129 - Backdraught and Underventilated Fires with Dr. Ricky Carvel

I've just been interviewed by my idol , pat Flynn , in the Smart Passive Income podcast and , if you've listened to the Wednesday's episode few days ago , I've covered a little bit on what we talked with Pat and I've built a whole episode about what FireSafe Engineering profession is for anyone who has no idea what FireSafe Engineering is and would love to learn from

us , inspired by the interview in the Smart Passive Income . If you came here for Fire Science in this episode I still have Dr Ricky Carvel with me from University of Edinburgh , and Fire Science is what we do today . So with Ricky we will discuss underventilated fires .

He told me that almost everything he has done so far scientifically be it the tunnel fires , be it the backdrop phenomena , lectures on combustion and stuff like that all of those touch underventilated fires in some way . So this , this subject , is definitely something he feels very comfortable in and we're having really nice discussion on those phenomenon .

This is some very useful science , not just for fire safety engineers , but definitely useful for firefighters , because smoke explosion phenomena are one of the reasons for line of duty , that's , an injuries for firefighters . So absolutely something they need to be aware of .

I wouldn't say they need to understand , because even as scientists we don't have a full comprehension of what those phenomena are . How do they work , where are the boundaries for them ? It's still under research , but in this episode I hope we get as close to that answer as possible . So I don't think this needs more introduction .

It's great fire science with a very enthusiastic guest , so please help me welcoming Dr Ricky Kovale from the University of Edinburgh and let's go . Welcome to the fireside show . My name is Vojci Wynchczynski and I will be your host .

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This opportunity is tailored just for you and if you would like to take it , please visit OFRConsultantscom for further details and instructions on how to apply . As everybody , I'm here again with Dr Ricky Kovale from the University of Edinburgh . Hey , ricky , hi there . Vojci , you sound less excited than last time .

So today we will talk about what happens in fires when they are under ventilated . So let's clear the air . What does it mean if fire is under ventilated , ricky ?

Your only products are going to be carbon dioxide and water vapor and nothing else , and that's it , and that's it , and that's pure combustion , that's perfect combustion , and it never happens in the real world . In the real world something gets in the way , ok .

And if you're in a building , in a box , the rate of air getting to where the fire is may not be enough to give air to the fire . So under ventilated simply means when we've got less oxygen than stoichiometrically the fuel wants . I guess we could talk about equivalence ratios .

We tend to talk about things as being fuel lean or fuel rich , so a rich mixture has not enough oxygen . It has an equivalence ratio greater than one .

Equivalence ratio higher than one obviously led to increased production of carbon monoxide , toxic products , incomplete products of combustion . So definitely something is happening to combustion when you have too much fuel . Perhaps we can dive in there why the chemistry would be different when it's fuel rich than when it's stoichiometric .

For that it's about 50 kilojoules per gram of methane , which is actually a lot of energy for something as small as a gram . Now if I was to take the same amount of methane and I give it too much air I give an excess of air we're still going to get 50 kilojoules of energy out per gram of methane .

It's just we're going to have some air left over at the end that wasn't used . So in the lean situation , the burning is the same as the stoichiometric situation . It's nice . It's nice and easy to quantify . It's nice and clean . When we go to the fuel rich situation , the chemistry gets a lot more messy .

So the methane all wants to burn but there's not enough oxygen for it to combine with . And what we find in terms of chemistry ? Once you start breaking these chemicals up into their component parts , you take a methane . It's one carbon to four hydrogens . Those four hydrogens are far more reactive than the carbon .

So , they're going to steal all the available oxygen , and if we've not got enough oxygen , what you're left with is carbon that doesn't have enough oxygen to make carbon dioxide , tries to make carbon monoxide , because that's a kind of a compromise as far as it's concerned . Sometimes it can , sometimes it can .

Sometimes you end up producing soot , very carbon rich mixtures . It's not pure carbon , but it's a very carbon rich solids which are messy , which are not nice to breathe .

But the whole energetics , the whole kinetics of the thing is different , because we're now no longer getting 50 kilojoules per gram out of the methane , because the methane isn't able to generate that much energy , because the chemistry is not working for it anymore . So we get a lot less energy produced .

