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Hydrodynamic characteristics of a 2D2D cyclone separator with a finned cylindrical body
Powder Technology 363 (2020) 541–558
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Hydrodynamic characteristics of a 2D2D cyclone separator with a finned cylindrical body Mahesh Dasar, Ranjit S. Patil ⁎ Department of Mechanical Engineering, BITS PILANI - K K Birla Goa Campus, NH-17(B), Air Port Road, Zuarinagar, Goa 403726, India
a r t i c l e
i n f o
Article history: Received 24 April 2019 Received in revised form 3 November 2019 Accepted 7 January 2020 Available online 10 January 2020 Keywords: Computational fluid dynamics Collection efficiency Triangular fins
a b s t r a c t Cyclone separators used for separating solid particles from the gases mainly employed in electricity generating units having fluidized bed boilers and in other industries such as cement industries. In the present study, cylindrical portion of the nonfinned (conventional) cyclone separator (nfcs) was reshaped by fixing triangular helical fins in order to improve its performance in terms of collection efficiency. Fluid dynamic characteristics like axial velocity, tangential velocity, pressure drops etc. were studied which influences the collection efficiency by varying the fin size (fs) such as 5.0 mm, 7.5 mm and 10.0 mm and also by varying fin pitch (fp) as 50.0 mm and 30.0 mm. With available experimental work, validation was accomplished for the nfsc before proceeding computational study on the novel triangular finned cyclone separators (fcs). For the particles' size less than 3 μm, comparatively proposed triangular fin with fs & fp as 7.5 mm & 30.0 mm respectively was giving improved collection efficiency than other selected separators. Improvement in the collection efficiency of triangular finned cyclone separators (fcs) was perceived from 5% to 10% over the conventional cyclone separator (nfcs). Main role of the cyclone separators is to separate the particles from gases which was unruffled after by inclusion of fins. Rather fin inclusion has played a dominant role in minimizing very fine particulate matter emissions which otherwise leads to severe health problems. © 2020 Elsevier B.V. All rights reserved.
1. Background Cyclone separators are useful for the separating solid particles from the gases mainly employed in electricity generating units having fluidized bed boilers and in other industries such as cement industries. This is very important function in order to collect the very fine particles to reduce the environmental pollution also to circulate the un-combusted coal particles back to the riser in case of circulating fluidized bed boilers. Fluid dynamic characteristics like axial velocity, pressure drop, tangential velocity etc. reported in detail through the computational study which influences the collection efficiency of the cyclone separators [1–5]. Literature reports the 2 phase flow simulations on the cyclone separator in order to study the behavior of two-phase flow and its influence on the collection efficiency [6]. It was reported that the separation efficiency of cyclone separators sinks by increasing cylinder diameter, mixture inlet area and size of top gas outlet [6]. Mathematical model was established by Avci and Karagoz [7] to predict the separation efficiency of cyclone separators. Further Zang et al. [8] has reported the effects varying parameters such as twist angle and outer body round corner ⁎ Corresponding author. E-mail addresses: [email protected] (M. Dasar), [email protected] (R.S. Patil).
https://doi.org/10.1016/j.powtec.2020.01.021 0032-5910/© 2020 Elsevier B.V. All rights reserved.
diameter on the performance of the hexahedral cyclone separator. It was observed that with increase in these parameters leads to the higher pressure drop and lower separation efficiency. Zang et al. [9] has also reported about novel dual stage multi cyclone separators (DSCS) in order to improve the flow distribution. For a given inlet velocity, DSCS gives higher collection efficiency than other selected cyclone separators. Cyclone separators' separation efficiency improvement was reported by Mariani et al. [10] by changing the conicity angle and length of vortex finder. By changing the structural deigns of the conventional cyclone separator to improve the repeatedly sinking permitted limits of suspended fine particles in atmosphere were reported by Wasilewski and Duda [11] and they have presented total of 57 different geometric configurations of cyclone separators. Nag and Gupta [12] has reported the effects of adding the fins on heat transfer characteristics. Various parameters such as particles and gas velocity which influences on the heat transfer rate by suspending the fins inside the barrel drum of the cyclone separators' were reported by Nag and Gupta [12]. However, it was observed that complete modification of the conventional cyclone separators' barrel wall by replacing it with triangular helical fins in order improve cyclone separators efficiency/performance has not reported yet. Therefore, present study focuses on computational investigations related to the performance of cyclone separators by replacing the
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Fig. 1. (a) Geometrical configuration of 2D2D type conventional cyclone separator (left) and geometrical dimensions of 2D2D type conventional geometry used in the present study (All dimensions are in mm). (b) Geometry of proposed helical triangular fin. (c) Proposed triangular fin based cyclone separators with fin having pitch 30 mm and sizes as 5.0 mm, 7.5 mm and 10 mm (from left to right). (d) Proposed triangular fin based cyclone separators with fin having pitch 50 mm and sizes as 5.0 mm, 7.5 mm and 10 mm (from left to right).
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Fig. 1 (continued).
cylindrical portion of the conventional cyclone separator with novel triangular fins in terms of separation/collection efficiency through the fluid dynamic characteristics like axial velocity, tangential velocity, pressure drop. 2. Numerical method Novel triangular finned cyclone separators' complex swirling two-phase flow simulation was accomplished with the help of Ansys-Fluent 15.0 research code. It was reported by many researchers [1,2,4,5,13] that, Reynolds stress model (RSM) was best suitable turbulence model over k-ε turbulence model for cyclone Table 1 Dimensions of the cyclone separator used in the present study.
separator simulation. Recent studies proposed RSM precisely unerring turbulence modeling since the cyclone separator simulation dealt with high swirling [14]. Most of the industries for the turbulent flow simulations uses Reynolds stress model [15]. Reynolds stress model is best suitable turbulence model for the complex three-dimensional simulations which are having strong streamline curvature, swirling motion with rotational flow [16]. Three dimensional conservation equation for the steady, incompressible and isothermal flow as described by Safikhani et al. [17] and Hesham m. El-Batsh [18]: ∂ui ¼0 ∂x j ρu j
D
H/D
h/D
De/D
B/D
S/D
a/D
b/D
200 mm
4.0
2.0
0.50
0.25
0.625
0.25
0.50
" !# ∂τ ij ∂ðui Þ ∂P ∂ ∂ui ∂u j ¼− þ μ þ ; þ ∂x j ∂xi ∂x j ∂x j ∂xi ∂x j
ð1Þ
ð2Þ
where, ui and u j Mean velocity in i and j directions, respectively. xj is the
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Fig. 2. (a) Cut section of computational grid of helical triangular finned cyclone separator showing the fins fixed on the barrel wall in the present study. (b) Cut section of computational grid of helical triangular finned cyclone separator at the vortex finder region in the present study. (c) Cut section of computational grid of proposed helical triangular finned cyclone separator utilized in the present study.
position, P is the mean pressure, ρ is the constant gas density, τij is the 0
0
Reynolds stress tensor given by τ ij ¼ −ρui u j . 2.1. Discrete phase model As the particles hit cyclone wall soon after entering through gas-inlet, the particles affected by the centrifugal force having the higher inward drag force are forced to move downward, similarly the particles'
having the lower inward drag force than the centrifugal force starts moving in upward direction. The particles affected by centrifugal force having the equal inward drag force to centrifugal force, they start rotating in equilibrium and starts moving down and hits the slant edge of the cyclone separator and are collected at the solid-outlet of the cyclone separator. Two phase flow was modeled with discrete phase model which is Eulerian Lagrangian approach in Ansys-Fluent 15.0. Isolated particles' flow simulation is performed with help of particle tracking method. Separation efficiency was obtained using Eulerian-
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Fig. 3. Mesh independence study carried out for 2D2D type conventional cyclone separator in the present study.
Fig. 4. Validation of the computational data related to normal cyclone separator using the experimental data of Wang et al. [5].
