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FIRE SAFETY => Fire Alarm Systems => Topic started by: Wiz on July 14, 2011, 03:24:11 PM
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Assistance required please gentlemen.
All I know about stratification (as in that likely to affect smoke from a fire) is that mentioned in clause 22.1 of BS5839-1:2002+A2:2008. I have been given the task to answer all questions raised by the following comments made by someone in respect of an installed fire detection system
There are 3 high level smoke detectors, their actuation may be delayed by smoke stratification. Calculations use 5 seconds as the time to AFD actuation, which may not be realistic at the best of times.
Smoke temp is said to reach 31 degrees C at 121 seconds, this is the time an evacuation should be complete.
But - smoke at 31 degrees C may not reach detectors for some time on a hot day, as ceiling temp could be around 31 degrees C
Can anyone throw some light on how the figures of 5 seconds, 31 degrees C and 121 seconds have been calculated and also any data on when and how it is ascertained that smoke stratification is likely to occur in any given size of area or construction and/or use of that area. Or point me in the direction of where I might find this information.
Finally, the above mentioned BS clause (22.1) which mentions 'stratification' is only a 'Commentary' clause, and the actual 'recommendation' clauses following the commentary, do not, as far as I can tell, contain any specific recommendations created to deal with the problem of potential stratification. The commentary does suggest that additional detectors might be required to be sited at a lower level if stratification might occur; How might this be achieved in an area with 4 vertical walls and a horizontal ceiling (other than with beam detectors - which might be impractical)?
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Dr Wiz
It appears on first reading that fire engineering calculations have been carried out to justify a particular design, probably a fire engineered solution, and one of the enforcement agencies has aked questions about the parameters used.
It is impossible to answer the detail of these questions. Whoever did the calculations appears to have assumed the time from initiation of the fire to the detection of smoke would be 5 seconds- surprisingly quick, and that the smoke temperature in the modelled scenario would only reach 31 degrees before everyone was outside the building. The enforcement agency quite rightly has pointed out that smoke at this temperature may not even reach the ceiling on a hot day as ambient temperatures will be of this order. So its not a problem for the fire alarm designer or the installer to resolve- the fire engineer needs to review the whole basis for their calculations from the beginning.
If you go down to the library CIBSE guide E is a good starting point to read up on this but it really should not be your or your clients problem.
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Cheers for that answer Prof. K.
Out of interest, who would you guess has originally calculated the modelling, and is 31 degrees C a typical figure for the heat from a typical fire at ceiling level in a space (exhibition display area) with a ceiling height of 6 metres?
Has anyone else got anything to add to the Prof.'s comments (not that I don't believe him - he is my hero ;))
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It all depends on what is burning and how long it has been burning for. What someone is suggesting is that 121 secs into the fire they have modelled there will be no significant increase in temperature at ceiling level. Tha might be true for a very small fire indeed or one which is growing very slowly but we cannot comment without knowing what there is to burn and what ignition sources there are. To suggest such rapid detection will occur together with such a slow rate of fire growth appears on the face of it to be contradictory.
I believe the comments on stratification were a signpost to say go away and reconsider the problem not to try and design a fire detection solution based around the scenario.
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I am much inclined to the Prof's lines of thinking. In an exhibition area with 6m ceiling height I would have though detection times of tens of seconds would be more realistic. Faster-growing fires will presumeably be the more hazardous but on the other hand the plume temperature of such a fire will increase much quicker as well and overcome any stratification sooner. (On that score I have seen temperatures under a 12m flat ceiling/roof construction reach between 35/40 degC due to a hot sunny day outside the building.)
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All of the above is true. But the actual process of stratification is not just dependant on the convection heat from a fire but also on the phenomena of inversion. Colder more dense air at height in a building sometimes acts as a ceiling itself and prevents the less dense hot air from rising through it. This effect is enhanced if there is a trap in the ceiling shape creating an inverted pool where there is no ventilation to allow air movement and is therefore dependant on the shape of the building. You can for example have a situation where hot smoky gasses on the ground or first floor are unable to rise to the third floor due to this effect. This is sometimes believed to be caused by the cooling of the plume but it is not always the case.
What is true………. is that fire dynamics are very complicated……
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Thanks for all your answers guys, which I am finding most informative and enlightening.
There appears to be so many 'variables' and 'what ifs' that it is obvious that there must be no simple answer to the question of how 'stratification' might occur in any instance. Indeeed the commentary in BS5839 states; It is usually difficult to predict with any degree of certainty the level at which stratification occurs....
