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GENERAL RESEARCH Flow in the Inlet Region in Tangential Inlet Cyclones Weiming Peng* and Pascal J. A. J. Boot Department of Chemical Engineering, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Alex C. Hoffmann* Department of Physics, University of Bergen, Allegt. 55, 5007 Bergen, Norway
Huub W. A. Dries, Jan Kater, and Andreas Ekker Shell Global Solutions, Shell Amsterdam, P.O. Box 38000, 1030 BN Amsterdam, The Netherlands
In this paper the flow pattern in a tangential inlet cyclone is studied by laser Doppler anemometry, with emphasis on the inlet region. The particular focus is on axial asymmetry in the flow, which was studied by determining radial profiles of the axial and tangential gas velocity components at four different angular positions. This was done at each of four axial flow stations. The results are shown graphically, discussed, and compared with the literature; in particular, they are compared with two studies: (a) an experimental investigation of the boundary layer flow in cyclones and (b) a study based on three-dimensional computational fluid dynamics (CFD) simulations. The results presented in this paper expose regions where the gas velocity differs considerably from that normally held to exist. Although the results generally agree qualitatively with numerical simulations, they differ considerably quantitatively: they show less axial asymmetry than do the CFD simulations. The results support the notion put forward by some other workers that a recirculatory flow pattern in the axial/radial directions exists in the upper part of the inlet region associated with secondary flows induced by the swirling motion in the boundary layer by the cyclone lid. This feature may have profound influence on the separation efficiency. The likely effects of the flow features on the separation efficiency of the cyclone and its reliability are discussed. Introduction Tangential cyclones are the most widely used dedusting devices in the processing industry. Among many other applications, they are an essential component in fluidized catalytic cracking units, which convert heavy feeds into lighter products by cracking large hydrocarbon molecules into smaller ones over a solid catalyst. Efficient, reliable dedusting downstream of the main plant is needed to meet ever more stringent environmental requirements and protect downstream equipment, such as turbines for power recovery. Meeting new stringent emission limits using improved cyclone technology is far superior to having to resort to more expensive and cumbersome dedusting alternatives. To improve our prediction of the separation efficiency in centrifugal dedusters and improve their performance, we need to develop our understanding of the complex swirling flow pattern. This paper focuses on an aspect that is often discussed in the literature but is not very well-known: the flow in the inlet region of tangential entry cyclones and the degree of axial asymmetry there. * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +47 55 58 28 76. Fax: +47 55 58 94 40.
Experimental observations of the flow pattern in cyclones have historically been made in four ways: (a) by flow visualization by introducing a tracer in the form of a very fine dust or smoke into the air stream entering the cyclone or at points within the cyclone, (b) by means of streamers suspended in the cyclone, (c) by probe measurements of the local velocity with either pitot tubes or hot-wire anemometers, and (d) by laser Doppler anemometry (LDA). Rosin et al.1 and Van Tongeren2 reported early experimental studies of the flow pattern in cyclones. Shepherd and Lapple3 used a pitot tube to determine the gas velocity in the body of a cyclone. Smith4 found that a conventional probe entered through a hole in the cyclone wall caused a strong asymmetrical disturbance of the flow. He, therefore, measured the axial and tangential velocity profiles in his 140-mm-diameter cylindrical cyclone by using probes with slender heads on 1-mm-diameter tubes spanning the diameter of the cyclone. This arrangement allowed the pitot tube probes to be accurately aligned along the diameter and meant that the measuring device did not compromise symmetry about the cyclone axis. Chanaud5 discussed the causes and effects of vortex asymmetry. Reydon and Gauvin6 observed asymmetry
10.1021/ie010226q CCC: $20.00 © 2001 American Chemical Society Published on Web 10/13/2001
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in the flow in a cyclone due to the single inlet persisting as far as the cone region and discussed the phenomenon. Iozia and Leith7 also found a difference in the tangential velocity profile between the opposite sides of a cyclone. When they compared the axial velocity profiles with the time-averaged axial velocity model of Bloor and Ingham,8 they found a difference in the closeness of fit between model and measurements between opposite sides of the cyclone in the lower cone, which, moreover, varied in time. Ogawa et al.9 studied the variation in flow profiles with orientation of the traverse diameter with respect to the inlet. The asymmetry and fluctuations of the core thwarted Hsieh and Rajamani’s10 attempts to measure radial velocities with a laser Doppler anemometer. They found that the flow was most asymmetric near the inlet and dust exit. Trefz11 studied the flow in the boundary layer under the cyclone lid by entering a three-bore probe through the lid at a number of tangential and axial positions and thus measuring the near-wall velocity