Mass of the air at the exit of the nozzle is constant. The entrainment of the surrounding in the jet increases the mass of the jet but decreases the velocity of the jet. The cumulative effect is that momentum flux within the jet remains constant. Entrainment of the surrounding and hence increase in mass of jet depends on the difference in the momentum flux within the jet and that of surrounding (note that as the jet is discharged into a still surrounding, the surrounding sets in motion). The entrainment of the surrounding will continue as long as the difference in the momentum flux exists.

Behavior of free (unconfined) and confined jet

A free jet has no confinement and hence can spread till the difference between the momentum flux of the jet and the surrounding becomes zero. Figure 27.4 shows the entrainment in the free jet