Fire Science Show
Fire Science Show
218 - Fire decay and cooling phases with Andrea Lucherini
What happens when the flames die down? It's a question rarely addressed in fire engineering, yet the decay and cooling phases of fires can be more dangerous than peak fire conditions. In this deep-dive conversation with Dr. Andrea Lucherini from Frisbee at ZAG in Slovenia, we uncover why these overlooked phases matter profoundly for structural safety.
Most engineers focus on protecting structures during the fully developed fire phase, but as Dr. Lucherini reveals, catastrophic failures can actually occur during cooling. We discuss a tragic case where seven firefighters died when a concrete structure collapsed, not during the fire's peak, but while they were extinguishing what appeared to be a dying fire. This sobering reality highlights how current testing methods fail to capture real-world risks—standard fire curves never decrease, creating a dangerous blind spot in our understanding.
The physics of cooling creates unique challenges for different building materials. Reinforced concrete might reach maximum temperatures in the steel reinforcement during decay rather than during peak fire. Steel structures face destructive tensile forces during contraction that can exceed the compressive forces experienced during heating. Mass timber presents particularly complex behaviour that may never truly enter a cooling phase without proper design considerations.
Perhaps most fascinating is how thermal boundary conditions transform as fires decay. When dense smoke thins, radiation patterns change dramatically, creating heat transfer scenarios that standard models fail to capture. These insights aren't just academic—they're essential for performance-based engineering approaches that prioritise realistic structural behaviour throughout a fire's entire timeline.
Andrea was kind enough to share these papers with me:
- Defining the fire decay and the cooling phase of post-flashover compartment fires: https://doi.org/10.1016/j.firesaf.2023.103965
- Thermal characterisation of the cooling phase of post-flashover compartment fires: https://doi.org/10.1016/j.ijthermalsci.2024.108933
- More information about FRISSBE project and team: https://www.frissbe.eu/
And I can shamelessly plug one of our own: https://onlinelibrary.wiley.com/doi/abs/10.1002/fam.2735
Cover image from experiments with Danny Hopkin that we have discussed here: https://www.firescienceshow.com/172-lessons-from-mass-timber-experiments-with-danny-hopkin/
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Hello everybody, welcome to the Fire Science Show. In today's episode we're diving a little bit into structural fire engineering and, thankfully, a bit of compartment fire dynamics along the structural fire engineering, so I feel a little bit more comfortable in there. I have invited Dr Andrea Lucherini from Frisbee at ZAG in Slovenia to discuss some research of his that was presented a few years ago at IAFSS in Japan and some other papers followed, and namely that research touches the decay and cooling phases of fires. I found this a very interesting, intriguing discussion. You perhaps remember some episodes ago I had an episode with Thomas Jeunet and Professor Zephus on failure of timber columns in the cooling phase. Today we talk more generally about what cooling and decay phases of fires are, and it's an interesting concept. If you use fire resistance as a proxy for your structural fire safety, you probably don't even need them. If you over design your structure, there's a good chance those considerations are not important to you. But if you're doing anything related to performance based engineering of the structural fire safety, the decay phase is something that you most likely need to account for, Because it's not necessarily the peak heat release of the fire when your structure is most thermally stressed, if I am allowed to say that In this episode we discuss how to define those decay and cooling phases, which on its own is quite a challenge.
Wojciech Wegrzynski:We discuss what happens at different types of structure reinforced concrete, steel, timber In the cooling phase. We talk about some physics, how the heat transfers through the structure, and we also discuss the challenges with setting up the correct boundary conditions for those phases, because the boundary conditions we use for fully developed fires may not really be the ones that you would like to use in the decay phase. So a lot of physics, a lot of structural fire engineering, but, given in a very approachable way, I've enjoyed it. I don't know much about structural fire engineering, so I am sure that you'll also like and enjoy that. And if you know a lot about structural fire engineering, you will definitely enjoy it. I mean, if you like fires and that's your part of the world of fire, you will enjoy it. Anyway, that's way more talking than did it. Let's spin the intro and jump into the episode. Welcome to the Firesize Show. My name is Wojciech Wigrzynski and I will be your host.
Wojciech Wegrzynski:The FireSense Show is into its third year of continued support from its sponsor, OFR Consultants, who are an independent, multi-award-winning fire engineering consultancy with a reputation for delivering innovative safety-driven solutions. As the UK-leading independent fire risk consultancy, OFR's globally established team have developed a reputation for pre-eminent fire engineering expertise, with colleagues working across the world to help protect people, property and the plant. Established in the UK in 2016 as a startup business by two highly experienced fire engineering consultants, the business continues to grow at a phenomenal rate, with offices across the country in eight locations, from Edinburgh to Bath, and plans for future expansions. If you're keen to find out more or join OFR Consultants during this exciting period of growth, visit their website at ofrconsultantscom. And now back to the episode. Hello everybody. I am joined today by Andrea Luccherini from Frisbee at Zag in Slovenia. Hello Andrea. Hello Wojciech, Nice to see you.
Wojciech Wegrzynski:Nice to see you again. We've just seen each other in Slovenia at the European Symposium for Fire Safety Science. Are you alive after it? That was intense for the organizers. I can only imagine.
Andrea Lucherini:That was very intense indeed. It took a little bit of time to prepare it because we started last year after the fourth edition. The fourth edition last year was in Barcelona, in Spain, and you know after that we started preparing the fifth one. Last week we had it. There were three very intense days plus a pre-day for heavy career researchers, but I don't know. You told me everything went smoothly, we were happy, everything went well.
Wojciech Wegrzynski:So I have not seen a single mess up really, so that's like that's excellent and I've enjoyed it thoroughly. Like you know it all, this ESFS conference always has this nice atmosphere of very like early career researcher oriented but I don't mean like first year PhD students, I mean like everyone between their start and seniority. It's just like a nice place you know, to stand up on the stage, give your presentation, get constructive feedback, get some experience with that, especially that we're like IFSS submissions in two weeks Exactly. It's a good moment to get your final, final feedback on your ideas, really right.
