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Numerical simulation and experimental study of liquid–liquid flow dispersion in conical spiral pipes
Chemical Engineering Research and Design 1 3 8 ( 2 0 1 8 ) 374–386
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Numerical simulation and experimental study of liquid–liquid flow dispersion in conical spiral pipes Ting Zhang a,b , Kai Guo a,b,∗ , Chunjiang Liu a,b , Aiguo Feng a,b , Hongwei Cai a,b , Siyuan Ren a,b a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
This paper presents the numerical simulation and experimental study of an immiscible
Received 18 March 2018
liquid–liquid flow dispersion in conical spiral pipes for oil–water separation. Flow patterns
Received in revised form 17 August
of oil–water flow in the pipe are identified. The flow characteristics such as pressure drop,
2018
cross sectional phase distribution, and outlet flow rate are obtained. In addition to flow
Accepted 3 September 2018
behavior, separation performance of the conical spiral pipes is examined under different
Available online 10 September 2018
operating conditions. The effects of geometric parameters such as conical angle, pipe diameter, pitch height, and outlet split ratio, on oil–water separation are revealed. Moreover, the
Keywords:
effects of inlet velocity, inlet oil concentration, and operating temperature on the separa-
Conical spiral pipe
tion are obtained. In addition, it has shown that surface treatment for the conical spiral
Oil–water two-phase separation
pipes has an impact on the oil–water separation. In particular, the V-shaped sawtooth sur-
Computational fluid dynamics
face microstructure can enhance the oil–water separation, and Graphene-coated surface
Separation performance
exhibits good separation capacity under high flow velocity.
Flow patterns
1.
© 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Introduction
With the frequent occurrence of spilling oil and chemicals leaking into groundwater in recent years, oily water separation has become an imperative technology (Shannon et al., 2008) to achieve the goals of energy conservation, floating oil recovery, and ecological protection (Wang et al., 2015). Spiral pipe is a simple device with a compact structure (Li et al., 2018). The advantageous structure produces centrifugal force, by which the dense phase flows along the outer side of the spiral pipe whereas the light phase flows along the inner side of the pipe. The characteristics of immiscible liquid–liquid two-phase flow in spiral pipe are complex, which may be affected by many factors mainly, including pipe geometrical parameters (Gammack and Hydon, 2001), fluid physical properties, operating conditions, and pipe surface microstructures. Up to date, some scholars conducted a few investigations on flow patterns in spiral pipes and revealed the liquid–liquid
∗
two-phase flow mechanisms (Grassi et al., 2008). Shi and Yeung studied the inconsistency degree of flow patterns in various horizon pipes. The ratio of the gravitational force to viscous force was proposed to characterize immiscible liquid–liquid two-phase flows (Shi and Yeung, 2017). Mansour et al. (2017) conducted numerical study of flow distribution in spiral pipes, and they introduced Reynolds number and Schmidt number to evaluate liquid–liquid mixing efficiency. They also found two optimal Re values which could lead to the best mixing performance in spiral pipes. Liu et al. identified that the curvature and torsion exert significant effect on liquid distribution and frictional pressure drop in spiral pipe. They established the correlation of two-phase frictional factor characteristics with Re, curvature and torsion in vertical downward spiral pipes (Liu et al., 2017). Zhao et al. studied the flow-field distribution of spiral-tube separators, in which the optimized gyration diameter, spiral coils, oil droplet diameter, and inlet flow rate were analyzed. Because outlet geometry
Corresponding author. E-mail address: [email protected] (K. Guo). https://doi.org/10.1016/j.cherd.2018.09.005 0263-8762/© 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Chemical Engineering Research and Design 1 3 8 ( 2 0 1 8 ) 374–386
Nomenclature Latin letters Coo-outlet [%] Oil concentration at oil outlet Cow-outlet [%] Oil concentration at water outlet D [mm] Average conical diameter [mm] Bottom diameter of conical spiral pipe Dbot [mm] Top diameter of conical spiral pipe Dtop d [mm] Pipe diameter doo-outlet [mm] Oil outlet diameter dow-outlet [mm] Water outlet diameter [–] Dean number De [%] Oil-phase collection efficiency E [mm] Pipe height H [mm] Pitch height h [round] Conical spiral round number n [–] Pitch ratio PR P [Pa] Pressure [Pa] Pressure drop between inlet and outlet P [m3 /h] Inlet flow rate Qin Qo−outlet [m3 /h] Oil-outlet flow rate Qw−outlet [m3 /h] Water-outlet flow rate Q¯ outlet [–] Outlet flow rate ratio [m] Contour arithmetic mean deviation Ra [m] Maximum contour height Ry Rz [m] Ten-point height average [–] Reynolds number Re r [–] Outlet split ratio [◦ C] Temperature T [m/s] Inlet velocity Uin [m/s] Velocity difference U Voo-outlet [m3 ] Oil volume at oil outlet Vow-outlet [m3 ] Oil volume at water outlet VWo-outlet [m3 ] Water volume at oil outlet VWw-outlet [m3 ] Water volume at water outlet [–] Winding ratio WR Greek letters ˛ [%] Volume fraction [◦ ] Conical angle [kg/m3 ] Density ω [◦ ] Spring length [Pa s] Dynamic viscosity Abbreviations CFD [–] Computational fluid dynamics [–] Reynolds stress model RSM [–] Volume of Fluid VOF
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Dean et al. identified the existence of rotating vortices in curved pipes, and revealed the relationship between the pressure gradient and flow rate (Dean, 1928). The transformation of secondary flows is deemed to a key factor accelerating oil–water separation (Frising et al., 2006; Jarrahi et al., 2011). Recently, scholars developed the super-hydrophobic surface (Misyura, 2017; Chen et al., 2016; Wu et al., 2011), which provides the opportunity to develop potential oil–water separation technologies. In the study, conical spiral pipes (see Fig. 1) with surface microstructures were fabricated. The oil–water two-phase flow characteristics in the pipes and separation efficiency were investigated by CFD calculations and experiments. Firstly, outlet flow rate, wall shear stress, phase distribution of oil and water, and two-phase flow pattern were obtained. Secondly, the effects of outlet structure, operating temperature, inlet oil concentration, and geometric structure of the conical spiral pipe on oil–water flow characteristics and separation performance were investigated. At the end of the study, conical spiral pipes with surface treatment were applied to oil–water separation process. And the collection efficiencies of four different pipes were measured.
2.
Methodology and apparatus
2.1.
Geometric structure
The geometric parameters of the conical spiral pipe are shown in Fig. 2. Dtop is the top diameter, Dbot is the bottom diameter, H is the pipe height, is the conical angle, and d is the pipe diameter. The average conical diameter of spiral pipe, D, is calculated by Eq. (1). D=
1 (Dtop +Dbot ) 2
(1)
For the conical spiral pipes, the top diameter, Dtop , is larger than the bottom diameter, Dbot . The round number of spiral pipe, n, is 2.5, and the height between spiral pipe rounds, i.e., pitch height h, changes from 15 mm to 50 mm. Detailed geometric parameters are shown in Table 1. To evaluate the separation performance of the conical spiral pipes, de-oiling process by the spiral pipe was studied as a basis, represented by Case 0# in Table 1.
2.2.
Numerical modeling
In this section, a three-dimension CFD model based on Reynolds-averaged Navier–Stokes equations was built. The
of spiral pipe is a critical problem on oil–water separation process, they optimized the structure of oil-outlet holes. It provided a basis for structural design and optimization of spiral pipe separators (Zhao et al., 2012). Kumara et al. characterized the flow patterns of oil–water flow in horizontal and inclined pipes by visual observations (Kumara et al., 2009). Simon et al. studied the oil–water separating process by means of nuclear magnetic resonance measurements (Simon et al., 2011). These studies gave lights on the investigation of oily water separation process by spiral pipes, but there are still some unclear flow characteristics and separation mechanisms need to be explored.
Fig. 1 – Structures of the spiral pipe and conical spiral pipe.
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Fig. 2 – Geometric parameters of the conical spiral pipe. turbulence model, multiphase model, and boundary conditions of the CFD model were described as follows. — Turbulence model The choice of turbulence model is very important for CFD calculation. Turbulent models with consideration of the physics of turbulent flows perform better in simulating highly swirl flows (Yang et al., 2010; Guo et al., 2016). In particular, the Reynolds stress model (RSM) takes into account all the effects of streamline curvature, swirl, rotation, and rapid changes in strain rate. Therefore, the RSM is able to give reliable predictions for complex flows such as highly swirling flows in combustors, rotating flow passages, and the stress-induced secondary flows in ducts (Ansys Inc., 2017). In the subsequent CFD calculations, the RSM was used to predict the flow in spiral pipes. Transport equations with six variants of the Reynolds stresses terms were written as follows:
∂Rij ∂t
where Cij is the transport convection term, Pij is stress production, Dij is turbulent and molecular diffusion term, εij is dissipation, ij is pressure strain, and Gij is buoyancy production term. — Multiphase model The Volume of Fluid (VOF) has shown to be able to track the oil–water interface of oil–water two-phase flow (Al-Yaari and Abu-Sharkh, 2011). Therefore, the VOF model was employed to predict the oil–water flow behavior and dispersion characteristics. In the VOF method, the momentum equation is shared by fluids, and a single momentum equation is solved throughout the domain (Ansys Inc., 2017). The two phases are considered as two separate phases, and the volume fraction equation is written as 1 ∂ [ (˛q q )∇ · (˛q q uq ) = 0] q ∂t
(3)
In the study, subscript q denotes water or oil. The volume fractions of water and oil satisfy
+ Cij = Pij + Dij − εij + ij + Gij
(2)
C = 0.09, Cε1 = 1.44, Cε2 = 1.92, ε = 1, k = 0.82.
