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Experimental and CFD study on effects of spiral guide vanes on cyclone performance
Advanced Powder Technology 29 (2018) 3394–3403
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Experimental and CFD study on effects of spiral guide vanes on cyclone performance Faqi Zhou a, Guogang Sun a,b,⇑, Xiaopeng Han c, Yong Zhang a, Wenqun Bi a a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, PR China Beijing Key Laboratory of Process Fluid Filtration and Separation, Beijing 102249, PR China c Handan Iron and Steel Refco Group Ltd, Handan 056000, PR China b
a r t i c l e
i n f o
Article history: Received 13 December 2017 Received in revised form 8 August 2018 Accepted 14 September 2018 Available online 25 September 2018 Keywords: Cyclone separator Spiral guide vane Collection efficiency Pressure drop Flow field
a b s t r a c t This paper presents an experimental and numerical study on a tangential inlet cyclone separator with a spiral guide vane which is not often researched. Numerical pressure drop results were in close agreement with the experimental data. The spiral guide vane was also found to considerably influence the velocity distribution, turbulence intensity, pressure drop and collection efficiency in the cyclone. A critical value of spiral guide vane turns appeared below or above which there was a marked increase in collection efficiency, pressure drop, and tangential velocity. Compared to a cyclone with zero spiral guide vane turn, the maximal decrease in collection efficiency in the cyclone with the critical spiral guide vane turns (one turn) was 2% approximately. The maximum-efficiency inlet velocity appeared to exist independent of spiral guide vane turns, as inlet velocity affected the radial distance traveled by the rebounded particles from the inner wall. The analysis of flow field in cyclones indicated that the flow field was improved with the spiral guide vanes employed to some extent. The results presented here may provide a workable reference for the effects of spiral guide vanes on the flow field and corresponding performance in cyclone separators. Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Cyclone separators are utilized in a variety of industries to separate solid particles. They are popular because they are low in cost, easily maintained, easily operated, free of moving components, and highly reliable even under extreme working conditions. Cyclone performance, primarily collection efficiency and pressure drop, is strongly affected by geometric parameters [1]. The inlet geometry is also a crucial parameter, as it controls the initial swirl flows which dominate the flow field inside the cyclone. The effects of conventional inlet geometry such as inlet width and height [2–4], single inlet versus spiral double inlets [5,6], normal inlets or declining inlets [7], or single inlet versus triple inlets [8] on cyclone performance have been studied extensively for many years. The spiral guide vane, special inlet geometry often employed in axial flow cyclones, has been investigated only in recent decades. Jiang et al. [9] found that the optimal helix angle of a spiral guide vane is 20° in an axial flow liquid-liquid hydrocy⇑ Corresponding author at: No. 18, Fuxue Road, Changping, Beijing 102249, PR China. E-mail address: [email protected] (G. Sun).
clone in regards to the flow velocity and the wear to the vanes. Hsiao et al. [10] developed a multi-stage cyclone system consisting of spiral guide vane in the axial flow cyclone stages to classify Lunar and Martian dust stimulants in a NASA space exploration study. The results showed the spiral guide vane reduced cutoff particle size, however, their cyclone only had a barrel diameter of 41.69 mm; its performance could not be predicted at larger dimensions. Later, similar study was investigated by Hsiao et al. [11] in an axial flow cyclone with one three-full-turn spiral guide vane. But the diameter of cyclone body was even smaller (only 29 mm). A mathematical model was proposed by Maynard et al. [12] to predict the particle penetration of an axial flow cyclone under laminar flow. Their model assumes that particle separation occurs in the vane section and in the cyclone body. To estimate the particle penetration in the vane section, the radial displacement of particles is evaluated with the implicit assumption that there is no particle mixing at any cross section in the helix channel. Chen et al. [13] and Tsai et al. [14] tested the collection efficiency of both solid (NaCl) and liquid (OA, oleic acid) nanoparticles in a small axial flow cyclone (30 mm body diameter) employed a spiral guide vane with three turns. They found that the smallest cutoff aerodynamic diameters for OA and NaCl nanoparticles reached about
https://doi.org/10.1016/j.apt.2018.09.022 0921-8831/Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
F. Zhou et al. / Advanced Powder Technology 29 (2018) 3394–3403
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Nomenclature
DP Vi Ci
g
dp a b
pressure drop (Pa) inlet air velocity (m/s) inlet particle concentration (g/m3) collection efficiency (—) particle diameter (lm) inlet height (mm) inlet width (mm)
21.7 nm and 21.2 nm, respectively. Tsai et al. [15] found that a spiral guide vane with one helix flow channel and three complete turns had higher collection efficiency than one with three halfturn helix flow channels in an axial flow cyclone (30 mm body diameter as well) for fine particle removal in vacuum conditions. Li et al. [16] proposed a radial-inlet cyclone separator with a spiral guide vane in which pressure drop and tangential velocity increased as the number of spiral guide vane turns increased; in their experiment, the grade efficiency first increased and then reached a smooth variation region with one and a half turns. A spiral guide vane with two turns was applied in a tangential inlet dual cyclone separation system with by Pan et al. [17]. And the cyclone performance was investigated by CFD approach with the RNG k-e turbulent model. The results indicated the spiral guide vane improved the separation efficiency of small particles. However, the RNG k-e model adopting the assumption of isotropic turbulence was proved unsatisfactory for the prediction of the flow field inside cyclones [18–20]. In summary, few researchers have explored the effects of spiral guide vane turns on the flow field or separation performance in cyclones, especially in tangential inlet cyclones and larger cyclone scale. There is not sufficient data to support the industrial design of cyclone separators, to this effect. In the present study, we attempted to fill this research gap by conducting experimental and CFD tests on a tangential inlet cyclone with a barrel diameter of 300 mm to investigate the effects of spiral guide vane turns on flow field and separation performance. We hope that the results presented here provide a workable reference regarding the detailed separation process and precise effects of spiral guide
De S h H B D L
diameter of gas outlet duct (mm) gas outlet duct length (mm) cylinder height (mm) cyclone height (mm) cone diameter (mm) body diameter (mm) spiral channel height (mm)
vanes on the flow field and separation performance of cyclone separators. 2. Experimental apparatus and methods 2.1. Experimental system and cyclone model The experimental system is shown in Fig. 1. Four tangential inlet cyclone separators (with the vane turns of 0–3) with barrel diameter of 300 mm were manufactured and the detailed geometries and dimensions were shown in Fig. 2 and Table 1, respectively. 2.2. Experimental procedure and methods We conducted all experiments at atmospheric pressure and ambient temperature. Particle-laden gas was forced into the cyclone inlet with a centrifugal fan. The air flow rate was controlled by a knife switch in the outlet pipe and measured with a Pitot-tube in the outlet pipe. Particles were fed into the inlet by a star-type feeder. The inlet velocity ranged from 9.7 to 29.8 m/s and the feeding particle concentration was 30g/m3 throughout the experiment. In a cyclone, most particles were separated from the gas and ultimately collected in the discharge bin underneath. After passing through the cyclone, the gas was passed through a filter to remove any remaining particles. The particle concentration in the inlet was controlled by the particles feeding time and simultaneously the rotational speed of the feeder. The pressure drops
Fig. 1. Schematic diagram of the experimental system. (1) particle bin, (2) particle feeder, (3) U tube, (4) cyclone separator, (5) particle discharge bin, (6) Pitot tube, (7) knife switch, (8) filter, (9) fan.
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Fig. 2. Schematic representation of the cyclone separators (From left to right: the vane turns are 0, 0.5, 1, 1.5, 2–4).
