International Journal of Fluid Machinery and Systems DOI: http://dx.doi.org/10.5293/IJFMS.2017.10.1.086 Vol. 10, No. 1, January-March 2017 ISSN (Online): 1882-9554 Original Paper

Numerical Simulation on the Performance of Axial Vane Type Gas-Liquid Separator with Different Guide Vane Structure

Yang Fan1, 2, Liu Ailan1 and Guo Xueyan1, 2

1 School of Energy and Power Engineering, University of Shanghai for Science and Technology, No. 516, Jungong Road, Shanghai, 200093, PR China, usstyf@126.com, ailanliuxia@163.com, xyguo@usst.edu.cn

2 Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering, University of Shanghai for Science and Technology, No. 516, Jungong Road, Shanghai, 200093, PR China

Abstract

In order to obtain high efficiency and low resistance droplet separation apparatus, axial vane type gas-liquid separators with different guide vanes were designed, and the RNG -k ε model as well as discrete phase model (DPM) were used to investigate the flow pattern inside the separators. It was shown that the tangential velocity distribution under different guide vanes have Rankine vortex characteristics, pressure distribution exhibits a high similarity which value becomes big as the increase of the blade outlet angle and the decrease of the guide vane numbers. The increase of the guide vane numbers and the decrease of the blade outlet angle could make separation improve significantly. The separation efficiency is almost 100% when the droplet diameter is bigger than 40μm.

Keywords: axial vane type gas-liquid separator; numerical simulation; guide vane; separation efficiency

1. Introduction Gas-liquid separator is an important equipment for two-phase separation in the oil, natural gas and chemical industry, which is

classified into tangential or reversal type separator and axial flow separator according to the direction of fluid flowing in the apparatus[1-3]. The axial type cyclone separator was first presented in 1948[4]. Compared with reversal type separator, the axial type one has simple structure and does not exist internal swirl flow within the cyclone, which pressure drop is much lower[5].

The axial vane type gas-liquid separator is mainly composed by a diversion cone, guide vanes, vortex tube and exhaust pipe. The working principle is that gas containing droplets flows into the separator, and then axial motion was turned into rotary motion after through the guide vanes, which provides a tangential acceleration to the mixture, thereby forming a distributed centrifugal field inside the separator. Due to the density difference between gas and droplets, the latter moves to the wall and is collected at the bottom, while the purified gas flows from exhaust pipe. Since the fluid enters axially and the swirl was generated by the guide vanes, the direction of airflow inside the cyclone doesn't change and the friction is low, which maintained the stability of the main airflow, the pressure loss is relatively smaller [6-8].

The flow in separator is a gas-liquid two-phase flow, and the separation process is complex. Many experiments were conducted to study flow pattern in the cyclone. Although consuming a lot of manpower and material resources, there is no universal results obtained.

Along with the rapid development of computer technology, modern computational fluid dynamics (CFD) techniques have proved to be a useful tool for the study of the gas-solid flow and separation phenomena that occur within a cyclone. The application of CFD technology has gradually expanded from aerospace to various industries related with the flow and heat transfer[9-12].One of the first CFD simulations of the fluid flow in an industrial cyclone was performed by Boysan et al. [13] using a hybrid turbulence model based on the algebraic stress model combined with the -k ε turbulence model. It detected that the standard -k ε turbulence model was inadequate to simulate flows with strong swirl because it led to excessive turbulence viscosities. Later, Fraser et al.[14] developed a modified -k ε model to simulate the three-dimensional flow in cyclones, proving that the modified -k ε model provide better agreement with the experimental data. Hoekstra and Derksen[15] made a comparative study to standard -k ε model and RNG -k ε model. Guangcai Zhou et al.[16] investigated the effect of helix angle and leaf margin on the flow pattern and the performance of the axial flow cyclone separator, which proved the reliability and accuracy of the simulation method. CFD technology has been proven to be an effective tool to investigate the two-phase flow and separation in a cyclone separator.

Received July 24 2016; revised October 13 2016; accepted for publication January 7 2017: Review conducted by Xuelin Tang. (Paper number O16024J) Corresponding author: Yang Fan, usstyf@126.com

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In this paper, flow pattern inside the axial type cyclone separator was simulated using FLUENT software, moreover, the influence of the structure parameters of the guide vanes on the flow field, pressure drop as well as separation efficiency was obtained.

