experimental and theoretical study of particle dispersion phenomena in a turbulent gas jet of the...

Experimental and Theoretical Study of Particle Dispersion Phenomena in a Turbulent Gas Jet of the Flash-Smelting Process by the Image Analysis Technique

YUTAKA YASUDA and H.Y. SOHN

The particle dispersion phenomena in a turbulent gas jet of the flash-smelting process have been investigated. The particle number density in a nonreacting jet was measured experimentally by taking photographs of the particles in flight and counting them using the image analysis technique. Numerical computations were also performed using a mathematical model for a turbulent particle-laden gas jet. The experimental particle number density data were then compared with the predicted results of the model. Good agreement was obtained between the experimental and predicted data. The effects of the parameters such as the injector design, particle loading, and air flow rate are discussed. Optimization of the concentrate burner design can be achieved with the help of experimental and theoretical simulations as performed in this work.

I. INTRODUCTION

F L A S H smelting is a pyrometallurgical process for smelting metal sulfide concentrates. In flash smelting, fine particles of sulfide concentrate are injected with the process gas through the concentrate burner, forming a turbulent particle-laden jet. Since the concentrate particles undergo oxidation reaction in flight, particle dispersion is a major factor in controlling the degree of oxidation of each concentrate particle. The configuration of the concentrate burner and the flow rate of the process gas significantly affect the particle dispersion in the jet. For improved burner design, it is necessary to under- stand how the particles disperse in the jet and how the dispersion phenomena are affected by the flow conditions and geometry of the burner. Experimental work on a simplified system can provide important information for the basic understanding of the particle dispersion phenomena.

On the other hand, development should be based on theoretical background. Hahn and Sohn ~1,31 and Hahn 12] developed a mathematical model to describe the various processes taking place in a flash furnace. The mathematical model provides a fundamental basis for the design and optimization of the concentrate burner. The model was verified by comparing the predicted results with measurements in a laboratory flash furnace and a pilot plant furnace. A detailed description of the particle behavior, which has a significant effect on the predicted results of the mathematical model, however, was not developed. The predictive ability of the mathematical model should be verified in terms of the particle behavior in a turbulent gas jet. It was the purpose of this research to carry out an experimental and mathematical simulation investigation to determine the effects of various oper- ating parameters on the particle dispersion in such a turbulent gas jet.

YUTAKA YASUDA, formerly Graduate Student, Department of Metallurgical Engineering, University of Utah, is Process Engineer, Saganoseki Smelter & Refinery, Nippon Mining and Metals Co., Ltd., Saganoseki, Oita, Japan. H.Y. SOHN, Professor, is with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112-1183.

Manuscript submitted May 5, 1994.

II. EXPERIMENTAL

To investigate the particle dispersion phenomena in a jet, the particle number density was measured under iso- thermal and nonreacting conditions. The experimental particle number density data were compared with the predicted results of the mathematical model.

A. Experimental Facility

The experimental apparatus is shown schematically in Figure 1. Pressurized air (500 to 800 kPa) was supplied by a compressor. A pressure regulator with an air filter was used to set the air flow rate, which was measured with a rotameter. A particle feeder was installed above the observation chamber. To minimize the pulsing of the particle feed, a tandem arrangement of an Acrison screw feeder and a Syntron vibratory feeder was used. The particles used in this work were silica sand nominally sized between 149 and 250/~m. The mass density of the silica sand was 2600 kg /m 3. The injector was made of coaxial acrylic tubes. The primary air and particles were supplied from the top through the inner tube, and the secondary air was injected from the side through the annulus. The observation chamber was a rectangular box made of acrylic plates and lined with glass plates to prevent the particles from sticking to the wall of the chamber. The height of the chamber was 1.22 m, and the width and length were both 0.61 m. At the bottom of the chamber, there was an outlet for the air. The chamber pressure was maintained at slightly below the atmospheric pressure ( - 5 mm H20). For the light source, five 250-W spot- lighting halogen lamps (Q250PAR38/SP, General Electric, Schenectady, NY) were placed on either side of the chamber, as shown in Figure 2. A black cloth was used for background. The particles were observed from the front of the chamber as white spots on a black background.

