126 - Where is the challange with tunnel smoke control? with Wojciech

Hello everybody, welcome to the Fire Science Show. Today I have an episode focused largely on smoke control and that is smoke control in tunnels. But I know it's niche subject so I promise I will keep it up. Interesting for any fire professional, because the things we will discuss today are important not just for tunnels but for any project in which you want to move air and smoke into a place where you want that air and smoke to be. Despite having a very large experience as an engineer designing smoke control in tunnels we've delivered 29 tunnels, if I'm not wrong. That's a big chunk of experience over the last 10 years. I felt a little reluctant on speaking about tunnels in the podcast because it's this odd imposter syndrome. There are many people way more fit than me to talk about a tunnel ventilation system. I have not written a book about it yet perhaps one day. But I've been encouraged by two things that happened to me and my team when I was in Japan. We've received quite a big award Pascal Award for the best ventilation system design, for actually a road tunnel ventilation system, and I've also been invited to a workshop for the directors of road administrations in European countries. So yeah, what those two things mean to me. I will tell you a little bit more just after the intro. Now, just please feel invited to the beautiful world of smoke control in tunnels. And my second promise is I will not talk that much about critical velocity, at least not in a positive way. Let's go. Welcome to the Firesize show. My name is Vojci Wynchczynski and I will be your host. This podcast is brought to you in collaboration with OFR Consultants, a multi-award winning independent consultancy dedicated to addressing fire safety challenges. Ofr is the UK's leading fire risk consultancy. Its globally established team has developed a reputation for preeminent fire engineering expertise, with colleagues working across the world to help protect people, property and environment. Ofr is calling all graduates, as it is opening the graduate application scheme for another year, inviting prospective colleagues to join their team from September 2024. If you take this opportunity, you'll be provided with fantastical practical immersion into fire engineering and a unique opportunity to work with leading technical experts in the field, while learning the skills critical to becoming a trust and consultant to clients. This opportunity is tailored just for you. If you would like to check out this opportunity, please visit OFRconsultantscom for fertile details and the instructions on how to apply. Now back to the world of smoke control in tunnels. So I promise to you I'll tell you more about the rationale behind doing this podcast episode Now. First of all, the Pascal Award this is a award given by the Polish Ventilation Association to the best ventilation design in Poland. It happens annually. It's not just smoke control award, it's for all ventilation, like HVC, cooling, heating, climate control and all of this stuff, smoke control being a little part of this world. And this year we've submitted our work on the road tunnel in Warsaw. It's a project that I started when I had a master's degree now and now professor, so that's a big chunk of my life. It started somewhere around 2015 and the first design was done by 2016, I believe. In this huge tunnel, we were obligated to design a transversal system, but we knew it's going to be tough to keep the smoke where we want it to be. So we've introduced quite a significant and strong longitudinal component to that system, making it something that did not exist in literature, actually, and in the end, fast forward many years ahead 2021, when we were commissioning the tunnel. This longitudinal component came very, very important when we have discovered the importance of wind on this particular project. So these are the tough lessons that we've learned by first designing and then commissioning a road tunnel. That led to many of my recent fire research projects and investigations and it's also something that I'm going to talk a lot more later in the podcast episode. And the second thing that broke my impostor syndrome is I've been actually invited by CEDR, that's an association of European Road Administration directors people who define the requirements for roads, tunnels included in their countries and they've asked me to speak out the designing tunnels. I gave a similar talk sometime ago to a different group from this organization. They really enjoyed it. Now I've been invited to give a workshop to the rest and I need practice and I thought I have this nice group of people who like to listen to me and if what I say is rubbish they will tell me. So I'm kind of practicing what I want to say to the directors in this podcast episode. I hope it goes good. So first of all, after this exciting introduction, please let me drop a very boring bump on you because we need to talk about some definitions. So in the world of tunnels, there are a few things that matter and that will change the whole project perspective. First, like what type of tunnel you're designing? Is it a road tunnel? Is it a train tunnel? Is it a metro system? Is it high speed railway? Each type of a tunnel will have different