Computational fluid dynamics and experimental study of the hydrodynamics of a bubble column and an air-water jet-stirred cell

A large number of flows encountered in nature and in many industrial processes areintrinsically multiphase flows. The efficiency and the effectiveness of multiphase flow processes strongly depend on the ability to model the fluid flow behaviour. Thus, a robust and accurate description of multiphase flow can lead to an increase in performance, a reduction in cost, and an improvement in safety for engineering systems. In recent years, Computational Fluid Dynamics (CFD) has become an indispensable predictive tool for gathering information to be used for design and optimization for fluid systems. In this thesis the hydrodynamics of two bubbly flow systems, a bubble column and a waterjet-agitated flotation cell (Hydrojet cell), were studied by means of numerical simulations. In order to validate the bubble column CFD simulations Particle Image Velocimetry (PIV) was used. An experimental investigation about bubble size distribution (BSD) along a water jet was carried out by means of image analysis. Because of high gas fraction and high velocity of the air/water streams used to agitate the Hydrojet cell, with the available equipment, no experimental measurements could be done to evaluate the velocity field of the cell. The thesis consists of three parts: theoretical part, bubble column study and Hydrojet cell study. In the theoretical part, first, a summary of fluid dynamics principles and an overview of the principal issues related to multiphase flow modelling were presented. Then a brief introduction to PIV and its application to two phase bubbly flow were given. Finally a review of the principle of the flotation process and its modelling were done in order to highlight the reasons for the low recovery of fine particles. Then the potentialities offered by the use of waterjets to fine particles flotation were presented. In the second part experimental and numerical studies of a bubble column were presented. PIV technique was used to determine the velocity field of a laboratory bubble column. A separation method for multiphase PIV was developed and tested. By means of the proposed method, the acquired mixed-fluid images were processed to obtain two sets of single phase images before PIV analysis. The velocity field was determined using a multi-pass crosscorrelation. Following three-dimensional time-dependent CFD simulations of a lab-scale bubble column were presented. The simulations were carried out using the Euler - Euler approach. Two different multiphase turbulence models, Shear Stress Transport (SST) and Large Eddy Simulation (LES), were tested, and different interfacial closure models reported in the literature were examined. When LES were used to model the turbulence instead of the SST model, much better agreement with the experimental data was found, provided that the drag, lift and virtual mass forces were taken into account. In the third part a preliminary experimental study, carried out in a rectangular flat cell, was presented. It was carried out to investigate the size distribution of bubbles generated by a moderate pressure water jet, by means of image analysis. This study showed the ability of water jets at moderate pressure to break an air stream into small bubbles. Increasing the pressure of the pump, smaller and more uniform bubbles were obtained. Then three-dimensional CFD simulations of the Hydrojet cell are presented. The Hydrojet cell, due to the exceeding computational burden, was simulated as a two-phase (gas-liquid) system, although actually it is a three-phase (gas-liquid-solid) system. Also in this case simulations were carried out using the Euler - Euler approach. The turbulence of the liquid phase was modelled with the SST model. The single reference frame technique was used to describe the movement of the waterjet lance. To achieve a homogeneous aeration in the region near the inlets different inlet velocity and rotational speed were tested. The results gave useful indications about the role of the four principal operating parameters: nozzles diameter, velocity of rotation of the lance, speed of the water jets and then pressure of the pump and inlet air flow rate. What emerges is the need of high rotational speed of the waterjet lance in order to ensure an uniform gas distribution within the mixing zone. This is not possible with the current apparatus. Thus in order to make the system suitable to produce an appropriate environment for the full development of the flotation process it is necessary to modify the system.