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Table 2 Final computation mesh used after completing the mesh dependency test for all 6 sets of helical triangular fin based cyclone separators. Fin shape
Pitch size Fin size Chosen number (mm) (mm) of nodes
Triangular 30 fin 50
5 7.5 10 5 7.5 10
585 K 596 K 604 K 564 K 571 K 575 K
Identification name henceforth
Y+ value measured
Set-1 Set-2 Set-3 Set-4 Set-5 Set-6
26.12 26.21 33.71 37.05 40.18 32.49
dxpi ¼ upi dt
ð3Þ
ð4Þ
where FD(ui − upi) is the drag force per unit particle mass, FD ¼
18μ C D Rep 2 24
ρp dp
ρp dp u−up Rep ¼ μ
ð5Þ
ð6Þ
Drag coefficient CD is a function of relative Reynolds numbers, which is in the form of: C D ¼ 24= Rep For 0 b Rep ≤1 For 0 b Rep ≤1000 C D ¼ 24= Rep 1 þ 0:15 Re0:687 p
Meshing was accomplished with the help of Ansys-ICEMCFD 15.0 meshing tool. Unstructured hybrid mesh was created with hexahedral cells at core regime of cyclone separator and three-layered prismatic cells to capture boundary layer physics near the wall. Tetrahedral cells were used to connect prismatic & hexahedral cells (refer Fig. 2(a, b & c)). Mesh quality above 0.2 was achieved for all sets (six) of the proposed triangular finned cyclone separators and overall around 80% mesh quality having the range from 0.9 to 1.
2.4. Computational details
Lagrangian approach [19]. Particle motion equation used was g i ρp −ρ dupi ¼ F D ui −upi þ dt ρp
2.3. Meshing
ð7Þ
In the present work, RSM model was enabled. Near wall treatment was carried out using the scalable-wall function with standard constants. Simulation parameters' settings for all the six sets of triangular finned cyclone separators and conventional (non-finned) cyclone separator was same. Hence for all the cyclone separators, mixture inlet (Fig. 1(a)) through which gas+solid mixture enters the cyclone separator was assigned with velocity-inlet boundary condition with 20 m/s with 3% turbulence intensity and hydraulic diameter as 0.06 [5,20]. Pressure-outlet boundary condition was applied at gas-outlet with 3% turbulent intensity and hydraulic diameter as 0.1 m. The no-slip with wall boundary condition was chosen for the solid outlet and cyclone body. Solution was controlled using 40 as flow-courant number, explicit-relaxation factor as 0.6 for the pressure, and 0.08 for the momentum. Steady state solver with Reynolds stress model was utilized by setting convergence criteria of 10−5. Many investigations are available [5,21,22] where the researchers have implemented and executed steady state simulations on cyclone separators using Reynolds stress model. Flow behavior by changing the outflow length was proposed by Wang et al. [5] carrying out steady and unsteady state computational simulations on cyclone separator. They also observed that, unsteady state solver may affect slightly on the flow field behavior. However qualitatively and quantitatively, axial & tangential velocity profiles were observed to be very close for the steady and unsteady state solver.
2.2. Modeling When the barrel height and conical length are double the barrel diameter; Lapple proposed it as a 2D2D type cyclone separators. 2D2D type cyclone separator was witnessed as highly performable and highly preferable in many industries among all other types of cyclone separator available [20]. In the present study, 2D2D type conventional (nonfinned) cyclone separator (nfcs) (Fig. 1(a)) was used with geometrical dimensions shown in Table 1. Geometry of proposed novel triangular helical fin (Fig. 1(b)) which illustrates the solid base (blue color) and hollow triangular fin (yellow color) fixed on the solid base. The solid base of fin differ in sizes to get different base sizes such as for fin size (fs) 5.0 mm, solid base size was 10 mm × 5 mm; for fs 7.5 mm, solid base was 15 mm × 5 mm; and similarly for fs 10.0 mm, the solid base size was 20 mm × 5 mm (refer Fig. 1(c & d)). The proposed triangular finned cyclone separator varies with three fs and two fin pitch (fp) variations, such as fs 5.0 mm with fp 30.0 & 50.0 mm; fs 7.5 mm with fp 30.0 & 50.0 mm; similarly fs 10.0 mm with fp 30.0 & 50.0 mm. Fig. 1 (c) shows the proposed helical triangular finned cyclone separators with fs variations from 5.0 to 10.0 mm from left to right with fp 30 mm. Fig. 1(d) shows the proposed helical triangular finned cyclone separators with fs variations from 5.0 to 10 mm from left to right with fp 50 mm. With intent to improve performance of cyclone separator on the bases of collection/separation efficiency, 6 sets of triangular fcs were proposed which varies with fs and fp. Optimized fs and fp will be found on the basis of fluid dynamic characteristics and collection/ separation efficiency through computational study.
2.5. Mesh dependency and validation Mesh numbers variations dependency on the results was carried out for the nfcs. Meshing was performed in ICEM 15.0 research code from Ansys using the same meshing parameters used during the simulation of triangular finned cyclone separators. Simulations were started with 337 K nodes and did the mesh independence study by almost doubling the mesh density such as 617 K nodes, 1200 K nodes. Simulation were performed in the Ansys Fluent 15.0 research code with the same solver setting which were used while simulating the triangular finned cyclone separator simulation. The simulation work was performed on the 617 K grids for the nfcs. As observed in Fig. 3 the tangential velocity profiles obtained at distance six hundred and fifty mm from the solid outlet with 617 K and 1200 K nodes were almost close while computational time was nearly doubling with 1200 K nodes when compared to 617 K nodes. Therefore, without compromising the accuracy of the results and also to be with lower computational time, 617 K nodes were chosen to validate the results with experimental results of Wang et al. [5]. Validation of the simulation settings used while performing the computational work for nfcs was carried out using the experimental data of Wang et al. [5] as shown in the Fig. 4. As observed in Fig. 4, less than 10% difference was observed between experimental tangential velocity profile by Wang et al. [5] and computational tangential velocity data for nfcs carried out in the present study.
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Fig. 5. (a) Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 5.0 mm with non-finned cyclone separator. (b). Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 7.5 mm with non-finned cyclone separator. (c). Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 10 mm with non-finned cyclone separator. (d). Comparisons of axialvelocity profiles between triangular fin based cyclone separator having fin pitch 50 mm and size as 5.0 mm with non-finned cyclone separator. (e). Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 50 mm and size as 7.5 mm with non-finned cyclone separator. (f). Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 50.0 mm and size as 10.0 mm with non-finned cyclone separator. (g). Comparisons of axial-velocities for 6 fin based cyclone separators.
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Fig. 5 (continued).
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Fig. 5 (continued).
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Fig. 5 (continued).
Fig. 6. Axial pressure drop for 6 sets of fin based and non-finned cyclone separators.
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Fig. 7. (a) Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 5.0 mm with non-finned cyclone separator. (b). Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 7.5 mm with non-finned cyclone separator. (c). Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 10 mm with non-finned cyclone separator. (d). Comparisons of tangentialvelocity profiles between triangular fin based cyclone separator having fin pitch 50 mm and size as 5.0 mm with non-finned cyclone separator. (e). Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 50 mm and size as 7.5 mm with non-finned cyclone separator. (f). Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 50.0 mm and size as 10.0 mm with non-finned cyclone separator. (g). Comparisons of tangential velocities for 6 fin based cyclone separators.
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Fig. 7 (continued).
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Fig. 7 (continued).
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Fig. 7 (continued).
