The most basic criterion for preventing ignition is to simply specify the maximum allowable exposing radiative heat flux at a level below which ignition cannot occur. Historically, this approach has been used when determining the safe separation distances between buildings.8 A value of 12 kW/m ² has been reported as the safe maximum radiation level for wood9 and is often used as the criterion for building-to-building fire spread. The minimum heat flux for ignition should not be confused with critical heat flux for ignition. To clarify the difference between these two terms, Janssens'10 definitions are adopted in this discussion:

• Minimum heat flux for ignition, the heat flux below which ignition under practical conditions (in bench scale test or a real fire) cannot occur

• Critical heat flux for ignition, an estimate of derived from a correlation of experimental data

The values reported for are extrapolated from experimental correlation when the time to ignition goes to infinity, thus making dependent on the model used for correlating the data. The critical heat flux has a role in fire hazard analysis but is not the best way for estimating the minimum safe radiation levels.11 The minimum heat flux level, , is obtained from experiments and requires a somewhat arbitrary decision as to how long a test should be conducted. Most testing criteria are on the order of 15 to 20 minutes. However, ignition times greater than 60 minutes have been reported in the literature.12, 13 Regardless of the...