129 - Backdraught and Underventilated Fires with Dr. Ricky Carvel

Hello everybody, welcome to the Fire Science Show. This is the Thanksgiving week in the US and for me it's also a very special week. 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. This podcast is brought to you in collaboration with OFR Consultants, a multi-award winning independent consultancy dedicated to addressing fire safety challenges. Ofr is UK's leading fire risk consultancy. Its globally established team has developed a reputation for preeminent fire engineering expertise, with colleagues working across the world to help protect people, property and environment. Internationally, its work ranges from the Antarctic to the Atacama Desert in Chile, to a number of projects across Africa. Ofr is calling all graduates, as it is opening the graduate application scheme for another year, inviting prospective colleagues to join their team from September 2024. By taking this opportunity, you'll be provided with fantastic practical immersion in the fire engineering and unique opportunity to work with the leading technical experts in the field, while learning the skills critical to become a trusted consultant to clients. 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.

I don't know, we'll see. We'll see how my excitement levels go through the show.

If anyone listening to the previous episode about how great a career of fire safety engineer is. Now we are going to the fire science as it is and I hope we sound as excited as when we were talking about the potential future, bright future, of new, young fire safety engineers in the market. 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?

OK. So if we're diving into the science, let's start with the chemistry. If you've got the right balance of a fuel to oxygen to burn perfectly chemistry types would call that stoichiometric burning yeah. Then if you've got hydrocarbon and oxygen, you've got them in the right balance. 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.

Yeah, that's what I wanted to ask. You're good, you can write your own podcast, so tell me about the equivalence ratio, because I think that's the concept that captures this.

OK. So if you've got the perfect balance of fuel to oxidizer, you've got an equivalence ratio of one. If you've got a number that's an equivalence ratio below one, then you've got a surplus of air, a surplus of oxygen. If you've got a number above one, then you've got not enough air, or possibly you could express that as too much fuel. 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.

I've had on this podcast, professor David Perser, where we have heavily focused on the toxicity of fires, and equivalence ratio was obviously a big part of that discussion and everyone knows David's history life history of researching how different things are produced at different levels of equivalence ratio. 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.

Well, if you want to go to perfect chemistry, if you take something like methane and you've got methane burning with the right amount of oxygen, you need two parts oxygen to one part methane and you can look up the textbook and you'll get a textbook value for the heat of combustion. 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.

I remember when I first took dry sail book to my hand. It starts with a chapter in chemistry and very early in the book there's a reaction of methane, you know, and in my high school it was very simple CH4 plus oxygen gives you CO2, h2o, we're done. 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.

No, that's a very good take. I mean in this semester at the moment, teaching in Edinburgh, I've been teaching the class in fire dynamics and the very first thing I do in my class is I teach them basic 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.

so let's now close the problem in a box, because you're not doing it just for fun, right, okay?

So I actually I mean we started talking about carbon monoxide production and toxicity and things like that that you talked about with David Perser, but Actually my interest is much more in the fire behavior end of things. Let's move slightly to what does what are the flames do? 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 had an initial energy source that gave energy to the chemical compounds that to form radicals. This source is gone and the radicals on the wrong don't produce.

yeah, yeah, so that your, your missile radicals, need to find enough oxygen to keep going. If they keep finding more methane, it doesn't get very exciting. 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.

Yes. So let's go, let's go there, let's go to back to us and smoke explosions, because the practical outcome of this research is that this is perhaps a very dangerous phenomenon. This is like immediately dangerous to firefighters, to who attend a fire, because we there's videos over the Internet. 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.

Oh you must, you must watch it, and it's both amazing and terrible at the same time. Okay, our legend in Edinburgh and this is a confirmed legend, but I'm still holding it to a legend rather than the fact is that Dougal Drysdale walked out of the cinema mid film backdraft. I've asked him about that and he has confirmed that he walked out. 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.

How do you shoot a movie when there's a smoke?

come on Well exactly, but you should. You should at least try. No offense to Ron Howard if he's listening. I hope Ron Howard does come and listen to your podcast and he's made some of my favourite movies and he also made backdraft. 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.

Every single one is a horrible tragedy.

yeah, yeah, and you can look at backdraft incidents and they happen fairly regularly. 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.

OK, we're in fires, we're used to diffusion flames. You have a tiny, I don't know 40 micron wide reaction zone where the flame is. So it's not that If you see a 1 square meter pan of heptane burning, it still has a tiny like 40 micron flame surface at which the reactions happen. It's just flowing around. But now flames.

The problem with diffusion flames that we're used to seeing is that they might have some fluctuations, but they tend to stay more or less in the same place. So a first approximation, they don't move, at least they don't move very fast. 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.

that's actually the place of reaction, exactly.