2.6. Mesh dependency study for triangular finned cyclone separators Mesh dependency on the results was carried out with the same procedure as explained in section 2.5 for nfcs for all 6 groups of triangular fcs. Geometry of the triangular fcs varies with fin size of 5.0 mm, 7.5 mm and 10.0 mm along with pitch variations as 50.0 & 30.0 mm. For the grid independency test, for the set 1 (refer Table 2), simulation was started with 376,707 nodes and did the mesh independence study by almost doubling the mesh density such as 585,902 nodes, 1,235,572 nodes. It was observed that results obtained with 585 K and 1235 K nodes were almost close while computational time was nearly doubling with 1235 K nodes in comparison with 585 K nodes. In the similar way mesh dependency study is carried out for remaining 5 sets of triangular finned cyclone separators and the selected mesh for further post processing work for all fcs with Y+ value are shown in the Table-2. 3. Results and discussion In this section, the output of triangular finned cyclone separators (fcs) in terms of fluid dynamics characteristics influencing the separation efficiency discussed in detail. Also results of the fcs by varying fin size (fs) and fin pitch (fp) were compared with conventional nfcs. 3.1. Axial-velocity profiles Collection efficiency of cyclone separators is strongly influenced by the axial velocity. Fig. 6(a-f) show the comparison of axial velocity profiles 6 groups of fcs with nfcs at various locations 300 mm, 500 mm and 700 above the bottom of cyclone separator. The different locations along the height of cyclone separators from
where the axial velocity data was extracted are, 300 mm which is in cone region, 500 mm which is at the end of cylindrical drum portion where helical fins turn ends and 700 mm which is just below the vortex finder in the cylindrical region of cyclone separators. The axial velocity profiles proposed by Wang et al. [5,23–27] are qualitatively similar to those presented in the current study for fcs and nfcs. As observed from Fig. 5(a-f) at 700 mm location because of the swirling flow and centrifugal force inside the cyclone separators, high self-weight particles were pushed towards wall and particles within the region measured 40 mm from the wall were moving downwards as the axial velocity is observed to be negative means axial velocity's direction was in downward direction. In this regime due to friction, axial velocity continuously decreases till the end of cylindrical portion of the cyclone separator and then increases due to sloping edges of the conical regime. Effects of fin's pitch and size variations at five hundred mm location on axial-velocity profile of fcs is as shown in Fig. 5(g). Influence of fin's pitch and size variations on axial velocity was observed to be very minute as realized from Fig. 5(g). However as shown in Fig. 5(g), fins with 30 mm pitch were having higher axial velocity in the upward direction than fins with 50 mm pitch at the core region of cyclone separator. Hence separation efficiency (section 3.4) for fins with 30 mm pitch was noted to be higher than that of fins 50 mm pitch. 3.2. Pressure field: axial pressure drop Axial pressure-drop (ΔP) measured for all 6 sets of fcs and a nfcs is as shown in Fig. 6. Axial pressure drop is the difference between the pressure measured at the start of helical fin and the end of helical fins which
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Fig. 8. (a) Comparisons of separating efficiency for the fin based cyclone separator having 30 mm pitch and cyclone separator without fin. (b) Comparisons of separating efficiency for the fin based cyclone separator having 50 mm pitch and cyclone separator without fin.
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Fig. 9. (a) shows the front and top view of particles' tracking for cyclone separator without fin at cylindrical region and (b) shows the front and top view of particles' tracking for cyclone separator without fin at cone region. (c) shows the front and top view of particles' tracking for fin based cyclone separator at cylindrical region and (d) shows the front and top view of particles' tracking for fin based cyclone separator at cone region.
is 208 mm and 493 mm respectively. With the same locations (208 mm and 493 mm pressure measuring points) pressure difference was calculated for nfcs.
From the Fig. 7, it should be noted that pressure drop measured for fin having pitch 30 mm and size 5.0 mm is highest among all the 6sets of fcs while it was lowest for fin having pitch 50 mm and size
Fig. 10. Reverse-flow locations in the cyclone separators with fin and without fin.
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10.0 mm. Pressure drop noted for nfcs was lower than pressure drop reported for any of the fcs. Increase in pressure drop is observed because of additional frictional losses due to modification made with cylindrical wall by fixing the fins. However, this loss can be compensated through improvement noted in separation (collection) efficiency (section 3.4) and possible reduction of heat loss from the hot wall of the cyclone separator by making possible heat transfer between hot gases flowing outside the fin and the water flowing inside the fin. It will be advisable to fix the fin on the inner wall rather than fixing it on the outer wall since heat transfer rate will increase due to increased surface area due to fins fixed on inner wall and it will also help to increase the separation (collection) efficiency. It was also a good sign that pressure drop decreases while separation/collection efficiency (section 3.4) increases with increase in fin's pitch. 3.3. Tangential velocity profiles Solid+air mixture flow inside the cyclone separators will be dominated by tangential velocity creating sturdy shear in radial direction and that creates centrifugal force which in-turn decides the particle separation [28]. Fig. 7(a-f) shows the comparison of tangential velocity profiles of 6 groups of triangular fcs with conventional nfcs at various locations 300 mm, 500 mm and 700 mm measured from the solid outlet along the height of the cyclone separator. The different locations along the height cyclone separators from where the tangential velocity data was extracted are, 300 mm which is in cone region, 500 mm which is at the end of cylindrical drum portion where helical fin-turns ends and 700 mm which is just below the vortex finder in the cylindrical region of cyclone separators. The tangential velocity profiles trends were observed qualitatively same trend as investigated by many researchers [29–35]. It is observed that tangential velocity in the region between 20 mm to 80 mm from the center (both left and right sides) slightly higher compare to conventional nfcs at 700 mm where the solid particles' concentration was high, this in-turn helps in improved collection/separation efficiency for the fcs. It is also observed from Fig. 8(a-g), as the tangential velocity of mixture flow decreases as flow moves downwards along the wall of cyclone separator and becomes zero at 100 mm radial distance from the center due to available friction between fluid & wall and no-slip boundary condition, which helps solid particles' stick to wall till it reaches collection point (solid outlet at the bottom of the cyclone separator). Comparisons of tangential velocity profile for 6 fcs is as shown in Fig. 7(g) which represents the effects of fin size and pitch variations made at five hundred mm above the solid outlet. Influence of fin size and pitch variations on tangential velocity was observed to be very minute as realized from Fig. 7(g). However tangential velocity for fin having 30.0 mm pitch and 7.5 mm size is slightly higher than all other fcs and the same was experienced with higher collection/separation efficiency. 3.4. Separation/collection efficiency Fig. 8(a-b) reveal the collection/separation efficiency comparisons for fcs and nfcs. Fig. 8(a) reveals the collection efficiency for various fin sizes with pitch 30 mm while Fig. 8(b) for pitch 50 mm. The improvement in the collection efficiency was noted for the fcs over the nfcs because of the improved tangential velocity (as shown in Fig. 7(a-f)) in the higher particle concentration region (close to inlet of cyclone separator i.e. about seven hundred mm above the bottom of cyclone separator) where tangential velocity forces the particles towards wall. Hence more particles were pushed (Fig. 9(c)) towards wall in fcs than nfcs (Fig. 9(a)). Subsequently improved axial velocity (as shown in Fig. 5(a-f)) helps further to push these particles in the downward direction towards bottom solid outlet. Additionally, helical path formed between the subsequent turns of the fin help to provide the space for
