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Aug. 18, 2021

014 - A joyrney through the scales of fire phenomena with Sara McAllister

014 - A joyrney through the scales of fire phenomena with Sara McAllister

Have you ever wondered how is a fire of a match or candle different from a wildfire? Or maybe rather, why is it different? What is it, that makes the fires at different scales behave in such a different manner? What are the phenomena that drive these fires, and are these the same phenomena across the scales? These are the questions I had in mind when starting the interview with a rising star of Wildfire Science – Dr Sara McAllister of the USDA Forest Service.

 

Together with Sara we go into a journey across the scales of fire phenomena, discussing what are the key phenomena at particular scales. From matchsticks, through cribs and compartment fires, to wildfires and megafires – probably the most comprehensive discussion on the scales of fire phenomena I’ve ever had. From this episode you will also learn a lot about burning down wood cribs and about the LOF&BE initiative within the International Association for Fire Safety Science.

 

Learn more about Sara at:

https://www.firelab.org/profile/mcallister-sara

Connect with Sara at LinkedIn:

https://www.linkedin.com/in/sara-mcallister-90b76917/

 LOF&BE group of IAFSS:
https://iafss.org/lofbe-webinar-series/

--- LinkedIn discussion ---

 https://www.linkedin.com/posts/fire-science-show_014-a-joyrney-through-the-scales-of-fire-activity-6833641588724432896-ZEhW

--- Useful links ---

 

The groundbreaking paper on role of convective heat transfer in the spread of wildfires:

https://www.pnas.org/content/112/32/9833

 

Prof. Rein explaining why it is a breakthrough:

https://www.pnas.org/content/112/32/9795

 

Videos of the experiments:

https://movie-usa.glencoesoftware.com/video/10.1073/pnas.1504498112/video-1

https://movie-usa.glencoesoftware.com/video/10.1073/pnas.1504498112/video-2

https://movie-usa.glencoesoftware.com/video/10.1073/pnas.1504498112/video-3

 

Papers on wood cribs:

https://link.springer.com/article/10.1007%2Fs10694-015-0536-4

https://link.springer.com/article/10.1007%2Fs10694-015-0543-5

 

1000 MW fire experiment:

https://journals.ametsoc.org/view/journals/bams/61/7/1520-0477_1980_061_0682_iavawa_2_0_co_2.xml

 

Our paper summarizing the research on wildfires and the role of wind in wildfire spread:

https://link.springer.com/article/10.1007/s10694-018-0748-5

Transcript
Wojciech Wgrzyski:

Have you ever wondered how diverse the world of fire realy is? I mean, in the end, it's all just non-premixed combustion and turbulent buoyant plumes, were entrained air is mixed with some gaseous fuel and releases energy within a chemical reaction. But yet, if you look at the matchstick or a candle flame a wood crib, b uilding or forest, it seems that this phenomena are completely different. And I often wondered what makes them differ so much from each other? What is the physical background of these fires and what really makes each of them unique at their scale. And this is a topic of today's episode, and I have a fantastic guest to talk this over with. She's a distinguished young scientist recipient of the Early Career Award from the International Association for Wildland Fire. She's burned 1000s of woods crepes in her career and participated in some research that has been praised as a paradigm shift in understanding wildland fires. And obviously, we're gonna discuss both of these topics today. She's also very involved in growing the community through Large Outdoor Fires and Built Environment (LOF&BE) workshops in the working group of the IAFSS. And we're cordially inviting you to join that. So yeah, let's welcome Dr. Sara McAllister. Let's hit the intro and jump into the episode. Welcome to the Fire Science Show. My name is Wojciech Wegrzynski, and I will be your host. Hello, everybody. Today I'm here with Dr. Sara McAllister from USDA Forest Service. Hello, Sara, great to have you.

Sara McAllister:

Thanks for inviting me.

Wojciech Wgrzyski:

My number one requested guests finally here. I'm really hyped about that. So Sara is famous for her science related to combustion and combustion fundamentals and applying this research on fundamentals into practical world of fire engineering, especially wildfire...engineering, can I say that? Wildfire engine? Yeah, probably not.

Sara McAllister:

Okay, let's call it a thing. It's a thing now

Wojciech Wgrzyski:

It's a thing now, it's been coined here. So, so a good place to start is a quote from H. Hottel, from paper from Dr. Guillermo Rein, who were praising your

research, and the quote was:

"a case have been made for fire being next to the life processes, the most complex phenomena to understand". And I absolutely agree trying to learn learn fire and flame. And I guess what are your thoughts about, this is fire easy?

Sara McAllister:

Well, you of course not. I mean, it is just even fire alone. I mean, it's, it's heat transfer, it's combustion, it's fluid dynamics, it's a lot of really difficult engineering topics all rolled into one. And then to make it even worse, there's wildland fire, which combines forest ecology, and you know, the way that plants live and grow and photosynthesize to really get into like the nitty gritty of how wildland fire works. So, yeah, it makes it easy. He adds even more layers of complexity on top of it. So yeah, no, definitely not an easy. I call it job security, though.

Wojciech Wgrzyski:

Yeah, even without typography and wind, probably, it's already crazy. And once you add these little complexities into the equation, it's a little bit more complex. So I had this idea for for this interview, like, I have top combustion scientists in front of me. So let's talk about how combustion affects fire and fire engineering. And I would love to work with you through the scales of fires. Because as someone who has done experiments on the on my tabletop, experiments in my lab, and burned buildings, I mean, it's all shiny and produces the smoke but behaves in a quite different manner. And I assume within outdoor fires, it's it's the same. Let's start with the small scales. Let's think about a match fire or a candle flame, you know, what would you say are the phenomena that the simplest phenomena that would drive this fires?