Fig. 1 Structure of axial guide vane separator

Fig. 2 Schematic diagram and three-dimensional model of guide vane

Table 1 Geometric dimensions of axial vane type gas-liquid separator

Notations Definition Value(mm) D Cyclone diameter 200 D0 Hub diameter 50 Dl Liquid-out diameter 20 Dg Gas-out diameter 140 H Total length of gas-liquid separator 1028 Hw The length of cyclone tube 880 Hg The length of exhaust pipe 400 Hb The height of guide vanes 100 α The blade outlet angle 5°~45°

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2. Geometrical model and boundary conditions 2.1 Geometrical model

Structure of axial guide vane separator and three-dimensional structure of guide vanes is shown in Fig. 1 and Fig. 2. The separator mainly comprises guide vanes, diversion-cone, outer cylinder wall, exhaust tube and discharge tube. The blade outlet angle α is the angle between the tangential direction and the horizontal direction. Figure 1 shows the notations of the axial guide vane gas-liquid separator and Table 1 gives their definitions and values. The blade outlet angle and the number of the vanes were variables during the simulations.

2.2 Grid generation and boundary conditions In this paper, CATIA was used to create axial guide vane type separators' geometrical model, and ICEM-CFD was used to

generate structural mesh. The grid space is smaller near the wall and the central axis of the regional, while which is larger far away from the wall. Meanwhile the boundary layer grids are divided near the cylindrical wall and the vane wall so as to access better flow of the wall in the separator. Due to a number of separators different structure of the gas-liquid separators is created, the total number of grid is about 4 million.

Fig. 3 Grid of the guide vane type gas-liquid separator

During the single-phase flow simulation, fluid is air and the inlet velocity is 6m/s. The Reynolds number 58.3 10Re = × . The outlets are set to be pressure boundary conditions, which is the standard atmosphere pressure. No slip boundary conditions are used in the wall. SIMPLE algorithm was adopted and the convection term was separated by the central difference scheme. In order to make the calculation easier to converge and shorten the calculation time, the time step is set to 10-5s, and then change to 10-4s after converges.

3. Numerical method 3.1 RNG -k ε turbulence model

The RNG -k ε model is the improvement of the standard -k ε model, in which Reynolds stress term is described as:

jii tj

j i

uuu ux x

ρ µ ∂∂′ ′− = + ∂ ∂

(1)

There is a direct relationship between the turbulent viscosity tµ and the turbulent kinetic energy k and the turbulent energy dissipation rateε . Turbulent kinetic energy k and turbulent energy dissipation rate can also be obtained by transport equation[17]:

( ) 2ik ef t ij ij

i j j

k uk k S St x x x

ρρ α µ µ ρε ∂∂ ∂ ∂

+ = + − ∂ ∂ ∂ ∂ (2)

( ) ( ) ( )3 220

1 2 3

1 /2

1i

ef t ij iji j j

CuC S S C

t x x x k kµ

ε ε ε

ρη η η ερερε ε ε εα µ µ ρβη

−∂∂ ∂ ∂+ = + − − ∂ ∂ ∂ ∂ +

(3)

Equation (2) and (3) is the governing equations of turbulent kinetic energy and turbulent energy dissipation rate in the RNG -k ε model. kα and εα are coefficients related to Prandtl number, and 1 2 0, , , , ef C Cε εµ η η and β are coefficients of

control equation, which is determined by the empirical formula. ijS represents the strain rate tensor, which is defined as:

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jiij

j i

uuSx x

∂∂= + ∂ ∂

(4)

The average flow in the cyclone and rotary flow is considered in RNG -k ε model. In ε equation, the last term of RHS is extra term, which takes the impact of mean strain rate on the dissipative term into account. All of this makes RNG -k ε model could deal with high strain rate and streamline curvature flow [18-19].

3.2 Discrete Phase Model (DPM) model In DPM model, the parameters of the continuous phase flow field are calculated in the Eulerian coordinates, and then the

motion trajectory of a single discrete phase droplet is calculated based on the flow field in the Lagrangian coordinate.