challenges to it, despite them being a simple pipe with two ends coming from one end going out the other end. Usually, in most cases, it's the details that make it complicated and the context that brings up a lot of challenges, especially to how do you handle life safety, because we're designing a life safety system and it's obvious that evacuation from a high speed train will be quite different process than evacuation from a traffic that stopped in the middle of a bi-directional tunnel. So, first of all, the type of tunnel is something that needs to be considered and your system will be crafted with that in mind. Second thing is is the tunnel unidirectional or bi-directional? Which means if vehicles are going both ways or they're going only in one direction in the tunnel, and this is critical because if you have vehicles going only in one direction, you can pretty safely assume that if your fire happens, the vehicles that are in front of that event have no idea that it happened, unless the tunnel tells them. So these vehicles are very fit to travel further until they escape the tunnel and they're essentially safe the people in vehicles who will find themselves behind the fire like they drive and approach the fire. These people are in danger because they have to stop and evacuate from their vehicles. So you have two distinct groups of people one of them is in danger and this group is approaching the fire. So when we design systems, we can craft them in a way that it pushes smoke and products into the direction of the exit of the tunnel, assuming that no one should be there. And this is simply the basis of most of the tunnel design we've done. If you have bi-directional traffic, the situation complicates a little because suddenly you have vehicles from both sides of the fire so you don't have a preferable direction to which you want to send the smoke out. So it takes a lot more engineering to craft a safe system that will allow people to escape and will manage their safety in the tunnel. So yeah, unidirectional, bi-directional, massive, massive difference. That really changes how you design your system. And now for the systems that we can design. Basically, we can move the air along the tunnel, we can extract the air at the ceiling of the tunnel or we can do a mix of both. We call it the longitudinal ventilation, where you push the smoke along your tunnel. We call it transversal ventilation when you have smoke vents on the top of your tunnel and you just extract your smoke to that point. There are also some, let's say, hybrid perhaps that's the wrong word. There are different solutions where you would have simply a one big, massive vent that acts like some mid extraction point in your tunnel. So the system is kind of longitudinal, but on a much shorter distance. You're not pushing the smoke up to the exit of the tunnel. You can have some semi-transversal solutions where, for example, you extract the smoke in the ceiling and you supply the air longitudinally, or you can extract longitudinally and supply at vents at the floor. Actually, I hate the system the most. It's extremely expensive and brings no value the projects which we also have found during our experience. There are many, many ways you can move the smoke inside this one directional pipe. Tunnel engineers hate when people call the tunnels pipes, so I'm very sorry tunnel engineers. There are many ways you can move the smoke around your tunnel. Of course, the way you choose will depend on the type of your tunnel, the type of traffic and how you want to carry your evacuation. It's a bigger part of a very large safety system. As, most important as you can imagine, we're talking about large systems, large ducts, a lot of space for jet fans. These are the things that will define the size of a tunnel in many cases. If your tunnel is too small to fit the jet fan, we need to find a way to make it bigger. If your tunnel needs a 20 square meter extraction duct for the smoke, we need to find space for that duct and, trust me, you will not be able to increase that by a one square centimeter in the future, because once the tunnel design is cast in stone, they build a tunnel boring machine or they start your digging operations and they bid it to that size and it's impossible to change it further. So in this case, the ventilation design must happen very early in the project and must be really high quality work. Otherwise you're risking the whole tunnel project if you fail at smoke control. The last thing that we need to cover is what fires are we talking about, and this is a challenging thing, because we obviously need a design fire and obviously the size of the system will depend on the design fire you choose. And, as I mentioned just seconds ago, the size of the system will also dictate the size of the tunnel in some way. So we often design for large truck fire scenarios like heavy goods vehicles fires. A typical value used in Poland is 100 megawatt fire. This is something that we designed the tunnels for. In case of railway tunnels it's a little more complicated because there is a little bit less data, or perhaps there's a lot of data, but it's a little bit all over the place. On the projects that we've been working with use 19 megawatt design fire based on an ICE train experiment in Germany. But I know there's a metro