International Journal of Science & Technology Volume 1, No 1, 65-74, 2006

The Performance of Two Phase Flow Systems in Pond Aeration Fahri OZKAN, Ahmet BAYLAR and Mehmet TUGAL Firat University, Construction Education Department, Elazig, TURKEY Firat University, Civil Engineering Department, Elazig, TURKEY [email protected] (Received: 13.01.2005; Accepted: 15.12.2005)

Abstract: 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 paper 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. When the gate of a high head outlet conduit is partly opened or a minimal amount of differential pressure exists between the inlet and outlet sides of the venturi tube, air suction occurs at air vents. In two phase flow systems, air that is entrained into the water will be 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 and venturi tube, and the rising air bubbles into pond. Moreover, high pressure in two phase flow systems also facilitates the dissolution of oxygen into the water. Therefore, two phase flow systems have an extremely high efficiency for transferring oxygen from air bubbles to water. It was observed from the results that two phase flow systems were more efficient in deep ponds than in shallow ponds. Keywords: Conduit, Venturi, Oxygen transfer, Aeration, Pond

Gölet Havalandırmasında İki Fazlı Akım Sistemlerinin Performansı Özet: Havalandırma, doğal metotlarla veya mekanik araçlarla su ve hava arasındaki temas alanının arttırılması yöntemidir. Uygun havalandırma ile gölet ekosisteminde oldukça önemli bir gelişme sağlanabilir. Bu çalışmada, yüksek basınçlı kapaklı konduit ve venturi tüplü iki fazlı boru akım sistemleri ile gölet havalandırması incelenmiştir. Yüksek basınçlı kapaklı konduit kısmen açılırsa veya venturi tüpünün giriş ve çıkışları arasında minimum bir basınç farkı meydana getirilirse, hava deliğinden hava girişi olur. İki fazlı akım sistemlerinde su içerisine çekilen hava küçük hava kabarcıkları şeklinde mansaba doğru itilir. Su içerisindeki oksijenin çözünmesi, yüksek basınçlı kapaklı konduit ve venturi tüpünün mansabındaki hava çekiminden ve gölet içerisine verilen hava kabarcıklarının yükselmesi ile olur. Ayrıca iki fazlı akım sistemlerindeki yüksek basınç ta su içerisinde oksijenin çözünmesini kolaylaştırır. Böylece, iki fazlı akım sistemleri oksijen transferinde oldukça yüksek verime sahiptirler. Sonuçlardan iki fazlı akım sistemleri derin göletlerde sığ göletlerden daha verimli olduğu görülmektedir. Anahtar Kelimeler: Konduit, Venturi, Oksijen transferi, Havalandırma, Gölet

1. Introduction Oxygen is a necessary element to all forms of life. The level of dissolved oxygen is one of the best indicators of overall water quality. It is removed by respiration of organisms and by organic decomposition. During respiration and decomposition, animals and plants consume dissolved oxygen and liberate carbon dioxide. Organic waste from municipal, agricultural and industrial sources may overload the natural system causing a serious depletion of the oxygen supply in the water. Water rich in nutrients

produces algae in quantity which upon decomposition deplete the oxygen supply. Fish kills are often associated with this process of eutrophication. Standards for dissolved oxygen vary. Habitats for warm water fish population should contain dissolved oxygen (DO) concentrations of not less than 4.0 mg/L. Habitats for cold water fish population should not be less than 5.0 mg/L. Aeration is the process by which the area of contact between water and air is increased, either by natural methods or by mechanical devices.

