188 - Fire Fundamentals pt. 13 - Porous solid fuels

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Hello everybody, welcome to the Fire Science Show.

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Today we're going back to your favorite Fire Fundamentals series, and I'm going back to one of my favorite all-time guests in the podcast, dr Sara McAllister from the US Forest Service, and last time I had Sara, more than 150 episodes ago, we've talked about the scales of fire phenomena.

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We did not have Fire Fundamentals series back then, but the approach that we've used in that episode was very close to this series.

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In today's episode I'm also taking you on a journey and we're going to talk about how stuff burns, one of the most fundamental and perhaps unanswered questions in the world of fire science.

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It's one of those things that when you start investigating it, every time you dig deeper, you find five new caveats, three more problems and some measurement that you lack, and that's the story of fire science and we fire engineers have to deal with that.

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Go away.

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You know putting our design fires based on some large-scale calorimetry mist, for example that I've covered in the podcast episodes previously.

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But if you would like to solve the burning of stuff from the first principles, it's not that easy and it's not that we're gonna tell you everything in this one podcast episode.

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The matter is too big to cover it in a podcast episode especially.

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There are a lot of things that we still don't know.

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But going into this thing of you know, knowing what you know, knowing what you don't know and having a good idea of what you don't know that you don't know, I think it's highly beneficial to broaden your horizons and think about those problems from a more fundamental perspective.

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In this episode we're going to go mostly through porous fuels, and those would be two very different things.

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You can think of porous foams, foam materials, polymers, or you could think about a collection of pine needles that's also porous fuel.

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Some characteristics will be shared between them, some will be completely different, and I think uncovering this world is very indeed fascinating to any fire safety engineer.

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So I hope it will help you look on your fire problems from a little different perspective in the future.

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So let's spin the intro and jump into the episode.

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Welcome to the Firesize Show.

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My name is Wojciech Wigrzyński and I will be your host.

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Hello everybody, I am here today joined by Dr Sara McAllister from the US Forest Service.

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Hey, Sara, good to have you back in the show.

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I'm happy to be here, thank you.

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And that's like three years or more.

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It's crazy how quickly time flies when you're having fun.

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I'm mind blown.

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Three years already.

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Yeah, I hope you have been having fun in the meantime as well.

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And I brought you to the podcast to geek out on fires, of course, and on burning stuff.

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I love discussing burning items with people An odd hobby, you could say and I would really love to discuss some stuff related to burning solid stuff in the podcast.

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Fine with that.

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Oh, that sounds great, let's do it.

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Solid fuels On the podcast I had some episodes about flame spread, about ignition.

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Today let's try and discuss how stuff burns.

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And yeah, every time I have to design a design fire for stuff I don't have, you know, an item in my NFPA or SFP handbook, I go through this madness how to define how big the fire will be.

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So if you were faced with a challenge like there's an item armchair, you don't have a reference for that how it's going to burn.

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And by how it's going to burn, and by how it's going to burn, I would say like the heat release rate in the end, Well, that's a I mean, that's a million dollar question, isn't it?

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Indeed.

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Come on Fire safety engineers, people who just graduate fire safety engineering.

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They're put in the seat of an engineer on a project and they're giving this task.

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Like here's your building, here's your fuels.

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Like, figure out what's the fire going to be?

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Like?

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People are challenged with this every day.

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I know it's a million dollar question.

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There's no answer.

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So maybe let's brainstorm how one could get to maybe not the worst answer possible.

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Well, I mean, you're talking to an experimentalist here, right?

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So I would obviously start with just by doing experiments, but I do understand that that's very hard for some if you don't have the right facilities to burn a whole sofa or to burn a whole mattress.

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Okay, so let's say I take my, let's say, nfpa 2.4, it has an appendix and tells you how much megawatts per square meter of pallets stuck to whatever height, a design fire would be.

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But I assume if I stack them like loosely and I stack them tightly it's going to be completely different fire outcome, right?

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Oh for sure.

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Yeah, I mean the porosity of the fuel bed or you know how much airflow that can get through there is really going to dramatically change how that fuel bed burns right.

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So we learned a lot burning wood cribs over far too many years.

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Where I mean there's, we go back to some of the very fundamental work done on that.

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Right, where there's sort of two regimes of a fuel bed.

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Right, it could be densely packed where the ventilation is driving the fire behavior, where it's ventilation limited.

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Or you can have them more loosely packed to where the point where it's more of the local heat and mass transfer that's driving the combustion.

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Right.

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So you can get if you take the same fuels, the same diameter, the same number of sticks and rearrange them in a very different way, you can get huge differences in the burning rate, like it could be double, whether or not you loosely pack them versus tightly pack them.

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And the other thing that I've learned by all of those crib burns is you know if you take the same crib, even if it is loosely arranged, and you put it directly on the floor, you're blocking a lot of the airflow that could happen through it.

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But if you lift it up like maybe seven, eight centimeters.

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You're allowing a lot more airflow through it and you can again very dramatically change the burning rate of it by you almost double it by giving it enough airflow through it.

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Is it the convective airstream from the fire driving that?

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What's the driver for this flow from underneath?

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Yeah, so it's that buoyancy right.

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So as it begins to burn, all of those gases rise and it's pulling in air from all of its surroundings.

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If you put it directly on the ground, it has to come in through the sides right.

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So the smaller the fuel elements, the smaller the little holes that it has to get through it.

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So it's going to have a harder time getting into the actual body of the fuel bed itself.

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If you've got big fuel elements, you've got bigger air gaps and you can get some airflow through it.