profiles. He did this with and without a vortex finder protruding into the cyclone. With the vortex finder, he also measured the boundary layer flow along the outer vortex finder wall. Trefz also collect samples of the dust flowing in the boundary layer under the lid, measuring the total material flow there and the size distribution of the dust. Trefz reported his velocities divided by the swirl velocity outside the boundary layer at the measuring point, so that his dimensionless tangential velocities always become unity at the edge of the boundary layer. Although this is the best way of showing the boundary layer flow, this means that his work does not give any information about absolute values for the velocities. This literature survey, which was done in the framework of a large project aimed at improving cyclone modeling and performance, concluded that very little quantitative information is available on the bulk flow in the inlet region and its degree of asymmetry. The knowledge of the flow pattern in the inlet section of conventional cyclones remains patchy. Lately, computational fluid dynamics (CFD) has gained enormouspopularity as a technique for predicting the flow field in gas cyclones. The separation efficiency is mostly predicted subsequently by Lagrangian particle tracking in a precalculated gas flow field. Threedimensional (3D) CFD can be very helpful for a detailed investigation of the gas-solid flow pattern inside the separator body. Such simulations have, until now, not been checked extensively against experimental results. Derksen and Van den Akker12 compare a number of aspects of their simulated flow pattern with experiment. The objective of this research was to determine the flow pattern in the inlet region of cyclones with a tangential inlet and to assess the degree of axial asymmetry and its significance for cyclone performance and reliability. Experimental Procedure Test Rig. The technique of LDA, which is nonintrusive, was used to measure the mean gas velocities without influencing the flow conditions. The rig was situated at the Shell Research and Technology Center in Amsterdam (SRTCA). A schematic of the rig is shown in Figure 1. In LDA the velocity of the gas is measured as the speed of small seed particles, considered to follow the gas flow faithfully. Two laser light beams are made to
Figure 1. Experimental setup.
cross inside the cyclone, which creates an interference pattern (the measuring volume) through which the seed particles fly. The light scattered by the particles is detected (this is called a “Doppler burst”), and the frequency of this is a measure of the speed with which the particles traverse the measuring volume. In classical LDA, each of the components of the gas velocity is measured separately. In newer systems, different velocity components can be measured in the same volume simultaneously using different colored laser lights, which are detected separately. If the direction in which the particle crosses the measuring volume is unknown, it can be determined by shifting the frequency of one of the beams (using a “Bragg cell”) to create a moving, rather than a stationary, interference pattern. The main advantage of LDA is that it is nonintrusive. In the case of cyclones, problems can be caused by the seed particles not faithfully following the strongly swirling gas flow and by the refraction of the laser beams in the cylindrical wall of the cyclone or swirl tube. In the present experiments, the LDA measurements were performed with a DANTEC four-beam, twocomponent, high-performance 5-W Ar-ion backscatter laser Doppler anemometer. This laser produced light of two different wavelengths: 514.5 and 488 nm. During the experiments only the 514.5-nm wavelength was used. The axial and tangential velocity components were measured separately. The frequency of one of the laser beams was shifted in a Bragg cell. A total of 95% of the seeding smoke particles were in the size range 0.5-1 µm. Each measurement was an average of about 10 000 individual measurements recorded over a period of about 10 min; the validation rate was 85%. To avoid bias, the LDA velocity analyzer was calibrated using a wheel spinning at a known rate. The root-mean-square velocity fluctuation was also recorded and was in the region of 1 or 2 m/s, depending on the position in the cyclone; these results are being analyzed. Dry air was fed into the cyclone by a 6-bar gauge network. The flow was controlled by a Bronkhorst mass flowmeter. To compute the volumetric flow from the mass flow, the pressure and temperature in the inlet of the cyclone were monitored with a Druck PTX-610-0 manometer and a Digitron PT100, respectively. Figure 2 shows a diagram of the cyclone used. The lower section of the vortex finder was tapered, and the details are given in the figure. Test Program. The tangential and axial velocity components were measured. The axial measurement stations are indicated in Figure 2. Measurements were performed at 9 different axial stations, each at 20 points in the radial direction. In this paper, the axial positions of the stations are given as the distance below the cyclone roof divided by the total length of the cyclone,
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Figure 4. Tangential gas velocity measured with LDA at a series of axial stations in the body of the cyclone. The highest station is just in the cylindrical section; the rest are in the cone. The darker the points, the lower the station. Figure 2. Construction of the cyclone with the axial measuring stations indicated.