Özkan, et.al

Aerators influence the rate of oxygen transfer from air to water by increasing turbulence and surface area of water in contact with air. Aerators are of two basic types: splashers and bubblers. An example of a splasher aerator is a paddle wheel aerator. It splashes water into the air to affect aeration. Splashing action also causes turbulence in the body of water being aerated. Bubbler aerators rely upon release of air bubbles near the bottom of a water body to affect aeration. A large surface area is created between air bubbles and surrounding water. Rising bubbles also create turbulence within a body of water [1]. Aeration performance of hydraulic structures has been studied experimentally by a number of investigators. These studies are reviewed by Wilhelms et al. [2], Chanson [3], Ervine [4], and Gulliver et al. [5]. Kalinske and Robertson [6] was the first to report on the air entrainment in a pipe. Since then a number of laboratory investigations into two phase flow systems have been carried out. These research works have dealt with air entrainment mechanisms in two phase pipe flow systems, but none has concentrated specifically on the oxygen transfer of two phase pipe flow systems. In this paper, experimental studies were conducted to investigate 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 is very important that sufficient circulation is provided within the pond so that all areas have proper oxygenation. By circulating water by two phase flow systems within the pond, stratification is eliminated and dissolved oxygen levels rebound. Higher oxygen levels yield greater biotic growth, including fish growth. Moreover, aeration by two phase flow systems can actually reverse the buildup of organics in the short-term and eliminate the foul odors as a result of the accelerated decomposition of organic material under oxygenated conditions. 2. Mechanisms of Air Entrainment When the gate of a high head outlet conduit is partly opened, a high velocity flow occurs downstream of the gate resulting in subatmospheric pressures. Theoretically, these pressures can be as low as the vapor pressure of water. When the conduit is connected to the atmosphere through an air vent located downstream of the gate, air suction occurs at air vent, as illustrated in Figure 1. The quantity of air depends on the entraining and carrying capacity of the flow and the drop in pressure (from the atmospheric value) behind the gate is a function of the gate opening [7].

Qa Sluice gate

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Air vent

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Figure 1. High head gated conduit flow system

When a minimal amount of differential pressure exists between the inlet and outlet sides of a venturi tube, a vacuum (air suction) occurs at air holes of venturi tube (Figure 2). When a pressurized operating (motive) fluid, such as water, enters the venturi tube inlet, it constricts toward the throat portion of the venturi tube and changes into a high velocity jet stream. The increase in velocity through the throat portion of the venturi tube, as a

result of the differential pressure, results in a decrease in pressure in the throat portion. This pressure drop enables air to be injected through the air holes and is dynamically entrained into the motive stream. As the jet stream is diffused toward the venturi tube outlet, its velocity is reduced and it is reconverted into pressure energy (but at a pressure level lower than venturi tube inlet pressure). The venturi tubes are high 66

The Performance of Two Phase Flow Systems in Pond Aeration

efficient, requiring less than 20 % differential to initiate suction. Qa /2 O

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Qa /2 Figure 2. Two phase pipe flow system using venturi tube

3. Background should be taken. The DO deficit is computed for each time that DO was measured during reaeration:

In a typical aerator test, water is deoxygenated with cobalt chloride and sodium sulfite Cobalt catalyzes the reaction between molecular oxygen and sodium sulfite. Waters that contain appreciable organic matter or an abundance of plankton should be avoided in aerator tests because oxygen may be added by photosynthesis or removed by respiration. Well water usually is suitable for aerator tests, but if well water has a high concentration of iron, oxidation of iron during the test will affect results. At least 8 or 10 DO measurements equally spaced in time

(1)

DO deficit = C s − C m

where Cs is the DO concentration at saturation (mg/L) and Cm is the measured DO concentration (mg/L). In this study, the saturation concentrations were determined by the chart of McGhee [8], as illustrated in Table 1.

Table 1. The solubility of oxygen (mg/L) in water at different temperatures and salinities from moist air with pressure of 760 mm Hg DO (g/m3) for stated concentrations of chloride, g/m3

Temperature

Difference per

(°°C)

0

5.000

10.000

15.000

20.000

100 g/m3 chloride

0 5 10 15 20 25 30

14.62 12.80 11.33 10.15 9.17 8.38 7.63

13.79 12.09 10.73 9.65 8.73 7.96 7.25

12.97 11.39 10.13 9.14 8.30 7.56 6.86

12.14 10.70 9.55 8.63 7.86 7.15 6.49

11.32 10.01 8.98 8.14 7.42 6.74 6.13

0.0165 0.0140 0.0118 0.0100 0.0088 0.0082 0.0075

The natural logarithms of DO deficits (Y) are plotted versus the time of aeration (X); the line of best fit is drawn by visual inspection or by aid of regression analysis. The oxygentransfer coefficient is adjusted to 20°C with the following equation:

(K L a ) 20 =

(K L a ) T 1.024 ( T −20)

where (KLa)20 is the oxygen transfer coefficient at 20°C (1/hr) and T is water temperature (°C). Results of oxygen transfer tests normally are reported on a clean water (tap water) basis. If the test cannot be run in clean water, the α-value must be determined for the water in which the aerator test was conducted and the test results adjusted [9]. The α-value is defined as:

(2) 67

Özkan, et.al

α=

(K L a ) 20 test water

The oxygen transfer coefficient is used to estimate the standard oxygen transfer rate for an aerator:

(3)

(K L a ) 20 tap water

SOTR = (K L a ) 20 x C s * x V x 10 −3

The α test can be conducted by using a laboratory scale aerator to determine (KLa)20 values for small samples of test water and clean, tap water [10]. When aerator tests are conducted in pond water, the oxygen transfer coefficient should be adjusted to a clean water basis as follows:

(K L a )' 20 =

(K L a ) 20 α

(5)

where SOTR is standard oxygen transfer rate (kg O2/h), Cs* is saturated DO at 20°C and standard atmospheric pressure (mg/L), V is aeration tank volume (m3), and 10-3 is factor for converting gram to kilogram. By definition, the SOTR is the amount of oxygen that an aerator will transfer to water per hour under standard conditions. Standard conditions are 0 mg/L DO, 20°C, and clean water [1]. An example of oxygen transfer test is provided (Table 2 and Figure 3).

(4)

where (K L a )' 20 is the adjusted ( K L a ) 20 .

Table 2. Calculation of the rate of change in the oxygen deficit during an oxygen transfer test for an aerator (T = 16 °C and Cs= 8.96 mg/L) Time

Measured DO

DO deficit

ln DO deficit

(min)

Cm (mg/L)

Cs-Cm (mg/L)

(mg/L)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

0.0 1.6 3.0 3.9 4.8 5.5 6.1 6.5 6.9 7.2 7.5 7.7 7.9

8.96 7.36 5.96 5.06 4.16 3.46 2.86 2.46 2.06 1.76 1.46 1.26 1.06

2.193 1.996 1.785 1.621 1.426 1.241 1.051 0.900 0.723 0.565 0.378 0.231 0.058

68

The Performance of Two Phase Flow Systems in Pond Aeration

2.5

T = 16 oC C s = 8.96 mg/L

2.0

(KLa)16 = 0.354 (1/min)

ln (C s - C m )

(KLa)20 = 0.389 (1/min)

1.5

(KLa)20 = 23.34 (1/h) V=1.5 m3 SOTR=0.321 kg O2 /h

1.0

0.5

0.0 0.0

1.5

3.0 4.5 Time (min)

6.0

7.5

Figure 3. Graph and calculations for an aerator performance test by the point method

4. Experimental Experimental investigations were conducted using experimental setups in the Hydraulic Laboratory at the Engineering Faculty of Firat University, Elazig, Turkey. General views of the experimental setups are given in Figures 4 and 5. The experimental setups consisted of water tank, water pump, water flowmeter, air flowmeter, venturi tube, gated conduit, bubble trap, flow control valve, release valve, water feed line, thermometer, and DO meter. All experiments were carried out in a 1.8 m3 water tank with glass-walls (0.75 m wide x 2.0 m long x 1.2 m high). The water depth in the water tank was varied as 0.50 m (V=0.75 m3) and 1.0 m (V=1.50 m3). The water in the experimental setups was

recirculated by a pump and the flow rate of water was measured using a water flowmeter. A rectangular conduit made of plexiglass with a finished inside section of 45 mm x 95 mm (wide x high) was used, as illustrated in Figure 4. The air vent consisted of a 14-mm ID transparent pipe. Sluice gate was made of metal and gate lip angle was selected as 45°. While water entered the flume under a sluice gate, a vacuum (air suction) occurred at air vent of high head gated conduit. Air entering air vent on high head gated conduit was measured with the help of an anemometer. Sluice gate openings were varied from 5mm to 30 mm in 5 mm steps. Flow velocities at gate section used in this study were equal to 6 m/s, 9 m/s and 12 m/s.