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But if you lift the whole thing up, that entrainment can happen from underneath and you can get the vast majority of the air through the fuel bed can come in from underneath.

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I did some work where I actually put those cribs in a box and fed air through it and was able to kind of back out how much air actually comes for combustion actually comes through the fuel bed and in most cases it can be up to a quarter of the air that's required to burn.

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All of the pyrolysis gases can come through the fuel bed and in most cases it can be up to a quarter of the air that's required to burn.

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All of the pyrolysis gases can come through the fuel bed.

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When I was like contemplating this type of fires, like I.

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Let's say, I have the crib and it burns and I can see by the shape of the crib that's getting smaller and smaller, that is burned like around the circumference of the crib right when I did not have this uplift of the crib versus floor.

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I like to think about that process.

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As you know, the air comes from the outside but while flowing through the crib it burns out the oxygen.

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So eventually the gases that reach the middle of the crib or could you know, reach deeper.

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They don't have sufficient oxygen to promote burning anymore, which doesn't mean they don't promote pyrolysis and they don't create new fuel.

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It's just this fuel burns way above my crib, not in the crib itself.

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So that was in my head, the explanation of different behavior of those cribs where they have different porosities.

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What's almost really interesting, too, is there's been some times where I've built a crib that's particularly dense and I don't.

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I could swear that it takes a really long time for the center of it to even start to char okay, so.

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I mean the the heat transfer within the fuel bed.

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If you're not getting that combustion down in the fuel bed, you're not getting the heat transfer to heat up the fuel elements in the center either and what if you close the sides of this porous fuel?

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So I know some experiments Heukingesson did this on tunnel fires when they had mock-ups of trucks and they had versions where it was just a bunch of pallets on a truck and they had versions where they had put a tarp around the truck to complete different fires.

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And the same would be with sprinkler test fuel cups which are in cardboards.

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Like if you had those cups outside of cardboards, that would be an inferno.

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When they are in cupboards again, you spread them into smaller chunks of fuel, right.

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Well, right, you're preventing that airflow to get through to feed the fire, right?

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I mean, even if you had a what under normal circumstances would be a loosely packed or an open fuel bed, you're restricting the ventilation and essentially making it one of those closed fuel beds.

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So if it doesn't have enough, you know, air to react in the fuel, you're you're really cutting off that, that mechanism, and not allowing it to be a porous fuel.

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Essentially, so, essentially, if an engineer wants to dig deeper into their porous potential fuel, they're going to have a hell of fun to figuring out and no real easy way to scale up or down their design fire.

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That's not a very reassuring finding Well you know, the sad thing is I burned cribs.

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I mean I burned over a thousand cribs and there's still so many questions that I have right.

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It's a very challenging problem because it's you know, you've got all of the normal issues of like a solid material burning, but then how they interact with each other and and how that then drives the fire behavior is is a very challenging problem.

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To perhaps make it simpler.

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So in our buildings we would find solid materials at different compositions, different states of matter, if you could say so.

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We have solid fuels.

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We have the same materials in the phone forms, we could have porous fuels.

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Can you help me identify how differently material starts to burn when we change the physical, let's say appearance of that item?

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or when we start foaming it or dropping it into smaller particles and putting that more in a lumped form of a porous crib, let's say and you make a good point about porous fuels, how they are different from a solid fuel, and to me there's almost two different ways to think of a porous fuel.

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Right, so there's continuous surfaces, say, like the foam of your sofa or the foam of your mattress or something.

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That's kind of one surface, one continuous material that has a bunch of holes in it, right, that allow for airflow to go through it.

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You know another way to think about porous material and porous fuels, however, are collections of fine fuels that come together to form a porous fuel bed.

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Thinking about you know I come from wildfire space, right?

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So thinking about like collections of needles on the forest floor could be a porous fuel bed.

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Or even a tree itself would be a porous fuel bed.

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Right, it consists of lots of different individual needles that are attached to branches, but the whole tree itself would be your fuel that you're concerned about, and that that very much, that ability to have airflow through that fuel does change dramatically the way it burns.

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Right, you think about a solid chunk of wood.

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It doesn't really want to burn very well on its own right.

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Right, Like you have to have a whole lot of external heat flux to help drive it.

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But once it begins to break down and char and crack and fissure, it can sustain itself a little bit better.

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Thinking about going along with this log and wood idea when it's like that log if it was out in the forest, and it begins to decay and gets what we call punky, it burns very differently than an intact tree would right, Because then you have there's different kinds of rot right.

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It either attacks the cellulose or the lignin.

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So you either have very fine stringing material of what's left over after the rot has worked right, and that can be very susceptible to ignition.

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It can smolder very easily, it can hold over fires for a really long time and then when wind comes along it can begin to flame again.

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So it's a very different process if you've got this ability to have airflow through the fuel bed versus if it's just a solid material that's totally impermeable to air.

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Yeah, but permeability and porosity, they always go together, because you could have a piece of polyurethane foam that would not be in perma.

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Right, that's how it seems.

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Closed cell versus open cell or whatever.

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Yeah, yeah, yeah, yeah, yeah.

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And I think, it being closed cell with air involved, there's still that unique heat transfer that can happen within those pores.

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But without that airflow you're going to have a different burning behavior.

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Right, the open cell foam would be different.

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And how would this heat transfer mechanisms be different than when you have a solid material, like if you would compare a porous material versus the same material in its bulk form?

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Well, and you know what I think about.

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Like heating to ignition and burning of a solid piece of wood, right, it's whatever is heating it up first.

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Right, it was radiant heat or convective heat.