B. Experimental Techniques

1. Photography The instantaneous images of particles were photo-

graphed with a Minolta Maxxum 8000i 35-mm camera

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 26B, JUNE 1995--637

Particle

I Screw Feeder

Particles and Primary Air

Primary Air Vibratory Feeder

Injector

Pressure L RegulatQr

Air

t I I I I

I I I I

I t

; \ I I

I I

Observation Chamber

Manometer

I ~ ~ -'~Exhaust Gas

z

Fig. 1 - Schematic representation of the experimental apparatus.

through a Promaster 3.5 to 4.5, 28- to 70-mm lens. The camera was mounted in front of the observation chamber on a tripod, and the distance between the camera and chamber was set at 200 mm. Before each experiment, the camera position was checked by taking pictures of a linear scale. To catch the image of moving particles, faster shutter speeds were desirable, but they caused weak images of particles to be missed. Several shutter speeds from 1/8000 to 1/1000 s were tested, and 1/4000 s was found to be the optimum shutter speed. The aperture was set at 3.5 to get the largest exposure. KODAK* T-Max

*KODAK is a trademark of Eastman Kodak Corporation, Rochester, NY.

3200 film, which was the most sensitive film available, was used.

2. Particle number density measurement Each photograph was digitized into an image of

512 • 480 pixels with 8-bit grayness scale resolution using an IBM PC*-based frame grabber. A PHILIPS** VCM-81 monochrome CCD camera with a PHILIPS VS-15R monochrome monitor was used as the video input device. In the digitized image, each pixel had a luminance intensity signal that was digitized with an arbitrary

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r-l'" ] L 4 ~ .

i ,? i

~4"'~ L . ~ .

Black Cloth

r J

0,: 1

/

Fig. 2--Schematic illustration of the lighting setup.

grayness scale from 0 to 255. A value of zero corre- sponds to zero luminance, and a value of 255 corre- sponds to the full-scale saturation in the luminance signal. A photograph of the linear scale was first digitized to calibrate the dimensions of the subsequent digitized image. The magnification of the photographs was adjusted so that each pixel was 0.25-mm wide • 0.21-mm long. The digitized images were saved on a floppy disk to be analyzed by a SUN-SPARC* work station. An image

*IBM PC is a trademark of International Business Machines Corp., Armont, NY.

**PHILIPS is a trademark of Philips Electronic Instruments Corp., Mahwah, N J.

*SUN-SPARC is a trademark of Sun Microsystems, Inc., Moun- tain View, CA.

analysis software program named MIA (Metallurgical Image Analysis) developed within the Department of Metallurgical Engineering at the University of Utah was used to analyze the digital images.

For the particle number density measurement, a horizontal profiling of the luminance intensity profile was used in this study. Figure 3 shows an example of horizontal luminance profile. In the figure, each peak cor- responds to the location of a particle. The wider peaks were considered to be due to the overlapping of multiple particles. The profiles were compared with the digitized images and pointed to the following:

638--VOLUME 26B, JUNE 1995 METALLURGICAL AND MATERIALS TRANSACTIONS B

<

0

0 o

..=

200

150

I00

50

80 100 120 140 160 180 200 220

Pixel Position (Number)

Fig. 3 - - H o r i z o n t a l luminance profi le.

(a) Background noises were seen throughout the profile, but they were not larger than 90 luminance intensity value. Therefore, the peaks of more than 90 luminance intensity value were identified as the particles. (b) Typical peaks at over 90 luminance intensity were 4-pixels wide (I mm). (c) In the region near the center of the jet, the peaks were sometimes wider than 4 pixels, due to the overlapping of multiple particles.

Since the mass average diameter of the particles measured using standard TYLER* sieves was 180/xm, the

* T Y L E R is a t r ademark of W.S . Tyler , Inc . , Mentor , OH.

particle diameter seen in the digitized image was much larger than the measured particle diameter. A similar phenomenon was reported by Longrnire and Eaton. t4J They noted that the area occupied by a particle was affected by the intensity of the light scattered by the particle.

