Fire Science Show
Fire Science Show
188 - Fire Fundamentals pt. 13 - Porous solid fuels
In this episode of Fire Fundamentals, together with Dr Sara McAllister, we dwell on how stuff burns... And it is far from an easy question. While the general theme of the episodes is porous fuels, we discuss them from different angles, highlighting the similarities and differences between foamed and permeable materials.
In this episode, we cover:
- role of permeability, entrainment and forced flows through porous fuel beds;
- differences in physical properties between porous materials and their bulk forms;
- ignition (flaming and smouldering) of porous fuels;
- natural and artificial fuels, open and closed cell fuels;
- hazards specific to porous fuels in wildfires and in building fires;
And also a bit of discussion on future research of Dr McAllister along with the need for canonical tests to characterize their flammabaility.
Thank you to the SFPE for recognizing me with the 2025 SFPE Fire Safety Engineering Award! Huge thanks to YOU for being a part of this, and big thanks to the OFR for supporting me over the years.
----
The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.
Hello everybody, welcome to the Fire Science Show. 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. 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. 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. 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. Go away. You know putting our design fires based on some large-scale calorimetry mist, for example that I've covered in the podcast episodes previously. 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. The matter is too big to cover it in a podcast episode especially. There are a lot of things that we still don't know. 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. In this episode we're going to go mostly through porous fuels, and those would be two very different things. You can think of porous foams, foam materials, polymers, or you could think about a collection of pine needles that's also porous fuel. 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. So I hope it will help you look on your fire problems from a little different perspective in the future. So let's spin the intro and jump into the episode.
Wojciech Wegrzynski:Welcome to the Firesize Show. My name is Wojciech Wigrzyński 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 the 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 planets. Ofr is constantly growing and involved in fire safety engineering of the most interesting developments in the UK and also worldwide. In 2025, ofr will grow its team again and is keen to hear from industry professionals who want to collaborate on fire safety features this year. Get in touch at ofrconsultantscom. Hello everybody, I am here today joined by Dr Sara McAllister from the US Forest Service. Hey, Sara, good to have you back in the show.
Sara McAllister:I'm happy to be here, thank you.
Wojciech Wegrzynski:And that's like three years or more. It's crazy how quickly time flies when you're having fun.
Sara McAllister:I'm mind blown. Three years already.
Wojciech Wegrzynski:Yeah, I hope you have been having fun in the meantime as well. And I brought you to the podcast to geek out on fires, of course, and on burning stuff. 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. Fine with that. Oh, that sounds great, let's do it. Solid fuels On the podcast I had some episodes about flame spread, about ignition. Today let's try and discuss how stuff burns. 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. 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. 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?
Wojciech Wegrzynski:Indeed. Come on Fire safety engineers, people who just graduate fire safety engineering. They're put in the seat of an engineer on a project and they're giving this task. Like here's your building, here's your fuels. Like, figure out what's the fire going to be? Like? People are challenged with this every day. I know it's a million dollar question. There's no answer. So maybe let's brainstorm how one could get to maybe not the worst answer possible.
Sara McAllister:Well, I mean, you're talking to an experimentalist here, right? 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.
Wojciech Wegrzynski: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. But I assume if I stack them like loosely and I stack them tightly it's going to be completely different fire outcome, right?
Sara McAllister:Oh for sure. 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. So we learned a lot burning wood cribs over far too many years. Where I mean there's, we go back to some of the very fundamental work done on that. Right, where there's sort of two regimes of a fuel bed. Right, it could be densely packed where the ventilation is driving the fire behavior, where it's ventilation limited.
Sara McAllister: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. Right. 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. 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. But if you lift it up like maybe seven, eight centimeters. 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.
Wojciech Wegrzynski:Is it the convective airstream from the fire driving that? What's the driver for this flow from underneath?
Sara McAllister:Yeah, so it's that buoyancy right.
Sara McAllister:So as it begins to burn, all of those gases rise and it's pulling in air from all of its surroundings.
Sara McAllister:If you put it directly on the ground, it has to come in through the sides right.
