I have a simple fire scene: As you can see, it's a ring with fire flowing up from it. How can I make the fire animation start with a small flame that grows, and eventually sets the entire ring on fire.
Quickest way I could work out: Use a 'screw' modifier on a circle, with an animated angle (and an empty to rotate the circle around), then use this as the emitter. You'd probably want to put it inside your torus object and make it not render. This works in Blender 2.81a (old smoke sim) and Mantaflow (November 26th 2019 build). here's what it looks like- https://youtu.be/7h7JBEYuu1Q
And here's the .blend file (old 2.81a smoke sim) to pull apart- https://www.dropbox.com/s/zfctf8g5x31j9iu/FireReveal01.blend?dl=0
I tried using a simple wipe white-over-black image sequence as a UV mapped texture on the emitter/flow object, but both the old smoke sim and Mantaflow ignored it (just got a quarter of the torus on fire that didn't move) so don't waste your time with that!