69

Özkan, et.al

Air vent DO meter o oo o

o o

o

o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o oo o o o o o o o o o o o o o o o o o o o o o oo o o o ooo o oo o o o o o o o o oo o o o o o o o oooo o o o

Flowmeter

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Thermometer

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Q A Sluice gate

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Flow control valve

Hw 45o Water pump

Figure 4.Laboratory high head gated conduit apparatus

Venturi tubes used in the experiments were made of PVC. The inlet and outlet diameters of the venturi tubes D were 36 mm, 42 mm and 54 mm. The ratio of throat diameter of venturi tube to inlet and outlet diameter of venturi tube Dt/D was 0.50 and 0.75. Throat length of venturi tubes was selected as Dt. Converging cone angle θ1 and diverging cone angle θ2 were 21° and 7°, respectively. At the throat portion of the venturi tubes, two holes with diameters of 5.0 mm were drilled through the wall. When a minimal amount of differential pressure existed between the inlet and outlet sides of the venturi tube, a vacuum

(air suction) occurred at air holes of venturi tube. A bubble trap for which the plan-view dimensions were 0.70 m x 0.75 m, was used to obtain air entering air holes on venturi tube using an air rotameter installed on its surface, as illustrated in Figure 5. While DO measurements were taken, the bubble trap was removed from the water tank. Flow velocities at inlet portion of venturi tubes were varied from 1.5 m/s to 4.5 m/s in 1.5 m/s steps for venturi tubes with Dt/D equal to 0.50 and 2.5 m/s to 7.5 m/s in 2.5m/s steps for venturi tubes with Dt/D equal to 0.75.

QA Air Q A /2 flowmeter Thermometer

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o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o oo o o o o o o o o o o o o o o o oo o o o o oo o o o o o o o oo oo o o o o oooo o

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Figure 5. Laboratory venturi tube apparatus

To obtain KLa, at first the tap water in the tank was deoxygenated by applying cobalt as cobalt chloride (CoCl2) and sodium sulphite (Na2SO3). In a typical aerator test, water is deoxygenated with cobalt chloride at 0.05–0.1 mg cobalt/L and sodium sulfite at 8–10 mg/L for each milligram per liter of DO. Cobalt catalyzes

the following reaction between oxygen and sodium sulfite:

Na 2SO 3 +

70

1 O 2 → Na 2SO 4 2

molecular

(6)

The Performance of Two Phase Flow Systems in Pond Aeration

The mixer was used to mix cobalt chloride and sodium sulphite with the water. The mixer was run until the DO was reduced to 0 mg/L. When the DO concentration again began to rise, readings were taken at timed intervals till DO increased from 0% saturation to at least 80% saturation. At least 12 DO measurements at equal time intervals were taken. During the experiments, DO measurements were taken using calibrated portable HANNA Model HI 9142 oxygen meter at the locations identified in Figures 4 and 5. The DO meter was calibrated daily, prior to use, by the air calibration method. Calibration procedures followed those recommended by the manufacturer. The calibration was performed in humid air under ambient conditions. The natural logarithms of DO deficits (Y) were plotted versus the time of aeration (X). The line of best fit was drawn by visual inspection or by aid of regression analysis. So the oxygen transfer coefficient at T °C (KLa)T was obtained. The oxygen transfer coefficient was adjusted to 20°C with Eq. 2 and standard oxygen transfer rate SOTR values were then calculated from Eq. 5. A thermometer was used to measure water temperature while DO measurements were taken.