So the flame itself is still very thin, but it moves very rapidly through a large space and, as I say in the classic backdraft scenario, it has to go back out the door where the air came in. And as the temperature goes up, the gas has expanded. 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.

Does it also mean that you could open the door for, let's say, 30 seconds? Close the door, walk away, and I mean later it happens, the door doesn't have to stay open.

I mean that's an interesting one. If you close the door again, you kind of rob the gravity current, you rob the mixing of its power. And yes, you're right, if you hold it open for long enough and you let enough air in and then close the door, you can still have an explosion inside. But usually that's not what we see. 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.

Well, you could bring a 5-meter or something to a fire, but that's not very nice. Well, bring a tube furnace with you to a fire scene.

The thing about the tube furnace is a very slow process. It's not the sort of thing it can't make a decision within 10 seconds and that's kind of what you need with it If you're going to be opening a door. You've got about 10 seconds of access to the inside of the room before you might want to close the door again. 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.

I would associate it with what you said before. Like you can have ignition out of the Ember or you can have auto ignition. Temperature of the cast is two slightly different mechanisms of how the mixture would ignite. One would be like a physical quantity, auto ignition temperature. You either have it or not. The other would be some sort of probabilistic. 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?

Yeah, so I mean, the problem is smoke is not a single fuel. Yeah, smoke is a very complicated mixture of a pyrolysis products, gases and all sorts of things. So we've done experiments with back draft with different fuels. So we did, and I should. 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.

You're in the show notes.

Okay, you can link in the show notes, excellent. But if you go to polyethylene, it was actually a slightly lower threshold, so you get ignition of that at a lower temperature. 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.

Now you mentioned there's a critical threshold temperature, so I guess a cooling is available strategy. I guess that's what's also advised to the firefighters to cool the glass layer.

So that's. That's what the firefighters certainly in the UK do when they're faced with this. 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.

Yeah, I just wanted to ask you, like you're speaking, risk and probability. So it's not that you can completely eliminate it, it can just reduce the probability, right yeah?

Yeah, yeah, so I mean the. The danger of opening the door is always there, but the danger of opening the door is mitigated if you're able to cool the environment sufficiently. That enables you to get in. 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.

So the best story I have and it's not a legend, because there's a YouTube video of that and I'll find it in the show notes is One of my friends, shimon cockat, who was here in the podcast twice and they were playing with the same thing that you were playing with. 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.

That sounds absolutely brilliant. I will watch that. If you're thinking of YouTube things, if you want to see Backdraft we've been talking about Backdraft. One of our University of Edinburgh videos on Backdraft is on YouTube, but I rather, if you want to see a very cool Backdraft, go to the Slow Mo Guys channel on YouTube. 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, ricky, we're almost running out of time, but there's a few more things under ventilated fires that we need to touch on. 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.

So rollover is something. I've actually got one of my MN students this year studying the question of rollover, largely because I want to know what it is, because I find firefighters use the phrase occasionally, sometimes rollover, sometimes flameover and then I look in the scientific literature and there's like two papers. 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.

So not a mixing phenomena, rather a heat transfer phenomenon.

I suspect there's a heat transfer phenomenon there. There may be a mixing phenomena going on, but at the moment, at the moment, as I say, we don't know I'm we're not even halfway through the project yet. The student is still trying to make sense of things. 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.

Fantastic. Hey, ricky, I know we had more on our agenda to speak, but I guess we can end up on this and I'll take you on your promise to deliver another talk when you are deeper into the rollover project. This. 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.

Yeah, in this case, the question I really want to know from firefighters, from people that actually use the word rollover or flamover is it just a stage in getting to flash over, or can it happen without flash over? That's what I'm trying to do. We're trying to detangle rollover, flamover from flash over. So can can you have rollover with no flash over? 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.

I confirm that it's easy to connect with you and you're actually answering emails Sometimes. Other colleagues Madembro. Anyway, ricky, thank you so much for coming to the Fire Science Show, both for the super interesting talk about what fire safety engineering is and even perhaps more enthusiastic talk about flammability limits and underventilated fires. It was genuine. 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.

Yeah, it's one of the best jobs ever. I mean, that's why I'm very happy in this job and when I'm finding I'm, when I get the age I have now reached. I found that some of my contemporaries are starting to talk about retirement. Ok, why? Why I'm not. I have no intention of retiring. You know you have to. 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 that's it. Thank you for listening. I hope, if you came here for fire science, fire science is what you got and you're satisfied with the product delivered. Under the related fires are certainly an interesting phenomenon and it makes chemistry really really complicated and messy and those very difficult to understand. 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.