Aeration is the process of bringing water and air into close contact in order to increase dissolved oxygen concentration. The concentration of dissolved oxygen is an important indicator of water quality because aquatic life lives on the dissolved oxygen in the water. The hydraulic structures can be accepted as the key components in increasing dissolved oxygen concentration because of the strong turbulent mixing associated with substantial air bubble entrainment at these structures. Closed conduit is a classic example of a hydraulic structure where aeration occurs. This work focused on determining the effect of conduit length on air-demand ratio and aeration efficiency in free-surface gated circular conduits. Experimental results showed that the Froude number had an important effect on the air-demand ratio and the aeration efficiency. The effect of the conduit length on the air-demand ratio and the aeration efficiency changed depending on the Froude number. It was demonstrated from the results that a free-surface gated circular conduit flow system had high efficiency in transferring oxygen from air bubbles to water. Moreover, a formula for the aeration efficiency was presented relating the aeration efficiency to the conduit length and the Froude number.
surface area associated with transfer volume
mean concentration of dissolved gas in water within control volume
dissolved oxygen concentration downstream of hydraulic structure
saturation concentration of dissolved oxygen at standard atmospheric pressure
dissolved oxygen concentration upstream of hydraulic structure
rate of change in concentration
aeration efficiency at 20°C
Froude number based on effective depth in conduit
acceleration of gravity
liquid film coefficient
air flow rate measured through air vent
water flow rate in conduit
water flow velocity at gate location
air-demand ratio ()
ratio of water cross-sectional flow area to conduit cross-sectional area
volume of liquid into which mass of gas dm diffuses in time dt
Gated conduits are hydraulic structures that involve high-velocity air-water flow. In large dams, they are commonly used for reservoir drawdown, sediment flushing, river diversion and environmental flow releases. In a gated conduit, a high-speed flow issuing from the gate drags and entrains a lot of air. If the air demand of the flow is not supplied, pressure reduction downstream of the gate causes cavitation. Usually an air vent is installed just downstream of the gate to supply enough air to the flow. Air that is entrained into the water is instantly forced downstream in the form of fine air bubbles. These fine air bubbles that create a large air-water surface area facilitate the solution of oxygen. The diffusion of oxygen into the water is usually greater in systems with fine air bubbles than systems with coarse air bubbles. This occurs because fine air bubbles present a greater surface area to the surrounding water than coarse air bubbles. Oxygen diffuses into the water at the surface, so a large surface area facilitates greater oxygen absorption.
Kalinske & Robertson (1943), Campbell & Guyton (1953), Sharma (1976), Stahl & Hager (1999), Speerli (1999), Speerli & Hager (2000), Ozkan et al. (2006, 2008, 2010, 2014), Escarameia (2007), Oveson (2008), Safavi et al. (2008), Mortensen (2009), Unsal et al. (2008, 2009) and Tuna et al. (2014) studied the air-demand ratio () and aeration efficiency (E20) in closed conduits and outlet works. However, the comprehensive literature search did not identify any published analytical or physical studies of the aeration efficiency in free-surface gated circular conduits. This paper reports an experimental investigation of a free-surface gated circular conduit flow system, including the effects of the Froude number (Fr) and conduit length (L) on the air-demand ratio and the aeration efficiency.
An air vent was installed immediately downstream of the gate. The air vent consisted of 14 mm inside-diameter pipes that had a length of 150 mm. The sluice gate lip angle was 45°. As water entered the flume under the sluice gate, a vacuum (air entrainment) occurred at the air vent of the free-surface conduit. An anemometer (Testo Model 435) was used to measure air velocity in the air vent. This measurement was accomplished by locating the anemometer at the center of the air vent. Each air velocity measurement was taken over a period of 60 seconds or longer. After obtaining a value for the air velocity, the air flow rate through the air vent was calculated. The anemometer used for air velocity measurements was accurate to ±(0.2 m/s + 1.5% of mv). Care was taken to ensure that the anemometer was always perpendicular to the direction of flow in the air vent to provide the most accurate measurements possible. Water flow rates were measured using a calibrated electromagnetic flow meter.
A calibrated dissolved oxygen meter (WTW Model Oxi 330i) was used to measure both the dissolved oxygen and the water temperature. The dissolved oxygen (DO) meter was calibrated using procedures following those recommended by the manufacturer. The sensor of the DO meter was immersed in the water to a depth of 0.20 m. Clean tap water was used throughout the experiments. The water in the storage tank was deoxygenated using the sodium-sulfite method. Cobalt chloride catalyzed the reaction between molecular oxygen and sodium sulfite. Each experiment was started by filling the storage tank and adding sodium sulfite (Na2SO3) and cobalt chloride (CoCl2). A stirrer was used to mix sodium sulfite and cobalt chloride with the water, until the DO was reduced to approximately 0. The aeration efficiency values were calculated from Equation (2) and then adjusted to 20 °C with Equation (3).
RESULTS AND DISCUSSION
The present study investigated the air-demand ratio () and aeration efficiency (E20) in free-surface gated circular conduits, and in particular, the effect of the Froude number (Fr) and conduit length (L) on the air-demand ratio and the aeration efficiency.
The results indicated that the free-surface gated circular conduit flow system had high aeration efficiency. The primary reason for this high aeration efficiency is that air is entrained into the flow in the form of a large number of fine bubbles. These air bubbles greatly increased the surface area available for mass transfer and hence the aeration efficiency.
where E20 is aeration efficiency at 20 °C, L is conduit length and Fr is Froude number based on effective depth in conduit.
This study was performed in an effort to better understand air-demand ratio and aeration efficiency of gated circular conduits with free-surface flow condition. For this purpose, a series of experiments was conducted in a free-surface gated circular conduit. The following conclusions can be drawn from the present study:
(a) The Froude number had an important effect on the air-demand ratio and the aeration efficiency. The air-demand ratio and the aeration efficiency increased with increasing the Froude number.
(b) The effect of the conduit length on the air-demand ratio and the aeration efficiency changed depending on the Froude number.
(c) At Froude numbers lower than 20, there was no effect of the conduit length on the air-demand ratio. However, at Froude numbers greater than 20, the air-demand ratio increased with increasing conduit length.
(d) At Froude numbers lower than 15, there was no effect of the conduit length on the aeration efficiency. However, at Froude numbers greater than 15, the aeration efficiency increased with increasing conduit length.
(e) The free-surface gated circular conduit flow system was very effective for oxygen transfer. Therefore, this system can be used as a highly effective aerator in aeration processes.
(f) A regression equation was obtained with a very high correlation coefficient, showing the effect of the conduit length and the Froude number on the aeration efficiency.
(g) Great care must be taken when scaling results from models of two-phase flows as size-scale effects may exist. Previous studies have shown that scale effects of air entrained within closed conduits are negligible. However, scaling of aeration data to prototype size is virtually impossible, largely due to the relative invariance of bubble size. Various model sizes may be necessary to determine the significance of size-scale effects of aeration efficiency in circular closed conduits between the different-sized structures.
(h) Additional research is needed to better understand the effect of conduit geometry on aeration efficiency.