Prior to Houdini 18, pyro simulations involving combustion used a fuel field. In hot areas, the fuel would ignite, contributing to the instantaneous burn field. This burn field would in turn refresh heat, which captured the emissive parts of the smoke. Finally, outputs would be generated from the accumulated heat and instantaneous burn fields.

The new recommended workflow requires the instantaneous burn to be generated in SOPs and brought in as part of sourcing. A good example of this is the Sparse Fireball shelf tool.

• The burn volume is created as the source of the initial detonation.

• This instantaneous burn is then merged with the flame and divergence fields of the object.

This approach has the advantage of predictability and more control. For instance, if you would like the explosive fuel to rise, you can animate the burn source with exactly the desired speed.

For scenarios that require spreading of flames over a fuel source, you can use the Pyro Source Spread SOP. This node simulates the spreading behavior throughout its input point cloud and is especially useful for controlling spreading fires.