160 - Fire Fundamentals pt 10 - Flame Spread with David Morrisset

Transcript

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Hello everybody, welcome to the Fire Science Show and welcome to Fire Fundamentals once again.

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Oh boy, this episode.

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I got to tell you some stuff about this episode before we begin.

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First, I re-invited David Morissette.

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You may remember him from the episode on burning upholstered chairs.

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It was a very popular episode of Fire Science Show and I enjoyed it thoroughly.

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So David is a PhD student at Edinburgh and almost a doctor, so he's just a few steps away, crossing my fingers for his viva and brilliant future.

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I actually love interviewing PhD students and postdocs.

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You know young researchers.

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It's really fun when you get to talk to legends of fire science and, of course, that's something that's also my dream to interview the biggest names.

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But interviewing, you know, the new generation, the ones who will become the legends of fire science, is a huge pleasure.

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Anyway, with David, we've chosen to jump into pretty deep water, and this perhaps is the most challenging, most difficult episode of the Fireside Show.

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Yet we're talking about flame spread.

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And boy, flame spread is a really complicated matter, so it's a friendly piece of warning.

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This episode is not easy, but here I would love to convince you why it's worth your time to listen to it.

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And simply, flame spread is something that drives so much of the fire safety in our buildings.

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Even the simplest way how we create design fires based on alpha t squared relation, you can tie it back to flame spread, the catastrophical fires that surprised us in some way.

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You could actually explain those surprises by studying flame spread.

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A lot of stuff goes down to flame spread and yet, because it's so difficult, it's very rare that people really understand it and in some aspects it's counterintuitive, which makes it even harder.

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So these are the points that we really like to highlight with David In this episode.

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I usually do the summary at the end of the episode, but this time I'll try to give you a brief overview of the episode beforehand.

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In this episode we discuss what makes flame spread so important for fire safety engineers.

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We discuss two modes concurrent and opposed flame spread, which basically means upwards and downwards, if you consider a vertical item burning, and in those two flame spreads you either have flame moving in the same direction as your flame spread or flame moving in an opposite direction than the flame spread.

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So these two modes will have completely different driving mechanisms, completely different phenomena that will define or that will influence how the flame spread behaves.

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And, as david highlights it's.

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It's not that we can always focus on one and forget the other.

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It's not that we can focus on one simplification.

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It can become pretty complex when you start having lateral flame spread and you really have to understand which mechanisms are driving your flame spread in a very specific application.

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When you listen to this episode, I highly encourage you to capture those bits of the conversation what drives the flame spread, what are the differences between the mechanisms and how we can use it for our advantage, how we can use the knowledge about flame spread, this new intuition related to Flamespread in designing more fire safe buildings, and if we succeed with that, then I'm gonna be super happy.

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And well, let's at least try.

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So let's spin the intro and let's do some Flamespread.

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Welcome to the Firesize Show.

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

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This podcast is brought to you in collaboration with OFR Consultants.

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Ofr is the UK's leading fire risk consultancy.

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Its globally established team has developed a reputation for preeminent fire engineering expertise, with colleagues working across the world to help protect people, property and environment.

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In 2024, ofr will grow its team once more and is always keen to hear from industry professionals who would like to collaborate on fire safety futures.

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This year, get in touch at OFRconsultantscom.

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Hello everybody, I'm here today with David Morissette from University of Edinburgh.

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

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Hey, wojciech, thanks for having me, it's good to be back.

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Good to be back.

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Last time we were talking about burning some chairs, and that was a very interesting talk.

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But there's another thing that you are well known in the fire community.

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That is the most beautiful videos of burning PMMA, the favorite fuel of all fire engineers and scientists.

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So today we're going to be talking about that second thing, which is well not PMMA.

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It's about flame spread and learning about flame spread in general.

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Now question is I mean, flame spread is some hardcore physics.

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To be honest.

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It's like really, really hard when you start digging that.

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What's the premise of studying that for you, like, why did you go into the footpath of Fernandez-Peyo, williams and other giants to study this ridiculously difficult thing?

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That is a wonderful question.

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What sets Flamespread apart, too, is if you look at it in terms of some of the fundamental topics that you see in fire science too, is, if you look at it in terms of the war, some of the fundamental topics that you see in fire science, whether it's ignition and flashovers or those, those compartment fire dynamics, those big core ideas, each of which have a chapter of their own in the textbook.

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Flame spread is kind of one of those categories, right, sort of a standalone, quintessential topic of fire science.

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And so when I was starting my phd, my supervisors, angus, law and rory, had sort of sat down.

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We're thinking about some of these fundamental problems that would be really rewarding to sink our teeth into, and so the one that came up was Flamespread.

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And if you again, like you just said, though, if you look at some of the forerunners of the field, right, you had some pretty heavy hitters, right.

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So in terms of the Flamespread literature, the likes of Howard Emmons, former DeRis, I mean the list can go on and on Dibble, drysdale, and these individuals are recognized both within the world of fundamental fire science and combustion science.

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So you have these amazing people looking at this problem over the last 50 years, so there's lots of good literature for anyone who's interested.

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Maybe we can link some stuff in the show notes.

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Absolutely, but there's still a lot to explore with this topic.

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Right, you have to understand what's happening in terms of the heat transfer from the flame to the solid.

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What's going on in the solid?

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The solid is then pyrolyzing, producing pyrolysis gases.

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Those pyrolysis gases then need to mix, and then they need to ignite, and then that flame needs to then again heat the solid, right?

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So you have this really interesting feedback loop between these solid and gas phase interactions, and then the decoupling of these problems is, you know, extremely intricate, and so understanding how does the gas phase and the solid phase interact across different configurations or different experimental conditions truly complex problem.

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And that's really where things get interesting, though, from a fire science perspective.

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I think you nailed it it combines so much of important physics at one model in which you cannot separate them.

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I mean, you need to understand ignition of the gas phase.

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You need to understand the radiation both from how the heat flux is emitted and also how the heat flux is absorbed, the solid phase, gas phase, heat transfer modes.

