Stratification

[CivvyFSO]

Re-reading the original post I have an idea what might be the problem.

Someone has a fire engineer who has created a mythical 5s detection time, and it seems that this is being used to support a particularly low (read: unlikely) claimed evacaution time of <121 seconds. If elsewhere in the very same report the smoke temp is shown to be only just getting to 30C at the time the report is claiming evacuation should be complete, then it doesn't take a genius to suggest that the 5S detection time might be a little bit of an exaggeration.

For info: To work out a smoke detector's activation time the detector is often assumed to be a heat detector with a low RTi (5), and an activation temp of 10C above ambient. Based on this, as far as any calcs would go, they are quite right in suggesting that while the temp is lower than this 31C, detection may be delayed, but this is (as Stu points out) normal behaviour of any growing fire, not true stratification. They are simply using this to show that the 5S is improbable, and using a comment from BS5839 to back that up, not that the fire alarm system is wrong in any way.

IMO the approval body should be going straight for the details behind the ASET time calculated by the engineers. The time to detection, if calculated properly, will be way above 5s. After that they should consider the time that it will take to raise the alarm (typically instantaneous if no investigation time is factored in to the alarm) the time it takes people to react, (anything from 30s upwards depending on the occupancy) and then the walking time to the exits, and/or any queueing behaviour expected.

I think that this is strictly the fire engineers problem (maybe a problem on paper only) and is not a shorftfall in the alarm provision. If for any reason you need detection quicker than the 120S (approx) in order to evacuate people particularly quick, then the risk level seems to be beyond the scope of normal detection requirements and something such as aspirating detection or a way of controlling the risk more effectively is required.

[kurnal]

I agree with civvy- well put.

[SamFIRT]

An inversion of cold air above warm air in the still air inside a building is as likely as a brick floating

Bricks and other solids do float phoenix.

A brick will float on a liquid if in a vessel if the sum of the density of the vessel and the brick is less than the displacement average density. A ship carries bricks in this way. A hot air balloon carries people in just this way. A brick will also balance on surrounding supporting more dense objects with less dense ones under it.ie balances on top of a test tube filled with water. A hovercraft balances on air. An aeroplane can fly. A ball can balance on a column of water.

I have observed the phenomena of inversion first hand and seen people rescued from locations above fires.

Fire service ICS high rise sectorising is based on a search zone above a fire.

Granted building construction and fire safety is a factor in this. But did I not say that it was complicated.

Theorists seem to discount observation…. discuss 

[CivvyFSO]

The brick/vessel argument doesn't really work. Hot smoke will be less dense than cold air, therefore it will rise (Or more accurately the cold air will fall due to gravity, displacing the less dense smoke and forcing it upwards) With the vessel anology you are comparing the density in the way the laws of physics do actually work, you have the less dense (on average) floating above the more dense.

I think that what you are describing with regards smoke is simply more to do with the neutral layer / stack effect which is really more to do with pressures brought on by changes in temperature and height.

Temperature inversion actually involves cold air being trapped under warm air, which is essentially the right way round until taken in the context of the planet and weather.

[Phoenix]

Thank you Civvy.

Sam, I am not entering into an argument about whether or not bricks float.

But I am having trouble visualising a brick in a test tube. 

And, for technical precision, how is the flight of an aeroplane analogous to a brick floating?

Returning to the topic, for this scenario I was looking at it as an open space, whereas you seem to have in mind a much more complex building geometry.  Fair enough.

Stu

[SamFIRT]

Even within an open space stratification occurs, with cold air remaining above, (provided the space is large enough relative to the energy).

Take for example a fire in the open air, on a still day; perhaps a bonfire in a field or park. The hot smoky gasses rise, (true the more dense molecules fall), and a convection current is set up. As the hot smoky gasses rise the heavier particulates reach a point where their buoyancy is outweighed by their density. Therefore the more dense particulates and then more dense molecules start to form layers. (The smoke can be observed to form layers or stratify) the atmosphere above the smoke is at the same temperature as it always was. If this were not the case then all smoke from all fires would always reach space.
 
The other factor in the problem inside a building, is the movement energy (Kinetic) and the mixing into the plume of hot smoky gas stream of the surrounding air; especially relevant to this discussion  if the plume is deflected from the vertical to the horizontal, and then possibly passing a ceiling well.  Hot gasses from a jet engine do not rise until they have lost their kinetic energy of lateral movement. Some surrounding atmosphere is drawn into the plume by Bernoulli’s principle, true, causing eddies and partially cooling the jet stream. However, the surrounding atmoshere is the same temperature it always was; (albeit rising slowly by the radiation from the hot gas stream). There is no convection as the lateral movement is greater than the movement caused by relative density. (Hence the aeroplane analogy).