I wonder, therefore, how BS can provide a whole load of recommendations for the siting of detection and including recommendations in areas with mounting heights of upto 18m for the most common, i.e. point, detection on the basis that stratification won't occur when it now seems obvious that some people see stratification as a major issue in preventing the detection from working and also that it is difficult to predict when it will or won't occur.
It seems to me that either BS recommendations should be valid for mounting heights of only up to such height that is determined to be that which stratification is very unlikely to occur from the type of fires most likely to occur, or there should be more recommendations in BS confirming how the problem can be overcome rather than the very basic information contained in the Commentary; supplementary detection may be provided at lower levels in the hope of detecting the stratified layer .
Alternatively, is it possible that the 'what if' scenarios of stratification are so unlikely, or the modelling requirements for calculation so onerous, that the likelihood of it being a real problem in a system designed to all the current BS recommendations, is so small that it should be ignored for most circumstances?
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Could I ask whether this system is for an engineered solution and if it is protecting some kind of special risk or as a compensatory factor? When has a 6m ceiling been 'high level' - the CIBSE guide talks about 10.5m as a limiting ceiling height.
There seems to be a whole lot of figures being bandied about in this query however I do agree that ceiling temperature (dependent on ceiling construction/air handling etc.) could be above 31o - I would also be interested to know what design fire is being used to get the figures of 31oC after 121 seconds. An interesting post!
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Golden, the smoke detectors in this area are part of an L1 system and have all been sited as per the recommendations of BS5839-1. As far as I know they are not installed specifically or in a particular manner as protection of some kind of special risk or as a compensatory factor.
I'm interested to hear your comments about the mounting height and the CIBSE guide. Are you saying that the CIBSE guide infers that the likelihood of smoke stratification is only at 10.5m and above? If so, can you provide more details?
The details in my original post in italics are as written by a Fire Officer. The document was handed to me for my opinion by a fire system installer who had not even heard of the term 'smoke stratification'. I have told him about the basics of stratification of smoke but could not answer anything about the facts and figures quoted in the document. Prof. Kurnal has given me the impression that, at least some of, the facts and figures in the document have obviously been calculated by an unkown third party and the Fire Officer is using them to warn of the likelihood of smoke stratification. I have had no input into the design or installation of the fire detection system.
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No Dr Wiz to be clear I am saying the fire officers comments have been misunderstood. I am sure the fire officer was NOT saying go away and install a fire alarm system that takes stratification into account, he was saying go away and tell the fire engineer who came up with these figures to
*$%^ &$$ undertake a formal review of his objectives methodology and parameters to determine whether they are appropriate.
Stratification may be a total red herring here. What we need to know is this- for the likely fire scenario and growth how long will it be before the fire is detected. That is the job of the fire engineer not the fire alarm engineer.
In simple terms as I read it the Fire Officers view was this :
"The fire engineer has presented a report that says we needent worry about what happens in the bui,llding in a fire because the alarm will be raised early and people will have enough time to leave before it gets hot and smokey. The trouble is that if we were to believe the report presented by the fire engineer and the numbers he chose to make his case, it is actually likely that on a warm day the smoke will never ever reach the detector so the alarm will never sound and the chance therefore of anyone hearing the alarm within 5 seconds, responding and leaving the building within the 121 second evacuation time that you claim is cloud cuckoo land sunny jim"
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Thanks prof. K.. Actually, my earlier understanding of the comments was basically how you have subsequently described it, and not anything else.
Obviously someone came up with the figures presented to the Fire Officer and I have no clue who that was. I certainly don't think it was the fire aalrm system installer (who had never heard of smoke stratification anyway!)
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I think kurnal has got to the root of this problem quite succinctly (as usual) but here is some peripheral information.
There will be no stratification in a room 6m high though, of course, it will take longer (as said above) for smoke to reach a detector at the ceiling. A smouldering fire might smoke log the space without the detector being reached but that is not strictly the same as stratification.
There is a model for stratification above an axi-symmetric fire that is quite simple to apply but even with with a very steep temperature gradient across the height of the space (i.e. where ambient air is much warmer at the top of the space compared with the base) typical values for the height that smoke will rise to are well in excess of the 10.5m mentioned above. For example, for a small 50kW fire in a 10m high room where ambient air is 20 degrees hotter at the ceiling than at the base (which is, frankly, much worse than would realistically be expected), the model predicts that smoke will rise to the full height of the room. What the model doesn't tell you is how quickly it will get there. For that, I would say you would need CFD (or a full scale test!).