project, for example, which has 77 megawatt fire indicated in the tunnel that they've burned. So the values can be all over the place. If you consider something like a spillage of oil from a large tanker, you can go with design fires of 200, 300 megawatts. That's also possible. So it's kind of a risk analysis. In our case. We have our jurisdiction that gives us the values. So the choice is not completely mine. It acts both ways. I would love to be able to be responsible for my design fire, because I could put suppression system into the tunnel and claim that my design fire is smaller, perhaps 30, 50 megawatts and design my ventilation system accordingly. But I don't have this liberty in Poland, so I'm not really able to craft my design fire on my own. That's a thing that needs to be considered in many projects. It should be a matter of risk analysis, and why it should be a really strong part of the risk analysis I will tell you after we cover the components of forces that are acting on the air in the tunnel. So that would be it for definitions, and this is also a good place for a safety disclaimer. So keep in mind this is a podcast episode and, despite the things come from my experience over many, many telling projects, it's still just my opinions on some things and I understand I have some strong opinions on some of the things in the world of ventilation. If you do your design, you should make your own opinions based on the peer reviewed science, on the literature that you can find. You can take into account what I'm saying in here. While they please figure out stuff for your own, I'm here to help you to do the best extent that I can. But yeah, please remember it's just a podcast, not a standard. Anyway, with this in mind, let's go Now. I'll tell you where I think is the real challenge in the tunnel ventilation design. If you look at the literature, you find this dreaded number called the critical velocity that everyone is super focused on, which is the velocity of air at which the smoke will not back layer, which means it will travel only in one direction. And most of the guidance, most of the research, is absolutely focused on determining the value of critical velocity and designing your system accordingly to that value. For me, I dislike this approach a lot because if you think about any flow and a fluid movement in any body or system, like car park and the corridor in the tunnel in the shopping mall, velocity is a product of forces acting on the flow. It's like kind of like temperature is a product of heat transfer. You know you're not adding 100 degrees to a body, you're having heat transfer conditions that result in 100 degrees increase in that body. In the same way, with velocity, you are not producing velocity with your devices. Your smoke control is not creating velocity, it's creating momentum, it's creating pressures, forces, and the outcome of all of those is the velocity you get. The same thing happens when you consider smoke bloom in a tunnel. Obviously, tunnel is confined space, so the smoke bloom can go only in two directions up and down. It's not that if you introduce velocity in your tunnel you will magically have conditions like a virtual piston that will push with the force of three meters per second on that smoke and make it move away. It doesn't work like that, like it's a fluid. So if two fluids meet, where one flows in one direction, that being the cold air pushed by the ventilation system, and you have a hot smoke layer moving in the opposite direction in your ceiling jet, it's not a piston of air that will move that ceiling jet. It's a very fragile momentum exchange that will happen at the interface between those two fluids that will eventually cause that ceiling jet to move back. You will eventually achieve some sort of equilibrium. It's not very instant and this is the thing that researchers find. So they find you in their scale model to obtain this very fragile, steady state equilibrium, balance and report the values at which the balance is queued at a certain distance and they call it the critical velocity. But in fact it's a product of a transient momentum exchange. It takes a while to achieve this narrow balance and it takes a very little to create conditions that disturb it. I won't even go into Reynolds number and Tourbouliens in scale models, which for me is very tricky to handle, to give you velocity to the third significant digit, like people love to tell that in literature. So as I see this, ventilation condition in a tunnel or in any building, it's not balance of velocities, it's a much more complicated fluid dynamics situation. Now the things are changing. The newest edition of NFPA 502 brought a very, very significant change into this approach to design. So some years ago to give you a full background I think I've covered it more in depth in the discussion with Arnold Dix Some years ago the NFPA committee has changed the regulations relating critical velocity based on small scale research, which resulted in a significant increase of the velocity required, which kind of led to a backlash from the engineering society because we didn't like those numbers, like they were way too high, the systems were way too big that we thought they're needed. They were not in line with full scale research from Memorial Tunnel or RunaHummer, the