The local particle number density at a certain axial position was measured as follows. An area of 20 pixels width and 50 pixels length (5 • t0.5 mm) was chosen as a unit area. The unit area was scanned horizontally at 10 different locations. Since the typical particle diameter was 4 pixels long, the vertical distance between horizontal scans was set at 4 pixels so that no particles were missed. In each horizontal scan of the area, the number of pixels that had more than 90 luminance intensity value was counted. The counted number of pixels was divided by four to correspond to the number of particles. The number of particles counted in each scanning was added in the 10 scans, and the result represented the local particle number density for that area.

III. NUMERICAL COMPUTATION USING A M A T H E M A T I C A L MODEL

A. Outline of the Mathematical Model

Numerical computations have been performed using the mathematical model for flash-smelting processes developed by Hahn and Sohn, [1"31 Hahn, t2j and Sohn. tSl In the model, the flash-smelting process was considered to be a combination of several important subprocesses. The turbulent fluid dynamics of a particle-laden gas jet is a

major part of this model. Heat and mass transfer, gas- particle-reaction kinetics, and radiative heat transfer are also incorporated in the model. The two-equation (k-e) model was used for the turbulence closure problem in the model. The particle phase was viewed from the Lagrangian framework. The detailed descriptions, in- cluding the simplified geometries of the axisymmetric jet and the single- and the double-entry injectors, can be found in References 1 through 3.

B. Data Transformation by Horizontal Integration

The experimental particle number density was obtained from a two-dimensional image of the three- dimensional system. Since the model calculated the volumetric (cross-sectional) particle number density, either its integration to obtain a horizontally projected image or a conversion of the experimental data to the volumetric particle number density was necessary to compare them. The experimental data were not sufficiently smooth to obtain the appropriate result by the latter approach. Thus, the mathematically simpler former method was used in this work. The integration converts the number of particles per unit volume calculated by the mathematical model to the projected number of particles per unit area as it appears in the photographs.

IV. RESULTS AND DISCUSSION

Experiments were conducted using three different injector designs: a single-entry type, an axial double-entry type, and a double-entry type with a radial distribution cone. For each injector design, the air flow rate and the diameter of the injector were varied.

A. Single-Entry System

The single-entry-type injector is shown in Figure 4(a). The inner tube tip was 250 mm above the outlet of the injector to premix the air and the particles. Three single- entry-type injectors with inner diameters of 1 in. (25.4 mm), 1/4 in. (6.4 ram), and 1/8 in. (3.2 mm) were tested. The particle feeding rate was fixed at 0.03 kg/min, and the air flow rate was varied from 0.005 to 0.084 Nm3/min. As a result, the axial gas velocity at the outlet of the injector was varied from 0.47 to 88.4 m/s . The particle loading, which was defined as the mass flow rate of the particle divided by the mass air flow rate, was varied from 4.63 to 0.28 kg-solid/kg- air.

Effect of injector diameter To examine the effect of the injector diameter,

experiments were carried out using different injector diameters and the same air flow rate. Figure 5 shows the comparison of the projected particle number density plots for three different injector diameters. Figure 5(a) shows the experimental plots, and Figure 5(b) shows the predicted plots. The air flow rate was fixed at 0.028 Nm3/min, and the particle loading was 0.82 kg-solid/kg-air for all three conditions.

The central particle number density for the 1-in. injector is twice as high as that of the other injectors. The width of the plot remains unchanged with the injector


(a)

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Injector Diameter (Varied 3.2-25.4 mm) ~

1 Particles

m

(b) Primary Air and

1111 Particles

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Inner Tube Tip I ~2 o ,Oon,Van ,\ [111 10mm I n n e ~ n ~ t O u t e r x ~[~ ~_- ! - ~ - - -

(Varied 12.7-22.0 ram) --~, ~ 25. mm

Fig. 4 - - S c h e m a t i c representations of injectors: (a) single-entry type and (b) axial double-entry type.

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Fig. 5 - - E f f e c t of injector diameter on projected particle number density: (a) experimental and (b) predicted (single-entry injector; air flow rate = 0.028 Nm3/min; 150 m m below the jet entry). - - 1 in. (25.4 mm), �9 . . . . . 1 /4 in. (6.4 mm), and . . . . . 1/8 in. (3.2 mm).

diameter. In the predicted results, the particle number density in the center of the jet decreases with the de- crease of the injector diameter. The width of the plot also becomes small as the injector diameter decreases.

