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.

----
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

----
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.

As a kid of 80s and 90s, a lot of the pop culture was around the post-apocalyptic scenarios following a nuclear exchange. I always thought a nuclear winter is a consequence of atomic bombs falling down, but now as a fire researcher I've learnt it is mostly about fires, after all. Today I sit down with Dr Stephen Welch (University of Edinburgh) to trace the fire-engineering-heavy pathway from mass urban ignitions to firestorms, through soot production and its injection into the stratosphere, up to global climate impacts. The uncomfortable twist is that the energy release from the fires can dominate the scenario, and the climate outcome depends on whether a fraction of that smoke gets high enough and stays there long enough to matter.

We dig into what makes a firestorm different from a big fire: the burned-area thresholds, the requirement for many structures burning at once, and the extreme winds a firestorm can generate on its own. Then we connect urban firestorms to what we’ve learned from megafires and satellite observations, including measured, non-zero global cooling effects, tracing reference datapoints that allow us to approximate the scale needed for a considerable climate impacts. That modern data from wildfires or volcanic eruptions becomes a rare validation anchor for a problem that historically relied on older assumptions and is inherently limited in terms of potential validation.

From there, we go straight into the messy details fire engineers care about: fuel load density, modern plastics, under-ventilated compartment fires, glazing failure, and why soot yields taken from small, well-ventilated tests can be misleading when soot mass is the whole point. All of those, but considering large city fire at once. We also talk modeling options, from CFD to coupled atmosphere simulations, and why “consistent engineering crudeness” is sometimes the only honest starting point. In fact, we dive pretty deep into soot formation and modelling, as the classic "yield" approach we use for everyday fire engineering breaks down at scale of fires discussed here. At the same time, the exact amount of carbon injected into stratosphere is the number deciding of the final impact, therefore it requires a better justification.

I am sure you will enjoy this conversation, as it ties multiple important aspects of fire engineering, tied all into a challenging and interesting research question. How do fires lead to a nuclear winter?

If you would like to read more, try out the thesis of Abhas Adhikari on the subject:

https://imfse.be/s/Abhas-Adhikari_Thesis.pdf

----
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.