088 - Modeling fires of natural fuels with Eric Mueller

Wojciech Węgrzyński: 0:00

Hello, everybody. Welcome to the Fire Science Show. As this episode is published, it's most likely eighth February in the morning. Got leaks in here, which means that's the last day for IAFSS paper submission. IAFSS says is like Christmas for fire science. And I am absolutely thrilled to see you all in Japan and see. What newest achievements of fire science there are because that's, that's the fuel for my podcast. And I hope your papers, went great.. As I assume mine is still in production. It ends up like this, every time I own every year, it's going to be a little better. I bring IAFSS up, not just because it's a coincidentally the deadline for paper submissions, , But I think quite a nice thing happened last IFSS the one that was supposed to be at the Waterloo, but ended up in online space due to pandemic. Um, maybe you recall if you've attended that. Uh, there was this best theses awards and there was this guy from Edinburgh. Uh, he gave like BBC style documentary about his research and everyone expected a PowerPoint. And that was absolutely outstanding scientific communication. And on top of that, he was presenting some really, really good science on. Burning living material plants, vegetation. In dead form But the material that comes from living plants. And, uh, I enjoyed that talk a lot now. Fast forward three years later. I really need this knowledge because we working on green wall systems and suddenly we realized that burning plants is not exactly the same thing as burning timber or burning plastics. We need to upgrade our knowledge and you know what? I have a podcast and. I am privileged to get, be the best people to learn from. So I did reach out to him. His name is Dr Eric Mueller. He's working at NIST now. And he's still researching. Vegetation fires and living fuels and tries to update the ways, how we model them. So I took all my tough questions. I've asked them to him. And here's the recording of that. So, lets spin the intro and jump into the episode. Before we start the episode, I would like to thank you once again. OFR Consultants for sponsoring this podcast, It has been already a month and I'm really enjoying this collaboration. It has brought a lot of good to the podcast. OFR Consultants are a multi award winning independent consultancy dedicated to addressing fire safety challenges. OFR is the UK is leading fire risk consultancy. It's globally established team has developed a reputation for permanent fire engineering expertise with colleagues working across the world to help protect people, property and planets. In the UK, that includes the redevelopment of the Printworks building in Canada water. One of the tallest residential buildings in Birmingham, as well as historic structures, like the National Gallery, National History Museum. And the National Portrait Gallery in London. Internationally it's work ranges from Antarctic to Atacama desert in Chile, and the number of projects across Africa in 2023 of our will grow with seam. And it's keen to hear from industry professionals who want to collaborate on the fire safety futures this year. Get in touch at OFRconsultants.com. And now back to the episode with Eric Mueller have fun.

Wojciech Wegrzynski: 3:42

Hello everybody. Welcome to the Fire Science Show. I'm today with Dr. Eric Mueller from NIST. Hey Eric, great to have you here.

Eric Mueller: 3:48

Hey, thanks for having me.

Wojciech Wegrzynski: 3:50

And we have a lot to talk about some stuff to differentiates, build environment and wildfire science in particular, I would love to talk about modeling, living fuels or just modeling vegetation in this podcast episode. First, I, I would love to, to hear your professional story, how, how you ended into wildfires and, are you doing now at nist?

Eric Mueller: 4:12

I mean, I guess the, the story to go back is, uh, it starts in my undergraduate degree, which was, engineering physics, basically applied physics. I think it's a great degree to get some fundamentals, but probably more suited to graduate school than to going out and finding a job. You missed some of that practical education you might get in, you know, traditional mechanical engineering or something like that. Uh, and so I was looking at graduate programs. I was based in Boston trying to do material science and was looking at the website of WPI and their material science webpage was broken. And so I started clicking around and I discovered fire protection engineering that way. And I thought, oh, this sounds interesting. And then it just so happens to coincide with, professor Albert Simeoni when he was starting, his first round of being a professor there. at W P I. And so I went to meet with him to, you know, see about being a graduate student and he of course is an expert in wildfires. And that started kind of my interest, particularly in that space. And I've always had a love of, you know, the natural environment and being outdoors and everything. So the fact that I've been able to go do these big field experiments and be out in the woods and everything, I love that. So it kind of took off from there. And yeah, I ended up in Edinburgh following Albert Simi over to Edinburg to do my PhD. , stay there for postdoc and now have come back to the us to work at NS to kind of carry on this, uh, wildfire research and particularly around the modeling.

Wojciech Wegrzynski: 5:31

So, so it's, uh, applied physics turned into wildfire. I, I guess that's, that's, that, that makes it even better. That makes it even better. so Eric, You have studied a lot of, a lot of fuels. I I remember your paper from IAFSS. Uh, we'll remember the, the brilliant presentation of that, uh, that will never be forgotten. You've said the standard too high for the rest of us to compete. Uh, thank you for that. Uh, but it, it was really great if. If you haven't seen that link in the show notes, so it is definitely worth, having the visual component to the great audio. Uh, here we, we just have the audio. I would love to. Learn about, modeling vegetation. So let's, uh, first try to, to make an introduction how modeling, uh, pine needles or, or a tree differentiates from modeling just, I don't know, solid piece of timber, which is also an organic material. And I would assume many processes are similar in, in each other. So, so what, what's the big deal? What's the differe?