The large projected particle number density at the center of the jet for the l-in. injector may give an erroneous impression of very high volumetric number density. This is due to the larger width of the jet, which gives high number density per area when projected on a plane. To further elucidate this, the predicted volumetric particle number density profiles for the three injector diameters are plotted in Figure 6. The volumetric particle number density in the center of the jet does not change with the injector diameter, but the width of the jet decreases with decreasing injector diameter.

Although the agreement between the experimental and the predicted results is reasonable, the variation of the width of the plot with the injector diameter is somewhat different. The width increases with the injector diameter

in the predicted results but does not change with the injector diameter in the experimental results. The jet spreads wider than predicted in the experiments for the 1/4- and 1/8-in. injectors. This is because the effect of particle collision with the wall that causes the particles to scatter radially, as schematically illustrated in Figure 7, is larger with a smaller-diameter injector. This effect was actually observed in the digitized image of the particle-laden jet from the 1/4-in. injector, which showed the presence of a large number of particles outside the region of high axial velocities) 6,71 This phenomenon became less significant when the injector tip diameter was increased to 1 in.

B. Axial Double-Entry System

For the axial double-entry system, the particles and primary air were supplied through the inner tube, while


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Fig. 6 - - E f f e c t of injector diameter on predicted volumetric particle number density (single-entry injector; air flow rate = 0.028 Nm3/min; 150 m m below the entry). - - 1 in. (25.4 mm), . . . . . . 1 /4 in. (6.4 mm), and . . . . . I / 8 in. (3.2 mm).

Air Air

Particles

o o

250 mm

Fig. 7 - - S c h e m a t i c illustration of the injector tip.

METALLURGICAL AND MATERIALS TRANSACTIONS B

secondary air was introduced through the annulus between the inner and the outer tubes, as shown in Figure 4(b). The tip of the inner tube and the outlet of the injector were flush. The position of the inner tube tip was also varied to examine its effect on the particle number density. The diameter of the outer tube was fixed at 1 in. Two different inner tubes with outer diameters of 1/2 in. (12.7 mm) and 7/8 in. (22.0 mm) were tested. The thickness of the inner tubes was 1/16 in. (1.6 mm). While the particle feeding rate was fixed at 0.03 kg/min, the primary air flow rate was varied from 0.014 to 0.028 Nm3/min. The secondary air flow rate was varied from 0.014 to 0.084 Nm3/min. The axial velocity of primary air at the outlet of the injector was varied from 0.83 to 14.9 m/s, and the velocity of secondary air was varied from 0.62 to 11.2 m/s.

The geometry of the axial double-entry-type injector is similar to that of the Venturi-type taj concentrate burner used for the flash-smelting furnace. Since the particle is injected separately from the process air, the particle dispersion takes place after the jet is injected. It is desirable that the particles disperse widely and uniformly into the jet. In Sections 1 and 2, the effects of the inner tube diameter and the axial position of the inner tube tip will be discussed.

1. Effect of inner tube diameter To examine the effect of the inner tube diameter for

the axial double-entry system, data for injectors with inner tubes of different outer diameters were compared. As a reference, the data for the single-entry 1-in. (25.4-mm-) diameter injector were also compared with those of the double-entry injectors.

Figure 8 shows the comparison of the projected particle number density for different inner tube diameters. Figure 8(a) shows the experimental plots, and Figure 8(b) shows the predicted plots. The air flow rates for the double-entry injectors were 0.014 Nm3/min for the primary and 0.084 Nm3/min for the secondary. The particle loading was 1.64 kg-solid/kg-air for the primary air and 0.23 kg-solid/kg-air for the total air. The air flow rate for the single-entry injector was 0.084 Nm3/min.

In the experimental results, the plot for the double- entry injector with the 1/2-in. inner tube shows a narrow peak in the center of the jet, while the plot for the 7/8-in. inner tube is a wider peak. The plot for the single-entry injector has a shape between the plots for two double- entry injectors. The peak heights of these three plots are almost identical. The predicted results are close representations of the experimental results. The plot for the double-entry injector with the 7/8-in. inner tube indi- cates the widest and smoothest profile of the three plots. The plot for the 1/2-in. inner tube shows a sharp peak in the center of the jet. The peak of the predicted plot for the 1/2-in. inner tube is again narrower than that of the experimental plot, as observed in the case of single-entry injectors. The plot for the single-entry 1-in. injector shows a similar profile to the plot for the double- entry injector with the 7/8-in. inner tube.