When the energy goes down , it starts messing , it makes things even worse for the chemistry and you start producing all the nasty stuff that you really don't want to get out there .

And in Degle's book there's like a list of 20 equations of what's happening in between those two stages , even for something as simple as methane and oxygen reaction . So you have all these radicals produced and everything . So when the combustion is happening in too fuel rich , simply , some of those reactions are easier to happen .

And apparently these are the earlier reactions , not the final ones . Yeah , so you get a lot of byproducts , the meat products of the reactions , and in the end you don't have enough oxygen to complete all of that . That's my take on the chemistry .

So I do the CH4 plus 2 OH goes to CO2 plus 2 H2O and then actually , oddly enough , the class that I taught just yesterday is the class that I called . Everything I Told you About Chemistry is Wrong . Where I go back and I revisit the chemistry and I say actually I told you this was one reaction is not , it's 10 .

Here they are looking around the room of students that are civil engineers . Mechanical engineers haven't done a lot of chemistry . They don't like that . One bit is like oh , you know , that's that thing that you taught us a simple , suddenly realize it's a whole more complicated . And yeah , it is the simplest . Hydrocarbon burning is a really complicated thing .

So just imagine , if you take a more complicated hydrocarbon , just how , just how many chemical pathways there are . It doesn't really bear thinking about okay .

Yeah , I mean that's essentially the question that that we've been looking at and I say we and I'm talking about a group of people at university of Edinburgh , some of my former PhD students , postdocs and a few msc students as well .

Over the years we've we've been playing with fires in restricted ventilation environments to try and see what happens , trying to try and better quantify the fire behavior , and that extends from things like back draft and smoke explosions Through to some fascinating stuff that we did with ceiling vented compartments and I don't mean a standard compartment with a door on

the side and event in the ceiling , I mean when there's only event in the ceiling because of compartment fire . If you've done any stuff in compartment fires , you know you're used to a root H is the ventilation factor in the window . If you put that window in the ceiling it's still got an A . But what's the ? What's the root H ? Yeah , it has no height .

So how does . How do we quantify air flow in that ? This is a project that we did a few years ago where we actually just we took a compartment , fire side vented and we let it grow to flash over and then we close the side vent and open the top and just watched what happened , depending on the size of the opening , depending on where the fuel was .

And there was some really fascinating behavior fires that unexpectedly go out and then reignite , so smoke , explosions , fires that detach themselves from this you know , because we've got a solid fuel inside the compartment , a crib or something like that Fires .

Actually , the flames detach themselves from that and search slowly , moving around inside the compartment , going , basically going anywhere . There was air and there was oxygen and the flame moves there and if it moves too far away from the fuel , then the heat in the pyrolysis , that feedback loop , is broken and the fire gets smaller .

As the fire gets smaller , the temperatures come down , the air , the box sucks more air in because the temperatures have gone down Suddenly , there's more air in the box and the fire reestablish itself .

Is fascinating stuff and one of the things I realized while watching these experiments play out , watching the students my msc students are mn students that were doing the experiments .

Is you really need to understand flammability limits to understand any of this ? And I think that's one of those things that's kind of crucial to understanding how a fire behaves is understanding flammability limits . And what I find through about a decade of teaching fire dynamics at Edinburgh is people readily understand how the lower flammability limit works .

At the lower flammability limit , you know if we're below the lower flammability limit . So suppose we're talking that methane . The lower flammability limit for methane is 5% methane in air ambient temperature . If we've only got 4% methane in air , people instinctively understand what it means that I've not got enough fuel to sustain the reactions , so diluted .

Yeah , if I try to ignite a mixture that's got 4% methane and 96% air , people completely understand without having me having to explain very much . That is fine , I understand . Not enough fuel can't ignite . You go to the other flammability limit again , atmospheric temperature , methane 15% methane is the upper flammability limit 15 , 15 .

So if I have , if I have , 16% methane and 84% air at ambient temperature and I try and ignite it , I can't and that's because I've got too much fuel , not enough . Oxygen is not much and the concept is not the concept of too much fuel , not enough oxygen makes perfect sense .

