- Email: [email protected]
CFD modelling of cyclonic-DAF (dissolved air flotation) reactor for algae removal
Engineering Science and Technology, an International Journal 22 (2019) 477–481
Contents lists available at ScienceDirect
Engineering Science and Technology, an International Journal journal homepage: www.elsevier.com/locate/jestch
Full Length Article
CFD modelling of cyclonic-DAF (dissolved air flotation) reactor for algae removal Hong Sok Oh a,b, Seon Hong Kang b, SookHyun Nam c, Eun-Ju Kim c, Tae-Mun Hwang c,⇑ a
Kyong-Ho Engineering & Architects Co., Ltd, Korea Department of Environmental Engineering, Kwangwoon University, Korea c Korea Institute of Civil Engineering and Building Technology, Korea b
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 11 December 2017 Revised 25 July 2018 Accepted 14 December 2018 Available online 19 December 2018
The purpose of this study is to simulate the Cyclonic-DAF tanks using CFD techniques for the optimal design. CFD simulations were performed on two different Cyclonic-DAF reactor shapes either having or not having a cone shaped settled sludge removal unit in the bottom area of the inlet in the CyclonicDAF reaction tank. An average flow of less than 10% within the Cyclonic-DAF was rarely found in both shapes while areas of less than 20% were only slightly found near the central concentrate waste pipe. Areas having an average flow of less than 50% were found around the concentrate waste pipe and were observed at a rate of less than 5% of the total volume. Efficiency test results of different Cyclonic-DAF (CDAF) tank height to diameter ratios set at 1:1, 1:35:1, 1.6:1, and 1.85:1 indicated similar efficiencies with turbidity at 91.9% and chlorophyll-a at 94.7%. Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Algae removal Dissolved air flotation DAF Cyclonic-DAF Computational fluid dynamics (CFD)
1. Background Recent climate shifts have propagated rapid change in the physical environments (retention time, etc.) of rivers and lakes. One example is the formation of algae along reservoirs and dammed areas that are used exclusively for water supply. The algae formation has become pervasive to the extent that it has become a social problem. Particularly in the case of various types of algae formations, cyanobacterial formations raise the need for fundamental inhibitive solutions due to their potential toxicity. Water supplies with large formations of cyanobacteria result in increased costs associated with the treatment of the water as well as a number of social and economic problems for its surrounding region [1–3]. Dissolved air flotation (DAF), one of the most representative physico-chemical processing technologies used to remove algae, is a vehicle mountable aqua-mobile freshwater lake water treatment technology. This technology removes suspended substances or algae in aquatic environments using physical treatment processes that employ the flotation of air. Cyclonic Dissolved Air Flotation (DAF), a dissolved air flotation method displaying improved performance, has a structure capable of forming twirling flows that
⇑ Corresponding author. E-mail addresses: (T.-M. Hwang).
[email protected]
(S.H.
Kang),
Peer review under responsibility of Karabuk University.
[email protected]
produce microbubbles that adhere to suspended matter and float to the surface of the water passing through the float separation tank. The Cyclonic-DAF is an improved type of dissolved air flotation method in that it ejects floating substances through an ejection pipe installed in the center of devices that employ it. However, such flotation separation devices where the suspended substances (referred to as ‘‘scum”) float to the surface face a complication where it is difficult to adjust water levels that ensure stable overflow of scum due to the aggregation of flotation towards the center of the water surface of the cylindrical float separation tank. This study analyzes the removal effects of different water levels through lab-scale tests. The study also undertakes solid modelling based on computational fluid dynamics for the purposes of discovering application implications during the manufacture of Cyclonic-DAF (dissolved air flotation) devices capable of being mounted on a vehicle to remove algae in rivers. 2. Method 2.1. Cyclonic-DAF A Cyclonic-DAF device consists of a cylindrical float separation tank, air-saturated water and microbubble inlet pipe, and a vertical double tube that drains concentrations. The principles of operation of a dissolved air flotation device are described as per the following. An inflow of microbubbles and air-saturated water into the