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The equilibrium equation of the droplet is:

( ) ( )x ppD p x

p

gduF u u F

dtρ ρρ−

= − + +

(5)

2

18 ReDD

p p

CFd

µρ

=

(6)

where ( )D pF u u−

is the drag force per unit mass of droplet; xF is other forces; u represent fluid relative velocity; pu is

droplet velocity; µ represent for fluid dynamic viscosity; ρ is droplet density; pd is the droplet diameter; Re represent for the

droplet Reynolds number; DC is the drag coefficient.

The velocity of the droplet in the trajectory pu is obtained by the time integral of the eq. (5).

pdx udt

=

(7)

The droplet trajectory can be obtained by solving eq. (7). Within a short time interval, assuming every term is constant, and the track equation of the droplet is simplified as:

( )1pp

p

duu u

dt τ= −

(8)

where pτ is the relaxation time of the droplets.

In DPM, second phase (discrete phase) is assumed to been thin enough (discrete phase volume fraction is less than 10%), thus the impact of discrete phase volume fraction on the continuous phase is neglected [20-22]. In this study, volume ratio of droplet is relatively small (less than 1%), and requirements of DPM is satisfied. Therefore, DPM model was adopted to track droplets' trajectory.

4. Simulation results and analyses In order to study the effects of the blade outlet angle on the flow field, pressure drop characteristics and separation efficiency,

separators with six numbers of guide vanes and five numbers of blade outlet angle namely 5°,15°,25°,35°and 45° respectively were simulated.

5α = ° 15α = ° 25α = ° 35α = ° 45α = °

Fig. 4 Distribution of longitudinal sectional static pressure of different blade outlet angle (unit: Pa)

4.1 Effect of blade outlet angle on the performance of the separator 4.1.1 Pressure drop varies with the blade outlet angle

Longitudinal sectional static pressure contours and pressure drop curves at different vane outlet angel were shown in Fig. 4 and Fig. 5. By comparing Fig. 4, it can be seen that the static pressure distribution in five cases are generally similar to each other, only the values are different. The pressure drop decreases radial from wall to centre and reach their minimum values in the inlet of the exhaust pipe, and even displays negative pressure. According to the graph, the static pressure of 5,15,25,35 and 45 degree

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blade outlet angle separator are 185Pa, 137Pa, 126Pa, 115Pa and 104Pa, respectively, which decreasing trend is obvious. Meanwhile, with the blade outlet angle increases, the exhaust pipe imports static pressure reduces gradually.

0 10 20 30 40 50

100

120

140

160

180

200

pres

sure

dro

p (P

a)

outlet angle/(degree)

Fig. 5 Pressure drop curves changes with different blade outlet angle (unit: Pa)

Pressure drop within the separator are mainly composed by the flow loss in and out of the guide vanes, near the exhaust pipe and the loss of the kinetic energy of the fluid. Under the same conditions, the smaller the blade outlet angel, the smaller the cross-sectional area when fluid flow out of the guide vanes, which will make the flow rate increase under same flux. All of these resulted the local resistance increases when the fluid leaving the guide vanes. As a consequence, the smaller the blade outlet angle, the greater the pressure drop. 4.1.2 Effect of blade outlet angle on tangential acceleration

To investigate the swirling intensity after the fluid through the guide vanes at different blade outlet angles, four horizontal lines passing through the axis of four planes were cut to analyze the ratio of tangential acceleration to gravity acceleration. The four lines position are shown in Fig. 6.

Fig. 6 The plane of different vertical height

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5α = ° 15α = ° 25α = ° 35α = ° 45α = °

Fig. 7 Tangential velocity contours with different blade outlet angles

(a) z=-200mm (b) z=-300mm

(c) z=-400mm (d) z=-500mm

Fig. 8 Variation of the ratio of tangential acceleration to gravity acceleration at different blade outlet angles

