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/

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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.

Pretty CFD flames are something we got used to, but they usually are a product of our design assumptions - the design fire. While they give us a lovely illusion of being "real" fires, predicting real compartment fire growth is a completely different game. We’re joined by Dr. Stephen Welch (University of Edinburgh) to get honest about what “modeling fire spread” actually means, how quickly results can become believable but wrong, and why validation has to be the foundation of any serious CFD fire modeling workflow.

We walk through the path his group has taken from traveling fires in large compartments to timber-lined compartment behavior, using FDS and detailed pyrolysis models. Along the way we talk about scaling up from cone and crib experiments, why wood sticks burn differently than flat timber, and why calibrated kinetics are often engineering tools rather than universal material truths. We also dig into the uncomfortable reality of scenario uncertainty: moisture, stick variability, crib deformation, and tiny geometric changes can meaningfully shift predictions.

One of the most practical takeaways is how opening location and ventilation momentum reshape fire spread, sometimes in ways classic simplified models won’t anticipate. We explore entrainment height, hot layer effects, soot and radiative fraction, and what it might take to credibly predict burnout and post-burner behavior in timber-lined compartments. If you care about structural fire design, traveling fire exposure, and the future of validated predictive modeling, this one is for you.

After this one, you will have a lot of reading... Start with these:

1. Liu, C., Dai, X., Ming, X.M. & Welch, S. (2026) Exploring fire dynamics of travelling fires in large open compartments with CFD, Fire Safety Journal 104663 (special issue IAFSS2026) doi:10.1016/j.firesaf.2026.104663
2. Liu, C., Dai, X., Ming, X.M. & Welch, S. (2025) CFD predictions of fire spread over wood cribs in large open-plan compartments: new insights, Fire Safety Journal special issue 156: 104443 doi:10.1016/j.firesaf.2025.104443
3. Dai, X., Alam, N., Liu, C., Nadjai, A., Rush, D. & Welch, S. (2024) “Scaling-up” fire spread on wood cribs to predict a large-scale travelling fire test using CFD, Advances in Engineering Software 189: 103589 doi: 10.1016/j.advengsoft.2023.103589
4. Dai, X., Gamba, A., Liu, C., Anderson, J., Charlier, M., Rush, D. & Welch, S. (2022) An engineering CFD model for fire spread on wood cribs for travelling fires, Advances in Engineering Software 173:103213 doi: 10.1016/j.advengsoft.2022.103213

And the cover image comes from the image in paper [2], Fig. 17. Stephens papers are full of beautiful images. Another reason to check them out.

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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.

A compartment fire is more than just “a fire in a room.” The moment flames, smoke, and heat are trapped by walls and fed through openings, the physics changes and so do the hazards we design for. We sit down with Dr. Vinny Gupta (University of Waterloo) to rebuild compartment fire thinking from the ground up, from what the enclosure does to airflow and smoke layers to why those effects still anchor modern structural fire safety.

We trace the roots of today’s design tools through post World War II research and the foundational work of Kunio Kawagoe and Philip Thomas. That takes us straight into the ventilation factor, opening factor, and energy balance logic that underpins so much of compartment fire modeling, including parametric fires and many “golden number” rules engineers carry from project to project. Then we get honest about the fine print: the assumptions, the limits, and what gets lost when we remember the solution but forget the context it came from.

From there, the conversation shifts to modern buildings that refuse to behave like classic small rooms: open-plan compartments, changing ventilation conditions, nonuniform smoke layers, and traveling fires that move across a floor plate instead of involving everything at once. We also dig into why fuel type can matter even when theory says it should not, especially once mass timber and CLT linings enter the compartment and amplify sensitivity to radiation, flow, and ventilation.

If you would like to dig further, my recommendations are:

• Kawagoe's summary of compartment fire research
• Jose Torero's revisiting compartment fire
• Vinny's paper on mechanisms of flame spread in large compartments
• Vinny's paper on different fuels and compartment fires

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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.