https://doi.org/10.1016/j.jestch.2018.12.003 2215-0986/Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
478
H.S. Oh et al. / Engineering Science and Technology, an International Journal 22 (2019) 477–481
flotation tank results in the flotation of suspended substances in a twirling stream. The suspended algae float to the water surface in a twirling stream and aggregates towards the center after which it is later drained by a central pipe. The water treated for the removal of algae forms a downflow in a twirling stream and is later drained through the treated water pipe. Large particles shift and settle towards the walls of the tank due to centrifugal forces and is drained through a concentrate pipe. The turbidity of the tested air-saturated water was 12.4 NTU, and the chlorophyll-a concentration was 53.1 mg/m3. Different height to diameter ratios of the cyclonic-DAF tank set to 1:1, 1.35:1, 1.6:1, and 1.85:1 were tested to determine their efficiencies. 2 mg/L of a 0.05% polyacrylamide high-polymer solution was used as the coagulant for the tests (See Fig. 1). 2.2. CFD modeling As DAF techniques have begun to be employed in water processing technologies, several researchers have undertaken studies regarding the mutual exchanges that occur between microbubbles and particles [4–6]. Following the late 1990s, computational fluid dynamics (CFD) technologies were used to undertake a number of process interpretations that resulted in a number of findings regarding its hydraulics [7]. In line with these trends, this study undertook simulations of Cyclonic-DAF tanks using CFD techniques for the purposes of finding the optimal design and operational parameters of a Cyclonic-DAF reaction tank that would remove suspended substances. During the simulation, subjects of interpretation were divided along a small cubic mesh or grid after which flow motion equation (continual momentum and energy equations) values satisfying initial and boundary conditions using the finite volume method were repeatedly found using a computer. The Cyclonic-DAF reaction tank was configured as shown in Fig. 2. 2.2.1. Model equations ANSYS CFX is used as CFD code in this study. With the addition of air bubbles in the flow field, the flow is no longer a single-phase flow and becomes configured into two phases. This is referred to as a multiphase flow. In order to simulate the two-phase flow of the air bubbles and water, the Eulerian-Eulerian Method, which separately calculates and couples the motion equations of each phase, was used. Hague et al. found that computational interpretation
results applied with the k-e turbulence model were closer to the experimental value than computational interpretation results that assumed the flow as laminar flow [8]. Transport equations of one or two turbulent quantities, like kinetic energy (k) or its dissipation rate (e), are derived and leads to the set of equations called k-e equations [9]. 2.2.2. Operating conditions Two different shapes of the Cyclonic-DAF tanks were tested under the following CFD simulation conditions. Flocculation was undertaken as pre-treatment of the DAF reaction tank with retention time of 9 min in the flotation tank. Operating conditions are as follows. The flow of air-saturated water was maintained at 10 m3/h and dissolved air was supplied at 0.1 m3/h. The total volume of the inflow of air-saturated water and dissolved air was fixed at 10.1 m3/h during the operation. Air was dissolved using a saturator typically at 5–6 bars of pressure. The size of the air bubbles is approximately 10–120 mm. It varied according to water temperature and pressure. The pressurized microbubbles are ejected through the upper area. 2.2.3. Mesh Shape No. 1 of the Cyclonic-DAF tanks was composed of approximately 223,000 nodes and 857,000 elements in the flow zone. Shape No. 2 was composed of approximately 266,000 nodes and 1,035,000 elements. The meshes were mixed with tetragonal and hexagonal lattices. Areas with heavy changes in flow were inserted with strata having more narrow lattices. 2.2.4. Boundary conditions One inlet condition and 3 outlet conditions were given to simulate the Cyclonic-DAF reaction tank using CFD analysis. The wall of the tanks was applied with a no-slip condition. A degassing boundary was configured to achieve mass balancing of the air. The k-e model was applied as the turbulence model. 3. Results and discussions 3.1. Cyclonic-DAF devices Table 1 shows the efficiency analysis results of the CyclonicDAF tank as having different height/diameter ratios of 1:1,
Fig. 1. Schematic of Cyclonic-DAF process.
H.S. Oh et al. / Engineering Science and Technology, an International Journal 22 (2019) 477–481
479
Fig. 2. C-DAF Reaction Tank Shapes.