Table 1 Geometrical dimensions of the cyclone used in this study. Structure
a/mm
b/mm
De/mm
S/mm
h/mm
H/mm
B/mm
D/mm
L/mm
Value
150
60
150
670
900
1650
110
300
112.5
were measured with U tube. Each experiment was repeated at least three times to ensure relative error below 0.5%. The averages were used for subsequent calculations. Talcum powder was used as the experimental particles with a physical density of 2700 kg/m3. The particle size distribution was measured by Mastersizer 2000 laser particle size analyzer. The volume mean particle diameter was approximately 17.56 lm. 3. Numerical simulation 3.1. CFD model The flow in a cyclone separator is characterized by a high swirl and an intense anisotropic turbulence. Numerous studies have verified that both RSM and LES are favorable approaches to simulate the flow field in cyclones [21–24]. However, the RSM model has its advantages regarding the low requirement of computer capacity and the short running time compared to the LES in simulation [22]. The LES provides a partially dissatisfying prediction in the boundary layer with relative coarse grids compared with the RSM [25]. In addition, while Shukla et al. [26] also reported that the mean velocities predicted by LES and RSM were very close. Given the accuracy of results and low computational cost, RSM was applied in this work to compute the gas flow field in the cyclone. Hence, the RSM is applied to predict the flow field in the cyclones. The main foundation of this model can be found in the reference [27]. We simulated the dispersed phase by tracking a large number of spherical dispersed particles through the converged flow field of continuous flow in the Lagrangian reference frame by using a two-way coupling method via discrete phase model (DPM). 3.2. Grid division and independence In this study, the seven cyclone geometries were meshed in a structured grid by means of Gambit. Multi-block structured hexahedral grids were generated in the entire domain of the cyclone as
shown in Fig. 3. We conducted a mesh independent analysis of the cyclone with 585,750, 850,833 and 1,295,350 hexahedral nodes generated by Gambit 2.3.6 of the 0-turn cyclone, respectively. The difference in cyclone pressure drops at the position Z = 750 mm between 585,750 and 1,295,350 hexahedral nodes is about 8.3%, and the difference in pressure drop between 850,833 and 1,295,350 hexahedral nodes is less than 4.7%. The relative error in tangential velocity at the position Z = 750 mm between 585,750 and 1,295,350 hexahedral cells is about 5.8%, and the same relative error between 850,833 and 1,295,350 hexahedral cells is less than 1.2% (Fig. 4). The grids with 850,833 hexahedral nodes produce grid-independent results as per the accuracy and economy of the simulation, as well as the small differences in pressure drop and tangential velocity between 850,833 and 1,295,350 hexahedral nodes. The total number of nodes and mesh quality are listed in Table 2. 3.3. CFD verification Fig. 5 shows a comparison between simulated and experimental tangential velocity and axial velocity data at the position Z = 810 mm in a 0-turn cyclone with inlet velocity of 20 m/s. The two sets of data are generally in good agreement. 3.4. Boundary conditions and simulation strategy A presumed uniform inlet velocity Vi = 20 m/s (flow rate 648 m3/h) was specified at the normal-to-inlet surface of the cyclone. The hydraulic diameter equaled 85.71 mm with turbulent intensity to be 3.72%. The outflow boundary condition was imposed at the outlet. Outflow condition assumes the fully developed flow at the outlet section. No-slip conditions are set at the wall. For the grid nodes near the wall, the standard wall function was applied. The air density of 1.225 kg/m3 and the viscosity of 1.7894 10 5 kg/m s were defined in this study. The solid particles density was 2700 kg/m3, and inlet solids concentration was 30 g/ m3.
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was selected as the time step size. The simulation was performed via the commercial software Fluent 14.0 on a work station with a CPU of Intel Xeon 2.9 GHz and RAM of 64 GB. 4. Results and discussion 4.1. General performance
Fig. 3. Grid representation of simulated cyclone (Taking the cyclone with two spiral guide vane turns as an example).
Fig. 4. The distributions of tangential velocity at the position Z =
750 in cyclone.
The finite volume method was used to discretize the partial differential equations of the model using the semi-implicit method for pressure linked equations consistent (SIMPLEC) method for pressure velocity coupling and quadratic upwind interpolation of convective kinematics (QUICK) scheme to interpolate the variables on the surface of the control volume. The implicit coupled solution algorithm was selected. In this simulation, the 10 5 convergence criterion accuracy for the calculations was applied, and 0.0002 s
4.1.1. Pressure drop Fig. 6 shows the effects of spiral guide vane turns on pressure drops in a cyclone separator with inlet velocity of 20 m/s. The predicted results were in close agreement with experimental data at a relative error within 6.6%. The pressure drops showed a critical value in the separator as spiral guide vane turns increased: the pressure drop markedly increased above or below this critical value of one turn. Fig. 7 shows the cyclone pressure drops trends with varying inlet velocity. As the inlet velocity was small (Vi = 9.7 and 15.0 m/s), the effect of the spiral guide vanes was negligible. Similar to previous hydrocyclone research by Patra et al. [37,38], at low velocity, the drag force and the extra resistance in the cyclone due to the spiral guide vanes (i.e., friction between the fluid and spiral guide vanes) oppose the fluid flow inside the cyclone separator. Due to the low velocity, drag force, and extra resistance due to the spiral guide vanes and centrifugal force vary little as the spiral turns continue. At low velocity, the combined effects of these two opposing forces are significant as compared with the centrifugal force alone. Thus, at low velocity, the effect of the spiral turns of guide vanes on the pressure drop is negligible. However, the changes in pressure drops with the increase of spiral guide vane number were more pronounced at higher inlet velocities (Vi = 20.0, 25.0 and 29.8 m/s). There was a nearly completely consistent critical pressure drop along with vane turns for higher velocities. And the trend was much more obvious especially at the inlet velocity of 29.8 m/s. Compared to the cyclone with zero vane turns, at the inlet velocity of 29.8 m/s, the maximal decrease in pressure drop with one vane turn was 8.7%; the maximal increase in pressure drop in the cyclone with two vane turns was 2.0%; the maximal increase in pressure drop in the cyclone with three vane turns was 1.5%. Vane turns above or below this critical value led to an increase in pressure drop, which marks an interesting departure from the results reported by Li et al. [16], who found that pressure drops in a radial-inlet cyclone consistently increased with the vane turns increasing. These differences would be discussed in greater detail below. 4.1.2. Collection efficiency We measured overall collection efficiencies by weighing the dust particles in particle discharge bin. The experimental results are shown in Fig. 8. A consistent critical overall collection efficiency value was observed with vane turns for almost all five velocities. The efficiency was lowest in the structure with one vane turn
Table 2 The total number of nodes and the mesh quality. Cyclone structure
Mesh characteristic Total number of nodes
0-turn 0.5-turn 1-turn 1.5-turn 2-turn 3-turn 4-turn
850,933 860,163 868,374 873,061 880,388 888,132 895,315
Maximum aspect ratio
4.1 4.1 4.1 4.1 4.1 4.1 4.1
Skewness range (0–best, 1–worst) Maximum
0–0.2
0.2–0.4
0.4–0.6
0.6–0.8
0.8–1.0
0.811 0.811 0.811 0.811 0.811 0.811 0.811
94.65% 93.89% 93.56% 93.33% 92.94% 92.18% 91.31%
4.29% 4.82% 5.11% 5.30% 5.67% 6.23% 6.98%
0.66% 0.82% 0.83% 0.86% 0.94% 1.04% 1.16%
0.32% 0.38% 0.41% 0.42% 0.45% 0.46% 0.46%
0.08% 0.09% 0.09% 0.09% 0.09% 0.09% 0.09%
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Fig. 5. Comparison between simulated and experimented velocities in cyclones.
Fig. 6. Comparison between simulated and experimented pressure drop in cyclone separators.