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the particles to gather and to move downwards towards following helical path. These particles experiences additional frictional force due to fin other than friction caused due to cyclone separator's wall which helps to avoid their movement towards the central core of air jet moving in the upward direction towards gas outlet at the top. Hence number of particles collected at bottom of fcs (Fig. 9(d)) were more than those with nfcs (Fig. 9(b)). Also as shown in Fig. 10, reversal of air jet takes place close to the bottom solid outlet for the fcs having fin with pitch 30 mm and size 7.5 mm hence chances of more particles' separation were increased with this fin (30 mm pitch, 7.5 mm size). 4. Conclusions Important conclusions can be made from the present study are: • Tangential velocity was observed to be more with fin based cyclone separator (fcs) than non-finned cyclone separator (nfcs) which helped to improve the separation or collection efficiency of fcs. Separation efficiency of fcs having fin with pitch thirty mm and size seven and half mm observed to largest among the selected fcs and it was giving about 10% more efficiency than nfcs. • Axial pressure drop increases by adding fins but it will be recovered by increase in the separating efficiency in comparison with nfcs. • Axial pressure drop decreases (about 20%) while collection/separation efficiency increases with increase fin's pitch. However, fin's sizes were not affecting much on the axial velocity, tangential velocity and collection/separation efficiency. • Helical pathway formed due to fins exerts additional friction on the particles along with friction exerted on them due to wall of the cyclone separator which helps to separate more number of particles from gas/air. • Overall more numbers of very fine particles (below three microns) were separated in case of fcs than nfcs. Declaration of Competing Interests The authors declare that they have no known competing financial interestsor personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements SERB - Govt. of India, Grant ID SB/FTP/ETA-0007/2014. References [1] M. Azadi, M. Azadi, M. Ali, A CFD study of the effect of cyclone size on its performance parameters, J. Hazard. Mater. 182 (1–3) (2010) 835–841. [2] A.J. Hoekstra, J.J. Derksen, H.E.A. Van Den Akker, An experimental and numerical study of turbulent swirling flow in gas cyclones, Chem. Eng. Sci. 54 (1999) 2055–2065. [3] L. Shi, D.J. Bayless, Comparison of boundary conditions for predicting the collection efficiency of cyclones, Powder Technol. 173 (1) (2007) 29–37. [4] G. Wan, G. Sun, X. Xue, M. Shi, Solids concentration simulation of different size particles in a cyclone separator, Powder Technol. 183 (1) (2008) 94–104. [5] B. Wang, D.L. Xu, K.W. Chu, A.B. Yu, Numerical study of gas–solid flow in a cyclone separator, Appl. Math. Model. 30 (11) (2006) 1326–1342. [6] M. Trefz, E. Muschelknautz, Extended cyclone theory for gas flows with high solid concentrations, Chem. Eng. Technol. 16 (1993) 153–160. [7] A. Avci, I. Karagoz, Effects of flow and geometrical parameters on the collection efficiency in cyclone separators, J. Aerosol Sci. 34 (2003) 937–955. [8] T. Zhang, C. Liu, K. Guo, H. Liu, Z. Wang, Analysis of flow field in optimal cyclone separators with hexagonal structure using mathematical models and computational fluid dynamics simulation, Ind. Eng. Chem. Res. 55 (2016) 351–365. [9] T. Zhang, K. Guo, C. Liu, Y. Li, M. Tao, C. Shen, Experimental and numerical investigations of a dual-stage cyclone separator, Chem. Eng. Technol. 41 (3) (2018) 606–617. [10] F. Mariani, F. Risi, C.N. Grimaldi, Separation efficiency and heat exchange optimization in a cyclone, Sep. Purif. Technol. 179 (2017) 393–402. [11] M. Wasilewski, J. Duda, Multicriteria optimization of first-stage cyclones in the clinker burning system by means of numerical modelling and experimental research, Powder Technol. 289 (2016) 143–158. [12] P.K. Nag, A.V.S.S.K.S. Gupta, Fin heat transfer studies in a cyclone separator of a circulating fluidized bed, Heat Transfer Eng. 20 (1999) 28–34.
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[13] K. Elsayed, C. Lacor, The effect of cyclone inlet dimensions on the flow pattern and performance, Appl. Math. Model. 35 (2011) 1952–1968. [14] J. Parekh, R. Rzehak, Euler–Euler multiphase CFD-simulation with full Reynolds stress model and anisotropic bubble-induced turbulence, Int. J. Multiphase Flow 99 (2018) 231–245. [15] M,.D. Slack, R.O. Prasad, A. Bakker, F. Boysan, Advances in cyclone modelling using unstructured grids, Trans IChemE., Part-A. 78 (2000). [16] Modeling Turbulent Flow, Introductory fluent training, Dec-2006 (www.southampton.ac.uk/~nwb/lectures/…/Turbulence_Notes_Fluent-v6.3.06.pdf). [17] H. Safikhani, M.A. Akhavan Behabadi, M. Shams, M.H. Rahimyan, Numerical simulation of flow field in three types of standard cyclone separators, Adv. Powder Technol. 21 (2010) 435–442. [18] Hesham M. El-Batsh, Improving cyclone performance by proper selection of the exit pipe, Appl. Math. Model. 37 (2013) 5286–5303. [19] B. Zhao, Y. Su, J. Zhang, Simulation of gas flow pattern and separation efficiency in cyclone with conventional single and spiral double inlet configuration, Trans IChemE., Part A., Chemical Eng. Res. and Des. 84 (A12) (2006) 1158–1165. [20] T. Mothilal, K. Pitchandi, V. Velukumar, K. Parthiban, CFD and statistical approach for optimization of operating parameters in a tangential cyclone heat exchanger, J. Appl. Fluid Mech. 11 (2) (2018) 459–466. [21] K. Udaya Bhaskar, Y. Rama Murthy, M. Ravi Raju, S. Tiwari, J.K. Srivastava, N. Ramakrishnan, CFD simulation and experimental validation studies on hydrocyclone, Miner. Eng. 20 (2007) 60–71. [22] Y. Rama Murthy, K. Udaya Bhaskar, Parametric CFD studies on hydrocyclone, Powder Technol. 230 (2012) 36–47. [23] N.L.P. M'Bouana, D. Lia, T. Yangjia, T. Yang, H. Lu, Numerical simulation of gas and particle flow in cyclone separators, AIP Conference Proceedings. 1547 (2013) 545–554. [24] P. Farzad, H.H. Seyyed, A. Goodarz, E. Khairy, Impacts of the vortex finder eccentricity on the flow pattern and performance of a gas cyclone, Sep. Purif. Technol. 187 (2017) 1–13.
[25] M. Dzmitry, G.A. Anders, S.L. Tord, Effects of the inlet angle on the flow pattern and pressure drop of a cyclone with helical-roof inlet, Chem. Eng. Res. Des. 102 (2015) 307–321. [26] G.B. Sakura, Y.T.L. Andrew, CFD simulation of cyclone separators to reduce air pollution, Powder Technol. 286 (2015) 488–506. [27] R. Utikar, N. Darmawan, M. Tade, Q. Li, G. Evans, M. Glenny, V. Pareek, Hydrodynamic simulation of cyclone separators, Computational Fluid Dynamics book, Chapter 11. 2010, p. 420 , (ISBN 978–953-7619-59-6). [28] C. Cortes, A. Gil, Modeling the gas and particle flow inside cyclone separators, Prog. Energy Combust. Sci. 33 (2007) 409–452. [29] Z. Faqi, S. Guogang, Z. Yuming, C. Hui, W. Qing, Experimental and CFD study on the effects of surface roughness on cyclone performance, Sep. Purif. Technol. 193 (2018) 175–183. [30] V. Kumar, K. Jha, Numerical investigations of the cone-shaped vortex finders on the performance of cyclone separators, J. Mech. Sci. Technol. 32 (11) (2018) 5293–5303. [31] P. Bunyawanichakul, M.P. Kirkpatrick, J.E. Sargison, G.J. Walker, Numerical and experimental studies of the flow field in a cyclone dryer, J. Fluids Eng. 128 (2006) 1240–1250. [32] O. Akiyama, C. Kato, Numerical investigations of unsteady flows and particle behavior in a cyclone separator, J. Fluids Eng. 139 (2017) 913021–9130211. [33] P.H. Jafari, D. Misiulia, J.G.I. Hellstrom, B.R. Gebart, Modeling of particle-laden cold flow in a cyclone gasifier, J. Fluids Eng. 141 (2019) 0213021–02130213. [34] H. Zhou, Z. Hu, Q. Zang, Q. Wang, W. Lv, Numerical study on gas-solid flow characteristics of ultra-light particles in a cyclone separator, Powder Technol. 3441 (2019) 784–796. [35] J. Zhang, Z. Zha, P. Che, H. Ding, W. Pan, Influences of inlet height and velocity on main performances in the cyclone separator, Part. Sci. Technol. 37 (6) (2018) 669–676.