Sara McAllister:

Well, I think that also depends on which way you're holding the match, right? Because this match where the, you know, it's burning from the top down, you're going to get a very different process. And if you're holding the match upside down and trying to not burn your fingertips, or even holding it sideways, so that the flame propagates kind of horizontally across it, right. So there is much like a candle. There's a lot of processes going on, right you need to transfer heat to unburned fuel to pi realize the fuel that fuel that has to mix with air and ignite, you know, the typical process. And but you know how that heat is transferred? really depends on the direction that that flame is propagating. Right. So if it's if it's downward, you can't the the unburned fuel can't necessarily, quote, air quotes on see the flame right radio isn't going to be very important for downward spread, you were talking much more about, you know, conducting some of the heat through the solid, you're also doing some gas phase conduction or convection. You're kind of probably splitting hairs at that point, right? Because the .. isn't zero, but it's pretty minimal. So is it conduction or is it convection gas phase. But you know, if you hold that match upside down, and your flame is propagating upwards, you have a lot more opportunity for radiant heat from that flame to get the unburned fuel to spread that fire. But you still have, you know, conduction to the solid phase, and you still have some convective heating. So, the as that matchstick gets bigger and bigger, or as those flames get bigger and bigger, that component, the balance between what is driven by convection versus radiation is gonna change, right. And that's, that's the complex problem of scaling.

Wojciech Wgrzyski:

When I was preparing for the interview, I was trying to think out the fire problem in which conduction would play the dominant role and actually couldn't figure one where it could be like the dominant for them a flame spread in a small scale, let's say, but I figured out that once you introduce heat sinks to the equation, conduction can become the driving force. So suddenly, you have all three ways of heat transfer, how you produce it, how it goes away, playing a significant role, even at the small scale, right?

Sara McAllister:

Yeah, and certainly the thickness of the fuel can change how those all three of those balanced, right, because if you've got a thermally fixed fuel versus a thermally, thin fuel, that role of convection or conduction, pardon me is going to be a little bit different. But you know, what's really interesting about wildland fire, is the importance of convection, right, because as you mentioned, the heat sinks, very thin, fine fuels, like a pine needle, if you're trying to talk about spread from one pine needle to the next, you really need to consider convection, heat transfer, both in terms of heating and cooling, right, if you were to put a clump of pine needles in front of just a pure radiant, you know, heat exposure, saying you know, you're sick and have a radiant heater, that's 40 kilowatts per meter squared of radiant heat flux, and you put a clump of pine needles in front of it, nothing's gonna happen because of convective heat transfer, like away from like, away, it'll cool. Even just by natural convection, there's enough airflow for those fine little fuels to cool off again, something even like a 40 kilowatt per meter squared heat flux. And conversely, they those pine needles heat up very quickly to convective heat transfer, if you've got some kind of flame contact, right. So the the context of the field really matters too in terms of if you're talking about big surfaces that are going to be much more receptive to radiant heat transfer, and not so sensitive to any kind of convective cooling, versus very small fuels. Like pine needles and especially the when they're dangling and air and a tree canopy. They're going to be very, very sensitive to convective cooling, and any little breeze or even just natural convection can cool them down from a pretty significant radiant heat flux.

Wojciech Wgrzyski:

Okay, so here your airstream because your object is so small, like a match or a needle, it just goes either is there and goes around it or doesn't. It does not exist. Let's go bigger scale, let's go item scale, single single burning item would crib, an armchair. And here there are different ways the air can go not only above and below your item. And this is this was also significant part of your research, right?

Sara McAllister:

Yeah, so I mean, if you're talking about a big impenetrable surface like a sofa or something, right, I mean, you're not going to get really much in the way of airflow through it right but very different from say like a wood crib where you have a porous fuel bed and you can get a lot more opportunity for airflow through it which you know, makes the fuel bed kind of burn much more uniformly right you can get in depth burning in something like a crib but you know, a couch is kind of going to likely burn from the outside in right we just because there's not enough air in them in it. So, you know when you when you're able to burn something like a porous fuel bed, you're going to get a lot of you know, radiant heat transfer within the fuel bed that's going to help drive it certainly convective heat transfer from the burning elements hitting other burning elements. And so that makes that kind of process much more complicated and also a lot more interesting.

Wojciech Wgrzyski:

We will come back to the woods cribs, I have like 72 questions about the woods keeps here because I'm passionate about burning wood cribs. And I want to...Yes, who, who doesn't like wood crib, right? And but here, okay, so far we are at the item scale, and here's still just the fire, let's let's let's put the item in the compartment. And suddenly the physics changes. But the fact that this is in a compartment and means it's constrained by the walls by the openings, suddenly, it's not just a buoyant plume and flame, suddenly, it's a smoke layer that has some temperature radiates heat downwards. So your heat fluxes are not just the ones that are within the source itself. But suddenly the fuel, the ones that's burning, the one that that is not burning, is like completely surrounded by, if you if you will, by the radiation and heat being transferred around. And suddenly the behavior of your wood crib of your sofa placed within the compartment will be completely different than one outdoors or in your wind tunnel. Right.

Sara McAllister:

Certainly, yeah. And I you know, the other thing is, you know, as everything gets scaled up in bigger and bigger, you're dealing with a lot more turbulence, which is making your flame thicker, which is making them sootier, which is making radiation be much more dominant of a heat transfer mechanism, as well. So I mean, all that's at play here as well, too.

Wojciech Wgrzyski:

Yeah. When you think about the classical theories for compartment fires, and you go into work of Thomas, and the Haramathy, and now what Jose Torero is doing, there was this way to separate this into two regimes, one where it's like ventilation driven. So you have this amount of air that can penetrate your compartment. And that pretty much dictates the amount of heat being produced in the fire. And in this, let's say, we have this more or less solved, maybe not completely, but we feel pretty comfortable for the last 30 years with this type of fires. And then there's the other regime in which you have more than sufficient amount of the air. And the processes inside are in a way chaotic, like 1they depend on where the air is where it can flow, where it burns, it's very complex, the temperatures are lower. And this this is very, very complex, very difficult to describe with simple empirical model as a model does not exist. And if I remember correctly, Howard Emmons said, it's gonna happen around 2050. And then Jose said, He's not so optimistic about it. So if these two guys are not sure if we can solve it, that's a powerful problem. But I was wondering like, in this case, the interior of the compartment is very, like, similar to the interior of your wood crib, because it's always also how deep can the air going through it, right?