Fig. 7. Variation of pressure drop with vane turns.
and highest in the structure with three vane turns. To a great extent, collection efficiency was affected by tangential velocity in a cyclone. Here, the tangential velocity in the cyclone decreased as the spiral vane turns increased from zero to one (Figs. 9 and 10), reducing the centrifugal force acting on the particles. As a result, there were fewer particles captured near the inner wall with little collection efficiency. The tangential velocity and centrifugal
Fig. 8. Variation of collection efficiency with vane turns.
force acting on the particles increased gradually as the spiral guide vane turns increased from one to three (Figs. 9 and 10). More particles were captured with higher collection efficiency under the latter conditions. Compared to the cyclone with zero vane turns, the maximal decrease in collection efficiency with one vane turn was 2%; the maximal increase in collection efficiency in the cyclone with two vane turns was 0.7%; the maximal increase in collection efficiency in the cyclone with three vane turns was 1.4%. A critical collection efficiency value appeared again with increasing inlet velocity due to the effects of the radial distance of the rebounded particles from the inner wall [28]. We define this critical velocity as the ‘‘maximum-efficiency inlet velocity” [28,29]. Here the maximum-efficiency inlet velocity was 25 m/s for all four cyclones. The rebounded particles obtained more kinetic energy at higher inlet velocities, to the point where the radial distance traveled by a rebounded particle was longer than the width of the downward gas flow. The particles ultimately rebounded back into the upward gas flow. Within the rapid, upward gas flow, the particles moved towards the vortex finder quickly with little separation. Therefore, the particles rebounded out of the downward gas flow escaped the cyclone separator without being captured. At lower inlet velocities, the rebounded particles possessed less kinetic energy, so the radial distance traveled by a given rebounded particle was shorter than the width of the downward gas flow. The particles in this case moved towards the wall again via centrifugal force. The loss of energy due to collision decreased the velocity of the particles; the radial distance of the particle was truncated
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Fig. 9. Tangential velocity distribution in cyclones.
ious spiral guide vane turns for both two positions. At the position of Z = 750 mm, compared to the cyclone with zero spiral guide vane turn, the decreases in maximal tangential velocity were 1.8 m/s in the cyclone with half one turns, 2.4 m/s in the cyclone with one turn, 1.8 m/s in the cyclone with one and a half turns, 1.6 m/s in the cyclone with two turns and 0.6 m/s in the cyclone with three turns; while the increase in maximal tangential velocity were 0.3 m/s in the cyclone with four turns. However, at the position of Z = 1150 mm, compared to the cyclone with zero spiral guide vane turn, the decreases in maximal tangential velocity were 1.3 m/s in the cyclone with half one turns, 2.4 m/s in the cyclone with one turn, 1.7 m/s in the cyclone with one and a half turns, 1.3 m/s in the cyclone with two turns and 0.4 m/s in the cyclone with three turns; and the increase in maximal tangential velocity were 0.4 m/s in the cyclone with four turns.
Fig. 10. Maximum tangential velocity varying with number of vane turns.
in the subsequent rebounding process. Particles not escaping the cyclone after the first collision were unlikely to rebound into the upward gas flow in the following motion and thus unlikely to be captured. The decrease in collection efficiency at high inlet velocities was primarily caused by the escape of rebounded particles after the first collision. 4.2. Gas flow field 4.2.1. Tangential velocity Tangential velocity is an important component of the gas flow in a cyclone due to its effect on the particle separation process and pressure drop. Fig. 9 shows a comparison of the tangential (Vt) velocity profiles at the axial positions of Z = 750 and 1150 mm, respectively. It was found that spiral guide vane turns significantly influenced the tangential velocity distributions. The tangential velocity profiles remained effectually unchanged in the core region, while tangential velocity profiles in the outer vortex showed a critical tangential velocity value as the vane turns increased. Above or below the critical turn value, there was a marked decrease in tangential velocity. The maximum tangential velocity occurred in the cyclone with four spiral guide vane turns, while the minimal tangential velocity appeared in the cyclone with one spiral guide vane turn. Fig. 10 shows the maximal tangential velocity varying with spiral guide vane turns at the positions Z = 750 and 1150 mm, respectively. There was a clear minimum tangential velocity at var-
4.2.2. Axial velocity Fig. 11 shows a comparison among axial (Vz) velocity profiles at the positions Z = 750 and 1150 mm, respectively. Two distinct regions are visible in the axial velocity profiles: an outer vortex region where the velocity profiles move downward and an inner region where they move upward. The effects of spiral guide vane turns were generally irregular on the axial velocity profiles in the outer region. While, at the position Z = 750 mm in the inner region, except for the cyclone with one spiral guide vane turn, axial velocities in other cyclones were all downward. And the axial velocities in the inner region were upward in all the seven cyclones at the position Z = 1150 mm. The axial velocity in the inner region in the cyclone with one spiral guide vane turn was lower than that in other cyclones as well.
4.2.3. Static pressure As shown in Fig. 12, the distributions of static pressure at the positions Z = 750 and 1150 mm in the seven cyclones formed V-type curves. The effects of spiral vane turns were negligible in the inner region and significant in the outer region. As spiral guide vane turns increased, the static pressure in the outer region showed a critical value. With the spiral vane turns increasing from zero to one, the static pressure in the cyclone decreased; the static pressure increased as spiral guide vane turns continued to increase. And the maximal static pressure occurred in the outer region of the separator with four spiral vane turns. The distributions of static pressure in the cyclone with zero spiral guide vane turns and that with three spiral vane turns were fairly close. The negative static pressure in the inner region increased gradually with the axial direction for all seven cyclones.
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Fig. 11. Axial velocity distribution in cyclones.
Fig. 12. Static pressure distribution in cyclones.
4.2.4. Turbulence intensity The distributions of turbulence intensity in cyclone separators with different spiral guide vane turns were similar at the position Z = 750 mm and Z = 1150 mm respectively, as shown in Fig. 13. And the turbulence intensity decreased gradually along the axial direction in most regions of cyclones except for the area near the wall. From the side wall to the core, the turbulence intensity increased to maximal value, and then decreased. The maximal turbulence intensity appeared at the dimensionless radial distance r/ R = 0.53–0.56 approximately. In addition, the magnitude of turbulence intensity in cyclones with different spiral guide vane turns at the position Z = 750 almost maintained a constant value except for the cyclone with zero spiral guide vane turn. And the turbulence intensity in the core region was higher than that near the wall. While, at the position Z = 1150 mm, the abrupt curves of turbulence intensity appeared in the core region of all seven cyclones. The turbulence intensity in the core region was lower than that near the wall. Meanwhile, with the increase of spiral guide vane turns, the turbulence intensity in cyclones gradually decreased, especially at the position Z = 750 mm. This indicated that the flow field was improved and was relatively stable with increase of spiral guide vanes turns to some extent. 4.3. Grade separation efficiency Fig. 14 shows the grade efficiency of cyclones with different spiral guide vane turns. The simulation results show that the spiral guide vanes have significant effects on the grade efficiency. The
separator with one spiral guide vane turn completely removed the particles with diameter greater than 7.7 lm, while the separators with other spiral guide vane turns collected all the particles with diameter greater than 6.3 lm. For the particles range 0.8– 7 lm, the grade efficiency of separator with one spiral guide vane turn was lower than that of other separators, for example, the grade efficiency of 4.2 lm particle in the separator with one spiral guide vane turn is 91.0%, while, it is 93.7%, 93.5%, 93.1%, 95.7%, 94.6% and 96.8% in the separators with the spiral guide vane turns of 0, 0.5, 1.5, 2–4 respectively. In addition, except for one turn, for the particles range 0.8–2.9 lm, the curves of grade efficiency in other separators are so close to coincidence. However, for the particles range 2.9–6.3 lm, the curve of grade efficiency is higher in separator with four spiral guide vane turns and lower in separators with both half one turns and one and a half turns. Moreover, the critical particle size in small particles reported by Chen et al [30], Hugi et al [31] and Li et al [32], that is below which the grade efficiency increases as the particle size decreases in the experimental research, was not found in the simulated results. There may be two reasons for the difference. The first reason is that the experimental results were obtained by laser diffractometry and the smaller particle is affected significantly by optical parameters [33]. The refractive index and extinction coefficient of particles SiO2 were adopted 1.54 and 0.01 respectively according to particles structure [34], not by experimental determined. Therefore, there is possibility of inaccuracy in choosing these parameters, and hence inaccuracy in determining the smaller particle size distributions. The second reason is that the smaller particles agglomerate into
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Fig. 13. Distribution of turbulence intensity in cyclones.
ticles interaction, therefore the experiment results and simulation results of smaller particles are different.