Sara McAllister:So the smaller the fuel elements, the smaller the little holes that it has to get through it. So it's going to have a harder time getting into the actual body of the fuel bed itself. If you've got big fuel elements, you've got bigger air gaps and you can get some airflow through it. 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. 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. 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. All of the pyrolysis gases can come through the fuel bed.
Wojciech Wegrzynski:When I was like contemplating this type of fires, like I. 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. I like to think about that process. As you know, the air comes from the outside but while flowing through the crib it burns out the oxygen. So eventually the gases that reach the middle of the crib or could you know, reach deeper. 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. It's just this fuel burns way above my crib, not in the crib itself. So that was in my head, the explanation of different behavior of those cribs where they have different porosities.
Sara McAllister: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. I could swear that it takes a really long time for the center of it to even start to char okay, so. I mean the the heat transfer within the fuel bed. 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?
Wojciech Wegrzynski: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. And the same would be with sprinkler test fuel cups which are in cardboards. Like if you had those cups outside of cardboards, that would be an inferno. When they are in cupboards again, you spread them into smaller chunks of fuel, right.
Sara McAllister:Well, right, you're preventing that airflow to get through to feed the fire, right? 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. 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.
Wojciech Wegrzynski: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.
Sara McAllister:That's not a very reassuring finding Well you know, the sad thing is I burned cribs. I mean I burned over a thousand cribs and there's still so many questions that I have right. 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.
Wojciech Wegrzynski:To perhaps make it simpler. So in our buildings we would find solid materials at different compositions, different states of matter, if you could say so. We have solid fuels. We have the same materials in the phone forms, we could have porous fuels. Can you help me identify how differently material starts to burn when we change the physical, let's say appearance of that item?
Sara McAllister: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. Right, so there's continuous surfaces, say, like the foam of your sofa or the foam of your mattress or something. 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. 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. Thinking about you know I come from wildfire space, right? So thinking about like collections of needles on the forest floor could be a porous fuel bed. Or even a tree itself would be a porous fuel bed. 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.
Sara McAllister:Right, you think about a solid chunk of wood. It doesn't really want to burn very well on its own right. Right, Like you have to have a whole lot of external heat flux to help drive it. But once it begins to break down and char and crack and fissure, it can sustain itself a little bit better.
Sara McAllister: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. It either attacks the cellulose or the lignin. 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. 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. 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.
Wojciech Wegrzynski:Yeah, but permeability and porosity, they always go together, because you could have a piece of polyurethane foam that would not be in perma. Right, that's how it seems.
Sara McAllister:Closed cell versus open cell or whatever. Yeah, yeah, yeah, yeah, yeah. And I think, it being closed cell with air involved, there's still that unique heat transfer that can happen within those pores. But without that airflow you're going to have a different burning behavior. Right, the open cell foam would be different.
Wojciech Wegrzynski: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?
Sara McAllister:Well, and you know what I think about. Like heating to ignition and burning of a solid piece of wood, right, it's whatever is heating it up first. Right, it was radiant heat or convective heat. If it's solid, all of that is happening basically right at the surface. Right, so you don't have any in-depth heating. When it's porous, you do have that potential. Right, so that radiant heat can penetrate further into the fuel. So you're heating up more of the fuel. But the other thing that's important for wildland fuels is these fuels are a lot more porous than felt generally. Right, you've got bigger gaps of air, and that actually makes the convective heating much more important, right? So imagine your tree that's out in the forest. You've got these needles dangling on the branches that are separated by a fair amount of space. 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.
Sara McAllister:So, it changes the balance of what's driving it between radiant and convection, depending on how porous that fuel is.
Wojciech Wegrzynski: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. Right.
Sara McAllister:Right, yeah, and you can't just make an assumption of it's one or the other right. 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.
Wojciech Wegrzynski:Which, like bringing us back to the initial question about how does an engineer deal with that? 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?
Sara McAllister:Right, yeah, exactly, I mean once it's began to burn. 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?
Wojciech Wegrzynski: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. So I also like understand they would be damaged very quickly. 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.