There would have to be unusual air movements around the top of the space to maintain the inversion mentioned above. An inversion of cold air above warm air in the still air inside a building is as likely as a brick floating. Having said that, HVAC systems could maintain the phenomenon.
Out of interest, I tried many fire growth scenarios using different fire growth rates, radiative heat loss coefficients, air entrainment coefficients, etc and I could get close to 31 degrees at 121 seconds but not spot on. It would seem about right for a slow to medium fire growth rate in such a space.
Coming back to practicalities and solutions, I'm surprised no one has suggested the use of beam detection angled diagonally down through the space to cover the maximum volume possible - possibly the use of two - to give the earliest detection and warning. Or an aspirating system. Point detection is problematic anyway on high ceilings, how do you get to the things to test and maintain them? I've seen fun with cherry pickers, cracking marble floors and breaking glass channelling screens. I've even come across people dangling out of loft access hatches to reach the things.
Stu
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Just to back up Stu's version:
Using FPETOOL, a slow fire gets to just less than 31C in 121s and a medium fire gets to just over 31C in the same time. Different methods get different results regarding the temp and velocity of the plume, i.e. Mowrer and Alpert, and I don't know which method FPETOOL uses but it would probably explain any such difference.
The person stating that if the smoke is the same temp of the room that it would stratify is almost right, but no account there has been taken of the temperature being an increasing value which would quickly pass the 31C mark.
I personally think that you have two problems here: the fire engineer not doing his job right, and the approving body possibly not having a proper grasp of what is going on.
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Thanks Pheonix and Civvy FSO for the additional interesting and informative comments.
To pick up on a couple of things that Stu mentioned;
(1) Using 'angled up/down beam detection' - is this a common solution to the problem of providing additional low-level protection in the UK? I personally have never seen it, but I did find it mentioned in a document of USA origin:
http://www.clubs.psu.edu/up/ashrae/Archives/Events2003-2004/Klote-PSU-Talk.pdf
I have never seen the possibility of using angled up/down beams mentioned in manufacturer's literature or BS5839-1 even though I can see that it could work and could be a good solution to situations where smoke stratification is likely.
(2) The difficulties of reaching point detectors mounted at height are well known. However, if we are talking about the most likely scenario, after installation, of just removing and replacing detectors, this is not normally too much of a problem at 6m. Obviously using beam detection can be a good option, but it is expensive. This is especially true when supplementary beam detection is also required at lower levels since the spacing between beams needs to be less. It is quite easy to end up with four beam detectors in a space where one might do the job before smoke stratification comes into the calculations
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No Dr Wiz to be clear I am saying the fire officers comments have been misunderstood. I am sure the fire officer was NOT saying go away and install a fire alarm system that takes stratification into account, he was saying go away and tell the fire engineer who came up with these figures to *$%^ &$$ undertake a formal review of his objectives methodology and parameters to determine whether they are appropriate.................
Upon reading your post again, Prof. K, I would now add something that I hadn't previously mentioned. The author of the comments had preceded them with the comment; We were told this would have BS5839 L1 cover, this is not the case .
Do you think it is right that the fire alarm system designer is being accused of totally failing to meet BS5839-1 on the basis of the possibility of smoke stratification not being taken into account by the fire alarm system designer?
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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.
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I agree with civvy- well put.
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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 ;)
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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.
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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
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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).
:)
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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. ;)
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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.
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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.
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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
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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.
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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.
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Especially for you:
http://www.nationalstemcentre.org.uk/elibrary/file/6558/Monkey_and_hunter_teacher_notes.pdf
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I certainly hope the monkey wasn't harmed during this experiment?
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Sam, you are wrong.
Not at all Civvy. What we have both said is entirely consistent. :D
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. :o
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Thanks again Civvy for your succinctness and accuracy.
Sam,
I really haven't a clue what you're on about. It may be me, it may be you, but our paths are at right angles and I cannot see us finding common ground. Here are some comments on your brief treatise.
