Powder Technology 226 (2012) 10–15

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Powder Technology

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Turbo air classifier guide vane improvement and inner flow fieldnumerical simulation

Qiang Huang a, Jiaxiang Liu a,⁎, Yuan Yu b

a College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, Chinab College of Mechanical and Electrical Engineering Beijing University of Chemical Technology, Beijing 100029, China

⁎ Corresponding author. Tel./fax: +86 10 64446432.E-mail address: ljxpost@263.net (J. Liu).

0032-5910/$ – see front matter © 2012 Published by Eldoi:10.1016/j.powtec.2012.03.026

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 May 2011Received in revised form 15 March 2012Accepted 24 March 2012Available online 2 May 2012

Keywords:Turbo air classifierInertia rotating vortexBowed guide bladesNumerical simulation

The straight guide blade of a turbo air classifier was modified to obtain ultrafine powder with narrow particlesize distribution, and to improve classification precision. The modification involved positively or negativelycurving the straight blade. The structural effects of the straight, positively bowed, and negatively bowedblades on the inner flow field of a turbo air classifier were all simulated with the FLUENT® software. Thenumerical simulations indicate great improvement in the annular region tangential and radial velocitydistributions of the positively bowed guide blade, compared with the straight and negatively bowed blades.Air velocity fluctuations on the outer cylinder of the improved guide blade are decreased in the heightdirection. Also, the radial velocity is decreased and the tangential velocity is increased using the positivelybowed blade. As a result, the classified particles are cut smaller than with the straight and negativelybowed blades. The inertia rotating vortex between blades is decreased, and the flow field becomes stable.Material classification performance experiments were also performed. The results thereof demonstrate thatthe positively bowed guide blade improves the classification precision and decreases the particle cut size.These actual experimental results confirm the computational fluid dynamics simulation results and thefeasibility of the modified guide vane in engineering applications.

© 2012 Published by Elsevier B.V.

1. Introduction

Ultrafine powder with narrow particle size distribution is thepreparation basis of structural and functional materials. Thesematerials are widely used in the chemical industry, in fine ceramics,as refractory materials, as electronic components etc. Ultrafinepowder is mainly generated by a turbo air classifier with a highclassification precision. However, a conventional turbo air classifierproduces a wide particle size distribution because of the constraintsin the sectional structure of the classifier. Therefore, the structure ofa turbo air classifier should be optimized.

In the past, studies for the structural improvement of a turbo airclassifier have been conducted mainly by experimentation [1–3]. Inrecent years, developments in computer technology have enabled theuse of computational fluid dynamics for structural optimization studies[4–6]. Yang [7] used such an approach to investigate theflow field in thepassageway of two rotor cage blades in a turbo air classifier.

Recent studies mostly focus on the structure of the turbo airclassifier rotor cage [8]. The present study focuses on the guideblade as an important functional unit in a turbo air classifier. The

sevier B.V.

guide blade maintains the identity of the annular region flow field.Li [9] studied the effect of L-shaped guide blades on classificationperformance, and showed that the particle back mixing problemwas avoided. However, air velocity fluctuations in the annular regionwere not greatly decreased. In the present study, curved guide bladesare newly introduced. The purpose of the curved style is to realize anideal flow field wherein air velocity is kept uniform in the annularregion. The flow conditions are simulated using the FLUENT®software, and are confirmed by actual material experiments.

2. Theoretic formulations

2.1. Classification principle of a turbo air classifier

A turbo air classifier schematic diagram is shown in Fig. 1 andpositively bowed guide blades are shown in Fig. 2. The powders tobe classified are fed through the feed port and fall to the distributionplate. By rapidly rotating the distribution plate, the powders arethrown outward and gravitationally drop into the annular region.Air flows in a tangential direction into the annular region throughthe volute shell. The annular region, a cylindrical space, has an outerdiameter coincident with the inner boundary of the guide blades. Itsinner diameter is the diameter of the rotor cage outer periphery.Under the action of the air flow and the rotor cage rotation, the raw

Fig. 1. Diagram of a turbo air classifier.