1.35:1, 1.6:1, and 1.85:1. The field experiments were undertaken to examine the turbidity and chlorophyll-a removal efficiencies and explore the design factors of different height/diameter ratios of Cyclonic-DAF devices. The results indicated that the 1:1 ratio yielded the optimal results having removal efficiencies of turbidity at 91.9% and chlorophyll-a at 94.7%. This indicated not only that a 1:1 height/diameter ratio is appropriate for the design and manufacture of vehicle mounted algae removal devices but also that it is the most appropriate structural ratio for the purposes of mounting such a device on a vehicle.
3.2. CFD modeling 3.2.1. Velocity distribution Fig. 3 shows the distribution of the water flow velocity of shapes 1 and 2. The smaller size of the reaction tank of shape 1 compared to that of shape 2 resulted in a smaller average internal flow. An observation (green area; relatively slow flowing areas) of the water velocity distribution near the central sludge ejection pipe indicates a wide area of distribution in the case of shape 2. The average flow velocity in the reaction tank of shape 1 was
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H.S. Oh et al. / Engineering Science and Technology, an International Journal 22 (2019) 477–481
Table 1 Results of efficiency by height and diameter ratio. No.
Height/ diameter ratio
Effective volume (‘)
Feed flow (‘/min)
Air bubble (‘/min)
Coagulant (m‘/min)
Operation time (hr)
Turbidity (NTU)
Chlorophyll-a (mg/m3)
1:1 1.35:1 1.6:1 1.85:1
81.7 98.4 80.2 91.8
8.2 9.8 8.0 9.2
2 2.5 2 2.3
33 39 32 37
1 1 1 1
12.4 1.0 1.2 1.1 1.4
53.1 2.8 2.9 2.9 3.5
Raw water 2 3 4
Fig. 3. A schematic presentation of the distribution of water flow velocities in Cyclonic-DAF reaction tanks.
0.281 m/s whereas that of shape 2 was 0.261 m/s. These results were due to the fact that the bottom structure of shape 2 became larger in area compared with that of shape 1.
3.2.2. Local velocity region and retention time distribution A feature of the Cyclonic-DAF reaction tanks is that the airsaturated water flows in towards the left from the central axis. The flow velocity energy induces the water within the reaction tank to form a counter-clockwise cyclonic flow. The retention time of the microbubbles within the reaction tank is increased and the microbubbles collide with the turbid matter. Shape 1 and shape 2 differ in terms of sloping within the reaction tanks. The effect of the difference is to eject heavy turbidity matters not removed by floating air bubbles by having them settle towards the bottom of the tank.
Fig. 4. A schematic presentation of regions having 10%, 20%, and 50% average flow velocities in Shape 1 when average velocity is 0.261 m/s.
Figs. 4 and 5 present regions having a 10% average flow velocity of the Cyclonic-DAF reaction in tank shapes 1 and 2. Regions having 20% and 50% average flow velocity were also shown. Generally, areas having less than 10% or 20% average flow velocity are considered dead zones. Such regions are removed from the total reaction tank volume to calculate the effective volume. Fig. 4 presents areas having 10%, 20%, and 50% average flow velocities in shape 1 when average flow velocity was 0.261 m/s. Regions with less than 10% average flow velocities in the reaction tank were rarely observed. Regions with less than 20% average flow velocities were found to occur slightly around the central sludge waste pipe. Regions with less than 50% average flow velocities were found near the sludge waste pipe at less than 5% of the total area.
Fig. 5. A schematic presentation of regions having 10%, 20%, and 50% average flow velocities in Shape 2 when average velocity is 0.261 m/s.
H.S. Oh et al. / Engineering Science and Technology, an International Journal 22 (2019) 477–481
481
Fig. 6. A schematic presentation of water retention time in the cyclonic-DAF tank.
