design and development of centrifugal atomizer

DESIGN AND DEVELOPMENT OF CENTRIFUGAL ATOMIZER Sarawut Gonsrang1, Thawatchai Plookphol2*, Sirikul Wisutmethangoon3 12

, Department of Mining and Material Engineering Faculty of Engineering, Prince of Songkla University, Hatyai 90112 Thailand 3 Department of Mechanical Engineering Faculty of Engineering, Prince of Songkla University, Hatyai 90112 Thailand

*Contact person: [email protected] Abstract: The point of this literature is to demonstrate the preliminary design of the centrifugal atomizer. The quantity of operating parameters was figured out by using the formulas that were proposed by Zhao, for example, the suitable a disk atomizer, consumption power, etc. The experimental centrifugal atomizer has been fabricated to carry out the test to compare the result, the maximum distance of droplet, with the calculations. The results of the experiments are discussed. Keyword: Centrifugal Atomizer, Atomizer Design, Lead-free solder, Powder Metallurgy 1 INTRODUCTION A Centrifugal atomization is an effective processing for production of fine metal powder, it have a side ranging from 50-500 microns with narrow side distribution ( standard deviation is around 1.3-1.5). This process is not only available for pure and alloy metal powder production with spherical shape and less impurity but also can be apply for spray deposition and spray coating. This project studies on a designing of centrifugal atomizer for the production of lead-free solder powder and an appropriate process parameters such as melt feed rate, diameter and geometry of atomizers. Model material is commercially pure Sn. 2 PRELIMINARY DESIGN 2.1 Consumption power The power, W, that is consumed to achieve the powder fabrication procees, the total power consumption

Faculty of Engineering Prince of Songkla University, Thailand

with respected to feed rate of melt onto the atomizer., is able to calculate by following equation;[6] 6𝛾𝑄 𝑊= (1) 𝜂𝑑50

When γ is a surface tension (N/m), Q is a liquid feed rate ( kg/ m3) , η is the energy efficiency of centrifugal atomization process and d50 is the median particle size of powder ( micron) . The efficiency energy of centrifugal atomization process, η, was proposed by Ryley [ 3] . Its value is 0.005. 2.2 Rotary atomizer For centrifugal atomization, The rotary atomizer is the most important component because its magnitude and geometry affect on median particle size and efficiency of powder fabrication directly. That is why a cluster of prior works tried to study on the atomizer and design a different shape of the atomizer. However, for calculation, we focused on flat disk only because the formulas and experimental data were published in many literatures, so the comparison between this work with prior works is available. The appropriate disk radius, R, can be predicted by using the repersent equation;[6] 𝑅=

1 𝜔

12𝛾

√𝜌𝜂𝑑

50

(2)

when ω is a rotational speed (rad/s) and ρ is an atomizing liquid density ( kg/m3) . Gennerally, the speed of electric motor always is given in revolutions per minute (rpm) so it need to multiply by convertion factor, π/ 30, before subtitude in the above model.

Faculty of Technical Sciences University of Novi Sad, Serbia

work, the droplet was assumed as a completely spherical particle. The coefficient can be calculated by using the expression below ;[4] 2

𝐶𝑑 =

24.0 𝑅𝑒

(1 +

𝑅𝑒 3 6

(6)

)

Fig. 1. Illustrate of the atomizer 2.3 Maximun distance of complete solidification Althogh a centrifugal atomization have been carried out for ages, but the trajectory and heat transfer of flying droplet have been less mentioned. Practically, the maximum distance that droplets need to become solid completely after spreaded off from the edge of an atomizer is a significant parameter because the atomizer vessel radius is equal to or greater than this value to avoid a deposition of a metal on the vessel wall. 2.3.1 Trajectory of droplets For the simplest case, it is possible to estimate a path of a single droplet with a simple projectile formula but, actually, the distance that was predicted by mentioned concept is too big to fabricate the real one, especially for an industial application. Thus it should not neglect the effect of the drag due to an air viscosity also to get more an accarate solution. For this work, the flight of a single spherical droplet and its thermal behavior were carrid out to predict the maximum diameter of an atomizer vessel and compare the result with the actual situation. An initial velocity of a flying droplet is difficult to predict the exact value because it is necessary to understand the mechanism of thin film formation on disk atomizer, this matter depend on a wetability of liquid on an atomizer, an atomizer geometry, the properties of melt, etc., but, for a simple cage, it is possible to estimate the velocity of melt at the edge of a disk, U, by the following model;[6] U=UT=ωR