to 0.75, the values of QA for all the diameters decreased with increasing flow velocity. It was observed from Tables 3-4 that for all values of Dt/D, the values of air entrainment rate QA and standard oxygen transfer rate SOTR increased as inlet diameter of venturi tube increased. For all the diameters and the values of Dt/D, the values of SOTR of two phase pipe flow systems using venturi tubes increased with increasing flow velocity. Results indicated that for all the diameters and the values of Dt/D, the values of SOTR of two phase pipe flow systems using venturi tubes increased as the water depth in the water tank increased. In high head gated conduit flow systems, the values of air entrainment rate QA and standard oxygen transfer rate SOTR for all the gate openings increased as flow velocity at gate section Vc increased, as illustrated in Tables 5-6. Moreover, in high head gated conduit flow systems, the values of QA and SOTR increased with increasing gate opening. It was observed from Tables 5-6 that for all the gate openings, high head gated conduit flow systems were more efficient in deep ponds than in shallow ponds. Heavily populated ponds may need supplemental air and ponds with a large amount of algae may need supplemental air at night when the plants are not making oxygen but consuming it. In two phase flow systems, air that is entrained into the water will be momentarily forced downstream in the form of small air bubbles. Two phase flow systems that release fine bubbles usually are more efficient than those that discharge coarse bubbles. This results because fine bubbles present a greater surface area to the surrounding water than larger bubbles. Oxygen diffuses into the water at the surface, so a large surface area facilitates greater oxygen absorption. Naturally, most ponds will undergo two stratification periods annually. Stratification creates two distinct layers of water that are separated by a transitional layer (thermocline). Because of the density differences between the layers, the lower layer (hypolimnion) is isolated from nearly all input of atmospheric oxygen while stratification persists. Without aeration, many ponds will develop an oxygen-deprived hypolimnion throughout the summer. The same ponds may also run the risk of oxygen depletion

5. Experimental Results and Discussion The prime purpose of this paper is to study air entrainment rate QA and standard oxygen transfer rate SOTR of two phase flow systems such as high head gated conduit flow systems and two phase pipe flow systems using venturi tubes. The dissolution of oxygen into the water results from the air suction downstream of high head gated conduit and venturi tube, and the rising air bubbles into pond. Moreover, high pressure in two phase flow systems also facilitates the dissolution of oxygen into the water. The results obtained from the study are presented and discussed in this section. In two phase pipe flow systems using venturi tubes with Dt/D equal to 0.50, the values of air entrainment rate QA for all the diameters increased with increasing flow velocity at inlet portion of venturi tube Vv up to a certain point (3.0 m/s) and then the values of QA decreased with a further increase of flow velocity, as illustrated in Tables 3-4. However, in two phase pipe flow systems using venturi tubes with equal 71

Özkan, et.al

under periods of extended ice cover. Ponds that undergo periods of oxygen depletion may be influenced to different degrees ranging from a decline in ecosystem efficiency to the extreme of experiencing episodes of fish kills. By disrupting the pond’s stratification, the pond’s ecosystem is supplied with adequate oxygen, preventing the

suffocating effect that would have otherwise occurred. Aeration by two phase flow systems can make considerable improvements in a pond ecosystem. By circulating water within the pond, stratification is eliminated and dissolved oxygen levels rebound.

Table 3. Venturi tube data for V=0.75 m3 D

Vv

Dt / D

Hw

QA 3

-4

(KLa)20

Cs*

V

SOTR

(1/h)

(g/m3)

(m3)

(kg O2/h)

(mm)

(m/s)

(-)

(m)

(m /s) x 10

36 36 36 42 42 42 54 54 54 36 36 36 42 42 42 54 54 54

1.5 3.0 4.5 1.5 3.0 4.5 1.5 3.0 4.5 2.5 5.0 7.5 2.5 5.0 7.5 2.5 5.0 7.5

0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

7.86 8.86 8.03 10.68 11.07 9.64 12.36 15.28 12.42 9.52 8.73 7.72 11.10 8.73 8.09 17.94 17.03 16.72

