Fire and Smoke Analysis Using FDS: How Engineers Predict Fire Behaviour

A fire safety design lives or dies on one question: will people have enough time to get out? Not "is there a sprinkler system" or "is there a smoke alarm" - those matter, but the real question is a race against time, measured in seconds, between how fast smoke and heat make a space untenable and how long it actually takes occupants to escape. Fire Dynamics Simulator (FDS) is the tool engineers use to run that race in advance - modelling fire growth, heat release and smoke transport in a specific building's geometry, so the answer is based on physics and evidence rather than a rule-of-thumb guess.

Key takeaways

• FDS is a specialised CFD tool, developed by NIST, purpose-built for fire-driven combustion, heat release and smoke transport.
• Every study starts with a design fire - a heat release rate curve representing a realistic worst-case scenario, not a generic assumption.
• The core safety comparison is Available Safe Egress Time (ASET) against Required Safe Egress Time (RSET), with a margin.
• Mesh resolution, boundary conditions and design fire selection all need to be defensible, documented choices - not default settings.

Table of Contents

• What Is FDS?
• How FDS Models Fire
• The Design Fire
• Key Outputs & Metrics
• The Simulation Process
• Applications
• Common Mistakes
• FAQ

What Is FDS?

Fire Dynamics Simulator is a Computational Fluid Dynamics tool developed by the U.S. National Institute of Standards and Technology (NIST), built specifically to model fire-driven fluid flow - combustion, heat release, buoyant smoke transport and radiative heat transfer. Unlike general-purpose CFD, FDS is tuned around the specific physics of low-speed, thermally-driven flow that characterises fires and their smoke plumes.

Free, validated, and widely adopted: FDS is free to use and has been extensively validated against real fire test data over more than two decades of development, which is a significant part of why it's become a standard tool in fire engineering practice worldwide, typically paired with Smokeview for visualising results.

How FDS Models Fire

FDS solves the governing equations for low-speed, thermally-driven flow, coupling combustion chemistry, heat release, radiative and convective heat transfer, and smoke/soot transport within the specific 3D geometry being modelled - walls, openings, ductwork, obstructions.

Physical process	What FDS predicts
Combustion & heat release	How fast the fire grows and how much energy it releases over time
Buoyant plume & smoke transport	How smoke rises, spreads and forms a layer within the space
Radiative & convective heat transfer	Temperature development at surfaces and within the smoke layer
Species transport	Concentration of toxic combustion products relevant to tenability

The Design Fire

Every FDS study starts with a design fire - a heat release rate (HRR) curve describing how quickly the fire grows, how large it becomes, and how it eventually decays. Real fires broadly follow a recognisable shape: a slow incipient phase, an accelerating growth phase, a peak governed by available fuel or ventilation, and a decay phase as fuel is consumed.

Incipient → Growth (often t²) → Fully developed (peak) → Decay

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The design fire is the single most consequential inputEvery downstream result - smoke layer height, temperature, tenability, ASET - depends on the design fire chosen at the start. It should be based on the actual fuel load and occupancy of the specific space and relevant published test data or code guidance, not picked arbitrarily or copied from an unrelated project.

Key Outputs & Metrics

Output	Why it matters
Smoke layer height	Whether smoke stays above head height long enough for safe evacuation
Temperature	Thermal tenability along escape routes and near occupants
Visibility	Whether occupants can see exit signage and wayfinding cues through smoke
Toxic gas concentration	Whether combustion products reach hazardous levels before evacuation completes
ASET (Available Safe Egress Time)	The time before conditions along the escape route become untenable

The central safety comparison is ASET versus RSET - Required Safe Egress Time, the time occupants actually need to detect the fire, react and evacuate, typically from separate evacuation modelling or code-based assumptions. A design is generally considered acceptable when ASET comfortably exceeds RSET.

Internal link: for a related look at how this applies specifically to parking structures, see our article on CFD & FDS for Basement Parking.

