Experimental and numerical investigations of a dynamic cyclone with a rotary impeller

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Chemical Engineering and Processing 47 (2008) 1861–1866

Experimental and numerical investigations of a dynamic cyclone with a rotary impeller Jinyu Jiao a , Ying Zheng a,∗ , Jun Wang b , Guogang Sun b a

Department of Chemical Engineering, University of New Brunswick, 15 Dineen Drive, P.O. Box 4400, Fredericton, N.B. E3B 5A3, Canada b Department of Chemical Engineering, University of Petroleum, Beijing 102249, China Received 27 September 2006; received in revised form 24 August 2007; accepted 14 October 2007 Available online 26 October 2007

Abstract This paper studies a dynamic cyclone with a rotary impeller inside experimentally and numerically. The experiment mainly focuses on the separation efficiency, while the numerical simulation describes the flow field in the dynamic cyclone. The discrete phase model (DPM) is also used to predict the fractional efficiency of the dynamic cyclone, and the predictions are compared with the experimental results. The dynamic cyclone has been demonstrated to be very helpful for increasing the separation efficiency when the impeller rotates at greater speeds. The simulation predictions prove that the tangential velocity distributions are mainly dominated by the rotational speed for the region of impeller and dominated by the inlet velocity for the region outside of the vortex finder. Both the experiment and simulation show that the effects of inlet velocity on the separation efficiency are different for different rotational speeds. © 2007 Elsevier B.V. All rights reserved. Keywords: Dynamic cyclone; Separation efficiency; Flow field; Simulation

1. Introduction Dynamic cyclones, as relatively new technologies, attract more and more attention from researchers for theory and application studies. The main idea of the dynamic cyclone is to introduce a rotary assembly into the conventional cyclone to enhance the centrifugal field. As well known, conventional cyclones are not effective when used for fine particle separation because the separation in cyclones relies on the centrifugal forces acting on the particles and fine particles have rather low inertia. It is difficult for conventional cyclones to separate particles with size of 10 ␮m or smaller from the gas stream for the limited centrifugal field inside [1]. In order to improve the conventional cyclone’s performance, additional devices have to be introduced into conventional cyclones to increase the centrifugal fields. Different assemblies have been introduced into conventional cyclones during the past decades to build different dynamic cyclones. The representatives of them are the double-rotor cenAbbreviations: CFD, computational fluid dynamics; DPM, discrete phase model; RNG, re-normalization group theory; RSM, Reynolds stress model. ∗ Corresponding author. Tel.: +1 506 4473329; fax: +1 506 4533591. E-mail address: [email protected] (Y. Zheng). 0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.10.015

trifugal dust collector [2], Cyclocentrifuge TM [3], the rotational particle separator [4], the cyclone-type separator [5], and the rotary classifier separator [6]. The common point for these dynamic cyclones is that the rotary components are introduced into the cyclone house. Then, the centrifugal flow field in the cyclone can be increased by two or more times with different rotational speeds. Apart from increasing the separation efficiency, the rotational components can also serve as selectors for different particle size loadings by adjusting their speeds. For the conventional cyclone, all the uncollected particles follow the gas stream out of the cyclone through the vortex finder. If some of these particles can be separated for the second time in the vortex finder, the separation efficiency is sure to increase. Ray et al. [7] designed a so called “Post Cyclone”, which is a cylindrical annular shell located on top of the vortex finder, to utilize the strong swirl already present in the vortex finder of a conventional cyclone. Lim et al. [8] changed the vortex finder into different shapes to compare the separation efficiency of the conventional cyclone. However, the swirling flow in the vortex finder has not been increased much for the limited geometry modification in these cases. Thus, the swirling flow in the vortex finder still cannot produce strong enough centrifugal fields to get the particles through it to be separated because the uncollected

