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Topics - 2sBlind

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PyroSim / 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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