Sara McAllister:I would say so, seryad, too. I think it also really depends, too, on the char formation ability of the material as well, right? Because if it's something that doesn't char and hold its, maintain its structure real, well then obviously that's going to. You're going to have a puddle, a puddle fire, instead of a porous material fire, right?
Wojciech Wegrzynski: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.
Sara McAllister:Especially, if you're able to. 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.
Wojciech Wegrzynski: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. Like you would not believe how, like you would, but the audience may not believe how big the difference was between those fires. 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? That's a question I always ask myself when I do a fire experiment using a crib.
Sara McAllister: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? I mean, even if you're building your campfire when you go camping, you're building a wood crib.
Wojciech Wegrzynski:Exactly.
Sara McAllister:Essentially right. So understanding how all that works. A tree is kind of like that same structure of individual elements arranged around itself to have the air flow through it. 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.
Wojciech Wegrzynski:However, I've seen some sorts of external facades made from individual lamella all the time.
Sara McAllister:Or even the latticework on the outside of the house. Latticeworks exactly.
Wojciech Wegrzynski:And even more on that, uh, the green facade stuff that we've been dealing more and more with. But but that's, that's literally your wildland fuel, put in a in a vertical orientation. 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? Do you have any observations on what makes one fuels more flammable than others?
Sara McAllister:Well, wildland fuels are a particularly tricky lot, right, because they're totally uncontrollable, right? 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. So you kind of never know what you're going to get with one of those. So, yeah, like, understanding wildland fuels gets very complicated in terms of the chemistry, because you don't ever really know what you have. 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.
Sara McAllister:Continuity is a big thing, right. 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. In a lot of places they're clumps of grass, right, but sometimes there's an invasive like. We've got a problem right now with this invasive grass called cheatgrass. 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. So things like that can change the continuity of the fuels.
Sara McAllister:Um, vertically also matters, right. 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. Obviously, the weather conditions are a huge, huge, huge factor. 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.
Wojciech Wegrzynski: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.
Sara McAllister:So I mean the wind does a lot of things right. So it does bend the flames over so you get better heat transfer right. It also ventilates the fuels. So you're thinking about your punky log on the ground Without wind and you get a little amber in it. It will probably smolder. 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. The other thing about wildland fires and wind are embers right? I think you had a story about starting a fire by some foam that got caught by the wind.
Wojciech Wegrzynski:I absolutely did. Yes, Right.
Sara McAllister:Right. 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.
Wojciech Wegrzynski:In the case where an ember would land on your porous fuel. Do those conditions also play a role in how quickly that spreads into that fuel?
Sara McAllister:Oh yeah, yeah. So that's. I mean, that is a big thing, right? Those embers have to land into something susceptible, right? So if that ember lands, say, on your solid wood deck, right on the top of a board, you're probably okay, right. So if that ember lands, say, on your solid wood deck, right on the top of a board, you're probably okay, right. If it lands, you know, in the crevice of a board of a deck, you know you might start to worry.
Sara McAllister: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. So you've got that fine material that's very fluffy. If that ember can get down in there, it can land in that punky log. And that punky log insulates it so you're protecting it from any kind of heat losses. 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. So that's a really big source of ignitions. And those logs can hold over for a long time too, right? 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.
Wojciech Wegrzynski: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? I assume that that's the same mechanism that was behind the past favorite ignition source. You know, cigarettes to a mattress like. It's a very similar scenario, right?
Sara McAllister:yeah, very, very much so, and it's also the um. I think guillermo has talked about zombie fires before. Right, I mean, it's a similar thing. Right, it's how you know, fires in the peat in the arctic can survive all winter, right, it's?
Wojciech Wegrzynski: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. 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? That's a very interesting distinction.
Sara McAllister:Yeah. 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? So rata got an ember that's buried in a really punky log. 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. 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? So where the heat losses are greater than the heat that's generated by that ember. 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.
Wojciech Wegrzynski: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. But yeah, it's something you could break your mind around because it's not that simple. If you study it, what type of material properties you would be looking for, Like density, thermal bulk?