only very big experiments in real tunnels that we have. So this was kind of reverted at some point and now they've introduced a new concept called the confinement velocity where instead of doing everything to stop your smoke from backliring yellow for some degree of backliring and you can capture this fragile balance at a specific distance, because the realization is that a little bit of backliring is not very dangerous and doesn't destroy the purpose of your system. So you can actually design with that phenomenon in mind. So now we're in a world where you not only have one value of velocity you want to act. You actually will have a backliring distance definition that you have to define as your objective for your system and with that in mind we have to be very careful and confident in designing this fragile balance. Now the world of research is not really following this that much, because all I see still being published on smoke control tunnels is critical velocity in one fire, two fires under a hole when one fires outside, one inside on curved tunnels and so on. These are all peer-reviewed papers in quite good journals, but they don't give you answers on how to define this backliring length and how to design it. This will be a challenge for us engineers in the future, but it's also an opportunity that we will be given tools to really craft our systems better than today. The reason why I feel the change in an FB502 is so paramount is that it breaks the need to design for one single velocity. It gives you an objective, and if we have one objective, perhaps we can suddenly talk about a lot more objectives, like over what distance we want to control smoke and how we want to achieve that smoke is closed in this space. This would refer not only to longitudinal systems but in a much stronger way, to transverse the systems, like we did in the project that was given the Pascal Award. How about we put more fixed design conditions on the evacuation and escape paths and how we keep them clean from smoke? Perhaps we need to have criteria that allow us to maintain buoyancy while transporting the smoke out and having layers in the tunnel, which now, in longitudinal systems, is largely impossible because of the critical velocity which makes everything mix and transport as one massive mix of smoke and air. How about firefighting approaches and how we consider where firefighters can enter the tunnel and what conditions they would like to expect? Do they really need to have a fully clear entry in every possible fire, or perhaps for a range of fires is okay if there is some back layering and they can deal with it? What about the growth of fire? How does our design influence the growth of the fire itself? I mean, if we blow too much air on the fire, you make air penetrate porous fuels more efficiently and you eventually build up larger fires. On the other hand, if you have efficient smoke control, you cool down the smoke and make the spread of the fire to neighboring vehicles much more difficult. So it's also a factor we could design for. So breaking the paradigm of critical velocity, you know, a single value for which we design opens a world into designing for many more design objectives like real performance objectives that will allow us to craft better systems in the future. I think we really need now to have a discussion on what do we want to achieve with our smoke control systems in tunnels and buildings and in all other places where we use smoke control systems, and such discussions should take place everywhere, because our criteria that forces us to maintain a certain amount of velocity, certain amount of flow, air changes per hour or any other dreaded measure that you can think about designing smoke control systems. They need to be replaced with performance criteria. That's my personal, perhaps a very strong opinion. It's not a pre-reviewed science, it's my opinion based on the experience I have, but I would love to see the world in which we design like this Now, if we could step back a little bit to the point where I told you it's a velocity, is a product of precious forces and everything. Let's consider what makes air move in the tunnel. It's a pretty important consideration if you want to design a smoke control system for any objective you want, because you want to have a certainty that air will move in a specific direction, with specific flow velocity or whatever parameter they use to measure it. So let's think about what components go into the equilibrium of the flow in the tunnel, and to me there would be six, seven components of that. Let's list them. First would be the fire. The second would be the resistance of the tunnel as pipe. The third would be external effects such as wind. The fourth would be the chimney effects. The fifth would be the piston effect. The sixth would be the performance of your ventilation system in your tunnel. And the last component that needs to be included in this perhaps not a force acting on your flow, but a very, very important thing to consider is the traffic itself and the evacuation process. How do they look and where do they go? So now, if we break those into pieces, what makes air move in the tunnel? Imagine you have a flat tunnel empty, nothing inside, quiescent conditions, outside, no thermal gradient, there obviously will