concept

Critical heat flux

The weakest radiant heating that can still set a solid alight. Formally it is the minimum incident heat flux, in kW/m², that drives a surface up to its piloted-ignition temperature faster than convection and re-radiation can carry the heat away; below it the surface settles at a steady warm temperature and never ignites at all. For dry cellulosic fuels such as wood, leaf litter and paper it is roughly 10-14 kW/m² with a pilot flame present, and 28-35 kW/m² if the material must ignite unaided. It is measured in a cone calorimeter under ISO 5660, and it is the number that sets how close one burning object can stand to the next before the fire jumps.