Fluid mixing in ladle of RH degasser induced by down flow

In order to understand the effect of down flow in RH ladle on mixing time, the relationship among flow patterns, circulation and mixing phenomena were examined by using three sizes of model RH. The temporal change in electric conductivity in ion-exchanged water was tracked after injecting KCl solution and the decoloration process of iodine color in ion-exchanged water was observed visually after injecting sodium thiosulfate solution. There were two mixing patterns in the ladle: One has a damping oscillation curve for a tracer response and the decoloration reaction occurred everywhere and uniformly, the other has a monotonical decreasing tracer curve after an overshoot and the decoloration delayed near the free surface of the ladle. The (t_M/t_R)/(D_leg/H) values decreased with the increase in Re below Re ≒ 1.8 × 10⁴ and it was kept to be constant above Re ≒ 1.8 × 10⁴ where t_M: a mixing time (s), t_R: a mean residence time (s) of fluid in the ladle, D_leg: inner diameter of down-leg (m) and H: length of jet axis between down-leg outlet and jet-impinged wall (m). In the region of Re > 1.8 × 10⁴, the mixing was controlled by the circulating flow, whereas in the region of Re < 1.8 × 10⁴ the mixing behavior was influenced by the stagnation zone near the free surface of the ladle.