Experimental study of bubble dynamics and transport

Sammanfattning: The thesis concerns experimental studies of bubble motion and the transport of large particles and bubbles. Bubbly flows are encountered in many engineering applications where the bubbles play an important role for the mass transfer between the phases, e.g. in stirred reactors. A special case of bubble transport is the cavitation phenomenon, which mainly has a negative effect on the performance of hydro machinery. The experiments are divided in three parts where the first part concerns the dynamics of large deformable single bubbles as well as a train of bubbles rising trough a fluid at rest under the influence of wall effects. The aim of this study is to develop experimental techniques to study these flows and provide information and validation data for numerical simulations for development of more accurate modelling of these types of flows. The second part deals with the transport of small spherical bubbles past a Kaplan turbine model where the focus is to provide data of the flow field and the bubble transport in a well defined geometrical configuration that can be simulated numerically with a rather reasonable ease. The third case deals with the motion of large spherical particles falling in the proximity of a wall and as a result of the interactions with the wall it rotates and moves in a manner quite different from a particle falling in an unbounded media. The purpose of this part of the study is to increase the knowledge on wall-sphere interactions and provide data for validation of numerical models. All of the experiments are developed in close collaboration with numerical simulations to include most of the numerical challenges that exist in these flows and confinements. To measure the large deformable bubbles with a high spatial and temporal resolution a shadowgraph technique is used. This enables detailed studies of the formation, release and deformation of these bubbles. The transport of bubbles in the Kaplan turbine model is investigated using a Particle Streak Photography method (PSP) and the flow field of the surrounding liquid phase is measured by Particle Image Velocimetry (PIV). For the case of the falling sphere a tracking algorithm is developed where the motion of the sphere is determined by tracking surface markers of different colours. Images of the falling sphere captured by a camera are analysed to compute the linear and the angular velocity of the sphere. The results of the single and multiple rising bubbles show that the process of release from an underwater nozzle of bubbles in this regime is violent and that the deformation begins immediately after the break-off from the nozzle. Disturbances at the surfaces of the bubbles can be seen as waves travelling along the surface and this seems to contribute to the large deformations that the bubbles undergo. The studies of the Kaplan turbine model provide information of the flow field and bubble transport data for numerical studies. The rotating blades of the model induce large scale vortices where the bubbles tend to stay. The investigation of the falling sphere shows that the sphere is greatly affected by the presence of the wall but also by the initial conditions at the release. Numerical studies that are also presented in the thesis further emphasize the influence of these effects.