Numerical simulation of the flow and the solid transport when tilting a holding furnace
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I. INTRODUCTION
AFTER aluminum is molten inside a “melting furnace,” it is transferred to a “holding furnace,” where its temperature is kept constant. The alloying is done at this step. During this quasi-rest time, which varies from 60 to 120 minutes, the solid inclusions present in the liquid aluminum should settle to the bottom of the furnace. After this step, the aluminum is poured through spouts into casting equipment to create the final products. The draining of the furnace lasts from 60 to 150 minutes. Typically, such a furnace contains 50 tons of aluminum (rAl 5 2.32 103 kg m23). Solid inclusions in aluminum range in size from 10 to 200 mm and in density from 3 to 5-103 kg m23. Inclusion concentrations vary from 10 to 200 grams per ton of liquid aluminum. The present study is concerned with both the holding and pouring phases of aluminum casting. During the holding stage, heat lost through the walls of the furnace cools the liquid near the wall, producing a horizontal temperature gradient in liquid aluminum. This temperature gradient generates a slow thermoconvective motion. The liquid flows down near the walls and rises up in the center of the furnace. The Prandtl number of aluminum is low (Pr 5 0.03); therefore, both thermal and velocity boundary layers are of the same order of magnitude. Assuming a horizontal temperature difference DT ' 0.05K (this value has to be validated), the descending velocity near the vertical walls Uo ' O (!gbDTH) ' 7 mm s21, where g is the gravity, b the thermal coefficient of volumetric expansion, and H the characteristic height of the aluminum in the furnace. From mass conservation (O (Uodc) 5 O(UbathL)), the characteristic ascending velocity inside the bath is approximately 0.3 mm s21. Therefore, the Rayleigh number for this flow is of the order of 106, indicating that the flow is weakly turbulent. E. PAUTY, formerly Research Student with the Centre de Recherche de Voreppe, is Manager, CERMEP, 38100 Grenoble, France. B. LABOUDIGUE, Researcher, is with the Centre de Recherche de Voreppe, 38340 Voreppe, Cedex, France. J. ETAY, Researcher, is with EPM-MADY` LAM-ENSHMG, 38402 St Martin d’Heres, Cedex, France. Manuscript submitted February 17, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
This thermoconvective flow promotes the settling of solid inclusions whose densities are higher than that of aluminum. The settling of inclusions takes place at the center of the bottom of the furnace. The way the pouring is done has significant consequences on the quality of the final products. Indeed, the pouring conditions determine whether inclusions that have settled during the holding time will be resuspended where they can be swept into the pouring flow. To prevent the inclusions from being resuspended, the level of the turbulence has to be as low as possible. The order of magnitude of the Reynolds UL number, Re 5 , is equal to 1000 for aluminum in this v particular case, indicating a low turbulence level. The goal of this numerical study was to increase the knowledge of both the flows
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