4.4. Discussion
Fig. 14. The grade efficiency in cyclones with different spiral guide vane turns.
large particles and then are separated, which bring high separation efficiency, and the smaller particles agglomeration do occur in the cyclone separator [35,36]. Therefore, the grade efficiency curve of smaller particles is non monotonic. While the CFD ignores the par-
Fig. 15 shows the streamline of fluid in cyclones with the increase of spiral guide vane turns. When there were no spiral guide vanes, the swirling motion of flow in the cyclone was generated by the tangential inlet. The helix angle of the swirling flow was significantly larger than that of the spiral guide vanes. As the spiral guide vane turns increased from zero to one, the spiral guide vanes destroyed the original flow trajectory through the tangential inlet in the cyclone gradually and forced gas to flow along the spiral channel of the guide vanes with smaller helix angle. A single spiral guide vane turn had basically destroyed the original flow trajectory, but the acceleration of the flow along the spiral channel was insufficient to recover. Certainly, this caused a decrease in the tangential velocity and swirl intensity in the cyclone as the spiral guide vane turns increased. In the cyclone, pressure drop includes losses due to acceleration, friction, and radial pressure gradient [37,38]. Decrease in losses due to acceleration and radial pressure gradient may dominate the pressure drop. Fewer guide vane spiral turns and reduced velocity in the
Fig. 15. Schematic representation of the gas velocity streamline in cyclones (From left to right: the vane turns are 0, 0.5, 1, 1.5, 2–4).
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cyclone minimize the loss due to friction in comparison to the losses due to acceleration and radial pressure gradient. The pressure drop thus decreases as spiral guide vane turns increase from zero to one (Fig. 7). Meanwhile, the decrease of tangential velocity led to decrease in centrifugal force acting on the particles. More particles would be hard to move to the side wall for being captured with little separation. As a result, the collection efficiency decreased (Fig. 8). As the spiral guide vane turns increased from one to four, the vanes begin to more effectively guide the flow gradually. The flow continued to accelerate along the spiral guide vane with smaller helix angle as the spiral channel increased, which increased the tangential velocity and swirl intensity of the cyclone; this gradually increased the losses due to acceleration and radial pressure gradient. Meanwhile, because of the increased flow velocity and the spiral guide vanes, the loss due to friction increased as well. Hence, the pressure drop increased as spiral guide vane turns increased (Fig. 7). Moreover, the increase of tangential velocity led to increase in centrifugal force acting on the particles. Much more particles would move to the side wall for captured with higher collection efficiency (Fig. 8). That is why vane turns above or below this critical value (one turn) led to an increase both in pressure drop and collection efficiency in cyclone. Although the collection efficiency and the stability of the flow could be improved to some extent of the cyclones with the spiral turns of guide vanes increasing to more than two, the manufacturing of the spiral guide vanes is complicated. Meanwhile, the spiral guide vanes may not be suitable for application with high temperature differences because of structural stress. 5. Conclusions In this study, the effects of spiral vane turns on the performance of a tangential inlet cyclone were investigated numerically and experimentally. The variations in flow velocity, pressure, turbulence intensity, and fluid trajectory in various cyclones were assessed to explore the manner in which the spiral vane turns influence the cyclone performance. The conclusions can be summarized as follows. (1) The spiral guide vane turns considerably influenced the collection efficiency, pressure drop, and tangential velocity in the cyclone. A critical spiral guide vane turn value, below or above which there was a marked increase in collection efficiency, pressure drop, and tangential velocity, was found. Compared to a cyclone with zero vane turn, the maximal decrease of collection efficiency in a cyclone with the critical value of spiral guide vane turns (one) was 2%. And for the particles range 0.8–7 lm, the grade efficiency of separator with one spiral guide vane turn was lower than that of other separators. (2) The maximum-efficiency inlet velocity appeared to be independent of spiral guide vane turns, as inlet velocity affects the radial distance traveled by the rebounded particles from the inner wall. (3) The effects of spiral guide vane turns were generally irregular on the axial velocity profiles in the outer region of cyclones. And the effects of spiral vane turns on static pressure were negligible in the inner region and significant in the outer region. The static pressure in the outer region showed a critical value (again, one spiral vane turn) as the number of turns increased as well. (4) With the increase of spiral guide vane turns, the turbulence intensity in cyclones gradually decreased, especially in the cylinder part. In addition, the curves of turbulence intensity
in the core region were relatively abrupt in the cone part. And the turbulence intensity in the core region was lower than that near the wall. However, the cases were opposite in the cylinder part. (5) The analysis of CFD results indicated that the flow field was improved and was relatively stable with increase of spiral guide vanes turns to some extent. Maybe it could provide a workable reference for the design and application of cyclone separator.
Acknowledgements This work was sponsored by the National Key Project of Basic Research, Ministry of Science and Technology of China [grant number 2014CB744304] and National Natural Science Foundation of China [grant number 21276274]. References [1] K.S. Lim, H.S. Kim, K.W. Lee, Characteristics of the collection efficiency for a cyclone with different vortex finder shapes, J. Aerosol Sci. 35 (6) (2004) 743– 754. [2] K. Elsayed, C. Lacor, Numerical modeling of the flow field and performance in cyclones of different come-tip diameters, Comput. Fluids 51 (1) (2011) 48–59. [3] A. Avci, I. Kragoz, A. Surmen, Development of a new method for evaluating vortex length in reversed flow cyclone separators, Powder Technol. 235 (2) (2013) 460–466. [4] K. Elsayed, C. Lacor, The effect of cyclone inlet dimensions on the flow pattern and performance, Appl. Math. Model. 35 (4) (2011) 1952–1968. [5] B. Zhao, Y. Su, J. Zhang, Simulation of gas flow pattern and separation efficiency in cyclone with conventional single and spiral double inlet configuration, Chem. Eng. Res. Des. 84 (12) (2006) 1158–1165. [6] B. Zhao, H. Shen, Y. Kang, Development of asymmetrical spiral inlet to improve cyclone separator performance, Powder Technol. 145 (1) (2004) 47–50. [7] Y.X. Su, A.Q. Zheng, B.T. Zhao, Numerical simulation of effect of inlet configuration on square cyclone separator performance, Powder Technol. 210 (12) (2011) 293–303. [8] D. Winfield, M. Cross, N. Croft, Performance comparison of a single and triple tangential inlet gas separation cyclone: a CFD study, Powder Technol. 235 (2) (2013) 520–531. [9] M.H. Jiang, S.Z. Chen, F. Li, J.L. Yu, L.X. Zhao, Simulation analysis and experimental study of deoiling hydrocyclone by compact axial-flow type, Chin. J. Oil-Gas Field Surf. Eng. 29 (9) (2010) 18–20. [10] T.C. Hsiao, D.R. Chen, L. Li, P.S. Greenberg, K.W. Street, Development of a multistage axial flow cyclone, Aerosol Sci. Technol. 44 (4) (2010) 253–261. [11] T.C. Hsiao, D.R. Chen, P.S. Greenberg, K.W. Street, Effect of geometric configuration on the collection efficiency of axial flow cyclones, J. Aerosol Sci. 42 (2) (2011) 78–86. [12] A.D. Maynard, A simple model of axial flow cyclone performance under laminar flow conditions, J. Aerosol Sci. 31 (2) (2000) 151–167. [13] S.C. Chen, C.J. Tsai, An axial flow cyclone to remove nanoparticles at low pressure conditions, J. Nanopart. Res. 9 (1) (2007) 71–83. [14] C.J. Tsai, S.C. Chen, R. Przekop, A. Moskal, Study of an axial flow cyclone to remove nanoparticles in vacuum, Environ. Sci. Technol. 41 (5) (2007) 1689– 1695. [15] C.J. Tsai, D.R. Chen, H. Chein, S.C. Chen, J.L. Roth, Y.D. Hsu, W.L. Li, P. Biswas, Theoretical and experimental study of an axial flow cyclone for fine particle removal in vacuum conditions, J. Aerosol Sci. 35 (9) (2004) 1105–1118. [16] Q.P. Li, Y.J. Xu, L.H. Du, J. Xie, J.W. Cheng, Y. Wang, Numerical simulation of JLX cyclone separator with guiding vane, Chin. J. Chem. Eng. 43 (1) (2015) 37–41. [17] C.J. Pan, Z.W. Jin, X. Feng, Research on the spiral guiding and the back-mixing preventing of cyclone separating devices, Chinese J. Chem. Ind. Eng. Prog. 31 (6) (2012) 1215–1219. [18] A.J. Hoekstra, J.J. Derksen, H.E.A.V.D. Akker, An experimental and numerical study of turbulent swirling flow in gas cyclones, Chem. Eng. Sci. 54 (13–14) (1999) 2055–2065. [19] B. Wang, D.L. Xu, K.W. Chu, A.B. Yu, Numerical study of gas-solid flow in a cyclone separator, Appl. Math. Modell. 30 (11) (2006) 1326–1342. [20] F. Qian, J. Zhang, M. Zhang, Effects of the prolonged vertical tube on the separation performance of a cyclone, J. Hazard. Mater. 136 (3) (2006) 822–829. [21] H. Safikhani, M. Shams, S. Dashti, Numerical simulation of square cyclones in small sizes, Adv. Powder Technol. 22 (3) (2011) 359–365. [22] M.D. Slack, R.O. Prasad, A. Bakker, F. Boysan, Advances in cyclone modeling using unstructured grids, Chem. Eng. Res. Des. 78 (8) (2000) 1098–1104. [23] S.K. Shukla, P. Shukla, P. Ghosh, Evaluation of numerical schemes using different simulation methods for the continuous phase modeling of cyclone separators, Adv. Powder Technol. 22 (2) (2011) 209–219.