Andrea Lucherini:No, I think it's the ESFSS, the European Symposium for City Science. They found with time a good recipe of, you know, having two and a half days of conference with a single track, as you said, mainly with every career, researchers, phd postdocs talking a bit of a bit, a big range of topics related to first city science and, you know, keep it on a decent and manageable numbers. So usually we talk about, you know, 150, 200 people. So what I'd heard so far, and myself since I was a phd student, is always a nice, pleasant conference.
Wojciech Wegrzynski:People live happy I would recommend this uh to to everyone. The next one is not next year.
Andrea Lucherini:Next year we have ifss part here, so 2027 so next year is international symposium for psychic science, which will be in june in france, and the idea of the european symposium will be every year but the year of the international symp. The European Symposium will be every year but the year of the International Symposium. So the next one will be in 2027, I think around the same period, so September, and will be in Prague, in Czech.
Wojciech Wegrzynski:Republic. Very nice, very lovely Looking forward to that and also excellent keynote and they will end up in the Farcense show eventually, sooner than later.
Andrea Lucherini:You have to grab them.
Wojciech Wegrzynski:It already has been discussed, it's already in the making, but anyway, I have not invited you to talk about conferences. We can spend a lot of time discussing conference experiences, but there are more interesting things in the realm of fire science than our troubles. I always wanted to talk with you about the decay phase of fires. You know the stuff that happens when the fire dies and and it does not magically disappear in our world. You are a structural engineer. I think it's a very interesting topic from the perspective of structural engineering and, in general, how fire safety engineering in terms of structures is performed worldwide.
Andrea Lucherini:So you already agreed to talk to K-Face, so first let's, like you, maybe tell me why, from your perspective, it's worth discussing what importance that carries for fire safety engineers, I mean the whole motivation behind my research that started, you know, more or less after my PhD about this topic, in the fact that I've seen over the years there has been more and more papers published and presentation related to fire decay, cooling, fire dynamics, related to, you know, the fire decay and cooling phase of compartment fires.
Andrea Lucherini:And what you see is that there are more and more publications.
Andrea Lucherini:But what I found, at least from my side, that I usually see it's between you know the solids or the structure, the compartment boundaries, and the fire dynamics, so the gases, the flames and the hot temperatures, so how these two aspects they work together.
Andrea Lucherini:Because what you see is that there is definitely, and there was definitely and there is still on some aspects, an inconsistency in why and how we treat these aspects, because usually there is confusion between you know what is the fire decay, what is cooling, how do you apply a boundary condition to a structure goes into discussion of okay, if you're trying to do a performance-based design once you have completed the fully developed phase of the fire, what happens afterwards. And people have also seen you have people in the past in podcasts discussing, for example, about different columns suffering this problem in different materials. So there was the whole motivation trying to shed some light, to try to give some definitions related to this aspect, more on the fire dynamics point of view, that of course apply to solid, so structural, materials, but as well as compartment boundaries. So the same concept even applied to a compartmentation wall, a door, if you want, want and so on.
Wojciech Wegrzynski:I appreciate your touching into compartment fire dynamics. I am anything but structural fire engineer, so that's probably the least comfortable part of fire science for me to talk about. But I'm very happy to talk about compartment fire physics, so you gave me some area of familiarity in there. Maybe a rough question, but do we have a history of building failures in decay cooling phase that you can think of or like? Is this problem really relevant for real-world fires?
Andrea Lucherini:You know, what you see is that for these big tragedies and now today we are talking on the 11th of September, which is a quite hot day the idea is that you know you usually don't have lots of tragedies, but usually in first-party science, the structure for engineering, you have a few. That usually creates a lot of movement, questions, and you know they challenge the status quo of what happened, why it happened. And this is the case, for example, for the failures or structural failures or tragedies during the fire, decay or cooling. And there have been a few examples outside. You know the research world, but the most famous one was the one in Switzerland.
Andrea Lucherini:There was I don't remember exactly the year, but it was about 15 years ago where there was an underground car park oh yeah, made of reinforced concrete, where mainly there was a fire in the underground car park oh yeah, made of reinforced concrete, where mainly there was a fire in, again, their ground car park. The firefighters get in, they start fighting the fire and then when you know they were extinguishing the fire, they were cooling down the structure. Mainly the fire looked extinguished, so they, you know, can be in danger and you think that it was part of the past. Then the whole structure collapsed with having, if I'm not mistaken, seven firefighters trapped and killed in the underground structure that collapsed on Danang. So this is an example of you know, possibly the danger and the problems are not always in the big flames and the big smoke and in very high temperatures. So you have to think that sometimes you can have a local failure that can trigger different failures that can possibly hand up to something very serious.
Wojciech Wegrzynski:Yeah, I remember the case study. We're dealing a lot with car parks, so that case study is brought a lot because it's also one of very few real-world incidents where actually we had fatalities in a car park fire and that one triggered, if I'm not wrong, a lot of development in the world of steel, structural steel and fire protection of steel. But regardless, I also think you know a lot of time when we think about fire safety, engineering is, how do we protect our structures from the fire? So your first goal is that structure has to survive the fire. Fire resistance framework, uh, being proxy of that the best example, but I I think it's it's the cooling phase and being able to uh, you know, account for the stuff that's happening back then it's. It's also pretty important when you have to fix the structure after fire. You know like you had a fire somewhere and now how badly it was for the structure and unless you capture the whole image till the very end, when it's ambient, you probably cannot judge how. I'm not sure how relevant that is.
Andrea Lucherini:You have to think I mean something that I always tell students when I teach this part is that you have to remember that. You know, in order to counteract the threat of fires so means mainly usually related to thermal effects so a structure that possibly start heating up and start losing strength and stiffness or start having local or global failures. You know, we usually use low thermal inertia materials, somehow thermal insulation, and this works very well when the fire is big and we have the fully developed face. But you don't have to forget that the same material works in the opposite way when you're trying to cool down the structure. Okay, so sometimes and this is something we can come back to later the fact that if you have an insulation material that insulates really well during the fully developed phase of the fire, also it doesn't enable fast cooling during the fire decay and cooling because the whole effect is a sort of lagged, everything is delayed. So possibly this is the whole concept, that if you have something that heats up very slowly, possibly all the effects are seen at a later stage, not when you have the big flames but, for example, when you have a bit later than what would happen in the fire.
