Radiant Exposure Separation Distance

A fire radiates a fraction of its heat release rate outward in all directions -- roughly 0.2 to 0.4 for most fuels, with 0.3 a common working value -- and treated as a point source that power spreads over the surface of a sphere, so the flux falls with the SQUARE of distance and doubling the separation quarters the exposure. The target flux is the decision, and it moves the answer a long way: widely used thresholds put piloted ignition of wood near 12.6 kW/m2, spontaneous ignition much higher, the pain threshold for bare skin around 2.5 kW/m2, and a commonly cited limit for a firefighter in full protective clothing operating for a sustained period well below the ignition figure. Protecting a wood wall is a different problem from protecting a person, so the threshold is an input rather than a constant. A 5 MW fire -- roughly a fully involved passenger vehicle -- at a radiative fraction of 0.3 radiates 1,500 kW and reaches the piloted-ignition threshold at 3.08 m, about 10 ft. Scale the fire tenfold to 50 MW and the separation goes only to 9.73 m, about 32 ft: a tenfold fire produced a threefold distance, because of the square root. Separation is a weak lever against fire size, which is why exposure protection is done with water and with construction rather than with distance alone. A screening estimate; a fire protection engineer, the applicable exposure-protection standard, and the incident commander govern.

radiated_power_kw = radiative_fraction x heat_release_kw; separation_m = sqrt(radiated_power_kw / (4 pi x target_flux_kwm2)); flux at r = radiated_power_kw / (4 pi r^2).

Point-source radiation model -- radiated power = radiative fraction x heat release rate, flux = power / (4 pi r^2), with a distance entered in feet converted to metres before the inverse square -- by name; public physics, standard in the fire-protection engineering literature (SFPE Handbook). The radiative fraction and the target flux threshold are entered, not bundled. A screening estimate; a fire protection engineer, the applicable exposure-protection standard, and the incident commander govern.

The inverse-square point-source model is public physics; the fire size, radiative fraction, and target threshold are entered by the user.

Estimate. AHJ and licensed professional govern.

Field names used by the API: heat_release_kw, radiative_fraction, target_flux_kwm2, evaluate_distance_ft, radiated_power_kw, separation_m, separation_ft, flux_at_distance

• Point source power spreads over a sphere; flux falls with the SQUARE of distancepublic radiation physics
• Radiative fraction roughly 0.2 to 0.4 for most fuels, 0.3 a common working value; entered, not bundledSFPE Handbook
• Threshold is the question wood piloted ignition, skin pain, and a clothed firefighter's sustained limit are very different numbersfire-protection engineering practice