Wildfire Knowledge Graph · Heat Transfer
The fraction of total fire heat release emitted as thermal radiation under a stated fire and measurement condition.
In plain language
The fraction of total fire heat release emitted as thermal radiation under a stated fire and measurement condition.
Why it matters
Radiative Fraction is useful because it separates one specific part of wildfire behaviour, exposure, operations or evidence from nearby concepts that can otherwise be mixed together.
What is radiative fraction?
Radiative fraction is the dimensionless fraction of the total heat released by combustion that is emitted from the fire as thermal radiation.
Common symbol: χr · Units: dimensionless.
Engineering relationship
χr = Q̇r / Q̇
Therefore, Q̇r = χr × Q̇, where Q̇ is total heat release rate and Q̇r is the radiative component.
Worked example
If a fire releases 20 MW and a model uses χr = 0.30, the modeled radiative output is 6 MW. That does not mean a target receives 6 MW: distance, flame geometry, view factor, orientation and atmospheric transmission still control the radiant heat flux received at the target.
Radiative fraction is not radiant heat flux
- Radiative fraction: source energy partition, dimensionless.
- Radiative output: total radiant power emitted by the fire.
- Radiant heat flux: radiant power received per unit area at a target, commonly kW/m².
Why one fixed value should not be used blindly
NIST describes radiative fraction as a key fire-science quantity and documents substantial variation with fuel, soot production, fire size and experimental conditions. Fire Dynamics Simulator guidance notes that values around 0.3–0.4 are common for many combustibles, while also explaining that the global radiative fraction can change with scale and smoke re-absorption.
A value such as 0.30 or 0.35 is therefore a model assumption, not a universal wildfire constant.
Wildfire engineering chain
Heat Release Rate → Radiative Fraction → Radiative Output → Geometry / Transmission → Radiant Heat Flux → Structure Exposure
Interpretation checklist
- Identify the heat-release rate used.
- State the source of the radiative-fraction value.
- Keep fuel type and fire scale attached to the assumption.
- Separate emitted radiation from received radiation.
- Document view factor, distance and atmospheric transmission separately.
- Carry uncertainty into downstream exposure calculations.
Related concepts and tools
Heat Release Rate · Thermal Radiation · Heat Flux · View Factor · Radiant Heat Flux Calculator
Technical references
NIST — Energetics of Small and Moderate-Scale Gaseous Pool Fires
NIST — Fire Dynamics Simulator Technical Reference Guide
Tecnobosque Reference Concept · reviewed September 2026. Model values and assumptions must remain attached to their experimental or modelling context.