(554g) Nonisothermal CFD Modeling of a Homogeneous Free Falling Jet During Melt-Blowing Slag Fiberization

Red mud fiberization is a process with remarkable potential, alleviating environmental pressure by transforming an a­­­luminum by-product into mineral wool, thus to various marketable products. A promising mineral wool process is molten slag fiberization via an impinging air jet, which avoids mechanical wear and rotating parts. The molten slag which must be removed before pig iron casting flows out of a heated ladle orifice at a high temperature (1600 °C) and adjustable flowrate, and forms a free-falling vertical jet which visibly radiates its excessive heat: at a given distance, a high-velocity impinging air jet meets the vertical melt jet perpendicularly (or at an angle), inducing intensive droplet generation, subsequent fiber elongation, collection and processing. The homogeneous, nonisothermal laminar molten slag stream emanates from the pot orifice and gradually experiences diameter reduction and momentum gain due to gravitational acceleration. Intense radiative cooling heat losses are evident on the free surface throughout the stream height. The stream bottom sustains enormous temperature gradients (> 400 °C) while slag enters the actual atomization (fiberization) zone.

The problem of free-surface homogeneous nonisothermal laminar flow under radiative cooling has first been tackled by Epikhin et al. (1981), with a rigorous mathematical (PDE) formulation but without adequate computational exploration of key dimensionless number (Re, Ca) effects. Conversely, Georgiou et al. (1988) and Adachi et a. (1990) published detailed numerical studies of vertical laminar Newtonian liquid jets, but without considering any simultaneous heat effects. Leroux et al. (1997) proved that the free surface boundary is reliably captured by quartic curves, while Novitskii and Efremov (2006) provided mineral fiber size phenomenological correlations. 

This paper focuses on high-fidelity CFD modeling of the molten jet flow under external cooling: the model encompasses all physicochemical phenomena (melt laminar free surface flow, radiative cooling) and considers several temperature-dependent slag transport properties in order to understand which operational degrees of freedom (manipulated variables) are useful to process optimization. Melt-blowing slag fiberization is studied rigorously via COMSOL Multiphysics^®, considering unperturbed laminar flow and radiative cooling in an axisymmetric unstructured grid. A sensitivity analysis of vertical slag stream temperature and flow profiles has been conducted in order to probe the heat effect (if any) of key macroscopic variables on slag as the latter enters the fiberization zone. The foregoing series of temperature distributions summarize the sensitivity analysis undertaken and illustrate that, while for some parameters CFD results appear almost indistinguishable, the quadratic and quartic nonlinearity of transport properties (viscosity and emissivity, respectively) produce clear (often accentuated) temperature gradients at the bottom of the molten slag stream. Temperature and its gradients therein are indeed vital for fiberization: a lower than appropriate temperature results in rapid slag solidification, thus hindering efficient nascent fiber generation. A higher than appropriate temperature may conversely result in excessive fine droplet generation and subsequent breakup, thus inducing the highly undesirable effect of molten slag atomization.

LITERATURE REFERENCES

1. Adachi, K., Tagashira, K., Banba, Y., Tatsumi, H., Machida, H., Yoshioka, N., Steady laminar round jets of a viscous liquid falling vertically in the atmosphere, AIChE J. 36(5): 738-745 (1990).
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