Sara McAllister:Wow, yeah. 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. We mentioned, like open cell versus closed cell, with the porosity, the permeability In wildland fuels. We talk a lot about bulk density, right the mass per volume. We also. We talk a lot about density, right, the mass per volume we also. We talk a lot about because we have accumulations of fine fuels. So we talk a lot about surface area volume ratio of the fine fuel and make up the bigger fuel. 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. I mentioned that whole. The needles. Igniting by radiation alone is really hard unless they're super dense right.
Sara McAllister: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?
Wojciech Wegrzynski:You said surface to volume ratio. Is there an easy way to measure that? It sounds like a pain to get that value.
Sara McAllister:Well for full island fuels. A lot of those have been measured for decades. 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. 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. But you know, back in the old day used to be calipers and a lot of patients okay.
Wojciech Wegrzynski:So I thought like you need like three-dimensional cd scan or something yeah, and they, they, they have them.
Sara McAllister:Now they're coming out where you know. 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.
Wojciech Wegrzynski: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?
Sara McAllister:Well, yeah, you make a fair point. 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. So it's often not just an ember, right? You're going to get pounded by a whole bunch of them all at once. 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.
Wojciech Wegrzynski:You've said about continuity. How do you assess that? How important is that? Are you expecting like a continuous spread over the fuel or it's jumping anyway from fuel patch to fuel patch to embers? How do you approach that?
Sara McAllister:Yeah, I mean that again is have another million dollar question right there, right? So is how clumpy is clumpy and what kind of continuity is necessary, and I mean that's an active area of research. 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? How wide do you need to plow through the fuels to make it stop? And that certainly depends on a number of things, right, like the fuels itself. Some fuels are way more effective at producing embers. Some grass tends to not be a super high generating ember and what embers they have are small, so they burn out quickly. What kind of weather you have? 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.
Wojciech Wegrzynski:Yeah, one thing that we and you also mentioned, vertical continuity, and that was very interesting to me. 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.
Wojciech Wegrzynski:They didn't ignite that easily like the rooting systems. So it felt like almost impossible to protect the vertical meadow from having a surface fire Like. 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. 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. So this vertical separation also feels quite interesting.
Sara McAllister:Right, right.
Sara McAllister: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.
Sara McAllister: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. So, yes, we're seeing a lot more crown fires. 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. 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. 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.
Sara McAllister:Right, yeah, so I mean, the more porous it is right, the more susceptible it is to that convective heat transfer, right? So that plume is going to heat it up. The hot gases are going to heat up something that's more porous and open a lot more easily than something that's dense.
Wojciech Wegrzynski:Okay, let's move to smoldering fire, because it also was a part of this discussion. You said they could ignite or they could smolder of this discussion. You said they could ignite or they could smolder. So how susceptible are porous fuels for smoldering ignition? And again, is this a difference between porous fuels and their very solid counterparts?
Sara McAllister:Right. 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? So something that can smolder really easily is definitely a lot more hazardous than, say, a solid piece of material that won't smolder. But remember, to smolder you have to be able to char. So not all materials are capable of smoldering, right? Some polymers will just melt and not smolder. You have to be able to char, so not all materials are capable of smoldering, right. Okay, Some polymers, right, will just melt and not smolder. 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. 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? 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.
Wojciech Wegrzynski:Why not? Melting is a prerequisite.
Sara McAllister:Well, you need the char, right. So smoldering combustion is a they call it a heterogeneous combustion. 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. So in order to have that kind of reaction, you need that char, the carbon surface for the air to attack and react on. So something that melts obviously doesn't have this ability to have this surface reaction and we're talking about surface area, right. So something that's porous. 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.
Wojciech Wegrzynski: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. They're like highly foamed materials, very lightweight. A ton of air pores inside the polystyrene foam, yeah, but as soon as you put any heat source away it just melts away. 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. 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
Wojciech Wegrzynski:item, you assume it remains on. 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. It's flammable by definition, but they would just not participate that much in those facades. 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. So suddenly the amount of melted material starts to become very important and you're just changing one hazard to another. I think it's a very interesting dynamic.