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Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Experimental and CFD study on effects of spiral guide vanes on cyclone performance Faqi Zhou a, Guogang Sun a,b,⇑, Xiaopeng Han c, Yong Zhang a, Wenqun Bi a a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, PR China Beijing Key Laboratory of Process Fluid Filtration and Separation, Beijing 102249, PR China c Handan Iron and Steel Refco Group Ltd, Handan 056000, PR China b
a r t i c l e
i n f o
Article history: Received 13 December 2017 Received in revised form 8 August 2018 Accepted 14 September 2018 Available online 25 September 2018 Keywords: Cyclone separator Spiral guide vane Collection efficiency Pressure drop Flow field
a b s t r a c t This paper presents an experimental and numerical study on a tangential inlet cyclone separator with a spiral guide vane which is not often researched. Numerical pressure drop results were in close agreement with the experimental data. The spiral guide vane was also found to considerably influence the velocity distribution, turbulence intensity, pressure drop and collection efficiency in the cyclone. A critical value of spiral guide vane turns appeared below or above which there was a marked increase in collection efficiency, pressure drop, and tangential velocity. Compared to a cyclone with zero spiral guide vane turn, the maximal decrease in collection efficiency in the cyclone with the critical spiral guide vane turns (one turn) was 2% approximately. The maximum-efficiency inlet velocity appeared to exist independent of spiral guide vane turns, as inlet velocity affected the radial distance traveled by the rebounded particles from the inner wall. The analysis of flow field in cyclones indicated that the flow field was improved with the spiral guide vanes employed to some extent. The results presented here may provide a workable reference for the effects of spiral guide vanes on the flow field and corresponding performance in cyclone separators. Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Cyclone separators are utilized in a variety of industries to separate solid particles. They are popular because they are low in cost, easily maintained, easily operated, free of moving components, and highly reliable even under extreme working conditions. Cyclone performance, primarily collection efficiency and pressure drop, is strongly affected by geometric parameters [1]. The inlet geometry is also a crucial parameter, as it controls the initial swirl flows which dominate the flow field inside the cyclone. The effects of conventional inlet geometry such as inlet width and height [2–4], single inlet versus spiral double inlets [5,6], normal inlets or declining inlets [7], or single inlet versus triple inlets [8] on cyclone performance have been studied extensively for many years. The spiral guide vane, special inlet geometry often employed in axial flow cyclones, has been investigated only in recent decades. Jiang et al. [9] found that the optimal helix angle of a spiral guide vane is 20° in an axial flow liquid-liquid hydrocy⇑ Corresponding author at: No. 18, Fuxue Road, Changping, Beijing 102249, PR China. E-mail address: [email protected] (G. Sun).
clone in regards to the flow velocity and the wear to the vanes. Hsiao et al. [10] developed a multi-stage cyclone system consisting of spiral guide vane in the axial flow cyclone stages to classify Lunar and Martian dust stimulants in a NASA space exploration study. The results showed the spiral guide vane reduced cutoff particle size, however, their cyclone only had a barrel diameter of 41.69 mm; its performance could not be predicted at larger dimensions. Later, similar study was investigated by Hsiao et al. [11] in an axial flow cyclone with one three-full-turn spiral guide vane. But the diameter of cyclone body was even smaller (only 29 mm). A mathematical model was proposed by Maynard et al. [12] to predict the particle penetration of an axial flow cyclone under laminar flow. Their model assumes that particle separation occurs in the vane section and in the cyclone body. To estimate the particle penetration in the vane section, the radial displacement of particles is evaluated with the implicit assumption that there is no particle mixing at any cross section in the helix channel. Chen et al. [13] and Tsai et al. [14] tested the collection efficiency of both solid (NaCl) and liquid (OA, oleic acid) nanoparticles in a small axial flow cyclone (30 mm body diameter) employed a spiral guide vane with three turns. They found that the smallest cutoff aerodynamic diameters for OA and NaCl nanoparticles reached about
https://doi.org/10.1016/j.apt.2018.09.022 0921-8831/Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
F. Zhou et al. / Advanced Powder Technology 29 (2018) 3394–3403
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Nomenclature
DP Vi Ci
g
dp a b
pressure drop (Pa) inlet air velocity (m/s) inlet particle concentration (g/m3) collection efficiency (—) particle diameter (lm) inlet height (mm) inlet width (mm)
21.7 nm and 21.2 nm, respectively. Tsai et al. [15] found that a spiral guide vane with one helix flow channel and three complete turns had higher collection efficiency than one with three halfturn helix flow channels in an axial flow cyclone (30 mm body diameter as well) for fine particle removal in vacuum conditions. Li et al. [16] proposed a radial-inlet cyclone separator with a spiral guide vane in which pressure drop and tangential velocity increased as the number of spiral guide vane turns increased; in their experiment, the grade efficiency first increased and then reached a smooth variation region with one and a half turns. A spiral guide vane with two turns was applied in a tangential inlet dual cyclone separation system with by Pan et al. [17]. And the cyclone performance was investigated by CFD approach with the RNG k-e turbulent model. The results indicated the spiral guide vane improved the separation efficiency of small particles. However, the RNG k-e model adopting the assumption of isotropic turbulence was proved unsatisfactory for the prediction of the flow field inside cyclones [18–20]. In summary, few researchers have explored the effects of spiral guide vane turns on the flow field or separation performance in cyclones, especially in tangential inlet cyclones and larger cyclone scale. There is not sufficient data to support the industrial design of cyclone separators, to this effect. In the present study, we attempted to fill this research gap by conducting experimental and CFD tests on a tangential inlet cyclone with a barrel diameter of 300 mm to investigate the effects of spiral guide vane turns on flow field and separation performance. We hope that the results presented here provide a workable reference regarding the detailed separation process and precise effects of spiral guide
De S h H B D L
diameter of gas outlet duct (mm) gas outlet duct length (mm) cylinder height (mm) cyclone height (mm) cone diameter (mm) body diameter (mm) spiral channel height (mm)
vanes on the flow field and separation performance of cyclone separators. 2. Experimental apparatus and methods 2.1. Experimental system and cyclone model The experimental system is shown in Fig. 1. Four tangential inlet cyclone separators (with the vane turns of 0–3) with barrel diameter of 300 mm were manufactured and the detailed geometries and dimensions were shown in Fig. 2 and Table 1, respectively. 2.2. Experimental procedure and methods We conducted all experiments at atmospheric pressure and ambient temperature. Particle-laden gas was forced into the cyclone inlet with a centrifugal fan. The air flow rate was controlled by a knife switch in the outlet pipe and measured with a Pitot-tube in the outlet pipe. Particles were fed into the inlet by a star-type feeder. The inlet velocity ranged from 9.7 to 29.8 m/s and the feeding particle concentration was 30g/m3 throughout the experiment. In a cyclone, most particles were separated from the gas and ultimately collected in the discharge bin underneath. After passing through the cyclone, the gas was passed through a filter to remove any remaining particles. The particle concentration in the inlet was controlled by the particles feeding time and simultaneously the rotational speed of the feeder. The pressure drops
Fig. 1. Schematic diagram of the experimental system. (1) particle bin, (2) particle feeder, (3) U tube, (4) cyclone separator, (5) particle discharge bin, (6) Pitot tube, (7) knife switch, (8) filter, (9) fan.