If I'm talking about like 90% methane and only 10% air , everyone's like , yeah , of course that's never going to burn . When I say 16% methane is too much fuel to 84% air , people like hang on , how , how does that work ? Yeah , how does it work . It's just a matter of once . You know a combustion reaction is a chain reaction .

So when you start something igniting and if you put a spark into a balloon full of any gas , if you put enough of an energy spark in it , what you're going to do is you're going to break some of the atoms in there apart . They're going to form radicals . Those radicals are going to go flying away and interact with the things they find .

If they find more radicals , they'll simply recombine and you get the same energy back that you put in . If they find other things that they can interact with , maybe you know you've got a methyl radical goes away and finds an oxygen . It breaks that oxygen apart and reacts with half of it .

And you've got an oxygen radical that goes away and reacts with something else . But the problem is if you've got too much fuel , not enough air , or indeed if you've got too much air , not enough fuel . It's essentially a similar problem . The chain reaction simply fizzles out . It can't sustain itself .

Those radicals don't encounter enough of the right thing to sustain the reaction .

So you need to have the right balance between oxygen and the fuel and I find is something that I spend a lot of time going over and repeating for my students and by about week nine and week ten and semester they've kind of got the idea , because I've said it so many times .

But I find , when it comes to understanding how flames work and the numbers are different , but the concepts are still the same , we can have fuel and oxygen and even high temperature sometime and have no flaming because we've not got the right balance with , we've not got the right mixture and so in those boxes I was telling you about , with the event on the top

, even though the temperatures inside the box maybe five hundred , six hundred , seven hundred degrees , if there's not enough air getting in there , you've got plenty of fuel . If there's not enough air , the flames go looking for the for the air and they move away from the solid fuel source .

They break the feedback loop , pyrolysis products stop being produced or something so produced , so much flames ghost around , but then the fire diminishes a bit more air is able to be sucked in through the hole and the fire may start in a and occasionally very explosive and unexpected way .

You can watch right back to us , even a movie , hollywood movie , back to have no idea if they capture back to us correctly in that movie . I don't think .

But he says he walked out because the acting was so terrible , and I actually use clips from that movie in one of my lectures in Edinburgh because the fire dynamics are hilarious , if you know what to look for .

I mean , we talked about underventilated fires at the start and all the toxic production and the smoke production , and there's a scene in the middle of the film it's one of the pivotal scenes in the film where there's two firefighters in a room , ever the whole building is burning and dire of it that , like every surface that could be burning is burning .

It's a well confined space . There's not many places for air to get in , so we're definitely should be in an underventilated space , and yet there's no smoke . There's no soup production .

I think once you've understood fire dynamics you need to go and watch backdraft just so you can have a laugh at some of the fire behavior . There's a great moment when a fire appears to be generating enough pressure to literally bend a door outwards and I know you do get fire pressure .

But this door is bending outwards and yet the firefighters are still able to push in against it , which I don't believe . And also there's a firefighter alive on the other side of the door . But anyway , I don't want to talk about that movie too much . But real backdraft , but real backdraft .

So I would also say , with apologies to our American listeners , that of course in the UK we spell backdraft correctly OK , e-a-c-k-d-r-a-u-g-h-t . Ok , not with an F , but anyway I wanted to ask you if your team backdrucked or backdrafted . It's the proper British spelling of the word You're speaking English in this box ?

Yes , absolutely , but backdraft is fascinating , but backdraft is also really scary Because backdraft is one of the things that kills firefighters and thankfully it's not many firefighters a year , but sadly

it's not zero firefighters a year .

And the problem is that I should explain what backdraft is , perhaps , if you want to imagine a scenario where we've had a fire inside a room in a building and the room is mostly sealed , so it's not necessarily perfectly sealed but what's happened is the fire has burned for an extended period of time and it's consumed most of the oxygen in the room .

So we've gone from a well-ventilated fire to a very under-ventilated fire , possibly not completely extinguished , but certainly in the scenario where we've not got very much air getting in there . So in parts of the space , perhaps even in the whole space , we end up in a situation where the fire has gone beyond the upper flammability limit in the room .

