A free, vertical jet of liquid plunging into a uiescent surface of the same liquid entrains the surrounding gas into the receiving liquid to form bubbles. The entrainment characteristics of such jets of Newtonian liquids of varying physical properties have been investigated by means of high-speed photography.
Although both laminar and turbulent jets entrain gas bubbles, the mechanisms governing the entrainment process of the two types of jets are clearly different. Entrainment by turbulent jets results from the disturbances on the free surface caused by the jet instability; entrainment by laminar jets is accomplished by the formation of a thin shell of gas around the jet at the point of entrance, by the development of oscillations in the shell, and by its subsequent breakup into bubbles.
Entrainment occurs only when the average jet velocity exceeds a certain critical value termed minimum entrainment velocity. For a laminar jet having a flat velocity profile at the point of entrance, the following correlation permits prediction of the minimum entrainment velocity:
where the dimensionless numbers are based on the liquid properties and the jet diameter at the point where the jet meets the surface of the receiving liquid.