Gas-solid two-phase flow system. Its basic dynamic process and phenomenon are the same as liquid-solid flow, but the density of continuous phase (gas) is not large. The movement of sand and snow in the wind, the pneumatic transportation of powdery materials through pipelines, the separation of solid particles, and the combustion and injection of powdery fuel are all examples of gas-solid flow. Solid fuel rocket engine and gas-solid fluidized bed involve more complicated physical and chemical processes of gas-solid flow.