Figure 3. Angular measuring stations in the inlet region of the cyclone.
Figure 5. Axial gas velocity at the axial stations below the vortex finder.
roof to dust exit: |z|/L. The stations were (i) four in the gas inlet area, (ii) one in the cylindrical part, near the junction between the cylinder and the cone, and (iii) four in the conical part. Velocity profiles were measured at four different angles relative to the inlet: 0°, 90°, 180°, and 234°. The angular positions are indicated in the top view in Figure 3. The air flow into the cyclone was approximately 200 Nm3/h, corresponding to an inlet velocity of about 10 m/s at inlet conditions of 1.04 bar and 20.4 °C. This corresponds to a Reynolds number based on the mean axial body velocity and the body diameter of the cyclone of 24 000.
zero axial velocity appears to move inward slightly as we move down the conical section of the cyclone. Flow Pattern in the Inlet Region of the Cyclone. Experimental results for the tangential and axial velocity profiles at the flow stations in the inlet region, that is, at |z|/L ) 0.022, 0.059, 0.096, and 0.134 and at angles of 0°, 90°, 180°, and 234°, are shown in Figures 6 and 7.
Results Flow Pattern in the Body below the Vortex Finder. Figure 4 shows profiles of the tangential velocity at the stations below the mouth of the vortex finder in the cyclone at the angle of 0°. Because the wall of the cylindrical section has dimensionless radius 1.0, the axial stations in the conical section have wall radii of less than 1. In all of the figures in this section, the markers representing the experimental results have been connected with thin, straight lines, so that the reader can more easily distinguish the shape of the different profiles. Figure 5 shows profiles of the axial velocity at the same stations. A dip in the axial velocity near the axis is clearly visible. This is a well-known feature in reverse-flow cyclones. The shape of the curves is more or less independent of the axial position. The locus of
Discussion Flow and Axial Symmetry. Figures 6 and 7 show that the tangential and axial velocity profiles change with circumferential position at all of the axial flow stations high in the cyclone. Thus, both of these velocity components exhibit significant axial asymmetry in the inlet region. Concentrating first on the swirl velocity, when we study the shape of the profiles in Figure 6, the outer part of the curves can be seen to be similar to those in the outer part of the separation space below the vortex finder (Figure 4). This type of swirl is described by the equation
vθ ) C/rn
(1)
where n is empirically found to lie in the range 0.5-1, taking on the latter value for completely loss-free swirl. The tangential velocity exhibits a maximum close to the wall of the vortex finder. Trefz11 found that the boundary layers on the outer wall of the vortex finder were thicker on the back of the vortex finder (at his angular stations 180° and 225°, his angular positions are defined like ours), becoming as thick as 30-40 mm
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Figure 6. Tangential velocity profiles at the flow stations in the inlet region: (a) |z|/L ) 0.022; (b) |z|/L ) 0.059; (c) |z|/L ) 0.096; (d) |z|/L ) 0.134.
Figure 7. Axial velocity profiles in the cyclone at the flow stations in the inlet region: (a) |z|/L ) 0.022; (b) |z|/L ) 0.059; (c) |z|/L ) 0.096; (d) |z|/L ) 0.134.
