Combustion & Reactions (REAC)¶

Define fuel properties and combustion reactions for fire simulations.

Overview¶

The REAC (reaction) namelist specifies fuel properties and combustion chemistry. FDS includes default fuels, but custom fuels can be defined for specific scenarios.

# Use default propane
sim.add(Reaction(fuel='PROPANE'))

# Custom fuel with specific properties
sim.add(Reaction(
    id='WOOD',
    fuel='CELLULOSE',
    soot_yield=0.015,
    co_yield=0.004
)

Default Fuels¶

FDS includes predefined fuels with realistic properties:

Common Default Fuels¶

# Hydrocarbon fuels
sim.add(Reaction(fuel='METHANE')      # Natural gas
sim.add(Reaction(fuel='PROPANE')      # LPG
sim.add(Reaction(fuel='ETHYLENE')     # Polymer precursor
sim.add(Reaction(fuel='ACETYLENE')    # Welding gas

# Liquid fuels
sim.add(Reaction(fuel='N-HEPTANE')    # Gasoline surrogate
sim.add(Reaction(fuel='ETHANOL')      # Alcohol
sim.add(Reaction(fuel='METHANOL')     # Wood alcohol

# Solid fuels
sim.add(Reaction(fuel='CELLULOSE')    # Wood, paper
sim.add(Reaction(fuel='POLYURETHANE') # Foam
sim.add(Reaction(fuel='POLYSTYRENE')  # Plastic

Fuel Properties¶

Heat of Combustion¶

# Heat released per kg of fuel burned
sim.add(Reaction(
    id='CUSTOM_FUEL',
    heat_of_combustion=25000.0  # kJ/kg
)

Soot Yield¶

# Mass of soot per mass of fuel
sim.add(Reaction(
    id='SOOTY_FUEL',
    fuel='N-HEPTANE',
    soot_yield=0.037  # 3.7% of fuel mass becomes soot
)

# Clean-burning fuel
sim.add(Reaction(
    id='CLEAN_FUEL',
    fuel='METHANE',
    soot_yield=0.001  # Very little soot
)

CO Yield¶

# Carbon monoxide production
sim.add(Reaction(
    id='FUEL_WITH_CO',
    fuel='PROPANE',
    co_yield=0.010  # 1% of fuel mass becomes CO
)

Radiative Fraction¶

# Fraction of energy radiated (not convected)
sim.add(Reaction(
    id='FUEL_RAD',
    fuel='PROPANE',
    radiative_fraction=0.35  # 35% radiated, 65% convected
)

# Sooty fire (more radiation)
sim.add(Reaction(
    id='SOOTY_FIRE',
    fuel='N-HEPTANE',
    radiative_fraction=0.45  # Higher radiation
)

Custom Fuel Composition¶

Stoichiometry¶

Define fuel chemical formula:

# Wood (approximated as cellulose C6H10O5)
sim.add(Reaction(
    id='WOOD',
    formula='C6H10O5',
    heat_of_combustion=15000.0,  # kJ/kg
    soot_yield=0.015,
    co_yield=0.004,
    radiative_fraction=0.35
)

# Polyethylene (C2H4)n
sim.add(Reaction(
    id='POLYETHYLENE',
    formula='C2H4',
    heat_of_combustion=43000.0,
    soot_yield=0.060,
    radiative_fraction=0.40
)

Fuel Composition Table¶

Material	Formula	ΔH_c (kJ/kg)	Soot Yield	CO Yield
Methane	CH4	50,000	0.001	0.001
Propane	C3H8	46,300	0.024	0.010
N-Heptane	C7H16	44,600	0.037	0.010
Ethanol	C2H6O	26,800	0.008	0.007
Cellulose	C6H10O5	15,000	0.015	0.004
Polystyrene	C8H8	39,800	0.164	0.060
PVC	C2H3Cl	16,400	0.172	0.063

Complete Examples¶

Pool Fire with Custom Fuel¶

from pyfds import Simulation

sim = Simulation(chid='pool_fire')
sim.add(Time(t_end=300.0))
sim.add(Mesh(ijk=Grid3D.of(60, 60, 40), xb=Bounds3D.of(0, 6, 0, 6, 0, 4)))

