Glossary¶

Definitions of fire safety, FDS, and PyFDS terminology.

A¶

Absorption Coefficient¶

Parameter controlling radiation absorption by gases and soot. Higher values mean more radiation is absorbed. Typical range: 0.1-10 1/m.

Activation Temperature¶

Temperature at which a device (detector or sprinkler) activates. Commonly 68°C for sprinklers, 57-74°C for heat detectors.

Adiabatic¶

No heat transfer through a boundary. In FDS, SURF_ID='INERT' creates an adiabatic wall.

ASET¶

Available Safe Egress Time. Time from fire ignition until conditions become untenable for evacuation.

Aspect Ratio¶

Ratio between cell dimensions. Ideal is 1:1:1 (cubic cells). Generally avoid ratios > 4:1 for accuracy.

B¶

Backing¶

Surface backing condition. Options: - 'INSULATED' - No heat loss through back - 'EXPOSED' - Both sides exposed to gas - 'VOID' - Back surface at ambient

Boundary Condition¶

Specification of behavior at domain boundaries. Common types: OPEN, MIRROR, INERT, custom SURF_ID.

C¶

CFD¶

Computational Fluid Dynamics. Numerical solution of fluid flow equations. FDS is a CFD model specialized for fire.

CFL Condition¶

Courant-Friedrichs-Lewy stability criterion. Limits time step based on cell size and flow velocity: Δt ≤ Δx / (|u| + c).

Characteristic Fire Diameter¶

D* - Dimensionless fire diameter used for mesh sizing. Calculated from heat release rate and ambient conditions.

CHID¶

Case IDentifier. Unique name for simulation. Used as prefix for all output files.

CO Yield¶

Carbon monoxide production per unit fuel mass burned. Typical range: 0.001-0.01 kg/kg for most fuels.

Combustion¶

Chemical reaction between fuel and oxygen that releases heat. FDS uses fast chemistry (mixing-controlled).

Convection¶

Heat transfer by fluid motion. Natural (buoyancy-driven) or forced (fan-driven).

Control Function¶

Logical function that activates/deactivates based on device inputs. Used with CTRL namelist.

D¶

Deardorff Model¶

Default LES turbulence model in FDS. Relates sub-grid scale turbulence to resolved flow field.

Device (DEVC)¶

Measurement point or detector. Records quantities like temperature, velocity, or species concentration.

DNS¶

Direct Numerical Simulation. Resolves all turbulent scales - too expensive for practical fire problems. FDS uses LES instead.

E¶

Emissivity¶

Surface radiative emissivity (0-1). Typical values: concrete 0.9, steel 0.7-0.95, aluminum 0.05-0.15.

Extinction Coefficient¶

Measure of smoke obscuration. Related to visibility: vis ≈ C/K where C ≈ 8 (light-reflecting signs) or 3 (light-emitting signs).

F¶

FDS¶

Fire Dynamics Simulator. NIST's CFD model for fire-driven fluid flow. Open source, extensively validated.

Flashover¶

Rapid transition to fully-developed fire where all combustible surfaces ignite. FDS can simulate post-flashover but not predict onset.

Fuel¶

Combustible substance. In FDS, characterized by chemical formula, heat of combustion, and yields.

G¶

Grid¶

Computational mesh. Defines spatial discretization of domain.

Grid Convergence¶

Process of demonstrating solution independence from mesh resolution. Run coarse/medium/fine meshes and compare.

H¶

Heat Flux¶

Rate of heat transfer per unit area (kW/m²). Components: - Convective: From hot gases - Radiative: From flames and hot surfaces - Total (gauge): Sum of both

Heat of Combustion¶

Energy released per unit mass of fuel burned (kJ/kg). Typical values: wood ~17,000, plastics 20,000-40,000.

Heat Release Rate (HRR)¶

Power output of fire (kW or MW). Key quantity for fire size characterization.