˛w + ˛o = 1.
(4)
Table 1 – Geometric parameters of the conical spiral pipes to be investigated. Case a
0 1 2 3 4 5 6 7 8 9 10 11 12 a
(◦ )
Dbot (mm)
Dtop (mm)
h (mm)
d (mm)
Pitch ratio (PR = h/d)
Winding ratio (WR = D/d)
0 15 30 45 60 90 15 15 15 15 15 15 15
83.5 50 50 50 50 50 50 50 50 50 50 50 50
83.5 117 194 300 483 300 117 117 117 103.6 90.2 76.8 70.1
50 50 50 50 50 50 50 50 50 40 30 20 15
10 10 10 10 10 10 8 6 4 10 10 10 10
5 5 5 5 5 5 6.25 8.33 12.5 4 3 2 1.5
8.35 8.35 12.2 17.5 26.65 17.5 10.4 13.9 20.875 7.7 7 6.3 6
For spiral pipes, Dbot = Dtop .
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Table 2 – Physical properties of oil and water. Physical Property 3
◦
Density (), kg/m (5 C) Dynamic Viscosity (), Pa s (5 ◦ C)
Table 3 – Comparison on the inlet pressure between simulation and experiment.
Water
Oil
998.2 0.0015
837 0.2205
— Boundary conditions and physical properties The boundary conditions are as follows: the velocity inlet boundary condition was prescribed at the inlet, and a constant oil concentration was prescribed at the inlet. The pressure outlet boundary condition was prescribed at the outlet. No-slip condition was applied in the wall surface. The boundary conditions for inlet, outlet, and wall were given as follows: Velocity inlet: Uin = U0 ; Pressure outlet: P = P0 ; No-slip wall: U = 0. For simplicity, the fluid is assumed to be incompressible. The density and dynamic viscosity of water and oil are shown in Table 2. The calculation is non-steady state. The time step is 0.001 s, and the total calculation time is 12.5 s. The First order upwind scheme is adopted for the discretization of momentum and volume fraction equations. The SIMPLE scheme is adopted for the velocity pressure coupling solution. The Tetrahedral mesh was created by GAMBIT. In addition, the mesh in the inlet zone and outlet zone was refined. The CFD model was solved by Ansys Fluent 18.0. In Case 1# , the grid number of the mesh is 115672, 344956, and 1834572. There is no obvious difference in the simulation results solved with different grid number. Therefore, the mesh consisting of 115672 cells was adopted in the following simulations to save computational resources.
2.3.
Experimental apparatus
The conical spiral pipe was made of polyurethane (PU) for easy observation of flow behavior. There are two outlets at the end of the conical spiral pipes, namely oil outlet and water outlet. As shown in Fig. 3, oil outlet is located in the right upper side of the pipe cross-section, and the distance between the oil outlet and the inner side of the pipe is 10 mm. Water outlet is located in the outer side of pipe, and the intersection angle between the water outlet and the pipe is 45◦ . Fig. 4 illustrates the flowchart of the oil–water separation experiment. The working fluid is a mixture of water and gear oil. The red gear oil was mixed with water by a magnetic stirrer, then the oil–water mixture was fed into the spiral pipe by a peristaltic pump. The size of the dispersed oil droplets was measured by a scale microscope. As shown in the picture captured by the micro camera (Fig. 4), the range of the oil droplet diameter is 0.5–2.5 m. The inlet velocity Uin , which was calculated based on the inlet cross sectional area, is in the range of 0.05–0.6 m/s. After fluid flowed through the pipe, oil and water were collected at the oil outlet and water outlet, respectively. The collection efficiency was obtained by measuring the volume and oil concentration at the outlets. The oil-phase collection efficiency, E, was calculated as follows:
E=1−
Cow-outlet Coo-outlet
=1−
Vow-outlet Vow-outlet +Vww-outlet Voo-outlet Voo-outlet +Vw o-outlet
(5)
where Cow-outlet and Coo-outlet are the oil concentration at water outlet and oil outlet, respectively. Vow-outlet and Voo-outlet
Inlet velocity
Experiment result Inlet total pressure (Pa)
CFD result Inlet total pressure (Pa)
Difference (%)
Uin = 0.1 m/s
4445
4299.34
3.39
represent the oil volume at water outlet and oil outlet, respectively. Vww-outlet and Vwo-outlet represent the water volume at water outlet and oil outlet, respectively. Apparently, larger value of E means better separation effect.
3.
Results and discussion
3.1.
Experimental validation of the CFD model
To validate the CFD model, the oil–water separation process was simulated and compared with the experimental result. The operating temperature is 5 ◦ C, and the inlet oil concentration is 10%. The geometric parameters of the conical spiral pipe are: = 15◦ , n = 2.5, h = 50 mm, Dbot = 50 mm, and d = 8 mm. In the experiment, the inlet pressure was measured by an on-line pressure transducer. Its measuring range is 0–20 kPa, and the accuracy is ±0.5 kPa. The experiment result of inlet total pressure was obtained by averaging the pressure value sampled every 10 s during a period of 180 s. The experiment and simulation results were shown in Table 3. The difference between the experiment and simulation is lower than 3.4%.
3.2.
Flow field analysis
3.2.1.
Flow rates at the outlet
Fig. 5 shows the experiment results of water-outlet flow rate Qw-outlet and oil-outlet flow rate Qo-outlet with a pipe diameter of 8 mm, 6 mm, and 4 mm under different inlet Re numbers. For most operating conditions, the flow rates at water outlet are higher than that of oil outlet. That is, there is a flow-rate difference between the water outlet and oil outlet. For the cases with d = 6 mm, the flow-rate difference of the conical spiral pipe is larger than that of spiral pipe. When the pipe diameter decreased to 4 mm, spiral pipe produced a relatively large outlet flow-rate difference. In the case of d = 8 mm, the flow-rate difference between the outlets is not obvious at lower inlet Re number. The outlet flow-rate difference is deemed to be related with the difference of wall shear stress between inner side and outer side of the pipe. It has shown that large velocity difference could induce oil droplet coalescence, and affect oil–water emulsification (Floury et al., 2000). Therefore, outlet flow-rate difference can reflect the flow behavior of oil–water two-phase flow in the pipes.
3.2.2.
Wall Shear stress
Fig. 6 displays the variation of simulated wall shear stress with the spring length (d = 8 mm, = 15◦ , and Uin = 0.1 m/s). In Fig. 6, black line and red line represent the wall shear stress at the outer and inner sides of the pipe, respectively. Obviously, the shear stress at the inner side is lower than that of the outer side. There is a larger difference of wall shear stress between the inner side and outer side in the conical spiral pipe. The wall shear stress at the inner side decreased due to the variation in the gyration geometry of the conical spiral pipe. Shear force could accelerate oil droplets deformation, and
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Fig. 3 – Structures of the oil outlet and water outlet.
Fig. 4 – Flowchart of the oil–water separation experiment. weaken the strength of the interfacial film, thus effectively improve the separation of oil–water mixture (Sjoblom, 2001). In the study, it can be called shear stress effects. Moreover, it is assumed that there exists a critical shear stress which is related to the oil–water separation performance. When the shear stress is less than the critical shear stress, the shear effect is beneficial to oil–water separation; on the contrary, when the shear stress exceeds the critical shear stress, the oil–water separation performance is supposed to deteriorate due to the occurrence of oil–water emulsification.
3.2.3.
Phase distribution
Fig. 7 shows the simulated phase distributions of oil–water flow at different cross-sections (ω = 90◦ , 180◦ , 270◦ , 360◦ , 450◦ , 540◦ , 630◦ , 720◦ , 790◦ , and 900◦ ). The structural parameters of the pipes are: n = 2.5, h = 50 mm, and d = 10 mm. In the figure, = 0◦ and ω = 900◦ denote the spiral pipe and pipe outlet respectively. Fig. 7 indicates that oil-phase flowed along the inner side of the pipe, whereas water-phase flowed along the outer side.
The interfacial turning angle changed with the variation of conical angle. The oil–water interface is wavy and oblique due to the effects of centrifugal force, shear stress and the spiral structure. The interface became distinct and continuous gradually as the increase of spring length.
3.2.4.
Flow pattern
The oil dispersion characteristics determine the oil–water collection efficiency. In the experiment, flow patterns were captured to investigate the stability of the coalesced oil film attached to the inner side of conical spiral pipes. Operating temperature is 5 ◦ C, and the inlet oil concentration is 10%. The range of inlet velocity is 0.05–0.5 m/s. As shown in Fig. 8(a), four different kinds of flow patterns were observed in the experiment. They are dispersion of oil droplet in water and water (Do & w), stretched oil droplet flow and dispersion of oil in water and water (SOD & Do & o/w & w), oil plugs in water and dispersion of oil in water and water (OP & o/w & w), and dispersion oil in water and water in oil (Do/w & w/o).