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Hydrodynamic characteristics of a 2D2D cyclone separator with a finned cylindrical body Mahesh Dasar, Ranjit S. Patil ⁎ Department of Mechanical Engineering, BITS PILANI - K K Birla Goa Campus, NH-17(B), Air Port Road, Zuarinagar, Goa 403726, India
a r t i c l e
i n f o
Article history: Received 24 April 2019 Received in revised form 3 November 2019 Accepted 7 January 2020 Available online 10 January 2020 Keywords: Computational fluid dynamics Collection efficiency Triangular fins
a b s t r a c t Cyclone separators used for separating solid particles from the gases mainly employed in electricity generating units having fluidized bed boilers and in other industries such as cement industries. In the present study, cylindrical portion of the nonfinned (conventional) cyclone separator (nfcs) was reshaped by fixing triangular helical fins in order to improve its performance in terms of collection efficiency. Fluid dynamic characteristics like axial velocity, tangential velocity, pressure drops etc. were studied which influences the collection efficiency by varying the fin size (fs) such as 5.0 mm, 7.5 mm and 10.0 mm and also by varying fin pitch (fp) as 50.0 mm and 30.0 mm. With available experimental work, validation was accomplished for the nfsc before proceeding computational study on the novel triangular finned cyclone separators (fcs). For the particles' size less than 3 μm, comparatively proposed triangular fin with fs & fp as 7.5 mm & 30.0 mm respectively was giving improved collection efficiency than other selected separators. Improvement in the collection efficiency of triangular finned cyclone separators (fcs) was perceived from 5% to 10% over the conventional cyclone separator (nfcs). Main role of the cyclone separators is to separate the particles from gases which was unruffled after by inclusion of fins. Rather fin inclusion has played a dominant role in minimizing very fine particulate matter emissions which otherwise leads to severe health problems. © 2020 Elsevier B.V. All rights reserved.
1. Background Cyclone separators are useful for the separating solid particles from the gases mainly employed in electricity generating units having fluidized bed boilers and in other industries such as cement industries. This is very important function in order to collect the very fine particles to reduce the environmental pollution also to circulate the un-combusted coal particles back to the riser in case of circulating fluidized bed boilers. Fluid dynamic characteristics like axial velocity, pressure drop, tangential velocity etc. reported in detail through the computational study which influences the collection efficiency of the cyclone separators [1–5]. Literature reports the 2 phase flow simulations on the cyclone separator in order to study the behavior of two-phase flow and its influence on the collection efficiency [6]. It was reported that the separation efficiency of cyclone separators sinks by increasing cylinder diameter, mixture inlet area and size of top gas outlet [6]. Mathematical model was established by Avci and Karagoz [7] to predict the separation efficiency of cyclone separators. Further Zang et al. [8] has reported the effects varying parameters such as twist angle and outer body round corner ⁎ Corresponding author. E-mail addresses: [email protected] (M. Dasar), [email protected] (R.S. Patil).
https://doi.org/10.1016/j.powtec.2020.01.021 0032-5910/© 2020 Elsevier B.V. All rights reserved.
diameter on the performance of the hexahedral cyclone separator. It was observed that with increase in these parameters leads to the higher pressure drop and lower separation efficiency. Zang et al. [9] has also reported about novel dual stage multi cyclone separators (DSCS) in order to improve the flow distribution. For a given inlet velocity, DSCS gives higher collection efficiency than other selected cyclone separators. Cyclone separators' separation efficiency improvement was reported by Mariani et al. [10] by changing the conicity angle and length of vortex finder. By changing the structural deigns of the conventional cyclone separator to improve the repeatedly sinking permitted limits of suspended fine particles in atmosphere were reported by Wasilewski and Duda [11] and they have presented total of 57 different geometric configurations of cyclone separators. Nag and Gupta [12] has reported the effects of adding the fins on heat transfer characteristics. Various parameters such as particles and gas velocity which influences on the heat transfer rate by suspending the fins inside the barrel drum of the cyclone separators' were reported by Nag and Gupta [12]. However, it was observed that complete modification of the conventional cyclone separators' barrel wall by replacing it with triangular helical fins in order improve cyclone separators efficiency/performance has not reported yet. Therefore, present study focuses on computational investigations related to the performance of cyclone separators by replacing the
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Fig. 1. (a) Geometrical configuration of 2D2D type conventional cyclone separator (left) and geometrical dimensions of 2D2D type conventional geometry used in the present study (All dimensions are in mm). (b) Geometry of proposed helical triangular fin. (c) Proposed triangular fin based cyclone separators with fin having pitch 30 mm and sizes as 5.0 mm, 7.5 mm and 10 mm (from left to right). (d) Proposed triangular fin based cyclone separators with fin having pitch 50 mm and sizes as 5.0 mm, 7.5 mm and 10 mm (from left to right).
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543
Fig. 1 (continued).
cylindrical portion of the conventional cyclone separator with novel triangular fins in terms of separation/collection efficiency through the fluid dynamic characteristics like axial velocity, tangential velocity, pressure drop. 2. Numerical method Novel triangular finned cyclone separators' complex swirling two-phase flow simulation was accomplished with the help of Ansys-Fluent 15.0 research code. It was reported by many researchers [1,2,4,5,13] that, Reynolds stress model (RSM) was best suitable turbulence model over k-ε turbulence model for cyclone Table 1 Dimensions of the cyclone separator used in the present study.
separator simulation. Recent studies proposed RSM precisely unerring turbulence modeling since the cyclone separator simulation dealt with high swirling [14]. Most of the industries for the turbulent flow simulations uses Reynolds stress model [15]. Reynolds stress model is best suitable turbulence model for the complex three-dimensional simulations which are having strong streamline curvature, swirling motion with rotational flow [16]. Three dimensional conservation equation for the steady, incompressible and isothermal flow as described by Safikhani et al. [17] and Hesham m. El-Batsh [18]: ∂ui ¼0 ∂x j ρu j
D
H/D
h/D
De/D
B/D
S/D
a/D
b/D
200 mm
4.0
2.0
0.50
0.25
0.625
0.25
0.50
" !# ∂τ ij ∂ðui Þ ∂P ∂ ∂ui ∂u j ¼− þ μ þ ; þ ∂x j ∂xi ∂x j ∂x j ∂xi ∂x j
ð1Þ
ð2Þ
where, ui and u j Mean velocity in i and j directions, respectively. xj is the
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Fig. 2. (a) Cut section of computational grid of helical triangular finned cyclone separator showing the fins fixed on the barrel wall in the present study. (b) Cut section of computational grid of helical triangular finned cyclone separator at the vortex finder region in the present study. (c) Cut section of computational grid of proposed helical triangular finned cyclone separator utilized in the present study.
position, P is the mean pressure, ρ is the constant gas density, τij is the 0
0
Reynolds stress tensor given by τ ij ¼ −ρui u j . 2.1. Discrete phase model As the particles hit cyclone wall soon after entering through gas-inlet, the particles affected by the centrifugal force having the higher inward drag force are forced to move downward, similarly the particles'
having the lower inward drag force than the centrifugal force starts moving in upward direction. The particles affected by centrifugal force having the equal inward drag force to centrifugal force, they start rotating in equilibrium and starts moving down and hits the slant edge of the cyclone separator and are collected at the solid-outlet of the cyclone separator. Two phase flow was modeled with discrete phase model which is Eulerian Lagrangian approach in Ansys-Fluent 15.0. Isolated particles' flow simulation is performed with help of particle tracking method. Separation efficiency was obtained using Eulerian-
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Fig. 3. Mesh independence study carried out for 2D2D type conventional cyclone separator in the present study.
Fig. 4. Validation of the computational data related to normal cyclone separator using the experimental data of Wang et al. [5].