Fig. 3. Grid representation of the turbo air classifier based on FLUENT®.

11Q. Huang et al. / Powder Technology 226 (2012) 10–15

materials split into coarse and fine powders. The cut size (d50) in theannular region can be described by

d50 ¼ 3CDρaRv2r =4ρpv

2θ ; ð1Þ

where CD is the resistant coefficient, which is a function of theReynolds number related to air velocity. The value of d50 varies withthe air velocity. To obtain powders with a narrow particle sizedistribution, the flow field in the separation face should be kept stable.

2.2. Principles of guide blade design

An efficient guide blade shape results in a uniform flow field in theannular region. According to the aerodynamic theory, a bowed bladewith a suitable lean angle can effectively decrease the adverse trans-verse pressure gradient in the end wall. Hence, the intensity of thepassage vortex is reduced, and energy loss is decreased consequently.Air flow resistance in the bend passage can also be reduced. As a result,air is rapidly distributed in the annular region, and the turbulenceintensity is decreased. Although some studies have reported no obviouslinear relationship between energy loss and classification precision, wecan conclude that classification precision increases with decreasedenergy loss through material experiments.

3. Model descriptions

3.1. Simulation conditions

The simulation was conducted at a rotor cage rotary speed of600 r/min and an air inlet velocity of 20 m/s. The temperature was298 K and the air density was 1.2 kg/m3. The present study mainly

Fig. 2. Diagram of positively bowed guide blades.

investigates the air flow field in an improved turbo air classifier, andneglects the particle effect on the air flow field (the particle volumefraction in the classifier was very small). A grid representation ofthe turbo air classifier is shown in Fig. 3. The rotor cage has an outerdiameter of 270 mm and a height of 140 mm. The annular region is45 mm wide, and the guide blade is inclined at a 15° angle. Giventhe low-velocity air flow into the turbo air classifier and the lowtemperature, air can be deemed incompressible, and the segregatedimplicit solver can be applied. In the present study, the semi-implicit method for pressure-linked equations algorithm was usedto couple pressure and velocity, and the second-order upwindscheme was used for all variables.

3.2. The gas-governing equation

Problems of large velocity and pressure gradients in the radialdirection must be dealt with to describe flow in a turbo air classifieraccurately. An appropriate turbulent model must be used tocharacterize the rotating turbulent flow. The Reynolds stress model(RSM) [10,11], which can describe anisotropic turbulence, was usedin the present work. Although the RSM has been proved as anappropriate turbulence model for turbo air classifier flow, it is com-putationally more expensive than other unresolved-eddy turbulencemodels. The governing equations for an incompressible fluid basedon the RSM can be written as

∂∂t ρu=

i u=j

� �þ ∂∂xk

ρuku=i u

=j

� �¼ − ∂

∂xkρu=

i u=j u

=

k þ p δkju=i þ δiku

=j

� �� �

þ ∂∂xk

½μ ∂∂xk

u=i u

=j Þ

� i−ρ u=

i u=

k

∂uj

∂xkþ u=

j u=

k∂ui

∂xk

!−ρβ giu

=j θ þ gju

=i

� �

þp∂u=

i

∂xjþ∂u=

j

∂xi

!−2μρΩk u=

j u=meikm þ u=

i u=mejkm

� �ð2Þ

where the velocity components are decomposed into the mean ui

and the fluctuating velocities ui (i=1, 2, 3). ρ, μ, P, δ, and xi are theliquid density, viscosity, pressure, Kronecker factor, and positionallength, respectively.

4. Simulation results and analysis

4.1. Comparison of velocity distributions in the annular region

Equation of the cut size shows that for a given powder material,when the velocity distribution is uniform, d50 can be kept constantin all points of the annular region.

Table 1Velocity deviations in the turbo air classifier with different guide blade style.