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particles in the conventional cyclone are mostly of fine particles. To separate the fine particles passing through the vortex finder, a rotary assembly has to be introduced into the vortex finder to increase the centrifugal field. In this work, a new dynamic cyclone was designed. The main construction of the dynamic cyclone is that a rotary impeller was introduced into the vortex finder of the conventional cyclone. The rotary impeller has two functions for fine particle separation. The first function is that the rotary impeller can intercept some of uncollected particles and stop them flowing into the vortex finder when the impeller is rotating. The other function is that the rotary impeller provides an increased centrifugal field in the vortex finder to collect the fine particles. The experiments have been conducted on the separation efficiency of the dynamic cyclone with the rotary impeller inside. Numerical simulation was also carried out to study the flow field and particle separation in the dynamic cyclone. 2. Experimental apparatus and methods The experimental apparatus is shown schematically in Fig. 1. The air was sucked into the dynamic cyclone tangentially. The particles were introduced to the inlet pipe vertically through the aperture and were carried by the air into the dynamic cyclone. The flow rate of inlet air was adjusted through a guide vane, and the duration and quantity of feed were controlled through the feeding system. The diameter and the length of the cylindrical section for the dynamic cyclone are 300 mm and 450 mm respectively. The vortex finder has a diameter of 200 mm and a length of 200 mm. An impeller consisting of 30 blades was introduced into the vortex finder coaxially. The impeller height is 200 mm and the clearance between the inner wall of the vortex finder and the tip blade is 10 mm. The impeller was driven to rotate with different speeds by a motor sitting on the top of the cyclone. The gap between the inner wall of the vortex finder and the outer edge of the impeller was 10 mm. The particle used in this experiment

is the talcum powder with the density of 2750 kg/m3 . In this experiment, the effect of the solid loading on the separation efficiency has not been considered because of low solid loading of particles. In cyclones, either total separation efficiency or fractional efficiency can be used to describe the separation performance. The total cyclone separation efficiency, η, is the percentage of the collected particles from the total particles entered into the cyclone. The fractional efficiency of a cyclone, ηi (dpi ), is the percentage of the collected particle from the total particle at the entrance for a given range of particle diameter. The relationship of the total separation efficiency and the grade efficiency can be represented by the following equation [9]: ηi (dpi ) = 1 − (1 − η)

fo (dpi ) fin (dpi )

(1)

fo (dpi ) and fin (dpi ) are the particle size distribution for a given range of particle diameter at the outlet and at the entrance of a cyclone, respectively. The analysis of particle size distribution has been done before and after each test run. 3. Simulation approach 3.1. Turbulence model In the field of modeling the confined swirling flow, an important issue is the accurate description of the turbulent behavior of flow. A number of turbulence models are available, such as the standard κ–ε model, the renormalization group (RNG) κ–ε model, and the Reynolds stress model (RSM). It has been demonstrated that RSM provide better accuracy than the standard κ–ε model and the RNG κ–ε model for the calculation of flow field in cyclone [10,11]. The RSM model involves calculation of the individual Reynolds stresses by using differential transport equations. The individual Reynolds stresses are then used to obtain closure of the Reynolds-averaged momentum

Fig. 1. The schematic diagram of experiment set up.

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equations. When using the RSM, beside the momentum and continuity equations, the Reynolds stresses transport equations can be written as: ∂ ∂ (ρui uj ) + (ρUk ui uj ) = Dij + Pij + Φij − εij ∂t ∂xk

CD is the drag force coefficient and can be approximated by using below correlations for a single rigid sphere CD =

(2)

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24 (1 + 0.15Re0.687 ) p Rep

CD = 0.44

ReP < 1000

ReP ≥ 1000

(5) (6)

where the terms Dij , Pij , Φij and εij are turbulent diffusion, stress production, pressure strain, and dissipation rate, respectively. Of these terms, Pij does not need any modeling, while Dij , Φij and εij have to be modeled to close the Reynolds stresses transport equations and have been documented everywhere on how to model these three terms [12,13].

The Stokes equation needs to be modified when considering the effect of the Cunningham correction. Middha and Wexler [16] have found that the Cunningham correction factor is about 1.17 for 1 ␮m particles and 1.085 for 2 ␮m particles. Cunningham correction factor is neglected as a majority of particles applied in this work are larger than 1 ␮m. The centrifugal force, F C is

3.2. Boundary conditions

Fc =

The inlet boundary is set as the uniform velocity inlet, and the turbulence quantities are also uniformly imposed at the inlet by using the correlations: kin = 3/2(Uin I)2 , and εin = 1.5 / l, where I = 0.16(Re )−0.125 , Re = ρ D U /μ, C␮0.75 kin D D g H in C␮ = 0.09, l = 0.07DH , and DH = 0.0978 m. The boundary condition at the outlet is based on a fully developed flow assumption where zero streamwise gradients apply for all flow variables. No-slip conditions were assumed at the wall. The body-fitted technique was used when the grid system was established, and the solutions were obtained with 396, 660 grid cells. The QUICK difference scheme was adopted for the convection and diffusion terms, and the SIMPLE algorithm was employed to handle the coupling between the continuity and the momentum equations. The convergence criteria were set as 10−4 for the continuity and velocities, and 10−3 for the other equations.

