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Fire Science ShowAuthor: Wojciech Wegrzynski
Fire Science Show is connecting fire researchers and practitioners with a society of fire engineers, firefighters, architects, designers and all others, who are genuinely interested in creating a fire-safe future. Through interviews with a diverse group of experts, we present the history of our field as well as the most novel advancements. We hope the Fire Science Show becomes your weekly source of fire science knowledge and entertainment. Produced in partnership with the Diamond Sponsor of the show - OFR Consultants Language: en-us Genres: Physics, Science, Technology Contact email: Get it Feed URL: Get it iTunes ID: Get it Trailer: |
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268 - Is critical velocity that critical?
Wednesday, 9 September, 2026
Critical velocity is supposed to be the clean moment when tunnel smoke control flips from unsafe to safe. But after too many real projects and too many review battles, I am very sure that such a single number cannot be a sole proxy of what is safe or unsafe... So why it has so much power over my design?In this solo episode I walk through why critical velocity became the world's favorite proxy for tunnel fire safety: it’s simple, it’s calculable, and it gives verifiers something rigid to check. But what is simple at first glance, can become quite messy in projects. Different code editions, different assumptions for hydraulic diameter or slope, and different approaches like hand calculations vs CFD can push the “right” value around. And the value becomes what we argue about, not the safety. In a tunnel, smoke movement is not one-dimensional either. Ceiling jets, momentum exchange, and the timing of fan activation can all change what it takes to control back-layering.From there, I share project realities that don’t fit the checkbox. Reversible ventilation where slope fights you, station-to-tunnel transitions where uniform velocity is physically impossible, and very wide rail spaces where applying small-tunnel models can imply absurd power demands. We also talk about what happens when you miss critical velocity: back-layering isn’t automatically system failure, and the distance and conditions matter far more than a binary pass/fail.Finally, we dig into the hidden conservatism inside typical tunnel ventilation design scenarios and the real-world cost of “just adding jet fans” once cabling, inverters, power supply, redundancy, and maintenance hit the full design. If you care about tunnel fire engineering, smoke control, CFD modeling, and practical fire safety design, this one is for you. Did you like it? Subscribe for more, share it with a colleague who reviews tunnel designs, and leave a review with your take: should tunnel fire safety rely less on one number and more on performance outcomes?Origins of the critical velocity are in this highly recommended episode: https://www.firescienceshow.com/157-revising-critical-velocity-with-conrad-stacey-and-michael-bayer/----The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.
