Numerical simulation of the complex flow field in a new recycling cyclone separator

The three-dimension flow field is simulated in a recycling cyclone separator using the Reynolds stress model (RSM) with Fluent 6.2. The results show that the unique design of the flow route of the recycling cyclone separator can improve the axial symmetry and the order of the inner flow field. The appearance of the eddy flow and the pressure drop of the recycling cyclone separator are also reduced. Moreover, the secondary circulation in the recycling cyclone separator is analyzed, which is useful for understanding the flow mechanism. This study provides a reference for further improvement of the recycling cyclone separator.


Introduction
Gas-liquid separation mostly uses gravity separators, swirl vane separators, cyclone separators, wire mesh demisters, static electricity demisters [1][2], etc.The filtration agglutination separator and the electrostatic separator, which are of high efficiency, can separate much thinner droplets.However, their industrial and universal applications are limited due to the disadvantages of strict requests of separating medium, complex structure and clean service, high construction cost, small process load, and low performance-to-cost ratio.The recycling cyclone separator is a representative equipment that solves these problems well and has been widely adopted in separation situations with great process load.
The cyclone separator introduced in this paper has a unique recycling pipe compared with the conventional cyclone separators, as shown in Figure 1.The recycling cyclone separator can make quadratic gas-liquid separation because of the existence of the recycle pipe, which enhances the gasliquid separation efficiency in the recycling cyclone separator and overcomes the shortcoming of the low gas-liquid separation efficiency in the conventional cyclone separator.Compared with other separators, the recycling cyclone separator has a wider range of applications.However, the threedimension swirling flow, the separation process occurring, and the three-dimension flow field in the recycling cyclone separator haven't been fully understood yet.The development of experimental and

Model description
The k-ε model, the RNG k-ε model, and the RSM-model [10][11][12][13] are commonly used in cyclone separator simulation.Although it is simple and speedy, the k-ε model is unsuitable for the flow in a cyclone separator with anisotropic turbulence because it adopts the assumption of isotropic turbulence.RNG k-ε model cannot predict the three-dimensional flow field well because its amelioration is based on the whorl stickiness hypothesis.RSM model forgoes the assumption of isotropic turbulence and solves a transport equation for each component of the Reynolds stress.It is the most applicable turbulent model for cyclone separator flow, even though it has the disadvantage of being computationally more expensive.
In the RSM, the transport equation is written as shown in formula (1): where the left two terms are the local time derivative of stress and convective transport term, respectively.The right eight terms can be shown as follow [8]: The turbulent diffusion term is shown in formula (2): ( ) The molecular diffusion term is shown in formula (3): The stress production term is shown in formula (4): The buoyancy production term is shown in formula (5): ( ) The pressure strain term is shown in formula (6): The dissipation term is shown in formula (7): The production by system rotation term is shown in formula (8): ( ) The source item: The time item: The convection term: The diffusion term: The source item is a generalized quantity that represents the sum of all the other terms in the nonstationary, convection and diffusion terms that can not be included in the governing equation.where, where where

Formatting the text
The simulations were performed on different cyclone separators with some geometries, as reported by Sun and Fan [9].The dimensions and the coordinate directions defined in the simulation of these recycling cyclone separators are shown in Figure 1 and Table 1.The computational domain employed for recycling cyclone separator simulation contains around 55,000 to 86,000 hexahedral and tetrahedral cells, depending on the recycling cyclone separator.Figure 2 shows the cell of the recycling cyclone separator 1.The computational cells were generated by dividing the whole recycling cyclone separator into 10 blocks and then meshing each block separately.