(3)

when UT is the pheriperal speed of an atomizer (m/s) and UR or the radial velocity is negleted. The Reynold number, Re, was used in the calculation is shown below;[2] 𝑅𝑒 =

𝑉𝐷 𝜈𝑎

(4)

when V is the speed of a single flying droplet (m/s), D is the diameter of a droplet ( m) and νa is the kinematic air viscosity at 1 atmospheric (m2/s). Drag force, fd (N), that exert on a flying spherical particle is able to be calculated by Eq. (5);[2] 1

fd = - ρaV2ACd 2

(5)

when A is the projected area of a droplet (m2), ρa is an air density ( kg/m3) and Cd is the drag coefficient. For this

Fig. 2. Free body diagram of a single flying droplet The equation of motion of a single droplet travels along its path in the air atmosphere is shown in Eq. (7); 𝑑2𝑦 𝑑𝑡 2 𝑑2𝑥 𝑑𝑡 2

𝑑𝑦 2

=−

𝜌( 𝑑𝑡 ) 𝐴𝐶𝑑

(7)

2𝑚

(8)

=g

when m is the mass of a droplet (kg) and g is an acceleration due to the gravity, 9.81 m2/s. 2.3.2 Heat transfer behavior During the flight, a flying droplet transfer heat to a surrounding to solidify themselves. After liquid metal has implined on the atomizer, some heat transfer to the atmosphere and some also loss during the formation of the thin film on a disk so it is possible to assume that the temperature of droplets equal to the melting point temperature. Heat that was remove to the surrounding is only the latent heat of fusion, QL (joules), a total heat of a single flying droplet is calculated by using the formula was shown below; QL=mL

(9)

when L is the specific heat of diffusion of a droplet ( j/kg) . The performance of heat convection from flying droplet to surrounding is decribed in term of dimensionless number that is Nusselt number, Nu. The relation of Nu with the convection heat transfer coefficient, h (W/m2∙ºC), is shown below; 𝑁𝑢 =

ℎ𝐷

(10)

𝑘

when k is the thermal conductivity of atmosphere ( W/ m∙ ºC) . To calculate the value of Nu, the relation between Nu, Re and Pr , Pr or Pradtl number is a dimensionless number, was preposed by Whitaker [1] as show in Eq. (11); 𝑁𝑢 = 2 + [0.4𝑅𝑒

1⁄ 2

+ 0.06𝑅𝑒

2⁄ 3 ]𝑃𝑟 0.4

𝜇

( ∞) 𝜇𝑠

1⁄ 4

(11)

when μ∞ is the dynamic vicosity of a surrounding (m2/s) and μs is the dynamic viscosity of a surrounding that close to the surface of a droplet (m2/s). The coefficient of convection heat transfer can be calculate by using Eq. ( 10) and Eq. ( 11) , after that the rate of convection heat transfer from a droplet to an environment is able to be calculated by the following equation;[1] 𝑄̇𝑐𝑜𝑛𝑣𝑒𝑐 = ℎ𝐴𝑠 (𝑇𝑠 − 𝑇∞ ) (12) when As is the surface area of a flying droplet (m2), Ts is the temperature at the surface of a droplet (ºC) and T∞ is the temperature of a surrounding. The calculation of the path and the thermal history of a single droplet was carried out by an iteration and compared the solution with the experimental result in next part. 3 EXPERIMENTAL The centrifugal atomizer apparatus was fabricated in our lab, it is shown in fig. 3, for 2 main points; one is to study the effect of operating parameters, those are an atomizer diameter, a pouring temperature and the shape of an atomizer, another is to study the maximum distance that the droplet need to solidify completely. The testing machine was invented simply, it has 3 major components consist of a low temperature crucible with a graphite rod stopper and nozzle, rotary atomizer is driven by universal motor, speed of driver can be adjusted by voltage regulator, and support. Nozzle diameters, a free fall under gravitation type, are 0. 5, 0. 8, 1. 0 and 2. 0 millimeters. Rotary atomizer diameters, flat disk and cup shape, are, 3.0, 4.0 and 5.0 centimeters.

interesting temperature. After the melt had reached the interested temperature, the motor was run at 20,000rpm rotating speed. The stopper rod was lift up then to deliver melt onto the center of an atomizer and recorded the whole time of the process to calculate an average metal feed rate. The liquid tin was forced to be a thin film by centrifugal force that was generated by rotating disk and broken up to be the horizontal flying ligaments or droplets. During the flight, the flying particles disintegrated continuously to be some finer droplets and transfer their heat to an environment to solidify themselves at the same time. After the process ended, the powder was sampled at the different distances, 1.0, 1.5, 2.0, 2.5 and 3.0 meter, as shown in fig. 4. to determined the relationship between the particle size and the position of particles. After that, all of samples were mixed together again to detemine the global median particle size of metal powder again. The maximum distace of metal that deposited on the scene receptor was also measured to consider the magnitude of an atomiser vessel.