Even within an open space stratification occurs, (it can,agreed, but not always) with cold air remaining above, (only if we're talking about climatic systems - which we're not) (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) (more dense molecules?! Why are we getting involved at the molecular level? Are you talking about the mixture of gases and air?), and a convection current is set up (Are you sure? The phrase 'convection current' is normally taken to mean a cyclical current driven by convective forces - for illustration, put the phrase into Google images. In the open, even on a still day, I think we'd be likely to witness vertical convection, certainly, followed by probable stratification and dispersion of the gases. I don't think we'd see the typical cyclical current taking place as we might if the fire was in a room. Maybe you mean 'convection.'). As the hot smoky gasses rise the heavier particulates (is that a posh word for particles?) reach a point where their buoyancy is outweighed by their density ('buoyancy is outweighed by their density,' you say. Is buoyancy not a product of relative densities? If you'll agree that buoyancy is directly related to density how can it become 'outweighed' by the very characteristic that is creating it?). Therefore the more dense particulates (there's that word again) and then more dense molecules (we're looking at the density of molecules again!) 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 (who said it was otherwise?). 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 (I think Civvy has amply demonstrated that this is nonsense). Some surrounding atmosphere is drawn into the plume by Bernoulli’s principle (Are you sure you have the correct effect? Has the Venturi effect anything to do with it?) , 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). (So is that why bricks float?)
Stu
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Your monkey and hunter experiment assumes both the target and gun are falling simultaneously.
Not at all. The gun is stationary (apart from a bit of recoil, maybe). The bullet is falling. Did you mean bullet?
Stu
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Phoenix
I believe your comments on my post only go to show further that my original post was correct. .........Bernoulli’s principle......... Has the Venturi effect anything to do with it? ....Hummm well I would think so. Perhaps Bernoulli would think so too.
Convection "heat transfer within the atmosphere involving the upward movement" or "circulatory movement in a liquid or gas, resulting from regions of different temperatures and different densities rising and falling in response to gravity"
Hot gasses from a jet engine do not rise until they have lost their kinetic energy of lateral movement (I think Civvy has amply demonstrated that this is nonsense).
I think Newton would disagree. As would Bernoulli.
I take your point about the bullet falling and not the gun. :)
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What all the above indicates to me is that smoke stratification is difficult to predict, explain and prove. This is obviously why BS5839-1 skims over it and provides no specific recommendations, as such.
It appears to be one of those 'what if' scenarios that includes so many variables which can therefore produce various differing results depending on which of the numerous variables slightly alters.
It would help if there were at least some basic 'rules of thumb' that could at least provide some small guidance, such as something like 'the likelihood of smoke stratification at heights less than 6m above the seat of a fire are minimal and should be ignored'. But no-one has mentioned anything like this.
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Where is our resident genius BLEVE when you want him?
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There is a 'rule of thumb' method, Wiz, but you probably won't like it because it's not something you can do in your head. The maximum expected height to which smoke will rise in an axi-symmetric plume above a fire is given (in m) by:
zmax = 5.54Qp1/4(ΔT/Δz)-3/8
Δt/Δz is the temperature gradient over the height of the atrium in Km-1.
Qp is the convective heat output of the fire in kW.
The model has supposedly been reasonably well validated. I find that it always gives pretty high values for zmax, sometimes they feel, intuitively, a little too high. Also, establishing the temperature gradient can be problematic.
But there you go, that's the published guidance.
Stu
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WOT NO BERNOULLIS?
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zmax = 5.54Qp1/4(ΔT/Δz)-3/8
Very impressive
Do you mean Qp = KW or KW per time frame because if not this is only a snapshot and Heisenberg’s uncertainty principle must apply
Also, establishing the temperature gradient can be problematic
. I should say that is an understatement. Nice looking formula though.
Who did the maths? Why 5.54, ¼ and -3/8 ?
But there you go, that's the published guidance
. By whom?
Also. How does the formula take into account the fuel type and therefore the HSG type being produced? How does it take into account the efficiency of combustion and whether the fire is burning in fuel or ventilation controlled conditions? Final question (for today) how does it relate to smouldering. Either in totality or pre or post flaming?
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if not this is only a snapshot and Heisenberg’s uncertainty principle must apply
Quantum physics is nothing to do with it at this scale. Stop trying to be clever.
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Stop trying to be clever
Oh dear am I being told off? :o That used to happen at school as well. In exactly the same way. :'( Usually when the teacher could not answer a question. How that brought back memories. :-\
Heisenberg’s uncertainty principle applies to Newtonian physics as well as quantum. Ie …the position and the velocity of an object cannot both be measured exactly, at the same time.
Arguably it applies to philosophy as well.
But never mind I can see I am being irritating. Just like I was at school when I kept asking questions. And I thought this was a forum for understanding and knowledge exchange ???.
End of posts
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Weren't we talking about smoke?
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Ok, It's late and I have nothing better to do.
Who did the maths?
Some bloke named Gunnar Heskestad I think.
Why 5.54, ¼ and -3/8 ?
Because that is what that clever bloke named Heskestad found.
By whom?
James A. Milke in the SFPE Handbook, but I believe that the main credit should go to Heskestad.
