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Fundamental Study on a New Thin Oil-film Recovery Device Based on the MHD Method
Available online at www.sciencedirect.com
ScienceDirect Aquatic Procedia 3 (2015) 50 – 58
International Oil Spill Response Technical Seminar
Fundamental Study on a New Thin Oil-film Recovery Device Based on the MHD Method Jiangjin Liua,b, Ling zhi Zhaoa,*, Ciwen Shaa, Yan Penga, Wei Anc, Zhaolian Wangd, Fengliang Liud a
Institute of Electrical Engineering; Chinese Academy of Sciences, Beijing, China b University of Chinese Academy of sciences, Beijing, China c China Offshore Environmental Service Ltd d Shandong Huate Magnetism Technology Co.,Ltd
Abstract The MHD maritime oil spill recovery method has many advantages in tackling the marine thin oil-film. The existing device, in which the MHD channel is arranged horizontally, can’t operate successfully under wave conditions and the oil and air bubbles can’t move through the MHD channel smoothly. A new method with the MHD channel arranged with a slight tilt angle is proposed to improve the effect of the thin oil-film recovery. In this paper, the hydrodynamic characteristics of the seawater and air two-phase flow under the action of electromagnetic force with the MHD channel arranged obliquely were studied numerically and the results show that the static pressure decreases gradually from the MHD channel’s inlet to the outlet. The flow rate decreases slightly as the tilt angle increases. The pressure rise and the effective power increase slightly as the tilt angle increases. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
© 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of International Oil Spill Response Technical Seminar. Peer-review under responsibility of China Offshore Environmental Services Ltd
Keywords: MHD; water-air two-phase flow; numerical analysis; thin oil-film recovery; VOF model
1. Introduction Nowadays, the marine oil spill is the most common and most serious problem of the marine pollution and draws more and more attention from people. And it would cause serious environmental problems and lead to huge economic losses simultaneously. In the area of marine oil-spill disposal, it’s called the thin oil-film when the thickness of the oil-spill is less than 1 mm. The thin oil-film on the sea would block the exchange of air and water
*
Corresponding author. Tel.:+0-086-10-82547047; fax: +0-086-10-82547046. E-mail address:[email protected]
2214-241X © 2015 The Authors. Published 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/). Peer-review under responsibility of China Offshore Environmental Services Ltd doi:10.1016/j.aqpro.2015.02.227
Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
and have adverse impact on the environment. At the same time, the iridescence generated by the micron-sized oilfilm is so clear in good weather conditions that it would cause the illusion that the accident is serious, which would interference with the evaluation and disposal of the accident. The thin oil-film has a very thin thickness, poor continuity, huge oil-water and oil-gas interfaces, which make the traditional oil spill recovery methods not applicable to it and its recovery becomes a very difficult problem. In order to dispose the increasingly frequent oilspill accidents with thin oil-film, such as the ConocoPhillips Bohai oil spill accident, it is very urgent to find new thin oil-film recovery methods. Aiming at the disposal of the marine thin oil-film, the MHD marine oil spill recovery method was proposed in 2002 (Sha, 2002). It is based on the different flow states of oil-film, air and seawater under the action of the electromagnetic force, gravity, buoyancy and interphase force, as shown in Fig.1. Ship
Magnet
Valve
Separation tank
S
Inlet
B j
FEMHD
Outlet
N
MHD channel
Fig.1. Principle of the MHD floating-oil recovery device
From then on, the study on the new technology has been carried out numerically and experimentally. The hydrodynamic characteristics of the seawater–air two-phase flow and the seawater-air-oil three-phase flow under the action of the electromagnetic force were studied (Zhang, 2006; Zhang, 2007; Ye, 2013). A demonstration test facility was manufactured and a series of experiments were done in 2005. And an oil recovering rate of 68 kg/h with the containing water ratio less than 5% was obtained with the MHD channel’s section of 29×40 mm 2, a magnetic field of 0.8 T, a 5 mm 46# lubricating oil (Peng, 2005; Sha, 2007). In 2012, a laboratory prototype with a 1000L/h oil-contaminated seawater processing capacity was developed in IEECAS and circulation loop experiments and flume tests were carried out. With a current density of 2300 A/m 2 and a diesel oil film of 3 mm, the contaminated seawater’s flow rate was 1100 L/h with the oil’s flow rate of 56 L/h. When the thickness of the oil film was less than 1 mm and even in micrometres, it could see the disappearance of the iridescent oil film (Peng, 2014). However, the flume tests showed that the device was very sensitive to the inlet’s water level. If the inlet’s water level is too high, the fluid absorbed into the MHD channel will be only seawater. On the other side, the oil and seawater can’t flow into the MHD channel if the inlet’s water level is too low. Generally speaking, the good recovery results can be achieved only at the designed water level with a slight fluctuation in millimetres. However, in the actual application, there always are waves with the amplitude in centimetres and even meters. To the device in Fig.1, the static pressure in the MHD channel increases from the entrance to the exit because of the MHD effect. As we all know, the oil and the air-bubbles always move towards the low pressure areas. So they move with a big resistance due to the pressure rise. Much air remains in the MHD channel, impeding the oil recovery and leading to an under-utilization of the MHD channel.
