Stratification

[BLEVE]

in my experience such calculations are overly conservative when compared to CFD.

[CivvyFSO]

Slow T^2 fire:

0.1*6^4/5*600^2/5 = 5.4 seconds

Moderate T^2 fire:

0.1*6^4/5*300^2/5 = 4.1 seconds

Fast T^2 fire:

0.1*6^4/5*75^2/5 = 2.3 Seconds

So it's not 5s for all t² fires then?

[BLEVE]

5.4 seconds or less it would seem

[SamFIRT]

Well….. at last ...........discussion.   

Is this calculation not for the height a smoke plume will reach in a given time? ? ? ?

How does it apply to stratification if it does not take account of variable density of the Hot Smoky Gasses (HSG), gas movement and differential pressure interplay and relativistic cooling?

[BLEVE]

You will not get stratification at 6 metres. The plume will increase in height as the fire progresses to the point of reaching the ceiling, where it becomes a ceiling jet.

[SamFIRT]

BLEVE

You will not get stratification at 6 metres. The plume will increase in height as the fire progresses to the point of reaching the ceiling, where it becomes a ceiling jet.

I believe you are making the assumption the fire is burning in fuel controlled conditions, therefore it is free burning (there is no smouldering) and it is therefore developing exponentially. I believe you are also making the assumption all the fuel is all the same. I would argue you are not taking pyrolysis into account with different volatiles being released at different stages as a fuel source with the secondary ignition of HSG. However, as that would argue against stratification with a greater increase in thermal activity in the plume I won’t mention it as I would be arguing against myself.  

It seems to me the calculations shown in this post so far rely on a stable output of an (x) MW fire. Ie a snapshot at maximum output or perhaps an average output over a time frame. Is this correct?

If this is the case they have no relevance whatsoever to real life. Stratification is a phenomenon of incomplete combustion and HSG production over time balanced against the volume of the space the HSG is trying to fill. Add into the mix ventilation of both the fire and the building space (atrium, hall, hanger, room, stairwell, lift shaft etc etc ) and then write a calculation. That would be interesting.  

Phoenix

It's all very theoretical and hence, possibly, unreliable when applied to the real world ...  ... due to the variability of conditions that actually exist compared to the assumed conditions that exist in the minds of the model designers

I could not agree more!

[CivvyFSO]

And over to you BLEVE.

[Phoenix]

in my experience such calculations are overly conservative when compared to CFD.

Agreed.  But the conservative solution is not always what is sought.  Coming from a FRS background, I fully appreciate that the conservative answer should always satisfy safety needs.  But now I work for a broader clientele and I have to moot that the conservative solution is, by definition, potentially onerous for the developer.  Any conservative solution leaves room for a more targeted and finely tuned solution.  Returning to the matter of CFD, this can, if properly done, lead to a solution that is suitable for both the developer and the AHJ.

Yes, I agree, there is a lot of importance contained in the use of the phrase, "if properly done."   

Stu

[BLEVE]

Working beyond realms of interweb all week.

Of course we are talking about incomplete combustion, why would we consider otherwise.

Room volume has no bearing on the maximum height of a plume and stratification.

In the case of the 6-metre height as set out in the original post, the heat release rate must exceed 7.95 kW in order to reach the advised ceiling height. This would result in a convective HRR of circa 4.77 kW.

Taking a slow T^2 fire, the plume would reach 6 metres at 52 seconds, 26 seconds if medium T^2 fire. excluding lag times as previously discudsed.

Ultimately, irrespective of fuel type, species production etc, it is the convective heat release rate, height of building and Delta T that will determine the height at which stratification may occur.

[SamFIRT]

Humm

Room volume has no bearing on the maximum height of a plume and stratification.

...it is the convective heat release rate, height of building and Delta T that will determine the height at which stratification...

A Dichotomy in your post methinks 

What we have here is a clash between academia and reality 

[CivvyFSO]

It isn't a clash at all.

The height of the plume is nothing to do with the volume of the room. It could be a 6m high plume in a 6000m³ compartment or a 6m high plume in a 12000m³ compartment, the stratification would occur at the same time/height given the same difference in temperature over the height.

You can raise countless arguments about many different apsects of fire growth, but their effects are negligible compared to the variables being considered.

BLEVE, T^2? shurely shome mishtake? t^2 perhaps? I expect better from you.

[Phoenix]

Whatever.....

Anyway, here is some stratification of smoke that I saw just last week drifting across the Dordogne.  Couldn't resist a photo.  How sad.

Now, what you gonna believe, me or your own eyes?




Stu

[SamFIRT]

Thanks Phoenix. 

I think that proves my point perfectly

[Golden]

Looks more like inversion to me - what time of day was it?

[Phoenix]

Are you just trying to stir it up, Golden!

It was evening.

Actually, now that you mention it I think it is inversion.  To be more precise it's stratification caused by local inversion.  But inversion in climatic terms, as I think Civvy pointed out some time long ago on this thread (I'm not going to look back through) is when warmer air sits on top of colder air, which is exactly the condition we expect to prevail inside an atrium (with the exceptions of the unusual circumstances Sam referred to again long ago).  I believe it's referred to as inversion for weather systems because normally we expect warmer air to be located closer to the ground and the air at higher levels to be cooler, which is absolutely not what we would expect inside a building (remember my floating brick?).

Now, I think we should all be able to agree* and just enjoy a quiet evening overlooking the Dordogne.

Stu

ps * Somehow I doubt it