An experimental study on bubble entrainment and flow characteristics of vertical plunging water jets

When a vertical liquid jet plunges into a liquid surface after passing through a surrounding gas phase it entrains a large amount of gas bubbles into the receiving pool, and forms a large submerged two-phase region with a considerable interfacial area. At the intersection of the plunging jet and the liquid surface, free-surface instabilities develop, and gas entrainment may be observed. In this study, a set of experiments were performed on plunging water jets injected vertically downward through short circular nozzles lN/dN ⩽ 5 onto a free water surface. The effect of the operation conditions including initial jet diameters dN, initial jet velocity VN, and jet length x1 on the flow characteristics such as the inception velocity of the gas entrainment Ve, the bubble penetration depth Hp, the gas entrainment rate Qa, the centerline jet velocity Vc, and the axial jet velocity distribution Vx below the free water surface were evaluated. A flow visualization technique using a CCD camera, which allowed simultaneous measurements of several magnitudes, was used to investigate such flows. This technique provided a direct measurement of the interfacial behavior between the entrained air bubbles and the liquid ambient. The results obtained showed that the nondimensional bubble penetration depth Hp/dN decreased with the dimensionless jet length x1/dN up to 25, after this point it was almost constant. Also, the bubble penetration depth was found to increase with the jet velocity and nozzle diameters. The entrainment rate tended to increase when the jet velocity increased and its functional dependence was divided into three regions depending on the jet velocity. The value of Qa was also found to increase as x1 and dN increased for the same jet flow rate. The jet centerline velocity decay Vc was measured and found to be a function of: the jet impact velocity V1 with the plunge water surface, the jet diameter d1 and the plunge depth x. The axial velocity distributions Vx/Vc were found to be approximately Gaussian distributions for all the cases when plotted against r/bu. Empirical relationships were proposed to predict the jet parameters and when were compared with the available experimental data and correlation of other authors a good agreement was found