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Table 2 Final computation mesh used after completing the mesh dependency test for all 6 sets of helical triangular fin based cyclone separators. Fin shape
Pitch size Fin size Chosen number (mm) (mm) of nodes
Triangular 30 fin 50
5 7.5 10 5 7.5 10
585 K 596 K 604 K 564 K 571 K 575 K
Identification name henceforth
Y+ value measured
Set-1 Set-2 Set-3 Set-4 Set-5 Set-6
26.12 26.21 33.71 37.05 40.18 32.49
dxpi ¼ upi dt
ð3Þ
ð4Þ
where FD(ui − upi) is the drag force per unit particle mass, FD ¼
18μ C D Rep 2 24
ρp dp
ρp dp u−up Rep ¼ μ
ð5Þ
ð6Þ
Drag coefficient CD is a function of relative Reynolds numbers, which is in the form of: C D ¼ 24= Rep For 0 b Rep ≤1 For 0 b Rep ≤1000 C D ¼ 24= Rep 1 þ 0:15 Re0:687 p
Meshing was accomplished with the help of Ansys-ICEMCFD 15.0 meshing tool. Unstructured hybrid mesh was created with hexahedral cells at core regime of cyclone separator and three-layered prismatic cells to capture boundary layer physics near the wall. Tetrahedral cells were used to connect prismatic & hexahedral cells (refer Fig. 2(a, b & c)). Mesh quality above 0.2 was achieved for all sets (six) of the proposed triangular finned cyclone separators and overall around 80% mesh quality having the range from 0.9 to 1.
2.4. Computational details
Lagrangian approach [19]. Particle motion equation used was g i ρp −ρ dupi ¼ F D ui −upi þ dt ρp
2.3. Meshing
ð7Þ
In the present work, RSM model was enabled. Near wall treatment was carried out using the scalable-wall function with standard constants. Simulation parameters' settings for all the six sets of triangular finned cyclone separators and conventional (non-finned) cyclone separator was same. Hence for all the cyclone separators, mixture inlet (Fig. 1(a)) through which gas+solid mixture enters the cyclone separator was assigned with velocity-inlet boundary condition with 20 m/s with 3% turbulence intensity and hydraulic diameter as 0.06 [5,20]. Pressure-outlet boundary condition was applied at gas-outlet with 3% turbulent intensity and hydraulic diameter as 0.1 m. The no-slip with wall boundary condition was chosen for the solid outlet and cyclone body. Solution was controlled using 40 as flow-courant number, explicit-relaxation factor as 0.6 for the pressure, and 0.08 for the momentum. Steady state solver with Reynolds stress model was utilized by setting convergence criteria of 10−5. Many investigations are available [5,21,22] where the researchers have implemented and executed steady state simulations on cyclone separators using Reynolds stress model. Flow behavior by changing the outflow length was proposed by Wang et al. [5] carrying out steady and unsteady state computational simulations on cyclone separator. They also observed that, unsteady state solver may affect slightly on the flow field behavior. However qualitatively and quantitatively, axial & tangential velocity profiles were observed to be very close for the steady and unsteady state solver.
2.2. Modeling When the barrel height and conical length are double the barrel diameter; Lapple proposed it as a 2D2D type cyclone separators. 2D2D type cyclone separator was witnessed as highly performable and highly preferable in many industries among all other types of cyclone separator available [20]. In the present study, 2D2D type conventional (nonfinned) cyclone separator (nfcs) (Fig. 1(a)) was used with geometrical dimensions shown in Table 1. Geometry of proposed novel triangular helical fin (Fig. 1(b)) which illustrates the solid base (blue color) and hollow triangular fin (yellow color) fixed on the solid base. The solid base of fin differ in sizes to get different base sizes such as for fin size (fs) 5.0 mm, solid base size was 10 mm × 5 mm; for fs 7.5 mm, solid base was 15 mm × 5 mm; and similarly for fs 10.0 mm, the solid base size was 20 mm × 5 mm (refer Fig. 1(c & d)). The proposed triangular finned cyclone separator varies with three fs and two fin pitch (fp) variations, such as fs 5.0 mm with fp 30.0 & 50.0 mm; fs 7.5 mm with fp 30.0 & 50.0 mm; similarly fs 10.0 mm with fp 30.0 & 50.0 mm. Fig. 1 (c) shows the proposed helical triangular finned cyclone separators with fs variations from 5.0 to 10.0 mm from left to right with fp 30 mm. Fig. 1(d) shows the proposed helical triangular finned cyclone separators with fs variations from 5.0 to 10 mm from left to right with fp 50 mm. With intent to improve performance of cyclone separator on the bases of collection/separation efficiency, 6 sets of triangular fcs were proposed which varies with fs and fp. Optimized fs and fp will be found on the basis of fluid dynamic characteristics and collection/ separation efficiency through computational study.
2.5. Mesh dependency and validation Mesh numbers variations dependency on the results was carried out for the nfcs. Meshing was performed in ICEM 15.0 research code from Ansys using the same meshing parameters used during the simulation of triangular finned cyclone separators. Simulations were started with 337 K nodes and did the mesh independence study by almost doubling the mesh density such as 617 K nodes, 1200 K nodes. Simulation were performed in the Ansys Fluent 15.0 research code with the same solver setting which were used while simulating the triangular finned cyclone separator simulation. The simulation work was performed on the 617 K grids for the nfcs. As observed in Fig. 3 the tangential velocity profiles obtained at distance six hundred and fifty mm from the solid outlet with 617 K and 1200 K nodes were almost close while computational time was nearly doubling with 1200 K nodes when compared to 617 K nodes. Therefore, without compromising the accuracy of the results and also to be with lower computational time, 617 K nodes were chosen to validate the results with experimental results of Wang et al. [5]. Validation of the simulation settings used while performing the computational work for nfcs was carried out using the experimental data of Wang et al. [5] as shown in the Fig. 4. As observed in Fig. 4, less than 10% difference was observed between experimental tangential velocity profile by Wang et al. [5] and computational tangential velocity data for nfcs carried out in the present study.
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Fig. 5. (a) Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 5.0 mm with non-finned cyclone separator. (b). Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 7.5 mm with non-finned cyclone separator. (c). Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 10 mm with non-finned cyclone separator. (d). Comparisons of axialvelocity profiles between triangular fin based cyclone separator having fin pitch 50 mm and size as 5.0 mm with non-finned cyclone separator. (e). Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 50 mm and size as 7.5 mm with non-finned cyclone separator. (f). Comparisons of axial-velocity profiles between triangular fin based cyclone separator having fin pitch 50.0 mm and size as 10.0 mm with non-finned cyclone separator. (g). Comparisons of axial-velocities for 6 fin based cyclone separators.
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Fig. 5 (continued).
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Fig. 5 (continued).
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Fig. 5 (continued).
Fig. 6. Axial pressure drop for 6 sets of fin based and non-finned cyclone separators.
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Fig. 7. (a) Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 5.0 mm with non-finned cyclone separator. (b). Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 7.5 mm with non-finned cyclone separator. (c). Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 30 mm and size as 10 mm with non-finned cyclone separator. (d). Comparisons of tangentialvelocity profiles between triangular fin based cyclone separator having fin pitch 50 mm and size as 5.0 mm with non-finned cyclone separator. (e). Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 50 mm and size as 7.5 mm with non-finned cyclone separator. (f). Comparisons of tangential-velocity profiles between triangular fin based cyclone separator having fin pitch 50.0 mm and size as 10.0 mm with non-finned cyclone separator. (g). Comparisons of tangential velocities for 6 fin based cyclone separators.
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Fig. 7 (continued).
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Fig. 7 (continued).
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Fig. 7 (continued).
