Atomization Mechanisms and Performance of an Internal-Mixing Linear Atomizer in Spray Forming Apllications
| Autor: | Ming-Shen Sheu, 徐明生 |
|---|---|
| Rok vydání: | 1999 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 87 The performance and mechanisms of an internal-mixing linear atomizer for spray forming applications are investigated in this research program. The atomizer is designed by mixing the molten metal and the atomization gas in the mixing chamber of the atomizer to enhance the atomization performance. The liquid and gas mixture is then discharged through a planar nozzle with openings of 12mm×1mm and 6mm×1mm to compare their performance. A controlling device controls the mass flow rates of the molten metal and the atomization gas. Results show that the atomization performance of the normal liquid (i.e. water or salad oil etc.), is quite different from that of the molten metal. For example, under the same operation pressure PL=3kg/cm2, PG=4kg/cm2, the spray angle, the droplet size (d10, d32), and the gas/liquid mass ratio (mgas/mliquid) of the salad oil are 25o,13m, 23m and 6, respectively. While for the molten metal of pure Pb, the spray angle, d10, d32, and the gas/liquid mass ratio is 35o, 5m, 13m and 0.17, respectively. The spray cone angle of the molten metal is larger than that of the salad oil. The droplet size of the molten metal spray is much smaller than the oil spray even though the surface tension of the molten metal is 10~30 times higher than oil. Furthermore, d10 as small as 5m is achieved. It is normally in the range of 100m by the conventional atomizer. It is attributed to the thermal expansion effects during the atomization of the molten metal at the elevated temperature. Results also show that the gas-to-liquid mass ratios required for achieving the fine spray of the molten metal are in the range of 0.04~0.17. In a comparison, the gas-to-liquid mass ratios required for the conventional air blast atomizer reported in the literature are normally as high as 1.0~16.8. Hence the atomization gas required in this atomizer is as low as 1~10% of those required in the conventional ones. It can be concluded that a new atomization mechanism has been developed in this research program. Results also show that the surface temperature of the metal deposit increases with time until it reaches the nucleation temperature of the metal, which is about 20% below its melting point. It is also found that the deposition rate depends not only on the mass flux of the metal spray but also on the sticking efficiency of the deposition process. The deposition rates essentially follow the trend of the patternation except in the central regions where the temperature is relatively low. The sticking efficiency is in turn determined by the solidification process of the droplets and the surface condition of the deposit. Hence the deposition performance of the metal spray is dependent on the patternation as well as the nucleation mechanism of the metal spray. It is also found that the deposit material becomes the porous structure when the spray droplets are small enough with a proper cooling process. The porosity formed in the interstice among the droplets is smaller than the gas pore as found in the conventional spray forming process. Analysis on the microstructure of the deposit shows that the grain size is about 5m. It turns out that the length scale of the porosity found in current research is much smaller than the grain size, which is useful in the development of the new material. |
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