To further examine the effect of the inner tube diameter, the predicted volumetric particle number density and axial air velocity are shown in Figures 9(a) and (b). The shape of the air velocity plot changes significantly

VOLUME 26B, JUNE 1995--641

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1

0 -5

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Fig. 8 - - E f f e c t of inner tube diameter on projected particle number density: (a) experimental and (b) predicted (double-entry injector; 150 mm below the jet entry; air flow rate = 0.014 Nm~/min for primary and 0.084 Nm3/min for secondary). - - Double-entry injector with 7/8-in. (22.0-mm-) o.d. inner tube. - . . . . . Double-entry injector with 1/2-in. (12.7-mm) o.d. inner tube. - . . . . Single-entry injector with l-in. (25.4-mm) i.d.

with the inner tube diameter. The plot for the injector with the 1/2-in. inner tube shows a sharp peak in the center of the jet. On the other hand, the plot for the injector with the 7/8-in. inner tube shows a smooth and wide velocity profile. The double-entry injector with a larger outer diameter tube appears to generate a wider and smoother velocity profile. The volumetric particle number density plot also varies significantly with the inner tube diameter. The plot for the injector with 1/2-in. inner tube has a very different shape from the other plots. The plot shows a high and narrow peak and has a steep gradient on both sides. For the single-entry injector, the

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(b)

Fig. 9 - - E f f e c t of inner tube diameter (a) on predicted volumetric particle number density and (b) on predicted axial air velocity (double-entry injector; 150 mm below the entry; air flow rate = 0.014 Nm3/min for primary and 0.084 Nm3/min for secondary). - - Double-entry injector with 7/8-in. (22.0-mm) o.d. inner tube. . . . . . . Double-entry injector with 1/2-in. (12.7-mm) o.d. inner tube. . . . . . Single-entry injector with 1-in. (25.4-mm) i.d.

width of the particle number density plot is close to that of the velocity plot, and the shape of the density plot looks similar to that of the velocity plot. It is reasonable that the single-entry injector disperses the particles well into the jet, because the particles and air are premixed inside the injector for the single-entry system. For the double-entry injector with the 7/8-in. inner tube, the particle number density plot is almost as wide as that for the single-entry injector but slightly narrower than that of the axial velocity plot. The appearance of the plot is smooth and similar to the velocity plot. The particles are dispersed widely in the jet using the double-entry injector with the 7/8-in. inner tube.


In summary, the particle dispersion phenomena as- sociated with the double-entry injector are greatly affected by the diameter of the inner tube. An injector with a larger outer diameter inner tube disperses the particles more widely in the jet. The diameter of the jet also increases when a larger diameter inner tube is used.

2. Effect of inner tube position To examine the effect of the axial position of the inner

tube tip, the positions were varied experimentally. The tested axial positions were 25 mm above, 10 mm above, and 10 mm below the outlet of the injector. The particle number density was measured at 150 mm below the jet entry. However, the model could not accommodate the experiments in which the inner tube position was varied. The numerical computation was performed only for the "flush" inner tube tip position. Therefore, the experimental results have been compared with the experimental and predicted results with the flush inner tube tip position. In the experiments, the other parameters were kept the same. The inner diameter of the injector was 1 in. (25.4 mm), and an inner tube of 7/8-in. (22.0 mm) outer diameter was used. The air flow rates were fixed at 0.014 Nm3/min for the primary air and 0.056 Nm3/min for the secondary air. The axial velocities were 0.83 m/s for the primary and 7.45 m/s for the secondary air stream. The particle loading was 1.65 kg-solid/kg-air for the primary air and 0.33 kg-solid/kg-air for the total air.

Figures 10 and 11, respectively, show the projected particle number density plots with the inner tube positions of 25 mm above and 10 mm below the outlet. In the figures, the experimental and predicted plots with the flush inner tube position are shown as references. The experimental plot for 25 mm above the outlet has a wider and smoother profile than that of the flush position. The peak height is slightly smaller than the plots for the flush position. The experimental plot for 10 mm below the

outlet is similar to the plots for the flush inner tube position, though the peak height of the plot is slightly higher.