Fig.2. Flume test facility
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Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
So the so-called impeller+oblique channel method is proposed to improve the effect of the marine thin oil-film recovery (Zhao, 2014). The basic structure of the new device is shown in Fig.3. The MHD channel is placed obliquely with a tilt angle of α. There is a rotating impeller in front of the MHD channel’s inlet. The rotating impeller consists of a large hub, short blades and two side plates. The short blades distribute evenly on the hub and form small grooves along with the two side plates. When the impeller rotates, the grooves transport oil-contaminated seawater to the duckbill inlet. Then the fluid will be absorbed into the MHD channel. Because of the big impeller, even though the water level’s fluctuation is relatively large, the oil-contaminated water would be trapped in the grooves and be transported to the duckbill inlet. Thus, this new method can work successfully under wave conditions. Designed water level
Impeller
MHD channel
Separation tank
Outlet Duckbill inlet
Fig.3. Impeller+oblique channel MHD thin oil-film recovery device
The pressure distribution in the MHD channel is governed by: 'P
P Pt
U g ( Lmhd Ly )sin D Ploss Pmhd
(1)
where ᇞP stands for the pressure difference between the entrance and exit of the MHD channel, P=ρgH0 is the entrance pressure and H0 is the vertical distance between the sea surface and the duckbill inlet, Pt is the exit pressure, ρ is the seawater’s density, g is the gravitational acceleration, Lmhd is MHD channel’s length in flow direction, Ly is the duckbill inlet’s length, α is the tilt angle of the MHD channel, Ploss is the flow loss and Pmhd is the pressure rise generated by the electromagnetic force. Because the oil and air-bubbles always flow from an area of high pressure towards an area of low pressure, the pressure should decrease from the entrance to exit to ensure the oil and air-bubbles move smoothly through the MHD channel. To achieve this, the pressure distribution must meet this condition:
'Pᷟ0
(2)
2. Numerical Study In order to know the pressure distribution in the MHD channel exactly, look for the optimal tilt angle of the MHD channel and provide instructions for the design, the hydrodynamic characteristics of the seawater and air in an electromagnetic field of the new MHD thin oil-film recovery device were studied numerically. 2.1. Physical model The physical model is shown in Fig.4. It mainly consists of an inlet tank, an MHD channel with a tilt angle of α, a separation tank and connecting pipes. The inlet tank is much bigger than the separation tank to simulate the sea area. The pressure distribution of the MHD channel is the emphasis, so there is no impeller in the model. Table.1 shows main parameters of the MHD channel and the magnetic field.
Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
Separation tank Inlet tank
Connecting pipe
MHD channel
Fig.4. Physical model Table1. Parameters of the MHD channel Item Value Height
100 mm
Ratio of width to height
3.6
Ratio of length to height
6
Magnetic field
0.9T
2.2. Mathematical model The VOF model is used to simulate the seawater-air two-phase flow in the electromagnetic field. In the VOF model, the variables and properties in any given cell are either purely representative of one of the phases or representative of a mixture of the phases, depending upon the volume fraction values. In other words, if the qth fluid's volume fraction in the cell is denoted as q, then the following three conditions are possible (Fluent Inc, 2006): q = 0: The cell is empty (of the qth fluid). q = 1: The cell is full (of the qth fluid). 0
ěα q =1
q
(3)
=1
In Eq.3, n stands for the number of the phases, for example, in this paper, there are two phases, i.e. seawater and air, then n=2. αq denotes the volume fraction of the qth phase. The tracking of the interface(s) between phases is accomplished by the solution of a continuity equation for the volume fraction of one (or more) of the phases. ªw
«
U T ¬ wW
& º D T U T D T U TX T »
¼
6D
T
Q
¦ P ST P TS
(4)
S
Where PTS is the mass transfer from phase q to phase p and PST is the mass transfer from phase p to phase q. The primary-phase volume fraction will be computed based on the following constraint:
w UX UXX S >P X X7 @U J ) wW
(5)
53
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Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
F
J B
(6)
&
&
Where - is the current density in the seawater, % is the external magnetic field intensity. The momentum equation shown above is dependent on the volume fraction of all phases through the properties ρ and μ. In Eq.5, the calculation of physical parameters like density, viscosity coefficient is based on the weighted average method of volume fraction. For example˖
U
n
¦a
q
Uq
(7)
q 1
2.3. Mesh In order to improve the calculation’s efficiency, structured hexahedron mesh is employed in the entire model. In general, the demand for hardware resources is greater if finer mesh is used. Considering the above aspects together, the whole model is divided into 1.08 million cells. The mesh of the MHD channel is shown in Fig.5.
