It refers to the state in which a substance is in, including gas, liquid, and solid. Multiphase flow means the existence of two or more phases. Mainly including gas-liquid, gas-solid, three-phase flow, etc. In multiphase flow, it mainly involves the "main phase" and "secondary phase". Main phase: usually considered as a continuous medium, accounting for the main part of the flow. Secondary phase: usually considered as a phase dispersed in the main phase, which can have multiple particle sub phases of different sizes