Heat Transfer Coefficient at Cast-Mold Interface During Centrifugal Casting: Calculation of Air Gap
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HORIZONTAL centrifugal casting is an important industrial process used especially for the production of high-quality seamless tubes and outer shells of work rolls. In this process, the effect of centrifuging is twofold. First, it is the fictitious centrifugal force making the production of axisymmetric hollow castings even possible by pushing the molten metal against the inner wall of the cylindrical mold. Second, the interaction between inertial forces and the vector of the gravitational acceleration induces the so-called pumping effect, responsible for thorough mixing,[1] the growth of fine equiaxed grains, and superior mechanical properties of the cast.[2,3] As with many other industrial processes, horizontal centrifugal casting has been studied with increased
JAN BOHACEK, ABDELLAH KHARICHA, and ANDREAS LUDWIG are with the Chair of Simulation and Modeling Metallurgical Processes, Metallurgy Department, Montanuniversitaet Leoben, Franz-Josef-Str. 18/III, 8700 Leoben, Austria. Contact e-mail: [email protected] MENGHUAI WU and EBRAHIM KARIMI-SIBAKI are with the Christian Doppler Laboratory for ‘‘Advanced Process Simulation of Solidification and Melting,’’, Franz-Josef-Str. 18/III, 8700 Leoben, Austria. Manuscript submitted October 26, 2016.
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attention, with the help of various numerical techniques, in order to gain a better understanding of the process and underlying physical phenomena. While some of the numerical studies concentrate more on simulating flow dynamics, such as the mold filling, waves propagating over the free surface, and complex buoyant flow patterns inside the molten metal,[4–11] others focus more on heat transfer and solidification, often assuming coupling with simple segregation models.[12–14] The latter is naturally more frequent within the centrifugal casting community. Solidification is usually modeled by means of applying the enthalpy method with appropriate rules for a liquid fraction evolution in the mushy zone. In order to construct useful and realistic heat transfer models, precise and accurate material properties and boundary conditions are necessary. Heat transfer coefficients are usually imposed at boundaries, generally being determined from empirical formulas for the Nusselt number. Materials properties are generally temperature dependent and must be specified for all zones, i.e., the casting, the mold, and the coating. The thickness of the coating, a kind of a refractory material, such as ZrO2, is usually small (~ 1 mm); therefore, in numerical models, it is often simplified by an assumption of the thin-wall (zero-capacity) model. The coating is applied on the inner surface of the mold in order to insulate the mold from high temperatures and also to control to a certain extent the solidification rate. The
general consensus is that it tends to stick firmly to the mold surface. A time-dependent scenario at the contact between the casting and the coating attached to the mold surface is perhaps one of the weakest points of all currently avai
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