It is found that the inner tube tip position affects the particle dispersion in the jet. The particles are dispersed more widely into the jet by moving the inner tube tip higher above the outlet. When the inner tube tip position is located below the outlet of the injector, the particle dispersion of the injector is similar to the injector with the flush inner tube position. When the inner tube tip is located above the outlet, a space exists inside the injector, surrounded by the inner tube tip and outlet of the injector. This space acts as a mixing chamber for the particles and air. The particles are dispersed widely in the jet because of the mixing before the injection.

This phenomenon can be applied for the concentrate burner design. To obtain wider dispersion of the particles for the Venturi-type burner, the inner tube tip should be located higher above the outlet of the burner. How- ever, the oxidation reaction may occur inside the injector during the premixing. Thus, the single-entry-type injector is not often used as the concentrate burner of the flash-smelting furnace. The reaction has to take place after the particles and reactant gas are injected into the furnace. The inner tube position of the Venturi-type burner must be determined after a careful consideration of the particle dispersion and reaction inside the burner.

C. Double-Entry System with Radial Distribution Cone

The double-entry-type injector with a radial distribution cone simulates the central-jet-distribution-type tgj concentrate burner that is used for the flash-smelting furnace. A cone is located below the outlet of the injector, and the primary air is injected radially through the cone. Since the radially injected primary air provides a radial velocity component for the particles, they are expected

x 10 5 10

)<

r

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-4 -3 -2 -1 0 1 2 3 4

x 10 ~

Radial Position (m)

Fig. 10--Effect of inner tube tip position (double-entry injector; 150 mm below the jet entry; air flow rate = 0.014 Nm3/min for primary and 0.056 Nm3/min for secondary). - - Experimental plot of flush position. �9 . . . . . Predicted plot of flush position. - . . . . Ex- perimental plot of 25 mm above the outlet.

x 10 s

X 7

5

m 4

Z 3

�9 :"~ 2

0-5 - ; "3 -2 -1 0 1 3 4

x 10 .2

Radial Position (m)

Fig. l 1--Effect of inner tube tip position (double-entry injector; 150 mm below the jet entry; air flow rate = 0.014 Nm3/min for primary and 0.056 Nm3/min for secondary). Experimental plot of flush position. �9 . . . . . Predicted plot of flush position. - . . . . Ex- perirnental plot of 10 mm below the outlet.


to be dispersed more widely than the axial double-entry system.

For this system, the primary air was introduced radially at the outlet of the injector. In order to accomplish this, a cone that had holes on its side with its bottom blocked and connected to the primary air supply system was placed below the outlet of the injector, as shown in Figure 12(a). The primary air was introduced through the central tube and then injected radially through the holes in the cone, while the particles were supplied through the annulus between the central and inner tubes. The cone is shown in Figure 12(b). The diameter of the cone was 13 mm, and the cone had 11 holes on its side. The diameter of the holes was 1.6 mm, and holes were located evenly around the circumference of the cone. The particle feeding rate was fixed at 0.03 kg/min. The primary (radial) air flow rate was varied from 0.007 to 0.028 Nm3/min, and the secondary air flow rate was varied from 0.014 to 0.056 Nm3/min. The radial velocity of the primary air at the outlet of the holes was varied

Secondary Air

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Primary Air Particles

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(a)

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Fig. 12- -Schemat ic representation of injector: (a) double-entry type with a radial distribution cone and (b) radial distribution cone.

from 6.21 to 24.1 m/s, and the velocity of the secondary air was changed from 1.86 to 7.44 m/s. The measurement was carried out at 150 mm below the jet entry.

The experimental geometry for this system was too complicated to be modeled in the computer code. In the mathematical model, it was assumed that the primary air was injected radially and uniformly at the tip of the inner tube and the particle could only have a radial velocity component. However, in the experiment, the particles fell from the feeder to the injector and then collided with the distribution cone. The primary (radial) air was injected through the holes in the cone. Therefore, the particle had both radial and axial velocity components. The radial velocity of the primary air was also not uniform, since the holes were located at discrete positions. There- fore, the model predictions and experimental data for a double-entry system with radial particle injection were not compared in this study. The experimental particle number density data were measured under different flow conditions and compared with each other.