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Fig. 5 – Experiment results of flow rates at water outlet and oil outlet with different pipe diameters: (a) d = 8 mm; (b) d = 6 mm; (c) d = 4 mm. It was found that the flow pattern was determined by the inlet velocity. In the experiment, Do/w & w flow pattern occurred at low inlet velocity, in which oil phase existed in
the form of oil droplets. When the inlet velocity increased, the oil droplets were stretched into stable oil film, then OP & o/w & w flow pattern formed. After that, the dispersed oil
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Fig. 6 – Simulation results of wall shear stress at the outer side and inner side of pipes: (a) Spiral pipe; (b) conical spiral pipe. droplets constituted consecutive and thick oil film, which indicates a flow pattern transition from OP & o/w & w to SOD & Do & o/w & w. With a further increase of inlet velocity, oil film became intermittent and unstable. Thick oil film was sheared into thin film and small oil droplets by the high-velocity water flow, and, as a result, flow pattern changed to OP & o/w & w. When flow pattern changed to Do/w & w/o, oil–water emulsification appeared which has an adverse effect on separation. It indicates that oil–water emulsification is related to increase of shear stress caused by the increase of flow velocity. Fig. 8(b) shows the visualization sketches of the conical spiral pipe (Case 1# ) and the spiral pipe (Case 0# ) with n = 0.25, 1 and 2. The oil–water interface in the conical pipes was found to be more distinct than that of the spiral pipes. It is easier to form thick oil film at lower spiral pipe round under the same inlet velocity. The oil–water interface became distinct with the increasing of n, especially in the conical spiral pipes. It may be attributed to the decrease of wall shear stress along the inner side of conical spiral pipes. From Fig. 8(c), the oil–water interface became unclear and unstable in spiral pipe as the increase of inlet velocity. When the inlet velocity increased to 0.3 m/s, the interface disappeared. On the contrary, the oil–water interface can be identified clearly in the conical spiral pipe. It implies that the conical spiral pipe has a larger critical wall shear stress. Thus, in a certain degree, conical spiral pipe can avoid the oil–water emulsification.
Fig. 8 demonstrates that the conical spiral structure is beneficial to form the flow pattern of SOD & Do & w/o. The flow pattern with continuous oil film and distinct oil–water interface may contribute to oil–water separation.
3.3.
Effect of outlet split ratio
Outlet structure of conical spiral pipe is a critical issue in oil–water separation process. In the study, a parameter outlet split ratio, r, was defined as the ratio of oil-outlet diameter to the sum of oil-outlet diameter and water-outlet diameter as follows:
r = doo-outlet /(doo-outlet + dow-outlet )
(6)
To investigate the effect of outlet split ratio, the inlet/outlets diameter ratio, which is defined as the ratio of inlet diameter to the sum of oil-outlet diameter and wateroutlet diameter, is set to be 0.625. The diameter of water outlet is 3 mm, 2.5 mm, and 2 mm; correspondingly, the diameter of oil outlet is 2 mm, 2.5 mm, and 3 mm. The outlet split ratio, r, equals 0.4, 0.5, and 0.6. The pipe diameter, d, is 8 mm. The oil-phase collection efficiencies with three different values of r were measured. Fig. 9 shows the variations of collection efficiency and dimensionless flow rate ratio with the inlet
Fig. 7 – Simulation results of cross sectional phase distribution of oil–water flow in spiral pipes.
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Fig. 8 – Flow patterns of oil–water two-phase flow: (a) Various oil–water flow patterns; (b) Flow pattern in different locations; (c) Flow pattern under different inlet velocities. velocity. Here, the outlet flow-rate ratio, Q outlet , was defined by Eq. (7).
Q outlet =
Qo-outlet Qw-outlet
(7)
where Qo-outlet and Qw-outlet are the flow rate at oil outlet and water outlet, respectively. Fig. 9 shows that higher collection efficiency can be achieved by reducing the outlet split ratio and inlet velocity. A maximum value of collection efficiency can be obtained with r = 0.4 and Uin ≈ 0.1 m/s. On the other hand, the pipe with lower value of r produced higher outlet flow-rate ratio. In addition, collection efficiency was found to be reversely proportional to the flow rate ratio. That is, higher flow rate ratio means a lower collection efficiency, and vice versa.
3.4.
Effects of temperature and inlet oil concentration
3.4.1.
Effect of temperature
Typically, dynamic viscosity (or viscous force) changes with the temperature changing. Therefore, operating temperature may have an impact on oil–water emulsification and separation performance. In this section, oil–water separation efficiency was measured under different operating temperatures. To begin with, the variations of dynamic viscosity of oil and water with the temperature were measured by a viscositymeter, as shown in Fig. 10(a) and (b). Dynamic viscosity of both the gear oil and water decreases as the increase of temperature in the range of 5 ◦ C–70 ◦ C. In the oil–water separation experiment, the oil concentration is 10%, and the inlet velocity is 0.20 m/s. The geometric parameters of the experimental pipe are: h = 50 mm,
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Fig. 9 – Effect of inlet velocity on oil-phase collection efficiency and outlet flow-rate ratio. Dbot = 50 mm, = 15◦ , and d = 8 mm. The collection efficiency was obtained and depicted in Fig. 10(c). From the figure, it can be found that the collection efficiency increased with the increase of operating temperature in the range of 5 ◦ C–40 ◦ C. When the temperature was raised up to 40 ◦ C, the collection efficiency began to fluctuate within a range. It indicates that higher operating temperature leads to better collection efficiency.
3.4.2.
Effect of inlet oil concentration
The effect of inlet oil concentration on oil–water separation performance was examined. The operating temperature is 10 ◦ C, the inlet velocity is 0.40 m/s, and the inlet oil concentration increases from 10% to 50%. The geometric parameters of the conical spiral pipe in the experiment are: h = 50 mm, Dbot = 50 mm, = 15◦ , and d = 6 mm. As illustrated by Fig. 11, higher oil concentration has an adverse effect on oil-phase collection efficiency. This is because it is more likely to form the flow patterns of OP & o/w & w and dispersion oil in water and water in oil (Do/w & w/o) with higher inlet oil concentration. These flow patterns could produce serious oil–water emulsification that hinders the oil–water separation.
3.5.
Effect of geometric parameter
3.5.1.
Effect of conical angle
The effect of conical angle on oil–water separation was investigated. The operating temperature is 5 ◦ C, the inlet oil concentration is 10%, and inlet velocity is 0.50 m/s. The range of conical angle is 0◦ –90◦ . Other geometric parameters are: h = 50 mm, d = 6 mm, and Dbot = 50 mm. First, the inlet-outlet total pressure drop P was obtained by CFD calculation. Fig. 12(a) shows that the trend of change of pressure drop with conical angle is smooth, except for = 85◦ . When conical angle increased to 85◦ , pressure drop increased greatly. This is mainly because the increase of pipe length. For the pipe with higher conical angle, an increase of conical angle leads to a significant increase of pipe length, and, consequently, results in the increase of pressure drop. It should be noted that there is a significant decrease of pressure drop when conical angle increased to 90◦ . The conical spiral pipe becomes a flat-type spiral pipe when conical angle increased to 90◦ . Compared with the conical spiral pipe with = 85◦ , the pipe length of
Fig. 10 – Effect of temperature on oil viscosity, water viscosity and oil-phase collection efficiency: (a) Oil viscosity; (b) Water viscosity; (c) Oil-phase collection efficiency.
the flat-type spiral pipe is reduced. Moreover, the geometric structure difference between the conical spiral pipe and the flat-type spiral pipe may induce the significant difference of flow resistance. In addition to pressure drop, the oil-phase collection efficiency was experimentally measured under different inlet velocities. Fig. 12(b) shows the variation trend of collection efficiency of spiral pipes and conical spiral pipes. It indicates that the conical spiral pipe with = 15◦ produced the highest oil-phase collection efficiency. It also demonstrates that
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h = 50 mm, Dbot = 50 mm, and = 15◦ . CFD results in Fig. 13(a) indicate that pipe diameter is inversely proportional to pressure drop. Apparently, larger pipe diameter can reduce the flow resistance, and consequently, produce lower pressure drop. In Fig. 13(b), experiment results indicate that the collection efficiency of conical spiral pipes is higher than that of spiral pipes. Moreover, the oil–water separation effect of conical spiral pipes with d = 8 mm or 6 mm are better than the pipe with d = 4 mm. The difference in efficiency may be attributed to the flow pattern transition.
3.5.3.
Fig. 11 – Effect of inlet oil concentration on oil-phase collection efficiency. compared to spiral pipe, conical spiral pipe exhibits better oil–water separation capacity.
3.5.2.
Effect of pipe diameter
The effect of pipe diameter on flow resistance and collection efficiency was also investigated. The operating temperature is 5 ◦ C, and the inlet oil concentration is 10%. The range of pipe diameter is 4–8 mm. Other geometric parameters are:
Effect of pitch height
The effect of pitch height on oil–water separation was studied. In the experiment, operating temperature is 5 ◦ C, and the inlet oil concentration is 10%. The range of pitch height is 15 mm–50 mm. It should be noted that the average conical diameter, D, increases as the increase of pitch height. Other geometric parameters are: d = 10 mm, and = 15◦ . Fig. 14 shows the variation of oil-phase collection efficiency with the pitch height under different inlet flow rates. In general, oil–water separation is affected by the pitch height. However, no clear trend was found for the influence of pitch height on the collection efficiency. On the other hand, as the increase of inlet flow rate, the collection efficiency decreased. This may be attributed to the occurrence of oil–water emulsification as discussed in Section 3.2.4.
Fig. 12 – Effect of conical angle on pressure drop and oil-phase collection efficiency: (a) Pressure drop; (b) Oil-phase collection efficiency.