not be any flow in the tunnel, although I would argue there always seems to be flow in tunnel. So I'm yet to see a tunnel in which the air doesn't move at all really. But let's assume for a second that when no of the components I've mentioned are present, the flow will be in a standstill. So let's think about it one by one. What did they do? First, fire, and to some extent the chimney effect will play in here as well. What the fire does? The fire creates a buoyant plume. It heats up the air, which means it decreases the density of that air. This is the reason why buoyancy forces act. So the hot air flies upwards and then moves along the ceiling of the tunnel, along in something we call the ceiling jet. Now, if the tunnel is inclined, so you have some height difference between the portals of your tunnel, the air will obviously move upwards, the heat air will obviously move upwards. So you may end up with quite unidirectional flow from the fire itself against the inclination of the tunnel. The same happens if you have significant temperature differences between the portals of the tunnel. You can also have a very strong thermal flow through the chimney effect. This is very important to consider because very rarely the tunnels are perfectly flat. You build tunnels for a reason. The reasons are usually to cross a mountain, so it's very odd to have the same height at both sides of the mountain. You may want to use a tunnel to cross underneath city center, for example, so you also have to go down and then go back up to your city center. You want to cross underneath like metro lines, like the one we did in Warsaw. The reason why engineering this tunnel was so difficult was because it had to go underneath a metro line, so it's very rare that tunnels would not be inclined at all. So it's always a thing to consider. Now much of the research is done on completely flat tunnels, taking out this component, which kind of is ridiculous. Anyway, the fire itself will create buoyant forces. That will create some sort of force and some sort of flow already. But also it is in a way a resistance, a throttling effect, and I had an episode with Swiss engineer Ingo Reis in the podcast about the throttling effect, because he has captured this in the best way from all the people I know on how much resistance to a forced flow does the fire itself add? Because you have an obstacle, you have a bunch of heated air in your tunnel that are mass produced by the fire and train and strong flows and all of that. And if you add mechanical ventilation on one side to that equilibrium, the ventilation has to act against those forces. So it's a resistance in the tunnel, a throttling effect, which needs to be investigated in your calculations. Another component of the forces is the resistances of your walls, like the simple friction of the walls, but not just walls or the equipment that is installed in the tunnel. If you consider the tunnel as a pipe I'm sorry tunnel engineers if you consider the tunnel as a pipe, there's Darcy Weissbach equation that allows you to calculate the pressure losses due to friction in your tunnel, and the same thing would work in a road tunnel. We have made a very interesting calculations with our friends from Silesian Technical University, professor Alexandren Gosza-Kruhl, which is peer reviewed and published, and I'll link it into the show notes where we go much more in depth on how to calculate that and what kind of resistances you can see in the tunnel. The next element I'll skip it in for a second, so I'll jump directly to fifth. God, I'm horrible at reading lists because none of the points are in order. Let's go to piston effect. This is a very important thing because if you have a fire hazard in your tunnel, it means you had vehicles in it. If you had vehicles in it, it means they had to travel to reach the tunnel or they are traveling through the tunnel right now, and if they are traveling, it means they're moving air with them through something we call the piston effect. And even if you stop because there's a fire, it's detected. You still have those vehicles that are escaping the tunnel. You still have some vehicles that are still going into the tunnel before you stop the traffic. If you have a train network, like a metro system, you still have all other trains going in a normal fashion until they reach a safe place and it may take many, many minutes until everything settles down to a quiescent stage. So piston effect is something that always occurs in tunnels. Always If you have fire, it means you had movement. If you had movement, you had piston effect. And at the same time, it's something we never take into account in our calculations, never. Seriously, it's the elephant in the room when you think about designing of smoke control, detection and other stuff in tunnels and this is something that with our friends from Stiletsingen University again, and my student Jakub, we are investigating. It's going to be the subject of Jakub's PhD on how to include those effects into initial conditions of our simulations, because we feel it is extremely important to include the initial conditions which is movement, not quiescent into considerations of tunnel fire safety. I simply feel that if we simulate our tunnels and the initial conditions are like 0 meters per second velocity