Sara McAllister:Yeah, it's almost a corollary there with the embers and spot fires, right? I mean you get this transport of hot burning material elsewhere that can start new fires on the new fuel, right? It's its own hazard in itself.
Wojciech Wegrzynski:And this permeability. How important is that? Because I know that polyurethane would smolder. I think it would. That's my assumption. Yeah, I think that's likethane would smolder, I think it would.
Sara McAllister:That's my assumption yeah, I think that's the classic example. Right, it's polyurethane.
Wojciech Wegrzynski:And it would smolder in depth right. It would not just smolder at the surface.
Sara McAllister: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.
Sara McAllister:They would start a smoldering ignition and then change the airflow and look for conditions where it would transition to flaming using polyurethane. So I mean that brings up the really important phenomena, though, of the transition to flaming right. 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.
Wojciech Wegrzynski: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. Yeah, how persistent that smoldering in those porous materials is. 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.
Sara McAllister:That's a very good point, right. So that's one of the hazards of smoldering is it's hard to detect. 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. 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.
Sara McAllister: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. 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. 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. I mean we're talking, if you think about, like peat fires, for example. Those can burn. 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. 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.
Wojciech Wegrzynski:I wonder if those smoldering fires will become a bigger and bigger issue in the build industry. 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. Like polystyrene for me would be a traditional material, whereas sheep wool is not a traditional building material for me. I wonder, is there also any distinct difference between artificial and natural fuels that you would immediately see?
Sara McAllister: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. You know it's, they could go either way, right, whether it's a melter or a charring material. But yeah, I think well, and there's a whole lot of you know, you mentioned sheep, like. 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. It's like almost like cotton batting or something right when that smolders really easily.
Sara McAllister:It can create. It's very receptive to ignition because it's got the fine fluffiness of it.
Wojciech Wegrzynski: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.
Wojciech Wegrzynski: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. One more thing, one more hazard. 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. 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. 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. So that that was pretty crazy.
Wojciech Wegrzynski: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. If you started in the front door of a fire brigade, we're doing that experiment on the firefighter's ground. So that's not a full coincidence, but we've definitely seen the challenge with with this large firebrand thing. Are firebrands like that like something you observe in the in the modern fires like? I also wonder? I don't want to to bring into any any like political topics, but I wonder those urban settlement generated firebrands are found materials and porous materials, a large part of that.
Sara McAllister:You know, I think all materials at some point become firebrands, right.
Wojciech Wegrzynski:Everything's a firebrand if the wind is strong enough, right.
Sara McAllister:So I mean, if we're talking about, you know, some of these big like wildland urban interface, wooly fires, got your urban settlements, yeah, I mean, like there you have all sorts of stuff that's burning, right, you've got roof tiles, you've got, you've got cars, you've got the insides of the houses themselves and you know, you got a huge amount of heat release. So you've got a whole lot of buoyant flow that's picking up anything that's not like nailed down, right, so that could be porous fuels and often, once it's burned for a little bit, right, if it's something that chars, it's now suddenly kind of a porous fuel. Right, if you take solid wood and you burn it out, you got the charcoal, it's, it's burned out, it's porous, it's going to be picked up and carried by the wind and still be smoldering. So I mean that's, I mean that's a huge, huge uh part of the the spread through the communities. Right, the wildfire stops at, you know, at the edge of the, of the, of the community, and then from there it's, it's almost virtually home-to-home ignitions and a lot of those are like the homes are close enough that could be radiant heat, but those embers are really really driving a lot of that, you know, and you see pictures of these.
Sara McAllister:You know fires afterward and you see a lot of sadly. You see a lot of green vegetation but the houses are gone right Because it's not burning continuously across it. It's leapfrogging from one house to the next and that's how you know like you can have the one lucky house out of the whole bunch make it through right. Half of the one lucky house out of the whole bunch make it through right. It's just because it happened to not be bombarded, or it was built to withstand these embers a little bit better, or that was the house that the firefighters focused on to put out the little fires as they crept up. But yeah, that mechanism is a big part of these urban disaster fires.
