Stratification

[CivvyFSO]

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Sometimes it is truly better to say nothing at all.

[nearlythere]

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[SamFIRT]

Hummm.

This is a debating forum; is it not?

To say that something is as it is, because someone else has said it is; or to quote one scientific principle and demonstrably not understand where that principle comes from; or to dismiss other factors as being irrelevant, or all too difficult; or to accept assumption; displays learning but not understanding.
 
Knowledge is the application of learning. Learning never stops and therefore knowledge must grow. I believe it grows best by debate, peer review and knowledge sharing.

Please feel free to disagree with anything I post. But please outline your disagreements from scientific principles. To simply mock is to display a narrow minded attitude. To mock collectively displays sycophancy.

[BLEVE]

I disagree

[Midland Retty]

I blame the weather personally. The clouds have stratified causing it to go very muggy. Everyone is all hot bothered and narky

[BLEVE]

I'd reckon pasquill gifford stability class D5 by the looks of it

[Phoenix]

I disagree

[SamFIRT]

Oh Dear.

 
 
 
 

[Wiz]

I have decided that potential smoke stratification is a subject best left to the experts who can, not only explain it clearly and concisely, but calculate it's effect with a high degree of certainty. Unfortunately, I haven't yet discovered any of these experts, so I will henceforth treat potential smoke stratification as we must many things in this life i.e.; know that it exists but hope that we never meet it!

[BLEVE]

The 5 seconds relates to the plume transport lag time, which is 5 seconds for all T squared and steady fires at a height of 6 metres.

121 seconds is presumably the time required to complete an evacuation of the premises.

Plume centre line temperature was presumably based on a medium T squared fire at 121 seconds. However, the ambiant temperature should have been added to the 30.6 degrees giving a centre line plume temperature of 48.6 degrees.

 

[CivvyFSO]

Surely the transport lag time depends heavily on the fire size, so in a t² fire the actual time from established burning, and the growth rate used, will alter the lag time significantly?

[BLEVE]

Dunno Civvy

But according to Newman, J.S. 1988. Principles of fire detection. Fire Technology 24(2): 11 6- 127.


Plume transport lag time(seconds) for T^2 fire is stated as tpl = cpl2 H^4/5 tg ^2/5

Where
tpl = transport time lag of plume, s (S); tg = growth time, S (S); H = height of ceiling above top of fuel (m); cpl2= 0.1 (kW);


Taking the above the lag times at 6 metres for

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


Hence this is my assumption for the selection of the 5 seconds in the original post and don’t call me Shirley , I sound like Toddy now, Nah, only if I used a bit more arrogance

 

[SamFIRT]

Are we to assume that ^ = to the power of? ie T^2 = T² ?

[BLEVE]

That's usually the case

[Phoenix]

It's all very theoretical and hence, possibly, unreliable when applied to the real world (including my input above) due to the variability of conditions that actually exist compared to the assumed conditions that exist in the minds of the model designers (i.e. the people who make up the equations).

I'm not criticising, these are our best guesses; but I'm just stating that we should always bear in mind that this is theory based on a number of simplifying assumptions.  And those assumptions may or may not be applicable in any particular case.

A scan through PD7974 part 4 Detection of fire and activation of fire protection systems should demonstrate our inability to predict time to detection with any reliability.  For example, here is a quote from it, "there is little in the way of validated models for the prediction of time to response for smoke detectors."

CFD can provide an answer if properly conducted (e.g. numerous runs with varying parameters).

Stu