everywhere, I don't think that's a really valid approach. Third thing on my list, perhaps the fifth thing I talk about, is wind. So, as you know, I'm a wind engineer as much as fire engineer and wind was something that was very surprising to us in the Warsaw Ring tunnel, for which we got this Pascal award I've mentioned before. When we were designing the Warsaw Ring tunnel, we knew that keeping the smoke in the very confined part of the tunnel between five extraction vents will be challenging because how big the tunnel is and how fragile the balance is. So we've decided along the tunnel components to that. We knew that if we somehow managed to have approximately one and a half meter per second flow at each ends of this part of the tunnel from which we extract the smoke. We were fairly confident that we will be able to keep the smoke where we want it to be. So we've defined our own design criterion and we followed it. Now, to be sure that you actually have this type of velocity on both ends means that the tunnel must be quite independent from the wind action, because if you have very strong wind outside, let's say six, seven meters per second, a lot of the falls exerted by this wind on the portal of your tunnel will turn into, let's say, dynamic pressure on your tunnel portal, essentially meaning there will be flow caused by the wind that will go into your tunnel, and it can be very, very large amount of air, definitely enough to disturb the fragile balance that we're creating. So in order to fight with that and that happened much later, to the stage when the tunnel was actually built and we could do measurements we have measured how much flow the wind creates inside the tunnel. It's not obvious and there's no simple solutions for that. There are no discharge coefficient calculators that will tell you that in the tunnel of this shape you will have this discharge coefficient, meaning this amount of pressure will be created by this type of wind. There's very limited research from the 70s, from Austria, and that's pretty much it. So we had to figure out for our particular tunnel, experimentally what the wind effect is, and we've actually measured that. I remember a funny story it took us like two months to get a really good image of the wind action on the tunnel. I remember one Saturday evening there was this very strange northern winds in Warsaw very rare thing in here. We don't have strong northern winds in here and it was that appointed we didn't have. So I packed my bag, I took my anemometer and I tell my wife I'm going to the tunnel to measure wind. And it's Saturday evening and she's like what the hell are you doing? And I'm like there is very rare wind occurring, I have to measure it. She gave me a very odd look but nevertheless I went, I measured it and it was the last data point that completed the collection and gave us let's say, closed windrows overview on the flows in the tunnel caused by the external wind action. That allowed us to define how much force introduced by the fans in the system we have to have to overcome the wind effects. So the tunnel is again independent from the wind. That was perhaps the most challenging thing in the entire project and I assume one of the reasons we've been given an award because we really gave it a lot of thought, a lot of measurements, modeling, to understand what is the best way to cancel wind. And for every possible wind happening in Warsaw, for every possible location of the fire inside the tunnel, we have an optimal way to start the jetfans in the tunnel to cancel that wind. So I was pretty much a big win. Someone could argue that perhaps you could make an adaptive control system that would just monitor the velocity and start jetfans accordingly. Well, perhaps you could. I'll touch on that at the end of the episode when we will be talking about future directions. So we'll go there. We'll go there. The last element of my equilibrium regarding the forces in our ventilation systems fans. On one way, you have jetfans that push the air long into the normally, again, jetfan is not the thing that takes air at its inlet and throws it further away, and this is not how jetfans move air in the tunnel. Think about Dyson hairdryers or hand dryers. It's about inducing flow, it's about entrainment. So they inject high velocity air stream into the tunnel, which means they inject a lot of momentum into the tunnel and then these air moves and mixes with the surrounding air, transferring momentum to it, and some distance from the jetfan like 50 hundred meters from the jetfan you don't have the jet stream anymore, you just have air moving at a velocity that is reflecting the amount of momentum that the jetfan has given to the surrounding air. So this is how jetfans operate. It's not about sucking something and throwing it further. It's about transferring momentum from the fan into the jet stream and from the jet stream into the surrounding air in the tunnel. It's a very important concept that you absolutely need to understand if you want to design any sort of longitudinal systems, be it tunnel ventilation, be it car parks. You need to understand that jetfans operate by momentum transverse critical. But you also have extraction devices, like your exhaust vents, which suck the air outside of the tunnel, and it's actually quite difficult