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You need to understand the pyrolysis.

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You can study pyrolysis on its own.

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It's a very valid thing to study another fundamental phenomenon, but here you have to couple it with everything else.

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Pyrolysis on its own, it's very valid thing to study, like another fundamental phenomenon, but here you have to couple it with everything else in dynamically changing environment and in decaying fuel.

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So I I also think, uh, the interplay of physics in here makes it interesting, but it's also important, right?

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I mean, when you about it, the world is not built from liquid fuels or PM literature.

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A lot of these principles that we look at in terms of how do we mathematically describe the process of flame spread can also be applied to gas fuels, liquid fuels, I mean some of the you know interesting sort of models to look at the propagation of a flame front through a gas cloud.

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Right is actually, you can draw a lot of similarities between you know john derriss's and Quinteri's equations for solid flame spread.

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There's a lot of really interesting overlaps there.

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But, like you said, I mean most of the sort of scenarios that we're looking at most fire scientists and as fire engineers is solid fuels.

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So I guess we'll probably limit this discussion to solid fuels, which I think makes sense.

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But a very easy link to engineering is think about one of the reasons that we care about flame spread is fire hazard in general is generally linked to the concept of fire growth.

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So if you have a heat release rate right, I remember this you know something we've talked about last time I was on right and it's such an important parameter in fire science All the engineering sort of analysis that that heat release rate is going to grow at a certain rate and that growth rate is therefore a really critical input into different bits of analysis, right, whether it's your you know cfd modeling or looking at sprinkler activation or anything, and the concept of fire growth is fundamentally linked to the concept of flame spread, right, I mean, one of it's actually really interesting if you sort of dig back deep enough into our you know, classical alpha t square fire right.

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One of the core ideas there is that you have a fire that's spreading radially, horizontally on a flat surface right, and then you, what you do is basically that time constant is associated with the idea of what are you know properties in which result in a certain way to spread.

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So there is this really you know intimate link between the concepts of flame spread and fire growth.

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And then if you take that and extrapolate that to the idea of fire growth is super important in the context anything to do with the heat release rate then these, the concepts that go into flame spread, are truly embedded in a lot of the things that we do as fire engineers.

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And you can see this pretty clearly if you go back to our podcast episode and you have a little flashback to what we talked about last time.

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If you burn an upholstered chair, you can see that the heat release rate developing over such a complex fuel package can very simply be linked back to some of these ideas of flame spread.

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Right, we talked about how there was a certain period of the heat release rate curve where we saw that there was flame spread upward, flame spread up the backrest of the chair that resulted in a very different growth rate than horizontal spread along, you know, say, the base of the chair.

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And you know, by linking these observations of flame spread you can actually start to articulate the mechanisms that are driving the heat release rate itself.

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I would also like to put one more reason why it's important.

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If you think about the growth of the fire, you know the Drysdale's plot of the phases of the fire.

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You know ignition into flashover, into fully developed fire.

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If you consider carefully, like, where's the space for fire safety engineering in in that plot?

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I mean, once the fire has flash over, there's not very much you can do besides, uh, fire resistance of your, of your walls, which I struggle to call fire science.

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Anyway, there's not much you can do.

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Right, the things that we can do as engineers are in those first phases, when the fire is growing, developing, building up, and in this period it's all flame spread, faster, slower, dominated by whatever other phenomena that we will talk about, but it's all flame spread.

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So yeah, I mean, even if the engineers will not understand, fully understand the flame.

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That's not the point that you can derive Quintyree's model on your own.

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The point is to understand what makes one flame spread very different from other, like what circumstances, what variables, what physical, environmental conditions make the difference, and that's the point of doing this interview.

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So let's perhaps dig into that.

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So first let's narrow it down to solid fuels.

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I think that that's a good framework to work with.

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So how would you even define the flame spread Like what's actually spreading?

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How would you like put it into words?

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no, that's, that's a that's.

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I think it's a great next step.

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So I think another, let's put it, maybe a couple more restraints on the problem too right, one thing we're on earth for the rest of this discussion, that's maybe a another discussion we can have I love that we have to put constraints like that in modern science.

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I, I love it.

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Yeah, let's go.

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Because all the physics starts to change, right?

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It's a fascinating problem, but for the sake of this discussion let's stick to Earth.

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And so now one thing that happens on Earth is buoyancy, right.

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So the sort of natural way to first separate the problem is to look at the two major categories that people generally associate with flame spread, and that's post-flow flame spread and concurrent.

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Now what's really interesting is the physics between these are effectively actually the same, but generally we like to constrain the problems.

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Just to you know, make the mathematics work out nicer.

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Right, and you'll see that becomes clear as we go through this.

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But if, for anyone who wants to sort of play along at home, you can see both of these orientations just by lighting a match, right, and that's kind of like the nicest way to sort of demonstrate this right.

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So if you strike a match and you hold it upright, so that your fingers are at the bottom, holding the base of the stick, and the actual flame is pointing upward right, the flame will slowly creep down the wood of the matchstick and this is an example of opposed flow of flame spread.

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Right, flame is moving down in the opposite direction of where buoyancy is entraining air right.

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So buoyancy is moving upwards and the flame is moving down right now.

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If you strike a match and you hold it in the opposite direction so now you're striking and you're putting the head of the stick down towards the ground what you'll see is the flame will spread up the matchstick in the direction in which buoyancy is moving.

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So all the hot gases, the flame itself, is all moving in the same direction in which you're spreading flame.

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Right, and this is concurrent flame spread.

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And what's first thing to note right away is, for anyone who's done this just trying to light a candle right, you'll notice that concurrent flame spread is much faster, right?

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So if you hold a match in that direction for too long, you're probably going to burn your fingers.

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So right away you notice there's this big difference between those two different regimes, right?

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But what's interesting is the physics that actually drives the problem like doesn't actually change that much between each of these two regimes of flight spread.

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You still need, basically, you have your flame and you need that flame to heat fuel ahead of the flame front, then the pyrolysis gases need to react in the gas phase and you complete that feedback loop that we talked about.

