Liquid Fuel Guide¶

This guide covers liquid fuel evaporation and combustion modeling in pyfds, including pool fires, liquid mixtures, and evaporation physics.

Overview¶

Liquid fuels require special treatment in FDS due to their evaporation behavior. When BOILING_TEMPERATURE is specified in a MATL namelist, FDS automatically switches to the liquid fuel pyrolysis model, which simulates:

Temperature-dependent evaporation
Heat of vaporization effects
Liquid surface regression
Coupling with gas-phase combustion

Basic Liquid Fuel Setup¶

Simple Liquid Fuel¶

from pyfds import Simulation
from pyfds.core.namelists import Material

# Create simulation
sim = Simulation(chid="liquid_fuel", title="Liquid Fuel Example")

# Define liquid fuel
methanol = Material(
    id="METHANOL_LIQUID",
    density=792.0,              # Liquid density (kg/m³)
    conductivity=0.2,           # Thermal conductivity (W/(m·K))
    specific_heat=2.51,         # Specific heat (kJ/(kg·K))
    # Liquid fuel parameters
    boiling_temperature=64.7,   # Boiling point (°C)
    spec_id="METHANOL",         # Vapor species
    heat_of_reaction=837.0,     # Heat of combustion (kJ/kg)
    absorption_coefficient=140.0  # Absorption coefficient (1/m)
)

sim.material_mgr.add_material(methanol)

Complete Pool Fire Setup¶

For a pool fire, you need both the liquid fuel and a surface to contain it:

from pyfds.core.namelists import Surface

# Liquid fuel material
methanol = Material(
    id="METHANOL_LIQUID",
    density=792.0,
    conductivity=0.2,
    specific_heat=2.51,
    boiling_temperature=64.7,
    spec_id="METHANOL",
    heat_of_reaction=837.0,
    absorption_coefficient=140.0
)

# Pool surface (liquid layer)
pool_surface = Surface(
    id="METHANOL_POOL",
    matl_id="METHANOL_LIQUID",
    thickness=0.05,  # Liquid depth (m)
    tmp_front=25.0   # Initial temperature (°C)
)

sim.material_mgr.add_material(methanol)
sim.material_mgr.add_surface(pool_surface)

Advanced Liquid Fuel Parameters¶

Heat of Vaporization¶

For accurate evaporation modeling:

fuel = Material(
    id="FUEL",
    density=800.0,
    conductivity=0.15,
    specific_heat=2.0,
    boiling_temperature=100.0,
    spec_id="FUEL_VAPOR",
    heat_of_reaction=1500.0,
    # Advanced evaporation
    heat_of_vaporization=500.0,  # Heat of vaporization (kJ/kg)
    mw=46.0,                     # Molecular weight (g/mol)
)

Liquid Mixtures¶

For multi-component liquids:

# Define individual components
ethanol = Material(
    id="ETHANOL_LIQUID",
    density=789.0,
    conductivity=0.17,
    specific_heat=2.44,
    boiling_temperature=78.5,
    spec_id="ETHANOL",
    heat_of_reaction=1367.0,
    absorption_coefficient=1140.0
)

water = Material(
    id="WATER_LIQUID",
    density=1000.0,
    conductivity=0.6,
    specific_heat=4.18,
    boiling_temperature=100.0,
    spec_id="WATER_VAPOR",
    heat_of_reaction=0.0,  # Non-combustible
    absorption_coefficient=1000.0
)

# Mixture surface (volume fractions)
mixture = Surface(
    id="ETHANOL_WATER_MIX",
    matl_id=["ETHANOL_LIQUID", "WATER_LIQUID"],
    thickness=0.05,
    matl_mass_fraction=[0.8, 0.2],  # 80% ethanol, 20% water by mass
    tmp_front=25.0
)

Validation Rules¶

Required Parameters¶

When BOILING_TEMPERATURE is specified:

