Why can a jet ejector pump draw in a second stream of fluid?

A jet ejector pump draws in a second stream of fluid because it utilizes a venturi nozzle to create a region of low pressure.1

As the primary fluid passes through the constricted throat of the venturi, its velocity increases significantly. According to Bernoulli’s law, this increase in speed results in a corresponding drop in pressure. This localized low-pressure area acts as a suction, which entrains or draws in the second stream of fluid from an inlet line.231

In industrial applications, such as steam jet ejectors used for creating a vacuum, the high-velocity primary fluid also transfers some of its momentum to the second fluid. The combined mixture then enters a diffuser, where its kinetic energy is converted back into pressure energy, allowing the system to compress the entrained gas from a low pressure to a higher discharge pressure.1