Figure 13 shows the projected particle number density plots for the axial double-entry injector with a radial distribution cone. The air flow rates were 0.014 Nm3/min for the primary air and 0.028 Nm3/min for the secondary air. The radial velocity of the primary air at the outlet of the holes was 12.41 m/s, and the axial velocity of the secondary air was 3.73 m/s. For the axial double- entry injector, an inner tube of 7/8-in. (22.0-mm) outer diameter was used, and the axial velocities were 0.83 m/s for the primary air and 3.73 m/s for the secondary air. The particle loading was 1.64 kg-solid/ kg-air for the primary air and 0.55 kg-solid/kg-air for the total air. The plot for the injector with a radial distribution cone is twice as wide as that of the axial double- entry injector and has a very different shape. The plot shows a wide region in which the particle number density is high. The region has an edge on each side, and the density gradient is steep at the edge. The multiple

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x 10 5 10

-0.08

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i i ..~6 -0o4 .0.02 ; o.b2 0o4 0.o6 0.()8 0.1

Radial Position (m)

Fig. 13--Compar ison between the results for the double-entry injector with a radial distribution cone and for the axial double-entry injector (150 mm below the jet entry; air flow rate = 0.014 Nma/min for primary and 0.028 Nm3/min for secondary). - - Double-entry injector with a distribution cone. - . . . . . Axial double-entry injector.


peaks seen for the injector with a radial distribution cone are due to the fact that the radially injected air flows out of discretely located holes. Between the holes, the particles fall with much less radial velocity. Overall, the double-entry injector with a radial distribution cone disperses the particles more widely than the axial double- entry injector.

The effect of the primary air flow rate on the projected particle number density was examined, as shown in Figure 14. The secondary air flow rate was fixed at 0.056 Nma/min with a linear axial velocity of 7.45 m/s. The primary air flow rate was varied from 0.007 to 0.028 NmS/min, and the radial velocity for the primary air at the outlet of the holes was varied from 6.20 to 24.31 m/s. The particle loading accordingly varied from 3.28 to 0.82 kg-solid/kg-air for the primary air and from 0.36 to 0.27 kg-solid/kg-air for the total air.

At 0.007 Nm~/min primary air flow rate, the plot shows a sharp peak and resembles that for an axial double-entry injector. By increasing the primary air flow rate to 0.014 Nm3/min, the plot becomes broader with fluctua- tions. The plot shows a wide high-density region instead of a peak. At 0.028 NmS/min primary air flow rate, the plot shows a wide and smoother profile. While the density at the central region is almost identical to the other plots, the plot has the largest width of the three plots. It means that the particles are dispersed widely as the radial air flow rate increases.

The effect of the secondary air flow rate was also checked. Figure 15 shows the projected particle number density plots for different secondary air flow rates. The primary air flow rate was fixed at 0.007 Nm3/min, and the radial velocity at the outlet of the holes was 6.20 m/s. The secondary air flow rate was varied from 0.014 to 0.056 Nm3/min with the axial velocity for the secondary air accordingly varying from 1.86 to 7.45 m/s. The particle loading was 3.28 kg-solid/kg-air

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Radial Position (m)

Fig. 1 4 - - E f f e c t o f the primary air flow rate (double-entry injector with a radial distribution cone; 150 m m below the jet entry) - - 0.007 Nm3/min for primary and 0.056 Nm3/min for secondary. �9 . . . . . 0.014 Nm3/min for primary and 0.056 Nm3/min for secondary . . . . . . 0.028 Nm3/min for primary and 0.056 Nm3/min for secondary.

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8

7

6

5

4

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Radial Position (m)

Fig. 1 5 - - E f f e c t of the secondary air flow rate (double-entry injector with a radial distribution cone; 150 m m below the jet entry). - - 0.007 Nm3/min for primary and 0.014 Nm3/min for secondary. �9 . . . . . 0.007 Nm3/min for primary and 0.028 Nm3/min for secondary. - . . . . 0.007 Nm3/min for primary and 0.056 Nm3/min for secondary.

for the primary air and from 1.09 to 0.36 kg-solid/ kg-air for the total air. The solid line shows the plot at 0.014 NmS/min secondary air flow rate. The dotted and dashed lines represent the plots at 0.028 and 0.056 Nm3/ min secondary air flow rates, respectively.