and also having a more severe velocity deficiency than that on the front. It is clear that parts b and c of Figure 6 are in agreement with this, with the tangential velocity dropping off earlier and more severely as we approach the vortex finder wall for the angles 180° and 234°. In addition, our results show that the absolute values of vθ in the bulk fluid are also lower at the back of the vortex finder. In general, the swirl velocity in Figure 6 clearly depends strongly on the angular position; i.e., the axial asymmetry is strong. This will be discussed further below. Turning now from the shape of the profiles to the absolute values of the swirl velocity, the absolute values in Figure 6, including those at the outer wall of the cyclone, are significantly higher than those expected at this inlet velocity and higher than those in Figure 4. It is well-known that constriction of the inlet jet as it enters the cyclone causes an acceleration of the flow from the inlet velocity, vin, to that at the cyclone wall,
vθw. Barth13 proposed to calculate vθw from
R ≡ vinRin/vθwR
(2)
R is the radius of the cyclone, and Rin is the radial position of the center of the inlet. For a slot inlet, Rin ) R - b/2. R is given by
R ) 1 - 0.4(b/R)0.5
(3)
Meissner and Lo¨ffler14 (see also ref 15) proposed first to calculate a velocity just after the inlet vθw*, which was high due to the inlet jet constriction,
vin b ) -0.204 + 0.889 vθw* R
(4)
and then calculate the final velocity at the cyclone wall vθw considering the retardation due to the friction with the cyclone wall:
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vθw )
[x
〈vz〉 fcylHcyl*
]
vθw* 1 1 + fcylHcyl* 4 2 〈vz〉
(5)
with
Hcyl* )
[
]
Hcyl a -arccos(1 - b/R) + R 2π R
(6)
where 〈vz〉 is the volumetric flow Q divided by the cyclone cross-sectional area πR2, Hcyl is the height of the cylindrical section, and fcyl is the friction factor, equal to about 0.0065 at low solids loadings. These equations give, for the present case, vθw ) 10.5 m/s from eqs 2 and 3, vθw* ) 12.7 m/s from eq 4, and vθw ) 11.6 m/s from eqs 5 and 6. The experimental results support the general idea of Barth and Meissner-Lo¨ffler of a high wall velocity in the inlet region (Figure 6), which is then reduced by wall friction to a value only slightly higher than the inlet velocity (Figure 4). In fact, the profile for |z|/L ) 0.334 in Figure 4 agrees well with the prediction of eqs 5 and 6. However, the wall velocities in Figure 6 are 13-16 m/s, considerably higher than the value calculated above for vθw*, particularly at 0°. Turning our attention from the tangential to the axial velocity profiles, Figure 7 shows that the mean axial flow is progressively more downwardly directed as we move down the inlet, as we would expect. The profiles for the top two flow stations (Figure 7a,b) show upward flow at the wall at three of the four circumferential positions. This indicates a circulating flow high in the region between the cyclone wall and the vortex finder. This is confirmed by the observations of Patterson and Munz,16 who inferred precisely such a recirculating flow in this region, with upward gas flow at the cyclone wall, from the dust striation patterns on the wall of their cyclone. Trefz11 suggest a boundary layer flow pattern in this region as sketched in Figure 8. This idea is wellknown: the strong radial pressure gradient in the swirl is balanced by the centrifugal force for fluid elements in the bulk, but in the boundary layers at the lid and cone, where the velocity is much smaller yet the pressure gradient prevails, the pressure gradient is a driving force for the boundary layer flows indicated (often called “secondary flows”). As mentioned, Trefz verified the boundary layer flow along the cyclone lid. As stated in the literature survey, we cannot compare our results directly with his because of the way he reported them, but our results can be seen to be qualitatively consistent with the near-wall flows sketched in Figure 8. The shapes of the curves in Figure 7d are somewhat odd. It is likely that we are seeing a transition from the shape high in the inlet region (Figure 7a,b) to that below the vortex finder (Figure 5). When looking at axial symmetry lower in the cyclone, we see that the axial velocity at the flow stations below the vortex finder in Figure 5 exhibits some asymmetry in the inner vortex with respect to the cyclone centerline. Velocity Profiles in the Tangential Direction in the Inlet Region and Qualitative Comparison with 3D CFD Simulations. Figures 9 and 10 show angular profiles of the tangential and axial velocity components about 6 mm from the outside wall of the vortex finder
Figure 8. Secondary flows along the walls in a cylinder-on-cone cyclone with tangential inlet.11
Figure 9. Tangential velocities at different flow stations as a function of the azimuthal angle: (a) near the vortex finder wall (r/R ) 0.55, about 6 mm from the outer vortex finder wall); (b) near the cyclone wall (r/R ) 0.96, 2.2 mm from the wall).