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The tangential velocity is the dominant velocity component in the separator and the key factor responsible for the centrifugal force. It has a direct effect on the droplet separate in efficiency as when the tangential velocity increases, the centrifugal force and separation efficiency will increase. Figure 7 is tangential velocity contours under different blade outlet angles. It can be seen that the tangential velocity distribution is well axisymmetric. Figure 8 is the ratio of tangential acceleration to gravity acceleration varies with radial direction under different blade outlet angles. The abscissa is the radial coordinate position of the separator and the ordinate is the ratio of tangential acceleration to gravity acceleration, while “a” represents tangential acceleration. According to Fig. 8, we can see that the tangential acceleration is similar to the "hump" distribution, which value increases firstly, and then decreases radial from centre to wall and reaches minimum value that equal to zero in the wall. This tangential acceleration distribution conforms to Rankine vortex characteristics, namely the center is characterized by a forced vortex distribution, and the outside is surrounded by the quasi free vortex distribution [23]. The Rankine vortex distribution is very advantageous to the separation of gas and liquid, as the centrifugal effect of the forced vortex at the center is conducive to make the droplet moves to external and the rotation strength to carry droplets of the external quasi free vortex is relatively lower, especially to large droplets, which make it is easier to be trapped in the wall. Figure 8 also shows that the ratio along the flow direction displays a decreasing trend generally in the separation section. The blade outlet angle has strong effect on the tangential velocity distribution. As the outlet angel decreases, the tangential velocity increases. The efficiency of the separator with 5° blade outlet angle is the highest, and which reduction tendency is also sluggish. The closer to the end of the separation section, the smaller the ratio and the more slowly the weakening trend.

When the fluid carrying the droplets flows into the separator, the axial motion is turned into rotary flow under the effect of the guide vanes, while the flow intensity along the axial direction gradually weakened due to the tube wall friction and viscous. If the vanes outlet velocity is 0v , the tangential and axial velocity are tv and zv , respectively (neglecting the radial velocity), then we can get the following relationship:

0 0t zv v cos v v sinα α= =， (9)

When 45α < ° , the tangential velocity is greater than axial velocity, so the smaller the α , the greater the tangential velocity. Tangential velocity provides centrifugal force to the fluid, so the greater the tangential speed, the stronger the centrifugal force. Therefore, the ratio of tangential acceleration to gravitation acceleration rises with the reduction of the blade outlet angle. While in the process of flowing, the vortex intensity will gradually weaken as the impact of resistance and wall friction, so the ratio reduced on the overall trend. The tangential acceleration is 2 /ta v r= . Near the axis, even though the tangential velocity is relatively small, the tangential acceleration will be very high due to the minimum radius value; But because of the weak disturbance near the axis and the effect of shear friction force near the wall lead to the ratio of tangential acceleration to gravity acceleration rises at first and then reduced along the radial direction. However, the blade outlet angle cannot be too small, or the vortex speed will decay rapidly and the liquid will not be sufficiently separated [24]. Furthermore, low axial speed will cause some un-separated fluid flow into the exhaust pipe directly, which will lead to the separation efficiency drops. 4.1.3 The separation efficiency of different blade outlet angle varies with droplet diameter

For studying the flow of the droplets in the separator, the droplet separation and trajectories with different guide vane structures are simulated by using DPM model in this section. The liquid phase is set to be water-liquid and the droplet size varies from 5 to 40μm. Droplets were injected from inlet surface and the inlet velocity is same to the air flow. The bottom surface of the cylinder wall, the liquid pipe wall and the water outlet are set to “trap”, namely the droplets were captured after collided with the wall. The other wall is set to “reflect” and the gas outlet is set to “escape”. The separation efficiency can be calculated by the following equation

= 100%t

in

NN

η × (10)

where tN represent the amount of droplet that were captured from the water outlet, and

inN is the total number that injected from the inlet. Figure 9 shows the separation efficiency under different blade outlet angle. According to Fig. 9, it can be seen that the blade outlet angle has a great impact on the separation efficiency. When the droplet diameter is less than 10μm, the separation efficiency of the five cases have a little difference. While when droplet size is greater than 10μm, separation efficiency increases obviously with the blade outlet angle decreases. When droplet size is bigger than 40μm, separation efficiency of five cases is almost close to 100%. Figure 10 is the trajectory distribution of five cases blade outlet angle when the droplet diameter is 20μm. It can be seen that with the same number of injection droplets, the smaller the blade outlet angle, the less droplets that flow from exhaust pipe, which means the higher the separation efficiency.