At a lower secondary air flow rate, the plot shows a wide high-density region inside the jet. The density value fluctuates but remains high inside the region. The area has a distinct edge and steep density gradient on each side. By increasing the secondary air flow rate, the area of the central high-density region decreases. At 0.056 Nm3/min secondary air flow rate, a density peak appears. It is found that the particle dispersion in the jet becomes limited and the area of the high-density region becomes smaller as the secondary air flow rate increases.

In summary, the double-entry injector with a radial distribution cone disperses the particles more widely than the axial double-entry injector. The dispersion phenomenon of the injector with a cone is different from that for an axial double-entry injector. The injector generates a high-density region inside the jet instead of a peak. The region has a distinct edge on each side, and the density value is maintained high inside the region. The diameter of the region is significantly affected by the primary (radial) and secondary (axial) air flow rates.

In industrial operation of the flash furnace, wider dispersion of the particles will promote the effective utili- zation of the furnace volume by improving uniform contact between the particles and gas. Therefore, the use of a radial distribution cone can effectively improve the performance of the concentrate burner, but the radial velocity of injection should be limited so as to prevent the particles impinging on the furnace wall.

Because of the difficulty in correctly incorporating the experimental conditions at the injector outlet in the computer code, a rather substantial modification of the numerical technique for these boundary conditions would


be necessary to simulate the performance of this type of injector.

V. CONCLUSIONS

The comparison between the experimental and predicted results points to the following conclusions.

1. The measured particle number density data were in fair overall agreement with the predicted results from the mathematical model. The agreement proved the validity of the mathematical model from the view- point of the particle dispersion in the jet.

2. For a single-entry injector, the injector diameter and air flow rate significantly affected the diameter and the particle number density of the jet.

3. For an axial double-entry injector, the particle dispersion in the jet was affected by the outer diameter and position of the inner tube and the air flow rate.

4. The double-entry injector with a radial distribution cone dispersed the particles more widely in the jet than the axial double-entry injector. The diameter of the jet and the density value were affected by the radial and axial air flow rates.

In this work, basic features of the three basic injector designs have been identified regarding the particle dispersion in the jet. Since the development for the concentrate burner will be based on these designs, this work can be a basis of the further improvement of the concentrate burner design. Furthermore, the model can be

used as a effective guide for designing a concentrate burner.

ACKNOWLEDGMENTS

The authors wish to express their thanks to Nippon Mining and Metals Co. of Tokyo, Japan, for their fi- nancial support of this work and their support of one of the authors (YY) for his graduate program. They also wish to express their gratitude to Professor R.P. King of the University of Utah for his help with the image analysis system and the computer software.

REFERENCES

1. Y.B. Hahn and H.Y. Sohn: Metall. Trans. B, 1988, vol. 19B, pp. 871-84.

2. Y.B. Hahn: Ph.D. Dissertation, University of Utah, Salt Lake City, UT, 1988.

3. Y.B. Hahn and H.Y. Sohn: Metall. Trans. B, 1990, vol. 218, pp. 945-58.

4. E.K. Longmire and J.K. Eaton: J. Fluid Mech., 1992, vol. 236, pp. 217-57.

5. H.Y. Sohn: Metall. Trans. B, 1991, vol. 22B, pp. 737-54. 6. Y. Yasuda and H.Y. Sohn: EPD Congress 1994, TMS-AIME,

Warrendale, PA, 1994, pp. 753-77. 7. Y. Yasuda: Master's Thesis, University of Utah, Salt Lake City,

UT, 1993. 8. L.L. Lilja and V.J. Makitalo: U.S. Patent No. 4,147,535, 1979. 9. Y. Anjala, J. Asteljoki, and P. Hanniala: in The Impact of Oxygen

on the Productivity of Non-Ferrous Metallurgical Processes, G. Kachaniwsky and C. Newman, eds., Pergamon Press, New York, NY, 1987, vol. 2, pp. 87-105.


experimental and theoretical study of particle dispersion phenomena in a turbulent gas jet of the...

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