Modeling the consequences of a fire is what most of us do day to day. Modeling how a fire actually grows across real materials, under changing heat flux and ventilation, is the part we tend to avoid, hiding behind a prescribed design fire. That gap is exactly what we tackle with Jason Floyd and Jonathan Hodges from UL Research Institutes, the developers of S-PYRO, a new engineering pyrolysis model already included in Fire Dynamics Simulator (FDS).

We walk through the origin story in railcar fire growth modeling, where “traveling fire” assumptions and fixed HRR curves can fall apart once the compartment becomes ventilation-limited. S-PYRO takes a more practical route to predictive fire modeling: it uses cone calorimeter data (multiple heat flux levels, and optionally multiple thicknesses) and scales burning rate and time so mass consumption stays consistent while the exposure conditions come from your CFD simulation. That means the local incident heat flux in FDS drives which cone response the surface should follow, instead of guessing the fuel output up front.

The hardest detail is also the most important: flame feedback. A burning sample sees extra heat flux from its own flame, and the cone heater flux can be partially absorbed by soot and hot gases. Jason and Jonathan explain how they built a “digital twin” cone using high-resolution FDS runs to generate lookup tables for flame heat flux and cone absorption across different fuel properties, plus how validation against large public cone databases reveals where the self-similarity assumption holds and where it breaks (dripping polymers, delamination, collapse, long-duration char oxidation, and complex assemblies).

If you’ve ever wanted fire growth modeling that’s more realistic than a scripted curve but more approachable than full kinetics, this is the roadmap. Subscribe for more fire science, share this episode with a colleague who uses FDS, and leave a review if it helps. What material or scenario would you try S-PYRO on first?

Resources:

• Jack Bono Award winning paper "An Approach for Flux and Thickness Scaling of Cone Calorimeter Data for Predicting the Pyrolysis of Materials" (improved SPyro)
• An engineering model for the pyrolysis of materials
• FDS github with all the resources

Cover image from a paper: J. Hodges et al., MODELING THE FIRE GROWTH IN A RAILCAR, Interflam 2025

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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.

ASET/RSET is the fire safety engineering idea that sounds almost too clean to argue with: the building must give you more time to escape than you need to get out. But once you try to prove it in a real project, the simplicity could and and the hard questions start. Where does “time zero” actually begin: ignition, detection, or alarm? What does it mean for tenability to fail in a place where nobody is anymore? And are we designing safer buildings, or just winning a seconds-on-a-spreadsheet contest?

I break down the Available Safe Egress Time and Required Safe Egress Time concept from both sides: the human evacuation model and the fire model. On the RSET side, we talk about detection and alarm strategy, pre-movement time distributions, and why congestion, stair capacity, and occupant behavior can dominate the result. On the ASET side, we get into CFD fire modeling, tenability criteria like visibility, heat, and toxicity, and the huge responsibility that comes with choosing a design fire that is both plausible and fail-safe. I also explain why fire location can completely change outcomes even when the heat release rate curve is identical.

We also tackle the famous criticism of ASET/RSET, where it truly breaks down (especially residential scenarios), and why I still find it useful in complex spaces like public venues and tunnels. To move beyond one “magic number,” I share practical alternatives like scenario-by-scenario comparisons, probabilistic thinking, ASET/RSET maps, and agent-based tenability approaches, plus a look at digital twin possibilities for smarter evacuation guidance.

If this helped you rethink how you use ASET/RSET in performance-based fire engineering, subscribe, share the episode with a colleague, and leave a review so more engineers can find it. What’s the toughest ASET/RSET argument you’ve had to defend?

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Three mentioned links in the episode:

Rino's pre evacuation time distributions: https://www.sciencedirect.com/science/article/pii/S0379711218302212
Vyto's paper here: https://onlinelibrary.wiley.com/doi/abs/10.1002/fam.1025
Tenability survey paper here: https://www.scopus.com/pages/publications/105046523596?origin=resultslist

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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.

In this episode I take you into practicality of introducing wind into fire safety engineering considerations, and focus on it's implementation as a part of fire oriented CFD analyses. In here, we dig into why wind is tied to so many catastrophic fires, why firefighter safety is often shaped by sudden wind shifts, and why designing for calm air can quietly turn fire safety into a coin toss.

I walk through what wind actually is from a fire engineering perspective: an atmospheric, turbulent phenomenon full of vortices and gusts, not a tidy uniform flow. We talk about the trap of modeling only the average wind speed, why gusts matter so much for smoke movement and vent performance, and how the wind profile changes with height and terrain roughness. From there, I connect the dots to computational wind engineering and the messy reality of cities, where nearby buildings, parks, towers, and “urban canyons” can completely reshape the local wind field right at your façade openings.

Then we get practical about CFD. I share the domain and mesh decisions that keep results honest (including blockage and downstream wake space), how to handle wind direction, and what turbulence modeling choices mean when you’re trying to couple wind and fire. Finally, I lay out when the hassle is worth it: constrained historic roofs, atria with limited openings, exposed natural smoke ventilators, high-rise mechanical floors, complex tunnel portals, architected natural vents with uncertain discharge coefficients, and roof features where vortex shedding can damage photovoltaics.

The key resource to implement what has been said in the episode:

- Wind and Fire Coupled Modelling—Part I: Literature Review

- Wind and Fire Coupled Modelling—Part II: Good Practice Guidelines

- Fire and Smoke Modelling Chapter

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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.