The results of computational fluid dynamics (CFD) modeling obtained by using FLUENT software with respect to the air entrainment at spillway aerators are compared with data from a physical model study, as well as the results of some empirical equations and prototype observations presented by other investigators. The air-entrainment rates obtained from the CFD analyses are in reasonable agreement with the prototype data and the values calculated from empirical equations, and are better than the physical model data, which include considerable scale effects. The numerical verification procedure in this study is based on the American Society of Mechanical Engineers (ASME) editorial policy statement, which provides a framework for CFD uncertainty analysis. Thus, validation of the CFD is discussed within the scope of this study.
The venturi system creates a pressure differential that forms a vacuum. As water flows through the tapered venturi orifice, a rapid change in velocity occurs. This velocity change creates a reduced pressure (vacuum), which draws air and liquid to be injected into the system. The air and liquid injection rates vary with the pressure differential across the venturi. Typical applications of venturi tubes are for injecting fertilizers, chemicals, ozone gas, air or oxygen into pressurized water systems. In this paper, experimental studies were conducted to investigate the effects of inlet and throat diameters of the venturi tube, pipe length downstream of the venturi tube, diameter of the suction pipe at the throat portion of the venturi tube, angle of the pipe downstream of the venturi tube, flow velocity at the inlet portion of the venturi tube and density and viscosity of the liquid injected into the venturi tube on air and liquid injection rate. It was observed from the results that venturi tubes had high air and liquid injection efficiencies.
Air may be entrained into flowing water by design or inadvertently. Air entrained into flowing water at a hydraulic structure may be beneficial from the standpoint of oxygen recharge. When the gate of a high-head outlet conduit is partly opened, a negative pressure draws the air in through the air vent. Air that is entrained into the water is instantly forced downstream in the form of small air bubbles. The solution of oxygen into the water results from the air entrainment downstream of the high-head gated conduit. Moreover, high pressure in high-head gated conduit flow systems also facilitates the solution of oxygen into the water. Numerous model studies and a few field measurements have been conducted to determine the amount of air demand by closed conduits. The literature search did not identify any published analytical or physical studies of the dissolved oxygen levels produced in gated conduits. Therefore, experimental studies were conducted to investigate aeration efficiency of high-head gated conduit flow systems by using a simplified experimental configuration. It was observed from the results that almost full oxygen transfer, up to the saturation value, occurred at high-head gated conduits. Accordingly, it was concluded that high-head gated conduit flow systems have an extremely high efficiency for transferring oxygen from air bubbles to water.
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