to introduce them into your calculations of forces in the tunnels because it's not an obvious force, it's an extraction point Can be done. The easiest way is to just run CFD and have the navier stalk solver resolve it for you. So that's probably the way we act. There's also a third thing you can inject air from semi-transversal or transversal inlet points, so you may have inlet points in your tunnel that inject air inside your tunnel and with those points, the interesting thing is they inject air into the tunnel but not in the direction of the flow, so you again need some momentum transfer to change the direction of the flow. It's a pretty difficult concept and we're actually in the middle of writing a paper about it, because it's something that the books don't tell you. But the practical outcome is that if you inject air throughout your tunnel, you need much more force on the fans to actually move the air where you want it to be. That's a practical outcome. Now I also mentioned there's traffic and evacuation, which takes a role. You need to understand why are you designing it and what probability of having this amount of people you will have in your tunnel. Because if you want to make it a risk analysis, you need to know consequences of your actions and to know consequences, you need to know how many people are in danger. Now, if we take all of these components together, look, we designed for specific fire, specific resistances, wind, chimney effects, piston fans, traffic. Now we put it all together into one design scenario in which you have the largest fire at the largest wind and perhaps at the biggest traffic, perhaps in a traffic jam. This is the typical design scenario for which you would be choosing the equipment you're using in a tunnel. And the difficulty is like how rare is that actually Like fires in tunnels? In most of the tunnels, if you run the calculations, they would happen once every two to five years. Let's say one in the four years. Now, if you look at the distribution of the size of fires in real tunnels, the biggest fires, the 100 megawatt-ish fires, are perhaps 1% of all the events in the tunnels. So we're talking about one in 100, that's already one to the four. So one in 400 years you would have such a massive fire. You can do the calculations on your own. Just look for tunnel fire statistics. Many road administrations publish those data so you can find the data that's most relevant to the place where you live, where you engineer, and just do the calculations for yourself. Then, if you had wind, we often designed for a 95th percentile of wind, which means one to twenty chance that this particular wind action will occur. So we designed the system that's already once in 400 years to one in the 20 chance of particular wind. So we end up in one to 8,000 years scenario and that's our baseline design scenario for the tunnel. Now it doesn't matter when you are designing with some very nice goals that you define for your system, but if you design for critical velocity and the wind action is acting against your velocity, it means you have to over design your ventilation system to accommodate for this wind action. Which means you heavily over design your system for this one in 8,000 years scenario. And the consequence is that if you don't have such a strong wind, if you don't have a such a strong fire, you simply create a very, very high velocity flow in the tunnel 5 meters, 6 meters, 7 meters per second. This kind of flow can actually cause the fire to grow bigger. So if you think about it, our safety system in the tunnel in one in 8,000 year scenario is the exact system we need for that scenario. But in all other fires that could happen over those 8,000 years, the system Perhaps can accelerate the growth of fire and make it kind of worse like. I'm not very comfortable with this idea. That's also why I am criticizing the concept of critical velocity. I think we need to find better ways to accommodate and Perhaps those most rare, most dangerous scenarios we can treat with a confining velocity concept, with Concept that some back layering is low in such a large fire. Some other counter measures, like understanding what traffic we have in the tunnel, understanding the communication between the tunnel and fire brigade to accelerate the extinguishing action, perhaps by introducing sprinklers into the tunnel or any water sprays into the tunnel that will handle this biggest event. And then design ventilation for a more feasible scenarios and more like everyday fires that you find in the tunnels, which would lead to a smaller ventilation system. Design still a system that that's very safe, and because we would have smaller ventilation systems, we would have smaller tunnels, and that are savings in hundreds of millions. My friends, this is the scale at which we are battling. The tunnels are extremely Expensive systems and, as I mentioned at the start, the ventilation design drives the size of the tunnel in a large way and Optimizing it can lead to insane savings on the whole project, which is essentially a cost on the society. Perhaps those hundreds of millions could be spent on safety elsewhere and actually improve safety in that case. So this is why I believe we need a different approach to designing tunnels. This is something that comes from my experience