Under the same flow rate, the smaller the blade outlet angle is, the larger the tangential velocity. The increased tangential velocity will lead to an increase in the centrifugal force and lead to an increase in the spiral ability of the gas flow after through the guide vanes, which will improve the separation efficiency. However, the blade outlet angle is not as small as possible. This phenomenon could be seen from the Fig. 9. The separation efficiency of the separator at the blade outlet angle of 15° is larger than that of 5 ° when the droplet diameter is 25μm. Although the tangential velocity is large enough at small blade outlet angle, the impulse force of the gas flow on the separator will increase and the axial velocity will become small, which will lead to the spiral ability of the gas flow decay faster. Consequently, the separation performance maybe been weakened as the flow have not sufficient separation space and separation time. In order to ensure a large separation efficiency, droplet size is preferably bigger than 20μm for different structures separator.

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Fig. 9 Curves of separation efficiency under different blade outlet angle changes with droplet size

5α = ° 15α = ° 25α = ° 35α = ° 45α = °

Fig. 10 The trajectory distribution of five cases blade outlet angle at 20μm droplet size 4.2 Effect of the numbers of guide vanes on the separation performance

In order to investigate different number of guide vanes on the separation performance, the separator with four, six, eight, ten and twelve vanes were simulated respectively. In the process, the blade outlet angle was 35 degree. The distribution of pressure drop and separator performance in the separator with different number of vanes is obtained. 4.2.1 Effect of the numbers of vanes on the separator pressure drop

The pressure contour and pressure drop versus of the separator with different number guide vanes were shown in Fig. 11 and Fig. 12, respectively.

According to Fig. 11, we can see that the separator's profile static pressure exhibit a very high similarity when changing the number of the guide vanes, and as the relatively thinner thickness of the vanes during the simulation, whose influence on the cross-sectional area could be neglected. In general, the more numbers of the vanes, the smaller the distance between the vanes and the stronger the rotary intense produced by the guide vanes, which is advantageous to droplet separation. However, more vanes number will cause an increase in the resistance and also lead the pressure drop increased sharply. As a result, the pressure drop increases with the increase of the vane numbers. While during the simulation, as the wall surface setup was simplified, which made the pressure drop is smaller than the practical pressure drop. Figure 11 also shows that the change of pressure drop is very slightly at different guide vanes, the effect of guide vanes on the pressure is mainly reflected in the vanes area and the exhaust pipe inlet region.

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4 guide vanes 6 guide vanes 8 guide vanes 10 guide vanes 12 guide vanes

Fig. 11 Distribution of longitudinal sectional static pressure of different guide vane numbers (unit: Pa)

Fig. 12 Pressure drop changes with different guide vanes

4 guide vanes 6 guide vanes 8 guide vanes 10 guide vanes 12 guide vanes

Fig. 13 Tangential velocity contours with different guide vane numbers

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4.2.2 Effect of the numbers of vanes on the velocity within the separator In order to study the velocity in the separator, 4 horizontal lines along the radial direction at the same position of the separation

section were intercepted to analyze the ratio of tangential acceleration to gravity acceleration. Figure 13 shows the tangential velocity contours with different guide vane numbers. Figure 14 is the ratio of tangential acceleration to gravity acceleration varies with the radius at different number vanes.

(a) z=-200mm (b) z=-300mm

(c) z=-400mm (d) z=-500mm

Fig. 14 Changes of the ratio of tangential acceleration to gravity acceleration at different number vanes

Fig. 15 Curves of separation efficiency under different number guide vanes changes with droplet size

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According to Fig. 13 and Fig. 14, it can be seen that the tangential acceleration distribution also conforms to Rankine vortex characteristics, namely the center is characterized by a forced vortex distribution, and the outside is surrounded by the quasi free vortex distribution. The ratio increases radial first and then decreases from wall to centre and reach zero at the wall. The tangential velocity contours shows that its distribution present a good axial symmetry, and with the increase in the number of vanes, the tangential velocity values on both sides also increase.