Andrea Lucherini:This is the case of, again, reinforced concrete structures.
Andrea Lucherini:For example, reinforced concrete structures, they have the magic concrete cover that they're always trying to protect the inside of steel reinforcement from the effect of the fire.
Andrea Lucherini:So you try to have a very thick concrete cover because, in a way, concrete is relatively good for elevated temperatures but steel is not.
Andrea Lucherini:So what you're trying to do do you're trying to create a thermal barrier for a steel reinforcement not to heat up, but we know the concrete intention is not that good.
Andrea Lucherini:So as soon as the tensile reinforcement, the concrete heat up, then loses capacity and you can imagine, since concrete is very high density yeah, so it's a high thermal inertia the whole effect on the bars, which usually sits a bit in, is not seen when the fire is the biggest. It's actually seen a bit later than that because the energy takes some time to actually go through the concrete and reach the steel. So this is one of the examples of some of the research that is going on this topic that focuses on really the delayed failure or the delayed critical situation in some low thermal inertia materials where you have reinforced concrete structure, where the maximum temperature and the steel reinforcement is actually achieved a bit later, so during the fire decay, during the cooling, rather than during the fully developed fire. And this happens the same for possibly, you know, steel structures protected with insulation material. Okay, because the energy has to penetrate.
Wojciech Wegrzynski:What changes if you have a low-density insulation material like mineral wool? It will not accumulate that much heat, but still it will make it very difficult for heat to penetrate.
Andrea Lucherini:Yeah, I mean you have to think also, you know what is the ratio and all of that. But the lower the thermal inertia material, the faster sort of response to what affects in the fire. So the idea is that you know if you protect the best. The substrate structure is always the best, because we don't have to forget that the heat fluxes that you achieve during the fully developed phase of the fire are the highest and sometimes depending we can discuss about it later during fire decay and cooling, this heat flux can be negligible compared to what we actually achieve during the fully developed phase when we have flashover, high temperature and smoke burning and all of this.
Wojciech Wegrzynski:Are decay cooling phase considerations relevant for all types of structures, or there are types of structures for which it's uh, yeah, definitely definitely.
Andrea Lucherini:You know. I mentioned to you the reinforced concrete structures and I mentioned the key importance of this concrete cover and the possible delayed highest temperature in the steel reinforcements or the tensile reinforcement. This thing happens in steel structures, for example, when they are highly and heavily protected. But for example in steel structures what you see also for example the Cardington fire test in the 90s that you see that after the full phase of the fire, let's say the high temperatures, steel particularly suffers from large thermal deformations. What happens when the materials start cooling down and start trying to take the original shape and the original length in particular, but now it's deformed. That what happens?
Andrea Lucherini:Mainly you start having very big forces at the connections. So possibly you know, at the connection, during the fully developed phase of the fire the steel element is trying to expand, so it's pushing sort of the connection to receive compressive stresses. But then imagine when it's cooling down the depth force changes because now the element is trying to go back to the original dimension. So now the connections bolted well that they could have big thin cell forces. This thin cell force is big when it's trying to contract, but it's even bigger when you have a deformed shape and a force applied on it that you have some dead ends.
Wojciech Wegrzynski:So that is the case, and for that you don't really need to go to like 500 degrees temperature to create some changes in the steel. This can happen already when you heat it up to a few hundred degrees, because it's just thermal response expansion and compression right, exactly.
Andrea Lucherini:And you could have a very big concentrated force at the specific ball. So the part of the connection that can trigger some failure and then you know, to cover the last big construction material is timber and we can discuss later about fire decay cooling construction and everything.
Andrea Lucherini:But the main topic related to timber. Timber loses mechanical properties, so strength and stiffness, at relatively low temperatures. So we know that by the time that still you know, loses you know strength and stiffness. So we usually say you know 500, 600 degrees as critical temperatures. Concrete, we usually say you know we typically the compressive strength is okay below 500 degrees In timber. We know that by 300 degrees, which is our assumed charring temperature, the strength and stiffness of timber is zero. So we have a very steep reduction of mechanical properties at relatively low temperatures. Relatively means relatively low temperature that we usually see achieving compartment fires. This becomes relevant because everything you know, the fully developed phase, but also the fire decay and cooling that has a big effect on the structure.
Wojciech Wegrzynski:Yeah, thank you. I think this is a very good summary of the response of materials to these decay phases. Perhaps we should clean up the phases of fires as well, if you would like to talk about that in the episode.
Andrea Lucherini:Traditionally, we usually have the growth phase, the fully developed phase, and then we usually discuss about fire, decay and cooling in a mixed manner. I tried to break the two apart and I think we can go through them one by one. What are the relevant parts of that?
Wojciech Wegrzynski:And of course let's frame it into the thermal boundaries in the compartment and the fire dynamics happening in between the structure and the fire itself. So let's go gold phase.
Andrea Lucherini:So the whole concept of first disclaimer before discussing this. I think and I'm talking about these phases as a typical compartment fire framework where we have a compartment of relatively small dimensions where we can assume some sort of generalized flashover over the compartment, because as soon as you have a huge compartment then we can start talking about traveling fire, growing fires and localized flashover and all these things, with this disclaimer if we take a small compartment, which is a typical design framework for any compartment in typical buildings. So we start with the growth phase. The growth phase, you know you do a lot of safety calculations. What we usually use are apothecary squares or some sort of like curves of heat release rate of the fire that they know as a function of a quadratic function or exponential, something like that.
Andrea Lucherini:But in general you have the fire, that you have ignition, the fire start growing. But usually you can have typically two zone, two zones within the compartment. So we have the cold zone and the hot zone, where in the cold zone we have an ambient temperature. In the hot zone we usually have a smoke layer that is building up and usually we assume that the smoke layer is not very dense yet. So we can discuss about optical thickness. But in general, you know we have a smoke layer that is building up, is collecting within the.