5.46 8.76 9.24 7.38 10.32 12.72 8.70 12.48 12.96 7.26 14.40 29.76 8.46 24.12 32.10 16.56 54.54 64.74

9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17

0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

0.038 0.060 0.064 0.051 0.071 0.087 0.060 0.086 0.089 0.050 0.099 0.205 0.058 0.166 0.221 0.114 0.375 0.445

Table 4. Venturi tube data for V=1.50 m3 D (mm)

Vv (m/s)

Dt / D (-)

Hw (m)

QA (m3/s) x 10-4

(KLa)20 (1/h)

Cs* (g/m3)

V (m3)

SOTR (kg O2/h)

36 36 36 42 42 42 54 54 54 36 36 36 42 42 42 54 54 54

1.5 3.0 4.5 1.5 3.0 4.5 1.5 3.0 4.5 2.5 5.0 7.5 2.5 5.0 7.5 2.5 5.0 7.5

0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

7.86 8.86 8.03 10.68 11.07 9.64 12.36 15.28 12.42 9.52 8.73 7.72 11.10 8.73 8.09 17.94 17.03 16.72

3.30 6.24 6.48 3.54 6.90 9.12 5.10 8.64 11.22 3.96 7.68 11.64 4.68 9.90 16.74 8.10 25.74 36.48

9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17

1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50

0.045 0.086 0.089 0.049 0.095 0.125 0.070 0.119 0.154 0.054 0.106 0.160 0.064 0.136 0.230 0.111 0.354 0.502

Under low oxygen conditions, decomposition takes place at a much slower rate than under

oxygenated conditions. As a result, organic material is decomposed at a very slow rate, in fact, usually 72

The Performance of Two Phase Flow Systems in Pond Aeration

slower than the rate that new organic material is being created. So pond rapidly becomes shallower as it fills with organic material. Simultaneously, gases bubble up from the muck, producing the foul rotten egg smell. Aeration by two phase flow systems can

actually reverse the build-up of organics in the shortterm (natural succession dictates that a pond will naturally fill in) and eliminate the foul odors as a result of the accelerated decomposition of organic material under oxygenated conditions.

Table 5. High head gated conduit data for V=0.75 m3 h (mm)

Vc (m/s)

Hw (m)

QA (m /s) x 10-4

(KLa)20 (1/h)

Cs* (g/m3)

V (m3)

SOTR (kg O2/h)

5 5 5 10 10 10 15 15 15 20 20 20 25 25 25 30 30 30

6.0 9.0 12.0 6.0 9.0 12.0 6.0 9.0 12.0 6.0 9.0 12.0 6.0 9.0 12.0 6.0 9.0 12.0

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

1.77 8.77 19.26 3.66 12.41 37.39 3.97 29.40 52.08 5.70 32.30 57.10 18.30 34.56 59.50 21.40 35.67 64.67

0.96 1.86 7.20 4.38 15.12 28.68 16.86 30.42 37.38 19.50 35.22 44.34 24.72 34.74 53.64 26.10 51.96 67.32

9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17

0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

0.007 0.013 0.050 0.030 0.104 0.197 0.116 0.209 0.257 0.134 0.242 0.305 0.170 0.239 0.369 0.180 0.357 0.463

3

Table 6. High head gated conduit data for V=1.50 m3 h (mm)

Vc (m/s)

Hw (m)

QA (m /s) x 10-4

(KLa)20 (1/h)

Cs* (g/m3)

V (m3)

SOTR (kg O2/h)

5 5 5 10 10 10 15 15 15 20 20 20 25 25 25 30 30 30

6.0 9.0 12.0 6.0 9.0 12.0 6.0 9.0 12.0 6.0 9.0 12.0 6.0 9.0 12.0 6.0 9.0 12.0

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.77 8.77 19.26 3.66 12.41 37.39 3.97 29.40 52.08 5.70 32.30 57.10 18.30 34.56 59.50 21.40 35.67 64.67