Wojciech Wegrzynski:Let's try and do some closing statements. Evidently, the solid fuels are quite a trick to understand at the way, where you could, just you know, calculate a design fire based on a bunch of individual, like simple physical quantities to describe fuels Do you have? If an engineer engineer is faced with such a problem, what should be their first steps in terms of reading more about it or learning more about it? Are there any specific things that could help people deepen their knowledge to the point where maybe it would be more comfortable for them?
Sara McAllister:Well, I think understanding how that fuel is arranged and how that arrangement affects its burning is a really important part of it, right? I mean, if you take the same fuel and arrange it different, you're going to have very different fire behavior. And understanding that limitation I think is an important first step. You may not be able to fully predict it, but just that appreciation that you know if you've got something very densely packed versus loosely packed, you're going to get very different fire behavior for the same material.
Wojciech Wegrzynski:I think in this podcast episode we also felt victim of, like, simplifying it into, you know, porous fuels, because that's a hell of a difference when you talk about, let's say, foamed material in your furniture, where those heat transfer phenomena will perhaps be the drivers, and it's about ignition that we worry the most and your, let's say, wood grips, where the permeability would be largely driving force. I mean, if you look at it, it's just a matter of what ignites and how big the fire is and how much it burns. So you eventually reach different parts of practical fire science, like one where you worry more about where will it ignite, one where it uh, it's more uh about how badly it will burn. A lot of, a lot of tough questions and they're both very important, right.
Sara McAllister:I mean how, how badly will burn is moot if it doesn't ignite in the first place. Right, burn A lot of tough questions. They're both very important, right. I mean, how badly will burn is moot if it doesn't ignite in the first place.
Wojciech Wegrzynski:Right, Exactly, Sara. So what's next for you? What's your next research? What are you working on today?
Sara McAllister:Well, you know. So you mentioned material properties and that's something that we're really interested in is how to characterize some of the material properties of forest fuels. Right, because we've long just assumed that the conductivity, density and all that just follow wood. But you know, a needle is not wood, so we're trying to come away with you know better material properties, so we understand. You know just how important it is for predicting fire behavior, right, you know it may be less important. You know just how different the density of a pine needle is in a raging crown fire where you've got intense, super intense flames.
Sara McAllister:But it might be a lot more important when you're trying to say understand whether your prescribed fire is going to burn or not, right, in much more mild conditions. So I'm trying to get a better handle on some of those questions. Understand whether your prescribed fire is going to burn or not, right, in much more mild conditions. So trying to get a better handle on some of those questions. You know I mentioned how, you know, challenging wildland fuels are, and that's part of it is we just we don't really have good material properties for them.
Wojciech Wegrzynski:And it's also so hard to generalize across world and different fuels you would find, because fuel in Poland will be probably very different to the fuel we find in Colorado, right, right, well, and even the fuel within Poland, whether it's summer versus winter could be very different.
Wojciech Wegrzynski:One thing that would be interesting, perhaps, and perhaps that's something that could interest you as a government agency, you as a government agency, if we had a set of tests to perform to quantify a fuel, you know, like a benchmark, like don't, don't write the paper unless you have those, the type of what you mean.
Sara McAllister:You bring up a very, very good point, and something that we have discussed for gosh, probably a decade at this point is there is no canonical test or problem for wildland fire, right? We don't have the equivalent of a pool fire, we don't have, you know, we have a cone calorimeter. But how to best test wildland fuels in those to properly characterize their flammability? Right? Because no matter what, you're taking it out of the context of the wildland fire. So how you're testing it still has to, you still have to keep that larger, you know picture in mind. But when you test it, are you testing it in a way that is going to characterize the real problem? And that's something I think we struggle with a little bit, because you know the way I test. You know pine needles is going to be different than you know somebody else's group. That's going to be different from somebody else's group.
