Combustion simulations produce explosive fireballs of flammable gases by default. You can adjust their settings to produce a variety of different scenarios, such as a campfire or a candle flame.
The size of emission sources and other objects has a large effect on the appearance of flames. Depending on the scale to which the scene is modeled, you should adjust scene_units_in_meters on the aero_solver_settings node. For example, if 1 grid unit represents 1 centimeter in the scene, set this value to 0.01.
To make a combustion simulation look more like a burning solid fuel and less like a fireball, you can control the size and height of flames. There are several ways to do this, using a combination of the following techniques:
expansion_scale on the combustion_settings node. This reduces the explosive nature of the combustion.radiative_cooling on the combustion_settings node. Cooler gases don't rise as quickly, but note that this generates more soot and heavier smoke. You can also increase temperature_diffusion, but this tends to smooth out the results.buoyancy on the aero_solver_settings node. This also makes the hot gases rise less quickly.burn_rate on the source_fuel node. This reduces the amount of heat generated. You may need to balance the burn_rate with the radiative_cooling to maintain a sustained flame. Lowering the ignition_temperature can help.There are several settings that help control the amount of smoke.
soot_formation_rate on the combustion_settings node directly affects the amount of soot, which is visible as smoke. Note that setting this to 0 or turning off emit_soot makes the simulation invisible, because the soot is stored in the voxel_fog_density channel — set fog_density to a value above 0 on the source_air node to see the simulation again.oxidize_soot on the combustion_settings node to eliminate a portion of the smoke. Note that this also affects the size of flames and the amount of heat. You may need to balance soot_oxidation_rate and oxygen_diffusion to achieve the desired height of flames.You can use one fuel source to ignite another fuel source, like a match that sets kindling on fire.
To do this, you need two separate chains of source_air and source_fuel nodes: the first one for the source that is already burning, and the second one for the source that gets ignited later. Each of these should be connected to the sources input of the simulate_aero node.
To prevent the second fuel source from igniting immediately, adjust its settings so that temperature on the source_air node is lower than ignition_temperature on the source_fuel node.
Animate the first source in the scene so that it approaches very close to or touches the second source. Two parameters on the combustion_settings node control how the second source ignites:
radiative_heating controls how fast the fuel heats up due to the nearby source.flame_propagation_speed controls how fast the fire spreads once it has been lit.