When material ignites from proximity to a heat source, it comes from convection, conduction, and / or radiation. The example below is often used and the materials are familiar to most people, so it is a terrific way of explaining to clients.
Conduction: Get an imaginary candle and an imaginary match (hey, I’m not going to tell you to start a fire – my insurance would kill me!) OK – now imaginarily strike the match and light the candle. Did the matchstick burn your fingers? I hope not – that’s because it is made of wood or cardboard or some other poor conductor of heat. If your candle is in a glass container, the glass might get hot (not all containers are appropriate for candle making btw) and many metals are terrific conductors of heat, I blogged about that way over here but I digress….
Convection: Hold your hand over the imaginary flame. Higher. Do you feel heat rising? You should. There are two things happening here – the heat radiating towards your hand, yes, but there is also convection: heat rises. This is huge. HUGE! This is the basic information you need to begin understanding how large fires create their own weather and all sorts of crazy things. My own personal connection to this is that when baking something in the oven, there’s no air being mechanically moved by my ordinary little oven, nothing fancy in there. But when I open the oven door, all this hot air comes out and literally blows my hair up. That is convection.
Radiation: Now hold your hand to the side of the imaginary flame. As long as you have an appropriate candle container, the sides should be cool enough to handle (conduction) and the hot air is rising upwards (convection), but there’s still heat coming off the flame sideways and all directions. That heat is being transferred to you by radiation.
I have to admit it took me a while to really get how this information could be applied to my own design work. I’ll get there in future posts, promise. Meanwhile ….
Direct contact is not necessary for ignition. That means you don’t have to hold the lit match flame right on the wick of the candle – if you get close enough and the wick is flammable (as all good wicks are), then the wick will ignite once it reaches its temperature of ignition. The wick is raised to that level by the transfer of heat through radiation, convection, or conduction.
This post came about because I believe that understanding the work of firefighting professionals is key to figuring out how to better design for a fire event. In order to explore the firefighting world, I took a fire science class at the local jr. college in early 2019. This was not a class about the landscape at all, but about fire combustion and behavior intended for students who want to become firefighters. It was somewhat informative; a chunk of the class was about getting a job in the service so that part was fun to watch, but not really my jam. However, hearing the instructor’s stories about fires and what it is like to be a firefighter indirectly helped me to think up new design challenges and possible solutions that I wouldn’t have had before. I’m grateful to Lt. Cody, retired Oakland FD for all his tales, academic challenges, and his heart for aspiring fire fighters.
I did not invent the candle example, I’m merely sharing it with you; I’ve included a link for an article I found helpful below.