So it's definitely a very high equivalence ratio . Burning has happened . The room is possibly hot internally , probably if it's hot . What happens ? Even if the flames go out completely or the fire dies out completely Because it's hot , pyrolysis continues to happen .

So you get flammable gases venting around your room but they can't burn in there because there's too much fuel , not enough oxygen . Now what happens ? Obviously , firefighters turn up to the scene . Their job was to put the fire out . They know there's a fire in the building . They know there's a fire in the room . They have to open the door to get there .

So they open the door and nothing happens , at least nothing visible happens . What's actually happened ? When you open the door , you're letting fresh air in and you're letting hot gases out . Because of the density difference , you'll get hot gases flowing out the top of the door and the cold air probably going in along the floor .

So imagine that the scenario I described . We've got this hot room full of potentially flammable gases , without enough oxygen . We're now letting oxygen in but depending on the size of the door , depending on how widely it was opened , it might not be a very efficient vent to let lots of oxygen in immediately .

So there's going to be some period of time for oxygen flowing in until we get a balance somewhere in the compartment where a flammable gas air mixture is created . And now it's an interesting exercise to do If you can imagine the scenario of a room full of fuel gas and a room full of oxygen and they mix when you open the door .

So we've got a flow of too much air , not enough fuel coming in and a room of too much fuel , not enough air but due to turbulence , the interface between the flow , they mix .

Now , if you actually think about that , we've got gas that's well above the upper flammable to the limit on one side and well below the lower flammable to the limit on the other side . If these two gases mix , it's actually impossible to mix them without at some point creating a flammable mixture .

Exactly , yeah , there will be an interface at which to the interface must be a flammable mixture . Now that doesn't necessarily mean it must ignite , because if there's no ignition source , if there's no hot spot , if there's no flaming there , you might get lucky .

You might get the situation where those gases mix , they don't ignite , they get further diluted until they're no longer a problem .

Unfortunately , what does happen regularly is the gases do mix , form that flammable mixture at a place or a time when it's either still hot enough to auto ignite If it was a very hot room , you could get auto ignition or if there's some embers , there's some residual burning somewhere , and that flow of air , that flow of mixed air , gets to where the embers are ,

then you get ignition of that mixture . Now , as soon as you've got ignition of that mixture , you've got a premixed flame . So that premixed flame is going to rapidly move through the flammable cloud , which basically extends through the compartment and back out through the door that's just being opened , which is where the firefighters are .

But if you've got a mixture of fuel and oxidizer mixed together and then you ignite it , the flame will go through the whole atmosphere very rapidly and you could be talking hundreds of meters a second , because it's the whole atmosphere .

You get an explosive force coming out through the door and , yeah , it can be fatal for the firefighters outside the door . The main problem for firefighters is , of course , the time it takes , because the process I've just described isn't an instantaneous process .

And that's actually coming back to the Ron Howard film backdraft , the one classic alleged backdraft incident . In the middle of the film from which the film gets its title , the guy opens the door and within a tenth of a second the whole building explodes . That doesn't happen in a backdraft .

In a backdraft what happens is you open the door and then all the fluid dynamics have to do their stuff to create the flammable mixture , to find the ignition source and depending on the size of your compartment , that could be 10 seconds , it could be two minutes . I mean , the uncertainty of the duration is immense .

Usually if you open the door briefly and close it again you don't get an explosion . And this is one of the problems we have , because I know fire brigade practice varies all around the world but in the UK at least our firefighters are trained .

If you're dealing with a fire in an enclosed space , you open the door , you look in , you check the signs , you close the door and you kind of repeatedly do this and every now and then you might spray some water into trying to cool the environment . But it's a visual thing . You're looking in saying is it safe to enter ? Is it safe to enter ?

And the problem when we've done our experiments with Backdraft is you can't tell visually whether it's safe to enter or not . You can only tell if you're able to get measurements of what's in the room . What would you need to measure so well ? This is actually treading in the toes of a project that I want to do in future but haven't yet done .

But what we actually need to know is you need to know the composition of the gas and you need to know the temperature of the gas , and if you could quantify both of those , you would be able , with some fairly complex chemistry , to work out if you've got a flammable mixture or not .