Fig. 5 shows the average flow velocity findings for shape 2. The average flow velocity in the reaction tank was 0.261 m/s. The local velocity distribution of each percentage group was not very different compared to that of shape 1, with only slight occurrences of dead zone regions having less than 10% and 20% average flow velocities. This finding indicated that the Cyclonic-DAF reaction tank had an advantage in that it could reduce dead zones. Fig. 6 shows the analysis results of how long on average the water molecules that flow in through the inlet stay within the tank. The results indicated a settling period of 480 s for shape 1 and 548 s for shape 2. The settling period of shape 2 was found to be 68 s longer, which was considered to be the case because of an increase in retention time due to the increased area associated with its cone shaped bottom. 4. Conclusion This study was undertaken to design and manufacture of vehicle mountable C-DAF reactor based on the field tests and CFD simulation. 1) The field experiments were undertaken to explore the design factors of different height/diameter ratios of Cyclonic-DAF device based on the treated water quality. The results indicated that the 1:1 ratio yielded the optimal results having removal efficiencies of turbidity at 91.9% and chlorophyll-a at 94.7%. 2) CFD simulations were performed on two different CyclonicDAF reaction tank shapes either having or not having a cone shaped settled sludge removal unit in the bottom area of the inlet in the Cyclonic-DAF reaction tank. The results indicated that the hydraulic behavior of inflow in the upper area of the inlet was similar whereas that of shape 2 (cone shaped bottom) was found to have a longer average retention time of approximately 68 s.
3) An average flow of less than 10% within the Cyclonic-DAF was rarely found in both shapes while areas of less than 20% were only slightly found near the central concentrate waste pipe. Regions having an average flow of less than 50% were found around the concentrate waste pipe and were observed at a rate of less than 5% of the total volume. In conclusion, an advantage of Cyclonic-DAFs found in this study is their capacity to reduce dead zones.
Acknowledgements This subject was supported by ‘‘Field Evaluation of Best Available Technique for Algae Control in River” (code 18AWMPB098632-02) funded by MOLIT(Ministry of Land, Infrastructure and Transport) References [1] D.R. De Figueiredo, U.M. Azeiteiro, S.M. Esteves, F.J.M. Gon Alves, M.J. Pereira, Microcystin-producing blooms-a serious global public health issue 1, Ecotoxicol. Environ. Saf. 59 (2004) 151–163. [2] M.Y. Han, W. Kim, A theoretical consideration of algae removal with clays, Microchem. J. 68 (2001) 157–161. [3] M. Teixeira, R.M.J. Rosa, Comparing dissolved air flotation and conventional sedimentation to remove cyanobacterial cells of Microcystis aeruginosa: part I: the key operating conditions, Sep. Purif. Technol. 52 (2006) 84–94. [4] J. Baeyens, I.Y. Mochtar, S. Liers, H. DeWit, Plugflow dissolved air flotation, Water Environ. Res. 67 (7) (1995) 1027–1035. [5] A. Eades, W.J. Brignal, Counter-current dissolved air flotation/filtration, Water Sci. Tech. 31 (3–4) (1995) 173–178. [6] J.K. Edzwald, Principles and applications of dissolved air flotation, Water Sci. Tech. 31 (3–4) (1995) 1–23. [7] M. Lundh, L. Jonsson, J. Dahlquist, Experimental studies of the fluid dynamics in the separation zone in dissolved air flotation, Water Res. 34 (1) (2000) 21–30. [8] J. Hague, C.T. Ta, M.J. Biggs, A. SattaryJ, Small scale model for CFD validation in DAF application, Water Sci. Technol. 43 (8) (2001) 167–173. [9] L. Davidson, An Introduction to Turbulence Models, Dept. of Thermo and Fluid Dynamics, Chalmers University of Technology, Gothenburg, 1997.