(Especially since the SFPE handbook seems to have an error as it is -3.8 in there, not -3/8.)
Also. How does the formula take into account the fuel type and therefore the HSG type being produced? How does it take into account the efficiency of combustion and whether the fire is burning in fuel or ventilation controlled conditions?
First of all, forgive me if I am being stupid but I have no idea what you mean by HSG.
It clearly does not concern itself with the fuel type, fuel amount or efficiency, All of those things would help to determine Q.
A useful analogy here would be as follows:
[Conversation in a pub]
Sam's mate: I worked out that my car does about 50mpg
Sam: But you would need to know about the aerodynamics of the car, the octance of the fuel, the rolling resistance, the weight carried in the car and loads of other things!
Sam's mate: No, I just filled it up, then the next time I filled it up, looked how many miles I had done and looked at how many gallons it took to fill up, divided the miles by the gallons, and it came to 50.
Sam: But what about when it is cold petrol doesn't burn as efficiently, surely you had to calculate that?
Sam's mate: No mate.
Sam: What about other loads, like having the air-con on? That alters the fuel consumption too!
Sam's mate: It does 50mpg on average, I worked it out.
Sam: But what about air pressure? What about the weight difference as you use more fuel?
Sam's mate: I am going home.
Sam: In your car?
Sam's mate: Yes.
Sam: Don't believe your sat-nav.
Sam's mate: Why?
Sam: Because Heisenberg's uncertainty principle proves that there is no way that a sat-nav can actually know where you are AND the speed you are going!
Sam's mate: I think I need some new friends.
Anyway..... The equation is usually used for smoke calculations in shopping malls and atria, the types of places where stratification could be a problem, therefore the required inlet air is calculated so ventilation controlled fires are not a problem. If you have a ventilation controlled fire then stratification is probably less of a problem than unburnt gases. And rather than force everyone to calculate stuff from first principles, some kind people such as Mr Milke and Mr Heskestad sometimes try to simplify things for us common people.
Final question (for today) how does it relate to smouldering. Either in totality or pre or post flaming?
It doesn't.
To be honest, if you are asleep in a room with such a slow burning fire, which never reaches a detector, and it kills you, I am afraid that you have just been very very unlucky.
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WOT NO BERNOULLIS?
Well some one is talking a load of bernoullis, that's for sure and it aint Civvy ;D
To be fair, I reckon John Klote should also get a mention as he had some contribution to Mr Milke ::)
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Well that's pretty clear, Bleve! :)
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Sometimes it is truly better to say nothing at all.
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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.
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I disagree :P
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I blame the weather personally. The clouds have stratified causing it to go very muggy. Everyone is all hot bothered and narky
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I'd reckon pasquill gifford stability class D5 by the looks of it
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I disagree :P
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Oh Dear.
:o
;)
;D
:-X
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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!
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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.
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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?
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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 ;D, I sound like Toddy now, Nah, only if I used a bit more arrogance
::)
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Are we to assume that ^ = to the power of? ie T^2 = T2 ?
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That's usually the case
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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
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in my experience such calculations are overly conservative when compared to CFD.
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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?
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5.4 seconds or less it would seem
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Well….. at last ...........discussion. :) 8)
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?
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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.
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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!
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And over to you BLEVE.
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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
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Working beyond realms of interweb all week.
Of course we are talking about incomplete combustion, why would we consider otherwise. ;D
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. :P
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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 ::)
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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.
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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?
(http://dc250.4shared.com/img/kdadyCo7/0.20523272684394633/DSC01465ss.jpg) (http://www.4shared.com/photo/kdadyCo7/DSC01465ss.html)
Stu
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Thanks Phoenix. :D
I think that proves my point perfectly
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Looks more like inversion to me - what time of day was it?
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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
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It isn't a clash at all.
BLEVE, T^2? shurely shome mishtake? t^2 perhaps? I expect better from you.
Tsk Tsk, Lower case t when inserted into a formula and Upper case T when described in text ;D
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Now, I think we should all be able to agree* and just enjoy a quiet evening overlooking the Dordogne
The Dordogne is beautiful ………… agreed 8)
Just got back from eastern med myself
Didn’t see any stratification though. :D
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How about inversion
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I've learned virtually nothing about smoke stratification from this thread other than it is simple to explain the effect, but difficult to calculate the likelihood of when and where it might occur, and that there is no agreement on this thread as to the mathematics used in any such calculations and of which criteria needs to be taken into account. No wonder BS5839-1 skims over it so lightly!