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Fig. 2. Schematic representation of the cyclone separators (From left to right: the vane turns are 0, 0.5, 1, 1.5, 2–4).
Table 1 Geometrical dimensions of the cyclone used in this study. Structure
a/mm
b/mm
De/mm
S/mm
h/mm
H/mm
B/mm
D/mm
L/mm
Value
150
60
150
670
900
1650
110
300
112.5
were measured with U tube. Each experiment was repeated at least three times to ensure relative error below 0.5%. The averages were used for subsequent calculations. Talcum powder was used as the experimental particles with a physical density of 2700 kg/m3. The particle size distribution was measured by Mastersizer 2000 laser particle size analyzer. The volume mean particle diameter was approximately 17.56 lm. 3. Numerical simulation 3.1. CFD model The flow in a cyclone separator is characterized by a high swirl and an intense anisotropic turbulence. Numerous studies have verified that both RSM and LES are favorable approaches to simulate the flow field in cyclones [21–24]. However, the RSM model has its advantages regarding the low requirement of computer capacity and the short running time compared to the LES in simulation [22]. The LES provides a partially dissatisfying prediction in the boundary layer with relative coarse grids compared with the RSM [25]. In addition, while Shukla et al. [26] also reported that the mean velocities predicted by LES and RSM were very close. Given the accuracy of results and low computational cost, RSM was applied in this work to compute the gas flow field in the cyclone. Hence, the RSM is applied to predict the flow field in the cyclones. The main foundation of this model can be found in the reference [27]. We simulated the dispersed phase by tracking a large number of spherical dispersed particles through the converged flow field of continuous flow in the Lagrangian reference frame by using a two-way coupling method via discrete phase model (DPM). 3.2. Grid division and independence In this study, the seven cyclone geometries were meshed in a structured grid by means of Gambit. Multi-block structured hexahedral grids were generated in the entire domain of the cyclone as
shown in Fig. 3. We conducted a mesh independent analysis of the cyclone with 585,750, 850,833 and 1,295,350 hexahedral nodes generated by Gambit 2.3.6 of the 0-turn cyclone, respectively. The difference in cyclone pressure drops at the position Z = 750 mm between 585,750 and 1,295,350 hexahedral nodes is about 8.3%, and the difference in pressure drop between 850,833 and 1,295,350 hexahedral nodes is less than 4.7%. The relative error in tangential velocity at the position Z = 750 mm between 585,750 and 1,295,350 hexahedral cells is about 5.8%, and the same relative error between 850,833 and 1,295,350 hexahedral cells is less than 1.2% (Fig. 4). The grids with 850,833 hexahedral nodes produce grid-independent results as per the accuracy and economy of the simulation, as well as the small differences in pressure drop and tangential velocity between 850,833 and 1,295,350 hexahedral nodes. The total number of nodes and mesh quality are listed in Table 2. 3.3. CFD verification Fig. 5 shows a comparison between simulated and experimental tangential velocity and axial velocity data at the position Z = 810 mm in a 0-turn cyclone with inlet velocity of 20 m/s. The two sets of data are generally in good agreement. 3.4. Boundary conditions and simulation strategy A presumed uniform inlet velocity Vi = 20 m/s (flow rate 648 m3/h) was specified at the normal-to-inlet surface of the cyclone. The hydraulic diameter equaled 85.71 mm with turbulent intensity to be 3.72%. The outflow boundary condition was imposed at the outlet. Outflow condition assumes the fully developed flow at the outlet section. No-slip conditions are set at the wall. For the grid nodes near the wall, the standard wall function was applied. The air density of 1.225 kg/m3 and the viscosity of 1.7894 10 5 kg/m s were defined in this study. The solid particles density was 2700 kg/m3, and inlet solids concentration was 30 g/ m3.
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was selected as the time step size. The simulation was performed via the commercial software Fluent 14.0 on a work station with a CPU of Intel Xeon 2.9 GHz and RAM of 64 GB. 4. Results and discussion 4.1. General performance
Fig. 3. Grid representation of simulated cyclone (Taking the cyclone with two spiral guide vane turns as an example).
Fig. 4. The distributions of tangential velocity at the position Z =
750 in cyclone.