Contents lists available at ScienceDirect
Engineering Science and Technology, an International Journal journal homepage: www.elsevier.com/locate/jestch
Full Length Article
CFD modelling of cyclonic-DAF (dissolved air flotation) reactor for algae removal Hong Sok Oh a,b, Seon Hong Kang b, SookHyun Nam c, Eun-Ju Kim c, Tae-Mun Hwang c,⇑ a
Kyong-Ho Engineering & Architects Co., Ltd, Korea Department of Environmental Engineering, Kwangwoon University, Korea c Korea Institute of Civil Engineering and Building Technology, Korea b
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 11 December 2017 Revised 25 July 2018 Accepted 14 December 2018 Available online 19 December 2018
The purpose of this study is to simulate the Cyclonic-DAF tanks using CFD techniques for the optimal design. CFD simulations were performed on two different Cyclonic-DAF reactor shapes either having or not having a cone shaped settled sludge removal unit in the bottom area of the inlet in the CyclonicDAF reaction tank. An average flow of less than 10% within the Cyclonic-DAF was rarely found in both shapes while areas of less than 20% were only slightly found near the central concentrate waste pipe. Areas having an average flow of less than 50% were found around the concentrate waste pipe and were observed at a rate of less than 5% of the total volume. Efficiency test results of different Cyclonic-DAF (CDAF) tank height to diameter ratios set at 1:1, 1:35:1, 1.6:1, and 1.85:1 indicated similar efficiencies with turbidity at 91.9% and chlorophyll-a at 94.7%. Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Algae removal Dissolved air flotation DAF Cyclonic-DAF Computational fluid dynamics (CFD)
1. Background Recent climate shifts have propagated rapid change in the physical environments (retention time, etc.) of rivers and lakes. One example is the formation of algae along reservoirs and dammed areas that are used exclusively for water supply. The algae formation has become pervasive to the extent that it has become a social problem. Particularly in the case of various types of algae formations, cyanobacterial formations raise the need for fundamental inhibitive solutions due to their potential toxicity. Water supplies with large formations of cyanobacteria result in increased costs associated with the treatment of the water as well as a number of social and economic problems for its surrounding region [1–3]. Dissolved air flotation (DAF), one of the most representative physico-chemical processing technologies used to remove algae, is a vehicle mountable aqua-mobile freshwater lake water treatment technology. This technology removes suspended substances or algae in aquatic environments using physical treatment processes that employ the flotation of air. Cyclonic Dissolved Air Flotation (DAF), a dissolved air flotation method displaying improved performance, has a structure capable of forming twirling flows that
⇑ Corresponding author. E-mail addresses: (T.-M. Hwang).
[email protected]
(S.H.
Kang),
Peer review under responsibility of Karabuk University.
[email protected]
produce microbubbles that adhere to suspended matter and float to the surface of the water passing through the float separation tank. The Cyclonic-DAF is an improved type of dissolved air flotation method in that it ejects floating substances through an ejection pipe installed in the center of devices that employ it. However, such flotation separation devices where the suspended substances (referred to as ‘‘scum”) float to the surface face a complication where it is difficult to adjust water levels that ensure stable overflow of scum due to the aggregation of flotation towards the center of the water surface of the cylindrical float separation tank. This study analyzes the removal effects of different water levels through lab-scale tests. The study also undertakes solid modelling based on computational fluid dynamics for the purposes of discovering application implications during the manufacture of Cyclonic-DAF (dissolved air flotation) devices capable of being mounted on a vehicle to remove algae in rivers. 2. Method 2.1. Cyclonic-DAF A Cyclonic-DAF device consists of a cylindrical float separation tank, air-saturated water and microbubble inlet pipe, and a vertical double tube that drains concentrations. The principles of operation of a dissolved air flotation device are described as per the following. An inflow of microbubbles and air-saturated water into the
https://doi.org/10.1016/j.jestch.2018.12.003 2215-0986/Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
478
H.S. Oh et al. / Engineering Science and Technology, an International Journal 22 (2019) 477–481