Andrea Lucherini:You know the volume of the compartment and typically we know that the curve of the temperature evolution of smoke layer is related to the heat risk rate of the compartment and typically we know that the curve of the, the temperature evolution of smoke layer, is related to the heat risk rate of the fire. So the faster the fire grows, the faster the smoke layer heats up. But then for related to the temperature conditions, usually we see, you see most of the for structural calculations and the growth phase is usually disregarded because the heat fluxes are relatively negligible unless you have an impinging flame on a very low soffit or your ceiling. That you can consider that. But the actual big heat fluxes that started happening in the fluid phase.
Wojciech Wegrzynski:Yeah, let's move there. If you have sufficient amount of heat in your structure, sufficient amount of radiation, you encounter flashover. I guess our friends fire safety engineers know what it is. Well, feel free to summarize the flashover phase and the fully developed phase. How does it work?
Andrea Lucherini:certain point the fire would lead to a fully developed phase.
Andrea Lucherini:Means is fully developed in terms of it tends to a steady state condition in terms of release of heat, because the heat release rate will be controlled by two different aspects it can be controlled by the ventilation, so the opening, so the amount of oxygen that actually is able to enter the compartment, or the fuel itself.
Andrea Lucherini:So if the fuel is somehow limited in an area or from geometry, then of course can be limited. But then if we think about the most critical condition that you usually look at as a critical case, which is the fully developed ventilation control fire, then we can assume that we have a spill plume, so the compartment doesn't have enough oxygen to fill the fire. So what happens if the pyrolysis gases, the combustible gases, they come out of the opening trying to reach fresh oxygen in order to fill the fire? And what we typically see? We see the spill plumes, so these flames that come out of windows or doors where we have an environment full of oxygen, so usually unlit conditions what was the heat, what's the dominant heat transfer mode and and what is the structure exposed to?
Andrea Lucherini:I mean you have to think that, since the heat release rate becomes very large. So what happens is the compartment quickly collects all the smoke and fills up with smoke, where the smoke is usually very dense, so we say that it's optic and thick. So this means that if, for example, we look at the ceiling or we look at the beam at the ceiling, what the beam is seeing is seeing only the smoke around it. So when you're trying to characterize the boundary condition of the thermal boundary condition to the structural element, you usually consider a convective term and a radiative term that is directly connected to the evolution of the gas phase. Yeah, so since the smoke is very dense and very dark, very black, so the energy, so both the radiation and convection is coming from the smoke. So the energy, so both the radiation and convection, is coming from the smoke. And here you achieve, of course, very high heat fluxes, which can go up to very high values 100 to 100 kilowatts per square meter, where actually the structure is very challenged at this stage.
Wojciech Wegrzynski:But in this phase the structure is not yet at the gas temperature. It's a continuous like. It's a sort of thermal equilibrium in terms of continuous heat transfer from the flame to the structure, to the deep parts of the structure.
Andrea Lucherini:You know right, I mean that for sure. You know you have to think always on an energy balance, because what I was focusing now is the energy that comes from the fire and goes into the solid. Then what happens to the solid? You know you could have material that takes a lot of time to heat up, like you know, a thick steel beam that has a very high inertia, because steel is well known as a very, very high density. So it takes a lot of time to actually heat up.
Andrea Lucherini:Of course, depending on the boundary condition, depending on the shape, depending on many things. But also you have to think that possibly the energy can also go somewhere else, can be conducted somewhere else, or if you have a thin wall, then the wall has a lot of energy coming from one side, but then it's cooling down from the other side. Then you have also losses from the other side. So it's an energy balance and there is a lot of fancy software and calculation tools to actually see when the energy comes from the fire, where it goes within the structure and also how the structure is challenged I always like it was fascinated by observing those surface thermocouples and in-depth thermocouples during my fire resistance tests.
Wojciech Wegrzynski:You know it's always like interesting to see how different things happen at different times and sometimes how long it takes for a structure to heat up, and sometimes like it's seconds, and then already you're getting the full power of fire in somewhere where you don't really want it to be. But okay, those are like the first phases.
Andrea Lucherini:The fire was fully developed and the main concept I want to say is that for these two phases, if you characterize well the smoke layer, then you get a quite good quantification of the thermal boundary conditions.
Wojciech Wegrzynski:That the structure will be exposed to. Yes, exactly, and, like you could say, it's fire safety engineers' bread and butter. This is what we do normally in the design, with the caveat proxy of fire resistance testing, which we'll come back to, Because basically those are the phases that fire. Maybe this is the moment to talk about fire resistance testing, because this is pretty much it for fire resistance testing. The ISO curve never goes down, it can only go up.
Andrea Lucherini:Exactly no, but this is the full concept, that is, the standard fire framework. Yeah, you know, the idea is that you disregard the growth phase, because if you look at the standard temperature curve or any other post-wishover curve that is implemented in a furnace, you have a huge heating rate at the beginning because, mainly, you want to achieve high temperature in very fast time.
Wojciech Wegrzynski:Have you ever? You have a furnace in Zagat have you ever looked into the fuel consumption in the early phase? It's really funny because, like we've done this exercise on the furnaces, I have a paper on it. It's published. I'll shamelessly plug it in the show notes.
Wojciech Wegrzynski:So we looked into that and in the first like minute or two minutes of of a furnace test you, for example, go to like three megawatt heat release rate in the furnace and then you get to like 600, 700 degrees on your sample. You go down, you take it down like by a half at least, like to one megawatt, 1.2 megawatt, very low. So. So you start the fire by insane peak for a few minutes and then you go down. And what's even funnier, we've done an exercise some time ago with Piotr Tafiło, a colleague from Polish Fire Academy, and he'd written a reverse zone model. So you gave a time-temperature relation to the zone model and it's trying to backtrack what the heat release rate would have to be in the compartment to create that condition, what the heat release rate would have to be in the compartment to create that condition. It was funny as hell because again we got this massive peak and then it's much lower to continue with this standard curve. Standard curve is really funny.