Fig. 13 – Effect of inlet velocity (or inlet Re number) on pressure drop and oil-phase collection efficiency under different pipe diameters: (a) Pressure drop; (b) Oil-phase collection efficiency.
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Fig. 14 – Variations of Re number, De number and collection efficiency with the pitch height under different inlet flow rates: (a) Qin = 0.0565 m3 /h; (b) Qin = 0.038 m3 /h; (c) Qin = 0.019 m3 /h. The secondary flows that influence the separation of oil–water mixture in conical spiral pipes are mainly generated by inertia force, viscous force, and centrifugal force. In this section, outlet Re number and De number were introduced to characterize the variation trend of oil-phase collection efficiency. Re number represents the ratio of flow inertia force to viscous force, whereas De number represents the ratio of inertia force, centrifugal force, and viscous force (Di Carlo et al.,
2007; Hou et al., 2013). The definitions of outlet Re number and De number in this study are given as follows:
Re =
Ud
d 12
De = Re
D
(7)
(8)
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Fig. 15 – Surface treatment of conical spiral pipe: (a) V-shaped microstructure pipe; (b) Graphene-coating pipe. Variations of outlet Re number and De number with the pitch height were also depicted in Fig. 14. Obviously, the variation trend of collection efficiency with the pitch height is different from that of Re or De. Nevertheless, similar trend between oil-phase collection efficiency and Re or De can be found under some specific conditions. For example, when the flow rate is 0.019 m3 /h, the variation of E was synchronized with De under the condition of h < 30 mm or h > 40 mm; whereas for Re number, its variation trend corresponded to the collection efficiency when the pitch height changed from 30 mm to 50 mm. When flow rate increased to 0.038 m3 /h, the variation trend of E is similar with that of De under the condition of 20 mm < h < 30 mm or h > 40 mm; whereas the positive correspondence relationship between E and Re was found under the condition of 30 mm < h < 50 mm. When the flow rate is 0.0565 m3 /h, the variation of E was synchronized with Re within the entire range of pitch height; whereas the positive correspondence relationship between De and E existed for h < 40 mm. From Fig. 14, it could be deduced that neither Re number nor De number is able to predict the variation trend of collection efficiency. Alternatively, a combination of Re and De is possibly an appropriate parameter to predict the changing tendency of oil-phase collection efficiency.
3.6.
Effect of surface treatment
The well-wetted wall surface with high roughness has been shown to exhibit underwater superoleophobicity and unique adhesive behaviors (Chen et al., 2016). In the study, four types of conical spiral pipe, smooth PU pipe, 3D-printed abrasive pipe, Graphene-coating pipe, and V-shaped surface microstructure pipe, were fabricated to investigate surface effect in oil–water separation. As shown in Fig. 15, V-shaped microstructure pipe is featured by the sawtooth surface microstructure. The number of the sawtooth is 36, and the height of the sawtooth is 2 mm. They are arranged uniformly around the surface of the conical spiral pipe. Graphene-coating pipe is fabricated by coating the inner side of the pipe with hydrophobic Graphene. The hydrophobic Graphene is composed of 8% Graphene powder, 1.5% carbon micro-tube, epoxy resin, and epoxy curing agents. Table 4 shows the surface roughness of the pipes measured by a surface roughness tester. Surface roughness was characterized by three parameters. Ra is the contour arithmetic mean deviation; Rz is the ten-point height average; Ry is the maximum contour height. The results indicate that smooth PU pipe has the smoothest surface, whereas V-shaped microstructure pipe has the roughest surface.
Table 4 – Surface roughness of the pipes. Pipe type
Ra (m)
Rz (m)
Ry (m)
Smooth PU Pipe 3D-printed abrasive pipe V-shaped microstructure pipe Graphene-coating pipe—inner side Graphene-coating pipe—outer side
0.10 14.72 21.34 0.18 9.07
0.37 51.81 61.98 0.46 44.90
0.56 68.28 83.68 0.84 54.29
Fig. 16 shows the oil-phase collection efficiency of the pipes under different inlet velocities. For lower inlet velocity (Uin = 0.1 and 0.2 m/s), V-shaped microstructure pipe obtained the best oil–water separation performance. When inlet velocity increased to 0.3 m/s, 3D-printed abrasive pipe produced the highest oil-phase collection efficiency. Compared to low inlet velocities, Graphene-coating pipe exhibits better oil–water separation capacity at Uin = 0.3 m/s. This may be attributed to the adhesion between the oil film and Graphene-coating surface, which contributes to the formation of the flow pattern of SOD & Do & o/w & w or OP & o/w & w and suppresses the oil–water emulsification.
4.
Conclusion
The oil–water two-phase flow dispersion characteristics and oil–water separation capacity of conical spiral pipes were investigated. Phase distribution, wall shear stress, and pressure drop of oil–water two-phase flow in conical spiral pipes were obtained by CFD calculations. Flow patterns of oil–water flow were identified by visual observations. Outlet flow rate
Fig. 16 – Collection efficiencies of the pipes under different inlet velocities.
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Chemical Engineering Research and Design 1 3 8 ( 2 0 1 8 ) 374–386
and collection efficiency under different operating conditions were measured experimentally. In the conical spiral pipes, oil and water flowed along the inner side and outer side, respectively. The wall shear stress at the outer side is larger than that at the inner side of the conical spiral pipe. There is an oil–water interface in the cross-section of the pipe. As the inlet velocity increased, flow pattern transition occurred in the pipe, and the interface became unclear or even disappeared. Oil–water emulsification is more likely to appear under relatively high inlet velocity. Effects of geometric structure and operating conditions on collection efficiency were studied. Either low inlet velocity or low inlet oil concentration is beneficial to oil–water separation. Conical spiral pipes can produced higher separation effect at high temperature due to the decrease of fluid viscosity. In particular, the oil-phase collection efficiency increased monotonously with the increase of temperature in the range of 5–40 ◦ C. In terms of oil-phase collection efficiency, conical spiral pipes exhibit better separation capacity than spiral pipes. Moreover, the conical spiral pipe with = 15◦ achieved the maximum oil-phase collection efficiency. The relationship between the pitch height and the separation performance are complex. Experimental results indicated that the collection efficiency was maximized with a pitch height of 40 mm for lower inlet flow rate. For a larger flow rate, the conical spiral pipe with a pitch height of 20 mm is preferred. In other hand, some parameters such as outlet flow-rate ratio, Re and De can reflect the separation effect. A combination of Re and De is possibly an appropriate parameter to predict the changing tendency of oil-phase collection efficiency of the conical spiral pipes. Effects of surface treatment on oil–water sedation were also studied. Collection efficiencies of four different kinds of conical spiral pipe were measured. It has shown that increasing surface roughness can enhance oil–water separation. And, increasing wall adhesion between the oil film and the surface is a promising method to suppress oil–water emulsification and improve separation performance under high inlet velocity.
References Al-Yaari, A.M., Abu-Sharkh, B.F., 2011. CFD prediction of stratified oil-water flow in a horizontal pipe. Asian Trans. Eng. 01, 68–75. Ansys Inc., Ansys Fluent Theory Guide, Release 18.0, 2017. Chen, H., Zhang, P., Zhang, L., Liu, H., Jiang, Y., Zhang, D., Han, Z., Jiang, L., 2016. Continuous directional water transport on the peristome surface of Nepenthes alata. Nature 532, 85–89. Dean, W.R., 1928. The stream-line motion of fluid in a curved pipe. Lond. Edinb. Dublin Philos. Mag. J. Sci. 5, 673–695. Di Carlo, D., Irimia, D., Tompkins, R.G., Toner, M., 2007. Continuous inertial focusing, ordering, and separation of particles in microchannels. Proc. Natl. Acad. Sci. U. S. A. 104, 18892–18897. Floury, J., Derumaux, A., Lardieres, J., 2000. Effect of high-pressure homogenization on droplet size distributions and rheological properties of model oil-in-water emulsions. Innov. Food Sci. Emerg. Technol. 1, 127–134. Frising, T., Noik, C., Dalmazzone, C., 2006. The liquid/liquid sedimentation process: from droplet coalescence to technologically enhanced water/oil emulsion gravity separators: a review. J. Dispersion Sci. Technol. 27, 1035–1057.