flotation tank results in the flotation of suspended substances in a twirling stream. The suspended algae float to the water surface in a twirling stream and aggregates towards the center after which it is later drained by a central pipe. The water treated for the removal of algae forms a downflow in a twirling stream and is later drained through the treated water pipe. Large particles shift and settle towards the walls of the tank due to centrifugal forces and is drained through a concentrate pipe. The turbidity of the tested air-saturated water was 12.4 NTU, and the chlorophyll-a concentration was 53.1 mg/m3. Different height to diameter ratios of the cyclonic-DAF tank set to 1:1, 1.35:1, 1.6:1, and 1.85:1 were tested to determine their efficiencies. 2 mg/L of a 0.05% polyacrylamide high-polymer solution was used as the coagulant for the tests (See Fig. 1). 2.2. CFD modeling As DAF techniques have begun to be employed in water processing technologies, several researchers have undertaken studies regarding the mutual exchanges that occur between microbubbles and particles [4–6]. Following the late 1990s, computational fluid dynamics (CFD) technologies were used to undertake a number of process interpretations that resulted in a number of findings regarding its hydraulics [7]. In line with these trends, this study undertook simulations of Cyclonic-DAF tanks using CFD techniques for the purposes of finding the optimal design and operational parameters of a Cyclonic-DAF reaction tank that would remove suspended substances. During the simulation, subjects of interpretation were divided along a small cubic mesh or grid after which flow motion equation (continual momentum and energy equations) values satisfying initial and boundary conditions using the finite volume method were repeatedly found using a computer. The Cyclonic-DAF reaction tank was configured as shown in Fig. 2. 2.2.1. Model equations ANSYS CFX is used as CFD code in this study. With the addition of air bubbles in the flow field, the flow is no longer a single-phase flow and becomes configured into two phases. This is referred to as a multiphase flow. In order to simulate the two-phase flow of the air bubbles and water, the Eulerian-Eulerian Method, which separately calculates and couples the motion equations of each phase, was used. Hague et al. found that computational interpretation
results applied with the k-e turbulence model were closer to the experimental value than computational interpretation results that assumed the flow as laminar flow [8]. Transport equations of one or two turbulent quantities, like kinetic energy (k) or its dissipation rate (e), are derived and leads to the set of equations called k-e equations [9]. 2.2.2. Operating conditions Two different shapes of the Cyclonic-DAF tanks were tested under the following CFD simulation conditions. Flocculation was undertaken as pre-treatment of the DAF reaction tank with retention time of 9 min in the flotation tank. Operating conditions are as follows. The flow of air-saturated water was maintained at 10 m3/h and dissolved air was supplied at 0.1 m3/h. The total volume of the inflow of air-saturated water and dissolved air was fixed at 10.1 m3/h during the operation. Air was dissolved using a saturator typically at 5–6 bars of pressure. The size of the air bubbles is approximately 10–120 mm. It varied according to water temperature and pressure. The pressurized microbubbles are ejected through the upper area. 2.2.3. Mesh Shape No. 1 of the Cyclonic-DAF tanks was composed of approximately 223,000 nodes and 857,000 elements in the flow zone. Shape No. 2 was composed of approximately 266,000 nodes and 1,035,000 elements. The meshes were mixed with tetragonal and hexagonal lattices. Areas with heavy changes in flow were inserted with strata having more narrow lattices. 2.2.4. Boundary conditions One inlet condition and 3 outlet conditions were given to simulate the Cyclonic-DAF reaction tank using CFD analysis. The wall of the tanks was applied with a no-slip condition. A degassing boundary was configured to achieve mass balancing of the air. The k-e model was applied as the turbulence model. 3. Results and discussions 3.1. Cyclonic-DAF devices Table 1 shows the efficiency analysis results of the CyclonicDAF tank as having different height/diameter ratios of 1:1,
Fig. 1. Schematic of Cyclonic-DAF process.
H.S. Oh et al. / Engineering Science and Technology, an International Journal 22 (2019) 477–481
479
Fig. 2. C-DAF Reaction Tank Shapes.
1.35:1, 1.6:1, and 1.85:1. The field experiments were undertaken to examine the turbidity and chlorophyll-a removal efficiencies and explore the design factors of different height/diameter ratios of Cyclonic-DAF devices. The results indicated that the 1:1 ratio yielded the optimal results having removal efficiencies of turbidity at 91.9% and chlorophyll-a at 94.7%. This indicated not only that a 1:1 height/diameter ratio is appropriate for the design and manufacture of vehicle mounted algae removal devices but also that it is the most appropriate structural ratio for the purposes of mounting such a device on a vehicle.
3.2. CFD modeling 3.2.1. Velocity distribution Fig. 3 shows the distribution of the water flow velocity of shapes 1 and 2. The smaller size of the reaction tank of shape 1 compared to that of shape 2 resulted in a smaller average internal flow. An observation (green area; relatively slow flowing areas) of the water velocity distribution near the central sludge ejection pipe indicates a wide area of distribution in the case of shape 2. The average flow velocity in the reaction tank of shape 1 was
480
H.S. Oh et al. / Engineering Science and Technology, an International Journal 22 (2019) 477–481
Table 1 Results of efficiency by height and diameter ratio. No.