# Custom gasoline surrogate (n-heptane)
sim.add(Reaction(
    id='GASOLINE',
    fuel='N-HEPTANE',
    heat_of_combustion=44600.0,  # kJ/kg
    soot_yield=0.037,
    co_yield=0.010,
    radiative_fraction=0.33
)

# Pool fire surface
sim.add(Surface(
    id='POOL',
    hrrpua=1500.0,  # kW/m²
    color='ORANGE'
)

# Circular pool (1m radius)
sim.add(Vent(
    xb=Bounds3D.of(-2, 2, -2, 2, 0, 0),
    surf_id='POOL',
    xyz=Point3D.of(3, 3, 0),
    radius=1.0
)

# Soot and CO monitoring
sim.add(Device(
    id='SOOT',
    quantity='SOOT DENSITY',
    xyz=Point3D.of(3, 3, 2)
)

sim.add(Device(
    id='CO',
    quantity='VOLUME FRACTION',
    spec_id='CARBON MONOXIDE',
    xyz=Point3D.of(3, 3, 2)
)

sim.write('pool_fire.fds')

Wood Fire¶

sim = Simulation(chid='wood_fire')
sim.add(Time(t_end=600.0))
sim.add(Mesh(ijk=Grid3D.of(50, 50, 30), xb=Bounds3D.of(0, 5, 0, 5, 0, 3)))

# Wood fuel properties
sim.add(Reaction(
    id='WOOD',
    fuel='CELLULOSE',
    heat_of_combustion=15000.0,
    soot_yield=0.015,
    co_yield=0.004,
    radiative_fraction=0.35
)

# Wood crib surface
sim.add(Surface(
    id='WOOD_CRIB',
    hrrpua=600.0,
    color='BROWN'
)

sim.add(Obstruction(
    xb=Bounds3D.of(2, 3, 2, 3, 0, 0.5),
    surf_id='WOOD_CRIB'
)

# Visibility (affected by soot)
sim.add(Device(
    id='VISIBILITY',
    quantity='VISIBILITY',
    xyz=Point3D.of(2.5, 2.5, 1.5)
)

sim.write('wood_fire.fds')

Plastic Fire (High Soot)¶

sim = Simulation(chid='plastic_fire')
sim.add(Time(t_end=600.0))
sim.add(Mesh(ijk=Grid3D.of(60, 50, 30), xb=Bounds3D.of(0, 6, 0, 5, 0, 3)))

# Polystyrene (very sooty)
sim.add(Reaction(
    id='POLYSTYRENE',
    fuel='POLYSTYRENE',
    heat_of_combustion=39800.0,
    soot_yield=0.164,  # High soot production
    co_yield=0.060,
    radiative_fraction=0.42  # High radiation due to soot
)

# Burning plastic items
sim.add(Surface(
    id='PLASTIC',
    hrrpua=800.0,
    color='BLACK'
)

sim.add(Obstruction(
    xb=Bounds3D.of(2.5, 3.5, 2, 3, 0, 1),
    surf_id='PLASTIC'
)

# Dense smoke monitoring
sim.add(Device(
    id='SOOT_CEILING',
    quantity='SOOT DENSITY',
    xyz=Point3D.of(3, 2.5, 2.9)
)

sim.add(Device(
    id='OPTICAL_DENSITY',
    quantity='OPTICAL DENSITY',
    xyz=Point3D.of(3, 2.5, 2.9)
)

sim.add(Device(
    id='VISIBILITY',
    quantity='VISIBILITY',
    xyz=Point3D.of(3, 2.5, 1.5)
)

sim.write('plastic_fire.fds')

Multi-Fuel Fire¶

sim = Simulation(chid='multi_fuel')
sim.add(Time(t_end=600.0))
sim.add(Mesh(ijk=Grid3D.of(80, 60, 30), xb=Bounds3D.of(0, 8, 0, 6, 0, 3)))

# Define multiple fuels
sim.add(Reaction(
    id='GASOLINE',
    fuel='N-HEPTANE',
    soot_yield=0.037,
    co_yield=0.010
)

sim.add(Reaction(
    id='WOOD',
    fuel='CELLULOSE',
    soot_yield=0.015,
    co_yield=0.004
)

# Gasoline pool fire
sim.add(Surface(id='GAS_POOL', hrrpua=2000.0, reac_id='GASOLINE'))
sim.add(Obstruction(xb=Bounds3D.of(2, 3, 2, 3, 0, 0.1), surf_id='GAS_POOL'))