Of course , no firefighter is going to be able to do that analysis , so what I want to do ultimately is to develop a methodology by which we can know what sensors we need and know the analysis ready in advance , so that we can actually detect what's in a room and be able to tell is it a flammable mixture or not .

But the problem is , at the moment we have no such technology .

Maybe if you leave that door open for 30 seconds , that's when the back draft occurs . So you've got a very short window . So it needs to be something that can analyze rapidly and currently we don't have that technology .

It's one of my project ideas that I haven't quite brought to the point of gaining funding , for sadly I'm not very good at getting funding for all these ideas . I have but one of these years . But it should be possible to analyze that gas and essentially give a green light or a red light to the firefighters to say yes , it's safer .

No , it's not , because the problem is , if the room is not tremendously hot , you open the door . It could be of the order of minutes before the back draft happens that the hotter the room , the faster the back draft .

But we've done experiments at small scale where our box was like a meter , a meter 20 long and We've had delays of up to 30 seconds at that scale which , scaled up , goes to several minutes at full scale . So the biggest danger of back drafts for the firefighters actually the uncertainty . We simply don't know how long it will take .

Is there a source with sufficient energy to ignite the mixture , with the probability of Existing in the space at the same time where you have this , your mixture , within the flammability limits ? So it kind of makes sense that you would have such a diversity in the physical phenomenon .

But coming back to the flammability limits , okay , you know , methane is five to fifteen percent . How much is smoke like ? What's the flammability limit like ? Where are we with this ?

I should acknowledge my former PhD student , dr Farion Wu , who's now back home in Taiwan at Changyong Christian University . There he went , did a lot of experiments with a lot of different parameters and we also use a few different fuels . And you say it's about the flammability limits .

But the problem is the flammability limits for the pyrolysis products of polypropylene are different from the flammability limits for the Pyrolysis products of polyethylene , and those are two pure fuels .

If you're dealing with a real compartment , you know that's had sofas , it's got timber furniture , it's got soft fabrics , the mixture , the soup of pyrolysis products you've got in there . I don't know , I don't know what the flammability limits are of that .

And that's one of the things that , before I can get to the point of developing this nice simple device that can tell you back draft can happen or not , we actually have to analyze a whole heap of different fuels and look at the flammability Properties of them all in isolation and then try and elevate your temperatures right and ?

And elevated temperatures Absolutely . I mean . That is one of the things that farion's PhD was based on . The idea was , if you look back at most of the literature and back draft , they've used methane . Now , methane ignites readily at room temperature .

What we did was we used the pyrolysis products from mostly from Polypropylene , polyethylene , and we found that there was a sort of cut-off temperature . You have to be at elevated temperatures before you get back drafts with these fuels . But the sort of threshold temperature was different depending on the fuel .

So for polypropylene , I think was had to be above 320 degrees . I think you can check our papers and see if I've got the right numbers . They are in the show notes .

So if you're trying to figure out in a real scenario where you've got multiple fuels , you need to know the temperature , you need to know the mixture , you need to know a lot of information that at the moment we're incapable of collecting .

So there is a fascinating and huge amount of research still to be to help improve the safety of firefighters against back draft .

You know they'll open the door , they take a second or two to look around , and then they squirt some water spray Water mist if you like into the hot upper layer inside the compartment , close the door , wait a few seconds , repeat , and do this as many times as they they feel is necessary .

What we haven't quite quantified is how many times you need to do that . What we did , though , was we had a series of research projects a few years ago , funded by the fire service research and training trust in the UK , where we looked at firefighting strategies

. I'll be at small scale . We did our experiments at small scale , so what are ? If we can only open one door , what are the risks associated that , if we can open two doors , have we improved things or not ?

And we found certainly the risk of back draft was considerably reduced If you can open two doors and get a through draft , because part of the problem with back draft if you is when you've got a single opening , and the single opening is Shared by the outflowing hot gases and the inflow cold gases , so you get turbulent mixing along the interface .

If you can get two openings and get an inflow in one and an outflow in the other , the same turbulent mixing doesn't happen . So you're actually generating dilution from the word go .