SPEC_ID must be provided (vapor species)
HEAT_OF_REACTION should be set (combustion energy)
Cannot use N_REACTIONS > 1
Cannot use multi-reaction arrays

Parameter Ranges¶

Boiling temperature: Typically 20-300°C
Heat of vaporization: 100-2000 kJ/kg
Molecular weight: 10-200 g/mol
Absorption coefficient: 100-5000 1/m

Surface Configuration¶

Pool Geometry¶

from pyfds.core.namelists import Obstacle

# Define pool boundaries
pool = Obstacle(
    id="POOL",
    xb=[2.0, 3.0, 2.0, 3.0, 0.0, 0.05],  # 1m x 1m x 5cm deep
    surf_id="METHANOL_POOL"
)

sim.geometry.add_obstacle(pool)

Ignition Sources¶

# Ignition surface
ignition = Surface(
    id="IGNITER",
    hrrpua=1000.0,  # Heat release rate (kW/m²)
    ignition_temperature=400.0
)

# Small ignition patch
igniter = Obstacle(
    id="IGNITER_PATCH",
    xb=[2.45, 2.55, 2.45, 2.55, 0.0, 0.01],  # Small patch
    surf_id="IGNITER"
)

sim.geometry.add_obstacle(igniter)

Combustion Chemistry¶

Gas-Phase Reactions¶

Define the combustion reaction for the vapor:

from pyfds.core.namelists import Reaction

combustion = Reaction(
    id="METHANOL_COMBUSTION",
    fuel="METHANOL",
    soot_yield=0.015,
    co_yield=0.001,
    radiative_fraction=0.35
)

sim.physics.add_reaction(combustion)

Multi-Step Chemistry¶

For more detailed combustion:

# Two-step combustion
step1 = Reaction(
    fuel="METHANOL",
    formula="C H4 O",
    co_yield=0.5,
    soot_yield=0.01
)

step2 = Reaction(
    fuel="CO",
    co_yield=0.0,
    soot_yield=0.005
)

sim.physics.add_reaction(step1)
sim.physics.add_reaction(step2)

Best Practices¶

Pool Fire Experiments¶

Grid Resolution: Use fine grids near the pool surface (≤1cm)
Boundary Conditions: Ensure proper vent boundaries
Ignition: Use small ignition patches to avoid unrealistic burning
Monitoring: Add thermocouples and flux gauges

Parameter Estimation¶

Boiling Temperature: Use literature values or measure experimentally
Heat of Vaporization: Calculate from vapor pressure data
Absorption Coefficient: Measure or use correlations
Combustion Properties: Use standard values or measure in burners

Common Issues¶

No evaporation: Check boiling temperature is below flame temperature
Unstable burning: Adjust absorption coefficient
Incorrect burning rate: Verify heat of vaporization
Cross-reference errors: Ensure SPEC_ID matches REAC fuel

Examples¶

Methanol Pool Fire¶

# Complete methanol pool fire setup
methanol = Material(
    id="METHANOL_LIQUID",
    density=792.0,
    conductivity=0.2,
    specific_heat=2.51,
    boiling_temperature=64.7,
    spec_id="METHANOL",
    heat_of_reaction=837.0,
    absorption_coefficient=140.0
)

pool_surf = Surface(
    id="METHANOL_POOL",
    matl_id="METHANOL_LIQUID",
    thickness=0.05,
    tmp_front=25.0
)

combustion = Reaction(fuel="METHANOL", soot_yield=0.015)

Gasoline Pool Fire¶

# Gasoline surrogate
gasoline = Material(
    id="GASOLINE_LIQUID",
    density=750.0,
    conductivity=0.12,
    specific_heat=2.0,
    boiling_temperature=80.0,  # Representative boiling point
    spec_id="GASOLINE_VAPOR",
    heat_of_reaction=44000.0,  # MJ/kg
    absorption_coefficient=1000.0,
    heat_of_vaporization=350.0
)

See the examples directory for complete working pool fire simulations.