Air entrainment in vertical dropshafts
This thesis describes an investigation into the phenomenon of air entrainment in vertical dropshafts conveying water from a high level to a lower level. The study initially commenced with hydraulic tests on scale models of a vortex-entry dropshaft arrangement. This subsequently expanded into a more detailed analysis of the mechanisms involved in the air entrainment, rejection and transport processes evident in the early stages of the work. An attempt has been made to define the criteria controlling these three modes of operation in air/ water systems. The physical process of initial entrapment by the plunging water film was researched extensively. The inability to accurately reproduce the bubble sizes in scale models conclusively shows that this aspect is fundamental to the understanding of the problem. A dimensional analysis of the parameters controlling the rate of entrainment in the system was undertaken in the investigations. A number of dimensionless groups were obtained to describe the relationships between the parameters involved in the process. This enables the rate of air entrainment to be predicted in any.system of dropshafts under certain flow and geometrical conditions. The downward passage of air bubbles in water (just one mode of operation in two-phase flow systems) was investigated to establish the air transport capabilities of various flows and hence the air rejection process. An empirical relationship was derived which describes the air void ratio at various discharges and hence enables many aspects of the transport of air bubbles to be studied. A series of values defining the limits of operation of the phases invovled in air entrainment were also identified, e.g. onset of air entrapment and air transport.