Quantifying boundary interaction of negatively buoyant jets.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The use of desalination around the world has increased substantially over the past several decades to provide a sustainable source of water. Desalination plants employ turbulent negatively buoyant jets to dispose brine effluent that is produced from the desalination process. The effluent is characterised by elevated density and contaminant levels and potentially poses a direct threat to the marine environment if the discharge does not dilute to acceptable concentrations. This aspect has led to an important area of research where numerous studies have been conducted to investigate the mixing characteristics of these dense discharges. These studies have quantified the flow behaviour using dilution and geometry measurements. Despite this focused effort, significant variations exist between different studies particularly with regards to the flow dilutions. These differences have recently been attributed to the inconsistent placement of a horizontal lower boundary near the vicinity of the discharge location. The influence of the lower boundary is still unclear and the present study addresses this issue through a comprehensive experimental program. Boundary effects are analysed by comparing the flow behaviour with and without imposed boundary influences in a stationary and uniform ambient. Experiments have been conducted using the Laser Induced Fluorescence (LIF) technique which produces two-dimensional concentration fields. Boundary influence is initially investigated using simple vertical flows discharged onto a raised horizontal circular platform with varying amounts of buoyancy. The flow impinges the lower boundary and spreads radially along the boundary. The behaviour of the flow field is characterised using dilution and velocity results, where velocity information was obtained from a previous researcher. The discharge height above the platform is varied and results are presented for a wide range of initial condition. Numerous flow parameters illustrate the influence of the boundary and are used to define the extent of the impingement/impact region. An integral model is developed specifically for non-buoyant jet flows and predictions are reasonably consistent with experimental results. This analysis is extended to inclined negatively buoyant jets where experiments have been conducted for three discharge angles of 30o, 45o and 60o above the horizontal. This flow configuration is more representative of discharges from desalination plants. Experiments conducted without boundary influences have a minimum clearance of 720 mm between the source and the bottom tank wall. Geometric and dilution results from these experiments are consistent across all parameters tested. Boundary influences are imposed on the measured flow region using the raised platform which is placed at various heights below the discharge source. Empirical dilution and geometric coefficients are compared to experiments without the raised platform and reveal important distinctions in behaviour between the two series of experiments. At the maximum height, differences between each data set are minimal. Near the return point and the platform location, the influence of the boundary is more apparent as measurements are shown to increase linearly with the source height parameter. This parameter is also used to investigate inconsistencies between results reported in the literature. Results from previous studies show an intuitive variation with the source height, suggesting that these discrepancies are in part associated with this boundary condition.