Study of air entrainment and oxygen transfer at pressure conduit

ABSTRACT PhD Thesis STUDY OF AIR ENTRATNMENT AND OXYGEN TRANSFER AT PRESSURE CONDUIT Fahri OZKAN Firat University Graduate School of Natural and Applied Sciences Department of Construction Education 2005, Page: 64 The amount of dissolved oxygen (DO) in the waters is very important parameter for water quality. Hydraulic structures have an impact on the amount of dissolved oxygen in a pond system, even though the water is in contact with the structure for only a short time. The primary reason for this accelerated oxygen transfer is that air is entrained into the flow, which produces a large number of bubbles. These air bubbles greatly increase the surface area available for mass transfer. The aeration properties of free surface flows have been studied widely in the laboratory and field over a number of years. However, the aeration efficiency properties and oxygen transfer efficiency of pressure conduits have not been studied. This study seeks the air entrainment of pressure conduits and how they affect the oxygen transfer efficiency. When the gate of a high head outlet conduit is partly opened, a negative pressure drew air in through air vent. Air that is entrained into the water is momentarily forced downstream in the form of small air bubbles. The dissolution of oxygen into the water results from the air suction downstream of high head gated conduit. Moreover, high pressure in high head gated conduit flow systems also facilitates the dissolution of oxygen into the water. It was observed from the results that approximately full transfer up to the saturation value occurred at the high head gated conduits. Therefore, high head gated conduit flow systems have an extremely high efficiency for transferring oxygen from air bubbles to water. XIThe 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 the air to be injected into the system. The injection rate varies with the pressure differential across the venturi. In present study, experimental studies were conducted to investigate the effect of inlet diameter of venturi tube, the ratio of throat diameter of venturi tube to inlet diameter of venturi tube, pipe length downstream of venturi tube, angle of pipe downstream of venturi tube and flow velocity at inlet portion of venturi tube on air injection of venturi tube. It was observed from the results that venturi tubes were high efficiency in the air injection and oxygen transfer efficiency. Aeration is the process by which the area of contact between water and air is increased, either by natural methods or by mechanical devices. Proper aeration can make considerable improvements in a pond ecosystem. This study investigates pond aeration by two phase flow systems such as high head gated conduit flow systems and two phase pipe flow systems using venturi tubes. It was observed from the results that two phase flow systems were more efficient in deep ponds than in shallow ponds. KEY WORDS: Oxygen transfer, Aeration, Aeration efficiency, Air entrainment, Two phase flow, Dissolved oxygen, Gated conduit, Venturi XII