Solidification of an anisotropic porous medium saturated with a binary alloy
Abstract
An experimental and theoretical study of the mixture solidification in the presence of a porous matrix phase has been performed to obtain fundamental understanding required for improved fabrication of composites by means of solidification. The emphasis was on the effect of the flow characteristics possessed by the porous matrix and dendrite arrays and on the interaction of hydrodynamics and transport of energy and species in the mushy and liquid regions. Artificial structures compatible with the flow visualization were constructed and served as the solid matrix of an anisotropic porous medium. The pressure gradient and filtration velocity data pairs were measured, and the regressed empirical relations were used in the analysis of the natural convection and the solidification studies. Natural convection fluid flow and heat transfer inside a rectangular enclosure partially filled with an anisotropic porous medium were investigated experimentally and theoretically. Differences in the velocity and temperature fields predicted for different ratio of directional permeabilities suggested the need to account for the anisotropic characteristics of the mushy zone in the analysis of solidifying ingots and castings. Solidification of an aqueous ammonium chloride solution was studied inside a square cross-section test enclosure with/without the porous matrix phase. The porous matrix phase affected the solidification of a binary alloy in the following ways: (1) the effective thermal conductivity, heat capacity and specific latent energy of phase change were altered; (2) the porous structure offered an additional resistance to the motion of the fluid both in the liquid and mushy regions; and (3) the migration of the separated crystals was considerably weakened. The predicted amount of macrosegregation occurring during the lateral solidification of hypereutectic composition solution was found to decrease when the permeabilities of the porous matrix phase and/or the dendrite array were decreased. In order to obtain a significant decrease in the amount of macrosegregation, the permeability of the porous matrix phase must be smaller than the critical value. The amount of macrosegregation was mainly controlled by the porous matrix permeability in the direction of gravity for the range of permeabilities larger than the critical value.
Degree
Ph.D.
Advisors
Viskanta, Purdue University.
Subject Area
Mechanical engineering|Materials science|Metallurgy
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