Air entrainment by plunging water jets

Abstract

Gas entrainment caused by the impact of liquid jets upon liquid pool surfaces is a subject which has received too little attention. This well-known phenomenon,which occurs In nature and in numerous industrial operations, has only recently received interest from scientific workers. The influence on product quality and applications in waste water treatment have been the main stimuli for their investigations. This thesis is restricted to the study of water jets travelling through air. The results are generally applicable to low viscosity jet systems. After a general introduction the observed entrainment mechanisms for turbulent waterjets are described. With increasing jet velocity the entrainment, which is at first erratic, becomes more continuous. This fact explains why a distinction must be made between low velocitv (v < 5 m/s) and high velocity entrainment. At low velocities entrainment is determined by the presence of surface deformations upon the jet and the recovery processes of the target liquid. With high velocities air is mixed with the jet and together with an accompanying air boundary layer is brought under the receiving pool surface. From the study of these mechanisms it was concluded that jet surface deformation caused by destabilising factors and cavity formation in the pool liquid needed thorough investigation. Although many publications have appeared describing jet disintegration, the influence of nozzle design, turbulence and air friction was unknown. Nozzle design and its effect upon the onset of turbulence,completely controls the surface distortions of the jet. Criteria have been established which define the issuing flow conditions. Air friction forces cause a spreading of the waterjet and a correlation is presented which relates the increase of jet diameter to the variables used. One chapter is entirely devoted to the cavity formation following the impact of drops upon liquid surfaces. A theoretical model is proposed which describes the cavity growth for single drop impact. Experimental verification fits the prediction. The impact of subsequent drops is also qualitatively examined. The results of the measurements, combined with relevant data of other authors are compared with the fundamental analyses and a number of formulae are presented t;lat permi t calculation of the entrainment rate. Mass transfer aspects, and in particular the transport of oxygen, are also treated. Mixing of the pool liquid as a result of the impact has been investigated and the size of the bubbles in the bubble swarm determined. With these two studies in mind equations have been derived which relate the oxygen transfer rates to the jet parameters. The penetration depth of the induced bubbles has also received attention. The remarkable result has been established that the simple measurement of this depth is sufficient to predict the amount of entrained air as well as the oxygen transfer rate. The main conclusion of the work is that under certain conditions jet aeration is very efficient and its use for waste water treatment is attractive.