Combustor is a 2D gas simulation where you can draw your designs and simulate their performance. The simulation is not based on the common "falling sand" concept like Noita or Powder Toy. Each cell can has information about all the gases and their temperature, pressure and other features which results in very realistic reaction behaviour.
Paint walls. Mix some gasses. Spark them. Then find out what you built.
Putting fuel next to oxidizer does not start a fire. Combustion needs temperature of 700 K and ambient air sits at 300 K, so you need an ignition source and a mixture rich enough to stay hot once it lights. Thin plume in open air? It flickers and dies. Box it in with walls, or turn the feed rate up, and it cooks.
Flip between Species, Pressure, Temperature, Velocity and Reactions views. You can also acess detailed metrics charts which will help you perfect your design.