Andrea Lucherini:Also because you have to consider that the furnace itself has an inertia. Yeah, yeah, yeah, and we are used to have these bricks-based furnaces and bricks and refectory materials as a usually very high density to be, you know, durable. But this means they have to pump in a huge amount of energy to heat them up, and heat them up very fast according to the curve, but then the whole framework of what you think about it. So, if you disregard so what you do, you do an engineering analysis or you need to engineer a simplification and say, okay, I don't care about the growth phase, so I immediately try to go into the fluid level phase, which means the time zero of my standard temperature time curve is flash over, pretty much. Flash over, yeah, exactly. And then what I focus? I focus on capturing the right duration of the fully developed fire, because this is what happens in a furnace. So you test for half an hour or one hour or two hours, or we can go to very high requirements.
Wojciech Wegrzynski:Welcome to Poland. Four hours for super tall buildings? Oh, for God's sake.
Andrea Lucherini:But the idea is what you're trying to do. You try to test the material for the fully developed phase and you know traditionally you were trying to. You created this standard temperature curve because it was representative of the worst case file you could have in a building, all of these things. But then in general, you focus on the duration in the fully developed phase and then if during that fully developed phase, none of the failure criteria are actually achieved, that'd be days.
Wojciech Wegrzynski:Yep, you're done. And now in the lab, as you say, we end the assessment at the end of the fire test, plus the uncertainty, but still a lot of things happen to the sample afterwards. The cooling phase in the furnace is also very different from the real-world cooling phase and it's actually very difficult to enforce a cooling phase on a furnace. We've actually attempted that and we also have a paper on Interflam this year about enforcing furnace to do that and we were not extremely successful because of the thermal inertia and everything you've mentioned before. So this is the reasons why we were not successful. But in the real compartment it doesn't magically disappear. Now you entered this decay phase which is the topic of this episode. So let's talk about Do you even have a sharp definition when the fully developed fire ends and when the decay phase starts? You said it's a steady-state fire. I would argue it's not a steady. I fully agree. It's more or less a steady state fire.
Andrea Lucherini:So when does it end? The short answer is it depends on the fuel, because you know, first of all, when do we assume or try to define the beginning of the fire decay? So the fire decay sits within the boundary of having the fuel that is no more able to sustain the heat release rate of the food at the bottom phase. So it starts decreasing naturally because it doesn't have enough fuel to consume until the fire becomes somehow negligible. So the heat release rate at a certain point becomes zero or almost zero. So that is the phase that I try to define as a far decay phase, because the far decay phase, we don't have to forget that. There is the phase that I try to define as a far decay phase because the far decay phase, we don't have to forget that there is a fire in that definition. So we assume there is a fire, but it's still no zero. So we have a hitter is rate which is non-negligible and it's still positive, so it's still adding energy to your compartment.
Andrea Lucherini:How this curve looks like, you know we can discuss later about models, but what I did, for example, in the paper that I published and presented in 2023 in japan and international symposium for city science, we look at the two extreme cases where we have, from one case, something that possibly decay very slowly like a wood creep, because a wood creep when you see timber. Timber is a charred material so it takes a very long time to make the heat-release rate curve to slowly decay. And what you see you often hands up, if you look at a little compartment fire test, that it often ends up as a sort of linear decay of the heat-release rate or something like that. But then if you go to the literature and you find a few cases there are not so many that instead of using a cellulosic fire they use a hydrocarbon fire like a pool fire, and I mentioned this test from India in that paper.
Andrea Lucherini:As soon as you close or as soon as the pool rounds out the fuel, the whole fire decay is super short, because at a certain point it's fully developed and as soon as the pool starts running out of fuel, then the far decay phase is very short. So naturally the far decay phase is fuel controlled. So fuel is what controls the heat release rate curve and therefore controls possibly the cooling and the mix of the hot smoke with the cold air coming from the compartment opening and so it defines, you know, typically the temperature curve of the smoke, but also defines how long it will take, because this phase can take minutes. If you have, like you know, hydrocarbon fuel that burns very fast, very quickly and runs out also very quickly, compared to a charring material like cellulosic material, like wood, that takes a lot of time to actually fully burn it and the cooling phase would that?
Wojciech Wegrzynski:I assume, with your logic, it would start at the end of the decay phase.
Andrea Lucherini:So I mean before moving to the cooling, I just wanted to highlight one aspect that, yeah, you know, similar to the growth phase the decay phase is is something comparable because again we go to a fuel control case characteristics where, in a different way, in the growth phase is heating up the smoke, instead in the decay phase is cooling down smoke. So the smoke depends on, of course, in the five hectares rate. But if you have to start thinking that the boundary conditions and the thermal boundary condition within the compartment would change, mainly because the smoke becomes less dense and becomes possibly also optically thin by the fact that the cold air mixes with the smoke.
Wojciech Wegrzynski:We absolutely need to define optically thick and optically thin. I see where you're going, I love it, but what does it mean? A smoke is optically thin versus smoke that is optically thick?
Andrea Lucherini:So the smoke is optically thick means it's very dense, so if you try to look into the smoke you see all the smoke. Instead, if the smoke is optically thin means you can see through the smoke at different degrees. Thin means you can see through the smoke at different degrees and of course it depends on the density, it depends on the distance that we are trying to see. This is something that I think is typically seen also in evacuation studies. I believe also other episodes you discuss about it. But the idea is that the smoke mixes up with clean, fresh air so it starts being less dense, less dark.
Andrea Lucherini:The beam that, for example, is at the ceiling starts seeing again some other things than the smoke within the compartment, and some other things means possibly hot linings that are still very hot because they've been heating up Some flames, because in the fire decay you can still have flames, smaller or bigger, depending on the fuel again, and you can see also.
Andrea Lucherini:You know the cold environment as well here, depending on the fuel again, and you can see also, you know the cold environment as well. Now the challenge becomes that you know, for your convective transfer to the element, for example the ceiling, the smoke layer is still a good proxy of that convective transfer coefficient. But if you want to try to solve in detail the radiation heat transfer, then becomes tricky because you have to start thinking how is your element exchanging heat by radiation with the surrounding environment? And the surrounding environments depends on view factor, the optical thickness of the smoke, but also how hot it is, because of course stephan bosman goes with the temperature to the power four so that from that point of view depends, you know, becomes more complex to solve it and also the emissivity of the of the layer will change exactly.