Results and discussion
Figure 3 shows that the static pressure presents the axial symmetry distribution well in the entire separation space.It decreases radially from the wall of separation space to the center of separation space, and a negative pressure zone appears in the center of the vent pipe.The pressure gradient is large along the radial direction.The characteristic of static pressure distribution is a certain pressure difference at the two ends of the recycling pipe, as shown in Figure 4, which provides the power for the second gasliquid separation.  .Calculated pressure drops between both sides of the recycling pipe of the recycling cyclone separator 1. Figure 5 shows the calculated tangential velocity contour.The tangential velocity contour, similar to the dynamic pressure contour, shows good axial symmetry distribution.The value of the tangential velocity on the wall and in the center of the flow field is zero.From Figure 5(B-B), it can be seen that high-speed gas enters from the inlet and is quickly accelerated up to 20-25m/s.Then the velocity gradually decreases as the gas spins down along the wall, even appearing in the reverse direction at point A, which would be the main cause that the short-circuiting flow and eddy flow come into being and the pressure drops increase.In order to overcome this problem, it is suggested that the inlet shape should be modified.
Figure 6 shows that the calculated tangential velocity achieves peak value at different heights of the wall of the vortex finder except for z = -900 mm.The calculated tangential velocity shows two distributed tendencies because of the existence of the vortex finder above z = -510 mm, as shown in Figure 6.The calculated tangential velocity once more achieves the peak value near the wall of the recycling cyclone separator, whereas underneath z = -510 mm, the calculated tangential velocity achieves peak value only once.Otherwise, the axial symmetry of the calculated tangential velocity is not good at z = -350 mm.It is attributable that the gas flow is not stabilized because of the inlet shape.Moreover, at z = -900 mm, the calculated tangential velocity changing and the value entirely is small, which is advantageous for maintaining the stabilization of the separated liquid phase.Tangential velocity distribution at different heights at inlet gas-velocity 15m/s in the recycling cyclone separator 1. Figure 7 shows the calculated axial velocity contour.It can be found that the axial velocity is upward spiral and good axial symmetry.The axial velocity reaches a peak value at the bottom of the vortex finder.The calculated axial velocity achieves a big value in the vortex finder, and it achieves the biggest value in the vent pipe.  .Axial velocity distribution vs. heights at inlet gas-velocity 15m/s in the recycling cyclone separator 1. Figure 8 shows the calculated axial velocity at different heights of the recycling cyclone separator.The axis of the axial velocity coincides nicely with the geometrical axis of the recycling cyclone separator.It can be seen from Figure 8 that the axial velocity shows the upward flow and the downward flow at each position.Meanwhile, the axial velocity value is relatively tiny out of the vortex finder, which is advantageous for the gas-liquid separation because the axial velocity plays a negative part in the gas-liquid separation.The axial velocity achieves peak value in the vortex finder.In the center of the vent pipe, the axial velocity achieves the biggest value.
Figure 9 shows the calculated radial velocity contour.The axis of symmetry of the radial velocity does not coincide with the geometrical axis of symmetry of the recycling cyclone separator, and it is not straight but curved.The radial velocity distribution in the recycling cycling separator based on the axially symmetric line is eccentric.The value of one side is positive, and the other is negative.The radial velocity with a high value is scattered in the entire interior space of the recycling cyclone separator, as shown in Figure 9.It mainly concentrates in the orifice surface since the gas flow expanding or shrinking abruptly when it flows from one pipeline to another.This phenomenon is the primary cause of pressure loss.Radial velocity distribution at different heights at inlet gas-velocity 15m/s in the recycling cyclone separator 1. Figure 10 shows the calculated radial velocity at different heights of the recycling cyclone separator.It can be seen that the radial velocity is mostly small from Figure 10, and it is unstable and alternates from a positive to a negative value.Such eccentric velocity distribution will cause enormous energy loss.Therefore, putting the radial velocity in order will be advantageous to recede the pressure drop of the recycling cyclone separator.
Figure 11 shows the changes of the pressure drop along with the increase of the inlet gas velocity when the interval between the vent pipe and the vortex finder is different.It can be seen that the influence is insensitive to the pressure drop when "k" takes a different value from Figure 11.
Figure 12 shows the gas velocity at different positions of the recycling pipe section when the interval between the vent pipe and the vortex finder is different.It can be seen that the gas velocity has a slight change when "k" takes different values from Figure 12.However, the gas velocity in the recycling pipe decreases slightly as "k" increases, whereas, this change is not apparent.  .All kinds of gasflow in recycling cyclone separator.Figure 13 shows that the secondary circulation can deteriorate the separation performance of the recycling cyclone separator.There are three regions where the secondary circulation formed because of axial velocity and radial velocity, as shown at points A, C, and D in Figure 13, respectively.Firstly, because of the collision between inlet gas flows and part of the gas flows inward, gas-flow A creates an eccentric circumfluence.Secondly, because of the low-pressure region of the bottom of the vortex finder and the massive gas-fluid influxes the vortex finder, gas-flow C brings short-circuiting flow C. Thirdly, because of the collision between upward flow of the separation of space and the downward flow the recycling pipe, gas-flow D creates an eddy flow.Eccentric circumfluence (A), short-circuiting flow (C), and eddy flow (D) can all cause intensive momentum transfer and energy loss.Their damage can be reduced by improving the configuration of the recycling cyclone separator.