Gammack, D., Hydon, P.E., 2001. Flow in pipes with non-uniform curvature and torsion. J. Fluid Mech. 433, 357–382. Grassi, B., Strazza, D., Poesio, P., 2008. Experimental validation of theoretical models in two-phase high-viscosity ratio liquid–liquid flows in horizontal and slightly inclined pipes. Int. J. Multiph. Flow. 34, 950–965. Guo, T.Y., Shi, X., Chu, G.W., Xiang, Y., Wen, L.X., Chen, J.F., 2016. Computational fluid dynamics analysis of the micromixing efficiency in a rotating-packed-bed reactor. Ind. Eng. Chem. Res. 55, 4856–4866. Hou, H.W., Warkiani, M.E., Khoo, B.L., Li, Z.R., Soo, R.A., Tan, D.S.W., Lim, W.T., Han, J., Bhagat, A.A.S., Lim, C.T., 2013. Isolation and retrieval of circulating tumor cells using centrifugal forces. Sci. Rep. 3, 1259. Jarrahi, M., Castelain, C., Peerhossaini, H., 2011. Secondary flow patterns and mixing in laminar pulsating flow through a curved pipe. Exp. Fluids 50, 1539–1558. Kumara, W.A.S., Halvorsen, B.M., Melaaen, M.C., 2009. Pressure drop, flow pattern and local water volume fraction measurements of oil-water flow in pipes. Meas. Sci. Technol. 20, 114004. Li, S., Cai, W., Chen, J., Zhang, H., Jiang, Y., 2018. Numerical study on condensation heat transfer and pressure drop characteristics of ethane/propane mixture upward flow in a spiral pipe. Int. J. Heat Mass Transf. 121, 170–186. Liu, X., Zhao, D., Liu, Y., Jiang, S., Xiang, H., 2017. Numerical analysis of the two-phase flow characteristics in vertical downward helical pipe. Int. J. Heat Mass Transf. 108, 1947–1959. Mansour, M., Liu, Z., Janiga, G., Nigam, K.D.P., Sundmacher, K., Thevenin, D., Zahringer, K., 2017. Numerical study of liquid–liquid mixing in helical pipes. Chem. Eng. Sci. 172, 250–261. Misyura, S.Y., 2017. The effect of Weber number, droplet sizes and wall roughness on crisis of droplet boiling. Exp. Therm. Fluid Sci. 84, 190–198. Shannon, M.A., Bohn, P.W., Elimelech, M., Georgiadis, J.G., Marinas, B.J., Mayes, A.M., 2008. Science and technology for water purification in the coming decades. Nature 452, 301–310. Shi, J., Yeung, H., 2017. Characterization of liquid–liquid flows in horizontal pipes. AICHE J. 63, 1132–1143. Simon, S., Pierrard, X., Sjoblom, J., Sorland, G.H., 2011. Separation profile of model water-in-oil emulsions followed by nuclear magnetic resonance (NMR) measurements: application range and comparison with a multiple-light scattering based apparatus. J. Colloid Interface Sci. 356, 352–361. Sjoblom, J. (Ed.), 2001. Encyclopedic Handbook of Emulsion Technology. Marcel Dekker, Inc., New York. Wang, B., Liang, W., Guo, Z., Liu, W., 2015. Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature. Chem. Soc. Rev. 44, 336–361. Wu, D., Wu, S.Z., Chen, Q.D., Zhao, S., Zhang, H., Jiao, J., Piersol, J.A., Wang, J.N., Sun, H.B., Jiang, L., 2011. Facile creation of hierarchical PDMS microstructures with extreme underwater superoleophobicity for anti-oil application in microfluidic channels. Lab Chip 11, 3873–3879. Yang, W., Wang, Y., Chen, J., Fei, W., 2010. Computational fluid dynamic simulation of fluid flow in a rotating packed bed. Chem. Eng. J. 156, 582–587. Zhao, L., Xu, B., Jiang, M., Li, F., Hua, Z., 2012. Flow-field distribution and parametric-optimisation analysis of spiral-tube separators. Chem. Eng. Res. Des. 90, 1011–1018.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Numerical simulation and experimental study of liquid–liquid flow dispersion in conical spiral pipes Ting Zhang a,b , Kai Guo a,b,∗ , Chunjiang Liu a,b , Aiguo Feng a,b , Hongwei Cai a,b , Siyuan Ren a,b a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
This paper presents the numerical simulation and experimental study of an immiscible
Received 18 March 2018
liquid–liquid flow dispersion in conical spiral pipes for oil–water separation. Flow patterns
Received in revised form 17 August
of oil–water flow in the pipe are identified. The flow characteristics such as pressure drop,
2018
cross sectional phase distribution, and outlet flow rate are obtained. In addition to flow
Accepted 3 September 2018
behavior, separation performance of the conical spiral pipes is examined under different
Available online 10 September 2018
operating conditions. The effects of geometric parameters such as conical angle, pipe diameter, pitch height, and outlet split ratio, on oil–water separation are revealed. Moreover, the
Keywords:
effects of inlet velocity, inlet oil concentration, and operating temperature on the separa-
Conical spiral pipe
tion are obtained. In addition, it has shown that surface treatment for the conical spiral
Oil–water two-phase separation
pipes has an impact on the oil–water separation. In particular, the V-shaped sawtooth sur-
Computational fluid dynamics
face microstructure can enhance the oil–water separation, and Graphene-coated surface
Separation performance
exhibits good separation capacity under high flow velocity.
Flow patterns
1.
© 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Introduction
With the frequent occurrence of spilling oil and chemicals leaking into groundwater in recent years, oily water separation has become an imperative technology (Shannon et al., 2008) to achieve the goals of energy conservation, floating oil recovery, and ecological protection (Wang et al., 2015). Spiral pipe is a simple device with a compact structure (Li et al., 2018). The advantageous structure produces centrifugal force, by which the dense phase flows along the outer side of the spiral pipe whereas the light phase flows along the inner side of the pipe. The characteristics of immiscible liquid–liquid two-phase flow in spiral pipe are complex, which may be affected by many factors mainly, including pipe geometrical parameters (Gammack and Hydon, 2001), fluid physical properties, operating conditions, and pipe surface microstructures. Up to date, some scholars conducted a few investigations on flow patterns in spiral pipes and revealed the liquid–liquid
∗
two-phase flow mechanisms (Grassi et al., 2008). Shi and Yeung studied the inconsistency degree of flow patterns in various horizon pipes. The ratio of the gravitational force to viscous force was proposed to characterize immiscible liquid–liquid two-phase flows (Shi and Yeung, 2017). Mansour et al. (2017) conducted numerical study of flow distribution in spiral pipes, and they introduced Reynolds number and Schmidt number to evaluate liquid–liquid mixing efficiency. They also found two optimal Re values which could lead to the best mixing performance in spiral pipes. Liu et al. identified that the curvature and torsion exert significant effect on liquid distribution and frictional pressure drop in spiral pipe. They established the correlation of two-phase frictional factor characteristics with Re, curvature and torsion in vertical downward spiral pipes (Liu et al., 2017). Zhao et al. studied the flow-field distribution of spiral-tube separators, in which the optimized gyration diameter, spiral coils, oil droplet diameter, and inlet flow rate were analyzed. Because outlet geometry
Corresponding author. E-mail address: [email protected] (K. Guo). https://doi.org/10.1016/j.cherd.2018.09.005 0263-8762/© 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Chemical Engineering Research and Design 1 3 8 ( 2 0 1 8 ) 374–386
Nomenclature Latin letters Coo-outlet [%] Oil concentration at oil outlet Cow-outlet [%] Oil concentration at water outlet D [mm] Average conical diameter [mm] Bottom diameter of conical spiral pipe Dbot [mm] Top diameter of conical spiral pipe Dtop d [mm] Pipe diameter doo-outlet [mm] Oil outlet diameter dow-outlet [mm] Water outlet diameter [–] Dean number De [%] Oil-phase collection efficiency E [mm] Pipe height H [mm] Pitch height h [round] Conical spiral round number n [–] Pitch ratio PR P [Pa] Pressure [Pa] Pressure drop between inlet and outlet P [m3 /h] Inlet flow rate Qin Qo−outlet [m3 /h] Oil-outlet flow rate Qw−outlet [m3 /h] Water-outlet flow rate Q¯ outlet [–] Outlet flow rate ratio [m] Contour arithmetic mean deviation Ra [m] Maximum contour height Ry Rz [m] Ten-point height average [–] Reynolds number Re r [–] Outlet split ratio [◦ C] Temperature T [m/s] Inlet velocity Uin [m/s] Velocity difference U Voo-outlet [m3 ] Oil volume at oil outlet Vow-outlet [m3 ] Oil volume at water outlet VWo-outlet [m3 ] Water volume at oil outlet VWw-outlet [m3 ] Water volume at water outlet [–] Winding ratio WR Greek letters ˛ [%] Volume fraction [◦ ] Conical angle [kg/m3 ] Density ω [◦ ] Spring length [Pa s] Dynamic viscosity Abbreviations CFD [–] Computational fluid dynamics [–] Reynolds stress model RSM [–] Volume of Fluid VOF
375
Dean et al. identified the existence of rotating vortices in curved pipes, and revealed the relationship between the pressure gradient and flow rate (Dean, 1928). The transformation of secondary flows is deemed to a key factor accelerating oil–water separation (Frising et al., 2006; Jarrahi et al., 2011). Recently, scholars developed the super-hydrophobic surface (Misyura, 2017; Chen et al., 2016; Wu et al., 2011), which provides the opportunity to develop potential oil–water separation technologies. In the study, conical spiral pipes (see Fig. 1) with surface microstructures were fabricated. The oil–water two-phase flow characteristics in the pipes and separation efficiency were investigated by CFD calculations and experiments. Firstly, outlet flow rate, wall shear stress, phase distribution of oil and water, and two-phase flow pattern were obtained. Secondly, the effects of outlet structure, operating temperature, inlet oil concentration, and geometric structure of the conical spiral pipe on oil–water flow characteristics and separation performance were investigated. At the end of the study, conical spiral pipes with surface treatment were applied to oil–water separation process. And the collection efficiencies of four different pipes were measured.
2.
Methodology and apparatus
2.1.
