THE EFFECT OF SURFACE MOVEMENTS ON NITROGEN MASS-TRANSFER IN LIQUID-IRON

Marangoni convection due to surface tension gradients set up in a nitrogen-liquid iron system at 1873 K has been mathematically simulated and experimentally confirmed by measuring the rate of nitrogen absorption into liquid iron and by observing the free surface motion of liquid iron via zirconium oxide particles using X-ray radiography. This surface flow was created by blowing nitrogen gas through a small lance over the free surface of the liquid iron contained in an alumina boat under resistance heating. Absorption measurements Indicated that the mass-transfer coefficient of nitrogen in liquid iron due to Marangoni convection (k'(m) = 1.5 x 10(-4) m/s) is slightly less than that due to induction stirring (k'(m) = 2.1 x 10(-4) to 1.5 x 10(-4) m/s). The free surface velocity induced by the surface tension gradient, with an initial composition difference of 425 to 10 ppm, was found to be of the order of 0.05 to 0.11 m/s. Mathematical modeling was also carried out to determine the velocity and nitrogen concentration profiles in the alumina boat. It was found that Marangoni convection creates mildly turbulent flows in liquid iron. Good agreements between the experimentally obtained data (nitrogen concentration and surface velocity) and the computed results were attained when a constant eddy viscosity which was five times the molecular value was assumed. This prescription allows both the predicted apparent mass-transfer coefficient and the predicted surface velocity to match the experimental results simultaneously. Note that the transport coefficients in the momentum and the convective-diffusive equations are correlated by the turbulent Schmidt number. The dimensionless mass-transfer correlation obtained for the present N-Fe system at 1873 K under resistance heating was found to be Sh = 0.104 Re-0.7 . Sc-0.7 with Sc = 79.5 (3500 < Re < 7700).