Wojciech Wegrzynski:Uh and and also like the transparency. I like that because if the smoke is not transparent, then this exchange of heat is only between the, the beam and the smoke. Then there's nothing else. The beam cannot see anything beyond the smoke so so it's all about the temperature of the smoke.
Wojciech Wegrzynski:When it's transparent, it also kind of stops shielding the beam from everything else. It stops being the averaging function of your compartment in terms of heat transfer. You suddenly are exposed. Yeah yeah, this is like many things in fire science it's very simple until you get very deep into that and then it becomes annoyingly difficult. I assume that convective heat transfer you said it's simple, but it's also like getting the coefficient right is probably a lot of fun. If I'm correct, you said that the K5 is a function of the fuel. Does it depend on the compartments? Well, I assume it would have a dependency on the compartment itself how big it is, how tall it is.
Andrea Lucherini:So this is some research we are trying to do is because you know there is plenty of data out there where you take different fuels, you put under the hoods and you do oxygen consumption, calorimetry and you can quantify fire growth rate or the fire growth index to get your AFOTI square for the growth phase. You can understand the fully developed phase of the fuel, control fire and also you can look at decay, of course. But you can start thinking that you know if you have a fuel within a compartment then of course the compartment, the flow fields, the oxygen level has an effect on the combustion of the fuel and therefore you know, you see there is very limited data on this. We are trying to do some research, research related to that, and there is very limited data on this. We are trying to do some research related to that and there is very limited information and research and it's something that hopefully you know. I don't know, I've been last time here at the Paracent show three years ago, maybe in three years' time, and I'll tell you everything about this.
Wojciech Wegrzynski:You're already invited, so can we move to cooling phase? Yes, I would really love to get the distinction between the decay phase and cooling phase, because in both, well, basically from the perspective of the structure, it's cooling down.
Andrea Lucherini:Exactly. So I will first discuss about the cooling phase from the compartment sides and then we'll discuss about what actually is cooling. Yeah, let's do that, yeah, please. So if we look at about the cooling phase of the fire, looking at a compartment level, so imagine our control volume is the compartment? Yeah, so you can imagine that when the fire dies so when there is no more heat release rate from the fire, mainly have huge flows coming in from the opening that the time for the gases to cool down is extremely fast because there is a very big flow and, of course, the highest the temperature within the compartment, the fastest, the more flow goes into the compartment. So if you look at gas temperature, within the order of minutes, if you don't have a heat release rate from the fire, then the gas temperature within the compartment becomes ambient temperature.
Andrea Lucherini:Now the problem, or the heat, comes from the aspect of the solids, because we know that solids take a much longer time to cool down compared to, you know, the compartment gases, and this clearly depends on their thermal inertia.
Andrea Lucherini:This depends on their thermal inertia and depends also on how much energy they actually received in the phases before growth phase, fluid phase and decay phase. So now the whole problem becomes simpler from one point of view. So if we go again into trying to study our beam at the ceiling of our compartment, then we can assume quite quickly the gas temperature that tends to ambient temperature. But then if we want to try again to solve the radiation, you have to think about how that element is exchanging heat by radiation with the surrounding environment. The surrounding environment means now we don't have flames anymore so that we can disregard. But this means that possibly your element can exchange heat by radiation with anything visible to the element within the compartment. It means a hot wall or a cold wall or a hot ceiling or a cold ceiling or a cold floor or a hot floor, you know. So now the whole problem becomes radiation dominant because the gases are more or less cool and so this goes into the compartment therm dynamics aspect related to the cooling phase.
Wojciech Wegrzynski:So the compartment conditions will shortly depend on the radiation of the structure to the compartment. Okay, exactly.
Andrea Lucherini:Because you know that if you have a thick concrete wall that takes a long time to heat up also is going to take a long time to cool down Good good, and this happens to any solid that will be in the compartment yeah, I mean this is obviously critically important for firefighters, but I'm very, I'm very sure they know that, like they, they feel the heat on this on the skin, so so they understand how much it takes to cool down the structure is.
Wojciech Wegrzynski:Actually, people who have not, who do not witness fires daily, may not appreciate how long it takes, because it takes a long time. But, yeah, please, structure.
Andrea Lucherini:Sometimes I ask directly to the students would you enter into a compartment where the fire has died? You would get in and you would feel uncomfortable and it's like, okay, what's I mean, apart from the gases? What actually is hitting you? And it's the radiation that comes from the linings, simple as that Structure.
Wojciech Wegrzynski:Let's do structure. What's the cooling phase?
Andrea Lucherini:looking from the perspective of the structure, Exactly so, because this is a discussion I had with many other researchers. Like, what is cooling? You know the short answer again depends. It's always it depends. But it depends what you're focusing on.
Andrea Lucherini:If you're focusing on the structure, you know we discussed earlier that in the far decay, depending on how long it takes and the hit-release rate curve of the far decay, you can still have a smoke layer that possibly has a significant temperature that possibly, if the structure has a very high thermal inertia, that temperature can still be higher than actual structure.
Andrea Lucherini:So if you think about the net heat flux into the structure, it's still positive because the material is still receiving heat from the environment. Okay, yeah, makes sense. So this means that we discussed and the similar thing in the cooling phase. In the cooling phase, if you have something that if your structure is relatively cold on the surface but isn't put in front of something that is very hot and it takes a very long time to cool down what ends up, you can still have a positive heat flux during cooling so you don't have a fire but still your solid is receiving energy from somewhere within the compartment. So that is when you discuss about cooling phase. From the structure point of view means to me the definition is when you discuss about cooling phase from the structural point of view means to me the definition is you need to have a negative heat flux at the structural surface. Means the total energy of the element is decreasing. Okay, because the energy is leaving the solid. So that's the general definition I will give.
Wojciech Wegrzynski:I like that. But the energy is not only going outside of the structure the way it came in. It is moving towards the region of the lowest temperature, which may be actually the opposite side of the wall usually.
Andrea Lucherini:Yeah, exactly. But if we think about cooling of an element of a solid, you know you have to think that your energy has to go somewhere else. But still, even if the energy would keep going in depth by conduction, the same energy also is able to reach the surface and therefore leave the solid. The boundary condition of the surface enables cooling, of course.