Radius(mm)

Straight guide blade Positively bowedguide blade

Negatively bowedguide blade

Tangentialvelocity

Radialvelocity

Tangentialvelocity

Radialvelocity

Tangentialvelocity

Radialvelocity

137 6 3.6 1.5 0.62 4.18 3.14145 6.3 4.11 1.39 0.65 4.16 3.14153 5.52 3.97 1.42 0.79 3.6 3.49

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Changes in the tangential and radial velocities in the level sectionof the turbo air classifier can describe the extent of velocityfluctuation. Such deviations can be calculated from

S2 ¼ 1n−1

Xni¼1

vi−�vð Þ2; ð3Þ

where S, vi, and v are the distribution deviation, velocity of differentgrid nodes in the circle, and mean velocity, respectively.

Fig. 4. (a) Tangential velocity distribution in a turbo air classifier equipped with a positively ba positively bowed guide blade. (c) Tangential velocity distribution in a turbo air classifier eqair classifier equipped with a negatively bowed guide blade. (e) Tangential velocity distribdistribution in a turbo air classifier equipped with a straight guide blade. (Different colors

Table 1 lists the deviations in tangential and radial velocities inthree equidistant circle lines. The radii of these circles are 137, 145,and 153 mm.

Overall, the annular region velocity deviation is decreased to agreater extent in the turbo air classifier with a bowed blade thanthat with a straight blade. The velocity deviation in the classifierwith a positively bowed blade is less than that with a negativelybowed blade.

Fig. 4 compares the tangential and radial velocity distributions in ahorizontal section of the turbo air classifier with three different guideblades. Different colors represent different velocity values. There is adistinct decrease in the velocity gradient between the bowed andstraight guide blades.

4.2. Comparison of flow fields in the passageway of the two rotorcage blades

Fig. 5 shows the revolving flow field characteristics.The inertia rotating vortex in the passageway of the two rotor

cage blades may result in large air radial velocity fluctuations in

owed guide blade. (b) Radial velocity distribution in a turbo air classifier equipped withuipped with a negatively bowed guide blade. (d) Radial velocity distribution in a turboution in a turbo air classifier equipped with a straight guide blade. (f) Radial velocityrepresent different velocity values).

Fig. 5. (a) Flow field of straight guide blade. (b) Flow field of positively bowed guideblade. (c) Flow field of negatively bowed guide blade. (Different colors representdifferent velocity values).

Fig. 6. Distribution of radial velocity.

Fig. 7. Distribution of tangential velocity.

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the rotor cage exterior edge [12]. The inertia rotating vortextransforms the fine particles (with small sizes) back into coarsepowder. Fig. 5 clearly shows the dramatic decrease in the vortexintensity of the bowed guide. For the straight guide blade (Fig. 4),the tangential velocity (V=8.5 m/s) of the rotor cage blade exterioredge is less than the air flow tangential velocity in the rotor cage

exterior edge. Consequently, air flow near the windward blades isrebounded by rotor cage blades [13]. The rebounded air flow thencounters the rotational direction of the rotor cage, enormouslydecreasing the tangential velocity as well as increasing the radialand axial velocities. Near the leeward blades, there is negativepressure that causes a vortex. This vortex has the same rotationaldirection as the rotor cage. The positive whirling would only vanishwhen the air flow tangential velocity is approximately equal to thetangential velocity of the rotor cage blade exterior edge. For thebowed guide blade (Fig. 4), the air flow tangential velocity slightlyincreased compared with the peripheral tangential velocity of therotor cage.

4.3. Comparison of velocity distributions in the height direction of therotor cage

Figs. 6 and 7 show the velocity distributions of the three guideblades in the outer cylindrical surface of the rotor cage. The airmotion is a plane motion, as well as a superposition of a quasi-free vortex and a sink point in the annular region. In time averageflow sense, this flow feature may lead to a Taylor column effectand to the formation of a uniform force field in the height direction[14]. Given the axial force effect and the air flow field turbulence onthe outer cylindrical surface of the rotor cage, a marked change inthe velocity distribution occurs.