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So the actual physical processes occurring throughout the problem are the same.

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It's just the mechanisms by which, say, the heat transfer gets to the solid fuel will change.

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So that's how we start to distinguish between these two different problems.

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So let's say, the flowchart of the flame spread would be that you have your flame, which is a source of heat, which then transfers that heat to the solid in front of the flame, wherever the front is, and with this heat transfer it heats up the the solid to a point where the gases or whatever's emitted from the solid can ignite and create a new flame, and this repeats and repeats, and repeats in an endless loop until the flame has moved through the entire fuel.

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Is that like good enough approximation?

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I?

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mean, I think that's like, that's a nice way to start framing the problem.

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I think that is a good way to look at it, right.

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So throughout both of these processes or any flame spread process, right, you know the core, same core phenomenon applied to it, right?

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But in the context of, let's say, a post-flow flame spread, uh, looking at, let's say, start with downward flame spread over slabs of pmma, right, the kind of like time scales that we're talking about here, right, if you have flame slowly creeping down and anyone who does, he works in a fire lab downward flame spread over pmma is usually on the order of about three mils per minute, right, so not racing down the solid by any means.

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But then concurrent flame spread, like upward flame spread, for example, is going to be, can be orders of magnitude faster, right, depending on the length scales involved and so on, because it's an unsteady process.

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And so in the context of fire engineering, upwards sort of concurrent flame spread and these sort of rapid flame spread rates immediately pose a significant issue in the context of fire safety, not to say, you know, also, opposed flow is another regime of flames where we really need to characterize.

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But it's interesting to note right away that there is, you know, in I'm sure we really need to characterize, but it's interesting to note right away that there is, you know, in terms of the implications that has to fire, fire engineering, you know, start to see that right away.

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And this is also if you start to take a step back and think about these two conditions, like the matches again hold them in opposite directions.

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It's clear that the conditions between these two scenarios are different.

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Right In one you have hot gases moving in the direction of flame spread, and then the other, the flame literally has to move in the opposition of the direction in which all the products of combustion in the flame is is moved, and so for the.

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For the topic of my phd, we actually chose to study primarily the opposed flow problem and that was largely to.

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Basically, we want to study a steady state process, this sort of canonical sort of condition, and resolve as much of the physics as we possibly could with high resolution measurements, right, you've mentioned.

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You observed that on the slabs of pmma and you you urge people to burn pmmas in their laboratories.

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I see that fuel becoming a very common choice when people study and illustrate those phenomena.

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What kind of makes this the perfect fuel for this type of research and how does it help you simplify the physics of what's happening around?

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Because I feel there's a secret to PMMA, actually in understanding these mechanisms.

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I mean this is really funny.

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I'm glad you asked that question.

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Well, there's the actual practical implications of why PMMA is useful and then there's sort of the history of why PMMA is used.

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So let's start with the history, because that's just a fun anecdote, and I think Rory might have even touched on this when he was talking about ignition.

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But if you looked at back in the day, in like the late 1970s not too late 1960s, there was research going on, particularly in different parts of the world.

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There was some research going on to the east coast of the United States looking into flame spread over PMMA, because PMMA was at the time used as rocket fuel, so it was actually something that was being used as a propellant, and so there was a lot of study of PMMA under high pressure as a propellant, and so understanding the rate of flame propagation was a surrogate, for how are you actually getting energy out of your propellant, basically?

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And so because there was existing data when the fire scientists came around to study the process of flame spread.

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Theoretically this is my my understanding of this is there was already existing data and they're like oh hey, let's just crack on with pmma and the nice thing about pmma now for everyone who continues to use it today is that all of, especially with ignition and flame, spread the sort of theoretical thermal models that we use.

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These processes require assumptions like you, you have to be a vaporizing solid, right?

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Basically you have to have a solid that once it heats to a sort of magic pyrolysis temperature, it just instantly vaporizes.

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Charring solids, melting polymer stuff like that cause some complexities.

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So if you get pma and you get a high quality cast PMA this is sort of a nice little anecdote to add there, because if you, depending on how you manufacture the PMA and depending on the cross-linking of the polymer, you can actually, if you extrude the PMA, for example, and there are other kinds of processing techniques that actually will result in the PMA dripping quite a bit, but if you get nice high quality cast PMA, then it acts pretty like a nice vaporizing solid.

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You don't see much melt flow, you definitely don't see any charring or anything like that, because it's polymer, but you end up with a solid that actually behaves pretty well.

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And we can compare this to these simple models that we've done.

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And for laymen, PMMA is basically acrylic glass, right?

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Correct.

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Sorry, I should have said that yes, it's acrylic For the last, like you know, four years.

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What's been going up in grocery?

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stores between you and the till right.

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Yeah, it's kind of what a career from being a rocket fuel to something that is supposed to stop the coronavirus from spreading right it's really an all-star, I mean brilliant, brilliant materials.

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Anyway you've mentioned.

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It does not go through the same phenomena as complex fuels would go, so there's no charring.

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Basically it's like 100% efficiency pyrolysis, which leaves not much behind.

00:20:33.678 --> 00:20:40.659
How does that translate to other substances that we would meet in the real world?

00:20:40.659 --> 00:20:46.025
Timber is something that's perhaps most interesting for most fire engineers nowadays.

00:20:46.025 --> 00:20:50.638
So is studying pmma also relatable to to timber structures.

00:20:51.381 --> 00:20:53.266
So I think that I mean, that's a great question, right.

00:20:53.266 --> 00:20:57.215
And I should also caveat heavily that pma still is still polymer.

00:20:57.215 --> 00:20:59.588
It will still, on a fundamental level, melt.

00:20:59.588 --> 00:21:05.232
It's just the point which, between that occurring vaporizing as pyrolysis gas, is so close.

00:21:05.232 --> 00:21:09.276
So there's going to be someone who's going to be listening and be like, oh it does, absolutely true.

00:21:09.276 --> 00:21:12.414
And there are people who understand the degradation kinetics of PMMA.

