Smokeview and hit the "q" key. Note how the angled walls change to a "stair-step" appearance and the door disappears. The latter reflects the width of the door hole and the cell size used in that area. So, the model you ran does not have a door, and your fire is in a sealed box.

See the attached file with a new door that spans two cells, and the wall.

Dave
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Dave McGill
firemodel.ca