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If it's solid, all of that is happening basically right at the surface.

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Right, so you don't have any in-depth heating.

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When it's porous, you do have that potential.

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Right, so that radiant heat can penetrate further into the fuel.

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So you're heating up more of the fuel.

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But the other thing that's important for wildland fuels is these fuels are a lot more porous than felt generally.

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Right, you've got bigger gaps of air, and that actually makes the convective heating much more important, right?

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So imagine your tree that's out in the forest.

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You've got these needles dangling on the branches that are separated by a fair amount of space.

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This is a porous fuel and if a big amount of radiant heat hits it, there's a whole lot of room for airflow, and so as those fuels begin to heat up, natural convection or any kind of wind or whatever can come in and actually cool them back down.

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So, it changes the balance of what's driving it between radiant and convection, depending on how porous that fuel is.

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So in this case you could say that the heat transfer is so much complicated because it happens like in three dimensions, not just at the surface.

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Right.

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Right, yeah, and you can't just make an assumption of it's one or the other right.

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You kind of have to do the work and find out what's actually what the balance is, if it's radiant heat or if it's convective heat, or how important that convective cooling is.

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Which, like bringing us back to the initial question about how does an engineer deal with that?

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That's already one point to consider, like if the form changes, the heat transfer phenomenon, and as an engineer like you work with simple terms like I don't know ignition temperature or radiant heat that's going to create the ignition condition inside, so you rarely would go that deep and in this case, even those simple terms, they would much differ if you have a porous variant of a fuel versus a solid variant of the same fuel, right?

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Right, yeah, exactly, I mean once it's began to burn.

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I mean there's different dynamics there as well, right, because if it's porous, it allows airflow through it, so you can actually entrain air into the fuel, into the actual fuel bed itself while it burns, right?

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And also like when I try to visualize to myself what the porous fuel is, I imagine it as a collection of like bubbles with very thin walls.

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So I also like understand they would be damaged very quickly.

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Is the damage progression in such a material, I don't know, easier, does it penetrate deeper and you could have a larger fuel production or pyrolysis in a foam material or porous material versus the same material in solid form.

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I would say so, seryad, too.

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I think it also really depends, too, on the char formation ability of the material as well, right?

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Because if it's something that doesn't char and hold its, maintain its structure real, well then obviously that's going to.

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You're going to have a puddle, a puddle fire, instead of a porous material fire, right?

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I've asked that because you know we often would quantify the heat release rate of an item per square meter where, again in this time, we're like somewhere in the cubic meter, but porous materials, it could be large items stacked together in some sort of like permeable way, like our wood crib.

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Especially, if you're able to.

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I mean, I go to the wood pallet and the wood crib fire right, where I mean you've got somewhat large elements that are better kind of stacked and arranged but due to the that inner material at the same time as the outer material, right, because that's you know, the whole thing will be burning, not just the outer rim.

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Um, so yeah, that's you know, a really important part of the whole process with those date materials after we've talked about wood grips three years ago, we've done since then some experiments, large scale experiments on timber compartments, and in those experiments we've done since then some experiments, large-scale experiments on timber compartments, and in those experiments we've used different types of wood creeps, from very permeable ones to some, let's say, not that permeable, and and the differences were absolutely massive.

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Like you would not believe how, like you would, but the audience may not believe how big the difference was between those fires.

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But I, I wonder, in realistic settings of offices, buildings, compartments, do we even have sets of fuels that would reassemble cribs or other way, are cribs a good representation of realistic buildings?

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That's a question I always ask myself when I do a fire experiment using a crib.

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Right, yeah and I see your question there, because nothing really comes to mind that looks and feels like a crib in the built environment, right, but I mean we do all the time out in the wildland environment, right?

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I mean, even if you're building your campfire when you go camping, you're building a wood crib.

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Exactly.

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Essentially right.

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So understanding how all that works.

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A tree is kind of like that same structure of individual elements arranged around itself to have the air flow through it.

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Man-made structures are a little bit less in that stacked Lincoln log house type of arrangement, unless you're talking about pallet fires in a warehouse or something right.

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However, I've seen some sorts of external facades made from individual lamella all the time.

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Or even the latticework on the outside of the house.

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Latticeworks exactly.

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And even more on that, uh, the green facade stuff that we've been dealing more and more with.

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But but that's, that's literally your wildland fuel, put in a in a vertical orientation.

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Uh, when you're testing those wildland fuels, what are the characteristics of the fuel that uh leads to, let's say, worse or larger fires?

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Do you have any observations on what makes one fuels more flammable than others?

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Well, wildland fuels are a particularly tricky lot, right, because they're totally uncontrollable, right?

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And so the dead needles that are on the ground are very different than the live needles up in the trees, right, and so the dead needles that are on the ground are very different than the live needles up in the trees, and understanding what's going on with the live needles is probably a whole episode into itself, because they're living, breathing, photosynthesizing fuels that change their chemical composition by the hour, practically, whether it's morning versus afternoon, versus winter versus summer.

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So you kind of never know what you're going to get with one of those.

00:20:45.696 --> 00:20:55.039
So, yeah, like, understanding wildland fuels gets very complicated in terms of the chemistry, because you don't ever really know what you have.

00:20:55.039 --> 00:21:05.945
But in terms of wildland fuels availability and what to understand when fires get really bad, right, there's always the classic example of, like we need to know the moisture content.

00:21:07.167 --> 00:21:08.990
Continuity is a big thing, right.

00:21:08.990 --> 00:21:19.884
So wildland fuels often are very clumpy, or at least a lot of natural native vegetation tends to be Like, particularly if you think about your native grasses.

