Abstract

Turbulent jet motions are prevalent in the natural environment and are essential in industrial applications, they can lead to complex multiphase flow. The flow structure and processes are essentially unsteady and turbulents. In this study, two different test-rigs were constructed to investigate first the horizontal buoyant gas jets and second the vertical plunging water jets, also an Integral Numerical Model was developed to predict the jet trajectories and the parameters and it was validated with the experimental results previously obtained. 
A flow visualization technique using a CCD camera, which allowed simultaneous measurements was used to investigate such flows. This enabled to record the behaviour of the two-dimensional trajectories with relative ease. However, this technique provides a direct measurement of the interfacial behavior between the gas jet or bubbles and the liquid ambient. Two different methods were used in this study to obtain and analyze the shadowgraph images denoted as the summation and the statistical and we have found that both methods yield almost identical results. 
In the first part of this work, experiments were carried out to investigate the behavior of horizontal buoyant gas jets in water ambient. The following magnitudes were obtained from recorded time-averaged images: gas jet pinch-off, oscillation, unsteadiness of the jet interface, jet penetration length, jet half width, and the expansion angle. Few experimental data and calculations on horizontal buoyant jet with large density variations can be found in the open literature.
Experimental results indicated that the penetration length of the gas jets is strongly influenced by the nozzle diameter and the Froude number as well as with the injection mass flow and jet momentum flow rate. Increasing the Froude number and the injector diameter leads to increase the jet unsteadiness. In addition, the maximum location before the jet pinch-off is shown to have a logarithmic relation with the Froude number for all the jet diameters. Empirical correlations are also developed to predict these parameters.
An integral model is developed to assist in the design and to monitor the performance of the experimental investigation. Unlike the other models, the trajectory of the buoyant-jet is divided into two regions named, momentum and buoyant-dominated due to the effect of momentum and buoyancy force respectively. Each region was studied individually under certain assumptions and governing relations. In addition, the local rate of entrainment is considered variable along the jet trajectory and consisted of two components; one is due to the jet momentum force while the other is due to the jet buoyancy force. In addition, an interfacial shear stress acting at the interface between the jet flow and the ambient in the opposed direction to the main jet momentum flux is considered. Also, an approximately assumption of the momentum of the entrained water droplets into the jet flow is considered. The jet trajectory, penetration lengths, half-width, and velocity along the jet axis defining the jet trajectory are predicted and solved as variables along the jet path. Predictions from the Integral Model are compared with data from the current experimental data and good agreement predictions were found.
In the second part of this work, a series of experiments were performed on the plunging water jets injected vertically downward through short circular nozzles onto a water surface. The effect of the operation conditions including initial jet diameters (dN), initial jet velocity, and jet length (x1) on the flow characteristics such as the inception velocity of the gas entrainment, 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. 
The results obtained showed that the bubble penetration depth (Hp/dN) decreases with the dimensionless jet length (x1/dN) up to 25, after this point 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 with increasing the jet velocity and their curves were divided into three regions depending on the jet velocity range. The value of Qa was found also to increases 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, 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 were compared with other available experimental data and correlations. A good agreement was found between predicted and experimental results.










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