Basics of fluid dynamics were discussed in former chapters. But the flow we studied there was a simple one such as only a liquid or a gas. As is well known, materials can change into three forms, gas, liquid, and solid. In industrial instruments like found in chemical and nuclear power plants, flow is more likely to be multiphase than single phase. For example, BWRs involve mixtures of vapor and liquid phases of water. Flow which is composed of more than two phases is called multiphase flow. Figure 11.1 is a schematic representative of multiphase flows. Besides these, liquid-liquid two-phase flow like oil and water is also possible.