Eric Mueller: 6:34

Yeah, sure. I mean, I think that's a good context to start in because we might think of, uh, yeah, whether it's pine needles or even branches on a forest floor or something like that. Some dead material that's wood. Effectively from that, more material or, or fundamental chemical sense? Probably very physics going on in terms of looking at the, the ignition around a, you know, decomposition, the thermal decomposition, uh, how it's paralyzing and what sort of, gaseous species it's creating and, and how that's burning. Of course, that's, you know, I'm simplifying, there's some nuance there, but we can say, you know, if we look at timber in the built environment and. Timber on the forest floor, not terribly different. So where are the big differences coming from? And uh, I think it really comes down to, uh, a, the environmental conditions that these materials are gonna be exposed to. So, of course, uh, moisture is a huge thing, uh, the potential moisture content of, of these different fuels. And again, if we get to dead fuels, the range that they can have, uh, is dictated sort of by this fiber saturation point. Similar to what you would find with timber, uh, in the built environment. So you can get up to, you know, maybe 30, 40% moisture content, it's going to vary a lot more seasonally or as weather patterns, blow through a certain area, let's say, than what you would find in the built environment. So you really have to consider this whole spectrum of, you know, very, very low moisture content all the way up to 30, 40%. Uh, and then the other component of the environment, of course, is the wind that you. Typically have to deal with inside of, uh, of the built environment. but how that's going to affect, uh, certain processes that will, let's say, uh, alter the ignition behavior. So, whether that's driving convective, heating or cooling, or the mixture of gases or providing oxygen for, you know, oxidative processes like smoldering. So all of these things come into play when we think about the wind. And then beyond the environment, we also have the structure, of the fuel. if you think about just one log, maybe that's not so different from a slab of timber, but as we start to get finer fuels in, in these sort of fuel matrices or fuel layers, like pine needles, as you mentioned on the forest floor, uh, now we really have a problem of porous media and so. uh, we start to have to think about all of the, the complexities that come into understanding porous media, where we have this mixture of air and solid and, you know, the proportions matter, but also, uh, the sizes of, say, your elements. You could have two materials with the same overall porosity, but made of very thin pine needles or very thick sticks, like in a wood crip or something. And there's going to be, you know, different behaviors there because we have maybe thermally thick or thermally thin, we have. different, potential for convective heating or cooling, different reradiation. So there's a lot of complexity that comes into the mix. And we have this porosity introduced.

Wojciech Wegrzynski: 9:21

In, in your talk, you, you had this, example where you've walked with a cell or, or fin element and put it on a tree to illustrate what does it mean that there's a, there's a. Fuel inside. Uh, I'm actually gonna make it into the cover of the episode so everyone knows what we are talking about, . So if you went to this episode and seen the cover, you know exactly what I'm talking about and it, so that's a very, very good illustration on the issue with modeling that with competition free dynamic tools, with with volume of fin development methods because. a, in a finished volume. The, the assumption is that it's uniform in the, it is whole volume. It's, it's just one block of matter that exists and is holy at the same state. Whereas, uh, with this tiny needles, you, you have a set of really tiny elements. It itch with its own physics insight. That's control volume. That essentially burned the, the, the gradients inside are enormous, but for the reasons of efficiency, computational efficiency, most likely we are unable to simulate them at this tiniest, tiniest scales. So we have to assume something for, for the whole control volume and that that's, The really, really challenging part, how does one, uh, simulate stuff that's so tiny within quite large quantum volumes in a way that it makes sense?

Eric Mueller: 10:50

Y. Yeah. I mean, I think as you say, there's, there's two ways we can make these models more sophisticated, right? Because right now we have this very crude representation. We have a course model, and so theoretically we could go the direction of trying to resolve in a lot of detail. Uh, the actual structure. And, you know, maybe if you're looking at just a, a tiny piece of a bit of pine litter, you could actually simulate the flow through that whole matrix, but that's not really currently at least sustainable, uh, or achievable with the, you know, computational resources we have. So the other approach is with these assumptions, these approximations submodels, that allow us to describe the exchanges between phases. And I think we have a, a good starting. . But you know, if we wanna more models, more sophisticated, as I say, and go that approach, uh, we might need to refine these submodels and figure out how we can describe the fact that it isn't uniformly distributed, even though we're not fully representing what it looks like.

Wojciech Wegrzynski: 11:49

and when we are learning that when you are developing these, these models, With your experiments, you go all the way down to the scale of a single needle. You, you take a a three branch. Uh, how, how do you approach that? I wonder like, how, how deep can you go? Because in my eyes I see, like if I want to model a needle, I could make a CFD model of a needle. But then a question is, should I model the boundary layer on the needle and go, you know, from one millimeter. Fin elements to, uh, let's say one 40th of a millimeter element to really capture the, whole structure of the flow around the single needle. So, so there really, you can really go far into, into making this, this very complicated. I wonder how far have you went? How, How far is sufficient?

Eric Mueller: 12:35

How far sufficient? That's a good question. I could tell you more easily how far I've gone, which I think is maybe bit too far. But, I mean the most that we, we did in detail when I was in Edinburgh is, uh, using X-ray ct micro CT to scan some samples that were of this kind of pine litter on the order of. let's say 10 centimeters in diameter of a cylindrical sample, you know, sort of a, a cone sized sample. And, uh, then you get this really, you know, micrometer resolution, uh, nice 3D image of all of your pine needles in space. And so you could mesh that up theoretically, I guess, and try to model it. I tried a little bit with, uh, with Latice, Boltzmann, uh, simulations, but it's well beyond anything I really understand. So , I kind of gave that up. I think that's an interesting scale to get to. Uh, if you do that kind of refined modeling to try to confirm some of the coarser approximations that we would make of, you know, what a bulk convective coefficient of this whole matrix. You know, we're not even talking about convection around a single cylinder. Uh, it's really. This whole matrix, effectively. And so, you know, you might be able to simulate something on that scale and try to confirm what you've shown experimentally, but getting into all of the combustion processes, trying to get the degradation of the fuel and the burning and everything at that scale, I think is a ways away.