Fig.5. The mesh of the MHD channel
2.4. Boundary conditions and initial condition The inlet tank and separation tank are exposed to the atmosphere, so the top surfaces of them are defined as the pressure inlet and pressure outlet respectively. The wall boundary is set as the stationary wall, i.e., the velocity is 0. An interior surface is set in the connecting pipe to measure the seawater flow rate in the solution. Initial water level just covers the exit of the MHD channel and the volume fraction of the seawater-phase in the corresponding area is 1. As to the pressure inlet and pressure outlet boundaries, the volume fraction of the air-phase is 1 and the volume fraction of the water-phase is 0. 2.5. Electromagnetic force The electromagnetic force is only performed on the seawater phase in the MHD channel and it is a function of the seawater’s volume fraction. The electromagnetic force is applied through a UDF (user-defined function) which is compiled in the C language. In this case, the electromagnetic force’s density is 1800N/m3. 2.6. Solver The pressure-velocity coupling is adopted in the SIMPLE method, together with the application of the GeoReconstruct method to track the flow interface. The time step is set as 0.001 second and the maximum of iterations per time step is 20. The inlet tank and separation tank are connected, so the seawater-air two-phase flow under the action of the electromagnetic force will get to a steady state. In this study, the flow field in the steady state is analyzed. The seawater’s flow rate through the flow surface of the connecting pipe is monitored to determine the steady state.
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Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
3. Results and Analysis Fig.6~Fig.9 show the flow field with α=15°and t=50s. Fig.6 shows the contour of the seawater’s volume fraction. The red colour indicates the volume fraction of the seawater is 1 and the blue colour represents it is 0. It can be seen that the seawater’s level of the separation tank is higher than that of the inlet tank due to the electromagnetic force. And there is only the seawater in the nozzle of the separation tank. 0
1
Fig.6. Contour of the seawater’s volume fraction(t=50s)
The distribution of the velocity is shown in Fig.7. It can be seen that the fluid in the MHD channel has a high velocity due to the electromagentic force. The fluid in the upper part of the separation tank also has a relatively high velocity because it’s impacted by the flow from the MHD channel. The flow velocity of the fluid in the MHD channel is up to 0.2m/s. 0
0.2
0.41
0.61
0.82
1.02
1.23
1.36m/s
Fig.7. Contour of the velocity (t=50s)
Fig.8 shows the streamlines with the color indicating the static pressure. It can be seen that the flow in the inlet tank is mainly concentrated in the upper part. The flow is relatively complex in the separation tank and there is a vortex which is adverse to the separation of oil and water. So in practice, an extending part should be added to the exit of the MHD channel to lower the fluid’s velocity in the upper part of the separation tank. 0
2017
4033
6050
8067
10084
12100
13445pa
Fig.8. Streamlines(t=50s)
Fig.9 shows the distribution of the static pressure. It can be seen that the static pressure decreases gradually from the inlet to the outlet of the MHD channel. The isobar has inflection points at the boundary of the MHD effective segment (the area with the electromagnetic field).
