What is continuum flow and free molecule flow?

Before describing the continuum flow, it is reasonable to know the meaning of the Knudsen number. Knudsen number is the mean free path of a fluid molecule divided by the length of the section in which the fluid is flowing.

Continuum flow is that flow that approaches zero Knudsen number. It can be achieved when the mean free path of a fluid molecule is tiny, or the section is enormous.

Free molecular flow is a type of flow in which the mean free path of the gas molecule is larger than the chamber.