Wojciech Wegrzynski: 13:57

you, you've mentioned convection. And, in, in my giant ignorance of the subject of, of living fuels, I have been recently exposed to, to those, uh, types of problems while trying to, do some experiments with, uh, with greenwall facade systems. And now we are, uh, oh boy, we're learning very quickly this difficult world of fire science and. I've, I've learned the importance of convection in these type of, of of fuels. I've learned that it's very difficult to ignite such fuels, uh, with, with just radiation because of their characteristics. And I've learned that. The convection in them is, is much more complicated in buildings. We, I guess as a community chose to like, pretend it doesn't matter and we just assume one hit transfer coefficient that goes from Euro code and it's so convenient. No one's trying to touch that. Uh, however, at this, at this, uh, scale of fuels, it seems to be fundamentally important, uh, to understand the convective heat transfer. So may, maybe you can enlighten me about the, the mode of heat transfer. Again, if you, if you could compare that to how a piece of wood would be ignited by radiation and convection. What role this, these modes played? How will be splendid?

Eric Mueller: 15:14

Yeah, I mean, sure if. , down to just flame spread again in something like, uh, a layer of pine needles, uh, versus on a slab of wood. And as you say, it comes down to the scales we're dealing with and how that differs, uh, in those two cases. And you have these very fine particles. And as you start to reduce the sort of characteristic, Diameter of, of your pine needle or whatever it is, then you're getting into a regime where it can cool very effectively. You have, uh, potentially a very high convective heat transfer coefficient for these elements. And so, um, whether that's even just due to, natural convection or you have some entrainment, uh, due to the buoyancy of the fire itself, or, you know, in a wildfire you might have a considerable amount of wind anyways. And so, it's not, certainly not that there isn't radiation and that that isn't important. and it depends on, again, what the problem you're looking at is. Um, if that's flame spread in, in fine fuels versus, uh, the potential for ignition of the facade of a structure, let's say. Or, you know, we had some Prescribed fire experiments that we had done, uh, out in New Jersey in the us And we have a great video of, uh, auto ignition, of a tree trunk, uh, due to the radiation from this big flaming front. Um, so we see these processes, uh, we see radiation playing a big role in certain contexts, but when it comes down to the ability of these fine fuels to ignite, uh, they can just cool very, efficiently by convection when they're just being heated by radiation. But as soon as they become bathed in a flame or some hot gases from the plume, , then you've lost all of that, uh, convective cooling and potentially you have significant convective heating. And so even if it's just very intermittent or quick pulses, uh, you know, there's been a lot of, uh, great work from, uh, the US Forest Service out in Missoula looking at these, uh, sort of very high frequency, convective heating processes and how they really are important to driving, uh, flame spread in a lot of condit.

Wojciech Wegrzynski: 17:04

Uh, I, I find that fascinating because, because that's like the concept that, uh, it can so effectively cool by convection while being so vigorously heated by radiation, it, it's immense to the listeners, we're, we're talking about heat fluxes of let's say 30 kilowatt per square meter that still do not ignite the branch because it, it has capability to, to cool down. That heat that that receives is, it's quite astonishing because I'm coming from a world where at 25 kilowatts per square meter, you have a flashover in your room and the game has ended. Right? So, so, so that's true. But as soon as you start to, to, to compressing that, if you have a, a bat, that, that, that is not a. Powerous medium that it loses the, the capability of, of convecting, cooling. And then, then you have the, and is, is it the same for like plants with leaves or is it, is it something very specific to to

Eric Mueller: 18:01

I mean, I, I think there's been some good demonstrations, uh, again from Missoula just looking at, you know, clumps of excelsior, of shreds of wood, basically, and just compacting them in front of a radiant panel. And there is a point at which you can compress them enough that you've, uh, reduced that efficiency of cooling. And so, uh, you know, it comes back to this whole idea. Vegetative fuels and what are the structural, components that matter, the way that we describe the structure. And so it's the size of individual elements, but it's also the way that they come together potentially in different clumps and clusters. And so if you had a very dense, uh, vegetative canopy, then maybe that's not cooling, uh, as effectively.

Wojciech Wegrzynski: 18:38

Okay. That's very interesting. But now, as a modeler and, and how the hell do you do that in, in, in a control volume that's like 20 by 20 centimeters. How, how do you adjust for that?

Eric Mueller: 18:47

Yeah. So that's the part where I think we, we needed to get more sophisticated with the way that we do things because right now, uh, you know, we conserve mass is, is our starting point. That's what we hope to do. So if you take the same control volume and increase its size and you still had the same, uh, cluster of vegetation in it, you are effectively changing its porosity. Uh, and so that's going to change the way that it, you know, interacts with the flow through drag and convection, the way that it attenuates radiation. . The hope or the way that it's working more or less right now is that we're not looking just one thing that's, uh, subgrid size. So that if you have a tree crown and you represent that with, thousands of grid cells, or you reduce that a little bit, then overall you're effectively still, conserving the same kind of, uh, properties and the way that it interacts with the flow around it. But yeah, as you start to push the bounds of that and you're resolving, trying to resolve, let's say a, an entire tree with one grid cell, then you're really violating. Some of these assumptions are really oversimplifying and so there might be ways that we need to look at, modifying the submodels. You know, there's ways when we look at, flame temperature, gas temperature is a classic one, you know, cfd right? Whereas you, uh, expand the grid sale size, you'll tend to under predict the temperatures because you're under resolving things like the flame, the actual flame sheet and things like that. Uh, and you know, there's ways to short sort of model probability, distributions of temperature to help improve your submodels. And so there's things that we can look at, uh, with vegetation in the same.

Wojciech Wegrzynski: 20:14

So basically what we need is a, is a bunch of submodels to resolve the drag ignition, the heat production within the control volume, and hopefully not violate the, the, the mass equation and, and accommodate for the, the fact that it's burning away.