00:21:19.884 --> 00:21:25.914
In a lot of places they're clumps of grass, right, but sometimes there's an invasive like.

00:21:25.914 --> 00:21:28.926
We've got a problem right now with this invasive grass called cheatgrass.

00:21:28.926 --> 00:21:44.030
That isn't a clumpy grass, so it comes in and makes this continuous layer of very fine, easily dried out grass that makes it go from one clump of grass to the next very quickly and it out and it and it burns like stink.

00:21:44.030 --> 00:21:47.944
So things like that can change the continuity of the fuels.

00:21:48.425 --> 00:21:50.368
Um, vertically also matters, right.

00:21:50.368 --> 00:22:13.469
So, um, whether or not you've got, uh, short trees underneath your tall trees and then shrubs underneath your short trees, um can really make a big difference on whether or not you have a surface fire where it's just burning on the surface fuels on the ground, maybe flames as high as your knees, up to whether it's up in those tree grounds and you've got 100 meter flight lengths.

00:22:13.469 --> 00:22:17.721
Obviously, the weather conditions are a huge, huge, huge factor.

00:22:17.721 --> 00:22:26.986
I mean we're seeing that right now when the fire is in LA, right, when you've got 150 kilometer an hour, bone dry winds, I mean there's not much you can do.

00:22:27.500 --> 00:22:41.050
I'll come back to continuity, but the weather here would be considered in terms how efficiently you can push the flame to the middle of the porous fuels to create those effects that we've started with, or yeah, yeah.

00:22:41.140 --> 00:22:44.150
So I mean the wind does a lot of things right.

00:22:44.150 --> 00:22:49.723
So it does bend the flames over so you get better heat transfer right.

00:22:49.723 --> 00:22:51.545
It also ventilates the fuels.

00:22:51.545 --> 00:22:57.490
So you're thinking about your punky log on the ground Without wind and you get a little amber in it.

00:22:57.490 --> 00:22:58.770
It will probably smolder.

00:22:58.770 --> 00:23:08.557
But if you blow a really strong wind on it, suddenly your logs, your big logs that would normally be smoldering, are now flaming and contributing to the fire front.

00:23:08.557 --> 00:23:13.131
The other thing about wildland fires and wind are embers right?

00:23:13.131 --> 00:23:18.480
I think you had a story about starting a fire by some foam that got caught by the wind.

00:23:18.640 --> 00:23:19.401
I absolutely did.

00:23:19.401 --> 00:23:19.721
Yes, Right.

00:23:20.080 --> 00:23:20.340
Right.

00:23:20.340 --> 00:23:33.976
So I mean, that's another big factor about wildland fires and wind is it's also leapfrogging itself by these embers that get ahead of itself, which are, you know, smoldering little debris that are Likely also porous.

00:23:35.009 --> 00:23:39.615
In the case where an ember would land on your porous fuel.

00:23:39.615 --> 00:23:46.664
Do those conditions also play a role in how quickly that spreads into that fuel?

00:23:47.045 --> 00:23:47.625
Oh yeah, yeah.

00:23:47.625 --> 00:23:48.152
So that's.

00:23:48.152 --> 00:23:50.199
I mean, that is a big thing, right?

00:23:50.199 --> 00:23:53.509
Those embers have to land into something susceptible, right?

00:23:53.509 --> 00:23:56.701
So if that ember lands, say, on your solid wood deck, right on the top of a board, you're probably okay, right.

00:23:56.701 --> 00:24:02.276
So if that ember lands, say, on your solid wood deck, right on the top of a board, you're probably okay, right.

00:24:02.276 --> 00:24:07.294
If it lands, you know, in the crevice of a board of a deck, you know you might start to worry.

00:24:07.535 --> 00:24:15.942
But if it lands in that punky log, you've got it ignition like 100%, because those punky logs are super ignitable, super receptive to ignition, right.

00:24:15.942 --> 00:24:20.334
So you've got that fine material that's very fluffy.

00:24:20.334 --> 00:24:24.761
If that ember can get down in there, it can land in that punky log.

00:24:24.761 --> 00:24:30.340
And that punky log insulates it so you're protecting it from any kind of heat losses.

00:24:30.340 --> 00:24:38.634
But then again it's still punky, it's porous, so it's got enough air to breathe so it can actually sustain ignition super easily.

00:24:38.634 --> 00:24:42.176
So that's a really big source of ignitions.

00:24:42.176 --> 00:24:46.279
And those logs can hold over for a long time too, right?

00:24:46.279 --> 00:24:57.146
So there's been instances where we know for sure that those logs have been ignited, say from a pile burn over the winter, and then four months later, later, they pop back up again right.

00:24:57.948 --> 00:25:07.256
Is this mechanism that you just described like the, the fact that the fuel is fluffy and it isolates the heat, that there are not that many heat losses?

00:25:07.256 --> 00:25:12.960
I assume that that's the same mechanism that was behind the past favorite ignition source.

00:25:12.960 --> 00:25:15.428
You know, cigarettes to a mattress like.

00:25:15.428 --> 00:25:17.096
It's a very similar scenario, right?

00:25:17.759 --> 00:25:19.853
yeah, very, very much so, and it's also the um.

00:25:19.853 --> 00:25:23.201
I think guillermo has talked about zombie fires before.

00:25:23.201 --> 00:25:24.872
Right, I mean, it's a similar thing.

00:25:24.872 --> 00:25:29.330
Right, it's how you know, fires in the peat in the arctic can survive all winter, right, it's?

