What exactly is a flame made of? And why does it have that specific shape?


What exactly is a flame made of? And why does it have that specific shape?

In: Physics

The flame is made of hot gas rising out of the “burning” matter and undergoing the actual chemical reaction of burning with oxygen. For example, if you are burning some firewood, the wood itself has to become hot enough first so that some of its compounds evaporate and become gas. This gas then mixes with oxygen in the air and burns, producing the flame. This reaction produces a whole lot of heat, keeping the wood hot and continuing to evaporate more compounds which can then burn and so on, until there aren’t any volatile (i.e. able to evaporate) compounds left in the wood.

The primary thing making a flame is heat. There might be different reasons for this heat which may be visible in the shape and color of the flame. The classical shape and color of say a candle fire or a bondfire is caused by several things. Firstly most of the heat comes from volotile compounds which evaporates from the heat and then burns in the air. So as these hot gasses rises they actually increase in temperature until these volotile compounds have burned up by which time the temperature decreases as the gasses mix with cold air. At the same time the hot rising air creates a low pressure at its base due to a lack of air such that surrounding cold air gets pushed into the base cooling it down. So the flame itself is quite small at its base, growing bigger in the center and then tailing off into invisible temperatures higher up. For the color, hot things will glow with a color based on its temperature. First infrared, then red, orange and eventually white and even hot blue. And there is some of this color in the flame. However when you heat up things to ions they will glow in a specific color based on the element. This usually gives off far more light. In the case of a classical fire they usually have a lot of sot and other carbon based compounds which glows yellow when ionized. This is why flames are usually yellow.

When you heat stuff up to high enough temperatures, it begins to glow. This is just the energy that’s been crammed in being released as heat. Red is a cool glow, then as we get through yellow and white and eventually bluish, things are hotter and hotter. This is how filament light bulbs work. The electricity makes them so hot that they begin to glow.

Fire is when stuff reacts with oxygen. This reaction creates gases (carbon dioxide and water vapour) and a lot of heat. The colour you see as a flame is just these gasses glowing because they are really hot.

The shape is basically because gravity means hot gases will float upwards. The hot gases released by burning begin to drift upwards. As they do, they cool down, meaning they stop glowing at some point. The gases in the middle of the flame stay hotter for longer because they are surrounded by other hot gases, so the middle of the flame is taller.

Here’s a [video](https://youtu.be/tMDKeBaLWDw) about it 🙂

Oh, also, the gravity thing means that flames in space (inside a ship) are spherical 🙂