0.54 0.97 3.72 2.40 6.84 18.66 5.76 18.00 30.36 9.90 23.34 36.54 14.04 26.40 44.46 15.90 29.94 56.52

9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17 9.17

1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50

0.007 0.013 0.051 0.033 0.094 0.257 0.079 0.248 0.418 0.136 0.321 0.503 0.193 0.363 0.612 0.219 0.412 0.777

3

6. Conclusions experiments were carried out to investigate pond aeration by two phase flow systems such as high head gated conduit flow systems and two phase pipe flow systems using venturi tubes. Results

A high level of dissolved oxygen is vital for the maintenance of healthy ponds. Aeration can make considerable improvements in a pond ecosystem. In this study, a series of laboratory 73

Özkan, et.al

indicated that two phase flow systems have an extremely high efficiency for transferring oxygen from air bubbles to water. It was observed that two phase flow systems were more efficient in deep ponds than in shallow ponds. Moreover, mixing of pond water by two phase

flow systems can reduce vertical stratification of temperature and chemical substances. Aeration by two phase flow systems can also be an important tool for reducing the accumulation of organic muck that builds up on the pond bottom.

7. Acknowledgments The studies reported herein were conducted at the Hydraulic Laboratory at Firat University, Elazig, Turkey. The financial support was provided by Firat University Scientific Research Projects (FUBAP). 8. References 1. Boyd, C. E. (1998). Pond water aeration systems. Aquac. Eng., 18, 9-40. 2. Wilhelms, S. C., Gulliver, J. S., and Parkhill, K. (1992). Reaeration at low-head hydraulic structures. Tech. Rep. HL-91, U S Army Engineer Waterways Experiment Station, Vicksburg, Miss. 3. Chanson, H. (1995). Predicting oxygen content downstream of weirs, spillways and waterways. Proc. Instn Civ. Engrs Wat., Marit. & Energy, 112, March, 20-30. 4. Ervine, D. A. (1998). Air entrainment in hydraulic structures: A review. Proc. Instn Civ. Engrs Wat., Marit. & Energy, 130, September, 142-153. 5. Gulliver, J. S., Wilhelms, S. C., and Parkhill, K. L. (1998). Predictive capabilities in oxygen transfer at

hydraulics structures. J. Hydr. Engrg., ASCE, 124 (7), 664-671. 6. Kalinske A. A., and Robertson, J. M. (1943). Closed conduit flow. Transactions of the Symposium on Entrainment of Air in Flowing Water, ASCE, 108, Paper No. 2205, 1435-1447. 7. Sharma, H. R. (1976). Air-entrainment in high head gated conduits. J. Hydraul. Div., ASCE, 102, (HY11), 1629-1646. 8. McGhee, T. J. (1991). Water Supply and Sewerage. McGraw - Hill International Editions, (6th ed.). 9. Boyd C. E., and Ahmad, T. (1987). Evaluation of aerators for channel catfish farming. Bulletin 584, Alabama Agricultural Experiment Station, Auburn University, Alabama, pp. 52. 10. Shelton J. L Jr., and Boyd, C. E. (1983). Correction factors for calculating oxygen-transfer rates of pond aerators. Trans. Am. Fish. Soc., 112 (1), 120-122.

9. Notation Cs Cs* Cm D Dt H (KLa)20 (KLa)'20 (KLa)T SOTR QA Qw T

saturation concentration of oxygen in water (mg/L) saturation concentration of oxygen in water at standard conditions (mg/L) measured DO concentration (mg/L) inlet and outlet diameter of venturi tube (mm) throat diameter of venturi tube (mm) gate opening (mm) mass transfer coefficient at 20 °C (1/h) adjusted (KLa)20 (1/h) mass transfer coefficient at T °C (1/h) standard oxygen transfer rate (kg O2/h) air entrainment rate (m3/s) water discharge (m3/s) water temperature (°C)

V Vc Vv α

74

water volume (m3) flow velocity at gate section (m/s) flow velocity at inlet portion of venturi tube (m/s) (KLa)20 test water / (KLa)20 tap water (-)