0
2241
4482
6722
8963
11204
Fig.9a. Contour of the static pressure(t=50s)
13445pa
56
Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
728
1321
1914
2507
3099
Inlet
3692pa
Outlet
Fig.9b. Contour of the static pressure in the MHD channel(t=50s)
Fig.10 shows the pressure distribution along the centreline of MHD channels with different tilt angles. The abscissa represents the length of the MHD channel in the flow direction. It can be seen that the static pressure decreases gradually from the inlet to the outlet in the case with α=15° and the static pressure’s gradient within the MHD effective segment is smaller because of the effect of the electromagnetic force. For the case with α=0°, the static pressure increases gradually in the MHD effective segment under the action of the electromagnetic force. The oil and air are non-conducting and unaffected by the electromagnetic force, they move from high pressure areas towards low pressure areas. As to the horizontal MHD channel (α=0°), the gradually increasing pressure would impede the oil and air bubbles move through the channel. To make things worse, the accumulated air would block the channel and reduce the effective flow area. As the volume fraction of the seawater becomes smaller, the electromagnetic force would decrease, which is adverse to oil recovery. With regard to the case of α=15°, the gradually decreasing pressure promotes the oil and air bubbles to discharge from the MHD channel. To some extent, air bubbles would carry oil to move towards the separation tank. As a result, arranging the MHD channel with a tilt angle changes the pressure distribution in the MHD channel totally and thus benefits the oil recovery. 3200 α=15° α=0°
2800 2400
P (pa)
2000
MHD effective segment
1600 1200 800 400 0 0
0.2
0.4
0.6
0.8
1.0
L (m) Fig.10. Pressure distribution on the centerline of the MHD channels
Fig.11 shows the relationship between the seawater flow rate Q, pressure rise Pr and the MHD channel’s tilt angle α. It can be seen that Q decreases slightly as α increases and Pr increases slightly as α increases. Fig.12 shows the relationship between the effective power W=QPr and α. It can be seen that W increases slightly as α increases.
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Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58 160
Q Pr
30
120
20
80
10
40
0
Pr (mmH2O)
Q (m3/h)
40
0 0
5
10
15
20
a (°)
25
30
Fig.11. Variation of Q and Pr with α 10 W
W (w)
8
6
4
2
0 0
5
10
15
20
25
30
a (0°) Fig.12. Variation of W with α
4. Conclusions The hydrodynamic characteristics of the seawater and air two-phase flow under the action of an electromagnetic force with the MHD channel arranged obliquely were studied numerically in this paper. The results show that: (1) The static pressure decreases gradually from the MHD channel’s inlet to the outlet and the static pressure’s gradient within the MHD effective segment is smaller because of the effect of the electromagnetic force. (2)For a certain MHD channel, the flow rate decreases slightly with the increase of the tilt angle of the MHD channel (α); the pressure rise and the effective power increases slightly with the increase of α. References Fluent Inc ,2006. FLUENT 6.3 Users’ Guide, Fluent Inc. Peng,Yan, et al ,2014. Research on marine floating-oil recovery technology based on magneto hydrodynamic method, marine of environmental science, 33, 592-597.
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Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58 Peng,Yan, et al ,2005. Fundamental Study on Oil Spill Recovery from Oil-Contaminated Seawater by MHD Method, Proc 15th ,2005Int MHD Energy Conversion Conf, Moscow, Russia, 2,319-322. Sha,Ciwen,et al ,2007. Study on oil separation and recovery from oil-contaminated seawater by MHD method, Collection of Technical Papers 38th AIAA Plasmadynamics and Lasers Conference, 2, 1140-1144. Sha,Ciwen,et al ,2002. Method and Device of MHD Marine Floating-oil Recovery, China Patent: 02142835. 2[P], 09-18-2002. Ye,Chong,et al,2013. 3D Numerical Analysis on Flow Process of Floating-oil Recovery Device by MHD Method, Proc.23rd,2013 International Offshore and Polar Engineering Conference, Anchorage, Alaska, USA, 3,1006-1010. Zhao,Lingzhi,et al ,2014. A Marine Thin Oil-film Recovery device Based on the MHD Method, China Patent: 103774630A[P], 01-23-2014. Zhang,Guoyan,et al ,2007. Analytical Study on Flow Process of Floating-oil Recovery Device from Oil-contaminated Seawater by MHD Method, Journal of hydrodynamics, Ser . B, 9, 195-200 . Zhang,Guoyan,et al ,2007. Numerical simulation of flow process in MHD floating oil recovery channel, Marine Environmental Science, 26, 3337. Zhang,Guoyan ,2006. Numerical Simulation on Flow Process of Oil Separation and Recovery Device from Oil-contaminated Seawater by MHD Method, Masters’ Thesis, Beijing, China: Graduate School of Chinese Academy of Sciences.
