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the gas hole is 2.0 mm. Fig. 4 shows the shape and the size of the porous plug. J. Jiang et al., Water modeling of molten steel flow in a multi-strand tundish with gas blowing 145 Fig. 3. Cases investigated in this study: (a) case 2, 180 L/h, 210 mm×56 mm; (b) case 3, 90 L/h, 210 mm×56 mm; (c) case 4, 90 L/h, 210 mm×56 mm; (d) case 5, 90 L/h, 105 mm×56 mm; (e) case 6, 60 L/h, 105 mm×56 mm; (f) case 7, 120 L/h, 210 mm×56 mm; (g) case 8, 120 L/h, 105 mm×56 mm; (h) case 9, 120 L/h, 105 mm×56 mm; (i) case 10, 120 L/h, 210 mm×28 mm; (j) case 11, 120 L/h, 210 mm×56 mm. Fig. 4. Porous plug used in the water experiments. 3. Results and discussion Fluid flow characteristics in the water model are listed in Table 2. The dead flow volume fraction in most tundishes with gas blowing is less than that in the prototype tundish, but case 2 is an exception, and the reason will be discussed later. Therefore, proper gas blowing can reduce the dead flow volume fraction in the tundish. However, it is noticed that the minimum residence time in case 1 is the longest, so gas blowing reduces the minimum residence time in the current tundish. This is different from the results obtained by Zhong et al. [2], whose results showed that gas blowing hardly influenced the minimum residence time. Perhaps this is due to the different tundish configurations used in the two investigations. 146 Int. J. Miner. Metall. Mater., Vol.17, No.2, Apr 2010 Table 2. Fluid flow characteristics in the water model Case tmin / s tmax / s tav / s Vdv / % Case 1 62 275 665 11.0 Case 2 42 412 658 12.0 Case 3 38 305 679 9.1 Case 4 54 351 685 8.3 Case 5 42 448 713 4.8 Case 6 52 446 710 5.0 Case 7 41 422 726 2.9 Case 8 61 402 709 5.2 Case 9 58 360 684 8.5 Case 10 59 373 677 9.4 Case 11 36 377 681 8.9 In cases 2 and 11, the minimum residence times are very short, 42 and 36 s, respectively. When the flow jets reached the bottom of the turbulence inhibitor, first, flew up and then flew into the 3 holes in the baffle. The flow coming from the front hole was divided into 2 parts, one part flew into the surface of the molten steel due to the driven force from bub- bles, and the other part flew into the middle outlets directly. Therefore, the minimum residence time in the middle outlets was much shorter than that in the side outlets because of the front hole in the baffle, which caused the flow in the tundish inhomogeneous. Therefore, the middle hole (shown in Fig. 1) on the baffle was covered in the following experiments. For cases 3 and 4, the gas flow rate is same, but the gas blowing location is different. According to the results in Ta- ble 2, the dead flow volume fraction in case 4 is 8.3%, smaller than that in case 3. Cases 3 and 5 have the same gas blowing location and gas flow rate but different areas of porous plug; however, the dead flow volume fraction in case 5 is smaller than that in case 3. For case 6, the gas flow rate is smaller than that in case 5, but case 6 gets a longer mini- mum residence time. Therefore, the gas blowing location, gas flow rate, and the area of porous plug gas greatly influ- ence the flow characteristics in a tundish, the gas blowing location near the baffle, smaller gas flow rate, and smaller area of porous plug are better for improving fluid flow characteristics. This conclusion also can be proved in the following experiments. For cases 7 and 8, the gas flow rate and area of porous plug are same, but the gas blowing location is different. The minimum residence time is only 41 s for case 7, but for case 8, the minimum residence time reaches 61 s. In case 7, all the flow from the side holes in the baffle flows into the sur- face along the bubbling curtain and then flows down quickly due to the absorbing force on the outlet, and reaches the middle outlets first. In case 8, the porous plug is far from the side holes, so part of the flow from the side holes can keep the same flow direction, and the other part will flow into the surface of the fluid along the bubbling curtain. The velocity of the second flow becomes slow because the first flow weakens the absorbing force at the outlet, so the minimum residence time in case 8 is longer than that in case 7. For the same reason, case 9 also achieved good experimental results; the minimum residence time is 58 s and the dead flow vol- ume fraction is 8.5% Typical RTD curves for case 1 (without gas blowing) and case 10 (with gas blowing), are shown in Figs. 5 and 6, re- spectively. It can be seen that the RTD curves in the tundish with gas bubbling curtain is obviously different from those without gas blowing. The peak concentration time of the tracer for the experiment with gas injection is significantly larger than that without gas blowing. The curves for the ex- periment without gas blowing fluctuate in the vicinity of the peak concentration, while the curves for the case with gas Fig. 5. RTD curves for case 1 (without gas blowing): (a) global RTD curve; (b) local RTD curve. J. Jiang et al., Water modeling of molten steel flow in a multi-strand tundish with gas blowing 147 Fig. 6. RTD curves for case 10 (with gas blowing): (a) global RTD curve; (b) local RTD curve. blowing reveal a smooth curve. This shows that the flow characteristics in the tundish can be improved by using gas blowing. It also can be noticed that both RTD curves for the outside outlet and inside outlet fit better in Fig. 6 than those in Fig. 5. For case 1, the minimum residence time at the middle outlet and side outlet is 52 s and 81s, respectively, but for case10, the minimum residence time at the middle outlet and side outlet is 59 s. Therefore, using gas blowing can reduce the difference of flows at the middle outlet and side outlet. Visual observation in the experiments with potassium permanganate solution as a dye tracer showed that the tracer decelerated before passing through the bubbling curtain and accelerated after flowing through the curtain. When it reached the bubbling curtain, the dye tracer changed the flow direction and reached the surface of fluid. Inclusions could be brought into the surface by the flow; and they were absorbed by top slag. The bubble produced by gas blowing could absorb small inclusion and also could provide the condition for inclusions collision and aggregation, which are good for inclusion removal. 4. Conclusions (1) Gas blowing can greatly improve the flow character- istics in the tundish with a turbulence inhibitor. It dramati- cally increases the peak concentration time, greatly de- creases the dead volume, and reduces the minimum resi- dence time. (2) The gas blowing location, gas flow rate, and the area of 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 bet- ter. (3) Using gas blowing can reduce the difference of the flow at the middle outlets and side outlets for the multi-strand tundish. (4) 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. References [1] L. Zhang and S. Taniguchi, Fundamentals of inclusion re- moval from liquid steel by bubble flotation, Int. Mater. Rev., 45(2000), No.2, p.59. [2] L.C. Zhong, L.Y. Li, B. Wang, M.F. Jiang, L.X. Zhu, L. Zhang, and R.R. Chen, Water modelling experiment of argon bubbling curtain in a slab continuous casting, Steel Res. Int., 77(2006), No.2, p.103. [3] M.J. Zhang, H.Z. Wang, A. Huang, Q.X. Meng, and H.Z. Gu, Water modeling of effect on steel liquid flow in the bottom gas blowing tundish of different porous materials, J. Wuhan Univ. Sci. Technol. Nat. Sci. (in Chinese), 29(2006), No.5, p.433. [4] M.J. Zhang, H.Z. Wang, A. Huang, H.Z. Gu, and D.Q. Liu,