Modeling ISO 3941 Fire Classes: A Practical Guide to FDS Implementation

Fires aren’t all created equal. In the world of fire protection engineering, understanding the nature of the fuel is the first step toward stopping it. This is where ISO 3941 comes in—it’s the international standard that categorizes fires into classes based on the fuel source.

But what if you aren’t just fighting a fire, but trying to predict one? That’s where Fire Dynamics Simulator (FDS), the gold standard in CFD (Computational Fluid Dynamics) modeling, becomes your best friend.

In this post, we’ll break down the ISO 3941 fire classes and show you how to represent each one in an FDS simulation.

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What is ISO 3941?

ISO 3941 (and its European equivalent, EN 2) classifies fires into specific categories. This helps engineers choose the right extinguishing agents and design appropriate suppression systems.

Class	Fuel Type	Typical Materials
Class A	Solid Materials	Wood, paper, textiles, plastics
Class B	Flammable Liquids	Gasoline, oil, paint, solvents
Class C	Flammable Gases	Methane, propane, hydrogen
Class D	Combustible Metals	Magnesium, aluminum, sodium
Class F	Cooking Oils	Fats and oils in kitchen appliances

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Simulating ISO 3941 Classes in FDS

In FDS, we don’t just “click a button” for a Class A or Class B fire. We define them using chemical reactions (&REAC) and surface properties (&SURF).

1. Class A: Solid Materials

Class A fires involve pyrolysis—the process where heat decomposes solid fuel into flammable gases.

• The FDS Strategy: You can either prescribe a Heat Release Rate (HRR) or use a “complex” solid phase model.
• FDS Code Snippet:Fortran&SURF ID='WOOD_SURFACE', COLOR='BROWN', PYROLYSIS_RATE=0.01 / &OBST XB=..., SURF_ID='WOOD_SURFACE' / Pro-tip: For simple designs, use HRRPUA (Heat Release Rate Per Unit Area) to skip the complex chemistry and just define how much energy the “wood” emits.

2. Class B: Flammable Liquids

These are essentially pool fires. The fuel evaporates and burns just above the liquid surface.

• The FDS Strategy: Define a liquid fuel (like Heptane) and apply it to a horizontal surface. FDS handles the evaporation based on the heat feedback from the flame.
• FDS Code Snippet:Fortran&REAC FUEL='N-HEPTANE' / &SURF ID='POOL_FIRE', HRRPUA=500, COLOR='BLUE' /

3. Class C: Flammable Gases

Class C fires involve a gas leak—think a broken pipe or a gas burner.

• The FDS Strategy: Instead of an obstruction, you use a vent that “blows” the gas into the room at a specific velocity.
• FDS Code Snippet:Fortran&REAC FUEL='METHANE' / &VENT XB=..., SURF_ID='BURNER', VEL=-0.5 / &SURF ID='BURNER', TMP_FRONT=20, COLOR='RED' /

4. Class D: Combustible Metals

These are the “nightmare” fires for simulators. They burn at incredibly high temperatures and often involve solid-phase combustion that FDS wasn’t originally built for.

• The FDS Strategy: Most engineers simulate Class D by focusing on the extreme radiant heat. You define a custom reaction with a very high heat of combustion and high radiative fraction.
• Note: FDS is generally a gas-phase simulator; modeling the actual melting and “crusting” of metal is highly advanced and requires custom thermophysical properties.

5. Class F: Cooking Fats (Class K in the US)

Class F fires have a unique characteristic: they have high flash points and stay hot for a long time.

• The FDS Strategy: Similar to Class B, but you must account for the high thermal energy of the oil. If you are simulating suppression (like a wet chemical system), you’ll need to focus on the cooling effect of the droplets.

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Summary Table for FDS Implementation

Fire Class	Primary FDS Command	Key Parameter
A (Solids)	&SURF / &MATL	IGNITION_TEMPERATURE, HEAT_OF_PYROLYSIS
B (Liquids)	&SURF	HRRPUA or LIQUID_FUEL properties
C (Gases)	&VENT	VEL (Mass flow rate of the gas)
D (Metals)	&REAC	RADIATIVE_FRACTION (High heat output)
F (Oils)	&REAC / &PART	SPECIFIC_HEAT (for suppression interaction)

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