Height/ diameter ratio
Effective volume (‘)
Feed flow (‘/min)
Air bubble (‘/min)
Coagulant (m‘/min)
Operation time (hr)
Turbidity (NTU)
Chlorophyll-a (mg/m3)
1:1 1.35:1 1.6:1 1.85:1
81.7 98.4 80.2 91.8
8.2 9.8 8.0 9.2
2 2.5 2 2.3
33 39 32 37
1 1 1 1
12.4 1.0 1.2 1.1 1.4
53.1 2.8 2.9 2.9 3.5
Raw water 2 3 4
Fig. 3. A schematic presentation of the distribution of water flow velocities in Cyclonic-DAF reaction tanks.
0.281 m/s whereas that of shape 2 was 0.261 m/s. These results were due to the fact that the bottom structure of shape 2 became larger in area compared with that of shape 1.
3.2.2. Local velocity region and retention time distribution A feature of the Cyclonic-DAF reaction tanks is that the airsaturated water flows in towards the left from the central axis. The flow velocity energy induces the water within the reaction tank to form a counter-clockwise cyclonic flow. The retention time of the microbubbles within the reaction tank is increased and the microbubbles collide with the turbid matter. Shape 1 and shape 2 differ in terms of sloping within the reaction tanks. The effect of the difference is to eject heavy turbidity matters not removed by floating air bubbles by having them settle towards the bottom of the tank.
Fig. 4. A schematic presentation of regions having 10%, 20%, and 50% average flow velocities in Shape 1 when average velocity is 0.261 m/s.
Figs. 4 and 5 present regions having a 10% average flow velocity of the Cyclonic-DAF reaction in tank shapes 1 and 2. Regions having 20% and 50% average flow velocity were also shown. Generally, areas having less than 10% or 20% average flow velocity are considered dead zones. Such regions are removed from the total reaction tank volume to calculate the effective volume. Fig. 4 presents areas having 10%, 20%, and 50% average flow velocities in shape 1 when average flow velocity was 0.261 m/s. Regions with less than 10% average flow velocities in the reaction tank were rarely observed. Regions with less than 20% average flow velocities were found to occur slightly around the central sludge waste pipe. Regions with less than 50% average flow velocities were found near the sludge waste pipe at less than 5% of the total area.
Fig. 5. A schematic presentation of regions having 10%, 20%, and 50% average flow velocities in Shape 2 when average velocity is 0.261 m/s.
H.S. Oh et al. / Engineering Science and Technology, an International Journal 22 (2019) 477–481
481
Fig. 6. A schematic presentation of water retention time in the cyclonic-DAF tank.
Fig. 5 shows the average flow velocity findings for shape 2. The average flow velocity in the reaction tank was 0.261 m/s. The local velocity distribution of each percentage group was not very different compared to that of shape 1, with only slight occurrences of dead zone regions having less than 10% and 20% average flow velocities. This finding indicated that the Cyclonic-DAF reaction tank had an advantage in that it could reduce dead zones. Fig. 6 shows the analysis results of how long on average the water molecules that flow in through the inlet stay within the tank. The results indicated a settling period of 480 s for shape 1 and 548 s for shape 2. The settling period of shape 2 was found to be 68 s longer, which was considered to be the case because of an increase in retention time due to the increased area associated with its cone shaped bottom. 4. Conclusion This study was undertaken to design and manufacture of vehicle mountable C-DAF reactor based on the field tests and CFD simulation. 1) The field experiments were undertaken to explore the design factors of different height/diameter ratios of Cyclonic-DAF device based on the treated water quality. The results indicated that the 1:1 ratio yielded the optimal results having removal efficiencies of turbidity at 91.9% and chlorophyll-a at 94.7%. 2) CFD simulations were performed on two different CyclonicDAF reaction tank shapes either having or not having a cone shaped settled sludge removal unit in the bottom area of the inlet in the Cyclonic-DAF reaction tank. The results indicated that the hydraulic behavior of inflow in the upper area of the inlet was similar whereas that of shape 2 (cone shaped bottom) was found to have a longer average retention time of approximately 68 s.
3) An average flow of less than 10% within the Cyclonic-DAF was rarely found in both shapes while areas of less than 20% were only slightly found near the central concentrate waste pipe. Regions having an average flow of less than 50% were found around the concentrate waste pipe and were observed at a rate of less than 5% of the total volume. In conclusion, an advantage of Cyclonic-DAFs found in this study is their capacity to reduce dead zones.
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