ScienceDirect Aquatic Procedia 3 (2015) 50 – 58
International Oil Spill Response Technical Seminar
Fundamental Study on a New Thin Oil-film Recovery Device Based on the MHD Method Jiangjin Liua,b, Ling zhi Zhaoa,*, Ciwen Shaa, Yan Penga, Wei Anc, Zhaolian Wangd, Fengliang Liud a
Institute of Electrical Engineering; Chinese Academy of Sciences, Beijing, China b University of Chinese Academy of sciences, Beijing, China c China Offshore Environmental Service Ltd d Shandong Huate Magnetism Technology Co.,Ltd
Abstract The MHD maritime oil spill recovery method has many advantages in tackling the marine thin oil-film. The existing device, in which the MHD channel is arranged horizontally, can’t operate successfully under wave conditions and the oil and air bubbles can’t move through the MHD channel smoothly. A new method with the MHD channel arranged with a slight tilt angle is proposed to improve the effect of the thin oil-film recovery. In this paper, the hydrodynamic characteristics of the seawater and air two-phase flow under the action of electromagnetic force with the MHD channel arranged obliquely were studied numerically and the results show that the static pressure decreases gradually from the MHD channel’s inlet to the outlet. The flow rate decreases slightly as the tilt angle increases. The pressure rise and the effective power increase slightly as the tilt angle increases. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
© 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Scientific Committee of International Oil Spill Response Technical Seminar. Peer-review under responsibility of China Offshore Environmental Services Ltd
Keywords: MHD; water-air two-phase flow; numerical analysis; thin oil-film recovery; VOF model
1. Introduction Nowadays, the marine oil spill is the most common and most serious problem of the marine pollution and draws more and more attention from people. And it would cause serious environmental problems and lead to huge economic losses simultaneously. In the area of marine oil-spill disposal, it’s called the thin oil-film when the thickness of the oil-spill is less than 1 mm. The thin oil-film on the sea would block the exchange of air and water
*
Corresponding author. Tel.:+0-086-10-82547047; fax: +0-086-10-82547046. E-mail address:[email protected]
2214-241X © 2015 The Authors. Published 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/). Peer-review under responsibility of China Offshore Environmental Services Ltd doi:10.1016/j.aqpro.2015.02.227
Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
and have adverse impact on the environment. At the same time, the iridescence generated by the micron-sized oilfilm is so clear in good weather conditions that it would cause the illusion that the accident is serious, which would interference with the evaluation and disposal of the accident. The thin oil-film has a very thin thickness, poor continuity, huge oil-water and oil-gas interfaces, which make the traditional oil spill recovery methods not applicable to it and its recovery becomes a very difficult problem. In order to dispose the increasingly frequent oilspill accidents with thin oil-film, such as the ConocoPhillips Bohai oil spill accident, it is very urgent to find new thin oil-film recovery methods. Aiming at the disposal of the marine thin oil-film, the MHD marine oil spill recovery method was proposed in 2002 (Sha, 2002). It is based on the different flow states of oil-film, air and seawater under the action of the electromagnetic force, gravity, buoyancy and interphase force, as shown in Fig.1. Ship
Magnet
Valve
Separation tank
S
Inlet
B j
FEMHD
Outlet
N
MHD channel
Fig.1. Principle of the MHD floating-oil recovery device
From then on, the study on the new technology has been carried out numerically and experimentally. The hydrodynamic characteristics of the seawater–air two-phase flow and the seawater-air-oil three-phase flow under the action of the electromagnetic force were studied (Zhang, 2006; Zhang, 2007; Ye, 2013). A demonstration test facility was manufactured and a series of experiments were done in 2005. And an oil recovering rate of 68 kg/h with the containing water ratio less than 5% was obtained with the MHD channel’s section of 29×40 mm 2, a magnetic field of 0.8 T, a 5 mm 46# lubricating oil (Peng, 2005; Sha, 2007). In 2012, a laboratory prototype with a 1000L/h oil-contaminated seawater processing capacity was developed in IEECAS and circulation loop experiments and flume tests were carried out. With a current density of 2300 A/m 2 and a diesel oil film of 3 mm, the contaminated seawater’s flow rate was 1100 L/h with the oil’s flow rate of 56 L/h. When the thickness of the oil film was less than 1 mm and even in micrometres, it could see the disappearance of the iridescent oil film (Peng, 2014). However, the flume tests showed that the device was very sensitive to the inlet’s water level. If the inlet’s water level is too high, the fluid absorbed into the MHD channel will be only seawater. On the other side, the oil and seawater can’t flow into the MHD channel if the inlet’s water level is too low. Generally speaking, the good recovery results can be achieved only at the designed water level with a slight fluctuation in millimetres. However, in the actual application, there always are waves with the amplitude in centimetres and even meters. To the device in Fig.1, the static pressure in the MHD channel increases from the entrance to the exit because of the MHD effect. As we all know, the oil and the air-bubbles always move towards the low pressure areas. So they move with a big resistance due to the pressure rise. Much air remains in the MHD channel, impeding the oil recovery and leading to an under-utilization of the MHD channel.