Sara McAllister:

Yes, yeah. So as you were talking, I was, you know, like, Oh, yeah, this is sounds like so cool that there is such a, that the analogy is very similar to wood cribs, right? Because there's the same two regimes of wood cribs, right. And so if they're very dense, the airflow just can't penetrate them very effectively. And so they're ventilation limited as well. So as you if you were to, you know, widen the spacing in the cribs and make them more and more open and loosely packed, then you get this transition to a different behavior. And so there's, there's two different relationships to predict behavior. For wood crib, just based on those relationships, as well. What's interesting is that the open fuel bed configuration in a wood crib is actually a lot more predictable than compartments. Okay, yeah, cuz at that point, they basically just assume that the, the elements of the crib are burning it, the individual rate given a particular like external heat flux, so it becomes only proportional to the diameter of the field particle. But when it's when it's confined, then there's a lot more variables about how the airflow can get in through, get in through there. And that's, that's actually the challenging regime. You know, what crib is trying to figure out what happens when the airflow is restricted how much air gets through and what how that changes? The burning behavior.

Wojciech Wgrzyski:

Okay, let's go bigger. Let's exit the compartment. Let's take a giant Christmas tree and place it outside because for some reason, the fire engineers are very passionate about burning not only with cribs, but also Christmas trees. Have you have you burned the Christmas tree? I have not yet.

Sara McAllister:

I have not burned a Christmas tree

Wojciech Wgrzyski:

Oh my god were in a club.

Sara McAllister:

No, but it's like, I work for the Forest Service. I've studied wildland fire and if not burn your Christmas tree. It's it's almost we're both earning our living burning up things and we have not burned a Christmas tree. I hope to graduate to a Christmas tree though very soon

Wojciech Wgrzyski:

Yeah, yeah, but let's imagine a giant object such as a tree and in the green room before we we've entered, you told me that trees, the Christmas tree don't look like that in the forest. And there there is something to that. How would a single item like? What would drive the fire of like a single giant tree?

Sara McAllister:

Well, so that's a good question, right, because I, as I was talking about, you know, like a single pine needle versus a clump of pine needles is going to be, you know, a very different context, because now you have a raise of fine fuels. And so certainly when you're talking about, you know, a fire that's transitioning from a surface fire, so just burning onto the ground into a crown fire, you're, you are igniting it from the bottom up. So you do have your candlestick analogy, where you know, things are going to be bathed in flame as it goes up. And so because of the context of the the needles, you're going to get a lot of that convective heat transfer from flame contact as a tree, kind of they call torching right, and you torch a tree from the bottom up. But when you have, you know, a big spread and crown fire, you know, you have much more of a horizontal propagation, right. So that's kind of a little bit different context. And often there is a ton of radiation. And for the longest time, we've always just assumed that it was radiation that was driving these fires, because, you know, it's it's certainly igniting some of the downed logs that are down on the ground, because they're bigger sir, there's just a huge number. And it's just a huge number. And, you know, especially as people like, that's what we feel when we stand next to a fire is the heat flux, right? So we just sort of like naturally apply that to a pine needle. But, you know, we're still looking at that same process of, you know, those dangling pine needles in the air are going to be very sensitive to any kind of cooling from convective heat transfer. So you still need that flame contact. But it happens in a very interesting way. Because of the way that the flames have to come, essentially, they have to come down and forward into the fuel bed, which doesn't make any sense when you've got big buoyant plumes that on earth with gravity that want to go up, right, so. So that's always been kind of the crux of trying to figure out how convection can be important in wildland fire is how can it go forward and down to keep igniting the fuels and it turns out that there's some really cool, fluid dynamic instabilities in in flames that create turbulent structures that actually do force the flames down and forward into the feel bad. It's I wish I could like draw pictures.

Wojciech Wgrzyski:

I know it's, this makes the podcasting hard, you have to describe it with words.

Sara McAllister:

Yes. So but what's really interesting is one of the things that our group kind of really hate to say the word discovered, because it's something that is so totally obvious now that we've pointed it out. You know, if you hand a kindergartener a crayon, and you ask them to draw fire, they're going to draw these peaks and troughs, right? It's going to be a jagged edge thing, right? And but those jaggedy edge, you know, flames are actually very important to the way that the heat is transferred. Because the stuff that goes on in the troughs of those jaggedy Peak flames, it's actually where all the action is happening. And you can actually get that convective heat transfer that goes down and forward and other fuel, but so. Yeah, so it's like it's a really complex problem you're talking about.

Wojciech Wgrzyski:

So if you'd like put a row of fire sscience show logos next to each other, you will obtain this, this pattern of peaking flames and holes between them. Yes, this is what you observe. But please don't worry about observing things that are obvious. I had this time at the tunneling conference where I've told people that after 100 years of observing the fire, scientists realized that when we blow air on fires in tunnels, they grow. That's what's our discovery, and people had a good laugh from that. But it really is like that, we just start considering the effects of longitudinal ventilation on on tunnel fires and making them bigger, not just the ability to remove smoke from down. Yeah. So looking at things that are in front of your eyes and actually understanding it in a completely different way than everyone before you understood. That's like, that's the Eureka moment. And I and I wish every listener in the audience of fire science show to to have this eureka moment in their life because it's it's, it's like, it's distinct clicks in your brain and nothing is the same anymore. Again, like, I mean, you've always seen this phenomenon and you have never considered it being the mechanism that may be the driving force of this fire. And now, by looking at the video is like, Oh, it's obvious, it's

Sara McAllister:

Oh, I think it became obvious only because we were, we did a whole bunch of experiments where we got rid of every other variable ever, right? I mean, because when you see these things, they're out in the field or you know, you're doing windtunnel tests where you've got, like, you know, pine needles spread across the table. But, you know, you always kind of blow them off, because you're like, well, there's clumps of fuel. So what we're seeing must be a result of there being clumpiness in the field, no matter how smooth, we try to make it, there's always clumps. So we always, we always saw them. But you know, it wasn't really recognized that it was a fluid dynamics thing. And not just because the food, the fuels were clumpy or something, you know,

Wojciech Wgrzyski:

Okay, we'll go deep into it first. Actually, you know, what, in your PNAS, and in the paper, first out by Mark Finey, and congratulations, for authoring this magnificent paper. And you're, you're one of the authors in this paper, at the site of the PNAS, there's a bunch of videos from your experiments, and they're excellent to watch. So I'm, what I'm gonna do is I'm gonna link in the show notes, the link to the paper, and link to the videos. And I think the paper is in open access, but if you cannot access it, I will help you steal that. So tell me, I'll help you get it. And for those who don't want to go that far, and click the link in the video, the mechanism we're talking about is that the fire in in the forest, because we now enter the forest, it's not a wall of fire, like you would see in Lord of the Rings movie. However, the physics made it move like that as well, probably in the movies. But it's not just a continuous wall of fire with the same height, it's like you will have a part of it that's very like high flame, and then you will have parte where the fire is almost non existent is like down bottom. And they change because as the fire moves, one grows bigger, the other grows slower. And by this movement, they forced this, let's say push of air that pushes the flame against the surface in front of it, like if the fire was breathing, breathing flame in a way and this mechanism, despite the fact that the fire must and the hot air must move upwards because of the buoyancy is the mechanism that actually forces that this hot air this flame to go like slightly to the front half meter to the front. How, how much did you observe it moved by one?

Sara McAllister:

So that's actually research, we're still doing? So okay, yes, exciting, how far forward those flame excursions go, and what temperature they're at. So that we can kind of know, put some sideboards on how that convection heating is happening.

Wojciech Wgrzyski:

But it's, it's the mechanism that, as you mentioned, radiation may not be there. The main but radiation is certainly helpful because you're pre heating your fuel. So it's very, very ready to be ignited right. And as soon as you remove this cooling effect of the cold air surrounding the needle, or the fuel in general, and replace it with hot flame and, and and products of combustion, then suddenly, the magic happens and your needle ignites. And on the scale of the forest, this is what moves it forward. And now, okay, you've set your forest on fire. That's not very nice, but let's go with it.

Sara McAllister:

Hey, it can be a good thing,

Wojciech Wgrzyski:

it could be a good thing. That's true. That's true.

Sara McAllister:

So that's another this is probably another soapbox topic.

Wojciech Wgrzyski:

But you told me we are not allowed to talk about fire management.

Sara McAllister:

So this is ecology. This is a lot of the forest particularly in the western part of the US evolved with fire, right? So they need fire to thrive and exist and natural natural process is a natural process that it evolved with. So you know, fire isn't necessarily a bad thing in our forests. It's just needs to be the right kind of fire at the right time.

Wojciech Wgrzyski:

Yeah, I think this this subject is something I have never considered it like this until I've met people like, like you like Professor Rein, like Professor Stoof. Once I've realized what they really say, it's really like clicked again, this moment that it's obvious it's in front of your eyes, if we allow the ecosystem to follow its natural, let's say circle of life. We're not gonna have this once in 1000 year fires every month, right? Okay, so let's go back to the fire that's just that's just been ignited because now the fire is quite big. And what's fascinating for me in let's say wildfires, is that the terrain and the wind will play such a tremendous role in where the fire will go. So if you have the exact same forest exact same source of emission on, let's say, top of the hill, bottom of the hill, this will be two completely different fires. If you have it in California, where you have the wind, the Santa Ana wind from the hills to the ocean, and if you will have it on the other side of the same mountains is gonna be completely different fire, right? So what's driving the fire hem? Because it's starts to seem it's being the external conditions that start to take the driver's seat, right?

Sara McAllister:

So it's very interesting because this is one of the they call it the fire behavior triangle. Right? So this is something that is so fundamental wildland fire, you know that it's, it's the fields weather and topography that drive wildland fire. That's our, that's the other fire triangle, right? You know, so we all know that a fire will spread faster uphill, or it'll spread faster in a canyon, we know it spreads faster when the wind blows. But you know, the funny thing is, is, we still haven't been able to exactly explain why, because we've argued about those mechanisms of heat transfers for so long. You know, I mean, there's, there's some of the more obvious explanations of you know, when the wind blows, the flame tilts over. So you're preheating your fuels faster, right? So whether or not that's because of radiation, or convection has been, you know, argued about still. So same thing with the uphill, right, as if a fire fence froze faster uphill, the flames kind of start tilting over because of the the relationship, the angles we're having, right. And then at some point, if your slope is steep enough, it will actually attach. And this is something that I think was studied in depth, after say, like the King's Cross fire, because you can process of where you get this, like, all of a sudden, the flame just sucks down to the surface, and then you can get like crazy, fast fires. And again, you know, the argument of whether or not the crazy fast fire spread is because of radiation or convection still ongoing and still debated. You know, a lot of it has to do with that context, like I said, of the size of the fuel, whether or not you're talking pine needles that are suspended in the air, or you're talking about, you know, wooden steps or something, right, it's very different.

Wojciech Wgrzyski:

In terms of inclination, I'm actually gonna link PhD theses of Professor Michael Gollner, because I found, I found it really good. And the fact he did his PhD on the matchsticks adds additional beauty to that, and props to props to Mike. And And what about the wind, because the wind is a is a phenomenon is a natural phenomenon when I talk with the fellow fire engineers, and they usually tend to replace wind with some, like just flat velocity inlet of a certain velocity, I always tell them, this is as if we just replaced the whole complexity of a fire with a single temperature value, which actually we do. But both of these concepts are equally horrible. And wind is a phenomenon, and what's fascinating about wind is that it's the wind at the surface is the outcome of the interplay at the upper levels of the atmosphere. So it does not necessarily be stable, it rarely is stable. So it changes the directions. I know that real difficulty in modeling the wildfires, wildland fires, if you, if you had the constant wind and the same slope, you would probably figure out an average velocity Actually, that's probably what the Rothermells model would give you. More or less. Yeah. And, and your game, you solve the problem. But your topography is rarely, like just uphill, and your wind is never constant value that would be unifying the space. So I think this is something that brings you additional challenge to, to what you're trying to model it. I mean, do you do also research that or maybe take it into account where when researching the fundamental scalesof the combustion or

Sara McAllister:

so there are certainly some that have been working on? You know, what that profile looks like? Right? Because as you say, it's not steady. And it's also dependent on the vegetation on the ground, right? Because you that you ultimately care about the wind that's hitting the flames, right. So the, okay, you're talking about the wind, very, very close to the surface of the ground, right. So that same wind in a field of grass is going to be different than that wind when it hits, say, a tree canopy. And so how about when profile changes when it hits those obstacles and how it decreases or accelerates it goes back into the open is certainly a huge challenge and trying to figure out how to do modeling a fire spread. You know, there's, you were talking about scale issues, too, is when fires get big enough they can actually make their own wind. That's a whole nother like can of worms to have to deal with right. And that can totally have very different feedbacks on how about fire burns is when it starts making its own weather.