The Simulation Process

Define design fire → Build geometry & mesh → Set boundary conditions → Run simulation → Extract outputs → Validate & report

1. Define the design fire: a heat release rate curve representing a realistic worst-case scenario for the specific space and fuel load.
2. Build the geometry and mesh: the building's walls, openings, ductwork and obstructions, divided into a computational mesh fine enough to resolve fire and smoke behaviour.
3. Set boundary conditions: material thermal properties, ventilation openings, mechanical smoke extraction and any active fire protection systems.
4. Run the simulation: heat release, temperature, smoke layer height, visibility and toxic gas concentration are tracked over the relevant time period.
5. Extract key outputs: tenability along evacuation routes is reviewed, comparing ASET against RSET at critical locations.
6. Validate and report: results are sanity-checked against known fire behaviour benchmarks and documented for design or regulatory review.

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Mesh sensitivity isn't optional: Running the same model at a finer mesh resolution and confirming results converge is a standard, expected part of a defensible FDS study - skipping it leaves the results open to legitimate challenge during design or regulatory review.

Applications

Application	What FDS evaluates
Basement & underground parking	Smoke extraction performance and evacuation tenability from a vehicle fire
Atria & large open spaces	Smoke layer development and the effectiveness of smoke exhaust systems
High-rise buildings	Stairwell pressurisation and smoke control performance during evacuation
Transport & tunnels	Smoke movement and critical velocity requirements for tunnel ventilation
Industrial facilities	Fire behaviour around process equipment and storage areas

Common Mistakes

• Choosing an unrealistic or unjustified design fire. Since every result depends on this input, an unsupported assumption undermines the whole study.
• Skipping the mesh sensitivity study. Results at an under-resolved mesh can be significantly inaccurate without any visible warning that they are.
• Testing only one fire location or scenario. Smoke development can vary significantly depending on where a fire starts relative to exits and ventilation.
• Comparing ASET to a casually estimated RSET. Both sides of the comparison need a defensible basis - a generous ASET means little against a loosely guessed RSET.
• Treating the report as a formality rather than a design tool. The most value often comes from using early results to improve the design, not just documenting a fixed layout after the fact.

Related reading: see our article on CFD & FDS for Basement Parking, explore our CFD & Fire Analysis Services, or contact our fire engineering team to discuss a project.

Frequently Asked Questions

What is FDS?

Fire Dynamics Simulator (FDS) is a computational fluid dynamics tool developed by the U.S. National Institute of Standards and Technology (NIST), purpose-built for modelling fire-driven fluid flow - combustion, heat release, buoyant smoke transport and heat transfer - in enclosed and open environments. It is free, widely validated, and commonly used alongside a visualization tool called Smokeview to review results.

How is the heat release rate curve for a design fire determined?

The heat release rate curve is typically based on the fuel type and fire load expected in the space, drawing on published test data for similar fuel packages (furniture, vehicles, storage materials) and applicable code or standard guidance for design fire selection. It is chosen to represent a realistic worst-case scenario appropriate to the specific occupancy, not an arbitrary or generic assumption.

What is a mesh sensitivity study and why does it matter for FDS?

A mesh sensitivity study compares simulation results at different mesh resolutions to confirm the results have converged and aren't simply an artefact of an under-resolved computational grid. Since FDS results can be sensitive to mesh size relative to fire size, this check is a standard part of a defensible FDS methodology.

What is the difference between ASET and RSET?

Available Safe Egress Time (ASET) is the time predicted by the fire and smoke simulation before conditions along an escape route become untenable - too hot, too smoky, or too toxic. Required Safe Egress Time (RSET) is the time needed for occupants to detect the fire, react and physically evacuate to safety, typically from evacuation modelling or code-based assumptions. A design is generally considered acceptable when ASET exceeds RSET with an appropriate safety margin.

Conclusion

Fire safety design ultimately comes down to a margin - the gap between how long a space stays survivable and how long people actually need to get out of it. FDS is what turns that margin from an assumption into a calculated, defensible number, grounded in the physics of how a specific fire would actually grow and spread through a specific building. Getting the design fire right, resolving the geometry properly, and checking the result against a real evacuation timeline is what separates a fire safety case that would hold up in an actual emergency from one that only looks convincing on paper.

Whether you're validating a new atrium's smoke control system or checking an existing building's evacuation margin, an FDS study replaces a general assumption about fire behaviour with a specific, tested answer for your actual space.

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