# Wood pallet fire
sim.add(Surface(id='WOOD_PALLET', hrrpua=600.0, reac_id='WOOD'))
sim.add(Obstruction(xb=Bounds3D.of(5, 6, 3, 4, 0, 1), surf_id='WOOD_PALLET'))

# Monitor species from both fires
sim.add(Device(
    id='SOOT',
    quantity='SOOT DENSITY',
    xyz=Point3D.of(4, 3, 2)
)

sim.write('multi_fuel.fds')

Radiation and Soot¶

Soot-Radiation Coupling¶

Soot increases radiative heat transfer:

# Clean fuel (low soot, low radiation)
sim.add(Reaction(
    id='METHANE',
    fuel='METHANE',
    soot_yield=0.001,
    radiative_fraction=0.15  # Low radiation
)

# Sooty fuel (high soot, high radiation)
sim.add(Reaction(
    id='HEPTANE',
    fuel='N-HEPTANE',
    soot_yield=0.037,
    radiative_fraction=0.33  # Higher radiation
)

Measuring Radiative Heat Flux¶

# Define sooty fuel
sim.add(Reaction(
    id='FUEL',
    fuel='N-HEPTANE',
    soot_yield=0.040,
    radiative_fraction=0.35
)

# Fire
sim.add(Surface(id='FIRE', hrrpua=1500.0, reac_id='FUEL'))
sim.add(Obstruction(xb=Bounds3D.of(2, 3, 2, 3, 0, 0.1), surf_id='FIRE'))

# Radiative heat flux at distance
sim.add(Device(
    id='RAD_HF_1M',
    quantity='RADIATIVE HEAT FLUX',
    xyz=Point3D.of(4, 2.5, 1),
    ior=1  # Facing fire
)

sim.add(Device(
    id='GAUGE_HF_1M',
    quantity='GAUGE HEAT FLUX',
    xyz=Point3D.of(4, 2.5, 1),
    ior=1  # Total (rad + conv)
)

Species Tracking¶

Oxygen Depletion¶

sim.add(Reaction(fuel='PROPANE'))

# Monitor oxygen concentration
sim.add(Device(
    id='O2_UPPER',
    quantity='VOLUME FRACTION',
    spec_id='OXYGEN',
    xyz=Point3D.of(3, 2.5, 2.5)
)

sim.add(Device(
    id='O2_LOWER',
    quantity='VOLUME FRACTION',
    spec_id='OXYGEN',
    xyz=Point3D.of(3, 2.5, 0.5)
)

CO and CO2 Production¶

sim.add(Reaction(
    id='FUEL',
    fuel='PROPANE',
    co_yield=0.010,  # CO production
    soot_yield=0.024
)

# Track CO (toxic)
sim.add(Device(
    id='CO_CONC',
    quantity='VOLUME FRACTION',
    spec_id='CARBON MONOXIDE',
    xyz=Point3D.of(3, 2.5, 1.5)
)

# Track CO2 (asphyxiant)
sim.add(Device(
    id='CO2_CONC',
    quantity='VOLUME FRACTION',
    spec_id='CARBON DIOXIDE',
    xyz=Point3D.of(3, 2.5, 1.5)
)

Combustion Efficiency¶

Fuel Lean vs Fuel Rich¶

# Well-ventilated fire (complete combustion)
sim.add(Reaction(
    id='COMPLETE',
    fuel='PROPANE',
    co_yield=0.005,  # Low CO
    soot_yield=0.015  # Low soot
)

# Under-ventilated fire (incomplete combustion)
sim.add(Reaction(
    id='INCOMPLETE',
    fuel='PROPANE',
    co_yield=0.020,  # Higher CO
    soot_yield=0.040  # More soot
)

Best Practices¶

1. Use Realistic Fuel Properties¶

# Good: Properties from literature/experiments
sim.add(Reaction(
    id='WOOD',
    fuel='CELLULOSE',
    heat_of_combustion=15000.0,  # From SFPE Handbook
    soot_yield=0.015,
    co_yield=0.004
)

# Avoid: Made-up values
sim.add(Reaction(
    id='MYSTERY_FUEL',
    heat_of_combustion=100000.0,  # Unrealistic!
    soot_yield=0.5
)