Wojciech Wgrzyski:

That's what I wanted to touch as the final act in the, in the journey through scales because we're entering the The Mega fires, when the wind is a product, what's happening in the upper layers of the atmosphere, but at some point the fire can enter the upper layers of atmosphere with with the buoyant plume. And when it does, it starts to be quite, let's say unfunny. I mean, it's, it's absolutely, like if you're a fire scientist, and you read about the physics of mega fires, it is fascinating, like, it is, like, absolutely fascinating. But at the same time, these are the moments where people actually die. This This are the most horrible fires, we had one of such fires in Poland in 1992, it was in the forest called Kunia Raciborska. It was a fire created by a train a train was stopping and it was producing a lot of a lot of metal sparks flying into the forest. And it ignited a quite a significant part of of a forest at the middle of a very dry season. And then you know, a lot of firefighters were went inside to fight the fire. And then unfortunately, the all the elements clicked for a mega fire, because you need to have a certain stability of the atmosphere to allow this fire to affect the upper layers of the atmosphere. They had the weather, like almost no movement is actually the best weather for such a thing had to happen because no movement can be replaced by the movement by the fire. And I have colleagues who have attended this fire and they told me it was like a fire storm like truly the the air start burning, the fire was everywhere. And you had literally seconds to hide, because you know it was coming. And in the pictures, you can see a lot of like trees being broken. And fires do not break trees, it's winds that does break trees, and the wind was created by the fire. So in a way the fire has created, let's say a local hurricane like effect in this region. And unfortunately, two of the firefighters have lost their lives in this fire. And this events happen happen. Yeah, they did these types of fires happen over over the world, especially lately, where we load this accumulation of the fuel in forests. And if the all the elements click, you get this very, very nasty, nasty physics. And you know what I've learned from about this the most there's scientific report by Pitts from NIST who's touching the post war fire research like nuclear wars. How can nuclear warhead create the fire to destroy a city

Sara McAllister:

We dont need nuclear fire to do it? We fortunately figured it out in World War Two,

Wojciech Wgrzyski:

but also in Well, it was not war fire, but it was post-earthquake fire if I'm not wrong in Japan, the great Kanto fire There was also a huge fire whirl that destroyed a whole. prefecture. So what's happening there? So eventually these the fire driving itself? So it's no longer external beings? It's it's the fire itself. Right,

Sara McAllister:

right. So you know, one of the things obviously, we've learned a lot from, like you mentioned that research into how the totally level cities unfortunately, but the same kind of things happens in wildland fire. As you mentioned, as fuels accumulate, you get a lot of stuff that burns for a very long time, our normal concept of wildland fire being just a line fire that moves through and propagates ignites fuels, they burn quickly, because they're the fine fuels, then they go out. So that that kind of concept to how we normally idealize a wildland fire falls apart. And so if you get fuels that burned for a very long time, you start getting bigger and bigger areas that are generating heat and burning simultaneously, much like what was happening, we unfortunately figured out how to do in Dresden. So you, you're able to get a long duration burning, which is, which allows for the sort of spin up of this atmospheric thing to happen, right, so you get a really wide area, you get a really wide plume of smoke above it. And so wide plumes, I mean, you're, you'll inevitably need to entrain some fresh air as a plume rises, right, exactly.

Wojciech Wgrzyski:

So the plume removes that air, turns it into smoke, let's say and removes it from the place from something must come into its place, right. And if you remove 1 million cubic meters of air, right, you need 1 million cubic meters of fresh air from somewhere and it's just going a suck it from

Sara McAllister:

from where to go and and as the plume is wider and wider, it's it's actually less than less effective in entraining the air up in the atmosphere. So it has to come in from down on the ground, as you mentioned, so it's pulling all that air that's going to replacement air to replace the stuff that would normally kind of try to get you know entrained in plume falls coming in in all the grounds now and that's that fire induced wind that you know can turn it into a total Firestorm and You know, sometimes like in Dresden, and in the that great earthquake fire in Japan, the Kanto fire, Yeah, if you get any kind of like flow blockage so that that entrained air starts coming in at an angle, you start rotating the whole bloody thing.

Wojciech Wgrzyski:

Okay?

Sara McAllister:

That's when you can get, you know, that whole, like, real true Firestorm where you get that rotation in there that makes it even worse.

Wojciech Wgrzyski:

I'm not sure if you know that research, I don't think is widely No, but there was an experiment on this in the 80s. In France, where they have used 1000 megawatt fire source as the source in their experiment. They were burning like this, let's say house sized heptane burners, over an area of one square kilometer to see if they can create an artificial whirl, like you just explained. Well, I think luckily for them, they did not create the whole world. But they have observed the beginning of this spinning motion of the mass of the air surrounding it was like 1000 megawatts when I started calculated how much I would have to spend on diesel to to create 1000 megawatt fire. I was like, okay, and we're not repeating that. And the environmental people would not be happy about this the scale of an experiment. But yeah, I'm gonna link it to the show notes. I think it's an open access. And that was this probably the biggest fire experiments anyone has ever conducted in

Sara McAllister:

around that same area, time and time. The US was experimenting a lot with it, too. So there was a called Project Flamebow, same. Oh, and they also trying to figure out this whole magnifier thing and so I don't, I can't remember numbers, we'll have to go back and look it up. But yeah, they were piling huge areas of like, Juniper bushes over a huge area in the middle of the note in the middle of nowhere and trying to get that same same thing. The guy Countryman and Project Flamebow, I think a lot of that stuff has been declassified,

Wojciech Wgrzyski:

I'm gonna, I'm gonna find it. And I'm gonna link it in the show notes if I can, and for some odd reason, I want the French to lose. And we don't know what was researching Soviet Russia that much. So maybe they were burning something even bigger. So we've... from matchstick, where you determine the flame regime by turning it up or down through compartment where the feedback from your obstacles is deciding into a fire where the odd puffing motion will be determining, up to a mega fire, where the fire is that determining itself and you've suddenly lost control it's and it's still essentially the same thing. You know, it's a turbulent transfer of air into a fuel mixture, and just burning a super tiny interface between them. That creates all this all this noise. It's kind of fascinating.