The finite volume method was used to discretize the partial differential equations of the model using the semi-implicit method for pressure linked equations consistent (SIMPLEC) method for pressure velocity coupling and quadratic upwind interpolation of convective kinematics (QUICK) scheme to interpolate the variables on the surface of the control volume. The implicit coupled solution algorithm was selected. In this simulation, the 10 5 convergence criterion accuracy for the calculations was applied, and 0.0002 s
4.1.1. Pressure drop Fig. 6 shows the effects of spiral guide vane turns on pressure drops in a cyclone separator with inlet velocity of 20 m/s. The predicted results were in close agreement with experimental data at a relative error within 6.6%. The pressure drops showed a critical value in the separator as spiral guide vane turns increased: the pressure drop markedly increased above or below this critical value of one turn. Fig. 7 shows the cyclone pressure drops trends with varying inlet velocity. As the inlet velocity was small (Vi = 9.7 and 15.0 m/s), the effect of the spiral guide vanes was negligible. Similar to previous hydrocyclone research by Patra et al. [37,38], at low velocity, the drag force and the extra resistance in the cyclone due to the spiral guide vanes (i.e., friction between the fluid and spiral guide vanes) oppose the fluid flow inside the cyclone separator. Due to the low velocity, drag force, and extra resistance due to the spiral guide vanes and centrifugal force vary little as the spiral turns continue. At low velocity, the combined effects of these two opposing forces are significant as compared with the centrifugal force alone. Thus, at low velocity, the effect of the spiral turns of guide vanes on the pressure drop is negligible. However, the changes in pressure drops with the increase of spiral guide vane number were more pronounced at higher inlet velocities (Vi = 20.0, 25.0 and 29.8 m/s). There was a nearly completely consistent critical pressure drop along with vane turns for higher velocities. And the trend was much more obvious especially at the inlet velocity of 29.8 m/s. Compared to the cyclone with zero vane turns, at the inlet velocity of 29.8 m/s, the maximal decrease in pressure drop with one vane turn was 8.7%; the maximal increase in pressure drop in the cyclone with two vane turns was 2.0%; the maximal increase in pressure drop in the cyclone with three vane turns was 1.5%. Vane turns above or below this critical value led to an increase in pressure drop, which marks an interesting departure from the results reported by Li et al. [16], who found that pressure drops in a radial-inlet cyclone consistently increased with the vane turns increasing. These differences would be discussed in greater detail below. 4.1.2. Collection efficiency We measured overall collection efficiencies by weighing the dust particles in particle discharge bin. The experimental results are shown in Fig. 8. A consistent critical overall collection efficiency value was observed with vane turns for almost all five velocities. The efficiency was lowest in the structure with one vane turn
Table 2 The total number of nodes and the mesh quality. Cyclone structure
Mesh characteristic Total number of nodes
0-turn 0.5-turn 1-turn 1.5-turn 2-turn 3-turn 4-turn
850,933 860,163 868,374 873,061 880,388 888,132 895,315
Maximum aspect ratio
4.1 4.1 4.1 4.1 4.1 4.1 4.1
Skewness range (0–best, 1–worst) Maximum
0–0.2
0.2–0.4
0.4–0.6
0.6–0.8
0.8–1.0
0.811 0.811 0.811 0.811 0.811 0.811 0.811
94.65% 93.89% 93.56% 93.33% 92.94% 92.18% 91.31%
4.29% 4.82% 5.11% 5.30% 5.67% 6.23% 6.98%
0.66% 0.82% 0.83% 0.86% 0.94% 1.04% 1.16%
0.32% 0.38% 0.41% 0.42% 0.45% 0.46% 0.46%
0.08% 0.09% 0.09% 0.09% 0.09% 0.09% 0.09%
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Fig. 5. Comparison between simulated and experimented velocities in cyclones.
Fig. 6. Comparison between simulated and experimented pressure drop in cyclone separators.
Fig. 7. Variation of pressure drop with vane turns.
and highest in the structure with three vane turns. To a great extent, collection efficiency was affected by tangential velocity in a cyclone. Here, the tangential velocity in the cyclone decreased as the spiral vane turns increased from zero to one (Figs. 9 and 10), reducing the centrifugal force acting on the particles. As a result, there were fewer particles captured near the inner wall with little collection efficiency. The tangential velocity and centrifugal
Fig. 8. Variation of collection efficiency with vane turns.
force acting on the particles increased gradually as the spiral guide vane turns increased from one to three (Figs. 9 and 10). More particles were captured with higher collection efficiency under the latter conditions. Compared to the cyclone with zero vane turns, the maximal decrease in collection efficiency with one vane turn was 2%; the maximal increase in collection efficiency in the cyclone with two vane turns was 0.7%; the maximal increase in collection efficiency in the cyclone with three vane turns was 1.4%. A critical collection efficiency value appeared again with increasing inlet velocity due to the effects of the radial distance of the rebounded particles from the inner wall [28]. We define this critical velocity as the ‘‘maximum-efficiency inlet velocity” [28,29]. Here the maximum-efficiency inlet velocity was 25 m/s for all four cyclones. The rebounded particles obtained more kinetic energy at higher inlet velocities, to the point where the radial distance traveled by a rebounded particle was longer than the width of the downward gas flow. The particles ultimately rebounded back into the upward gas flow. Within the rapid, upward gas flow, the particles moved towards the vortex finder quickly with little separation. Therefore, the particles rebounded out of the downward gas flow escaped the cyclone separator without being captured. At lower inlet velocities, the rebounded particles possessed less kinetic energy, so the radial distance traveled by a given rebounded particle was shorter than the width of the downward gas flow. The particles in this case moved towards the wall again via centrifugal force. The loss of energy due to collision decreased the velocity of the particles; the radial distance of the particle was truncated
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Fig. 9. Tangential velocity distribution in cyclones.
ious spiral guide vane turns for both two positions. At the position of Z = 750 mm, compared to the cyclone with zero spiral guide vane turn, the decreases in maximal tangential velocity were 1.8 m/s in the cyclone with half one turns, 2.4 m/s in the cyclone with one turn, 1.8 m/s in the cyclone with one and a half turns, 1.6 m/s in the cyclone with two turns and 0.6 m/s in the cyclone with three turns; while the increase in maximal tangential velocity were 0.3 m/s in the cyclone with four turns. However, at the position of Z = 1150 mm, compared to the cyclone with zero spiral guide vane turn, the decreases in maximal tangential velocity were 1.3 m/s in the cyclone with half one turns, 2.4 m/s in the cyclone with one turn, 1.7 m/s in the cyclone with one and a half turns, 1.3 m/s in the cyclone with two turns and 0.4 m/s in the cyclone with three turns; and the increase in maximal tangential velocity were 0.4 m/s in the cyclone with four turns.
Fig. 10. Maximum tangential velocity varying with number of vane turns.
in the subsequent rebounding process. Particles not escaping the cyclone after the first collision were unlikely to rebound into the upward gas flow in the following motion and thus unlikely to be captured. The decrease in collection efficiency at high inlet velocities was primarily caused by the escape of rebounded particles after the first collision. 4.2. Gas flow field 4.2.1. Tangential velocity Tangential velocity is an important component of the gas flow in a cyclone due to its effect on the particle separation process and pressure drop. Fig. 9 shows a comparison of the tangential (Vt) velocity profiles at the axial positions of Z = 750 and 1150 mm, respectively. It was found that spiral guide vane turns significantly influenced the tangential velocity distributions. The tangential velocity profiles remained effectually unchanged in the core region, while tangential velocity profiles in the outer vortex showed a critical tangential velocity value as the vane turns increased. Above or below the critical turn value, there was a marked decrease in tangential velocity. The maximum tangential velocity occurred in the cyclone with four spiral guide vane turns, while the minimal tangential velocity appeared in the cyclone with one spiral guide vane turn. Fig. 10 shows the maximal tangential velocity varying with spiral guide vane turns at the positions Z = 750 and 1150 mm, respectively. There was a clear minimum tangential velocity at var-
4.2.2. Axial velocity Fig. 11 shows a comparison among axial (Vz) velocity profiles at the positions Z = 750 and 1150 mm, respectively. Two distinct regions are visible in the axial velocity profiles: an outer vortex region where the velocity profiles move downward and an inner region where they move upward. The effects of spiral guide vane turns were generally irregular on the axial velocity profiles in the outer region. While, at the position Z = 750 mm in the inner region, except for the cyclone with one spiral guide vane turn, axial velocities in other cyclones were all downward. And the axial velocities in the inner region were upward in all the seven cyclones at the position Z = 1150 mm. The axial velocity in the inner region in the cyclone with one spiral guide vane turn was lower than that in other cyclones as well.
4.2.3. Static pressure As shown in Fig. 12, the distributions of static pressure at the positions Z = 750 and 1150 mm in the seven cyclones formed V-type curves. The effects of spiral vane turns were negligible in the inner region and significant in the outer region. As spiral guide vane turns increased, the static pressure in the outer region showed a critical value. With the spiral vane turns increasing from zero to one, the static pressure in the cyclone decreased; the static pressure increased as spiral guide vane turns continued to increase. And the maximal static pressure occurred in the outer region of the separator with four spiral vane turns. The distributions of static pressure in the cyclone with zero spiral guide vane turns and that with three spiral vane turns were fairly close. The negative static pressure in the inner region increased gradually with the axial direction for all seven cyclones.