And even in the rare occasions when in our experiments the flare up let's not call that a back draft when they , when the fire , reestablished itself , it reestablished itself in a much gentler way and perhaps Mmm , not in the path , but on the out path . Well , actually what we found with with the two openings is , generally it didn't come out either path .

It was an entirely in compartment thing . So the people outside the compartment were okay . But yeah , we had these , these projects . Look at that . And again , in the show notes I'll put links to the videos because we've produced some training materials for the fire brigade based on that .

We then went from from looking at the well , if you can open a door as well as spray some water , what effects does that have ?

So we looked at opening the door , spraying water , in closing the door , repeating , and we found that the key in a Fire driven by a solid fuel so we were using solid fuels here , so not gas burners or anything was to to bring the temperature of the , the gas layer , down Below 200 .

If you can bring the , the upper layer temperature in the compartment down below 200 , then then the risk of Battered after smoke explosion more or less dropped to zero was the timber . PMMA or some complex fuels .

Now you're asking me , it was mostly polypropylene I think we did most of these experiments with , although I think there was some playing around with with fuel , but most it was mostly polypropylene . And yeah , in those experiments we found you , if you can cool the the hot gas layer down to about 200 , then the risk of back draft was diminished .

I'll never say it goes to zero .

But actually we then went on to a third project where we looked at the ultra high pressure lance , the cutting extinguishing system , which essentially certainly the way it sold it does away with the need to open the door , because you've got this means of drilling as basically a pinhole , a keyhole through through the barrier and through the thing , whatever's in your

way , and then cooling the environment beyond before you've introduced any air in , because it's the . The back draft problem is one of if you introduce the air . So if you can introduce cooling without air and you can bring it down again below 200 degrees , then your chances of successful entry are much , much greater than otherwise .

And we had a lot of fun playing with water spray systems in different injection points and we were actually reasonably surprised . We had a .

Everything was done at small scale , but we had a sort of two room setup , so the fire was in one room and then there was an adjacent room , so there was a half wall in between them , and One of the questions we had was does it matter ? Does it matter where the injection point is ? Do you have to hit the fire ?

And we found actually for cooling purposes , it doesn't matter , you don't have to know where the fire is on the other side of the wall , you just have to get enough cooling agent in because , having seen the things in action , it's a very turbulent mixing process . That happens , goes on . Yeah , so actually it floods . It floods the whole space .

If it's an open space and the water drops get everywhere , and so you don't need to hit the fire directly . You just need to cool the environment sufficiently .

They were deliberately creating conditions in a compartment where it would be Extremely hot , so they would be opening the doors and closing them , not to just view If the fire is still there , but to keep the fire in there , but keep heat inside as well , and eventually created extremely hot room .

Then they've put the clans against the door and after some training because that was some Not the first try they did it . They tried to focus the lens in a way that all of the Water will go into the gas , not hitting the wall .

So they told me they didn't want to cool the wall , they wanted to hit as much gas as quickly as possible and what they've done like obviously the first idea you have the , if I'm not wrong , the water expands 1830 times from its original volume when it changes into the gas . Perhaps the number is wrong , but it's roughly the scale .

But also it cools very effectively what's around you , so it Increases the density , changes the volume . So what they've done ?

Actually they've hit the point at which the decrease of volume due to , like , increase in density was bigger effect than evaporation of water mist and they actually created an implosion that sucked in the door , like the guy on the video puts the water lance against the door , presses a button and the door immediately sucked into the room . So there is .

I second your opinion on very interesting fluid dynamics happening in the room and I promise a YouTube video . It's gonna be there . I promise it . I'll make sure and find me the video . It has a really odd Polish title which makes me unable to find it immediately , but I will .

They've done a nice video of Backdraft and you can watch Backdraft in slow motion and it's a beautiful thing to watch . Definitely Well , better in slow motion than in any real world , Absolutely yeah , but when you see it in slow motion , you realise just to what extent the problem is , because it's slow and huge at the same time .

Because , yeah , we found that . You know , we talked about the Backdraft coming out through the door . You've opened the question of how big is the fireball that comes out . In our experiments it seems to be the size of the room determines the size of the fireball .

