Equiaxed dendritic solidification with convection: Part III. Comparisons with NH 4 Cl-H 2 O experiments
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I.
INTRODUCTION
SIMULTANEOUS liquid and solid flow is an important, but little understood, phenomenon in equiaxed alloy solidification. Convection in the liquid melt is caused by both thermal and solutal buoyancy forces, while the movement of free solid crystals is due to the density difference between solid and liquid under the influence of gravity. The resulting solid-liquid multiphase flow pattern strongly depends on the microstructure of the equiaxed crystals which, in turn, is governed by grain nucleation and growth mechanisms. Wang et al. I~) have shown that the settling characteristics of equiaxed crystals in the free particle regime and the permeability of the liquid melt through a packed bed of equiaxed grains are functions not only of the solid and grain fractions but also the multiple length scales characteristic of an equiaxed dendritic morphology. Because the coupled solid-liquid flow causes structural and chemical inhomogeneities in final solidified products, a fundamental understanding of the multiphase transport phenomena coupled with the grain nucleation and growth mechanisms is required. In a multiphase model formulated by the present authors for equiaxed dendritic solidificationt2] (hereinafter referred to as part I), the multiphase transport occurring on the macroscopic (system) scale has been coupled with the grain nucleation and growth mechanisms taking place on various microscopic scales. It has been further shown in Reference 3 (hereinafter referred to as part II) that such a micro-macroscopic modeling approach is capable of predicting grain structure and macrosegregation. The objective of the pres-
C. BECKERMANN, Associate Professor, is with the Department of Mechanical Engineering, The University of Iowa, Iowa City, IA 52242. C.Y. WANG, Assistant Professor, is with the Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, HI 96822. Manuscript submitted February 9, 1995. 2784--VOLUME 27A, SEPTEMBER 1996
ent article, therefore, is to provide preliminary validation of the multiphase model through comparisons with carefully designed aqueous ammonium chloride equiaxed solidification experiments. As a transparent analog for metallic alloys, the NHaC1H20 solution has been extensively used to study thermosolutal convection during columnar solidification. Notable work includes that by Beckermann and Viskanta,[4i Christenson and Incropera,[5] Chen and Chen, [6J McCay et aL, t7] Neilson and Incropera,[S] and Magirl and Incropera.[9] In contrast, there exist relatively few experimental studies of equiaxed solidification using NH4CI-H20. The aqueous solution was used to visualize the qualitative features of equiaxed solidification in low gravity environments, t~~ and quantitative measurements combined with visualization of two-dimensional (2-D) equiaxed dendritic solidification of an NH4C1-H20 mixture were recently obtained by Beckermann et al. t~2~ A number of interesting characteristics of the solidification process were found, which include nucleation, fragmentation, rec
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