2.6. Mesh dependency study for triangular finned cyclone separators Mesh dependency on the results was carried out with the same procedure as explained in section 2.5 for nfcs for all 6 groups of triangular fcs. Geometry of the triangular fcs varies with fin size of 5.0 mm, 7.5 mm and 10.0 mm along with pitch variations as 50.0 & 30.0 mm. For the grid independency test, for the set 1 (refer Table 2), simulation was started with 376,707 nodes and did the mesh independence study by almost doubling the mesh density such as 585,902 nodes, 1,235,572 nodes. It was observed that results obtained with 585 K and 1235 K nodes were almost close while computational time was nearly doubling with 1235 K nodes in comparison with 585 K nodes. In the similar way mesh dependency study is carried out for remaining 5 sets of triangular finned cyclone separators and the selected mesh for further post processing work for all fcs with Y+ value are shown in the Table-2. 3. Results and discussion In this section, the output of triangular finned cyclone separators (fcs) in terms of fluid dynamics characteristics influencing the separation efficiency discussed in detail. Also results of the fcs by varying fin size (fs) and fin pitch (fp) were compared with conventional nfcs. 3.1. Axial-velocity profiles Collection efficiency of cyclone separators is strongly influenced by the axial velocity. Fig. 6(a-f) show the comparison of axial velocity profiles 6 groups of fcs with nfcs at various locations 300 mm, 500 mm and 700 above the bottom of cyclone separator. The different locations along the height of cyclone separators from
where the axial velocity data was extracted are, 300 mm which is in cone region, 500 mm which is at the end of cylindrical drum portion where helical fins turn ends and 700 mm which is just below the vortex finder in the cylindrical region of cyclone separators. The axial velocity profiles proposed by Wang et al. [5,23–27] are qualitatively similar to those presented in the current study for fcs and nfcs. As observed from Fig. 5(a-f) at 700 mm location because of the swirling flow and centrifugal force inside the cyclone separators, high self-weight particles were pushed towards wall and particles within the region measured 40 mm from the wall were moving downwards as the axial velocity is observed to be negative means axial velocity's direction was in downward direction. In this regime due to friction, axial velocity continuously decreases till the end of cylindrical portion of the cyclone separator and then increases due to sloping edges of the conical regime. Effects of fin's pitch and size variations at five hundred mm location on axial-velocity profile of fcs is as shown in Fig. 5(g). Influence of fin's pitch and size variations on axial velocity was observed to be very minute as realized from Fig. 5(g). However as shown in Fig. 5(g), fins with 30 mm pitch were having higher axial velocity in the upward direction than fins with 50 mm pitch at the core region of cyclone separator. Hence separation efficiency (section 3.4) for fins with 30 mm pitch was noted to be higher than that of fins 50 mm pitch. 3.2. Pressure field: axial pressure drop Axial pressure-drop (ΔP) measured for all 6 sets of fcs and a nfcs is as shown in Fig. 6. Axial pressure drop is the difference between the pressure measured at the start of helical fin and the end of helical fins which
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Fig. 8. (a) Comparisons of separating efficiency for the fin based cyclone separator having 30 mm pitch and cyclone separator without fin. (b) Comparisons of separating efficiency for the fin based cyclone separator having 50 mm pitch and cyclone separator without fin.
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Fig. 9. (a) shows the front and top view of particles' tracking for cyclone separator without fin at cylindrical region and (b) shows the front and top view of particles' tracking for cyclone separator without fin at cone region. (c) shows the front and top view of particles' tracking for fin based cyclone separator at cylindrical region and (d) shows the front and top view of particles' tracking for fin based cyclone separator at cone region.
is 208 mm and 493 mm respectively. With the same locations (208 mm and 493 mm pressure measuring points) pressure difference was calculated for nfcs.
From the Fig. 7, it should be noted that pressure drop measured for fin having pitch 30 mm and size 5.0 mm is highest among all the 6sets of fcs while it was lowest for fin having pitch 50 mm and size
Fig. 10. Reverse-flow locations in the cyclone separators with fin and without fin.
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10.0 mm. Pressure drop noted for nfcs was lower than pressure drop reported for any of the fcs. Increase in pressure drop is observed because of additional frictional losses due to modification made with cylindrical wall by fixing the fins. However, this loss can be compensated through improvement noted in separation (collection) efficiency (section 3.4) and possible reduction of heat loss from the hot wall of the cyclone separator by making possible heat transfer between hot gases flowing outside the fin and the water flowing inside the fin. It will be advisable to fix the fin on the inner wall rather than fixing it on the outer wall since heat transfer rate will increase due to increased surface area due to fins fixed on inner wall and it will also help to increase the separation (collection) efficiency. It was also a good sign that pressure drop decreases while separation/collection efficiency (section 3.4) increases with increase in fin's pitch. 3.3. Tangential velocity profiles Solid+air mixture flow inside the cyclone separators will be dominated by tangential velocity creating sturdy shear in radial direction and that creates centrifugal force which in-turn decides the particle separation [28]. Fig. 7(a-f) shows the comparison of tangential velocity profiles of 6 groups of triangular fcs with conventional nfcs at various locations 300 mm, 500 mm and 700 mm measured from the solid outlet along the height of the cyclone separator. The different locations along the height cyclone separators from where the tangential velocity data was extracted are, 300 mm which is in cone region, 500 mm which is at the end of cylindrical drum portion where helical fin-turns ends and 700 mm which is just below the vortex finder in the cylindrical region of cyclone separators. The tangential velocity profiles trends were observed qualitatively same trend as investigated by many researchers [29–35]. It is observed that tangential velocity in the region between 20 mm to 80 mm from the center (both left and right sides) slightly higher compare to conventional nfcs at 700 mm where the solid particles' concentration was high, this in-turn helps in improved collection/separation efficiency for the fcs. It is also observed from Fig. 8(a-g), as the tangential velocity of mixture flow decreases as flow moves downwards along the wall of cyclone separator and becomes zero at 100 mm radial distance from the center due to available friction between fluid & wall and no-slip boundary condition, which helps solid particles' stick to wall till it reaches collection point (solid outlet at the bottom of the cyclone separator). Comparisons of tangential velocity profile for 6 fcs is as shown in Fig. 7(g) which represents the effects of fin size and pitch variations made at five hundred mm above the solid outlet. Influence of fin size and pitch variations on tangential velocity was observed to be very minute as realized from Fig. 7(g). However tangential velocity for fin having 30.0 mm pitch and 7.5 mm size is slightly higher than all other fcs and the same was experienced with higher collection/separation efficiency. 3.4. Separation/collection efficiency Fig. 8(a-b) reveal the collection/separation efficiency comparisons for fcs and nfcs. Fig. 8(a) reveals the collection efficiency for various fin sizes with pitch 30 mm while Fig. 8(b) for pitch 50 mm. The improvement in the collection efficiency was noted for the fcs over the nfcs because of the improved tangential velocity (as shown in Fig. 7(a-f)) in the higher particle concentration region (close to inlet of cyclone separator i.e. about seven hundred mm above the bottom of cyclone separator) where tangential velocity forces the particles towards wall. Hence more particles were pushed (Fig. 9(c)) towards wall in fcs than nfcs (Fig. 9(a)). Subsequently improved axial velocity (as shown in Fig. 5(a-f)) helps further to push these particles in the downward direction towards bottom solid outlet. Additionally, helical path formed between the subsequent turns of the fin help to provide the space for
