Experiments on Stratified Multiphase Plumes
Buoyant plumes are flows of great interest to engineers and geologists alike with applications varying from chemical engineering processes to explosive volcanic eruption columns. Single-phase plumes have been well studied and the flow’s behaviour is generally well understood, however many examples in both industry and the natural world consist of a secondary dispersed phase of either bubbles or dense particles. The presence of multiple phases in a plume significantly complicates the flow dynamics, especially when rising through density stratified environments such as the ocean or the atmosphere. In this thesis, four experimental studies are presented investigating the dynamic behaviour of both particle-laden plumes and bubble plumes in stratified ambients. Experiments on stratified particle-laden plumes have enabled five steady-state flow regimes to be identified and subsequently characterised using a criterion for the onset of ambient convection and the ratio of particle to fluid buoyancy flux at the source. Measurements of plume height are compared to theoretical models within the literature and are shown to be successful for pure plumes rising through quiescent environments, however not for those rising through particle-induced convection. Through discrete measurements taken within the convection column surrounding the plume, predictions for the local values of density and particle concentration are produced allowing the plume heights to be predicted through only a minor adjustment to simple plume theory. Other models to predict the radial extent of ambient convection, secondary intrusion height and various particle concentrations are also presented. Weak bubble plumes were experimentally studied to understand both the dynamics of the spreading intrusions and the mass transfer of dissolved species to the environment. Bubble plumes with large non-dimensional slip velocities create multiple spreading events and upon comparison with well known scaling, were determined to spread three times slower than an isolated intrusion at large radial distances when viscosity is important. Measurements along the spreading intrusions showed the presence of a mixing region where concentration of dissolved species decays and a non-mixing region where concentration remains constant. A diffusion model is developed and successfully compared to both local measurements and experimental images.