00:25:29.351 --> 00:25:56.298
they're buried down underneath the snow in these porous stuff, materials, and they just, they can kind of cook themselves, uh, over the winter and then resurface in the summer when things start to dry out and warm up let's perhaps reiterate this, this mechanism, because I think it could be very relatable to many sources of ignition and many, many types of fires, and could also be perhaps interesting to any fire investigators that are listening to us.

00:25:56.298 --> 00:26:11.696
So how exactly this source of energy as like low energy, doesn't ignite a porch, but ignites this porous fuel what exactly happens when it lands, when it would be on a solid one and when it's on a porous one?

00:26:11.696 --> 00:26:13.595
That's a very interesting distinction.

00:26:14.719 --> 00:26:15.039
Yeah.

00:26:15.039 --> 00:26:25.071
So I think this all comes down to well gosh, the way I think about combustion in general, like all of it is, it's all of the interplay between heat generated and heat lost, right?

00:26:25.071 --> 00:26:30.221
So rata got an ember that's buried in a really punky log.

00:26:30.221 --> 00:26:41.144
You know there's only so much heat generation that that ember is going to produce, and if you bury on a log where it can insulate itself, that changes that balance between heat generated and heat loss, and so you get that ignition.

00:26:41.144 --> 00:26:51.865
But if it's sitting on the top of a big chunk of wood, that ember is losing heat to the wood itself, it's losing heat to the environment, and so that tips it the other way, right?

00:26:51.865 --> 00:26:54.993
So where the heat losses are greater than the heat that's generated by that ember.

00:26:54.993 --> 00:27:07.015
So I kind of go through life thinking about this balance of heat generated and heat lost, because it's really, you know, it's like how smoldering happens, it's how ignition happens, it's how extinction happens.

00:27:07.329 --> 00:27:18.580
I wonder if this is something that the engineer who's facing a problem of designing their fire could solve for, Because on the one hand, you know it's like you're painting this as a simple heat balance.

00:27:18.580 --> 00:27:24.838
But yeah, it's something you could break your mind around because it's not that simple.

00:27:24.838 --> 00:27:31.614
If you study it, what type of material properties you would be looking for, Like density, thermal bulk?

00:27:31.954 --> 00:27:32.557
Wow, yeah.

00:27:32.557 --> 00:27:46.826
So I guess if you've got a porous fuel right, yes, you're looking beyond just the usual density, heat capacity, thermal conductivity, thermal diffusivity, in general right, and you do need to start thinking about a bit.

00:27:46.826 --> 00:27:53.193
We mentioned, like open cell versus closed cell, with the porosity, the permeability In wildland fuels.

00:27:53.193 --> 00:27:56.095
We talk a lot about bulk density, right the mass per volume.

00:27:56.095 --> 00:27:57.161
We also.

00:27:57.161 --> 00:27:58.191
We talk a lot about density, right, the mass per volume we also.

00:27:58.191 --> 00:28:01.606
We talk a lot about because we have accumulations of fine fuels.

00:28:01.606 --> 00:28:06.936
So we talk a lot about surface area volume ratio of the fine fuel and make up the bigger fuel.

00:28:06.936 --> 00:28:17.593
But again, how those are arranged will and how dense they are does matter and does drive that balance between heat generation and heat loss.

00:28:17.593 --> 00:28:18.355
I mentioned that whole.

00:28:18.355 --> 00:28:18.999
The needles.

00:28:18.999 --> 00:28:23.416
Igniting by radiation alone is really hard unless they're super dense right.

00:28:24.271 --> 00:28:31.258
You've changed that balance because you're adding a lot more heat loss if they're spread apart than if they're really close together, right?

00:28:32.512 --> 00:28:34.179
You said surface to volume ratio.

00:28:34.179 --> 00:28:35.895
Is there an easy way to measure that?

00:28:35.895 --> 00:28:38.759
It sounds like a pain to get that value.

00:28:39.210 --> 00:28:40.275
Well for full island fuels.

00:28:40.275 --> 00:28:42.837
A lot of those have been measured for decades.

00:28:42.837 --> 00:28:56.392
So I think there's some really new cool technology now these days where you can use cameras and automated processes to take a picture of something and it will measure all of that for you.

00:28:56.392 --> 00:28:57.930
But back in the old day it used to be calipers and a picture of something and it will measure all of that for you.

00:28:57.930 --> 00:29:01.558
But you know, back in the old day used to be calipers and a lot of patients okay.

00:29:01.859 --> 00:29:07.988
So I thought like you need like three-dimensional cd scan or something yeah, and they, they, they have them.

00:29:08.028 --> 00:29:09.432
Now they're coming out where you know.

00:29:09.432 --> 00:29:27.192
All you have to do is, you know, drop a whatever thing it is that you want to know into this little machine and it uses cameras and then just spits out a number for you but there also must be some like if you're talking about ignition from a small uh, smoldering sources, there must be also a scale.

00:29:27.231 --> 00:29:41.798
So if it falls down on a, on massive logs that are just stacked in a crib, they're probably still behave like a solid fuel to that little ember and then behave like a large one, right?

00:29:42.049 --> 00:29:43.355
Well, yeah, you make a fair point.

00:29:43.355 --> 00:29:53.603
One tiny little ember in a great big batch of fuel may not be enough, but at the same time, when you're talking about wildland fires, there's millions of embers.

00:29:53.603 --> 00:29:55.998
So it's often not just an ember, right?

00:29:55.998 --> 00:29:59.791
You're going to get pounded by a whole bunch of them all at once.