Wojciech Wegrzynski:Are there simple models that allow you to estimate quickly how much is going to go in, how much is going to go out? Because I think that's probably interesting. Alternatively, you can go finite element modeling and hours and hours of simulations much is going to go out. Because I think that that that's uh probably interesting. Alternatively, you can go finite element modeling and like hours and hours of simulations. I mean, it's already painful to do it for an hour of a fire. Now you have to do five hours of cooling phases.
Andrea Lucherini:It's probably expensive yeah, I mean the main, the main problem you have there. You know, if we discuss about fire decay, you know we go all there in the characterization of this decay branch. That usually is quite unknown how people usually solve it. They take the magic curve from, for example, eurocode 1, where you assume that 70% of the total fuel load is combusted during the fully developed phase of the fire. 30% of the fuel is actually released during the fire decay and then you draw a straight line. So you know you have a linear decay of heat release rate. That is something you can do. We try to do something more fancy for research-wise, but I don't have results yet. What I can tell you is to me it doesn't decay linearly.
Wojciech Wegrzynski:Maybe it's a good approximation, but I'll tell you more when we've tried that experimentally in a furnace, we've tried a Eurocode 1 approximation of the decay phase and we tried to force the furnace to give me that decay phase because we were comparing traveling fires with standard fires, with Eurocode, short hot fire, and what we found, even if we put the ventilation in the furnace to the maximum and, trust me, it's a lot of air to put, a lot of very cold air to put. You know, we cool furnaces as well. We try to do it in a few minutes, you know, to match this, this decay which you just said, linear decay, and boy, it was like impossible, like just impossible due to physics. Like impossible, like just impossible due to physics, like impossible due to heat transfer of fundamentals and very nonlinear because actually if you look at the equations, they go with different power laws I mean Newton's law of cooling goes with an exponential decay.
Andrea Lucherini:Yeah, If you're trying to force a linear decay, you're trying to force something on physics.
Wojciech Wegrzynski:So now if, let's say, a fellow fire engineer would like to put something like a decay cooling phase in their considerations for the structure, what would be the recommendation? How to define it? Where to even start getting reliable data? That actually is representative of fire physics we have to think that.
Andrea Lucherini:you know you have to start looking at how you characterize a boundary condition during these phases, because what I mentioned to you, what we usually assume as one simple proxy of having a good characterization on the smoke layer during the growth phase and the fully level phase, I told you that you know, during the fire decay and cooling phase you have a different characteristic temperature that can have a big influence on that, which means flames, fuel linings, gases they could have all of them somehow an influence on that. If you have a good character, if you have a well-defined case and you have a good approximation of your fuel, then that way you can just put different sensors, for example in CFD, and try to characterize the heat flux and get the different components of the heat flux and then trying to see how actually the really well-defined boundary condition that you would have to your element, just to mention. You know, we know that FDS, for example, doesn't do the greatest job in underventilated fires and you can imagine that if the full development phase of the fire is not well characterized or well predicted, then of course you're going to have an influence to the other phases, and something that I don't want to. I always say that or I say at the end we don't want to. I always say that or I say at the end we don't have to forget that the fire decay, cooling phase, are strictly connected to the fully double phase and the crow phase.
Andrea Lucherini:What happens before? Because, as I told you earlier, you know, if you have a very fast short fire, maybe the fire decay and the cooling phase may have a very important effect. But if you have a very long fire then impossibly, the part of the cooling phase of the fire decay may have a very important effect. But if you have a very long fire then impossibly the part of the cooling phase of the fire decay can have a secondary effect because actually the structure is much more challenged during the fully double phase. What we did? Simply we put and we published last year, at the beginning of last year, a paper that we try to characterize the pure cooling phase.
Andrea Lucherini:Pure cooling phase means we neglected completely the fire, so there is no fire decay, so we have the fully double phase and then immediately, in a very unphysical way, the heat release rate goes to zero, which is very unphysical.
Andrea Lucherini:But then in that case we can start putting the assumption I mentioned earlier. So our gas temperature tends quickly to ambient and if you want to characterize this radiation temperature you can take the linings, do a heat transfer model for the linings and you can model the evolution of the surface temperature of the linings in Cooley and that surface temperature, based of course on the view factor, can characterize how positively the radiation that is received by your element according to that line is. Because we said high thermal inertia line is can possibly release a lot of heat and takes a very long time to cool down. But if you have highly insulated line is with very low thermal inertia, that surface temperature drops very quickly as soon as you have ambient temperature in the gas phase. So you can solve that. I am in favor of sending you the link you can link to the episode.
Wojciech Wegrzynski:There are simple ways to do this, I'm very happy to do that and in the cooling phase. So no longer the fire is driving that. So it will be only the size and the bulkness of the element of the structure that will drive that, which kind of relates to how much heat it accumulated over the time Exactly.
Andrea Lucherini:Because the thermal inertia of the material, of the element, can be much bigger than the thermal inertia of the other materials. So at that point the element, its thermal inertia, let's say, is much more important than what happens around. So in that case that is the driving parameter. And this is the case I told you earlier Heavy thick steel section, highly parameter. And this is the case I told you earlier Heavy thick, thin section, highly insulated. Then it's going to cool down naturally according to its own image.
Wojciech Wegrzynski:And for the last few minutes let's drop timber into the mix, because I know it's like timber is probably a whole episode on its own, but it must be very different. One it kind of participates in the fire. Two, if you have a burning exposed surface of the timber, it's the heat transfer is not going to be towards the timber that much, like it's going to radiate away as well. Like it, burning timber can be hotter than the gases. That that's what carmen shown in her phd, which I think is very interesting observation, and also all the processes which are exothermic that happen in the team, like char oxidation etc. It's hard to say Well, I even struggle by your definition when the cooling phase starts, when the net heat flux is minus. Do we even get to that point with the mass timber structure that was exposed?
