Jiang_et_al_2010

Prévia do material em texto

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 ofthe 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. 
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