00:29:59.791 --> 00:30:08.159
It's actually kind of scary to watch videos from you know these fires, right, because it's just, it's, say, your ordinary fuel, if we can use that term.

00:30:08.179 --> 00:30:09.339
You've said about continuity.

00:30:09.339 --> 00:30:20.095
How do you assess that?

00:30:20.095 --> 00:30:21.395
How important is that?

00:30:21.395 --> 00:30:30.015
Are you expecting like a continuous spread over the fuel or it's jumping anyway from fuel patch to fuel patch to embers?

00:30:30.015 --> 00:30:31.380
How do you approach that?

00:30:31.910 --> 00:30:35.861
Yeah, I mean that again is have another million dollar question right there, right?

00:30:35.861 --> 00:30:43.195
So is how clumpy is clumpy and what kind of continuity is necessary, and I mean that's an active area of research.

00:30:43.195 --> 00:30:52.022
Is what counts as a fuel break, right, when you're, say, trying to stop a fire, how big do you need to separate the fuels from each other?

00:30:52.022 --> 00:30:55.605
How wide do you need to plow through the fuels to make it stop?

00:30:55.605 --> 00:30:59.535
And that certainly depends on a number of things, right, like the fuels itself.

00:30:59.535 --> 00:31:04.340
Some fuels are way more effective at producing embers.

00:31:04.340 --> 00:31:11.719
Some grass tends to not be a super high generating ember and what embers they have are small, so they burn out quickly.

00:31:11.719 --> 00:31:13.615
What kind of weather you have?

00:31:13.615 --> 00:31:21.837
Right, because if you've got a really strong wind and those flames are really far tilted over, you're going to get a lot better, a lot bigger windbreak for sure.

00:31:22.892 --> 00:31:27.436
Yeah, one thing that we and you also mentioned, vertical continuity, and that was very interesting to me.

00:31:27.436 --> 00:32:05.421
With our limited attempts on vertical meadows, on those green facades, we've seen that when you dry them enough they're extremely susceptible to fire, like super easy to ignite them, but but it'll be some sort of a surface fire and the ground or the mixture of ground roots, you know in which they're, they were embedded and actually when the plant is alive for many, many months, there's more, more roots than ground in its pot which probably any person who likes plants realizes that the root systems can be enormous.

00:32:05.951 --> 00:32:09.238
They didn't ignite that easily like the rooting systems.

00:32:09.238 --> 00:32:17.250
So it felt like almost impossible to protect the vertical meadow from having a surface fire Like.

00:32:17.250 --> 00:32:26.884
Even if I did like two, three meters separation between those patches, like it was a very little, tiny source that would be enough to ignite the next patch.

00:32:26.884 --> 00:32:39.006
If I did not have, like direct flame contact between my source my, let's say, wind and plume and the pots, they would not go into flaming fire that easily.

00:32:39.006 --> 00:32:42.878
So this vertical separation also feels quite interesting.

00:32:43.480 --> 00:32:43.862
Right, right.

00:32:43.890 --> 00:32:55.674
And so in wildland fire we talk a lot about ladder fuels, right, because that's what creates a ladder of fuel up from the surface fire where most ignitions actually start right Up into the tree grounds.

00:32:56.410 --> 00:33:06.559
And we're facing a lot of forest conditions these days because we haven't had a lot of fire in a lot of our landscapes that historically would have had fire, and so now we're seeing a lot more of this vertical field continuity.

00:33:06.559 --> 00:33:08.637
So, yes, we're seeing a lot more crown fires.

00:33:08.637 --> 00:33:18.492
I mean for many reasons, including, you know, just everything is hotter and drier these days, but also because there is this continuity, right, hotter and drier these days.

00:33:18.492 --> 00:33:23.448
But also because there is this continuity, right it's, it just walks itself up into the tree crowns really easily by, like you say, just by easily flame contact.

00:33:23.448 --> 00:33:51.103
And when you've, you know you're talking vertical right, all your heat's going up, so it's you have to have huge separations really to not get that flame contact, to just have the fire walk right up into the tree crowns whereas when you have more dense fuels, this separation distances perhaps could be lower because of having them less porous, and maybe dense is worse way, because I think it's the porosity that makes them extremely dangerous.

00:33:51.768 --> 00:33:58.717
Right, yeah, so I mean, the more porous it is right, the more susceptible it is to that convective heat transfer, right?

00:33:58.717 --> 00:34:00.192
So that plume is going to heat it up.

00:34:00.192 --> 00:34:06.724
The hot gases are going to heat up something that's more porous and open a lot more easily than something that's dense.

00:34:07.490 --> 00:34:13.523
Okay, let's move to smoldering fire, because it also was a part of this discussion.

00:34:13.523 --> 00:34:18.253
You said they could ignite or they could smolder of this discussion.

00:34:18.253 --> 00:34:20.297
You said they could ignite or they could smolder.

00:34:20.297 --> 00:34:23.063
So how susceptible are porous fuels for smoldering ignition?

00:34:23.063 --> 00:34:29.702
And again, is this a difference between porous fuels and their very solid counterparts?

00:34:30.244 --> 00:34:30.403
Right.

00:34:30.403 --> 00:34:41.916
I think that the smoldering of a porous fuel is a really critical part to the ignition and the initial start of a lot of fires, because that's the shortcut to flaming, right?

00:34:41.916 --> 00:34:51.342
So something that can smolder really easily is definitely a lot more hazardous than, say, a solid piece of material that won't smolder.

00:34:51.342 --> 00:34:53.554
But remember, to smolder you have to be able to char.