Fig.2. Flume test facility
51
52
Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
So the so-called impeller+oblique channel method is proposed to improve the effect of the marine thin oil-film recovery (Zhao, 2014). The basic structure of the new device is shown in Fig.3. The MHD channel is placed obliquely with a tilt angle of α. There is a rotating impeller in front of the MHD channel’s inlet. The rotating impeller consists of a large hub, short blades and two side plates. The short blades distribute evenly on the hub and form small grooves along with the two side plates. When the impeller rotates, the grooves transport oil-contaminated seawater to the duckbill inlet. Then the fluid will be absorbed into the MHD channel. Because of the big impeller, even though the water level’s fluctuation is relatively large, the oil-contaminated water would be trapped in the grooves and be transported to the duckbill inlet. Thus, this new method can work successfully under wave conditions. Designed water level
Impeller
MHD channel
Separation tank
Outlet Duckbill inlet
Fig.3. Impeller+oblique channel MHD thin oil-film recovery device
The pressure distribution in the MHD channel is governed by: 'P
P Pt
U g ( Lmhd Ly )sin D Ploss Pmhd
(1)
where ᇞP stands for the pressure difference between the entrance and exit of the MHD channel, P=ρgH0 is the entrance pressure and H0 is the vertical distance between the sea surface and the duckbill inlet, Pt is the exit pressure, ρ is the seawater’s density, g is the gravitational acceleration, Lmhd is MHD channel’s length in flow direction, Ly is the duckbill inlet’s length, α is the tilt angle of the MHD channel, Ploss is the flow loss and Pmhd is the pressure rise generated by the electromagnetic force. Because the oil and air-bubbles always flow from an area of high pressure towards an area of low pressure, the pressure should decrease from the entrance to exit to ensure the oil and air-bubbles move smoothly through the MHD channel. To achieve this, the pressure distribution must meet this condition:
'Pᷟ0
(2)
2. Numerical Study In order to know the pressure distribution in the MHD channel exactly, look for the optimal tilt angle of the MHD channel and provide instructions for the design, the hydrodynamic characteristics of the seawater and air in an electromagnetic field of the new MHD thin oil-film recovery device were studied numerically. 2.1. Physical model The physical model is shown in Fig.4. It mainly consists of an inlet tank, an MHD channel with a tilt angle of α, a separation tank and connecting pipes. The inlet tank is much bigger than the separation tank to simulate the sea area. The pressure distribution of the MHD channel is the emphasis, so there is no impeller in the model. Table.1 shows main parameters of the MHD channel and the magnetic field.
Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
Separation tank Inlet tank
Connecting pipe
MHD channel
Fig.4. Physical model Table1. Parameters of the MHD channel Item Value Height
100 mm
Ratio of width to height
3.6
Ratio of length to height
6
Magnetic field
0.9T
2.2. Mathematical model The VOF model is used to simulate the seawater-air two-phase flow in the electromagnetic field. In the VOF model, the variables and properties in any given cell are either purely representative of one of the phases or representative of a mixture of the phases, depending upon the volume fraction values. In other words, if the qth fluid's volume fraction in the cell is denoted as q, then the following three conditions are possible (Fluent Inc, 2006): q = 0: The cell is empty (of the qth fluid). q = 1: The cell is full (of the qth fluid). 0
ěα q =1
q
(3)
=1
In Eq.3, n stands for the number of the phases, for example, in this paper, there are two phases, i.e. seawater and air, then n=2. αq denotes the volume fraction of the qth phase. The tracking of the interface(s) between phases is accomplished by the solution of a continuity equation for the volume fraction of one (or more) of the phases. ªw
«
U T ¬ wW
& º D T U T D T U TX T »
¼
6D
T
Q
¦ P ST P TS
(4)
S
Where PTS is the mass transfer from phase q to phase p and PST is the mass transfer from phase p to phase q. The primary-phase volume fraction will be computed based on the following constraint:
w UX UXX S >P X X7 @U J ) wW
(5)
53
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Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
F
J B
(6)
&
&
Where - is the current density in the seawater, % is the external magnetic field intensity. The momentum equation shown above is dependent on the volume fraction of all phases through the properties ρ and μ. In Eq.5, the calculation of physical parameters like density, viscosity coefficient is based on the weighted average method of volume fraction. For example˖
U
n
¦a
q
Uq
(7)
q 1
2.3. Mesh In order to improve the calculation’s efficiency, structured hexahedron mesh is employed in the entire model. In general, the demand for hardware resources is greater if finer mesh is used. Considering the above aspects together, the whole model is divided into 1.08 million cells. The mesh of the MHD channel is shown in Fig.5.