Figure 10. Axial velocities at different flow stations as a function of the azimuthal angle: (a) near the vortex finder wall (r/R ) 0.55); (b) near the cyclone wall (r/R ) 0.96).
and 2.2 mm from the inside wall of cyclone body, respectively. These plots give direct information about angular asymmetry in the flow. The radial and axial positions have been chosen for comparison with the results of the 3D CFD simulations of Hoekstra et al.17,18 Parts a and b of Figure 9 show that the tangential velocity is highest at the azimuthal angle of 0° and does not change dramatically in the region 0-90°. In the
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region 90-234°, it decreases significantly and accelerates in the region 234-360°. At 360° the flow joins the flow entering the annulus through the inlet. Considerable axial asymmetry in the inlet region of the cyclone is thus evident both near the vortex finder wall and near the wall of the cyclone body. Results from the 3D CFD simulations of Hoekstra et al.17,18 are also included in these figures. We can only compare qualitatively, because their cyclone geometry was slightly different (they used a Stairmand HE geometry which has a slightly larger vortex finder). They used an inlet velocity of 15 m/s rather than the 10 m/s used here; we have scaled their results in that proportion for comparison. We can best compare our LDA results at |z|/L ) 0.134, corresponding to |z|/D ) 0.45 with their CFD simulation at |z|/D ) 0.5. Our radial positions of r/R ) 0.55 and 0.96 are somewhat equivalent to theirs of 0.63 and 0.97. The shapes of the tangential velocity profiles can be seen to agree quite well particularly near the vortex finder (Figure 9a). Both LDA and CFD show the same trends and significant degrees of asymmetry. Also here we see that our tangential velocities are considerably higher than those of Hoekstra et al., although their wall velocities are also higher than their inlet velocity. Despite the significant asymmetry in the inlet region, the conclusion of Hoekstra et al. was that the axial and tangential velocity components in the separation zone of the cyclone were quite similar for 3D and 2D simulations. The agreement between LDA and CFD in the axial velocities (Figure 10a,b) is rather poor. Our LDA results show a higher degree of axial symmetry, and the measured velocities are for |z|/L ) 0.134, close to the expected mean value of -1.65 m/s. The profiles of Hoekstra et al. are more nonuniform tangentially. The expected mean value for their axial velocity is -1.69 m/s. Effect of the Flow Asymmetry on the Cyclone Separation Performance. The results expose a region of low near-wall velocity on the “back” of the vortex finder, away from the inlet. Powder deposition often occurs in regions with low near-wall gas velocity. If, in a given process, there is a tendency for powder deposition, the back of the vortex finder might well be the region where problems are first encountered. It is worth exploring whether a vortex finder slightly off-center would lead to more uniform velocities around its outside wall and, therefore, to less dust deposition there. In fact, Trefz11 did look at this issue, trying out two off-center positions. He found that moving the vortex finder 0.1 vortex finder diameters in the direction of 180° made the problem worse, while moving it the same distance in the direction of 255° led to better axial symmetry in the flow near the outer vortex finder wall. Conclusions 1. The velocity distribution in a tangential inlet cyclone has been charted, with particular attention to the inlet region. 2. In the inlet region, tangential near-wall velocities higher than those expected were found. The tangential wall velocity reduced the cyclone body to attain the expected value just under the vortex finder. 3. A recirculating flow was found high in the inlet region, confirming inferences drawn from the dust