According to Fig. 14 it can be seen that the more the number of the vanes is, the greater the ratio. The maximum value of ratio is close to 150, and the overall trend is gradually decreased in the separation section, which was in conformity with the rotation intensity gradually weakness after the guide vanes. The ratio increases first and then decreases along the radial direction. This distribution is because the small disturbance and weak rotation intensity of the airflow at the center of the separator, and from the axial to outward, the swirling strength became stronger and the centrifugal force of the droplet became larger as well due to the guide effect of the vanes, but the velocity decreased rapidly near the wall. This phenomenon is advantageous to improve the separation efficiency as during the droplet moves from axial to wall, the centrifugal force is bigger at first and then becomes weaker approaching the wall, which avoid the strong collision with the wall. According to the picture we can also see that the ratio of tangential acceleration to gravity acceleration of four vanes along the flow direction decrease more slowly than that of six and eight vanes, and the value is even larger than six and eight vanes at the axial position, which indicate that the rotation intensity and pressure drop of 4 vanes are smaller than 6 and 8.

4 guide vanes 6 guide vanes 8 guide vanes 10 guide vanes 12 guide vanes

Fig. 16 The trajectory distribution of five kinds of vanes numbers at 20μm droplet size

4.2.3 The separation efficiency at different vanes varies with droplet diameter The separation efficiency varies with droplet size under different number guide vanes is shown in Fig. 15, and the trajectory

distribution of five cases at 20μm droplet size is shown in Fig. 16. According to those two figures, it can be seen that size of the droplet has a very important effect on the separation efficiency, which is not so obvious at small diameter. While the diameter reaches 10μm and above, the efficiency apparently rises with the increases of the vanes at the same droplet diameter. Six and eight guide vanes separators’ efficiency were relatively similar, but higher than 4 vanes; When the droplet diameter was bigger than 40μm, the efficiency of the six types separator with different number guide vanes was almost 90%.

Figure 16 also shows that under the same number of droplet injection, the more the number of guide vanes, the more droplets collected from water-outlet and the higher the separation efficiency. Two phase flow in gas-liquid separator is a strong three-dimensional turbulent flow and the movement is very complicated. The motion of small droplets has great randomness and easily affected by the air turbulence. The increased number of the vanes will lead to an increase in centrifugal force. The excessive number of the guide vanes is at the expense of the pressure loss in the separator. The increase number of the guide vanes will aggravate the collision between the droplets and increase the pressure loss, which will also make the large droplets coming together broken into small droplets. All of those phenomena are not conducive to the water droplets collection.

5. Conclusion In this paper, the axial guide vane type gas liquid separator with different blade outlet angle and different number of vanes

were simulated, and the pressure drop characteristics, velocity distribution as well as separation efficiency at different droplet sizes were analyzed. Based on the presented results, the following conclusions can be drawn:

(1) The pressure distribution with different guide vane structures exhibits very high similarity. The pressure drop has an

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obvious increasing trend with the decrease of the blade outlet angle and the increase of the number of the guide vanes. The low pressure zone within the separator mainly appears in the vicinity of the rotary vane, leaf axis rear center and the entrance of exhaust pipe.

(2) The tangential velocity distribution conforms to Rankine vortex characteristics, namely the center characterized by a forced vortex distribution, and the outside surrounded by the quasi free vortex distribution. The tangential velocity increase radial from centre to wall and reach zero at the wall of the cyclone. The Rankine vortex distribution is very advantageous to the separation of gas and liquid, as the centrifugal effect of the forced vortex at the center is conducive to make the droplet moves to external and the rotation strength to carry droplets of the external quasi free vortex is relatively lower, especially to large droplets, which make it is easier to be trapped in the wall.

(3) The separation efficiency increases with the increase of the droplet size. Both the decrease of the blade outlet angle and the increase number of the guide vane could improve the separation efficiency, but which will also make the pressure drop rise. When the droplet diameter is less than 10μm, the separation efficiency of different cases have a little difference. While when droplet size is greater than 10μm, the separation efficiency increases obviously with the blade outlet angle decreases. The separation efficiency of all cases is almost close to 100% when the droplet size is bigger than 40μm. In order to ensure high separation efficiency, droplet size is preferably bigger than 20μm for different structures separator.

Acknowledgments The study is supported by Science and Technology Commission of Shanghai Municipality (Grant Nos. 13ZR1428700,

13DZ2260900).

Nomenclature

tN Amount of droplet be captured zv Axial velocity

inN Total number of droplet in inlet α Blade outlet angle

p∆ Pressure drop η Separation efficiency v Inlet velocity µ Dynamic viscosity

0v Outlet velocity ρ Density

tv Tangential velocity pτ Relaxation time of droplet

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