Andrea Lucherini:For that point of view you have to break the problem in two cases. So the first case is that we're trying to study something that sits in front of a timber burning wall or burning ceiling, or are we trying to study the actual wall that is combusting and during the compartment fire? So without you know, carmen, I think we already discussed a lot with also previous guests about the fully developed phase. If we start thinking about fire decay, the whole timber problem, I define the fire decay, the end of fire decay. You have negligible heat retreat in the fire. Of course, if timber is exposed it contributes to the fire. If timber doesn't achieve state distinguishment or distinguishment somehow, then we still contribute to the fire. So we could lead to the fact that you never enter a pure cooling phase because the fire decay could last forever, because wood is never going to extinguish until.
Andrea Lucherini:I always tell students that in a timber compartment your fuel load density or fuel load is infinite because everything is made of fuel. Once you run out of fuel it means you don't have a compartment anymore. So that goes into a very detailed and problematic complex problem on trying to achieve self-extinguishment or extinguishment of the fire within a compartment with exposed combustible linings, so non-poly timber. But then it goes into a fire safety engineer problem of understanding the relationship of radiation between walls. Because I told you earlier, since in the far decay you have a clean environment, so also walls can start seeing each other. And if you have a burning wall in front of a burning wall, so this means they start exchanging heat V-factor one very nice so they feed each other and it's never going to extinguish.
Wojciech Wegrzynski:Absolutely. We've seen that experimentally. Actually, I think it's even more interesting for structures where you have partial exposure of the timber, because the interaction between the timber surfaces and your hot walls also you know timber there is some critical point at which it starts to self-extinguish. I leave that for another podcast episode. There's a lot of people researching that. But there is a point where it starts to self-extinguish. I'll leave that for another podcast episode. There's a lot of people researching that. But there is a point where it starts to self-extinguish and if you have very hot, dense walls heating it up, this point is also going to change in relationship to the timeline of the fire as we reach the timber. And we've reached pretty much the end of the interview. There's one more important question to you. I saw a position open for a fully sponsored PhD student under supervision of Dr Luccherini on timber. Maybe that's a good point to talk about that one.
Andrea Lucherini:That's a position that we've been looking for a PhD to join our team at Frisbee at ZAG, the Slovenian National Civil Engineering Building Institute, and you know what is the good aspect of this? You know I'm researching on these aspects and you've seen that already raised during the interview that there are some aspects that are not well known. We can do more research. So the position is open and we are looking actively for candidates, but the topic is not defined. I always try to, so we have the pleasure of having a non-project-related PhD position that this is the topic I want to work, but there's no specific topic I want to work and we can customize the project depending on the candidate.
Wojciech Wegrzynski:So it's more like a region of interest. And then you find a specific topic. I think it's an interesting position and if there's anyone willing to try this adventure, go to Slovenia and do some fire testing and fire experiments in Zaga. Contact Andrzej, and if you do well, you'll end up in the fire science show. I promise you that for sure.
Andrea Lucherini:Feel free to reach me. You know how to reach me.
Wojciech Wegrzynski:Any final recommendation to fire engineers who would like to start accounting for decay phases better for them. I took from this episode the controlling boundary conditions and, appreciating the fact that the gas boundary condition will change tremendously, I think that for me, for compartment fire scientists, that's an interesting observation and a good takeaway.
Andrea Lucherini:I mean the main concept that I tried to communicate during this interview is the fact that fire decay and cooling phase, starting from the definition, is not a simple thing and they're highly connected to what happened before, so the previous phases.
Andrea Lucherini:One thing I wanted to mention that possibly all the proxies that we developed to characterize the boundary condition during the fully developed phase possibly don't work during the fire decay and cooling phase because, I told you earlier, there are different characteristic temperatures or characteristic properties that would have an effect on the boundary conditions.
Andrea Lucherini:So the idea is that try to read, try to ask around how to do your job in the best way related to these phases, and then what I say is that you know fire decay and cooling phase matters when you want to get it right and you want to get it as close as possible to reality, namely following a performance based approach. Because it could be that if you have a compartment fire that will last for half an hour and then you connect the fire decay and cooling phase, then yes, it will be relevant in that case. But if you would apply a two-hour standard fire curve to the thin element, possibly you are on the safe side. So you can always do the classic civil engineering approach let's make it bigger, let's make it more critical, and then we are safe. So think always about you know how to simplify, but also how you can ensure that level of safety, that for having a structure or an element or a building or a compartment properly designed.
Wojciech Wegrzynski:Fantastic. Thank you very much. It's been a while since Structural Fire Engineering has been in the FireSense show, so I appreciate this take and I hope to see you again in the FireSense show.
Andrea Lucherini:Thank you very much Someday, thank you. Thank you very much, wojciech, bye-bye.
Wojciech Wegrzynski:And that's it. I hope you've enjoyed this one. I guess we can quantify this as a decay phase. Those definitions are quite funny when you really want to narrow them, precisely because for different types of structures, some of them will be true, some of them will not be true. Like think of timber. When does it stop being a net generator of heat flux? Huh, probably when it's it's gone. So yeah, fun times with definitions, but we sometimes need them. Sometimes you need to calculate the length of the fire, calculate the length of the decay phase, to set your models accordingly.
Wojciech Wegrzynski:I feel the interesting pain that Andra brought up in this ambiguity, a thing that I really, really enjoyed in this episode, and I've already said that a few minutes ago in the interview the boundary condition and the radiation, the optically thin, optically thick smoke. This is a really powerful finding. To be honest, I've never considered it like that. I have not been taught like this to think about the different smoke properties in those phases. It's one of those things that's very powerful. When you know it, it's quite obvious, but to figure it out not that easy. I already see some uses of this way of thinking to other aspects of far engineering, already talked with andrea about it. It's promising, so there's a good chance we'll do some research together after this. Because, yeah, it's, it's simply inspiring for other areas.
Wojciech Wegrzynski:Let's say, without spoiling too much, I wonder what you take of this episode. I really like Andrzej's way of explaining teaching. He's such a promising young researcher I even love to call him young. He's quite an advanced researcher nowadays and leading doctorates, etc. So yeah, dr Andrzera, good job there. Mate, thanks for coming to the Fire Science Show. Okay, I think this would be it for today's episode. I hope you've enjoyed your weekly dose of fire science and engineering, and what else I can say? Next week, another dose of fire science and engineering will be waiting here for you. I hope to see you there. Thank you, cheers, bye.