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the particles to gather and to move downwards towards following helical path. These particles experiences additional frictional force due to fin other than friction caused due to cyclone separator's wall which helps to avoid their movement towards the central core of air jet moving in the upward direction towards gas outlet at the top. Hence number of particles collected at bottom of fcs (Fig. 9(d)) were more than those with nfcs (Fig. 9(b)). Also as shown in Fig. 10, reversal of air jet takes place close to the bottom solid outlet for the fcs having fin with pitch 30 mm and size 7.5 mm hence chances of more particles' separation were increased with this fin (30 mm pitch, 7.5 mm size). 4. Conclusions Important conclusions can be made from the present study are: • Tangential velocity was observed to be more with fin based cyclone separator (fcs) than non-finned cyclone separator (nfcs) which helped to improve the separation or collection efficiency of fcs. Separation efficiency of fcs having fin with pitch thirty mm and size seven and half mm observed to largest among the selected fcs and it was giving about 10% more efficiency than nfcs. • Axial pressure drop increases by adding fins but it will be recovered by increase in the separating efficiency in comparison with nfcs. • Axial pressure drop decreases (about 20%) while collection/separation efficiency increases with increase fin's pitch. However, fin's sizes were not affecting much on the axial velocity, tangential velocity and collection/separation efficiency. • Helical pathway formed due to fins exerts additional friction on the particles along with friction exerted on them due to wall of the cyclone separator which helps to separate more number of particles from gas/air. • Overall more numbers of very fine particles (below three microns) were separated in case of fcs than nfcs. Declaration of Competing Interests The authors declare that they have no known competing financial interestsor personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements SERB - Govt. of India, Grant ID SB/FTP/ETA-0007/2014. References [1] M. Azadi, M. Azadi, M. Ali, A CFD study of the effect of cyclone size on its performance parameters, J. Hazard. Mater. 182 (1–3) (2010) 835–841. [2] A.J. Hoekstra, J.J. Derksen, H.E.A. Van Den Akker, An experimental and numerical study of turbulent swirling flow in gas cyclones, Chem. Eng. Sci. 54 (1999) 2055–2065. [3] L. Shi, D.J. Bayless, Comparison of boundary conditions for predicting the collection efficiency of cyclones, Powder Technol. 173 (1) (2007) 29–37. [4] G. Wan, G. Sun, X. Xue, M. Shi, Solids concentration simulation of different size particles in a cyclone separator, Powder Technol. 183 (1) (2008) 94–104. [5] B. Wang, D.L. Xu, K.W. Chu, A.B. Yu, Numerical study of gas–solid flow in a cyclone separator, Appl. Math. Model. 30 (11) (2006) 1326–1342. [6] M. Trefz, E. Muschelknautz, Extended cyclone theory for gas flows with high solid concentrations, Chem. Eng. Technol. 16 (1993) 153–160. [7] A. Avci, I. Karagoz, Effects of flow and geometrical parameters on the collection efficiency in cyclone separators, J. Aerosol Sci. 34 (2003) 937–955. [8] T. Zhang, C. Liu, K. Guo, H. Liu, Z. Wang, Analysis of flow field in optimal cyclone separators with hexagonal structure using mathematical models and computational fluid dynamics simulation, Ind. Eng. Chem. Res. 55 (2016) 351–365. [9] T. Zhang, K. Guo, C. Liu, Y. Li, M. Tao, C. Shen, Experimental and numerical investigations of a dual-stage cyclone separator, Chem. Eng. Technol. 41 (3) (2018) 606–617. [10] F. Mariani, F. Risi, C.N. Grimaldi, Separation efficiency and heat exchange optimization in a cyclone, Sep. Purif. Technol. 179 (2017) 393–402. [11] M. Wasilewski, J. Duda, Multicriteria optimization of first-stage cyclones in the clinker burning system by means of numerical modelling and experimental research, Powder Technol. 289 (2016) 143–158. [12] P.K. Nag, A.V.S.S.K.S. Gupta, Fin heat transfer studies in a cyclone separator of a circulating fluidized bed, Heat Transfer Eng. 20 (1999) 28–34.
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[13] K. Elsayed, C. Lacor, The effect of cyclone inlet dimensions on the flow pattern and performance, Appl. Math. Model. 35 (2011) 1952–1968. [14] J. Parekh, R. Rzehak, Euler–Euler multiphase CFD-simulation with full Reynolds stress model and anisotropic bubble-induced turbulence, Int. J. Multiphase Flow 99 (2018) 231–245. [15] M,.D. Slack, R.O. Prasad, A. Bakker, F. Boysan, Advances in cyclone modelling using unstructured grids, Trans IChemE., Part-A. 78 (2000). [16] Modeling Turbulent Flow, Introductory fluent training, Dec-2006 (www.southampton.ac.uk/~nwb/lectures/…/Turbulence_Notes_Fluent-v6.3.06.pdf). [17] H. Safikhani, M.A. Akhavan Behabadi, M. Shams, M.H. Rahimyan, Numerical simulation of flow field in three types of standard cyclone separators, Adv. Powder Technol. 21 (2010) 435–442. [18] Hesham M. El-Batsh, Improving cyclone performance by proper selection of the exit pipe, Appl. Math. Model. 37 (2013) 5286–5303. [19] B. Zhao, Y. Su, J. Zhang, Simulation of gas flow pattern and separation efficiency in cyclone with conventional single and spiral double inlet configuration, Trans IChemE., Part A., Chemical Eng. Res. and Des. 84 (A12) (2006) 1158–1165. [20] T. Mothilal, K. Pitchandi, V. Velukumar, K. Parthiban, CFD and statistical approach for optimization of operating parameters in a tangential cyclone heat exchanger, J. Appl. Fluid Mech. 11 (2) (2018) 459–466. [21] K. Udaya Bhaskar, Y. Rama Murthy, M. Ravi Raju, S. Tiwari, J.K. Srivastava, N. Ramakrishnan, CFD simulation and experimental validation studies on hydrocyclone, Miner. Eng. 20 (2007) 60–71. [22] Y. Rama Murthy, K. Udaya Bhaskar, Parametric CFD studies on hydrocyclone, Powder Technol. 230 (2012) 36–47. [23] N.L.P. M'Bouana, D. Lia, T. Yangjia, T. Yang, H. Lu, Numerical simulation of gas and particle flow in cyclone separators, AIP Conference Proceedings. 1547 (2013) 545–554. [24] P. Farzad, H.H. Seyyed, A. Goodarz, E. Khairy, Impacts of the vortex finder eccentricity on the flow pattern and performance of a gas cyclone, Sep. Purif. Technol. 187 (2017) 1–13.
[25] M. Dzmitry, G.A. Anders, S.L. Tord, Effects of the inlet angle on the flow pattern and pressure drop of a cyclone with helical-roof inlet, Chem. Eng. Res. Des. 102 (2015) 307–321. [26] G.B. Sakura, Y.T.L. Andrew, CFD simulation of cyclone separators to reduce air pollution, Powder Technol. 286 (2015) 488–506. [27] R. Utikar, N. Darmawan, M. Tade, Q. Li, G. Evans, M. Glenny, V. Pareek, Hydrodynamic simulation of cyclone separators, Computational Fluid Dynamics book, Chapter 11. 2010, p. 420 , (ISBN 978–953-7619-59-6). [28] C. Cortes, A. Gil, Modeling the gas and particle flow inside cyclone separators, Prog. Energy Combust. Sci. 33 (2007) 409–452. [29] Z. Faqi, S. Guogang, Z. Yuming, C. Hui, W. Qing, Experimental and CFD study on the effects of surface roughness on cyclone performance, Sep. Purif. Technol. 193 (2018) 175–183. [30] V. Kumar, K. Jha, Numerical investigations of the cone-shaped vortex finders on the performance of cyclone separators, J. Mech. Sci. Technol. 32 (11) (2018) 5293–5303. [31] P. Bunyawanichakul, M.P. Kirkpatrick, J.E. Sargison, G.J. Walker, Numerical and experimental studies of the flow field in a cyclone dryer, J. Fluids Eng. 128 (2006) 1240–1250. [32] O. Akiyama, C. Kato, Numerical investigations of unsteady flows and particle behavior in a cyclone separator, J. Fluids Eng. 139 (2017) 913021–9130211. [33] P.H. Jafari, D. Misiulia, J.G.I. Hellstrom, B.R. Gebart, Modeling of particle-laden cold flow in a cyclone gasifier, J. Fluids Eng. 141 (2019) 0213021–02130213. [34] H. Zhou, Z. Hu, Q. Zang, Q. Wang, W. Lv, Numerical study on gas-solid flow characteristics of ultra-light particles in a cyclone separator, Powder Technol. 3441 (2019) 784–796. [35] J. Zhang, Z. Zha, P. Che, H. Ding, W. Pan, Influences of inlet height and velocity on main performances in the cyclone separator, Part. Sci. Technol. 37 (6) (2018) 669–676.