00:21:12.414 --> 00:21:15.773
But on a macro level, we can actually observe these phenomena.

00:21:16.665 --> 00:21:21.257
But the fault that you're getting at is then let's compare that to a slab of timber or a slab of polyethylene.

00:21:21.257 --> 00:21:22.417
So you're looking at polyethylene, right.

00:21:22.417 --> 00:21:25.278
So you're looking at poly material like polyethylene, which is, you know, burn it.

00:21:25.278 --> 00:21:29.881
It's a dripping mess, and I'm sure you've seen this with you know many cladding tests that you've done, right.

00:21:29.881 --> 00:21:34.066
So then, all of a sudden, you start talking about the process of dripping and melting.

00:21:34.165 --> 00:21:42.617
Right now, all of a sudden, the entire concept of the heat transfer mechanisms that allow for flame spread to occur are now occurring on a material that's flowing.

00:21:42.617 --> 00:21:46.508
How does that complicate the problem for charring solid, like we talked about?

00:21:46.508 --> 00:21:48.011
This is a feedback loop, right?

00:21:48.011 --> 00:21:53.416
So so you know, in terms of the gas phase, flame is then feeding, you know, you transfer to the solid.

00:21:53.416 --> 00:21:56.470
The solid is then pyrolyzing as you char, you start to sort of.

00:21:56.470 --> 00:22:04.623
The char layer effectively regulates the amount of heat transfer that reaches the pyrolysis region, right, um, and then that's going to change your gas phase problem, right?

00:22:04.623 --> 00:22:07.159
It's going to start changing the amount of pyrolysis that gets into the gas phase.

00:22:07.159 --> 00:22:09.150
It's going to change the amount of heat transfer that can be provided.

00:22:09.150 --> 00:22:14.954
Most labs of timber can actually support flame spread without an external heat flux because the charring behavior.

00:22:14.954 --> 00:22:26.698
So that's why you have to study charring salts in something like the lift or you know sort of like those sort of standardized tests with with radiant panels or radiant heating conditions perhaps we also need to define pyrolysis in this series.

00:22:26.984 --> 00:22:33.532
So what's the pyrolysis process that we refer over to back and back, and we will until the end of the episode, all the time.

00:22:35.306 --> 00:22:37.354
That's another great point of clarification there.

00:22:37.354 --> 00:22:41.959
So when we're talking about pyrolysis, we're talking about basically the thermal degradation of the solid.

00:22:41.959 --> 00:22:44.749
So we're saying think about the match again, right?

00:22:44.749 --> 00:22:46.211
So I have this match, this piece of wood.

00:22:46.211 --> 00:22:49.699
In order for it to burn, the flame has to exist within the gas phase.

00:22:49.699 --> 00:23:00.592
So what we need to do is we need to first take this solid and break it down into a gaseous fuel, and so, again, that's a very simplified version of the process for people who study pyrolysis kinetics.

00:23:00.592 --> 00:23:14.732
So, forgive me, but the is that you basically you take heat, transfer into the fuel and you create a flammable gas that then burns the flame sheet it's difficult discussion because it's at the same time hard fire physics and.

00:23:14.913 --> 00:23:19.848
But for people who studied, like you and me, it's like the obvious fire physics.

00:23:19.848 --> 00:23:26.319
So we need to nail the very good balance between obviousness and hard-core-ness of the physics.

00:23:26.319 --> 00:23:28.874
But anyway, these are important things.

00:23:28.874 --> 00:23:50.472
So, once again, the loop of having a flame, having a heat transfer from that flame, the fuel to undergo some physical change, be it pyrolysis, be it melting, igniting the products of that physical transition process, and having another flame that can transition elsewhere, that's, that's the beauty of flame transfer.

00:23:50.472 --> 00:23:54.950
Now you've said that in case of charring or timber, that's actually the case.

00:23:54.950 --> 00:23:58.016
That's very often brought up by layman.

00:23:58.016 --> 00:24:06.719
Actually, you know, timber doesn't burn that much each chars, but the processes in there are much more complex.

00:24:06.719 --> 00:24:13.295
Can phenomenal natural charring have been like an insulated layer?

00:24:13.295 --> 00:24:15.820
Can we repeat those artificially?

00:24:15.820 --> 00:24:21.693
On materials, how do we make materials less prone to fire spread, knowing about those things?

00:24:22.178 --> 00:24:34.971
So I think I'm going to I guess this is going to be kind of a roundabout answer, right, because I think we'll take a crack at responding to that here but I think actually the the way to progress from here is to say, okay, we know, we have things like timber or other solids that behave differently, right?

00:24:34.971 --> 00:24:39.593
I think the next step we should actually talk about is we've been throwing around terms like heat transfer and whatnot.

00:24:39.593 --> 00:24:46.535
But let's actually dig into that a little bit more and let's see how those start to change as you start to change things like charring terms or charring solids or whatever.

00:24:46.535 --> 00:24:57.586
But I do think, basically, you need to define this framework for what we call the controlling mechanisms of flame spread right, and what we need to do is we need to very systematically study those under different conditions, right?

00:24:57.586 --> 00:25:03.450
So, um, there has been work on charring solids and flame spread right, and it was done by, say, like the likes of atreia and and colleagues.

00:25:03.450 --> 00:25:05.554
But there's still, obviously there's still many things.

00:25:05.554 --> 00:25:10.009
I'm sure we can more information to sort of dig into and more studies to more studies to do there.

00:25:10.009 --> 00:25:16.134
But it's basically, if we can tie everything back to this, this framework of what actually dictates this process of flame spread.

00:25:16.134 --> 00:25:22.529
I think that's sort of the the direction, so I've been thrown around this term controlling mechanisms of flame spread this is a term that was coined.

00:25:22.529 --> 00:25:32.756
Amazing paper that was, uh, written by friend, is carlos, france, pale and harana, and basically they outline this framework, or what are the things that actually control the flange red process, right.

00:25:32.756 --> 00:25:34.162
So I'd recommend reading this paper.