Sara McAllister:

Ultimately, the combustion still happens on millimeter scales, but now you're talking about kilometers, you know.

Wojciech Wgrzyski:

Okay, so let's go back to some something more pragmatic because I really enjoyed your research on wood cribs. And it's not the only that I am I love burning wood cribslike every fire engineer does and wood cribs are essential tool for our engineering, you were obviously researching that to understand the roles of fuel geometries in in the fires better. But for me, it's a very practical paper on how to design wood cribs better. And I actually can can see now how important the design of the source is actually, because the effects you've described in your paper were quite profound. Maybe you can bring us a little closer to like what you've done and what you found, because I found it fascinating and they wanted to hear it from you.

Sara McAllister:

Okay, so yeah, this started did not eight, nine years ago, when we were you know, sitting around the lab kind of puzzling. Okay, well, how can we describe how a particular fuel burn and a wildland fire burns? Right, whether or not we're talking about tree canopies, or the pine needles on the ground, and basically decided that like, you know, we can't study the real fuel because it's too complex, complicated, right? There's just so much variability that we can't see the forest through the trees. There, right, like, we can't figure out the processes because we just there's so much, you know, variability from one tree to the next, right,

Wojciech Wgrzyski:

you needed a more repeatable tree. So you've choppped it down to wood cribs

Sara McAllister:

drop down. And, you know, we were like, Well, hey, look, there's all this literature in the fire protection engineering, we're using wood cribs, right? Because, you know, they're used so often as ignition sources, you know, and there's this these great papers back from the 60s that kind of came up with this beautiful theory with the two different regimes When digging it, but then you start digging into the papers. And you know, they're the the cribs that they used were were very sort of, they were all cubic, they were all looked like perfect little cubes, right? So they all had perfect ratio of lengths of 10 times their thickness and the 10. And with the same amount, a layer, so they came out perfect unison. So, obviously, the first question is, well, what happens when it's not a cube? Do these relationships still hold? So. That was where we started was just playing around with what we thought was going to be a simple project of, oh, let's just throw some different things at it and see what happens. Right. And so we started building cribs have a wide, different wide different geometries, right? So things that were short and fat and tall and skinny looks like chimneys, just to see if that those typical relationships held right. And discovered along the way that sometimes is the answer. So it turns out when the fuels get thin relative to their length, so basically, if you've got thin fuels in a wide kind of area, relationships in the literature don't hold her well. Okay, but there is. So I mean, these are the ones by, by by gross by block, you know, the kind of the fundamental ones that everybody kind of Heskestad the fundamental ones that everybody kind of relies on. And they work great if as long as you can stay towards the cubic side side of cribs. So if you start building your cribs out of, say, like one millimeter diameter sticks, it gets a little messy, right, especially if you're making them 10 centimeters long or even longer, right. So it turns out that there is a little known correlation by Thomas that actually works really well for all of them. But it doesn't clearly delineate those cool tube regimes of you know, densely packed fuel beds versus loosely packed fuel bed. So it's a little more uncomfortable to use, because you can't, you know, calculate a crib porosity, and say, it's kind of like this, okay, but it works well. But along the way, we also noticed that the burning behavior is very sensitive to how far off the ground it is. Right. So there had been only a little bit of work in blocks, PhD thesis that looked at that the distance between the crib and the ground and how that affects things. Turns out, it's a lot bigger difference, particularly for those fuel beds that are, you know, say, shortened squat or made of thinner fuel elements. So that was a whole nother line of work that we did, where we were looking at the effect of the distance of, you know, the height off the ground and the feel of it, and found that, you do have to lift it up quite a bit higher than we thought, in order for that to not be a variable to not restrict the airflow through the fuel bed. Because it turns out that the airflow coming from underneath is a huge, huge driving factor in how, how a crib burns. Oh, so from there, I mean, we have tried to answer all sorts of questions like what happens if you have like a wind or forced ventilation? We've looked at moisture content, we've even tried to answer some of those questions about those, you know, those huge mega fires, right, and the the restriction of the plume entrainment above it, how that influences how that has a feed back into the burning underneath. So yeah, I've, in some ways, sadly spent, like eight or nine years, hurting cribs, with probably 1000s of cribs at this point. Which is why I say I hope to graduate to trees. But I learned a lot about that, you know how the flow through the fuel that affects how it burns and how important particularly the flow from underneath can be. So here that's really important information for trying to predict sick crownfire behavior and how crownfre will burn versus the surface fire will burn where you don't have, you know, airflow that can come underneath

Wojciech Wgrzyski:

what's what I've found fascinating about this work was that you found that when the fuel was thinner than actually more when there was decreased flame spread, and with thicker fuel it increased. So it's not like I would expect it always increases the so this spread velocity

Sara McAllister:

that was a really some really interesting and somewhat frustrating paper and experiment set of experiments, right? Because we wanted to know the effect of wind and how, you know, change things, right? Because it makes sense that, particularly if you've got a densely packed field, but that's ventilation limited. It should make sense that if you if you can get more air into it, it'll burn better because it's you know, better there's a better mixture

Wojciech Wgrzyski:

propagate the flaming insdie, right, right, in a way.

