Fire Spread
The purpose of this video is to address Fire Spread
I selected a solid, specifically wood, because of its availability but thought it was worth mentioning that Fire Spread will vary greatly due to a variety of factors, including the material type and phase state of the material.
As you can see, I’m doing a piloted ignition using a propane torch. I have taken a thick solid piece of wood and chopped it into smaller pieces because Fire Spread occurs more rapidly over thin solids with high surface area. In trying to ignite the whole logs, heat is conducting to the core of them and there isn’t sufficient heat to maintain a flame.
I’m also lighting the fire at the bottom of the wood because Fire Spread occurs faster upwards than downwards or horizontal. This is due to the fact the heat rises and so the material above the fire is going to start to pyrolyze and ignite much faster than any fuels below the fire. As heat and combustion products rise this also allows for fresh air to be drawn in underneath the fire, providing oxygen. Flames also can become taller as they spread upward, covering a greater area and therefore producing greater heat.
As we’re watching this fire, we are seeing flaming combustion which is a gas phase process. And we see that we have a diffusion flame.
We now have heat transfer occurring through radiation coming off the flames and hot surfaces of the wood, conduction of heat from direct contact of one piece of wood to another, and convection heating the air above the fire and being carried away as it’s replaced by fresh air.
Cool wood is being heated to the temperature at which its pyrolysis is fast enough that the concentration of combustible vapor above the wood surface exceeds its lean flammability limit. Flames continue to extend and pyrolyze the next area of cool fuel.
If I were using a material that could melt or drip, that would be an additional means of Fire Spread.
I’m not sure of the moisture content of this wood but it has been outside in a covered area and that’s just another factor to consider in how fast it will burn. If it had been indoors where it’s warmer and dryer it would be ready to pyrolyze at a faster rate.
Wood in general can be 40-50% cellulose which is a polymer and oxidizes primarily into carbon dioxide, water vapor, and of course heat.
Wind is another large factor in Fire Spread. If wind is pushing flames toward additional fuel, that is going to cause faster fire spread but it could also slow fire spread by pushing flame away from fuels.
My current fire tetrahedron consists of my fuel, wood. My oxidizing agent, atmospheric oxygen, heat introduced by my torch, and uninhibited chemical chain reactions occurring to sustain the fire.
Once the fire has died down, we can see smoldering of the remnants of the wood fuel. Smoldering is generally limited to porous materials that form a carbonaceous char. Char is a good thermal insulator that retains enough heat to sustain a reaction and so if I add another couple pieces of wood, you can see that pyrolysis is still occurring and our non-flaming combustion will reignite into flames with the radiative enhancement of these two pieces trapping heat between them.