00:25:34.162 --> 00:25:36.107
It's pretty awesome, but a classic.

00:25:36.107 --> 00:25:37.270
Largely.

00:25:37.571 --> 00:25:42.016
The controlling mechanisms break into sort of like two big categories, being heat transfer, right.

00:25:42.016 --> 00:25:50.269
So something we've already talked about and we can break that down even further from there, right, but heat transfer mechanisms basically control how is the material ahead of the flame front being heated, right.

00:25:50.269 --> 00:25:56.823
How is that heated, being pyrolized and then the flame sort of continues along its way, right, how does that process from a thermal perspective?

00:25:56.823 --> 00:26:15.268
Right now, the other mechanism they talk about in the paper and we will just briefly talk about here, but for completion I think it's worth talking about is other gas phase phenomena associated with the flame itself, right, and this includes things like chemical reaction, kinetics or mixing ahead of the flame front, and so the cases that we're normally interested in as engineers.

00:26:15.710 --> 00:26:21.053
We generally focus on what we'll call the thermal regime, right, everything is just heat transfer to the solid at the flame front.

00:26:21.053 --> 00:26:24.589
Right, the process of flame spread can be described through heat transfer.

00:26:24.589 --> 00:26:27.817
Right, and that's where largely we'll be focusing the discussion here.

00:26:27.817 --> 00:26:39.348
Right, but it's worth noting that there's another regime of flame spread, coined as the kinetic regime by Fernandes-Beyer, that all of a sudden the mechanisms that actually control the process start to change a little bit.

00:26:39.348 --> 00:26:39.589
Right.

00:26:39.884 --> 00:26:48.740
But in the thermal problem, right, this is where we see things like the models presented by Driss and Quateria, you know, it's treated as basically a balance of terms, an energy balance.

00:26:49.184 --> 00:27:00.931
You can basically create this model where you say I'm going to take this bit of solid, I know that I have a flame spreading along the surface, I'm going to take this bit of solid, I'm going to raise this bit of solid to a pyrolysis temperature right, and that's the feedback loop we've been talking about.

00:27:00.931 --> 00:27:03.654
All of a sudden, now it's pyrolyzing, it's producing pyrolysis gases.

00:27:03.654 --> 00:27:11.752
From that ratio of how much heat transfer am I providing ahead of the flame front as a ratio to how much energy does it take to actually pyrolyze this solid?

00:27:11.752 --> 00:27:27.721
What balances those two terms is basically your flame spread rate, and so your flame spread rate adjusts to the balance of those two turns, and so that's basically how most thermal models work, is they're just looking at the solid itself and saying if I can approximate the heat transfer at the flame front, I can appreciate how much energy it would take to pyrolyze the fuel.

00:27:27.721 --> 00:27:30.532
The flame spread rate balance those two terms.

00:27:31.305 --> 00:27:32.607
I'm not sure if balance is.

00:27:32.607 --> 00:27:37.959
If I may come into an argument with you, I'm not sure if balance is the correct way.

00:27:37.959 --> 00:27:38.848
It's just an outcome.

00:27:38.848 --> 00:27:52.434
You know, I like how nature is complex and sometimes you could have processes that are an outcome of 20, 30, 50 variables and interactions between them, Yet the nature does not go into an issue.

00:27:52.434 --> 00:27:53.790
Oh my God, there's too many variables.

00:27:53.790 --> 00:27:55.673
I cannot like find a solution.

00:27:55.673 --> 00:28:00.074
There always will be a solution and it's simply an outcome.

00:28:00.074 --> 00:28:03.375
And for us the balance is what you get out of the equation.

00:28:03.375 --> 00:28:06.634
Yet nature will find a way, that's true.

00:28:07.125 --> 00:28:08.853
I mean, it's all a matter of perspective, right?

00:28:08.853 --> 00:28:15.999
So from the perspective of these like simple, quote, unquote, simple, I mean they're really complex mathematics, simple looking equations, right.

00:28:15.999 --> 00:28:20.751
That is what I mean in terms of it's a balance, right, it's literally an energy balance, right?

00:28:20.751 --> 00:28:21.273
But you're right.

00:28:30.164 --> 00:28:33.796
And when you start compilingiling, you know, compounding all the relevant physics, right, it's an outcome of all of these different processes.

00:28:33.796 --> 00:28:48.007
Yeah, the place that I wanted to segue you into was more like practical, or maybe not practical, more more like real world consequences of the very basic phenomena that we're talking about in here, like why fire retarded materials would spread the flame slower than non fire retarded.

00:28:48.007 --> 00:28:56.781
Why found material would spread fire faster than the same material, but in a slab form, right.

00:28:56.781 --> 00:29:03.115
Why porous or or a dust will behave different than than the slab of the same material.

00:29:03.115 --> 00:29:04.317
Because it's all.

00:29:05.220 --> 00:29:10.701
You could probably track most of those questions back to the fundamental flame spread equations.

00:29:10.701 --> 00:29:15.115
Let's try the insulative material, formed materials, because it's.

00:29:15.115 --> 00:29:25.877
It's something that marked the change of times in fire safety and fire science when we moved from the classical furniture in your apartments into the modern furniture.

00:29:25.877 --> 00:29:40.590
Everyone's seen the videos demonstrating a fire in a 1970s house and the modern house full of foamed material and the consequence is striking in the time to flash over and stuff like that.

00:29:40.590 --> 00:29:47.013
But you can trace that back down to the flame spread and fundamental physics, right.

00:29:47.355 --> 00:29:48.317
Sure, I mean absolutely.

00:29:48.317 --> 00:29:57.164
I think everything that you mentioned in that list of potential interesting topics to talk about, I think it absolutely all ties back to everything that we've just said, this feedback particularly.

00:29:57.164 --> 00:30:02.765
You know the heat transfer mechanisms and you know and how that results in the establishing of pyrolysis and so on.

00:30:02.765 --> 00:30:06.859
So I mean for foams, but I mean what's, what is the characteristic that makes foam so useful?

00:30:07.160 --> 00:30:07.320
for me.