Eric Mueller: 20:28

Yeah.

Wojciech Wegrzynski: 20:29

that,

Eric Mueller: 20:30

exactly. And, and we, that's how you know what we have now and the approach we take now. It's just how can we make that maybe less grid sensitive or less resolution sensitive.

Wojciech Wegrzynski: 20:38

well you were working at nso. I guess you are familiar with this computer code fds and, I've seen in literature. I think it's even in the, examples within fds, there's this bo by case, I think it's using some sort of lagrangian particles to simulate, the vegetation or, or fuels where I guess, uh, each branch of like pieces of vegetation are split into, into smaller lagrangian particles, which are then spread around within, uh, control volumes. Um, any comments on, on that way of modeling?

Eric Mueller: 21:09

Yeah. I mean that is, Most of, or at least half of my day job, I guess, is working on, uh, fds and, uh, trying to improve the vegetation sub modeling there. So, the Lagrangian particle approach, I think it, it's helpful to consider it as like a numerical construct in a way. It's, for the most part, in terms of fds, it's just a handy way for us to add some, exchange terms, some source and sync. between the gas and solid phase within a grid cell. So there's no real point to have more than one particle within a grid cell because, you're just recalculating the same exchange, but you don't have any more information between two needles within one grid cell, right? Because it's the same average temperature, it's the same average flow field. So you know, the same integrated radiation intensity. All of these things are the same. You. calculating it twice, one for each needle, whereas you can calculate it once and just weight it by the number of needles that you have within a cell. So I think it's, yeah, a little bit confusing sometimes for people to look at this and it looks like, oh, it's a tree, and all of these particles are needles and everything. And in a way they are, they're representing them, but they're really just placeholders for more of a Eulerian approach where it's just the source and sync, uh, within particular grid cells, you know, representing that cell.

Wojciech Wegrzynski: 22:26

That's very interesting. I, I didn't, know that. so, so it's more like a clever representation of a fuel that's easier for the user to maybe defined. But in the end, as soon as this Lagrangiajn particle within the control volume is transferred into the fuel, that's, that goes into reaction. It essentially means it doesn't matter if it was 1 35 allian particles, it's, it's now just another burner that's emitting fuel into your control volume. And at the scale of that control volume, you are burning it, right?

Eric Mueller: 22:57

Yep. Yeah, and everything's just waited to, again, conserve that mass that you have initially a vegetation.

Wojciech Wegrzynski: 23:02

And, and the same for the ignition conditions of that Lagrangian particle. Like they're all solved at the level of that control volume. So if I have seven in my control volume, they should technically. Ignite at the same exact time and act like one.

Eric Mueller: 23:17

right? Yeah. There's no, there shouldn't be any temperature difference between these different, particles.

Wojciech Wegrzynski: 23:22

And.

Eric Mueller: 23:23

we could have them, you know, uh, Waited, let's say, based on some randomized position within the grid cell to get a temperature, that's, you know, if one pine needle is closer to one side of the grid cell and you have a higher temperature in the next cell, then you can wait the,

Track 1: 23:36

temperature

Eric Mueller: 23:37

of the particle or what it's exposed to. But for now, the approach is really just to, know, have them in the center of the sale effectively.

Wojciech Wegrzynski: 23:44

as you now expose yourself that you're working on this, I'm happy to ask you a hundred more questions. and is the flow field in any way, uh, affected by the existence of vegetation in this control? Volumes in terms of like the produce drag,

Eric Mueller: 24:00

So it's, uh, just a bulk, drag force term, uh, in the same way that you would have the convective term, uh, you know, applied to the entire gas volume that you have there. It's just. based on the density of vegetation you have and the geometry of the particles that you assume. . Then there's empirical correlations, and that's again, some space where we have a lot of work to do, I think, to try to improve the correlations that we rely upon now, or at least confirm their validity across. I mean, the wide range of plant types and geometry that you see out in nature. A bed of pine needles is one thing. And you know, if you look at the, the papers I've had, I've spent a lot of time looking at a bed of pine needles because it is easier to describe in a lot of ways than a lot of other vegetation. But even. We hope to apply this approach, know, to more complex plant types, shrubs, trees, and things that you would find in the wildland urban interface. Then we have to understand, uh, the validity of these submodels.

Wojciech Wegrzynski: 24:54

And to, to what extent it is important to include stuff like drag, in modeling the vegetation fuels, if I omitted it completely, am I introducing a huge error to my calculation, uh, of, of what importance is that factor?

Eric Mueller: 25:08

Yeah, I mean, that's a, a good question. A, a difficult one, but one we're trying to get a better handle on. Uh, actually we're putting together kind of a medium scale wind tunnel here at NIST and, know, do some more studies around plant drag, and especially when objects are burning and everything. But I, it certainly depends upon the type of vegetation and the density that you're looking at. So, uh, if you. A very low density forest canopy, let's say. Then maybe you're not introducing as much air, but if you look at like a, a dense conifer, you know, think of a Christmas tree or something like that, and you know, you impose some flow on it, you'll actually have very little flow penetrating all the way, through the tree and. the kinds of convection that you have on the, on the leeward side of the tree, for example, can be significantly affected by that drag. And then if you were to have another tree downstream of that, suddenly it's no longer exposed to this kind of ambient wind that you have. So yeah, I think you can introduce quite a bit of error if you just were to ignore the drag entirely.

