應用計算流體力學軟體，模擬不同密度的氧化物顆粒在收集池內沉澱的狀況，並應用水模擬實驗，驗證其實際沉澱距離與模擬結果之間的準確性。水模實驗中顆粒在水中沉澱的現象，應用斯托克斯定律(Stoke’s law)完成類比氧化雜質在鋁液中的狀況。當流量為3.219×10-3 m3/s條件下，粒徑1mm、密度3000kg/m3之氧化物顆粒，當擋板距離為40mm時，沉澱距離達到最小值357mm。
The recycled aluminum contains various oxide inclusions. One of these inclusions is the aluminum oxide newly formed on the free surface of aluminum melt as air contacts the melt surface. In the case of surface turbulence, this oxide and air are entrained into the melt to form folded bifilm defect. The bulk density of this defect with entrapment is sometimes very close to
the density of the liquid aluminum. As a result, the folded bifilm is difficult to deposit in the holding furnace. Eventually, the bifilms are entrained into mold cavity during the aluminum casting. They weaken the strength and reduce the elongation of the castings. Therefore, in a holding furnace an optimized design of the settling tank within a limited size is required to remove oxide inclusions from the molten recycled aluminum in a short time.
The purpose in this study, designing a settling tank, is to consistently supply clean aluminum alloy melt without oxide inclusions. Geometry of the internal chamber in the tank were utilized to separate the oxides from molten aluminum. There are three zones designed in the tank, a melting zone, a diffusing zone, and a collecting zone; and, their functions are introducing aluminum melt into the tank, reducing the velocity of the melt, and depositing oxide inclusions, respectively.
A computational fluid dynamics software was used to model the deposition of oxide inclusions with various densities in various tank designs. Water-analogy method was employed for validating the modeling results. In water experiment, using the Stoke law, the phenomena of particles settling in water was analog to oxides in aluminum melt. When the flow rate is 3.219x10-3m3/s, the distance of baffle in the tank is 40 mm, the oxide inclusions with a particle size of 1mm and a density of 3000 kg/m3 the minimum deposition distance is 357 mm.
Keywords: settling tank, aluminum recycling, oxide inclusions, bifilm defects