as a tunnel engineer. I'm designing those systems according to the current regulations, current standards, and I feel they are lacking in many aspects and I feel we can do so much better introducing performance based engineering into the concept. Now, last thing I want to mention is some future directions for small control devices in tunnels. So I observe there is some development happening around the world to make systems that adapt their performance to what's happening inside the tunnel. We were also defining such systems at ITP some years ago. We've called it some lots more control. It was meant for Buildings, not tunnels, but the same concept actually works in tunnels. We've confirmed that already. We know that those are based on PADs, pis, other Regulators that measure the velocity in your tunnel and match the performance of the fans. I think it's very interesting. There are challenges ahead, first of them being the fire. The presence of the fire, which changed the flow, changes the densities, makes this Steering operation much more difficult. I also believe there are significant issues with time lag, how long it takes to speed up the air in the tunnel, how long it takes to slow it down. But it's something we are very actively working on. As I mentioned, my student, jakub, is working on initial conditions and the impact of the traffic on the conditions in the tunnel. So we can count for that. Designing ventilation systems. One thing we still need to solve, perhaps some sort of a holy grail is at what conditions we can transport smoke longitudinally but not disturb the buoyancy. We know that it happens. If you have a flow of, let's say, one and a half meter per second, you will transport the smoke longitudinally in your tunnel and you will transport it as a layer, not through the whole cross-section of your tunnel, which gives a chance of those being in a cold air layer below To actually survive and evacuate. So this is some sort of a holy grail, especially for bi-directional tunnels, and it would be great to actually have Defined conditions in which this thing happens. And finally, another future direction is is use of machine learning in detecting fires, in in measuring how big the fire is. Already there are algorithms that can tell you, based on the measurements of temperature inside the tunnel, how big and where the fire is. And if you know that, you can choose your ventilation system operation accordingly. You don't have to always blow with maximum flow, as for this one in a thousand years scenario, but you could actually match the performance of your system to the things that are happening outside. Same for wind. For wind, we did it already in Warsaw we measured the wind and we set up the jet fan component of our system Accordingly to what's happening outside to counter the wind. So you can build smart system that reacts to the events happening in and out of the tunnel and give you the best performance that you need for it. And this is developed in tunnels, and I many times in the podcast I've said tunnels are the space of the of the quickest innovation in all of the fire science and engineering. So if we develop such systems for tunnels, five, ten years down, you will have them in car parks and in civilian buildings, let's say so. I highly hope that this developments go great and that we can achieve something interesting in here. So that would be it for today's rant about tunnel ventilation systems and some experiences we had. I Kind of fail not criticizing the critical velocity, which I promise I'm sorry for that. I promised I'll make it less tunnel ish. So I hope you've enjoyed the bits about the flow. If you're not a tunnel engineer, if you are a tunnel engineer, I really hope you've enjoyed the entirety of it. If it's rubbish, tell me, because I have to give this speech in one month the directors of European roads so I better give a good talk in there. It's much easier when you have slides and, yeah, just an audio content is so much harder. It's good training. So thank you for giving me an opportunity to train myself and make some thoughts I have more clear. I hope I will convey them to the road administrators in a good way. And, yeah, I'm championing better solutions in tunnels. I'm championing better performance based design objectives. I'm championing moving away from singular critical velocity in favor of Some other goals that we can achieve that are not necessarily a single value for all scenarios, but more based on what's happening in and outside of the tunnel. I'm championing into including the obvious things like traffic in the analysis to make them more realistic, and I'm championing a safer tunnels at more reasonable price. I hope that's something that we will be able to achieve in the future. Thank you for staying here with me. Let me know what you think. If you would like to talk about tunnel fire safety design, you can give me a call. I'm very happy to talk with everybody about it. It's something that I love doing and and again, I've realized we need to write more papers about it. So instead of my random opinions, I can give you pre-review science. The ones that I have already is in the show notes, and I promise you there is a lot more coming from us on this space. Thanks for being here with me. See you here next Wednesday. Bye you.