00:30:07.320 --> 00:30:12.318
Well, I like to sit on the foam much more than I like to sit on on slabs of polyurethane.

00:30:12.318 --> 00:30:13.842
I don't even know if I ever seen a slapped form of polyurethane.

00:30:13.842 --> 00:30:17.769
I don't even know if I've ever seen a slab form of polyurethane, you know.

00:30:17.769 --> 00:30:24.957
But from the fire perspective I guess it's all into the insulative properties and we didn't even show them.

00:30:24.957 --> 00:30:25.839
It's difficult.

00:30:26.430 --> 00:30:33.176
The fire fundamental series is most difficult for me and my guests because we don't have a blackboard where you can draw the model.

00:30:33.176 --> 00:30:44.932
But you need to understand that when you have a blackboard where you can draw the model, but you need to understand that when you have a flame spread and you have heat transfer to your solid fuel, the fuel is not an infinitely thin piece of material.

00:30:44.932 --> 00:30:49.262
Technically it could be, but that's a different case, a different regime.

00:30:49.262 --> 00:30:53.422
Let's say you have a heat transfer from your surface downwards to the fuel.

00:30:53.422 --> 00:30:58.455
You're gonna have fuels that's transferring that heat very well into its depths.

00:30:58.455 --> 00:31:08.282
Like if you had a slab of copper, it would be beautiful uniform temperature over a large bulk of the material.

00:31:08.282 --> 00:31:15.076
If you have a foam material, you suddenly have extremely hot surface and not that much heat transfer into it.

00:31:15.136 --> 00:31:17.801
So because you don't have as much think of the energy balance.

00:31:17.801 --> 00:31:19.265
As you know, there's a loss term.

00:31:19.265 --> 00:31:25.903
If you, like you said, you have a slab of copper, all of a sudden whatever heat you're receiving at the surface is just being conducted into the depth of the solid.

00:31:25.903 --> 00:31:37.515
But if you have something that is like a foam right, all of a sudden that term in your energy balance changes dramatically, and so then you have energy available to go to pyrolysis.

00:31:37.515 --> 00:31:42.775
That term you're making fun of calling it a balance, but you go back to the balance there, yeah, and if you remove that loss term, then all of a sudden what happens to your flame spread rate?

00:31:42.775 --> 00:31:57.471
You have more heat being provided for pyrolysis, your flame spread rate adjusts, and so I think actually, yes, all of these practical examples can be linked back to this sort of fundamental idea of looking at you know what are the processes that control okay, now it's kind of funny because it's it's.

00:31:57.612 --> 00:32:06.621
It strikes me now that you know the way how many people would refer to the fire phenomenon and it triggers me a lot like a media always focus on temperature.

00:32:06.621 --> 00:32:10.198
You know, the temperature of fire was so vicious the firefighters could not get in.

00:32:10.198 --> 00:32:14.134
The temperature of the flame was so high like temperature of the flame, come on it's.

00:32:14.134 --> 00:32:16.520
It's like it's like almost always the same.

00:32:16.520 --> 00:32:18.243
Go read forman williams or something.

00:32:18.243 --> 00:32:34.089
But anyway, in here actually presenting this in a very naive or or physically incorrect but lay understandable way, that this, this leads to more temperature on the surface and this leads to less temperature in in the surface.

00:32:34.089 --> 00:32:42.232
You know, having foam material with more temperature and bulk material with less temperature is actually a way to describe that difference, right?

00:32:42.934 --> 00:32:44.577
but it comes down to less about.

00:32:44.577 --> 00:32:45.440
I mean, yes, the temperature.

00:32:45.440 --> 00:32:48.335
The temperature is just a manifestation of the heat transfer, right?

00:32:48.335 --> 00:33:03.744
So if you link it back to the idea that it really was being regulated or controlled when you have something like different solids is how much heat transfer is occurring, how much heat is staying occurring, how much heat is staying in the solid, how much heat is being conducted further, how much heat is being sent ahead of the flame front through either the gas phase or the solid phase?

00:33:03.744 --> 00:33:17.125
It all starts to change the process, right, and I should say too, when we're talking about it, doesn't matter whether we're talking about concurrent flame spread or opposable flame spread, right, if we're talking about heat transfer, right, right, there's two major pathways for this process to occur.

00:33:17.125 --> 00:33:20.906
Right, okay, you have gas phase heat transfer and you have solid phase heat transfer.

00:33:21.307 --> 00:33:24.749
Imagine, let's go back to our matchstick example, right, and let's focus on you.

00:33:24.749 --> 00:33:28.930
Just the opposed flow start, right, but you have this flame sort of slowly creeping down this matchstick.

00:33:28.930 --> 00:33:35.317
Right now, the flame itself is going to be transferring heat both through the gas phase and through the matchstick itself.

00:33:35.317 --> 00:33:37.925
So there's going to be you know well, the matchsticks probably may be a bad example.

00:33:37.925 --> 00:33:55.502
Uh, it's gonna be thin, solid, but if you look at more like practical solid fuels right, like large slats of pma, right, there's going to be a thermal gradient through the solid right and then that thermal gradient is going to result in conduction through the solid ahead of the plant and what's dominant in in that case, when the matchstick is pointing upwards?

00:33:55.563 --> 00:33:56.444
Is it the gas phase?

00:33:56.444 --> 00:33:57.515
Is it the solid phase?

00:33:58.230 --> 00:34:13.474
Now when we're looking at matchsticks, right, that's a slightly different context than most practical fuels, because we're looking at thin fuels, right, and thin fuels you can read about this in the works by, you know, fernandes-payer and all the grades of the 80s and so there's little to no thermal gradient through a thin, solid right.

00:34:13.474 --> 00:34:16.737
So because of that, the gas phase processes start to dominate the problem.

00:34:16.737 --> 00:34:21.202
And so in the limiting cases, right under thin fuels, gas phase processes dominate.

00:34:21.202 --> 00:34:31.253
Now if we get to thicker fuels, practical fuels, practical quotes, you know what I mean or the typical fuels we're used to, then it's not so trivial.