Sara McAllister:

So I mean, the obvious first choice would be to put them in one of our wind tunnels and burn them right but It turns out that some of the results were kind of dependent on the actual experiment setup and the way that the wind would either go through it or around it, depending on the density of the fuel. But so we weren't in effect, able to control the amount of flow through the fuel bed. Because we had this, you know, external effect of it being, you know, a crib in a wind tunnel, and the flow knows where it goes, you know, that was the kind of the, you know, the, the key finding there is that when the fuels were thin, it created enough flow resistance so that the wind went around the fuel bed and stuff through it. And so that's, you know, kind of the explanation of you know, why we saw kind of unexpected results.

Wojciech Wgrzyski:

But this is, this is fascinating, because you found that the role of the aerodynamics of your fuel source and I know, mean the internal aerodynamics, but just let's say the bluff body aerodynamics, is key to the way how, generally air can be transported inside the fuel where it can be burned. And you will see the same phenomenon in wildfire, because it's also here, where you're, while you have very flat, and let's say, I aeropodynamic, the uniform source. But regardless, it's the aerodynamics, we'll we'll get their mind how deep it goes. But outside of the forests, in your paper, you've mentioned the tunnels that you've observed similar effects in like experiments done by tunnel fire scientists. And the one I think about it, it's also like, wow, that really was like, in a way like that. And it's, it's very frustrating to know what you're found to look at these experiments. Because here, you've exposed a source of uncertainty that can change, let's say, one or two significant digits in the experiment, like you can turn a 20 megawatt fire into 100 megawatt fire if you if you do the aerodynamics, well, and that is not necessarily even the factor of the wind velocity. In your tunnel, it can be just this actually alignment of the, of the way how air can go around the object and not

Sara McAllister:

Yeah, similarly, how far up off the ground, you prop it up, if you allow the air to come in from underneath, you're gonna have a different burning behavior than if you block it off.

Wojciech Wgrzyski:

Oh, you must be really good at setting up barbecues. This kind of knowledge is like, priceless at the grill party.

Sara McAllister:

But it's also frustrating, because then it's like, you know, we have all of these standardized tests that rely on you know, cribs, as you know, your your heat input to the whole compartment or, you know, whether or not you're just in the flammability of decks or something, it's but, you know, we have to be very careful that how to build the crib as is intended and burn it as an intended because if you miss something critical, like oh, you know, putting it straight on the deck or lifting up off the deck, you can get different answers. And then your input is not the same as you would expect

Wojciech Wgrzyski:

when when reading this. This paper, I was thinking about where in fire engineering week, you'd actually use this, like crib research. And one thing that came to my mind was informal settlements. Because this densely packed arrays of small buildings are similar to your density are loosely packed with wood cribs. Because these buildings are fairly easy to ignite, they produce large amounts of heat, very quickly, there probably is, again, this similar to crownfire, this buoyant, push, convective push of, of heat to the next building. And I was wondering if this if this, what you observed that thewood cribs would actually translate to the larger scale?

Sara McAllister:

Well, and you know, that's an interesting application that I hadn't thought of, right, because I live in wildland fire land. So I was only thinking about applying it to wildland fires, but, um, you know, I mean, that's the real hope is that what we learn about, you know, the processes can be applicable at, you know, different scales, right, and understanding how how the ventilation plays a role, how the, you know, how that has a feedback with the way it burns. And, you know, that is very application that, you know, I think that might be a worthy research project right there

Wojciech Wgrzyski:

to see. Yeah, and it's, and it's definitely super useful to anyone who's building a wood crib.

Sara McAllister:

You need to justify that in the paper somehow, right? Well, I'm still burning wood cribs.

Wojciech Wgrzyski:

Oh, right. Oh, yeah. Don't Don't, don't don't get me started on justifying things in the papers. Actually, in your paper, I found this excellent quote, and I'm gonna read it up and it was for your experimental discussion, and I absolutely love it and all the post docs and students listening to the podcast, like, take something to write and write it down. "Though no particular experiment design scheme was followed the fuel but parameters were varied in an exploratory manner probing for unexpected or nonlinear behaviors". This is good I'm going to break it. It's like it's the most scientific way for writing. We guessed the sizes because we wanted to see the most varied set that we could. That's, that's that is. So that's actually priceless. That's my take from the prep to this podcast. Okay, last thing I wanted to talk with you. And it's also connected to, to all the things that we've discussed. You're very involved in, in the IAFSS, and namely, in the group called large outdoor fires and built environment. This, I see this initiative as very valuable to the world of fire science. And I would like you to introduce it to everyone listening to the podcast, maybe someone would like to contribute or participate in it, because I think it's worth it.

Sara McAllister:

Yeah, I completely agree. So, so the history of this went back to the iafss symposium in Lund. We had, we were invited. So Samuel Manzelo and Sayaka Suzuki, and I are the co-leaders of this working group. And Sam was invited to lead a workshop prior to the symposium on the large outdoor fire topic, because it's become, obviously is becoming a more and more pressing issue, right? It's affecting people worldwide. And so we had this great workshop with a lot of interest from people and then iafss invited us to make it into this permanent marking group. So the idea is that we can bring the entire fire protection engineering community together to kind of work on problems of of large outdoor fires. And when we say large outdoor fires, we're talking about wildland fires, we're talking about wildland urban interface fires, urban fires, and informal settlement fires. Because the reality is, as at this point, any single fire can be all four of those, right? It can start in the wildland spread to a community, whether that is a traditional built community or an informal settlement. And then those structures can then ignite other structures and become an urban fire.

Wojciech Wgrzyski:

Yeah, and includes like management of communities, and also the management of emergency procedures. Right, right, firefighters.

Sara McAllister:

So I mean, obviously, we're tasked with this huge group are a huge topic, right? I mean, that's large after forest and built environment. So we kind of had to identify, you know, sort of some priority topics. And so we've broken the working group into three subgroups. So we have one on ignition resistant communities, that's looking at some of the the thought and the standards and the exposures that structure will experience, they're also looking at, and looking at mitigation strategies for preventing that. And that, you know, can range from, you know, ways of building and constructing and for informal settlement fires, but it can also include doing things like fuel treatments, in the wildland fire, right? This is where my passion comes in is, you know, you can harm your structure all you want. But if you've got a super intense crownfire coming at it, it's going to be much more difficult to harden that structure than as the fire was a much more tame, you know, surface fire, right. So if you can reduce the exposure, then you can have a much higher survivability chance. So so that's one of our subgroups. Another subgroup is the emergency management and evacuation subgroup. So that's looking at, you know, how do we evacuate people? How do we manage the the warnings and getting people out? And, you know, looking at case studies of where we are, where we haven't, and some of the lessons that have been learned?