striation pattern and measurements of the boundary flow in the literature. 4. A considerable deviation from axisymmetry was found in the inlet region. 5. Where comparison was possible, our results agreed with those of Trefz.11 His and our results can be combined to give a complete map of the flow pattern in the upper section of the cyclone. 6. The experimentally determined velocities agreed reasonably well with the 3D CFD study of Hoekstra et al.;17,18 the tangential velocities agreed much better than the axial ones. 7. The motion of the vortex under the mouth of the vortex finder was found to be slightly off-center with respect to the cyclone axis. List of Symbols b ) width of the cyclone inlet C ) constant in eq 1 D ) cyclone body diameter fcyl ) friction factor of the cylindrical section Hcyl ) height of the cylindrical section L ) total length of the cyclone n ) index in eq 1 R ) radius of the cyclone body ) D/2 Rin ) radial position of the center of the inlet with respect to the cyclone axis r, θ, z ) components of a cylindrical coordinate system vin ) inlet velocity vθ ) tangential velocity vθw ) velocity at the cyclone wall vθw* ) velocity at the cyclone wall just after the inlet in the Meissner-Lo¨ffler model 〈vz〉 ) mean axial velocity ) Q/πR2 Q ) volumetric gas flow rate z ) axial coordinate starting at the cyclone lid Greek Letters R ) parameter of Barth for the acceleration of the inlet jet
Literature Cited (1) Rosin, P.; Rammler, E.; Intelman, W. Principles and Limits of Cyclone Dust Removal. VDI Z. 1932, 76, 443. (2) Van Tongeren, A. J. A Modern Dust Collector. Mech. Eng. (Am. Soc. Mech. Eng.) 1935, Dec, 753. (3) Shepherd, C. B.; Lapple, C. E. Flow Pattern and Pressure Drop in Cyclone Dust Collectors. Ind. Eng. Chem. 1939, 31, 972. (4) Smith, J. L. An experimental study of the vortex in the cyclone separator. J. Basic Eng. 1962, 84, 602. (5) Chanaud, R. C. Observation of oscillatory motion in certain swirling flows. J. Fluid Mech. 1965, 21, 111. (6) Reydon, R. F.; Gauvin, W. H. Theoretical and experimental studies of confined vortex flow. Can. J. Chem. Eng. 1981, 59, 14. (7) Iozia, D. L.; Leith, D. Effect of Cyclone Dimensions on Gas Flow Pattern and Collection Efficiency. Aerosol Sci. Technol. 1989, 10, 491. (8) Bloor, M. I. G.; Ingham, D. B. The flow in industrial cyclones. J. Fluid Mech. 1987, 178, 507. (9) Ogawa, A.; Nagasaki, K.; Sugiyama, K. Theory of the fractional collection efficiency for the axial flow cyclone dust collector. Part. Sci. Technol. 1994, 12, 243. (10) Hsieh, K. T.; Rajamani, R. K. Mathematical model of the hydrocyclone based on physics of fluid flow. AIChE J. 1991, 37, 735. (11) Trefz, M. Die vershiedenen Abscheidevorgange im hoher un hoch beladenen Gaszyklon unter besonderer Berucksichtigung der Sekundarstromung. Forschritt-Berichte VDI; VDI-Verlag GmbH: Dusseldorf, Germany, 1992, Vol. 295. (12) Derksen, J. J.; Van den Akker, H. E. A. Simulation of Vortex Core Precession in a Reverse-Flow Cyclone. AIChE J. 2000, 46, 1317.
Ind. Eng. Chem. Res., Vol. 40, No. 23, 2001 5655 (13) Barth, W. Berechnung und Auslegung von Zyklonabscheidern auf Grund neuerer Unterscuhungen. Brennst.-Wa¨ rmeKraft 1956, 8 (1), 1. (14) Meissner, P.; Lo¨ffler, F. Zur Berechnung de Stro¨mungsfeldes, im Zyklonabscheider. Synopse in: Chem. Ing. Tech. 1978, 50, 471. (15) Mothes, H.; Lo¨ffler, F. Prediction of particle removal in cyclone separators. Int. Chem. Eng. 1988, 28 (No. 2), 231. (16) Patterson, P. A.; Munz, R. J. Gas and particle flow patterns in cyclones at room and elevated temperatures. Can. J. Chem. Eng. 1996, 74, 213. (17) Hoekstra, A. J.; Derksen, J. J.; Van den Akker, H. E. A. An Experimental and Numerical Study of the Turbulent Swirling
Flow in Gas Cyclones. Chem. Eng. Sci. 1999, 54, 2055. (18) Hoekstra, A. J.; Derksen, J. J.; Van den Akker, H. E. A. A CFD Study on the Performance of a High-efficiency Gas Cyclone. Proceedings of the International Symposium on Computational Technologies for Fluid/Thermal/Structural/Chemical Systems with Industrial Applications; ASME-PVP: Fairfield, NJ, 1999; Vol. 397-2, p 219.
Received for review March 12, 2001 Accepted August 27, 2001 IE010226Q