Fig. 4. Schematic of the runway and the scene receptor 4 RESULTS AND DISCUSSION

Fig. 3. Illustrate of the centrifugal atomization apparatus 3.1 Testing Material Acording to the aim of this study, SAC 305 is the model solder but in fact it is not able to perform all of the experiments by using these material because it is costly,so that commercially pure tin metal is a better chioce because it is a basal material of those solder, low melting temperatur, good oxidation resistance and easily to produce powder. In addition, the properties of tin metal is available on the prior literatures. 3.2 Procedure The centrifugal atomization processes were carried out in an air atmosphere, it was started by heating a tin metal in the crusible till it became molten metal at the

3.1 Atomizer design After liquid tin has just been poured onto a disk atomizer, some heat transfered from melt to a surrounding rapidly thus the premature solidification of melt or a skull happened on a disk atomizer. It was found that a solid metal did not struck on the disk but it was thrown around by centrifugal force, all the skulls are a metal flake. While the process has carried on for several seconds when a rate of heat transfer from melt to an atomizer equal to a rate of heat lost from an atomizer to an environment, in the other word, the system was the steady state, the rate of a skull formation decreased and went out soon.

Fig. 5. Typical skulls formed on a disk atomizer

For a ligament formation regime, in case of using a flat disk, it was found from the experiments that it is not necessary to preheat the atomizer, because the system was able to develop itself to be the steady state. But if the cup atomizer was employed with the same regime, the deposition of metal was found at the initial process and took a longer time to be the steady system than using a flat disk. So the preheating system should be installed if cup atomizer is employed to avoid a motor overloading or unsuccessfull powder fabrication. Another case that preheating is recommended is the low feed quantity because a rate of heat lost from an atomizer is greater than a rate of heat transfer from melt to an atomizer so the weight of skull formation increase rapidly untill it could not be removed by a centrifugal forced. Xie et al. [ 5] proposed the powder was prepared by using cup atomizer is finer than a disk atomizer by 25% but, unfortunatelly, they did not tell what the comparison index is. However, a trend of the result in fig. 6. is in line with their work. The shooting angle of droplets was not a zero degree when a cup atomizer was employed, the droplet need a space before fell down again. Thus the melt deliver should be as a tube and put into the chamber to avoid a deposition on upper wall.

3.2 Median particle size and morphology Fig. 6. shows the trend of median particle size of metal powder that was atomized at the different superheating temperatures. Higher temperature led to finer particle because a viscocity of melt reduced, however, a rising pouring temperature also lead to be more oxidation on the surface of flying droplet and resist the continuous disintegration. Fig. 7. show the trend of median particle size with respected to nozzle opening, different average feed rates are 150, 29, 16 and 7 kg/hr respectively. Median particle size reduce if the feed rate increase but when feed rate is 7 kg/ hr it is extremely different from the trend, median particle size is very high. The cause of the odd is the unbalance of heat transfer, the rate of heat transfer from molten metal was always less than the rate of heat loss from atomizer to surrounding so the process could not be steady state process by itself.

Fig. 8. Effect of disk diameter on median particle size at a 20,000rpm rotating speed and poured through 2.0mm nozzle openning

Fig. 6. Effect of superheating temperatures and atomizer geometries on median particle sizes of a tin powder at a 20,000rpm rotating speed and poured through 2.0mm nozzle openning.

Fig. 7. Effect of nozzle opening on median partcle size of a tin powder at a 20,000rpm rotating speed and 50 degree C superheating temperature.

Fig. 8. show the relation between disk diameter and median particle size, the trend was not clear but could estimated that the higher disk diameter lead to finer powder because of a higher peripheral speed. However, it is imposible to use a very large atomizer because of wetability of melt. Altrough motor is able to generate high torque, finer powder production do not achieve with a large disk. The melt will start taking off when a kinetic energy is greater than surface energy.

Fig. 9. Median particle sizes at different distances.