Wojciech Wgrzyski:

This will be a podcast episode. Im sure. This is such a Yeah, such an interesting topic, how to actually manage community wise. So you remove people in the perfect time and you convince them they need to leave, even though the fire is not at their footsteps, right? Because when it is, it's too late. Right? It's difficult.

Sara McAllister:

Yes. I mean, this is a matter of, you know, warning people ahead of time. It's also, you know, looking at strategies for the routes to take, right? I mean, often there's more than one way of getting out, which is the most effective, which, you know, and how do you do phase evacuations? Or do you, you know, put a blanket call of everybody out? There's a lot of lessons learned to that, you know, we're doing case studies of for all sorts of fires, but you know, the, what we've learned from evacuating before hurricanes and stuff like that, is, you know, sources of information. There's like there's a lot of great people that are doing a lot of work on this. So I look forward to an episode on that.

Wojciech Wgrzyski:

Yeah, I have a guest in mind, I will make it happen.

Sara McAllister:

Awesome. So the the third working or third subgroup that we have is the large outdoor firefighting. So this subgroup is looking at sort of kind of doing a survey some of the tactics that are used worldwide and other things and some of the impacts of those tactics, right. So sometimes there's in ecological impacts of say, spraying everything with fire retardant. And there's health impacts as the way we fight fires. But there's also, you know, one of the things that they're working on right now is a survey of UAVs, unmanned aerial vehicles, and how they're being used in firefighting at all, you know, scales at the fire. So, you know, looking at some of those tactics and seeing if there's any kind of way to sort of help to unify the way things are done.

Wojciech Wgrzyski:

And the future looks bright in this field with forecasting. And I had Xinayn Huang in my podcast, he was talking about the use of AI. And it's also seems, it could be promising for for this. And, yeah, I definitely see these efforts of the group as very needed. Because the tech, the subject we're tackling is bigger than any of us, like, it's bigger than your agency is bigger than my Institute. It's not something that a single person can solve, like, like Hottel said, it's next to life processes, the most complex to be understood. And I think through collaborations like that, through workshops, like held on IAFSS, it has certainly moved us a little bit closer into solving the issue, and at least gave us the tools, I am very lucky to be in both the iafss Research Subcommittee and the Workshop Committee for the next ifss. So I'm, I'm your supporter, and I would love LOF&BE to be a part of this, of this process, because it's I think it's essential and the way how it's helped, like really getting people together, to think about the subject that separate into, like subtasks. But in, let's say, democratical way, not prestige seeking way, not, you know, not necessarily competing about who will be the first to figure out how retardant affects the environment, but working in a way together on pieces of the puzzle to figure out the whole image in the end. That is, that is a fantastic way to collaborate. And I think this is this to be emphasized, because we absolutely need more of that in fire science at every part of the fire science.

Sara McAllister:

Yeah. And the more the merrier. So it is, it is free and easy to join. If you go to the iafss website, we do have a website there. With joining instructions. It's simple little Google Form used to get added to the mailing list, and then you'll hear about all of the work that we're doing. We also have our little monthly seminar series, so you can hear about that.

Wojciech Wgrzyski:

That's it. Yeah, I'll share the resources in the show notes. Because it's definitely worth to follow. And and see for yourself, and maybe you can contribute in a way. So Sara thank you very much for coming here. It was a huge pleasure to talk with you about burning things for living. It's, it has certainly been a lot of fun. And I hope the audience enjoyed that as well. Maybe we have armed someone to be a better incinerator.

Sara McAllister:

thanks again, for inviting me. I also really had a good time. And it's always fun to talk about burning things like

Wojciech Wgrzyski:

he said, definitely, definitely. We're gonna do that. Again, when you solve the mystery of the length of billions push on the wildfires, then then save some time for for part two. Thank you very much. Yeah, thanks. Cheers. Cheers. Wow. Well, the journey that was the world of fire is really diverse and fascinating. For me personally, there are some practical takeaways from this talk. First, I really have to watch out when I'm building my wood cribs. And check this porosities and ratios of scales of that of the woods crib. Because, yeah, it's a factor in the design. And maybe it should be accounted for when we do compartment fire experiments with with wood cribs our fuel, that's definitely something to look more into and consider in the future experiments. And also, as Sara mentioned, it's quite important when you have standardized test methods that work with wood cribs that you have to make sure that crib is built to the same specification every time you build it, because some changes in the structure of the would crib that will change how the air can enter and move through the width crib penetrated will change the fire behavior of it and essentially change the fire. So that definitely is something to be considered when performing fire tests with wood cribs. And from the other things that were discussed. I think the one that fascinated me the most was the convective cooling of pine needles, where this convective push of flame was necessary to create the conditions for the ignition of this porous fuel. And this is like truly unique because I'm so used to define critical heat fluxes at which things ignite or even temperatures at which item will ignite in my simulations that I wouldn't think it's going to work like that. But even at a very high rate in heat flux, you still may need this physical contact or, or this hot air surrounding the item to to ignite it, especially when we're talking about such a small item. So that was something definitely new to me and, and definitely very interesting. So I hope you like this twist in the podcast to do to move into wildfires a bit. Because it's pretty imp rtant subject and even if you re not researching or wor ing with wildfires, probably jus as I am, in my family, you re probably the the family fir person. And you're probably ask d a lot of these questions abo t the ongoing events in the wor d, the huge fires in the Med terranean in USA, on Sib ria, and to know a little bit better, you will be able to giv better, more scientifically acc rate answers and just spread thi good practices of wildfire man gement and dealing with fir s because it seems we have to earn how to live with fires fro now on. So thank you a lot for listening. And yeah, see you nex Wednesday. This was the fire science show. Thank you for listening and see you soon.