Geometric structure
The geometric parameters of the conical spiral pipe are shown in Fig. 2. Dtop is the top diameter, Dbot is the bottom diameter, H is the pipe height, is the conical angle, and d is the pipe diameter. The average conical diameter of spiral pipe, D, is calculated by Eq. (1). D=
1 (Dtop +Dbot ) 2
(1)
For the conical spiral pipes, the top diameter, Dtop , is larger than the bottom diameter, Dbot . The round number of spiral pipe, n, is 2.5, and the height between spiral pipe rounds, i.e., pitch height h, changes from 15 mm to 50 mm. Detailed geometric parameters are shown in Table 1. To evaluate the separation performance of the conical spiral pipes, de-oiling process by the spiral pipe was studied as a basis, represented by Case 0# in Table 1.
2.2.
Numerical modeling
In this section, a three-dimension CFD model based on Reynolds-averaged Navier–Stokes equations was built. The
of spiral pipe is a critical problem on oil–water separation process, they optimized the structure of oil-outlet holes. It provided a basis for structural design and optimization of spiral pipe separators (Zhao et al., 2012). Kumara et al. characterized the flow patterns of oil–water flow in horizontal and inclined pipes by visual observations (Kumara et al., 2009). Simon et al. studied the oil–water separating process by means of nuclear magnetic resonance measurements (Simon et al., 2011). These studies gave lights on the investigation of oily water separation process by spiral pipes, but there are still some unclear flow characteristics and separation mechanisms need to be explored.
Fig. 1 – Structures of the spiral pipe and conical spiral pipe.
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Fig. 2 – Geometric parameters of the conical spiral pipe. turbulence model, multiphase model, and boundary conditions of the CFD model were described as follows. — Turbulence model The choice of turbulence model is very important for CFD calculation. Turbulent models with consideration of the physics of turbulent flows perform better in simulating highly swirl flows (Yang et al., 2010; Guo et al., 2016). In particular, the Reynolds stress model (RSM) takes into account all the effects of streamline curvature, swirl, rotation, and rapid changes in strain rate. Therefore, the RSM is able to give reliable predictions for complex flows such as highly swirling flows in combustors, rotating flow passages, and the stress-induced secondary flows in ducts (Ansys Inc., 2017). In the subsequent CFD calculations, the RSM was used to predict the flow in spiral pipes. Transport equations with six variants of the Reynolds stresses terms were written as follows:
∂Rij ∂t
where Cij is the transport convection term, Pij is stress production, Dij is turbulent and molecular diffusion term, εij is dissipation, ij is pressure strain, and Gij is buoyancy production term. — Multiphase model The Volume of Fluid (VOF) has shown to be able to track the oil–water interface of oil–water two-phase flow (Al-Yaari and Abu-Sharkh, 2011). Therefore, the VOF model was employed to predict the oil–water flow behavior and dispersion characteristics. In the VOF method, the momentum equation is shared by fluids, and a single momentum equation is solved throughout the domain (Ansys Inc., 2017). The two phases are considered as two separate phases, and the volume fraction equation is written as 1 ∂ [ (˛q q )∇ · (˛q q uq ) = 0] q ∂t
(3)
In the study, subscript q denotes water or oil. The volume fractions of water and oil satisfy
+ Cij = Pij + Dij − εij + ij + Gij
(2)
C = 0.09, Cε1 = 1.44, Cε2 = 1.92, ε = 1, k = 0.82.
˛w + ˛o = 1.
(4)
Table 1 – Geometric parameters of the conical spiral pipes to be investigated. Case a
0 1 2 3 4 5 6 7 8 9 10 11 12 a
(◦ )
Dbot (mm)
Dtop (mm)
h (mm)
d (mm)
Pitch ratio (PR = h/d)
Winding ratio (WR = D/d)
0 15 30 45 60 90 15 15 15 15 15 15 15
83.5 50 50 50 50 50 50 50 50 50 50 50 50
83.5 117 194 300 483 300 117 117 117 103.6 90.2 76.8 70.1
50 50 50 50 50 50 50 50 50 40 30 20 15
10 10 10 10 10 10 8 6 4 10 10 10 10
5 5 5 5 5 5 6.25 8.33 12.5 4 3 2 1.5
8.35 8.35 12.2 17.5 26.65 17.5 10.4 13.9 20.875 7.7 7 6.3 6
For spiral pipes, Dbot = Dtop .
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Table 2 – Physical properties of oil and water. Physical Property 3
◦
Density (), kg/m (5 C) Dynamic Viscosity (), Pa s (5 ◦ C)
Table 3 – Comparison on the inlet pressure between simulation and experiment.
Water
Oil
998.2 0.0015
837 0.2205
— Boundary conditions and physical properties The boundary conditions are as follows: the velocity inlet boundary condition was prescribed at the inlet, and a constant oil concentration was prescribed at the inlet. The pressure outlet boundary condition was prescribed at the outlet. No-slip condition was applied in the wall surface. The boundary conditions for inlet, outlet, and wall were given as follows: Velocity inlet: Uin = U0 ; Pressure outlet: P = P0 ; No-slip wall: U = 0. For simplicity, the fluid is assumed to be incompressible. The density and dynamic viscosity of water and oil are shown in Table 2. The calculation is non-steady state. The time step is 0.001 s, and the total calculation time is 12.5 s. The First order upwind scheme is adopted for the discretization of momentum and volume fraction equations. The SIMPLE scheme is adopted for the velocity pressure coupling solution. The Tetrahedral mesh was created by GAMBIT. In addition, the mesh in the inlet zone and outlet zone was refined. The CFD model was solved by Ansys Fluent 18.0. In Case 1# , the grid number of the mesh is 115672, 344956, and 1834572. There is no obvious difference in the simulation results solved with different grid number. Therefore, the mesh consisting of 115672 cells was adopted in the following simulations to save computational resources.
2.3.
Experimental apparatus
The conical spiral pipe was made of polyurethane (PU) for easy observation of flow behavior. There are two outlets at the end of the conical spiral pipes, namely oil outlet and water outlet. As shown in Fig. 3, oil outlet is located in the right upper side of the pipe cross-section, and the distance between the oil outlet and the inner side of the pipe is 10 mm. Water outlet is located in the outer side of pipe, and the intersection angle between the water outlet and the pipe is 45◦ . Fig. 4 illustrates the flowchart of the oil–water separation experiment. The working fluid is a mixture of water and gear oil. The red gear oil was mixed with water by a magnetic stirrer, then the oil–water mixture was fed into the spiral pipe by a peristaltic pump. The size of the dispersed oil droplets was measured by a scale microscope. As shown in the picture captured by the micro camera (Fig. 4), the range of the oil droplet diameter is 0.5–2.5 m. The inlet velocity Uin , which was calculated based on the inlet cross sectional area, is in the range of 0.05–0.6 m/s. After fluid flowed through the pipe, oil and water were collected at the oil outlet and water outlet, respectively. The collection efficiency was obtained by measuring the volume and oil concentration at the outlets. The oil-phase collection efficiency, E, was calculated as follows:
E=1−
Cow-outlet Coo-outlet
=1−
Vow-outlet Vow-outlet +Vww-outlet Voo-outlet Voo-outlet +Vw o-outlet
(5)
where Cow-outlet and Coo-outlet are the oil concentration at water outlet and oil outlet, respectively. Vow-outlet and Voo-outlet
Inlet velocity
Experiment result Inlet total pressure (Pa)
CFD result Inlet total pressure (Pa)
Difference (%)
Uin = 0.1 m/s
4445
4299.34
3.39
represent the oil volume at water outlet and oil outlet, respectively. Vww-outlet and Vwo-outlet represent the water volume at water outlet and oil outlet, respectively. Apparently, larger value of E means better separation effect.
3.
Results and discussion
3.1.
Experimental validation of the CFD model
To validate the CFD model, the oil–water separation process was simulated and compared with the experimental result. The operating temperature is 5 ◦ C, and the inlet oil concentration is 10%. The geometric parameters of the conical spiral pipe are: = 15◦ , n = 2.5, h = 50 mm, Dbot = 50 mm, and d = 8 mm. In the experiment, the inlet pressure was measured by an on-line pressure transducer. Its measuring range is 0–20 kPa, and the accuracy is ±0.5 kPa. The experiment result of inlet total pressure was obtained by averaging the pressure value sampled every 10 s during a period of 180 s. The experiment and simulation results were shown in Table 3. The difference between the experiment and simulation is lower than 3.4%.
3.2.
Flow field analysis
3.2.1.
Flow rates at the outlet
Fig. 5 shows the experiment results of water-outlet flow rate Qw-outlet and oil-outlet flow rate Qo-outlet with a pipe diameter of 8 mm, 6 mm, and 4 mm under different inlet Re numbers. For most operating conditions, the flow rates at water outlet are higher than that of oil outlet. That is, there is a flow-rate difference between the water outlet and oil outlet. For the cases with d = 6 mm, the flow-rate difference of the conical spiral pipe is larger than that of spiral pipe. When the pipe diameter decreased to 4 mm, spiral pipe produced a relatively large outlet flow-rate difference. In the case of d = 8 mm, the flow-rate difference between the outlets is not obvious at lower inlet Re number. The outlet flow-rate difference is deemed to be related with the difference of wall shear stress between inner side and outer side of the pipe. It has shown that large velocity difference could induce oil droplet coalescence, and affect oil–water emulsification (Floury et al., 2000). Therefore, outlet flow-rate difference can reflect the flow behavior of oil–water two-phase flow in the pipes.
3.2.2.