Fig.5. The mesh of the MHD channel
2.4. Boundary conditions and initial condition The inlet tank and separation tank are exposed to the atmosphere, so the top surfaces of them are defined as the pressure inlet and pressure outlet respectively. The wall boundary is set as the stationary wall, i.e., the velocity is 0. An interior surface is set in the connecting pipe to measure the seawater flow rate in the solution. Initial water level just covers the exit of the MHD channel and the volume fraction of the seawater-phase in the corresponding area is 1. As to the pressure inlet and pressure outlet boundaries, the volume fraction of the air-phase is 1 and the volume fraction of the water-phase is 0. 2.5. Electromagnetic force The electromagnetic force is only performed on the seawater phase in the MHD channel and it is a function of the seawater’s volume fraction. The electromagnetic force is applied through a UDF (user-defined function) which is compiled in the C language. In this case, the electromagnetic force’s density is 1800N/m3. 2.6. Solver The pressure-velocity coupling is adopted in the SIMPLE method, together with the application of the GeoReconstruct method to track the flow interface. The time step is set as 0.001 second and the maximum of iterations per time step is 20. The inlet tank and separation tank are connected, so the seawater-air two-phase flow under the action of the electromagnetic force will get to a steady state. In this study, the flow field in the steady state is analyzed. The seawater’s flow rate through the flow surface of the connecting pipe is monitored to determine the steady state.
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3. Results and Analysis Fig.6~Fig.9 show the flow field with α=15°and t=50s. Fig.6 shows the contour of the seawater’s volume fraction. The red colour indicates the volume fraction of the seawater is 1 and the blue colour represents it is 0. It can be seen that the seawater’s level of the separation tank is higher than that of the inlet tank due to the electromagnetic force. And there is only the seawater in the nozzle of the separation tank. 0
1
Fig.6. Contour of the seawater’s volume fraction(t=50s)
The distribution of the velocity is shown in Fig.7. It can be seen that the fluid in the MHD channel has a high velocity due to the electromagentic force. The fluid in the upper part of the separation tank also has a relatively high velocity because it’s impacted by the flow from the MHD channel. The flow velocity of the fluid in the MHD channel is up to 0.2m/s. 0
0.2
0.41
0.61
0.82
1.02
1.23
1.36m/s
Fig.7. Contour of the velocity (t=50s)
Fig.8 shows the streamlines with the color indicating the static pressure. It can be seen that the flow in the inlet tank is mainly concentrated in the upper part. The flow is relatively complex in the separation tank and there is a vortex which is adverse to the separation of oil and water. So in practice, an extending part should be added to the exit of the MHD channel to lower the fluid’s velocity in the upper part of the separation tank. 0
2017
4033
6050
8067
10084
12100
13445pa
Fig.8. Streamlines(t=50s)
Fig.9 shows the distribution of the static pressure. It can be seen that the static pressure decreases gradually from the inlet to the outlet of the MHD channel. The isobar has inflection points at the boundary of the MHD effective segment (the area with the electromagnetic field).
