In this episode of fire fundamentals we sit down with Professor Simo Hostikka from Aalto University to cover radiation in fires, both from the angle of physical phenomena and ways to model it. In this episode we cover following topics:

• feel less mysterious, from blackbody basics and role of radiation actually does inside the CFD N-S equation.
• Spectrum and emissivity to real engineering outcomes like heat flux, tenability
• Radiation’s two roles in fire CFD: target heat flux and the gas energy source term
• Emission versus absorption and why Kirchhoff’s law is spectral, not just a single number
• Spectrum intuition using Planck, Wien’s law, and why T to the fourth explodes heat flux
• View factors as a hazard mental model for layers, panels, and distance effects
• Why gases are strongly non-gray while soot often looks smooth and easier to approximate
• How FDS uses the finite angle method, why 104 directions exists, and how updates are staged in time, how to manage spatial and temporal resolution of the radiation
• Ray effect and numerical diffusion, when you can see the error and when you cannot
• Other radiation models such as Monte Carlo, and when they are worth it.
  
  

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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.

While we can get pretty far with a very simple approximation of what a fire is in our fire cfd, at some point our simplications are not enough. And there is a plenty of features and phenomena, for which we simply need a better tool to handle - carbon monoxide, soot, extinction, flashover behavior, and what happens when ventilation disappears. At the IAFSS symposium, we sit down with Professor Bart Merci (Ghent University), fresh off delivering the Howard Emmons Invited Plenary Lecture, to talk about what it really takes to model turbulent combustion in real fires without asking practitioners to become full-time combustion scientists.

We start with the engineering reality check: you do not get unlimited mesh resolution, unlimited runtime, or the luxury of endless sensitivity studies. As Bart says - "you need to pick your battles". That practical constraint shapes everything, from whether LES is a smart choice to how you treat the “unseen” physics inside a CFD cell. Bart breaks down turbulence in plain terms, explains why the largest eddies dominate entrainment and smoke movement, and shows how mesh decisions can quietly decide whether LES outperforms unsteady RANS in practical smoke control and compartment fire problems.

Then we go deep on sub-grid combustion models. We unpack why infinitely fast chemistry can be acceptable in well-ventilated flames yet collapses in under-ventilated conditions, where toxicity, soot, and extinction dominate the risk picture. Bart explains a finite-rate, autoignition-informed approach that uses detailed chemistry offline to tune simplified reactions, then applies flamelet concepts and turbulence measures to predict reaction rates and species production inside each cell, including ignition and extinction behavior without relying on a guessed “critical flame temperature.”

We close with what’s next: validation in compartments, microgravity as a brutal test of “universality,” and why advanced non-intrusive diagnostics could finally improve near-wall heat transfer and flame-surface interaction. If you care about CFD, FDS modeling limits, fire dynamics, and the future of practical fire safety engineering, you’ll want this one.

If you would like to read more on the topic, here is Bart's paper that accompanied his brilliant lecture. Figure 3 is what we discuss at the end of the episode.

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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 timber column can survive the heating phase of a fire resistance test and still collapse later, after the flames are gone. We know there is so much more to structures in fires than the test demonstrates, but how much exactly do we know about timber nowadays? In this episode we try to dive deeper and discuss mass timber fire safety, structural fire engineering, and what a fire resistance rating does and does not tell us.

I’m joined by Dr. Felix Wiesner from the University of British Columbia, this year’s IAFSS Proulx Award recipient, to unpack his review on mass timber load-bearing capacity in fire across scales. We start where most design decisions begin: full-scale furnace tests and the practical reality that many modern timber elements are too large, too new, or too costly to test under load. From there we dig into the reduced cross-section method, charring rate assumptions, and the controversial “zero-strength layer” that turns heated wood into a simplified design allowance, even as uncertainty and code-to-code differences persist.

Then we turn to the decay phase and delayed failure, connecting recent column results to the bigger question of performance-based design for compartment fires that heat and cool. To model that behaviour, we need credible links between temperature, strength reduction, and elastic modulus reduction, and we need to care about how the data were generated: steady-state oven tests versus transient tests where timber is loaded first and heated with steep gradients.

Finally, we go down to the microscale and nanoscale, where moisture migration and even hydrogen-bond changes in cellulose help explain why “loaded while heating and cooling” can permanently reshape capacity. If you work with mass timber buildings, timber fire design, Eurocode approaches, or structural safety in fire, this is a deep reset on what matters most.

Read about the IAFSS Awards here https://www.iafss2026.com/awards

Read the whole paper with a more in-depth view on the subject "From nano-to megastructure: A review of mass timber load-bearing capacity in fire"

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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.