2. Match Fuel to Application¶

# Gasoline spill
sim.add(Reaction(fuel='N-HEPTANE')  # Good surrogate

# Natural gas leak
sim.add(Reaction(fuel='METHANE')  # Appropriate

# Wood structure
sim.add(Reaction(fuel='CELLULOSE')  # Reasonable approximation

3. Consider Soot Impact¶

# For visibility/radiation studies, soot is critical
sim.add(Reaction(
    id='FUEL',
    soot_yield=0.037,  # Don't neglect!
    radiative_fraction=0.33
)

# Monitor soot effects
sim.add(Device(id='VISIBILITY', quantity='VISIBILITY', xyz=Point3D.of(3, 2.5, 1.5)))
sim.add(Device(id='SOOT', quantity='SOOT DENSITY', xyz=Point3D.of(3, 2.5, 2)))

4. Document Fuel Sources¶

# Clear documentation
# Fuel properties from:
# - Heat of combustion: SFPE Handbook, 5th Ed.
# - Soot yield: Mulholland & Croarkin (2000)
# - CO yield: Tewarson (2008)
sim.add(Reaction(
    id='POLYURETHANE',
    fuel='POLYURETHANE',
    heat_of_combustion=25000.0,
    soot_yield=0.131,
    co_yield=0.042
)

Common Issues¶

Too much/too little soot

Cause: Incorrect soot yield

Solution: Check literature values for fuel type

# Typical soot yields:
# - Methane: 0.001
# - Propane: 0.024
# - Gasoline: 0.037
# - Wood: 0.015
# - Polystyrene: 0.164
sim.add(Reaction(fuel='PROPANE', soot_yield=0.024)  # Realistic

Wrong radiative fraction

Cause: Inconsistent with soot production

Solution: Higher soot → higher radiation

# Clean fuel
sim.add(Reaction(fuel='METHANE', soot_yield=0.001, radiative_fraction=0.15))

# Sooty fuel
sim.add(Reaction(fuel='N-HEPTANE', soot_yield=0.037, radiative_fraction=0.33))

Unrealistic CO levels

Cause: Incorrect CO yield

Solution: Use measured yields (typically 0.001-0.060)

# Well-ventilated
sim.add(Reaction(co_yield=0.004))

# Under-ventilated
sim.add(Reaction(co_yield=0.020))

Advanced Topics¶

Simplified vs Detailed Chemistry¶

FDS uses simplified combustion (mixture fraction approach):

# FDS default: Simple, fast
sim.add(Reaction(fuel='PROPANE')  # One-step chemistry

# For most fire simulations, this is sufficient
# Detailed chemistry (not in FDS) needed only for:
# - Flame chemistry research
# - Pollutant formation mechanisms
# - Ignition transients

Heat Release Rate vs Fuel Mass¶

# HRR specified directly (most common)
sim.add(Surface(id='FIRE', hrrpua=1000.0)  # kW/m²

# Mass flux (advanced)
sim.add(Surface(
    id='FIRE',
    mass_flux=0.020,  # kg/m²/s
    reac_id='PROPANE'  # Uses fuel heat of combustion
)

Fuel Property Reference¶

Gaseous Fuels¶

Fuel	Formula	ΔH_c (kJ/kg)	Soot Yield	Radiative Fraction
Methane	CH4	50,000	0.001	0.15
Propane	C3H8	46,300	0.024	0.30
Ethylene	C2H4	47,200	0.059	0.35

Liquid Fuels¶

Fuel	Formula	ΔH_c (kJ/kg)	Soot Yield	Radiative Fraction
Methanol	CH4O	20,000	0.001	0.15
Ethanol	C2H6O	26,800	0.008	0.20
N-Heptane	C7H16	44,600	0.037	0.33

Solid Fuels¶

Fuel	Approx. Formula	ΔH_c (kJ/kg)	Soot Yield	Radiative Fraction
Cellulose	C6H10O5	15,000	0.015	0.35
Polystyrene	C8H8	39,800	0.164	0.42
Polyurethane	C7H12O2N2	25,000	0.131	0.40

Next Steps¶

Fire Sources - Apply fuel properties to fires
Materials & Surfaces - Combustible surfaces
Devices - Monitor combustion products
Examples - Complete fire scenarios

Examples →