Wojciech Wegrzynski:We don't have that consensus on how to best test for these plantability properties. But look, if I'm testing building materials, I'm not in a much better position. If I had a cone calorimeter, I already had a lot, and probably much more than my standards told me to have, because I could go away with a single burning item test. I just put 30 kilowatt fire against the wall. Yes, no, that's the answer I do. Polish fire spread on a vertical surface. Yes, no pass fail, no more information. So I don't even have, you know, information to scale from. If we go back to the poor fire engineer from the beginning of an episode who's dropped on a problem to solve, it's not even the inability to scale data to the realistic problem, it's, it's the non-existence of data because we don't have this canonical set of of tests that would quantify the properties like, at best, I would have, like heat of combustion from sfp handbook. Maybe I would have cone for two to three irradiances. If I had that, I'm already rich, right, but not even that.
Wojciech Wegrzynski:And especially if we talked about porous fuels. Perhaps that's the reason behind this episode. Like 10 centimeter by 10 centimeter sample can only tell you about one scale of their behavior, but that's not the full story, right well for sure one you know if you're testing in your cone calorimeter the port with porous fuels, do you?
Sara McAllister:do you tape off the sides? Do you allow airflow through? We block it. How much do you do put in a basket? Do you like use a solid container, like we don't have these. Like consensus building, like I mean, maybe that is a call for you know something to do in a working group or something is how the working groups, please.
Wojciech Wegrzynski:No, we love them and we need them, but as a community.
Sara McAllister:We have to like, really, I think, step back and ask questions of like are we testing the right things, right to characterize the, the behavior? And I guess that goes for building fires as well. But I think the engineering approach to wildland fire is a much younger field than the actual fire protection engineering, right, I mean, historically, wildland fire has been studied by ecologists and foresters and stuff like that, oncologists and foresters and stuff like that. Granted, you know, we, if we go back like the, our spread models that we have right now are from dick rothenwell who you know was was an engineer. But those are the exceptions, not the rules, right?
Wojciech Wegrzynski:so we does not have not had as concerted of an effort, of an engineering approach to ask these questions I'm really looking forward what uh you and your colleagues in the wildland world uh, figure out problems like this one, because I'm very sure if you find any solution to the problem, I'm pretty sure my side of the world building fires will also benefit. And likewise I hope if we figure out something smart, we will be able to transfer those good solutions to the world of forest fires.
Sara McAllister:I'm keeping my fingers crossed.
Wojciech Wegrzynski:Same here. I hope you guys are first.
Sara McAllister:I'm thinking the same thing. Please figure it out for us please.
Wojciech Wegrzynski:Anyway, Sara, if we figure it out, we'll make a podcast episode out of this, and thanks for coming today to the Forest Science Show.
Sara McAllister:Oh, thank you. Thanks for having me.
Wojciech Wegrzynski:And that's it. Thank you for listening.
Wojciech Wegrzynski:If you think about it, if a layperson approaches a fire scientist, they would assume that asking like how stuff burns or why stuff burns, a fire scientist will have an immediate, like elevator pitch style answer to that question, whereas in reality some stuff is really complex in our discipline like ridiculously complex and in this podcast episode we've tried to nail the combustion or burning of porous fuels from many angles, as many as I could think of, to give you a very broad idea about things that matter when stuff burns and I hope those things that were highlighted the entrainment of air through the porous bed, the heat transfer problem, solving the heat transfer, the insulation of foams or porous materials, the differences between open and closed cell porous materials, some interesting aspects of leaf fuels, some interesting aspects of artificial fuels, some interesting aspects of artificial fuels. You found it all in this episode and I hope it really helped you look at the fire problems from a different angle. And the question I have in the beginning that was a hard one. Really I thrown a curveball on Sara how would you estimate how a fire will burn? That's a hell of a question and I wonder if someone has has really great answer to that.
Wojciech Wegrzynski:I'll be definitely looking into more episodes of fire fundamentals focus on design fires because, uh, one thing is scientific approach, first principles, understanding the physics. One thing is practical use, and I know there are a lot of scientists that are pursuing the topic of design fires, so I'll bring those up into the podcast episodes for sure. And for now, thanks for being here with me and I hope you've enjoyed the Far Fundamentals series this week and next week I invite you once again to Far Ascent show on Wednesday New content. See you there. Bye, thank you.