Figs. 6 and 7 also clearly show that for the positive bowed guideblade, the radial and tangential velocity distributions are fairlyuniform and fluctuate only slightly in the height direction. These

Fig. 8. Tromp curve (600 r/min).

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findings are on account of the low turbulence intensity in the annularregion. Furthermore, the mean tangential velocity in the heightdirection is increased, whereas the mean radial velocity significantlydecreased, compared with the straight and negatively bowed blades.These effects benefit the removal of large particles. As a result, theproportion of coarse powder mixing with the fine powder decreases.With regard to the negatively bowed guide blade, there is no effectiveimprovement on the velocity distribution.

5. Material classification experiments

Classification experiments were conducted to confirm the per-formance of the classifier with different guide blades. The experi-ments were performed at rotation speeds of 600 and 1200 r/min, aswell as an air inlet velocity of 20 m/s. The material aluminum oxidewas fed at the rate of 36 kg/h. The positively bowed and the straightguide blades were used. After classification, the coarse powderswere weighed and were analyzed using an LS-POP laser particlesizer according to Eq. (4):

ηD ¼ ηCRC Dið Þ−RC Diþ1

� �RO Dið Þ−RO Diþ1

� � ð4Þ

The partial coarse powder classification efficiencies at differentrotation speeds were obtained. The characteristic particle diametersD25, D50, and D75 are taken from the Tromp curves shown in Figs. 8and 9. The classification sharpness index K is calculated according

Fig. 9. Tromp curve (1200 r/min).

to K=D25/D75. Notably, the K of the turbo air classifier withpositively bowed blades is higher and the cut size is smaller thanthat of the straight blade at 600 and 1,200 r/min rotation speeds.Therefore, the experimental results are in good agreement withthe simulation results.

In addition, K increases and cut size decreases at a higher rotationspeed (1,200 r/min); that is, when the rotation speed matches theappropriate air inlet velocity.

6. Conclusions

(1) In a turbo air classifier with a positively bowed guide blade, thevelocity deviation is significantly decreased along circle lines ofdifferent radii in the annular region. The tangential and radialvelocity distributions are also uniform. According to theequation for cut size [Eq. (1)], fine powders with narrow par-ticle size distribution can be achieved by keeping constantthe velocity in the separation surface (defined as the outerrotor cage cylindrical surface).

(2) Guide blade structural variations dramatically influence theflow field in the passageway of the two rotor cage blades. Forthe bowed guide vanes, the air flow tangential velocity isapproximately equal to the tangential velocity of the rotorcage blade exterior edge. Consequently, the intensity of theinertia rotating vortex in the passageway of the two rotorcage blades is evidently decreased. Decreased vortex intensityis apparently advantageous for material classification.

(3) With positively bowed guide blades, the tangential and radialvelocity distributions of air flow in the height direction of theseparation surface are fairly uniform and only slightly fluc-tuate. Also, the mean radial velocity in the height direction isalso increased, whereas the mean tangential velocity isdecreased (compared with the straight and negatively bowedblades). The cut size is subsequently decreased.

(4) Actual classification experiments indicate that a turbo airclassifier with positively bowed guide blades produces smallercut size and a higher classification precision index than withstraight guide blades. The experimental results confirm thesimulation results.

(5) The further research will be focused on more detailed curvingangle that leads to the optimized performance.

List of symbols

d50 cut sizeCD drag coefficientρa density of air flowρp particle densityR outer semi-diameter of rotor cagevr radial velocityvθ tangential velocityρ liquid densityμ liquid viscosityP liquid pressureδ Kronecker factorui time average velocityxi positional lengthS distribution deviationvi velocity of different grid nodes in the circle

Acknowledgments

This project was supported financially by the National NaturalScience Foundation of China (No. 51074012).

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