00:34:31.253 --> 00:34:32.295
It actually gets really interesting, right.

00:34:32.315 --> 00:34:33.356
So it depends on a lot of things.

00:34:33.356 --> 00:34:35.579
It depends are we looking at opposed flow versus concurrent flow?

00:34:35.579 --> 00:34:38.244
Are we looking at flows under forced flow conditions?

00:34:38.244 --> 00:34:40.818
I mean flame spread under forced flow conditions?

00:34:40.818 --> 00:34:43.739
Are we looking at flame spread under external heating conditions?

00:34:43.739 --> 00:35:04.684
All of these variables, right, affect that feedback loop, and so then the balance of how much the solid versus the gas phase plays a role starts to change, right, and that's actually the balance of how much gas phase heat occurs and solid ac transfer occurs has been kind of the entire focus of my phd, okay, and so if we look at let's say, focus a little bit on.

00:35:04.704 --> 00:35:05.427
Concurrent spread occurred means the.

00:35:05.427 --> 00:35:10.539
The flame is touching your fuel and we're spreading the same way as the flame is pointing right.

00:35:10.679 --> 00:35:16.621
Yes, you ignite as loud as the mma and the flames are upwards right and the flame is spreading up the surface right.

00:35:17.070 --> 00:35:33.873
Now if you think about this in sort of in this context right Again, when we said all of the hot gases in the flame were sort of moving in the direction of the flame spread and so if you think about heat transfer processes that occur, this process is largely dominated by heat transfer in the gas phase, particularly radiation from this flame, right.

00:35:33.873 --> 00:35:50.579
So if you can imagine the flame sheet over the surface of this fuel, that's pyrolyzing radiation from the flame plays a large role in this and you can see this in the works like there's a great PhD by Isaac Leventon who looked into upward flame spread over various pollen fuels and this measure of radiation at the surface.

00:35:50.579 --> 00:35:52.103
That's really interesting work.

00:35:52.103 --> 00:36:09.157
You can also sort of see this Now the question of whether or not gas phase or solid phase is dominant in this process for concurrent flame spread can also be demonstrated by saying what happens if we replace a slab of pma with discrete little bits of pma kind of stacked on top of each other with a little air gap.

00:36:09.157 --> 00:36:30.960
So you imagine like instead of taking a continuous piece of pma, you add little air gaps and you say basically now I have no mechanism for solid phase conduction through the solid, so let's just look at, you know, gas phase heat transfer and you actually get similar orders of magnitude of flame spread, meaning that in this context heat transfer to solid, for you know, these sort of homogenous, continuous solids, might not be as important.

00:36:30.960 --> 00:36:39.438
And so the gas phase process sort of seems to run the show here and there's lots of studies that have sort of looked into this sort of idea.

00:36:39.438 --> 00:36:40.463
But this also should be heavily caveated with.

00:36:40.463 --> 00:36:45.701
All of these are looking at, you know, very nice slabs of non-melting PMMA, non-charring materials.

00:36:45.829 --> 00:36:57.922
But the idea is, when your plant spread is moving in the direction of induced flow, right, gas phase process, the process at least of gas phase heat transfer, begins to play a significant role in how are we getting heat transfer ahead of the solid.

00:36:57.922 --> 00:37:03.925
But that story sort of starts to change when we look at opposed flow of flames, because this isn't as simple right now.

00:37:03.925 --> 00:37:12.300
If we're sort of looking at, let's say, downward flame spread, we know for upward flame spread you have this really nice orange, sooting, luminous flame, right.

00:37:12.300 --> 00:37:24.541
But for, for anyone who's ever seen like a nice downward flame spread experiment, right again, if you just light a match and watch the leading edge for something like a downward flame spread experiment is a sort of very nice blue flame, right.

00:37:24.702 --> 00:37:26.474
It's just something that is very non-soothing.

00:37:26.474 --> 00:37:28.121
It's, uh, right at the leading edge.

00:37:28.121 --> 00:37:30.269
It's very sort of gorgeous blue right.

00:37:30.269 --> 00:37:36.278
It's very different if you think about the flame in terms of its interaction with the solid right at that leading edge.

00:37:36.278 --> 00:37:39.985
That's very different than a big sort of turbulent sooting flame.

00:37:39.985 --> 00:37:45.661
So for down flame spread, we have to think about how is that flame then transferring heat ahead of the flame front?

00:37:45.661 --> 00:37:46.001
What does that?

00:37:46.041 --> 00:37:46.342
look like.

00:37:46.342 --> 00:37:50.041
I guess because it's blue, non-sooty, the radiation must be very low.

00:37:50.041 --> 00:37:51.554
So I actually have no idea.

00:37:51.554 --> 00:37:54.717
Like there's no convection because it's away.

00:37:54.958 --> 00:37:57.757
There you go, so that actually have no idea, like there's no convection because it's it's away.

00:37:57.757 --> 00:37:58.884
There you go, so those, exactly that's exactly the point.

00:37:58.884 --> 00:37:59.547
That is really interesting, right.

00:37:59.547 --> 00:38:04.652
And so for the opposed flow case, in terms of the physics driving the problem, our natural intuition as engineers kind of goes out the window.

00:38:04.652 --> 00:38:10.438
Right, because if the flow is going the other way, we can't treat it like convection in the traditional sense that we normally do.

00:38:10.438 --> 00:38:19.706
Heat transfer coefficient if it's a nice, not sitting flame, radiation still exists, but it's probably negligible compared to other heat transfer coefficient if it's a nice, not sitting flame, radiation still exists, but it's probably negligible compared to other heat transfer mechanisms.

00:38:19.706 --> 00:38:28.036
And so you know, the intuition for the opposed flow problem is very different than looking at upward flame spread, you're left with conduction, but in the gas phase right correct, exactly.

00:38:28.117 --> 00:38:33.059
So, in terms of the gas phase, this is another cool thing you can do at home light a match and sort of stare really closely at it.

