Heat Flux is radiant energy and can be defined as the heat flow rate per unit area of flow path and is generally expressed in kilowatts per meter squared. Radiant energy becomes the dominant form of heat transfer as a fire moves through the development stages and can be responsible for the preheating and ignition of fuels. The potential for ignition of thermally thin objects increases at 10 kW/m2 and above and at 20 kW/m2 and above for thermally thick materials. Additionally, upper layer smoke temperatures in the 500o to 600o Celsius range can generate heat flux to the floor in the 20-25 kW/m2 range and is commonly associated with the onset of flashover. A formula for determining ¡§point-source¡¨ heat flux is expressed as:

This formula is relatively accurate if applied more than two flame diameters from the flame and is based on the assumption that all of the energy is received uniformly over a sphere radius from the flame.
For entry into the scientific calculator the equation is expressed as:
FLUX = RADFRA * RHR/(4Ĉ(DIST^2))

Where:

A. FLUX = RADIANT HEAT FLUX IN KILOWATTS PER METER SQUARED
B. RADFRA = FRACTION OF COMBUSTION ENERGY LOST BY FLAME AS RADIATION (Xr Factor)
C. RHR = RATE OF HEAT RELEASE IN KILOWATTS (Q DOT)
D. DIST = DISTANCE BETWEEN CENTER OF THE FLAME AND TARGET

Note: RADFRA relates to how efficiently a fuel radiates energy and can be expressed as a fraction of energy radiated relative to the total energy released. It is not a constant for a given fuel but will vary between .15 for low soot fuels such as methane to .60 for high soot fuels such as polystyrene. It should also be noted that the distance from the source to the target is measured from the axis line of the flame, not from the flame edge.

To establish some sort of table of distances, you would need to know what fuel was burning, how big an area was burning and how effectively it was radiating its energy. Safe distances would theoretically be different for different fuels. As Dennis indicated, Flux is inversly proportional to the square of the distance for example doubling the distance away from a 1500 kW fire from 1 meter to two meters drops the Flux from 41 kW/m^2 to 10 kW/m^2. Common flux related values are: 1 kW - Sunlight, 6 kW - Pain in 8 sec, 16 kW - Blistering in 5 sec, 20 kW - Cellulosic Materials begin to ignite.

An additional warning lies in that these formulas are for approximation and estimation, they are in no way a guarantee that the value will not be above that calculated.

Thanks,

Bobby