Stack effect and natural smoke venting

[Martin]

I am trying to get my head around this. In the fire safety engineering (FSE)community FDS is frequently used for modelling natural smoke ventilation in large spaces. However, based on my understanding of FDS, modelling differences in the so called hydrostatic pressure requires different zones and hence using functions like the HVAC feature. I have not experienced that these functions are used or even discussed.   

The stack effect caused by differences in the so-called hydrostatic pressure is mostly mentioned in connection to high rice buildings. However, as the smoke layer heats up the hydrostatic pressure differences can be a significant driving force even in relatively low buildings.

My conclusion would then be that most simulations in FSE and even some research is missing a major driving force in smoke ventilation and will then have major errors. Information or discussions regarding these matters is scarce. Is anyone familiar with validation work done on natural smoke ventilation that deals with these matters?

[Randy McDermott]

Natural ventilation in FDS is accounted for because the background pressure used in the equation of state is a function of height, z.  It does not require a zone.  But if different zones exist, the mean background pressure will adjust accordingly.  This is discussed in the tech guide.  Also, see the "stack_effect.fds" case in the Atmospheric_Effects directory of the verification suite.

https://github.com/firemodels/fds/blob/master/Verification/Atmospheric_Effects/stack_effect.fds

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[Martin]

Thanks for the quick reply. The type of simulations I am refering to are large buildings volumes where the engineers most often try to capture the effect of inlet and exhaust vents by using holes and extending the mesh beyond the hole to capture the so called "vena contracta" effect.

Would not this mean that the background pressure would be the same across the smoke vent or inlet vent since z is the same and the hole domain is within the same zone?

A more correct approach might be to use open vents but this again would not capture the  "vena contracta" effect.

[Randy McDermott]

OPEN vents would not be correct at all, as you said, they do not take the vena in to account.

If a window is open and resolved by the grid then the hydrodynamic pressure will force the flow in the appropriate direction.  The thermodynamic pressure is correctly the same both inside and out side the building at the same height z.

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[Martin]

Sorry, I have some trouble getting this. Would not this mean that simulations of natural ventilation only rely on the dynamic forces and not the forces caused by differense in temperature inside and outside (stack effect/ hydrostatic pressure difference)

fredag 31. august 2018 14.27.30 UTC+2 skrev Martin følgende:

[dr_jfloyd]

You could try something like this:

Set TMPA to the outside temperature.

Make the inside of the building a different zone

Use the localized leakage to add multiple leakage paths over the height of the building.

Use INIT to set the inside temperature of the building

Use TMP_INNER to set the steady state wall temperature profile for the exterior and interior surfaces (otherwise they will initialize at TMPA)

You will want to add a set of constant mass flow supply and exhaust vents inside the building. Aim for a typical building air change rate and with that mass flow set the temperature to compensate for the expected leakage flow into the building plus losses through the walls. 

Without a fire and the natural ventilation openings closed run this for a period of time until the leakage flows and interior temperature stabilize.

Then start the fire.

[Martin]

Thank you for the suggested procedure.

I see have been bit quick at drawing conclusions. Got lost in the theory when trying to explain some discrepancies.

That leads me back to one of my questions. Is there any validation work done with natural ventilation in large spaces? P Coyle and V. Novozhilov did a comparison with the work done by T. Yamana and T. Tanaka with FDS 4 https://pdfs.semanticscholar.org/5398/be70492b4fbb63d3e768886f56f9ed961383.pdf

I have tried to replicate this with FDS 6.6 and are getting lower smoke layer heights than the experiment. Any similar experience?  

[Salah Benkorichi]

There was some work I've seen before of Bart Merci group but not sure if it addresses all of your points, as it was more to do with the resolution of the opening, 

do some googling and you might find other researchers who conducted other studies on this.

Analysis of the impact of the inlet boundary  conditions in fds results for air curtain flows in the near-field region

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