 

[kurnal]

Fascinating. Makes me wonder how a fire in zero gravity would behave, such as within a space capsule. Persumably growth would be by direct burning only and the fire would develop in a spheriod shape. Smoke would not move at all other than by diffusion. Combustion oxygen would have no route to reach the seat of the fire and so growth would be slow. And the brick and the test tube  would do their own thing.

[John Webb]

Fascinating. Makes me wonder how a fire in zero gravity would behave, such as within a space capsule. Persumably growth would be by direct burning only and the fire would develop in a spheriod shape. Smoke would not move at all other than by diffusion. Combustion oxygen would have no route to reach the seat of the fire and so growth would be slow. And the brick and the test tube  would do their own thing.

Work was done some years ago by Martin Shipp and others from the Fire Research Station in microgravity flights regarding fire behaviour in such conditions; this was in connection with protecting the International Space Station. 

[CivvyFSO]

Even within an open space stratification occurs, with cold air remaining above, (provided the space is large enough relative to the energy).

This to me is simply a hot plume, moving up, entraining more air, losing its energy, and eventually cooling to the same level as the ambient temperature. It would only be a small difference in density between the air containing smoke particles and the clear air by this point. This is no different to any standard convection current, hot air goes up, loses its heat, cold air comes back down.

And gravity has the same effect on things regardless of any horizontal kinetic energy.

[CivvyFSO]

Persumably growth would be by direct burning only and the fire would develop in a spheriod shape.

http://www.youtube.com/watch?v=8IJ74IvpBlU&feature=related

[SamFIRT]

And gravity has the same effect on things regardless of any horizontal kinetic energy

So why do rockets, arrows, stones thrown from slingshots etc etc fly? Regardless of gas plumes.  

Humm... I think you are mistaken Civvy. Acceleration due to gravity is only 10 m/s squared (9. 8 ). Newtonian physics says acceleration forces greater than this will overcome g and the greater the acceleration the less effect g will have on it either directly or as an imposing force creating a resultant vector.

[CivvyFSO]

My apologies to the people with an interest in fire safety.

Arrows/stones will be affected equally by gravity regardless of their speed, to move away from the ground there must be a vertical force greater than 9.8ms^-2. Once any initial upwards force is removed gravity will then slow its vertical speed at the rate of 9.8ms^-2 and then when its vertical speed is 0 it will start to accelerate back down at 9.8ms^-2. Any horizontal force is completely unrelated to gravity until you are going at such speeds where the curvature of the earth comes in to play

A rocket or plane flying horizontally requires upward lift greater than the force of gravity to keep it in the air. A rocket flying vertically is still subject to the forces of gravity, once it can exert a vertical force greater than gravity it will indeed move upwards, but gravity will still be exerting the vertical force. Ignoring the lessening of gravity the further away you get from the planet, and ignoring drag, even if the rocket was travelling 5,000mph, once the vertical force was removed, the speed will diminish at the rate of 9.8m/s each second.

Also, if what you are saying is true then wouldn't skydivers spend a while flying alongside the plane after jumping out? They don't, they drop like a sack of s**t.

In summary: Look up projectile motion.

Sam, you are wrong.

[CivvyFSO]

Especially for you:

http://www.nationalstemcentre.org.uk/elibrary/file/6558/Monkey_and_hunter_teacher_notes.pdf

[nearlythere]

I certainly hope the monkey wasn't harmed during this experiment?

[SamFIRT]

Sam, you are wrong.

Not at all Civvy. What we have both said is entirely consistent.  

Your monkey and hunter experiment assumes both the target and gun are falling simultaneously.

My analogy is for an energy stream. Like a stream of water emitting from a hose. Of course it falls, in an arc. And two things dropped at the same time will fall equally regardless of mass. Galileo. as shown on the moon.

My point is if the flow is great enough then gas above it will not fall through the flow. Rather it will be partially entrained by it; like smoke being drawn out of a window when a jet of water is shot through it in order to ventilate. Or trapped above it as the acceleration due to gravity is less than the forces in the stream.

But if you all feel this is straying off topic when what we are discussing is the production of stratification then I apologise. I thought it was a discussion forum.