00:34:53.554 --> 00:34:54.798
So not all materials are capable of smoldering, right?

00:34:54.798 --> 00:34:56.001
Some polymers will just melt and not smolder.

00:34:56.001 --> 00:34:58.469
You have to be able to char, so not all materials are capable of smoldering, right.

00:34:58.469 --> 00:35:02.139
Okay, Some polymers, right, will just melt and not smolder.

00:35:02.139 --> 00:35:10.929
So you have to have the right balance of openness to be able to get the airflow through it and to sustain a little bit of a chemical reaction.

00:35:10.929 --> 00:35:22.445
But also, especially in the initial stages of a fire, when you've got a lot of heat losses, right, you need to be able to protect it from any kind of the heat loss itself, right?

00:35:22.445 --> 00:35:28.262
So that's that going back to that balance between heat generated and heat lost when we're talking about the start of a fire.

00:35:28.829 --> 00:35:29.291
Why not?

00:35:29.291 --> 00:35:30.657
Melting is a prerequisite.

00:35:31.592 --> 00:35:32.889
Well, you need the char, right.

00:35:32.889 --> 00:35:38.103
So smoldering combustion is a they call it a heterogeneous combustion.

00:35:38.103 --> 00:35:49.157
So normally when you talk about flaming, it's gases reacting with gases, but smoldering is the air reacting directly with the surface of a material, and that's usually the char.

00:35:49.157 --> 00:35:57.103
So in order to have that kind of reaction, you need that char, the carbon surface for the air to attack and react on.

00:35:57.103 --> 00:36:05.840
So something that melts obviously doesn't have this ability to have this surface reaction and we're talking about surface area, right.

00:36:05.840 --> 00:36:06.994
So something that's porous.

00:36:06.994 --> 00:36:13.775
With the char you get a whole lot of surface that's available for that oxygen to react on, right Versus something that's solid.

00:36:15.030 --> 00:36:24.557
I ask this question because we have some materials like polystyrene which are very interesting from this perspective in the building environment because they are porous.

00:36:24.557 --> 00:36:27.480
They're like highly foamed materials, very lightweight.

00:36:27.480 --> 00:36:36.034
A ton of air pores inside the polystyrene foam, yeah, but as soon as you put any heat source away it just melts away.

00:36:36.034 --> 00:36:50.423
So you have those effects I would assume endothermic of melting I guess that must be an endothermic reaction but also, you know, just physical thing of the fuel moving away from the source.

00:36:50.423 --> 00:37:04.641
It's like a physical separation distance that suddenly appears between your ignition source, which I assume would be non-movable, even if you have a pile of firebrands, if you have a cigarette or something or a burning

00:37:06.391 --> 00:37:07.856
item, you assume it remains on.

00:37:07.856 --> 00:37:15.230
So for many years in Poland, for example in facades, we didn't have really a big problem with EPS facades, which could be surprising.

00:37:15.230 --> 00:37:25.038
It's flammable by definition, but they would just not participate that much in those facades.

00:37:25.038 --> 00:37:45.289
Whereas in recent years I would not be so sure about that statement anymore, because we've started putting like insane amounts of EPS on the facades, like 20 centimeter layers, you know, to really get those U values to the max, you know to get this zero energy thing, and now the problem shifted into puddles burning underneath the building.

00:37:45.289 --> 00:37:52.664
So suddenly the amount of melted material starts to become very important and you're just changing one hazard to another.

00:37:52.664 --> 00:37:54.476
I think it's a very interesting dynamic.

00:37:55.110 --> 00:37:58.400
Yeah, it's almost a corollary there with the embers and spot fires, right?

00:37:58.400 --> 00:38:06.204
I mean you get this transport of hot burning material elsewhere that can start new fires on the new fuel, right?

00:38:06.204 --> 00:38:07.536
It's its own hazard in itself.

00:38:08.409 --> 00:38:10.056
And this permeability.

00:38:10.056 --> 00:38:11.581
How important is that?

00:38:11.581 --> 00:38:14.054
Because I know that polyurethane would smolder.

00:38:14.054 --> 00:38:14.536
I think it would.

00:38:14.536 --> 00:38:15.137
That's my assumption.

00:38:15.137 --> 00:38:17.461
Yeah, I think that's likethane would smolder, I think it would.

00:38:18.204 --> 00:38:21.139
That's my assumption yeah, I think that's the classic example.

00:38:21.139 --> 00:38:21.922
Right, it's polyurethane.

00:38:22.871 --> 00:38:25.320
And it would smolder in depth right.

00:38:25.320 --> 00:38:27.498
It would not just smolder at the surface.

00:38:27.829 --> 00:38:38.416
Well, I'm going back to some of the work that my lab did when I was a grad student, right, because they studied the transition to flaming a lot using polyurethane foam.

00:38:39.018 --> 00:38:45.998
They would start a smoldering ignition and then change the airflow and look for conditions where it would transition to flaming using polyurethane.

00:38:45.998 --> 00:38:52.119
So I mean that brings up the really important phenomena, though, of the transition to flaming right.

00:38:52.119 --> 00:39:15.853
Smoldering is its own hazard, but it is a shortcut to flaming, and that's where you know really the hazard lies, is that's where, suddenly, now you have not just a little smoking pile over in the corner, now you suddenly have a real fire right, and understanding that is really important, and I think we're just starting to get a good handle on that, but there's still a lot of work to be done there.

00:39:16.273 --> 00:39:41.777
I think for fire safety engineers it's important to be able to distinguish which items or materials would be susceptible to these small ring fires, because that means you need substantially lower ignition sources around them to trigger that, and if they can develop into a flaming fire, that sounds like a huge hazard for any building.