Saffman lift force was first proposed by Saffman [14] and modified by Mei [17] as,    uθ 1/2 9.66 1/2  d  Ff = | u−u  p |(μρ)  (8) πdp dr 

3.3. Particle phase To calculate the trajectories of particles in flow, the discrete phase model (DPM) was used to track individual particles through the continuum fluid. Since the particle loading in a cyclone is rather small (<10%), one-way coupling can be assumed, which means that the presence of the particles does not affect the gas flow field. The translation motion of an individual particle at any time in a cyclone, is determine by Newton’s second law of motion, which can be written as mp

d up = F D + F C + F S + F G dt

(3)

where mP and uP are the mass and velocity of particles. F D , F C , F S , F G are drag force, centrifugal force, Saffman lift force [14], and gravitational force, respectively. Other forces such as Basset force (due to the development of a boundary layer around the dispersed phase particles), and Brownian force are insignificant and are neglected in this study [15]. The drag force can be written as F D =

π u−u  P |( u−u P) CD ρdp2 | 8

(4)

π ρp dp3 ω2 R 6

Gravitational force is expressed as,   ρp − ρ F G = mp g ρp

(7)

(9)

Once the particle velocity field is calculated from the force balance discussed above in Eq. (3), the trajectories of all the particles can be identified using the following equation: dx =u p dt

(10)

Collection efficiency statistics were obtained by releasing a specified number of monodispersed particles at the inlet of the cyclone and by monitoring the number escaping through the outlet. Collisions between particles and the cyclone walls were assumed to be perfectly elastic [18]. 4. Results and discussion 4.1. Effects of rotational speeds The effects of the impeller rotational speeds on the grade efficiencies of the dynamic cyclone are shown in Fig. 2 for two gas inlet velocities. It can be seen that the rotary impeller has great effects on the grade efficiencies of the dynamic cyclone. The grade efficiency increases with the higher rotational speed. It should be noted that the rotational speed has a more obvious effect on the grade efficiency for particles with diameter less than 10 ␮m than for particles larger than 10 ␮m. The grade efficiency increase about 25% for particles smaller than 10 ␮m when increasing the rotational speed from 1000 rpm to 3600 rpm under the operating condition of the inlet velocity of 10 m/s. However, for particles greater than 10 ␮m, the grade efficiencies for different speeds come closer with the increasing particle diameter and reach to the same level eventually as demonstrated by the grade efficiency curves as shown in Fig. 2. The reason is that

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in which no gas flows, region II refers to the blade controlling area inside the vortex finder, and region III is the area outside of the vortex finder. As shown in Fig. 3, the rotational speed only dominates the tangential velocity distribution in region II and has nearly no effect on the tangential velocity distribution in region III. Thus, the rotary impeller is only responsible for the particle separation in region II. It can still be observed that the highest magnitudes of the normalization tangential velocity in region II are greater than those in region III with the rotational speed greater than 2000 rpm, demonstrating that the rotary impeller can separate the fine particles that cannot be collected in region III. 4.2. Effects of the inlet velocity

Fig. 2. Effects of different rotational speeds on fractional efficiencies.

most of the particles greater than 10 ␮m have more chances to be separated before they go into the vortex finder, indicating that most of the uncollected particles in the vortex finder are less than 10 ␮m. Thus, the rotary impeller located in the vortex finder has a great effect on the grade efficiency for the particles smaller than 10 ␮m, and it can safely be concluded that the rotary impeller is very useful for fine particle separation. The centrifugal force coming from the tangential velocity is mainly responsible for the particle separation. Fig. 3 shows the effects of the rotational speeds on the tangential velocity distributions at two elevations in the dynamic cyclone. The dynamic cyclone has been split into three regions from inner to outer with region I, region II, and region III. Region I is the solid body

Fig. 3. Effects of rotational speeds on tangential velocity distributions.

The effects of the inlet velocity on the total separation efficiency of the dynamic cyclone are shown in Fig. 4. It is remarkably noted that the effects of gas inlet velocity on the separation efficiency are different for different rotational speeds of the impeller. When the rotary impeller rotates at speeds less than 2000 rpm, the separation efficiency slightly increases with the increasing gas inlet velocity. However, the separation efficiency drops about 2% with the inlet velocity increasing from 10 m/s to 20 m/s for rotational speeds greater than 2000 rpm. In the dynamic cyclone, the higher gas inlet velocity leads to two effects on the flow field. On one side, the tangential velocity increases as the gas inlet velocity increases in the region III outside of the vortex finder, as shown in Fig. 5, which increase is helpful to enhance the separation efficiency. On the other side, the increase of the gas inlet velocity incur the higher average axial velocity in the vortex finder, which can result in the shorter residence time of the particles inside the vortex finder and decrease the chance of particles being collected for the second time.

