Welcome to the exciting world of FX simulation! It is a broad subject but we hope Eddy will make the learning process both productive and fun!
FYI - improved documentation for Eddy is being worked on as we speak and will be available soon as well.
Combustion simulation can be a bit tricky to get a handle on (we hope to improve this in the future) but here is a rundown of the basics concepts to get you started:
The basic principle of the combustion model is the fire triangle – making things burn requires heat, fuel and oxygen. Oxygen is already present in the air and thus you don’t need to explicitly add it to the simulation. Instead you control your combustion process by adding fuel and heat.
Heat is injected into the simulation using an emitter connected to the temperature input of the combustion element and fuel is injected using an emitter connected to the fuel input.
Fuel contains energy that is released as heat when the fuel burns. The resulting hot gases and hot soot makes up the bright parts of the flame through a process called black-body radiation.
Fires, although fundamentally the same process, can look very different – a slow burn like a candle or campfire looks very different from a fast burn like an explosion or the flame of a blowtorch. The specific behavior of the fire is controlled using the combustion parameters on the combustion element. I’ll walk you through them in the order they become relevant to the combustion process:
Ignition temperature: This is the temperature the heat channel must exceed to ignite the fuel. So to start the combustion process you’ll need to add some heat (ideally in contact with the surface of the fuel region) – exceeding this threshold.
Fuel combustion threshold: For combustion to happen fuel and air must be mixed. If a region of the simulation contains only air there is no fuel to burn and if there is only fuel there is no air (oxygen) for the fuel to react with. The minimum amount of air needed for the fuel to burn is controlled using the ‘Fuel combustion threshold’ parameter. Regions in the simulation where the fuel value is higher than this threshold will not burn until more air has been mixed in. Note that even if you emit a block of fuel with a fuel density above this threshold the surface of that region can still ignite as there is plenty of air outside the fuel region with which it can mix.
Combustion temperature: Once the fuel ignites it releases heat as it burns. The combustion temperature controls how much heat is released per unit of fuel spent.
Fuel burn rate: This parameter controls how fast the fuel burns – i.e. how many units of fuel is consumed per second under nominal conditions.
Fuel expansion: When fuel burns it is turns into hot gases that typically take up more volume. This parameter controls how much the fuel expands as it burns (expansion per unit fuel consumed) – leading to the burning region expanding. An explosion is an example of rapid expansion due to very fast combustion.
Fuel carbon amount: This parameter controls how much soot (smoke density) is produced per unit of fuel consumed. Higher values lead to a sootier, smoky burn whereas lower values lead to a cleaner flame.
Cooling rate: This parameter controls how fast the hot gases generated from the combustion cools down. The hot gases loose heat quickly through heat radiation. This is the heat you feel when looking at a fire from a distance (you are not in contact with any hot parts of the fire and the air is not hot but you can still feel the heat). Higher values means the gases that make up the flames cool down quicker, lower values mean they cool more slowly. This affects the visual look of the flame. Objects that are hot enough will glow due to a process called black-body radiation. Faster cooling means the flames of the fire will dim more quickly.
A lot to take in I’m sure but I hope this brings you some clarity! Don’t hesitate to ask if anything is unclear - there are no dumb questions.