Modeling and Measurements of Intermixing Time in a Water Model of a Four Strand Steelmaking Tundish System

Intermixing of two different grades of steel in a 17 tonne, 4 strand bloom casting has been investigated in a ~0.3 scale, geometrically and dynamically similar, isothermal water model tundish system. In this, intermixing phenomena were studied by simulating mixing of flowing water in the tundish, having different dissolved tracer (electrolyte) concentrations. Electrical conductivity measurement technique was employed to monitor concentration of dissolved electrolyte near the strands as a function of time and thereby determine intermixing time. For each experimental condition, three measurements were made and based on such an average intermixing time was estimated. Reproducibility was found to be always within ±15 %. Influence of key operating variables such as, inflow rate from ladle, net outflow rate from tundish, residual liquid volume left over from the previous grade etc. on the duration over which intermixing occurs (referred to as in the text as the intermixing time) was investigated. It was found that any increase in residual volume as well as inflow rate tends to prolong intermixing time. In contrast, influence of outflow rate was quite the opposite. Furthermore, while variation of intermixing time among strands was only marginal, tundish interior design (viz., presence of flow modifiers, pouring box etc.) was found to have considerable influence on intermixing time. Flow phenomena (observed visually through the dispersion of KMnO4 solution) in the given tundish was found to be practically symmetrical about the transverse centre-line and so was the associated intermixing time. Embodying a large number of experimental data, explicit correlations for intermixing time were derived in terms of principal operating variables through dimensional analysis and regression. Two different versions of correlations, applicable respectively to a flat bottom as well as a wedge fitted tundish systems, were developed. In SI unit, these are respectively represented as:τint.mix=6.38×Vres0.86Qin0.48Qout,T−1.32\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {{\uptau}}_{{{\text{int}}.{\text{mix}}}} = 6.38 \times V_{\text{res}}^{0.86} Q_{\text{in}}^{0.48} Q_{{{\text{out}},{\text{T}}}}^{ - 1.32} $$\end{document}τint.mix=1.43×10−2Vres1.7Qin−0.02Qout,T−1.82\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {{\uptau}}_{{{\text{int}}.{\text{mix}}}} = 1.43 \times 10^{ - 2} V_{\text{res}}^{1.7} Q_{\text{in}}^{ - 0.02} Q_{{{\text{out}},{\text{T}}}}^{ - 1.82} $$\end{document}in which, Qin is the input flow rate, Qout,T is the net outflow rate, Vres is the residual liquid volume and τint.mix is the intermixing time corresponding to a degree of 95 % homogeneity. Finally, adequacy and appropriateness of the proposed correlations are assessed both from theoretical and experimental stand points.