Measurement of Air Entrainment During Pouring of an Aluminum Alloy
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AIR entrainment occurs in free surface flows when surface discontinuities entrain air bubbles that are transported into the flow. For many decades air entrainment at the air-water interface of free surface flows has been an important research topic in hydraulics and multiphase fluid mechanics. Two reviews documenting the scope and progress of experimental and computational air entrainment research are presented by Chanson.[1,2] These reviews together provide a valuable perspective of past and recent developments, as they are nearly a decade apart. They document considerable advancement in that time and that air entrainment flows are important in many engineering disciplines and applications. As an example, entrained air reduces the water handling capacity of storm water management and flood control systems, and the systems are designed to minimize air entrainment.[1,2] The experimental work
FRANCISCO V. GUERRA, LUCAS ARCHER, RICHARD A. HARDIN, and CHRISTOPH BECKERMANN are with the Department of Mechanical Engineering, University of Iowa, Iowa City, IA 52242. Contact e-mail: [email protected] Manuscript submitted June 2, 2020; accepted October 2, 2020.
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presented here investigates air entrainment in a liquid metal. It contributes not only to the understanding of the casting filling process, but also to understanding other metallurgical processing involving pouring or transferring liquid metal. Air entrainment occurs at flow disturbances at the free surface and is evidenced by the formation of bubbles. A common air entraining flow during metal casting is the plunging jet shown in Figure 1(a). An example of this flow during casting is the filling of a downsprue. Air entrainment by plunging jet flows have been investigated extensively for water as the fluid medium.[3–11] From the authors’ review of the literature, air entrainment measurements have never been performed or published for liquid metals. As a result of air entrainment during pouring and transfer of liquid metals, oxide inclusions form in ferrous and non-ferrous metals. These inclusions form when liquid metal is exposed to oxygen during mold filling. Oxide inclusions in aluminum alloy castings form as thin solid aluminum oxide films. These inclusions are troublesome as they cannot be removed after the casting process and decrease the performance and life of cast aluminum parts.[12] Air entrainment during mold filling is believed to be a leading source of the oxygen that reacts with the metal to form inclusions.[12,13]
In this study, a system is developed to measure the volume of gas entrained by a jet of liquid metal plunging into a pool during filling. The gases used are air and argon. Here the pool of liquid metal is formed during filling of a mold cavity since the metal will solidify. Therefore, the part of the measurement system containing the pool is also called a mold. The measurement system is shown in the schematic drawing provided in Figure 1(b) and is described in more detail in the next section of the
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