So if you've got a room that's five metres long , you're going to get a fireball that's going to come about five metres out through the door , more or less , and that is crazy if you're a firefighter .

So at the beginning you said that it's all about understanding what the flame is , and I think , if you use thermoghosting flames , I know there are those things called the rollover flames that firefighters observe in the fires , and before the chat we had a short discussion and I know they are somehow related to under ventilated fires , so perhaps let's at least try

briefly cover those .

There's one by Jim Quintery back in the 70s and he seems to have done one paper and then moved on and there's one or two others . But in the scientific literature it's not something that comes out . The word does not appear in Drysdale's book , the word does not appear in Quintery's book , it's not in the Carlson and Quintery you know all our standard textbooks .

I think it appears once in the SFB handbook and that's got like 3,000 pages long and it's just mentioned in passing . So I want to know what rollover or flameover actually is .

So I've got a student looking in at the moment and she's thoroughly confused at the moment because there are many and different definitions of it , some of which are completely contradictory . But what generally it seems to be is ? It's the ignition of gases in the upper layer in a compartment fire . What we're trying to get a handle on is what that is .

Now I'm assuming it's something to do with flammability limits . It's possibly when we've got a fuel rich upper layer that , at the temperature it's at , is not flammable . But if you , if you elevate the temperature , it suddenly passes the threshold and becomes flammable .

Maybe , if you have me back in the future I'll give a better answer in that , but I dig that , I dig that and it is fascinating . And , and please , if your listeners out there are listening to this before March 2024 and you have any particular insight into rollover or flamover and what it is , please drop me an email . I want to know more .

I want to know all of the crazy and confusing different definitions of it , because I don't think it's a well defined thing and what I'm hoping my students able to do is to actually quantify how undefined it is and maybe , maybe , just maybe , come up with a unified description of what we're talking about .

There is , there is an NFPA description , there is an NFPA definition , but it would seem that a lot of people don't necessarily stick to that definition when they use the words and the ghosting flames .

Ghosting flames are very fun to watch because when we did them in the ceiling vented compartments , it's basically like the fire slows down and you watch it and you think you're and suddenly I'm watching . You know , I talked to the slow mo guys there , something that you think I'm watching flames in slow motion , but actually it's happening in front of my eyes .

It's a really it's mesmerizing . I would go as far as to say it's mesmerizing If you get the chance to , to watch an experiment with ghosting flames . It's fascinating . But it's just a fire on the point of suffocation , trying trying desperately to stay alive . And I always anthropomorphize fire .

It has needs , it has wants and it's trying to find the air it needs to breathe , to sustain itself . And it's a beautiful thing to watch .

This would be very interesting , and perhaps I would invite some firefighting instructor for that as well , because it could be actually quite interesting to clash the experience between , you know , firefighters and fire science . That's , that's what I tried to do .

That's a question that I want to know the answer to . So if anyone has an answer , please , please , send me an email . I'm easy to find on the internet . I have a reasonably unique surname , so you'll find me .

You see people , we are excited about those things . We are happy to see fires roll over . We're not happy if they cause harm to people , but we're happy finding solutions for that . I love it . I love this job .

You have to physically remove me from the building because I'm I'm going to keep going for as long as I can because the job's fun . Yeah , indeed , ricky . Thank you so much . No , you're very welcome . See you next time .

And this is also a knowledge that needs to be understood because these phenomena are a potential source of harm to firefighters and we want our firefighters to be as safe as they can in fires . That's a hard enough job without having exploding basements and stuff like that .

So understanding backdrops and other smoke explosion phenomenon surely leads to safer world everywhere

. I hope you've enjoyed both episodes this week . Next week we're back to normal schedule of our podcast , so I will be seeing you here next Wednesday on Monday I hope to release the beta version of the book of fire , the course on basics of fire safety engineering , or at least resources were to find them to self educate yourself .

So you probably would like to be looking out for that . Once again , the book of fire dot com is where you can find it . And yeah , pretty busy weekend for me Finishing the book of fire . I'll have a nice time . I hope you also have a great time . See you next week . Thank you Bye . This was the fire science show .

Thank you for listening and see you soon .