Fig. 11. Maximum distance of each experiments compare with theoretical distance at different superheating temperatures when rotating speed is 20,000rpm and atomizer is 50mm disk.

Vertical Distance, meter

0.7 58 72 97 131 175

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Horizontal Distances, meters Fig. 10. Theoretical trajectories of droplet when the rotating speed is 20,000rpm, Atomizer is 50mm disk and superheating temp. is 50 degree C 3.3 Trajectory and thermal history Fig. 9. shows the distributions of powder size, the powders were sampled at different distances and sieved to analyze a median size of each sample. From the graph, coarse particles flew farther than fine particles because they have a bigger mass, mass is inert property of body, so air drag have less affect on them. Thus most of powder should land close to the atomizer if a rotating speed of the atomizer increase because size of particle reduce. The theoretical trajectory of particle is shown in fig. 10. and the comparison of theoretical resalt and experimetal result is shown in Table 1,experimental data was used to compare is powder that was carried out by pouring at the 50 degree C superheating to be similar to the condition of the used formulas. From Table 1, the theoretical distance of fine particles close to the experiment, on the contary, trajectory of coarse particles extremely different from the actual situation because sampling area is near the scene receptor so the coarse particles could not fly more they impacted and droped on this area. Table 1. Median size at the different distances Med. Size(μm) 58 72 97 131 Exp. 100 150 200 250 Distances(cm) Theo. 99 138 213 315

175 300 441

The fig. 11 show the completely solidified lenght of droplet, the maximum lenght that is no metal deposition on the scene, solid line is the testing result and dashed line is the mathematical result. The initial assumption of this work is a median size of powder and maximum distance would have reduced if the pouring temperature had increased, but the experiments show a difference from the assumption. The maximum distance increased with increasing superheating temperature because of Snoxide film. The high temperature melt of Sn easily became Sn-oxide and resisted the continuously disintegration of melt instead of become a very fine powder. However, this matter will be tested again by performing the experiment in vacuum atmosphere. At higher superheating temperature, droplets took longer to remove heat and become solid particle so the deposition on the scene has farther length than lower superheating temperature. The mathematical result was calculated by putting the median size of powders those were prepared with different superheating temperature in the model. The condition of the model is droplet lost their heat during was pouring onto disk and formatting a thin film on a rotating disk till its temperature is near melting point. But, in fact, at the time droplet went from the edge of disk, their temperature is still higher than the melting point so it took a longer time to achieve a completely solidification. At 50 and 100 ºC, the forecasting distance is greater than the actual measure. 5 CONCLUSIONS 1) Median powder size can be decreased with increasing diameter of atomizer, increasing superheating temperature and decreasing quantity of feed rate. 2) Geometry of atomizer; Powder is prepared by using cup atomizer is finer than using flat disk atomizer. However, employing cup atomizer also enlarges mass of skull. 3) Premature solidification of melt on disk or skull always takes place at the early time of process and takes several seconds to reach steady state; vibration of rotary component will be generated then by unbalance mass. The skull can be reduced by increasing feed rate, increasing super heating and employing preheating atomizer 6 ACKNOWLEDGEMENT The author would like to thank Department of Mining and Material Engineering, Prince of Songkla University for laboratory area and sieve analysis equipment and Center of excellence in Nanotechnology at Prince of Songkla University for financial support.

7 REFERENCES [1] Çengel, Y.A., “ External Heat Convection” , In Heat Transfers: A Practice Approach Second Edition in SI Units, Pp. 367-418, Singapore: McGraw Hill, 2004. [2] Munson, B.R., Young, D.F. and Okiishi T.H., “External flow past bodies”, in Fundamentals of Fluid Mechanics 5th edition, Pp.614-683, WILEY, 2006. [3] Ryley, D. J. , “ Experimental determination of the atomizing efficiency of a high-speed spinning disk atomizer” British journal of applied physics, 10:Pp. 9397, 1959. [4] Teunou, E. and Poncelet, D. , “ Rotary disc atomization for microencapsulation applicationsprediction of the particle trajectories” Journal of Food Engineering, 71:Pp. 345-353, 2005. [5] Xie, J.W., Zhao, Y.Y. and Dunkley, J.J., “Effect of processing conditions on powder particle size and morphology in centrifugal atomization of tin” Powder metallurgy , vol. 7:Pp. 168-172, 2004 [6] Y. Y. Zhao, “ Considerations in Designing a Centrifugal Atomizer for Metal Powder Production” J. Material and Design, 27: Pp. 740-750, 2006.