Wojciech Wegrzynski: 26:04

a and once you advance from modeling a single plant to a whole forest canopy, I guess it becomes fundamental from the reasons you've just mentioned that y you then will have like the first layer that, is, burning, that's exposed to wind and the rest of the forest shielded from the wind,

Eric Mueller: 26:22

Yeah. I mean, these things, you should be able to kind of stack up the effects and you can see the attenuation of the flow into the, into the canopy. , it depends on the scale you're looking at, but you can get away with perhaps less, uh, description or detail in the description you have of the actual canopies. So you look at like, there's been a lot of work done around just flow profiles in forest canopies, as you can imagine for, you know, seed dispersion or, carbon exchanges and in forests and things like that, like this. And basically they just model the forest canopy as a. like a homogenous layer, and it will vary vertically to represent the canopy and maybe you have a sub canopy space, but you're not looking at treaty to tree gaps and things of that size because eventually deep within a forest, things typically are homogenous. Homogenous enough that you can get away with that representation,

Wojciech Wegrzynski: 27:11

Ma man, we went far away from boundary layer on a single needle

Eric Mueller: 27:15

right.

Wojciech Wegrzynski: 27:15

huge forest model as a sponge. That's, that's, that's pretty interesting, uh, how far science can, can, can get you. Um, if modeling drag would be important for the control volume. What about the change of drag as the, as the vegetation burns out? That's another interesting aspect. Uh, what happens as, as it's changing because it's a very transient phenomenon. It, it'll take not very long, uh, based on, uh, Maryland's, uh, Christmas tree burning tradition. It doesn't take a long time to, to burn the, the pine tree. So, so it's very transient.

Eric Mueller: 27:50

Yeah, sure. And, and the conditions under which it's burning, right? You think about, the production of, uh, firebrand and, you know, I know you've had talks with other guests about this kind of stuff, that's gonna change how much material you're, you're losing in a, a given instant because things will just be mechanically detaching and flying away, and so you're losing a lot of drag that way, potentially. And so capturing that is, difficult, let's say , but again, something that we're trying to get at more experimentally first, and. Uh, you know, build some more sophistication into the cell models we have. We can represent it if you want. Now, you know, effectively as mass goes away, then the drag coefficient has to, or the drag force, let's say, has to adapt to that. So it will effectively reduce its drag. And there's even ways to kind of hack in the detachment of particles and everything like that. But, you know, we prefer to be based on some better understanding of the.

Wojciech Wegrzynski: 28:41

Are you doing these measurements like wind tunnel burning trees,

Eric Mueller: 28:44

Yeah. Yep. Well, that's the, the plan moving forward,

Wojciech Wegrzynski: 28:48

That's, that's gonna be fun, man. That's interesting. Um, going back to the, uh, the smallest scale, so I understand and the image of, of how one model is, uh, fuel. What, what about the, like the chemistry, the kinetic, uh, the pyrolysis, uh, evaporation of, of moisture content, uh, how do you introduce that and.

Eric Mueller: 29:10

Yeah, I mean that's a, that's a great question. And that's the, you know, a whole other side of the complexity, which I. we have a lot of questions to answer and I would say it's a big field. There's some excellent researchers looking at this kind of work, but it's also a field where we have some established techniques that we can apply from, you know, traditional, more traditional built environment, fire engineering, right? So the same kinds of apparatus you would use to get at some fundamental parameters of, you know, Heat of com of combustion or look at the specific heat and the different degradation peaks in a TGA or a dsc. And so, you know, we can apply all of these tools and come up with some different pyrolysis models. And again, there's a whole range of complexity there because there's. , really so many reactions occurring in pyrolysis. So do we treat that as just kind of one broad reaction? That's when we get to larger scales where people try to apply these models. That tends to be the approach because there's just so much going on that, it's hard to, to get into the details here and, and we're not even certain that the sensitivity around, some of these details is such that it warrant. 13 step reaction, that's still kind of a, a big question, right? Is what level of detail do we need to go to, uh, in the sort of degradation kinetics, for example, or in the combustion chemistry. so I think it's just an iterative process, right? As we figure out more about the structure and how we can model at that scale, uh, we can then apply some of these more fundamental, kind of chemical models and look at the sensitivity between the two of them and advance from there.

Wojciech Wegrzynski: 30:40

When we are doing studies in, in the build environment, we, we have this, let's say, array of tools that are typically used from. Cone calorimetry to, to identify the ignition and the fundamental aspects of combustion of elements through, let's say, single burning item, which is a test where you test a performance of an element of a building till like large scale furniture, Cal meters were, the whole things would be burned, uh, at, at once to, to identify large scale behavior. what, what's, what's the. Pathway in, in wildfire research for development of fuel models. You use the same tools or how do you approach that?

Eric Mueller: 31:20

Yeah, I think a lot of the same tools are certainly, uh, useful and applicable. there's again, on the material side then certainly there's a lot to be learned. and on sort of the medium scale side, I would say sort of a furniture colorimeter, then I think, again, there's a lot that can be done to characterize the burning of individual items. And then, you know, you don't have to go to the full complexity of the sort of FDS CFD model we've been talking about, just because, you know, that's what I'm familiar with. But, example, in, again, in Missoula, in the Forest Service, I know they're working on sort of, you know, next generation operational fire spread models, and that's based on. Measurements of burning rates of, you know, different poorest structures at kind of that medium scale. So there's a lot that we can learn from these, you know, kind of engineering tests. The issue, or the, the caution I would give is, uh, kind of to the space in the middle of, let's say the cone scale, which again, I think there's a lot that we can learn, about flammability and things like that. particularly maybe in. Forest litter structures or you know, dead fuels like the pine needles we've been discussing. But the issue perhaps about applying some of that knowledge to the full range of complexity of structure we have is when you create this sample to put in the cone, how does that compare to the structure that you have in a real tree? You know, are you taking apart the needles and putting them in a basket or are you. putting a single branch and just kind of holding it in the cone and then is it really exposed to all the same heat flux and so can we apply the same methodology to understand the ignition of it? And so just the fact that we don't have these kind of nice plainer objects or nice, you know, geometric configurations that we have in construction materials, I think makes it. Not, uh, unuseful or unhelpful to apply these tools, but we just have to be cognizant of how we're applying the results that we get from 'em and how that compares to the real structure we find in a forest.

