| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 94 === The goal of this dissertation is to produce the ultra fine sprays for different applications. Both water spray and metallic spray were characterized in this research program. The atomizer was designed with internal impinging mechanism and the characteristics of the spray are measured by a Malvern Spraytec particle analyzer, a two-component phase Doppler particle analyzer (PDPA) and a particle image velocimeter (PIV). Results show that the Sauter mean diameter below 10μm has been achieved with GLR of 0.14 in the test of water atomization. The minimum mean drop size can be lowered to 4.0μm under low pressure conditions using this particular atomizer. Such performance cannot be easily achieved with the conventional nozzle design. Results also show that better atomization performance can be achieved by increasing the internal impinging angle and the orifice diameter. An empirical formula of SMD, in terms of operating conditions and nozzle length scale is also presented in this research program.
Furthermore, instantaneous flow image shows the intermittence of the spray jet as injected with internal impingement. On the other hand, a large-scaled sinusoidal flow structure along the axial direction is observed when the spray jet is injected without internal impingement. Hence flow impingement inside the atomizer has strong effects on the structure of the spray jet because of the enhanced mixing processes between the liquid and gas phases. It turns out that the spray jet with internal impingement has a wider and more uniform distribution. Measurements of the distribution of the spray droplets and volume flux justify the above observation.
The production of the extra-fine metal powder was further optimized using the L18(21x37) scheme of Taguchi method. The goal is to produce the metal powder with particle size less than 15μm. Optimization analysis shows that the atomization gas, the melt inlet diameter, the nozzle outlet diameter, the melt injection pressure and the materials are the control parameters among the eight design factors. Confirmation test with the optimized conditions indicates that the accumulative volume of the powder within 0~15μm as high as 56.9% can be obtained. That is, more than half of the powder is within extra-fine range. The results of melt spray were also compared with water spray in this dissertation.
The classification for metal powder shows that the mechanical vibration sieving of particles below 45μm is difficult due to the high surface energy and the floating of small particles. The former one causes the agglomerated phenomenon that reduced the filtration quality of fine particles. The irregular distribution of the mesh and irregular shape of particles also cause some problems. On the other hand, by the classification with the air-jet sieving, the particles carried by the air flow could reduce the agglomerated and floating effects. Thus it has a better performance as compared to that of the mechanical vibration sieving. However, the classification performance is still limited due to the existence of the sieve. It is concluded that the classification of particles smaller than 25μm is difficult by the classifiers with sieves. Finally, the experiments of the aerodynamic classifier with centrifugal force were carried out in the classification of the ultra-fine particles. By adjusting the settings of the operating parameters, the classifications of particles with cut points ranging from 1μm to 30μm had been performed successfully.
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