00:39:41.777 --> 00:39:51.233
Yeah, how persistent that smoldering in those porous materials is.

00:39:51.233 --> 00:40:00.222
I've seen chunks of CLT that smoldered all the way through, so I assume once it started it must be very difficult to quench that right.

00:40:20.264 --> 00:40:21.387
That's a very good point, right.

00:40:21.387 --> 00:40:23.210
So that's one of the hazards of smoldering is it's hard to detect.

00:40:23.210 --> 00:40:27.420
You don't always know that it's smoldering way down deep right, or sometimes it's smoldering a wall and you won months, zombie fires and things like that.

00:40:27.420 --> 00:40:39.918
Even you know insulation in homes and maybe not the insulation but foam materials like can just hold on to in ignition for a very long time before you know maybe something changes or it slowly propagates.

00:40:39.965 --> 00:40:54.391
I mean some small smoldering reactions can propagate I mean we're talking maybe millimeters an hour sometimes in some, some of the cases like, like really really slowly, so it will take them a very long time to reach the surface to where you might see them.

00:40:54.391 --> 00:41:02.518
And then you do have this in-depth reaction that you have to figure out how to quench, and sometimes you know if you're just trying to put water on it.

00:41:02.518 --> 00:41:12.405
Sometimes it's really down deep and it's really hard to get that water all the way down to where all of the heat is right, really down deep, and it's really hard to get that water all the way down to where all of the heat is right.

00:41:12.405 --> 00:41:14.112
I mean we're talking, if you think about, like peat fires, for example.

00:41:14.112 --> 00:41:14.452
Those can burn.

00:41:14.452 --> 00:41:19.809
I mean there's been peat fires that have burned for centuries, that we can't put up, or coal seams and mining areas, you know.

00:41:19.809 --> 00:41:28.615
So getting the whatever your extinguishing agent all the way down deep, deep, deep, deep to where some of that heat is, can be really challenging.

00:41:29.505 --> 00:41:35.498
I wonder if those smoldering fires will become a bigger and bigger issue in the build industry.

00:41:35.498 --> 00:41:48.619
Actually, because you know we're insulating like crazy our buildings and we're using more and more weird materials that say non-traditional materials like polystyrene for me would be a traditional material, whereas sheep wool is not.

00:41:48.619 --> 00:41:52.695
Like polystyrene for me would be a traditional material, whereas sheep wool is not a traditional building material for me.

00:41:52.695 --> 00:42:03.460
I wonder, is there also any distinct difference between artificial and natural fuels that you would immediately see?

00:42:04.865 --> 00:42:18.393
Well, I think almost all natural fuels that I could think of have the ability to smolder, right, they're going to be charring material and they're going to smolder um more artificial stuff I mean, we're talking polymers and things like that.

00:42:18.393 --> 00:42:23.253
You know it's, they could go either way, right, whether it's a melter or a charring material.

00:42:23.253 --> 00:42:27.108
But yeah, I think well, and there's a whole lot of you know, you mentioned sheep, like.

00:42:27.108 --> 00:42:36.440
I don't know if anybody's ever actually studied sheep's wool in terms of its flammability, but it ought to be pretty far up there, because it is a very fine, fluffy material.

00:42:36.440 --> 00:42:40.492
It's like almost like cotton batting or something right when that smolders really easily.

00:42:41.827 --> 00:42:42.568
It can create.

00:42:42.568 --> 00:42:46.898
It's very receptive to ignition because it's got the fine fluffiness of it.

00:42:49.085 --> 00:43:03.668
Each individual particle is a very small particle which is going to ignite really easily and then as a collective it insulates itself I I had this episode of the podcast with uh with german scientists is now in dbi patrick zutthoff, when we talked about natural fuels.

00:43:04.530 --> 00:43:25.813
I don't think patrick studied sheep's wool in particular, but he did a lot about hemp and all the types of hay and it was very interesting to observe the smoldering progression within the walls and potential transitioning into flaming on the opposite side of the wall, which is perhaps quite dangerous.

00:43:25.813 --> 00:43:29.253
One more thing, one more hazard.

00:43:29.253 --> 00:43:40.237
You've teased it a little bit, I sent it to you in the email, but we actually had the situation where we were burning a facade on a large BS rig.

00:43:40.237 --> 00:43:48.858
That's a pretty large facade, like nine and a half meter tall, and we had it insulated with I believe that was pir foam.

00:43:48.858 --> 00:44:05.407
It was a plastic material in a foam form that definitely chars, and we just had those, you know, huge chunks of foam that were eventually de-attaching themselves from the facade and flying away with the wind and we've actually started the wildfire with them.

00:44:05.407 --> 00:44:06.809
So that that was pretty crazy.

00:44:07.228 --> 00:44:16.240
Luckily, the wildfire was like directly in front of the doors to the fire brigade, which certainly helps if you want to put a fire down.

00:44:16.240 --> 00:44:21.878
If you started in the front door of a fire brigade, we're doing that experiment on the firefighter's ground.

00:44:21.878 --> 00:44:29.704
So that's not a full coincidence, but we've definitely seen the challenge with with this large firebrand thing.

00:44:29.704 --> 00:44:36.617
Are firebrands like that like something you observe in the in the modern fires like?

00:44:36.617 --> 00:44:38.027
I also wonder?

00:44:38.027 --> 00:44:49.597

188 - Fire Fundamentals pt. 13 - Porous solid fuels

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