Wojciech Wegrzynski: 33:16

Yeah, that, that's what we've learned the hard way by trying to do the cone on, on the plants. It, it just behaves a little, uh, a little different due to a lot of reasons that, that were already mentioned. And I also know that in wildfire science, they're using this interesting, uh, holders that imitate the pity of the, of the fuel and load some air to go. To go through that. That's also very, an interesting concept, but also in, wildfire science, as you mentioned, the environmental conditions that are important from first the moisture content, but that I guess, that you could mimic with, with normal tools of build environment. But the other, the, the wind is, is one thing that seems very fundamental and it is not very. Important. I mean, it is important in building fires. Of course, that wind completely changes the, the, the compartment fire dynamics, and no one's questioning that. But we just don't apply that during most of our, uh, object tests in, in, in for building environment. Whereas in, in forest fire, it doesn't, it seems to not have, uh, much point to, to do. No wind conditions because, uh, if wind is so, so impactful to the penetration of the fuel with the, the heat and, the ignition and everything, it, it seems, uh, paramount to have wind included in this large scale research that there's one complexity, but also the slope, as you mentioned this, the slope, uh, will also act, uh, because the way how the, the stuff will be ignited, that, that's well known. That slopes are fundamental for that. So, so I guess you also need a lot of outer research because I don't think you have a sloped full scale Winston yet to, to play with it.

Eric Mueller: 34:56

Yeah, not yet. That would be something. But

Wojciech Wegrzynski: 34:58

be something right.

Eric Mueller: 34:59

Yeah. I mean, at the end of the day, I think it's important to always try to connect to this larger scale. Understanding, and I've been fortunate in my own research to get to go out and do, you know, large scale tests in the forests. And I think you know yourself how challenging they can be, but how rewarding it can be as well. And you know, so we've had a lot of fun trying to pull these off, but they're good for what they're good for, right? You get, uh, you get conditions which are, let's say, more realistic, although in a wildfire, I'm not sure that we'll ever have a planned experiment that has the same, you know, intensity of Uncontained crown fire, let's say, but you're getting closer to, to real conditions. so that's, that's great. Your observations are becoming more relevant, but you're losing so much control, over the input variables and whether or not you can manipulate them in a way to get some findings that you can actually relate back to your models, let's say. So there's some difficulties.

Wojciech Wegrzynski: 35:49

I just had Rory in the podcast and he had this crazy idea of putting a, a piece of forest on a, on a scale and just measure the massless during this experince That that could, that that could actually work. That could be funny. please tell me more about the experiments. I, I recall, uh, these, um, beautiful videos that you've shown from inside the forest fires in which you. Not only observing the, the spread of a fire inside a forest, but also measuring the conditions, uh, at which this spread occurs. Like what was learned from this experiment and how it relates, to these attempts to build, some sort of surrogate models that will help us understand the, the fires of of forests.

Eric Mueller: 36:32

probably what was learned, the most valuable finding was how difficult it is to do these experiments and, and how maybe we need to instrument them moving forward. And it is challenging because you have a field model, right? And so you want to, to have, let's say, field measurements of, temperature or, uh, Species concentration or heat fluxes, you wanna have point measurements all over the place and in the forest. It's just very difficult to get out there and, and lay out these instruments and have them operate successfully and even know exactly how the fire's going to spread once it's been ignited. And you know, the wind is just gonna do what it's gonna do. But I think the key was trying to. These larger scale observations, which are a little bit easier to make in, in such a big test. So just what is the spread rate of the fire? And you know, what maybe what does the fire contour look like? Uh, we had some nice remote sensing, some aerial imagery so we could look at, you know, the actual shape of the fire and the depth of the fire front and everything. And so these are nice. Kind of a macro scale comparisons that you can make if you then go do some modeling. But because our model is attempting to describe these things based on much more fundamental processes, we also want to have measurements of heat, fluxes of flow, really in and around the fire front. And so we tried to get at that, with these experiments. And yeah, it would be nice to do a lot more and get a

Track 1: 37:51

lot

Eric Mueller: 37:51

more measurements.

Wojciech Wegrzynski: 37:52

And, and how does a successful model look like? Like what should it achieve? Is it about predicting the, the spread rate? Is it about predicting the size of the fire in terms of heat emitted? Is it the model that predicts the amount of smoke produced? I know. Like how, how, when would you say that Okay. My model is, is successful at, at, at doing.

Eric Mueller: 38:14

Yeah, I, I mean certainly it's application specific, but I think two big objectives are, are, as you mentioned, the spread rate is a big one. that tends to come more on the operational side of things. You know, if you're deploying firefighters to an actual fire and you wanna know where it's spreading and how quickly, and so, , it's still important for these detailed CFD models, but may, it may not even be the main parameter that you're interested in, in getting, is the spread rate, but I think it's important

Wojciech Wegrzynski: 38:40

But, uh, sorry, but the large scale of spread rate, like within the forest, how many meters per minute or whatever the, the, the unit is on a large scale, not necessarily just on a pine needle, how quickly a, a pine needle burns out, right? So, so that, that spread rate,