Conclusions
The axial symmetry of pressure drops and velocity of the recycling cycling is better than those in conventional cyclone separators, which is related to abundant parts within the recycling cycling.The good axial symmetry reduces pressure drops and the probability that the turbulent flow produces, improving the order of gas-fluid and separation efficiency.
The collision between different gas flows is the main reason for the short-circuiting flow, which is probably the key to decreasing the collision to design new cyclones with high separation efficiency and a low-pressure drop.

Figure 1 .
Figure 1.Definition of the recycling cyclone separator dimensional parameters.Table1.Dimensions of the recycling cyclone in simulation.

Figure 2 .
Figure 2. Mesh for the recycling cyclone separator.Table 2. The different recycling cyclone separator in simulation.

Figure 3 .
Figure 3.The static pressure contours (Pa) at inlet gas-velocity 15m/s in the recycling cyclone separator 1.

Figure 4
Figure 4. Calculated pressure drops between both sides of the recycling pipe of the recycling cyclone separator 1. Figure5shows the calculated tangential velocity contour.The tangential velocity contour, similar to the dynamic pressure contour, shows good axial symmetry distribution.The value of the tangential velocity on the wall and in the center of the flow field is zero.From Figure5(B-B), it can be seen that high-speed gas enters from the inlet and is quickly accelerated up to 20-25m/s.Then the velocity gradually decreases as the gas spins down along the wall, even appearing in the reverse direction at point A, which would be the main cause that the short-circuiting flow and eddy flow come into being and the

Figure 5 .
Figure 5.The tangential velocity contours (m/s) at inlet gas-velocity 15m/s in the recycling cyclone separator 1.

Figure 6 .
Figure 6.Tangential velocity distribution at different heights at inlet gas-velocity 15m/s in the recycling cyclone separator 1. Figure7shows the calculated axial velocity contour.It can be found that the axial velocity is upward spiral and good axial symmetry.The axial velocity reaches a peak value at the bottom of the vortex finder.The calculated axial velocity achieves a big value in the vortex finder, and it achieves the biggest value in the vent pipe.

Figure 7 .
Figure 7.The axial velocity contours (m/s) at inlet gas-velocity 15m/s in the recycling cyclone separator 1.

Figure 8
Figure 8. Axial velocity distribution vs. heights at inlet gas-velocity 15m/s in the recycling cyclone separator 1. Figure8shows the calculated axial velocity at different heights of the recycling cyclone separator.The axis of the axial velocity coincides nicely with the geometrical axis of the recycling cyclone separator.It can be seen from Figure8that the axial velocity shows the upward flow and the downward flow at each position.Meanwhile, the axial velocity value is relatively tiny out of the vortex finder, which is advantageous for the gas-liquid separation because the axial velocity plays a negative part in the gas-liquid separation.The axial velocity achieves peak value in the vortex finder.In the center of the vent pipe, the axial velocity achieves the biggest value.Figure9shows the calculated radial velocity contour.The axis of symmetry of the radial velocity does not coincide with the geometrical axis of symmetry of the recycling cyclone separator, and it is not straight but curved.The radial velocity distribution in the recycling cycling separator based on the axially symmetric line is eccentric.The value of one side is positive, and the other is negative.The radial velocity with a high value is scattered in the entire interior space of the recycling cyclone separator, as shown in Figure9.It mainly concentrates in the orifice surface since the gas flow expanding or shrinking abruptly when it flows from one pipeline to another.This phenomenon is the primary cause of pressure loss.

Figure 9 .
Figure 9.The radial velocity contours (m/s) at inlet gas-velocity 15m/s in the recycling cyclone separator 1.

Figure 10 .
Figure10.Radial velocity distribution at different heights at inlet gas-velocity 15m/s in the recycling cyclone separator 1. Figure10shows the calculated radial velocity at different heights of the recycling cyclone separator.It can be seen that the radial velocity is mostly small from Figure10, and it is unstable and alternates from a positive to a negative value.Such eccentric velocity distribution will cause enormous energy loss.Therefore, putting the radial velocity in order will be advantageous to recede the pressure drop of the recycling cyclone separator.Figure11shows the changes of the pressure drop along with the increase of the inlet gas velocity

Figure 12 .Figure 13
Figure 12.Calculated gas velocity in the recycling pipe of the recycling cyclone separator 1,3,4 at inlet gas velocity is 15 m/s.

Table 1 .
Dimensions of the recycling cyclone in simulation.

Table 2 .
The different recycling cyclone separator in simulation.