Wall Shear stress
Fig. 6 displays the variation of simulated wall shear stress with the spring length (d = 8 mm, = 15◦ , and Uin = 0.1 m/s). In Fig. 6, black line and red line represent the wall shear stress at the outer and inner sides of the pipe, respectively. Obviously, the shear stress at the inner side is lower than that of the outer side. There is a larger difference of wall shear stress between the inner side and outer side in the conical spiral pipe. The wall shear stress at the inner side decreased due to the variation in the gyration geometry of the conical spiral pipe. Shear force could accelerate oil droplets deformation, and
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Fig. 3 – Structures of the oil outlet and water outlet.
Fig. 4 – Flowchart of the oil–water separation experiment. weaken the strength of the interfacial film, thus effectively improve the separation of oil–water mixture (Sjoblom, 2001). In the study, it can be called shear stress effects. Moreover, it is assumed that there exists a critical shear stress which is related to the oil–water separation performance. When the shear stress is less than the critical shear stress, the shear effect is beneficial to oil–water separation; on the contrary, when the shear stress exceeds the critical shear stress, the oil–water separation performance is supposed to deteriorate due to the occurrence of oil–water emulsification.
3.2.3.
Phase distribution
Fig. 7 shows the simulated phase distributions of oil–water flow at different cross-sections (ω = 90◦ , 180◦ , 270◦ , 360◦ , 450◦ , 540◦ , 630◦ , 720◦ , 790◦ , and 900◦ ). The structural parameters of the pipes are: n = 2.5, h = 50 mm, and d = 10 mm. In the figure, = 0◦ and ω = 900◦ denote the spiral pipe and pipe outlet respectively. Fig. 7 indicates that oil-phase flowed along the inner side of the pipe, whereas water-phase flowed along the outer side.
The interfacial turning angle changed with the variation of conical angle. The oil–water interface is wavy and oblique due to the effects of centrifugal force, shear stress and the spiral structure. The interface became distinct and continuous gradually as the increase of spring length.
3.2.4.
Flow pattern
The oil dispersion characteristics determine the oil–water collection efficiency. In the experiment, flow patterns were captured to investigate the stability of the coalesced oil film attached to the inner side of conical spiral pipes. Operating temperature is 5 ◦ C, and the inlet oil concentration is 10%. The range of inlet velocity is 0.05–0.5 m/s. As shown in Fig. 8(a), four different kinds of flow patterns were observed in the experiment. They are dispersion of oil droplet in water and water (Do & w), stretched oil droplet flow and dispersion of oil in water and water (SOD & Do & o/w & w), oil plugs in water and dispersion of oil in water and water (OP & o/w & w), and dispersion oil in water and water in oil (Do/w & w/o).
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Fig. 5 – Experiment results of flow rates at water outlet and oil outlet with different pipe diameters: (a) d = 8 mm; (b) d = 6 mm; (c) d = 4 mm. It was found that the flow pattern was determined by the inlet velocity. In the experiment, Do/w & w flow pattern occurred at low inlet velocity, in which oil phase existed in
the form of oil droplets. When the inlet velocity increased, the oil droplets were stretched into stable oil film, then OP & o/w & w flow pattern formed. After that, the dispersed oil
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Fig. 6 – Simulation results of wall shear stress at the outer side and inner side of pipes: (a) Spiral pipe; (b) conical spiral pipe. droplets constituted consecutive and thick oil film, which indicates a flow pattern transition from OP & o/w & w to SOD & Do & o/w & w. With a further increase of inlet velocity, oil film became intermittent and unstable. Thick oil film was sheared into thin film and small oil droplets by the high-velocity water flow, and, as a result, flow pattern changed to OP & o/w & w. When flow pattern changed to Do/w & w/o, oil–water emulsification appeared which has an adverse effect on separation. It indicates that oil–water emulsification is related to increase of shear stress caused by the increase of flow velocity. Fig. 8(b) shows the visualization sketches of the conical spiral pipe (Case 1# ) and the spiral pipe (Case 0# ) with n = 0.25, 1 and 2. The oil–water interface in the conical pipes was found to be more distinct than that of the spiral pipes. It is easier to form thick oil film at lower spiral pipe round under the same inlet velocity. The oil–water interface became distinct with the increasing of n, especially in the conical spiral pipes. It may be attributed to the decrease of wall shear stress along the inner side of conical spiral pipes. From Fig. 8(c), the oil–water interface became unclear and unstable in spiral pipe as the increase of inlet velocity. When the inlet velocity increased to 0.3 m/s, the interface disappeared. On the contrary, the oil–water interface can be identified clearly in the conical spiral pipe. It implies that the conical spiral pipe has a larger critical wall shear stress. Thus, in a certain degree, conical spiral pipe can avoid the oil–water emulsification.
Fig. 8 demonstrates that the conical spiral structure is beneficial to form the flow pattern of SOD & Do & w/o. The flow pattern with continuous oil film and distinct oil–water interface may contribute to oil–water separation.
3.3.
Effect of outlet split ratio
Outlet structure of conical spiral pipe is a critical issue in oil–water separation process. In the study, a parameter outlet split ratio, r, was defined as the ratio of oil-outlet diameter to the sum of oil-outlet diameter and water-outlet diameter as follows:
r = doo-outlet /(doo-outlet + dow-outlet )
(6)
To investigate the effect of outlet split ratio, the inlet/outlets diameter ratio, which is defined as the ratio of inlet diameter to the sum of oil-outlet diameter and wateroutlet diameter, is set to be 0.625. The diameter of water outlet is 3 mm, 2.5 mm, and 2 mm; correspondingly, the diameter of oil outlet is 2 mm, 2.5 mm, and 3 mm. The outlet split ratio, r, equals 0.4, 0.5, and 0.6. The pipe diameter, d, is 8 mm. The oil-phase collection efficiencies with three different values of r were measured. Fig. 9 shows the variations of collection efficiency and dimensionless flow rate ratio with the inlet
Fig. 7 – Simulation results of cross sectional phase distribution of oil–water flow in spiral pipes.
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Fig. 8 – Flow patterns of oil–water two-phase flow: (a) Various oil–water flow patterns; (b) Flow pattern in different locations; (c) Flow pattern under different inlet velocities. velocity. Here, the outlet flow-rate ratio, Q outlet , was defined by Eq. (7).
Q outlet =
Qo-outlet Qw-outlet
(7)
where Qo-outlet and Qw-outlet are the flow rate at oil outlet and water outlet, respectively. Fig. 9 shows that higher collection efficiency can be achieved by reducing the outlet split ratio and inlet velocity. A maximum value of collection efficiency can be obtained with r = 0.4 and Uin ≈ 0.1 m/s. On the other hand, the pipe with lower value of r produced higher outlet flow-rate ratio. In addition, collection efficiency was found to be reversely proportional to the flow rate ratio. That is, higher flow rate ratio means a lower collection efficiency, and vice versa.
3.4.
Effects of temperature and inlet oil concentration
3.4.1.
Effect of temperature
Typically, dynamic viscosity (or viscous force) changes with the temperature changing. Therefore, operating temperature may have an impact on oil–water emulsification and separation performance. In this section, oil–water separation efficiency was measured under different operating temperatures. To begin with, the variations of dynamic viscosity of oil and water with the temperature were measured by a viscositymeter, as shown in Fig. 10(a) and (b). Dynamic viscosity of both the gear oil and water decreases as the increase of temperature in the range of 5 ◦ C–70 ◦ C. In the oil–water separation experiment, the oil concentration is 10%, and the inlet velocity is 0.20 m/s. The geometric parameters of the experimental pipe are: h = 50 mm,
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Fig. 9 – Effect of inlet velocity on oil-phase collection efficiency and outlet flow-rate ratio. Dbot = 50 mm, = 15◦ , and d = 8 mm. The collection efficiency was obtained and depicted in Fig. 10(c). From the figure, it can be found that the collection efficiency increased with the increase of operating temperature in the range of 5 ◦ C–40 ◦ C. When the temperature was raised up to 40 ◦ C, the collection efficiency began to fluctuate within a range. It indicates that higher operating temperature leads to better collection efficiency.
3.4.2.
Effect of inlet oil concentration
The effect of inlet oil concentration on oil–water separation performance was examined. The operating temperature is 10 ◦ C, the inlet velocity is 0.40 m/s, and the inlet oil concentration increases from 10% to 50%. The geometric parameters of the conical spiral pipe in the experiment are: h = 50 mm, Dbot = 50 mm, = 15◦ , and d = 6 mm. As illustrated by Fig. 11, higher oil concentration has an adverse effect on oil-phase collection efficiency. This is because it is more likely to form the flow patterns of OP & o/w & w and dispersion oil in water and water in oil (Do/w & w/o) with higher inlet oil concentration. These flow patterns could produce serious oil–water emulsification that hinders the oil–water separation.
3.5.
Effect of geometric parameter
3.5.1.