HRRPUA¶

Heat Release Rate Per Unit Area (kW/m²). Specifies fire intensity on surfaces.

HVAC¶

Heating, Ventilation, and Air Conditioning. Can be modeled in FDS with ducts and fans.

I¶

IJK¶

Cell count triplet (I, J, K) for mesh in x, y, z directions. Product I×J×K = total cells.

Init (INIT)¶

Initial conditions. Set temperature, velocity, or species at start of simulation.

IOR¶

Index of Orientation. Indicates surface normal direction: ±1 (x), ±2 (y), ±3 (z). Used with VENT and surface devices.

K¶

K-factor¶

Sprinkler discharge coefficient relating flow rate to pressure: Q = K√P. Units: LPM/bar^0.5 or GPM/psi^0.5.

L¶

LES¶

Large Eddy Simulation. Turbulence model that resolves large scales and models small scales. FDS default approach.

Layer Height¶

Height of smoke layer interface. Important for smoke control and tenability.

Latch¶

Control property. If true, maintains activation state permanently. Used for sprinklers and alarm devices.

M¶

Material (MATL)¶

Thermal properties of solids: conductivity, specific heat, density, emissivity.

Mesh (MESH)¶

Rectangular computational grid defined by IJK (cell counts) and XB (bounds).

Mixture Fraction¶

Conserved scalar representing fuel/air mixing. FDS uses single mixture fraction for combustion.

MLRPUA¶

Mass Loss Rate Per Unit Area (kg/s/m²). Alternative to HRRPUA for specifying burning rate.

N¶

Namelist¶

FDS input format. Group of parameters enclosed in &NAME ... / delimiters. PyFDS methods create these.

NIST¶

National Institute of Standards and Technology. Developer of FDS.

O¶

Obstruction (OBST)¶

Solid object in domain. Defined by XB bounds and optional SURF_ID for thermal properties.

Open Boundary¶

Boundary at ambient pressure where flow can enter/exit freely. Created with SURF_ID='OPEN'.

P¶

Plume¶

Rising column of hot gases above fire. Characterized by temperature, velocity, and entrainment rate.

Pressure Solver¶

Algorithm for computing pressure field that ensures mass conservation. FDS uses FFT (single mesh) or iterative methods (multiple meshes).

Pyrolysis¶

Thermal decomposition of solid fuel into gaseous products. Can be modeled in detail or simplified with prescribed HRRPUA.

Q¶

Quantity¶

Measured physical variable. Examples: TEMPERATURE, VELOCITY, HEAT FLUX, VISIBILITY. Specified in DEVC.

R¶

Radiation¶

Heat transfer by electromagnetic waves. Dominant in fires. FDS solves radiation transport equation with ~100 angles.

Radiative Fraction¶

Fraction of combustion energy emitted as radiation (0-1). Typical: 0.3-0.4 for most fuels.

Ramp (RAMP)¶

Time-varying function. Used to specify fire growth curves, decay, or other time-dependent inputs.

RSET¶

Required Safe Egress Time. Time needed for complete evacuation. Must be less than ASET for safety.

Reaction (REAC)¶

Combustion reaction. Defines fuel properties: formula, heat of combustion, soot yield, CO yield.

RTI¶

Response Time Index. Thermal inertia of heat detector or sprinkler (m·s)^0.5. Typical: fast=30-50, standard=80-100, slow=150-250.

S¶

Simulation¶

PyFDS class for building FDS input files. Created with Simulation(chid='name').

Smokeview¶

FDS visualization program for viewing 3D results. Included with FDS download.

Soot Yield¶

Soot production per unit fuel mass burned (kg/kg). Typical range: 0.001-0.1. Affects radiation and visibility.

Species¶

Chemical components: fuel, oxygen, products, nitrogen. FDS tracks mass fractions.

Sprinkler¶

Fire suppression device. Activates at temperature, sprays water to cool and suppress.

