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To evaluate the effect of actual cooling rate of liquid steel in the ladle on the metallurgical performances of a tundish, a transient and coupled computational model was developed to reveal the flow fields, temperature fields, residence time distribution of the molten steel and the inclusion removal efficiency in a typical single-strand tundish with given geometry and process parameters. The results showed that, with the decrease of the ladle stream cooling rate, the temperature difference of bulk flow at the outlet of tundish over a normal casting period decreased from 11.3 to 2.6 K, and the dead volume fraction of the tundish decreased from 17.58% to 14. 35%, while the inclusion removal efficiency was increased especially for the inclusions with the diameter less than 50 μm, whose removal ratio could be increased by 20.62%. When the cooling rate was less than 0.3 K · min-1 , however, the variation rates of the three evaluation criterions above declined significantly, which suggested that a critical value existed for the effect of the cooling rate of ladle stream on the tundish performances. The establishment of the critical ladle stream cooling rate should be very important to achieve persistent metallurgical properties of tundish over the whole casting stage, together with the reasonable ladle insulation design.

This study produced a novel characterization of the troposphere-to-stratosphere transport （TST） over the Asian monsoon region during boreal summer, using a comprehensive analysis of 60-day backward trajectories initialized in the stratosphere. The trajectory datasets were derived from the high-resolution Lagrangian particle dispersion model （FLEXPART） simulation driven by the wind fields acquired from the National Center for Environmental Prediction （NCEP）. The results indicate that the distribution of residence time （tTST） of tropopause-crossing trajectories in the lowermost stratosphere represents a horizontal signature of the Asian summer monsoon. Vertically, the distribution of tTST can be roughly separated into two layers： a consistent lower layer with tTST 〈5 days forming a narrow band, corresponding to a layer-3 km thick following the location of the tropopause, and an upper layer at a larger distance from the local tropopause. The maximum residence time was -20 days, especially within the Asian high anticyclone consistent with its confinement effects. In general, the overall geographical distribution of dehydration points was not coincident with the location of tropopause crossing. TST trajectories, which were initialized in the stratosphere, underwent their Lagrangian cold points mostly in the tropics and subtropics 1 4 days after the TST event; they were characterized by a wide range of temperature differences, with a mean value of 3-12 K. The vertical extent of the influence of tropospheric intrusion on the Asian monsoon region in the stratosphere exhibited a peak at -16.5-18.5 km, and the uppermost height was -21 km.

A detailed mathematical procedure of the optimization of the fluid flow in a tundish water model with and without flow control devices （weir and dam） was carried out using the commercial CFD eode FLUENT 6.0. The （k- ε） two-equation model was used to model turbulence. The residence time distribution （RTD） curves were used to analyze the behavior of the flow in tundish. The location of flow control devices in the tundish was studied. The results show that the flow modifiers play an important role in promoting the floatation of nonmetallic inclusions in steel. Comparing the three geometric configurations that are considered （bare tundish, weir, weir＋dam）, the tundish equipped with the arrangement （weir＋dam） is a best and optimal geometric configuration of tundish.

A 1∶2.5 scale tundish model was set up in laboratory for a six-strand billet continuous casting tundish with different configurations to investigate fluid flow characteristics under different operational conditions by measuring residence time distribution curves.It was found that minimum residence time,maximum concentration time and average residence time of the three strands on the same side of the tundish with the former configuration under normal operation,that is,six strands were open,were small and non-uniform and the tundish had large dead volume fraction.Vortexes easily formed on the liquid surface in the pouring zone of the tundish.The fluid flow characteristics in the tundish with the optimal turbulence inhibitor and baffles were improved and became less non-uniform among the strands.Vortexes were not found on the pouring zone surface in the optimal tundish.For non-normal operation,that is,one strand was close,it was important to choose which strand to be closed for maintaining flow characteristics of the rest two strands.It was found from this investigation that fluid flow characteristics in the optimal configuration tundish with closing strand 2 were better than those with closing strand 3 on the same side.

In a multistrand,the outlet near the inlet produces short circuiting flow.This leads to the formation of dead zones inside the tundish,and consequently,the mean residence time decreases.In the present study,numerical investigation of mixing inside a delta shaped tundish with sloping boundaries was carried out by solving the Navier-Stokes equation and employing the standard turbulence model.To decrease the dead zone volume inside the tundish,the effect of closing the outlet near the inlet for a small amount of time and further opening it on the mixing behavior of the tundish was studied.The outlets near the inlet were closed for varying amount of time,and the transient analysis of fluid flow and the tracer dispersion study were carried out to find the mixing parameters of the tundish,namely,mean residence time and the ratio of mixed to dead volume of the tundish.An optimum closure time of the near outlet has been found,which yields best mixing inside the tundish.The numerical code was validated against the experimental observation by performing the tracer dispersion study inside a multistrand tundish and the reasonably good match between the experimental and numerical results in terms of residence time distribution （RTD） curves.The results obtained from the present study confirm the strong role of choosing the right time for opening and closing the outlets to get improved characteristics for the fluid flow and mixing behavior of the tundish.The educational version of computational fluid dynamics （CFD） software PHOENICS was used to solve the governing equations and interpret the results in different forms.