Eric Mueller: 38:55

Yeah, yeah, and and certainly you want to try to. to get that spread rate, to be able to predict that accurately. But there's also a lot of complexity going on in the model. And so I think it's very easy in a way to achieve that spread rate, uh, for the wrong reasons or with the, you know, errors canceling out incorrect submodels. And so then I think the next step is if you're able to characterize it, which is difficult in the field, but getting that heat release rate or getting some metric of the energy release or the burning rate. So when we do this in the. . The other thing is, you know, if you put it on a load cell or whatever, or as Rory said, maybe you could put a forest on a load cell and you can get that burning rate, then that gives you a little bit more confidence that the spread rate you're getting is because you have at least the right energy release from the fire. Uh, and then of course, you can get into more details about are you getting the heat fluxes right, that are driving the spread rate and all of this. , you know, for the end goals, for the objectives we want to achieve. I think being able to supply that information about spread rate and about energy release, because then you can say something about the buoyancy and. predicting the smoke products somehow accurately from a combustion model would be nice, but we can at least approximate what they'll be from certain materials and then supply this energy release and an emissions factor to some larger scale model, let's say, and they can look at, you know, long range transport of the smoke. So I think being able to get that heat release.

Wojciech Wegrzynski: 40:18

And what's the end use of that? Is pre partners? Is it, uh, response.

Eric Mueller: 40:24

Uh, I mean for a, a very detailed kind of CFD model, I would say not for response. Yeah. Much more so for preparedness. We're trying to work a bit more with nist, with some fire managers and look at, uh, prescribed fires and help them when they go and plan how they're going to conduct a prescribed fire, where they're going to light the forest and what ignition pattern they're going to use. Um, those are the kinds of things that you have. Potentially a lot more time to study ahead of time. And you have these kind of idealized design scenarios. You know, what if I do this, what if I do this? And so you have the opportunity to actually run, you know, a more detailed model and you might need some more detail in your model to be able to understand. the interactions say of two fire lines, if you have two simultaneous ignitions, how do those come together? If you don't have enough detail on the interaction between the atmosphere and the buoyancy and the plume, then you're not gonna capture that. So we're trying to find some middle ground of, you know, capturing enough physics to give the sorts of answers that fire managers are looking for. But, you know, not requiring a research level project every time you want to get.

Wojciech Wegrzynski: 41:27

As a smoke control engineer. I must ask you about, about smoker and we, we've just kept talking about, about fire and to what extent you are capable of, of modeling the the smoke. Production in, in such fires and to what extent you're able to scale from your bench research to to, to drill fire forest fires.

Eric Mueller: 41:47

it's quite crude right now, or at least what's been done. I think the capability is there to improve this, so you can. Actually look at or try to solve for the conditions that you would have, in different parts of the fire. You can get some very deep flame fronts, let's say. And so you can have very different oxygen conditions and, uh, you can have a certain amount of fuel that's consumed in smoldering behind the flame front and so on. And so you know, some rough emissions factors that you can look at now that maybe will help you understand. how much, uh, of a certain product you're producing, uh, you know, under this condition or that condition. But if we can provide some greater detail, in terms of what is the actual oxygen concentration deep within this volume front, and for how long are the fuels exposed to that and under what heating conditions, then we can provide, uh, some more detail. But then we have to connect that back to the detailed chemistry, which for now we've. Put to the side just to, you know, focus on the flame spread. So I think it's a long term objective, but we still have some work to there to get a better prediction of actually what is produced.

Wojciech Wegrzynski: 42:49

That's a really interesting world of, of fire science, like one hand, you know, all these complexities that are related To the tiny scale of the parts of your fuel on the other end, the necessity to, to have a bulk representation of that to to make it useful, quick, reliable, robust, especially when you encounter like predictive modeling and you would like to give real time. considerations to the firefighters on the front. You cannot tell them, okay, now give me three weeks because my CFD must calculate it must be must be it. It kills the the point. Uh, so a very interesting set of challenges, uh, to overcome as we are. Transition into more, let's say, sustainable, uh, world. Or maybe we are just, more considerable about sustainability in the build environment. I know from my own example that, uh, implementations of technologies including, uh, living, uh, walls, like greenwall systems, green roof systems, there's a huge rise of that, uh, amount of solutions. Also with, uh, increasing, care about, wildfire Urban interface. Uh, Areas more and more fire engineers are involved in some sort of design or, considerations related to the, the threats typical for, for such, uh, aures. My prediction is that more and more fire engineers will have to deal With, uh, vegetation or living fuels, you who has crossed that path, uh, uh, what would be your advice for the fire engineer who's, uh, suddenly exposed to, to having to deal with, with life or, or vegetation, uh, fuel in, in their professional career for the first time?

Eric Mueller: 44:38

Good question. Uh, I mean, I think I would try to. , look at what's been done, perhaps more in the wild and urban interface context because it's at least a bit more mature of a field in some ways, perhaps than living walls, let's say. and there you can at least get some sense of, uh, what we understand about say vegetation flammability. And, you know, there's some good work going on to understand. rather than telling people we can't have any plants around their house in the weld and urban interface, maybe plants of this species will be more appropriate, less flammable than plants of another species. And so, trying to understand what are the kind of broad parameters that describe that plant that make it less flammable. that's perhaps a good starting place. and then, yeah, I think you just have to try. Be aware of the complexities that make these materials different from what you're used to encountering. So, you know, getting back to the porosity. And then unfortunately with living fuels like a green wall, it gets even more complex than kind of the dead fuels. We've talked a lot about. So far, , and there's, there's a lot that's just not known yet, I think, or, you know, we have a broad understanding of, of the complexity that's there, but maybe not a good description of the physics behind it. You know, we have observations of, uh, live fuels burning still with a lot of moisture. Content are still releasing a lot of moisture, which is not the way that we describe how dead fuels burn, at least in our models. we. , there's some studies showing that, uh, def flammability of live fuels can actually increase with moisture content over certain ranges. And so yeah, there's a lot of complexity there, which is a little bit outside of my wheelhouse, but, uh, I think we just have to be careful moving into the space to be aware of how different it can be from the materials that you're probably used to working with.