Stratification¶

Vertical temperature/density gradients. Hot layer above, cool layer below. Gravity must be enabled.

Surface (SURF)¶

Boundary condition specification. Can define fire source, thermal properties, velocity, or temperature.

Conditions safe for human survival. Criteria include: - Temperature: < 60°C at head height - Visibility: > 10 m for large spaces, > 5 m for familiar areas - CO: < 1,400 ppm for 30 min - Heat flux: < 2.5 kW/m²

Thermal Conductivity¶

Material property (W/m/K). Rate of heat conduction through solid. Insulator: < 0.1, metal: > 10.

Time Step¶

Temporal discretization Δt. Auto-calculated by FDS from CFL condition. Can set initial value with DT.

Turbulence¶

Chaotic fluid motion with eddies. FDS uses LES to model turbulent fire plumes.

V¶

Validation¶

Comparison of model predictions with experimental data. FDS has 100+ validation studies.

Vent (VENT)¶

Boundary patch. Can be open, supply/exhaust, or any custom surface condition.

Visibility¶

Distance at which signs are visible through smoke (m). Calculated from extinction coefficient.

Viscosity¶

Fluid resistance to shear. Dynamic viscosity (Pa·s) or kinematic viscosity (m²/s).

Volume Fraction¶

Species concentration as fraction of total volume (0-1). Alternative to mass fraction.

W¶

Wall Function¶

Model for heat transfer at solid boundaries. Accounts for convection, radiation, and conduction.

X¶

XB¶

Coordinate bounds: (x₀, x₁, y₀, y₁, z₀, z₁). Defines spatial extent of mesh, obstruction, vent, or device.

XYZ¶

Point coordinates: (x, y, z). Used for point devices.

Common Abbreviations¶

Abbreviation	Meaning
ASET	Available Safe Egress Time
BC	Boundary Condition
CFD	Computational Fluid Dynamics
CFL	Courant-Friedrichs-Lewy
CHID	Case IDentifier
CO	Carbon Monoxide
DNS	Direct Numerical Simulation
FDS	Fire Dynamics Simulator
HGL	Hot Gas Layer
HRR	Heat Release Rate
HRRPUA	Heat Release Rate Per Unit Area
HVAC	Heating, Ventilation, Air Conditioning
LES	Large Eddy Simulation
MLRPUA	Mass Loss Rate Per Unit Area
NIST	National Institute of Standards and Technology
RANS	Reynolds-Averaged Navier-Stokes
RSET	Required Safe Egress Time
RTI	Response Time Index
UL	Underwriters Laboratories

Units¶

Common FDS Units¶

Quantity	FDS Unit	SI Unit
Length	m	m
Time	s	s
Temperature	°C	K (internally)
Pressure	Pa	Pa
Density	kg/m³	kg/m³
Velocity	m/s	m/s
Heat flux	kW/m²	W/m²
HRR	kW	W
Conductivity	W/m/K	W/m/K
Specific heat	kJ/kg/K	J/kg/K
Mass fraction	kg/kg	kg/kg

Unit Conversions¶

# Temperature
celsius = kelvin - 273.15
fahrenheit = celsius * 9/5 + 32

# Heat flux
kW_per_m2 = BTU_per_ft2_s * 11.356

# HRR
kW = BTU_per_s * 1.055
MW = kW / 1000

# Pressure
bar = Pa / 100000
psi = Pa / 6894.76

# K-factor
LPM_per_bar05 = GPM_per_psi05 * 14.41

PyFDS-Specific Terms¶

Method Chaining¶

Returning self from methods to enable: sim.add(Time(...).add(Mesh(...).surface(...).

Validation¶

Checking simulation configuration for errors before writing. PyFDS validates automatically.

FDS File¶

Text file with FDS namelists. Generated by sim.write('file.fds').

Results¶

Output from FDS simulation. Loaded with FDSResults('chid') for analysis.