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International Journal of Minerals, Metallurgy and Materials Volume 17, Number 2, April 2010, Page 143 DOI: 10.1007/s12613-010-0204-0 Corresponding author: Jing-she Li E-mail: lijingshe@ustb.edu.cn © University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2010 Water modeling of molten steel flow in a multi-strand tundish with gas blowing Jing Jiang1), Jing-she Li1), Hua-jie Wu2), Shu-feng Yang1), Tao Li1), and Hai-yan Tang1) 1) School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China 2) Engineering Research Institute, University of Science and Technology Beijing, Beijing 100083, China (Received: 2 April 2009; revised: 20 May 2009; accepted: 2 June 2009) Abstract: Fluid flow characteristics in a four-strand tundish with gas blowing were studied by water modeling experiments. It is found that gas blowing can greatly improve the flow characteristics in the tundish with a turbulence inhibitor. It dramatically increases the peak concen- tration time, and greatly decreases the dead volume, and reduces the minimum residence time. The gas blowing location, gas flow rate, and porous plug area greatly influence the flow characteristics in the tundish; the gas blowing location near the baffle, smaller gas flow rate, and smaller porous plug area are better for improving the fluid flow characteristics. Using gas blowing can reduce the difference of flows at the middle outlets and side outlets for the multi-strand tundish. Bubbles produced by gas blowing can absorb small inclusions and provide the condition for inclusion collision and aggregation. Therefore, introducing gas blowing into a tundish and combining the turbulence inhibitor can improve inclusion floating and removal, and the cleanness of molten steel can be advanced. Keywords: continuous casting; tundish; water modeling; gas blowing; porous plug 1. Introduction Besides using weirs, dams, baffles with holes, and turbu- lence inhibitors, gas blowing is another important way to improve the fluid flow and inclusion removal in the tundish. The principle of this method is that the generated small bub- bles from the tundish bottom can form a bubbling curtain that works as a dam and as an efficient transporting mecha- nism of inclusions toward the bath surface to be captured by a suitable slag [1]. A few studies have been carried out for investigating the flow and inclusion behaviors in a tundish with gas blowing. Zhong [2] investigated the effects of the argon bubbling curtain on the flow characteristics in a 1:3 reduced scale two-strand tundish water model. It was found that gas blowing hardly influenced the minimum residence time but significantly increased the peak concentration time and shortened the tail of resident time distribution (RTD) curves. Zhang [3-5] studied the fluid flow and inclusion motion in an argon bottom blown tundish by water modeling and mathematical modeling, which found that the position of gas blowing and gas flow rate had remarkable effects on the steel fluid flow and the RTD curve. In the same way, Ramos-Banderas et al. [6] found that higher flow rates of gas lead to the decrease in the plug flow fraction of the fluid in the tundish. Yamanaka et al. [7] investigated the tundish using argon bubbling through porous plugs, and they claimed a 50% improvement in the removal of inclusions in the 50 to 100 mm range. Tao [8] reported that the inclusion index of the slab cast with argon bubbling was decreased from 0.733-0.898 mg/kg to 0.421-0.433 mg/kg. Investiga- tions of gas bubbling in continuous casting tundishes were carried out by Marique and a 25%-50% improvement in the removal of inclusions was reported [9]. In previous literatures, all the studies focused on the one- or two-strand tundish, and the reports on gas blowing in a multi-strand tundish are few. In the present article, the ef- fects of gas blowing on the flow characteristics in a four-strand tundish with 25 t capacities were investigated in a reduced scale water model. The optimum porous plug area, 144 Int. J. Miner. Metall. Mater., Vol.17, No.2, Apr 2010 the optimum location of the porous brick, and the optimum gas flow rate were determined by measuring the RTD curves in water modeling experiments. The prototype tun- dish configuration studied is shown in Fig. 1. A turbulence inhibitor and a baffle with 3 holes were used in the tundish. Fig. 1. Original tundish configurations (unit: mm). 2. Experimental procedure In the water modeling experiments, to insure the similar- ity of fluid flowing between the model tundish and the pro- totype tundish in isothermal and non-reactive systems, geo- metrical and dynamic similarities must be satisfied between the two vessels. In the present work, the ratio of the geomet- rical similarity of the model tundish to the prototype is cho- sen to be 1: 2.5. Dynamic similarity requires respecting si- multaneous equality of both turbulent Reynolds and Froude numbers, but it is impossible to keep the condition satisfied in reduced scale modeling studies. The computational and experimental studies of Singh and Koria [10] showed that the magnitude of turbulent Reynolds number under the tur- bulent flow range in different tundishes is very similar. Therefore, Froude numbers of the model and prototype tun- dishes are maintained to be equivalent in this work. Ac- cording to the Froude similarity criterion (Frm=Frp, where, m—model, p—prototype), the characteristic length: Lm=λLp=0.4Lp (1) the characteristic velocity: Um=λ0.5Up=0.63Up (2) the volumetric flow rate: Qm=λ2.5Qp=0.10Qp (3) The casting speed is 0.5 m/min, and the diameter of the round billet is 280 mm×325 mm. The parameters of the pro- totype tundish and model tundish are shown Table 1. A sketch map of the experimental apparatus is shown in Fig. 2. The RTD curve of the fluid flowing in the tundish can be obtained by the stimulus-response technique to in- vestigate the effect of different tundish configurations on the fluid flow characteristics in the tundish. Before measuring, the liquid levels of the ladle and the tundish were raised to the predetermined height. Then, the tundish nozzles were opened. After attaining the steady-state flow condition, 200 mL KCl saturated solution was used as a tracer and was in- jected into the water stream flowing through the ladle nozzle. One conductivity probe connected to a conductivity meter was installed below one of the outlets of the tundish to measure the instantaneous concentration of the tracer as a function of time. The measurement data were plotted with a recorder and input into a computer to construct RTD curves. From the RTD curves, the minimum residence time (tmin), peak concentration time (tmax), and mean residence time (tav), could be obtained for every experiment. Considering there was fluid exchange between the fluids in the dead zone and in the active zone, the flow model proposed by Sahai and Emi [11] was employed in this work to calculate the dead volume fraction (Vdv), but the fractions of plug flow and well mixed volumes were still calculated with the modified mixed model from Ahuja and Sahai [12]. Table 1. Parameters of the prototype tundish and model tun- dish Parameter Prototype Model Flow volume per nozzle / (L·h−1) 2730 276 Diameter of the nozzle / mm 80 32 Depth of liquid / mm 900 360 Distance between two nozzles / mm 621 248 Depth of nozzle penetration / mm 150 60 Fig. 2. Sketch map of water modeling experiments. In this study, 11 cases (case 1 is the prototype tundish) were investigated, in which 4 different locations of gas blowing, 4 different gas blowing rates, and 3 sizes of porous plugs were studied. Fig. 3 shows the details of the experi- mental cases. The porosity of porous plugs used in the ex- periment is 15%, and the average diameter of