Wojciech Wegrzynski: 46:24

two things I've learned working with these green wall systems and, uh, vegetation fuels. One thing is that the fact that at the beginning of your fire, the, the plants were very moist, does not guarantee you that they will not dry out during the fire. And fires seem to be pretty decent that drying out. Uh, Plan. So if it goes for long enough, it's gonna be dry enough to ignite that. That's one thing that we've learned and, and quite interesting, uh, to, to observe. The second thing is, Ivy, the, the ivy plant. It, it's ridiculous. We cannot dry the damn thing. It just refuses to drop its moisture content. That's an amazing plant. I, I was astonished like seeing other plants around on the wall being like dry. You could crack them in your fingers and it's. Damn thing refuses to die. It's so resilient. It, it's fascinating to observe that. Probably more fascinating for us than for the Ivy. But, uh, but, uh, yeah, it's, it's, it's, it's, it's, certainly an interesting world that, uh, I was maybe not very ready, uh, for

Eric Mueller: 47:25

Yeah, and I think like that's where a lot of this complexity comes in then with the live plants is it's no longer just wood, let's say chopped up and rearranged into different interesting structures and, and now the actual. Plant physiology and morphology comes into play and does it have something like a waxy layer, like, know, certain plants have to really encapsulate that moisture and then that can affect how it actually then releases the moisture when exposed to a high heat flux. And so things get fastly more complicated pretty quickly.

Wojciech Wegrzynski: 47:54

and we didn't, didn't even go into like fire brands production and, what happens in, in, in, in the complex fields where you mix different sorts of like, Plants. You have bush layer, you have crown layer. The, Interplay between the, the, the scales of the first fire. So there is so much more to unravel here. Eric, thank you very much for this in insightful talk. I think for anyone who would have to deal with, vegetation fires or living fuels, I think it was very interesting to learn about some sort of complexities that, uh, I don't think I would cons. I know I did not consider when dealing with my building fires. I've just learned them really recently exposed to this, uh, new, fascinating, and, and difficult world. thank you so much. I'm gonna link, uh, your IAFSS paper and, uh, and the video. It was the award for, for the best thesis, uh, by the F Ss. So congratulations on that. That's, uh, that's actually quite a big achievement. So that's a, that's a statement to the quality of the work that you, you have been doing. If someone would like to, to take a next step and just go, go and read something immediately, what would be the first thing that comes to your mind? Someone should go and, and see, uh, after this.

Eric Mueller: 49:14

well, actually, I was just looking at. Wildland Fire Behavior is a textbook, uh, that came out not so long ago from Missoula, from the US Forest Service, uh, mark Finney, Sarah McAllister, Tobin Grump Trump, and Jason Forer. And if you're interested in kind getting into the space of these different fuels and understanding what's important, I think that's a really nice textbook that kind of gives an overview, uh, of a lot of the topics that we've been covering and probably in much , greater detail or

Wojciech Wegrzynski: 49:43

Ah, you cannot fit much into one hour podcast episode, but it's a good starting point for everyone. I'll link to that, book, and maybe you'll find, we'll find together some more resources to sharing the show notes, so, so people know where to, where to go to.

Eric Mueller: 49:58

Yep.

Wojciech Wegrzynski: 49:59

Eric, thank you very much for coming all the best in your developments of surrogate fuel models for fds. I, I actually, I, I would love to have a really good fuel model within fds. I, I, I think the whole community would, uh, benefit from that. So I'm looking forward to your achievements at NIST all the best, and, and thank you

Eric Mueller: 50:18

All right. Thanks. Thanks so much for having me.

Wojciech Wegrzynski: 50:20

Cheers, man.

Wojciech Węgrzyński: 50:22

And that's it. I told you was going to be good. And I got access to all my questions, which makes me very happy and actually helped us proceed with our research for which I'm very thankful to Eric. After finalizing the recording. , Eric contacted me as he was reflecting on our discussion, uh, related to the grid size and porous media. You know, the one that's on the. Cover of the podcast all then. And he said that that there's one more concept that, that needs to be clarified. So if all vegetation that is contained within a numerical cell and it is increased in size. In fact, the porosity must go up because we need to conserve mass in that cell. But if instead we, we split the cell in half,. We would have half the volume and health mass in each cell. So porosity would not be changed. So when we have many cells across a piece of vegetation, generally we can make small, moderate adjustments to their size and conserve porosity, which from Eric's point is, is. Variable. So in the end, the grid sensitivity, maybe we have little overstated. That and, It can be used to play with the models and conserving the things that matter. So that's additional clarification. From Eric, I think an important one and I hope useful one. Um, back to the episode itself. I hope you got something interesting from it. I was absolutely shocked to learn. Well, maybe not shocked. I was surprised by my ignorance. The hats. Uh, the convective cooling makes such a big difference in living fuel fires. And I mean, they tell you that. The you take it as granted, but only when you see this samples not igniting from really large heat fluxes imposed on them. You start to realize, wow, this, this. This is really. Impactful on the fire behavioral of the, of the whole system. So definitely a biggest learning for me. And I hope you also got some learnings in here. No. That'll be it for today's episode. I guess I should go back to my IAFSS submission. Now, after this episode, I'm even more pumped up to get a good IAFSS is paper. This conferences are just amazing. And yeah, that's the home for fire science. So I guess that's a good place for me to be. So thanks once again. See you here next Wednesday and hopefully see you in Japan. Tsukuba in October. It's going to be great. Cheers. Bye.