Effect of conical angle
The effect of conical angle on oil–water separation was investigated. The operating temperature is 5 ◦ C, the inlet oil concentration is 10%, and inlet velocity is 0.50 m/s. The range of conical angle is 0◦ –90◦ . Other geometric parameters are: h = 50 mm, d = 6 mm, and Dbot = 50 mm. First, the inlet-outlet total pressure drop P was obtained by CFD calculation. Fig. 12(a) shows that the trend of change of pressure drop with conical angle is smooth, except for = 85◦ . When conical angle increased to 85◦ , pressure drop increased greatly. This is mainly because the increase of pipe length. For the pipe with higher conical angle, an increase of conical angle leads to a significant increase of pipe length, and, consequently, results in the increase of pressure drop. It should be noted that there is a significant decrease of pressure drop when conical angle increased to 90◦ . The conical spiral pipe becomes a flat-type spiral pipe when conical angle increased to 90◦ . Compared with the conical spiral pipe with = 85◦ , the pipe length of
Fig. 10 – Effect of temperature on oil viscosity, water viscosity and oil-phase collection efficiency: (a) Oil viscosity; (b) Water viscosity; (c) Oil-phase collection efficiency.
the flat-type spiral pipe is reduced. Moreover, the geometric structure difference between the conical spiral pipe and the flat-type spiral pipe may induce the significant difference of flow resistance. In addition to pressure drop, the oil-phase collection efficiency was experimentally measured under different inlet velocities. Fig. 12(b) shows the variation trend of collection efficiency of spiral pipes and conical spiral pipes. It indicates that the conical spiral pipe with = 15◦ produced the highest oil-phase collection efficiency. It also demonstrates that
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h = 50 mm, Dbot = 50 mm, and = 15◦ . CFD results in Fig. 13(a) indicate that pipe diameter is inversely proportional to pressure drop. Apparently, larger pipe diameter can reduce the flow resistance, and consequently, produce lower pressure drop. In Fig. 13(b), experiment results indicate that the collection efficiency of conical spiral pipes is higher than that of spiral pipes. Moreover, the oil–water separation effect of conical spiral pipes with d = 8 mm or 6 mm are better than the pipe with d = 4 mm. The difference in efficiency may be attributed to the flow pattern transition.
3.5.3.
Fig. 11 – Effect of inlet oil concentration on oil-phase collection efficiency. compared to spiral pipe, conical spiral pipe exhibits better oil–water separation capacity.
3.5.2.
Effect of pipe diameter
The effect of pipe diameter on flow resistance and collection efficiency was also investigated. The operating temperature is 5 ◦ C, and the inlet oil concentration is 10%. The range of pipe diameter is 4–8 mm. Other geometric parameters are:
Effect of pitch height
The effect of pitch height on oil–water separation was studied. In the experiment, operating temperature is 5 ◦ C, and the inlet oil concentration is 10%. The range of pitch height is 15 mm–50 mm. It should be noted that the average conical diameter, D, increases as the increase of pitch height. Other geometric parameters are: d = 10 mm, and = 15◦ . Fig. 14 shows the variation of oil-phase collection efficiency with the pitch height under different inlet flow rates. In general, oil–water separation is affected by the pitch height. However, no clear trend was found for the influence of pitch height on the collection efficiency. On the other hand, as the increase of inlet flow rate, the collection efficiency decreased. This may be attributed to the occurrence of oil–water emulsification as discussed in Section 3.2.4.
Fig. 12 – Effect of conical angle on pressure drop and oil-phase collection efficiency: (a) Pressure drop; (b) Oil-phase collection efficiency.
Fig. 13 – Effect of inlet velocity (or inlet Re number) on pressure drop and oil-phase collection efficiency under different pipe diameters: (a) Pressure drop; (b) Oil-phase collection efficiency.
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Fig. 14 – Variations of Re number, De number and collection efficiency with the pitch height under different inlet flow rates: (a) Qin = 0.0565 m3 /h; (b) Qin = 0.038 m3 /h; (c) Qin = 0.019 m3 /h. The secondary flows that influence the separation of oil–water mixture in conical spiral pipes are mainly generated by inertia force, viscous force, and centrifugal force. In this section, outlet Re number and De number were introduced to characterize the variation trend of oil-phase collection efficiency. Re number represents the ratio of flow inertia force to viscous force, whereas De number represents the ratio of inertia force, centrifugal force, and viscous force (Di Carlo et al.,
2007; Hou et al., 2013). The definitions of outlet Re number and De number in this study are given as follows:
Re =
Ud
d 12
De = Re
D
(7)
(8)
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Fig. 15 – Surface treatment of conical spiral pipe: (a) V-shaped microstructure pipe; (b) Graphene-coating pipe. Variations of outlet Re number and De number with the pitch height were also depicted in Fig. 14. Obviously, the variation trend of collection efficiency with the pitch height is different from that of Re or De. Nevertheless, similar trend between oil-phase collection efficiency and Re or De can be found under some specific conditions. For example, when the flow rate is 0.019 m3 /h, the variation of E was synchronized with De under the condition of h < 30 mm or h > 40 mm; whereas for Re number, its variation trend corresponded to the collection efficiency when the pitch height changed from 30 mm to 50 mm. When flow rate increased to 0.038 m3 /h, the variation trend of E is similar with that of De under the condition of 20 mm < h < 30 mm or h > 40 mm; whereas the positive correspondence relationship between E and Re was found under the condition of 30 mm < h < 50 mm. When the flow rate is 0.0565 m3 /h, the variation of E was synchronized with Re within the entire range of pitch height; whereas the positive correspondence relationship between De and E existed for h < 40 mm. From Fig. 14, it could be deduced that neither Re number nor De number is able to predict the variation trend of collection efficiency. Alternatively, a combination of Re and De is possibly an appropriate parameter to predict the changing tendency of oil-phase collection efficiency.
3.6.
Effect of surface treatment
The well-wetted wall surface with high roughness has been shown to exhibit underwater superoleophobicity and unique adhesive behaviors (Chen et al., 2016). In the study, four types of conical spiral pipe, smooth PU pipe, 3D-printed abrasive pipe, Graphene-coating pipe, and V-shaped surface microstructure pipe, were fabricated to investigate surface effect in oil–water separation. As shown in Fig. 15, V-shaped microstructure pipe is featured by the sawtooth surface microstructure. The number of the sawtooth is 36, and the height of the sawtooth is 2 mm. They are arranged uniformly around the surface of the conical spiral pipe. Graphene-coating pipe is fabricated by coating the inner side of the pipe with hydrophobic Graphene. The hydrophobic Graphene is composed of 8% Graphene powder, 1.5% carbon micro-tube, epoxy resin, and epoxy curing agents. Table 4 shows the surface roughness of the pipes measured by a surface roughness tester. Surface roughness was characterized by three parameters. Ra is the contour arithmetic mean deviation; Rz is the ten-point height average; Ry is the maximum contour height. The results indicate that smooth PU pipe has the smoothest surface, whereas V-shaped microstructure pipe has the roughest surface.
Table 4 – Surface roughness of the pipes. Pipe type
Ra (m)
Rz (m)
Ry (m)
Smooth PU Pipe 3D-printed abrasive pipe V-shaped microstructure pipe Graphene-coating pipe—inner side Graphene-coating pipe—outer side
0.10 14.72 21.34 0.18 9.07
0.37 51.81 61.98 0.46 44.90
0.56 68.28 83.68 0.84 54.29
Fig. 16 shows the oil-phase collection efficiency of the pipes under different inlet velocities. For lower inlet velocity (Uin = 0.1 and 0.2 m/s), V-shaped microstructure pipe obtained the best oil–water separation performance. When inlet velocity increased to 0.3 m/s, 3D-printed abrasive pipe produced the highest oil-phase collection efficiency. Compared to low inlet velocities, Graphene-coating pipe exhibits better oil–water separation capacity at Uin = 0.3 m/s. This may be attributed to the adhesion between the oil film and Graphene-coating surface, which contributes to the formation of the flow pattern of SOD & Do & o/w & w or OP & o/w & w and suppresses the oil–water emulsification.
4.
Conclusion
The oil–water two-phase flow dispersion characteristics and oil–water separation capacity of conical spiral pipes were investigated. Phase distribution, wall shear stress, and pressure drop of oil–water two-phase flow in conical spiral pipes were obtained by CFD calculations. Flow patterns of oil–water flow were identified by visual observations. Outlet flow rate
Fig. 16 – Collection efficiencies of the pipes under different inlet velocities.
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and collection efficiency under different operating conditions were measured experimentally. In the conical spiral pipes, oil and water flowed along the inner side and outer side, respectively. The wall shear stress at the outer side is larger than that at the inner side of the conical spiral pipe. There is an oil–water interface in the cross-section of the pipe. As the inlet velocity increased, flow pattern transition occurred in the pipe, and the interface became unclear or even disappeared. Oil–water emulsification is more likely to appear under relatively high inlet velocity. Effects of geometric structure and operating conditions on collection efficiency were studied. Either low inlet velocity or low inlet oil concentration is beneficial to oil–water separation. Conical spiral pipes can produced higher separation effect at high temperature due to the decrease of fluid viscosity. In particular, the oil-phase collection efficiency increased monotonously with the increase of temperature in the range of 5–40 ◦ C. In terms of oil-phase collection efficiency, conical spiral pipes exhibit better separation capacity than spiral pipes. Moreover, the conical spiral pipe with = 15◦ achieved the maximum oil-phase collection efficiency. The relationship between the pitch height and the separation performance are complex. Experimental results indicated that the collection efficiency was maximized with a pitch height of 40 mm for lower inlet flow rate. For a larger flow rate, the conical spiral pipe with a pitch height of 20 mm is preferred. In other hand, some parameters such as outlet flow-rate ratio, Re and De can reflect the separation effect. A combination of Re and De is possibly an appropriate parameter to predict the changing tendency of oil-phase collection efficiency of the conical spiral pipes. Effects of surface treatment on oil–water sedation were also studied. Collection efficiencies of four different kinds of conical spiral pipe were measured. It has shown that increasing surface roughness can enhance oil–water separation. And, increasing wall adhesion between the oil film and the surface is a promising method to suppress oil–water emulsification and improve separation performance under high inlet velocity.
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