Fig. 4. Effects of inlet velocities on total separation efficiencies.

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Fig. 6. Comparisons of fractional efficiencies between experiment and simulation.

Fig. 5. Effects of inlet velocities on tangential velocity distributions.

The impeller functions less on particle separations because of limited centrifugal force produced on the particles when the impeller is static or rotates at speeds less than 2000 rpm. Thus, the separation efficiency slightly increases as the gas inlet velocity increases, similar as the conventional cyclone. When the impeller rotates at high speed greater than 2000 rpm, the impeller is capable of driving more particles to move to the vortex finder wall. The particles must have enough residence time staying in the vortex finder to make sure that they can be collected by the wall of the vortex finder. However, the particle residence time in the vortex finder decreases as the gas inlet velocity increases, thus indicating that more particles do not have enough time to be collected by the wall. Thus, the separation efficiency decreases as the gas inlet velocity increases for the higher rotational speeds. Fig. 5 also shows that the effects of the gas inlet velocity on the tangential velocity distribution are different for region II and region III when the impeller rotates at different speeds. The tangential velocity increases as the gas inlet velocity increases in spite of different rotational speeds in region III, while in region II, the gas inlet velocity can just affect the distribution of the tangential velocity with the impeller rotating at low speeds. When the impeller rotates at greater speed, the tangential velocity distribution in region II is only dominated by the rotary impeller with greater speeds, indicating that the tangential velocity distribution does not change with different gas inlet velocities. 4.3. CFD predictions on separation efficiency CFD has been recently used to predict the separation efficiency in the conventional cyclone, and some reasonable agreements have been obtained [18–20]. All these simulations used the discrete phase model to trace the particle trajectories to calculate the fractional efficiency. However, the performance of the discrete phase model in more complicated cyclones such as

Fig. 7. CFD predictions of effects of inlet velocities on fractional efficiencies.

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the dynamic cyclone still need experimental validation. Fig. 6 shows the comparison of the fractional efficiency between the experiment and the predictions in the dynamic cyclone with inlet velocity of 10 m/s at different rotational speeds. It can be seen that the simulation predictions do not make good agreements with the experiment results. The predicted fractional efficiency is much higher than the experimental one. However, it can be observed that the trend of the change of fractional efficiency with the rotational speeds is represented well by the simulation. Thus, the technique of CFD with the discrete phase model is still very useful to study the effect of operating conditions on separation efficiency in a dynamic cyclone, although it cannot precisely predict the separation efficiency. Fig. 7 shows the CFD predictions of the effect of inlet velocity on the fractional efficiency with two different rotational speeds of 1000 rpm and 3000 rpm. For the low rotational speed at 1000 rpm, the separation efficiency increases as the inlet velocity increases, while for the high rotational speed at 3000 rpm, the separation efficiency decreases when the inlet velocity increases. The simulation predictions of the effect of inlet velocity on efficiency coincide with experimental results, as shown in Fig. 4. 5. Conclusions The dynamic cyclone with a rotary impeller inside was studied experimentally and numerically. The experiment shows that the rotational impeller has a great effect on the separation efficiency when rotating at higher speeds. The greater rotational speed, the higher separation efficiency is. The simulation predictions on the flow field show that the tangential velocity distribution in the region of impeller is mainly controlled by the rotational speed, while in the region outside of the vortex finder, the tangential velocity distribution is mainly controlled by the gas inlet velocity. Both the experiment and simulation show that the effects of inlet velocity on the separation efficiency are different for different rotational speeds. For low rotational speeds less than 2000 rpm, the separation efficiency increases with higher inlet velocity greater than 2000 rpm, while for high rotational speeds, the separation efficiency decreases as the inlet velocity increases. Acknowledgements Financial support from the Pan-Atlantic Petroleum Systems Consortium and Atlantic Innovation Fund is gratefully acknowledged. Appendix A. Nomenclature CD dp DH f I k mp Re

drag coefficient particle diameter (␮m) hydraulic diameter (m) particle size distribution turbulent intensity turbulent kinetic energy (m2 /s2 ) particle mass (kg) Reynolds number

u up Uin Ut z

gas velocity (m/s) particle velocity (m/s) gas inlet velocity (m/s) tangential velocity (m/s) dynamic cyclone axial position (mm)

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