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Fig. 11. Axial velocity distribution in cyclones.
Fig. 12. Static pressure distribution in cyclones.
4.2.4. Turbulence intensity The distributions of turbulence intensity in cyclone separators with different spiral guide vane turns were similar at the position Z = 750 mm and Z = 1150 mm respectively, as shown in Fig. 13. And the turbulence intensity decreased gradually along the axial direction in most regions of cyclones except for the area near the wall. From the side wall to the core, the turbulence intensity increased to maximal value, and then decreased. The maximal turbulence intensity appeared at the dimensionless radial distance r/ R = 0.53–0.56 approximately. In addition, the magnitude of turbulence intensity in cyclones with different spiral guide vane turns at the position Z = 750 almost maintained a constant value except for the cyclone with zero spiral guide vane turn. And the turbulence intensity in the core region was higher than that near the wall. While, at the position Z = 1150 mm, the abrupt curves of turbulence intensity appeared in the core region of all seven cyclones. The turbulence intensity in the core region was lower than that near the wall. Meanwhile, with the increase of spiral guide vane turns, the turbulence intensity in cyclones gradually decreased, especially at the position Z = 750 mm. This indicated that the flow field was improved and was relatively stable with increase of spiral guide vanes turns to some extent. 4.3. Grade separation efficiency Fig. 14 shows the grade efficiency of cyclones with different spiral guide vane turns. The simulation results show that the spiral guide vanes have significant effects on the grade efficiency. The
separator with one spiral guide vane turn completely removed the particles with diameter greater than 7.7 lm, while the separators with other spiral guide vane turns collected all the particles with diameter greater than 6.3 lm. For the particles range 0.8– 7 lm, the grade efficiency of separator with one spiral guide vane turn was lower than that of other separators, for example, the grade efficiency of 4.2 lm particle in the separator with one spiral guide vane turn is 91.0%, while, it is 93.7%, 93.5%, 93.1%, 95.7%, 94.6% and 96.8% in the separators with the spiral guide vane turns of 0, 0.5, 1.5, 2–4 respectively. In addition, except for one turn, for the particles range 0.8–2.9 lm, the curves of grade efficiency in other separators are so close to coincidence. However, for the particles range 2.9–6.3 lm, the curve of grade efficiency is higher in separator with four spiral guide vane turns and lower in separators with both half one turns and one and a half turns. Moreover, the critical particle size in small particles reported by Chen et al [30], Hugi et al [31] and Li et al [32], that is below which the grade efficiency increases as the particle size decreases in the experimental research, was not found in the simulated results. There may be two reasons for the difference. The first reason is that the experimental results were obtained by laser diffractometry and the smaller particle is affected significantly by optical parameters [33]. The refractive index and extinction coefficient of particles SiO2 were adopted 1.54 and 0.01 respectively according to particles structure [34], not by experimental determined. Therefore, there is possibility of inaccuracy in choosing these parameters, and hence inaccuracy in determining the smaller particle size distributions. The second reason is that the smaller particles agglomerate into
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Fig. 13. Distribution of turbulence intensity in cyclones.
ticles interaction, therefore the experiment results and simulation results of smaller particles are different.
4.4. Discussion
Fig. 14. The grade efficiency in cyclones with different spiral guide vane turns.
large particles and then are separated, which bring high separation efficiency, and the smaller particles agglomeration do occur in the cyclone separator [35,36]. Therefore, the grade efficiency curve of smaller particles is non monotonic. While the CFD ignores the par-
Fig. 15 shows the streamline of fluid in cyclones with the increase of spiral guide vane turns. When there were no spiral guide vanes, the swirling motion of flow in the cyclone was generated by the tangential inlet. The helix angle of the swirling flow was significantly larger than that of the spiral guide vanes. As the spiral guide vane turns increased from zero to one, the spiral guide vanes destroyed the original flow trajectory through the tangential inlet in the cyclone gradually and forced gas to flow along the spiral channel of the guide vanes with smaller helix angle. A single spiral guide vane turn had basically destroyed the original flow trajectory, but the acceleration of the flow along the spiral channel was insufficient to recover. Certainly, this caused a decrease in the tangential velocity and swirl intensity in the cyclone as the spiral guide vane turns increased. In the cyclone, pressure drop includes losses due to acceleration, friction, and radial pressure gradient [37,38]. Decrease in losses due to acceleration and radial pressure gradient may dominate the pressure drop. Fewer guide vane spiral turns and reduced velocity in the
Fig. 15. Schematic representation of the gas velocity streamline in cyclones (From left to right: the vane turns are 0, 0.5, 1, 1.5, 2–4).
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cyclone minimize the loss due to friction in comparison to the losses due to acceleration and radial pressure gradient. The pressure drop thus decreases as spiral guide vane turns increase from zero to one (Fig. 7). Meanwhile, the decrease of tangential velocity led to decrease in centrifugal force acting on the particles. More particles would be hard to move to the side wall for being captured with little separation. As a result, the collection efficiency decreased (Fig. 8). As the spiral guide vane turns increased from one to four, the vanes begin to more effectively guide the flow gradually. The flow continued to accelerate along the spiral guide vane with smaller helix angle as the spiral channel increased, which increased the tangential velocity and swirl intensity of the cyclone; this gradually increased the losses due to acceleration and radial pressure gradient. Meanwhile, because of the increased flow velocity and the spiral guide vanes, the loss due to friction increased as well. Hence, the pressure drop increased as spiral guide vane turns increased (Fig. 7). Moreover, the increase of tangential velocity led to increase in centrifugal force acting on the particles. Much more particles would move to the side wall for captured with higher collection efficiency (Fig. 8). That is why vane turns above or below this critical value (one turn) led to an increase both in pressure drop and collection efficiency in cyclone. Although the collection efficiency and the stability of the flow could be improved to some extent of the cyclones with the spiral turns of guide vanes increasing to more than two, the manufacturing of the spiral guide vanes is complicated. Meanwhile, the spiral guide vanes may not be suitable for application with high temperature differences because of structural stress. 5. Conclusions In this study, the effects of spiral vane turns on the performance of a tangential inlet cyclone were investigated numerically and experimentally. The variations in flow velocity, pressure, turbulence intensity, and fluid trajectory in various cyclones were assessed to explore the manner in which the spiral vane turns influence the cyclone performance. The conclusions can be summarized as follows. (1) The spiral guide vane turns considerably influenced the collection efficiency, pressure drop, and tangential velocity in the cyclone. A critical spiral guide vane turn value, below or above which there was a marked increase in collection efficiency, pressure drop, and tangential velocity, was found. Compared to a cyclone with zero vane turn, the maximal decrease of collection efficiency in a cyclone with the critical value of spiral guide vane turns (one) was 2%. And for the particles range 0.8–7 lm, the grade efficiency of separator with one spiral guide vane turn was lower than that of other separators. (2) The maximum-efficiency inlet velocity appeared to be independent of spiral guide vane turns, as inlet velocity affects the radial distance traveled by the rebounded particles from the inner wall. (3) The effects of spiral guide vane turns were generally irregular on the axial velocity profiles in the outer region of cyclones. And the effects of spiral vane turns on static pressure were negligible in the inner region and significant in the outer region. The static pressure in the outer region showed a critical value (again, one spiral vane turn) as the number of turns increased as well. (4) With the increase of spiral guide vane turns, the turbulence intensity in cyclones gradually decreased, especially in the cylinder part. In addition, the curves of turbulence intensity
in the core region were relatively abrupt in the cone part. And the turbulence intensity in the core region was lower than that near the wall. However, the cases were opposite in the cylinder part. (5) The analysis of CFD results indicated that the flow field was improved and was relatively stable with increase of spiral guide vanes turns to some extent. Maybe it could provide a workable reference for the design and application of cyclone separator.
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