00:38:33.059 --> 00:39:00.438
You'll see that at the leading edge of the flame you have an actual gap between the fuel surface and the flame itself, right, and so the process that controls the gas phase, heat transfer, uh, for something like downward flame spread is the conduction across that standoff distance, right diffusion, basically from the flame to the salt, which is something that it's kind of a sort of a very abstract concept it's super counterintuitive, because you would not think about conduction in gas phase that much.

00:39:00.980 --> 00:39:02.911
However, on the other side, it's.

00:39:02.911 --> 00:39:09.608
We're actually talking about length scales that are tiny, like nanometer scale.

00:39:09.608 --> 00:39:12.454
We're talking about quantum fire physics.

00:39:12.454 --> 00:39:13.056
It's.

00:39:13.056 --> 00:39:13.657
It's.

00:39:13.657 --> 00:39:20.358
It's not the scales that you would be used to in in which the, the convective heat transfer, buoyancy and stuff like that would dominate.

00:39:20.579 --> 00:39:26.329
Yeah, yes, true, and, and I mean the, the length scales of the standoff distances is more on the scale of millimeters.

00:39:26.329 --> 00:39:31.226
Right, it's very, very small, but it's not, you know, it's not impossibly unimaginable.

00:39:31.226 --> 00:39:36.318
You can observe it with your eyes, which is which is kind of cool in the context of seeing these experiments, right.

00:39:36.318 --> 00:39:46.277
But I think, I mean, I love that example of just like, let's take a step back and look at concurrent flame spread upward, flame spread over slough of pma, right, we know that, you know heat transfer is the star of the show.

00:39:46.661 --> 00:39:48.230
How is heat transfer getting ahead of the flame front?

00:39:48.230 --> 00:39:54.972
Right, you see, it's driven by gas phase processes, by, like radiation and convection and those kinds of heat transfer, primarily radiation.

00:39:54.972 --> 00:40:00.532
But then you look at the downward problem, right, and that main mechanism radiation goes out the window.

00:40:00.532 --> 00:40:03.278
So now, how the heck is this process happening?

00:40:03.278 --> 00:40:07.722
And so in the gas phase, it's dominated by this process of gas phase conduction, right?

00:40:07.722 --> 00:40:21.204
But equally, if you actually look at the solid phase and this is something that you know, we spent a lot of time measuring if you actually resolve the temperature profiles in the solid, you'll also see that there is a substantial amount of heat transfer going through the solid itself.

00:40:21.829 --> 00:40:27.943
Now, these thermal gradients don't exist for most upward cases, because the gas basis is basically dominating the process.

00:40:27.943 --> 00:40:30.217
You don't see as much conduction through the solid.

00:40:30.217 --> 00:40:33.391
However, for downward flame spread, you know, and we published this in a paper in Fire Safety Journal.

00:40:33.391 --> 00:40:50.818
However, for downward flame spread, you know, and we published this in a paper in Fire Safety Journal what's really interesting is if you actually start to look at the balance between how much heat transfer is being received at the surface from the gas phase versus how much heat transfer is being conducted through the solid, the order of magnitude they start to line up, not even order of magnitude in terms of total value.

00:40:50.818 --> 00:40:56.000
They start to more or less balance each other out for downward flame spread in particular.

00:40:56.000 --> 00:41:00.001
And so, in terms of wrapping your mind around, where is this heat coming from?

00:41:00.001 --> 00:41:06.463
There is heat transfer going through the solvent itself, ahead of the flame front, but also transferring through the gas phase as well.

00:41:07.204 --> 00:41:11.585
Okay, you've defined the control regimes as opposed and concurrent.

00:41:11.585 --> 00:41:16.827
What if you have a horizontal fuel like neither upwards neither downwards?

00:41:20.909 --> 00:41:22.076
Excellent, that's a great question.

00:41:22.076 --> 00:41:23.643
Yes, everything's been defined so far as upward and downwards, right.

00:41:23.643 --> 00:41:32.653
But the process of calling something a post-flow flame spread is truly defined by saying if you zoom in really closely to the leading edge of the flame, it's really easy to see for downward flame spread.

00:41:32.653 --> 00:41:44.282
But if you zoom into the leading edge right, your flame is moving in one direction and then the entrained air that's sort of reaching that leading edge, has to be moving in the opposite direction, right.

00:41:44.282 --> 00:41:49.490
So if you think about it from that reference frame, there's actually a lot of different kinds of opposed flow flame spread right.

00:41:49.490 --> 00:41:53.463
Horizontal spread over, say, like a surface, like a spread over a table right, that's also opposed flow.

00:41:53.463 --> 00:41:58.077
Lateral spread over a wall right, that's also a post-flow lateral spread over a wall.

00:41:58.077 --> 00:42:08.735
So if you ignite a wall and it spreads laterally, so sideways, as studied by you know conteria quite a bit, the development of you know test standards like the lift, um, that's also a post-flow flame spread.

00:42:08.735 --> 00:42:14.257
Now what's really interesting is is those are, you know, in the category of post-flow flame spread.

00:42:14.257 --> 00:42:17.556
But that's actually something that I've studied quite a bit over my phd.

00:42:18.036 --> 00:42:22.815
The heat transfer mechanisms start to again change though, between, even though these are all post-flow flame spread.

00:42:23.717 --> 00:42:34.139
I mean, just think about that in your mind's eye, right, how the leading edge of a downward flame spread, uh sort of problem is this nice blue leading edge, but imagine, you know, a horizontal flame, right?

00:42:34.239 --> 00:42:40.059
This is basically, like you know, pyramid of it's no longer the same sort of boundary condition, right?

00:42:40.059 --> 00:42:42.994
All of a sudden, things like radiation start to come back into the picture.

00:42:42.994 --> 00:42:52.101
For the lateral condition, right, you have a very luminous boy wall flame, and so you start to get a little bit of radiation at the leading edge as well, right, something that you didn't see for the downward case.

00:42:52.101 --> 00:43:18.889

160 - Fire Fundamentals pt 10 - Flame Spread with David Morrisset

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