Hostile Fire Events: Flashover
Fire breathes. I remember, early in my career, we were routinely asked to burn brush piles for some of the residents in our District. Several nights a week we stood in front of towering piles of sticks, twigs, branches, and brambles.
The orange glow of the flames lulling us hypnotically as we stood there watching it until the increasing heat finally resonated on our faces and forced us to step back further and further as the fire intensified. We wore turnout gear, but no SCBAs since we were out in the open, so our faces were exposed to the increasing, intense heat.
I also recall the loud sound of the air being sucked into the rising plume of heated gasses rising to the sky. This inflow of cooler air to the plume is also called entrainment. I also remember being surprised by how strongly I could feel the air current being sucked in by the fire as it created its own wind.
Remember in the 1st Law of Thermodynamics: Energy cannot be destroyed, but it can be changed to other forms of energy and matter. As the fire consumes the fuel, it changes the fuel to energy (heat, light), and matter (smoke, which is carbon particles, carbon monoxide-CO, carbon dioxide-CO2, hydrogen cyanide-HCN, many other toxic gasses, soot and ash).
Fire burns either in unconfined spaces- out in the open (like the brush piles I described earlier)- or in confined spaces- rooms with ceilings and walls.
Confined spaces, are usually called compartments or containers, and obviously vary in shape and size. A smaller fire in a large container (like a small fire in a warehouse) burns and behaves like a fire in an uncontained space, at least initially. The smaller the compartment the faster things change, and the more you need to watch the changing conditions.
Normal room air is about 21% oxygen. Fire will continue to burn even as the oxygen levels drop to 14-16% in room temperatures of 70F (21C). As long as there is an abundant amount of oxygen available, the fire is considered fuel-controlled and it will burn as long as there is a fuel source.
Unconfined, outside fires are normally fuel-controlled. As fire burns in an enclosed compartment, it consumes available oxygen in the compartment. Once there is less air (oxygen) than fuel the fire is considered ventilation-controlled. As more of the available oxygen is consumed, the smoke becomes more carbon-laden, dense, and darker. The smoke takes on a turbulent, “rolling” or “boiling” appearance.
This is never a good sign.