0
2241
4482
6722
8963
11204
Fig.9a. Contour of the static pressure(t=50s)
13445pa
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Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58
728
1321
1914
2507
3099
Inlet
3692pa
Outlet
Fig.9b. Contour of the static pressure in the MHD channel(t=50s)
Fig.10 shows the pressure distribution along the centreline of MHD channels with different tilt angles. The abscissa represents the length of the MHD channel in the flow direction. It can be seen that the static pressure decreases gradually from the inlet to the outlet in the case with α=15° and the static pressure’s gradient within the MHD effective segment is smaller because of the effect of the electromagnetic force. For the case with α=0°, the static pressure increases gradually in the MHD effective segment under the action of the electromagnetic force. The oil and air are non-conducting and unaffected by the electromagnetic force, they move from high pressure areas towards low pressure areas. As to the horizontal MHD channel (α=0°), the gradually increasing pressure would impede the oil and air bubbles move through the channel. To make things worse, the accumulated air would block the channel and reduce the effective flow area. As the volume fraction of the seawater becomes smaller, the electromagnetic force would decrease, which is adverse to oil recovery. With regard to the case of α=15°, the gradually decreasing pressure promotes the oil and air bubbles to discharge from the MHD channel. To some extent, air bubbles would carry oil to move towards the separation tank. As a result, arranging the MHD channel with a tilt angle changes the pressure distribution in the MHD channel totally and thus benefits the oil recovery. 3200 α=15° α=0°
2800 2400
P (pa)
2000
MHD effective segment
1600 1200 800 400 0 0
0.2
0.4
0.6
0.8
1.0
L (m) Fig.10. Pressure distribution on the centerline of the MHD channels
Fig.11 shows the relationship between the seawater flow rate Q, pressure rise Pr and the MHD channel’s tilt angle α. It can be seen that Q decreases slightly as α increases and Pr increases slightly as α increases. Fig.12 shows the relationship between the effective power W=QPr and α. It can be seen that W increases slightly as α increases.
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Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58 160
Q Pr
30
120
20
80
10
40
0
Pr (mmH2O)
Q (m3/h)
40
0 0
5
10
15
20
a (°)
25
30
Fig.11. Variation of Q and Pr with α 10 W
W (w)
8
6
4
2
0 0
5
10
15
20
25
30
a (0°) Fig.12. Variation of W with α
4. Conclusions The hydrodynamic characteristics of the seawater and air two-phase flow under the action of an electromagnetic force with the MHD channel arranged obliquely were studied numerically in this paper. The results show that: (1) The static pressure decreases gradually from the MHD channel’s inlet to the outlet and the static pressure’s gradient within the MHD effective segment is smaller because of the effect of the electromagnetic force. (2)For a certain MHD channel, the flow rate decreases slightly with the increase of the tilt angle of the MHD channel (α); the pressure rise and the effective power increases slightly with the increase of α. References Fluent Inc ,2006. FLUENT 6.3 Users’ Guide, Fluent Inc. Peng,Yan, et al ,2014. Research on marine floating-oil recovery technology based on magneto hydrodynamic method, marine of environmental science, 33, 592-597.
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Jiangjin Liu et al. / Aquatic Procedia 3 (2015) 50 – 58 Peng,Yan, et al ,2005. Fundamental Study on Oil Spill Recovery from Oil-Contaminated Seawater by MHD Method, Proc 15th ,2005Int MHD Energy Conversion Conf, Moscow, Russia, 2,319-322. Sha,Ciwen,et al ,2007. Study on oil separation and recovery from oil-contaminated seawater by MHD method, Collection of Technical Papers 38th AIAA Plasmadynamics and Lasers Conference, 2, 1140-1144. Sha,Ciwen,et al ,2002. Method and Device of MHD Marine Floating-oil Recovery, China Patent: 02142835. 2[P], 09-18-2002. Ye,Chong,et al,2013. 3D Numerical Analysis on Flow Process of Floating-oil Recovery Device by MHD Method, Proc.23rd,2013 International Offshore and Polar Engineering Conference, Anchorage, Alaska, USA, 3,1006-1010. Zhao,Lingzhi,et al ,2014. A Marine Thin Oil-film Recovery device Based on the MHD Method, China Patent: 103774630A[P], 01-23-2014. Zhang,Guoyan,et al ,2007. Analytical Study on Flow Process of Floating-oil Recovery Device from Oil-contaminated Seawater by MHD Method, Journal of hydrodynamics, Ser . B, 9, 195-200 . Zhang,Guoyan,et al ,2007. Numerical simulation of flow process in MHD floating oil recovery channel, Marine Environmental Science, 26, 3337. Zhang,Guoyan ,2006. Numerical Simulation on Flow Process of Oil Separation and Recovery Device from Oil-contaminated Seawater by MHD Method, Masters’ Thesis, Beijing, China: Graduate School of Chinese Academy of Sciences.