The easiest way to develop an understanding of the large differences in the flow properties for the different designs is to compare the velocity contours along the longitudinal plane, as shown in Figure 10.8. It immediately becomes apparent that the differences are mainly caused by the amounts of suppression delivered by each turbulence inhibitor design. The flow in each alternative design will now be discussed to better understand what is happening. However, it must be noted that this general comparison of turbulence inhibitors does not account for the optimal dimensions of each design. This means that the alternative designs may possibly perform better if they are optimised for the tundish being used. This comparison therefore only serves as a general discussion of possible advantages and disadvantages of each design strategy.
112 Figure 10.8 - Comparison of velocity contours along the longitudinal centre plane for the four turbulence inhibitor designs: a) TI, b) TI-S, c) TI-R and d) TI-C
10.4.1. TI-S
In the case of the turbulence inhibitor without flanges, the turbulence inhibitor effectively became a low dam placed very close to the inlet. The result is that nearly no turbulence suppression took place and that a very strong circulation pattern developed in the tundish, as shown in Figure 10.9. This explains the multiple peaks and the short residence times. Although the residence times were relatively short, the TI-S did prevent short-circuiting from taking place and delivered surface directed flow.
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10.4.2. TI-C
With the addition of the chamfers in the turbulence inhibitor, the flow path of the melt is smoothened. Therefore, suppression will take place, but will be reduced from that of the TI case. The vector plot along the longitudinal symmetry plane, shown in Figure 10.10, shows that the flow is indeed similar to that of the original turbulence inhibitor, as a similar circulation pattern develops above the inner strand. This similarity can also be seen in the C- curves, where the TI-C and TI cases are the only ones with only one peak forming. However, the reduced suppression leads to faster circulation in the tundish, resulting in much shorter residence times. It is possible that the angle of the chamfer could be used to control the suppression and achieve an optimum effect.
Figure 10.10- Vector plot along the longitudinal symmetry plane for the TI-C design
10.4.3. TI-R
The most surprising of the results were obtained for the round design, since similar results to the original TI design were expected. However, very poor performance is predicted, compared to the original turbulence inhibitor design. The velocity contours in Figure 10.8 show that a very large amount of suppression occurs due to the circular shape forcing the melt to converge on the inlet stream. This results in a dramatic change in the flow pattern, with clockwise circulation forming on the transverse planes, as shown in Figure 10.11. As a result the flow in the direction of the back of the tundish is reduced significantly.
114 Figure 10.11 - Vector plot on the transverse plane through the inner strand of the TI-R case
The effect of this is evident when comparing the C-curves of the separate strands in Figure 10.12. Due to the slow movement of flow in the direction of the back of the tundish, tracer reaches the inner outlet long before it arrives at the outer outlet. This leads to major differences between the individual strands, with the performance of the inner strand being highly inferior to that of the outer strand.
115 Figure 10.12 - C-curves for the individual strands for a tundish using the round turbulence inhibitor design
10.5. Surface Turbulence
From the comparison of tundish flow patterns for the four turbulence inhibitor designs discussed it is evident that the design of the inhibitor has a very strong effect on the resulting flow. Therefore, surface turbulence should also change with the different designs. A comparison of turbulence kinetic energy levels on a horizontal plane 4cm from the tundish surface, shown in Figure 10.13, displays these changes. The case with the straight turbulence inhibitor shows very different contours from the other three cases. The very high turbulence values near the inlet region are caused by the lack of suppression causing high velocity upward flow reaching the surface. Additionally, the strong circulation at the surface causes significant turbulence levels across most of the surface.
Figure 10.13 - Comparison of turbulence kinetic values near the surface for the four turbulence inhibitor designs: a) TI, b) TI-R, c) TI - S, d) TI-C
116 The TI-C and TI-R cases show roughly similar turbulence patterns to the original turbulence inhibitor design, with high turbulence centred on the inlet and low turbulence towards the back ends. However, both designs lead to increased turbulence near the surface. For the case of the chamfered turbulence inhibitor this is caused by insufficient suppression, which allows high velocity upward flow to reach the surface. In the tundish with the round turbulence inhibitor the explanation is less obvious, with the most likely explanation being increased turbulence generated by the convergence of flow around the inlet stream.
Figure 10.14 depicts the relative turbulence values near the surface for the four different designs of turbulence inhibitors. Previously it was found that the upwards flow formed by turbulence inhibitors will increase the surface turbulence. These results indicate that the design of the turbulence inhibitor will have an important effect on the amount of turbulence generated near the surface, with the maximum turbulence being nearly four times more for the TI-S case than for the original turbulence inhibitor design.
Figure 10.14 - Comparison of maximum and average turbulence kinetic energy levels for the four different designs of turbulence inhibitors on a horizontal plane near the melt surface
However, it should be noted again that to give full meaning to these results, a study of the turbulence levels associated with slag entrainment is required.
10.6. Strand Similarity
Although the comparison of the numerical and physical model results indicated that the numerical model does not predict the strand variation as accurately as it predicts overall properties, an interesting conclusion regarding strand similarity can be reached from the comparison of the different turbulence inhibitors. This is best observed by comparing a plot of the individual C-curves of the TI-S case, shown in Figure 10.15, with that of the TI-R case, shown previously in Figure 10.12. Clearly the TI-S configuration has much greater strand similarity than the TI-R case. Also, returning to physical modelling results; it was shown that
117 the bare case had more similar C-curves for the individual strand than for the cases using turbulence inhibitors.
Figure 10.15 - Individual strand C-curves for the TI-S tundish configuration
It is therefore hypothesised that in setups with no or little turbulence suppression, strong circulation patterns will form that dominate the flow in the tundish. Therefore, fluid follows very similar pathways to the inner and outer strands, causing higher strand similarity. For example, in the TI-S case the largest portion of the flow follows a path upwards from the turbulence inhibitor, along the surface, down the back wall and across the bottom towards the inlets. A portion of the flow leaves at the outlets and the rest joins the circulation again. The tracer reaching the inner outlet therefore follows a similar pattern to that of the one reaching the outer strand, except for an additional short time that it takes to cross the distance between the two strands.
In the case of turbulence suppression, the aim was to eliminate these strong circulations in order to reduce turbulence and to increase residence time. However, this caused higher numbers of smaller circulations to form, which resulted in different pathways for the tracer to the individual outlets.
The conclusion is that turbulence suppression could lower strand similarity in the tundish, if not implemented properly. The TI-R case perfectly illustrates that one strand can have exceptional properties, while melt reaching the other will probably contain large inclusions. Such large differences in product quality will complicate tundish practice. Therefore, turbulence inhibitor designs for multi-strand tundish must aim at producing similar steel from all the outlets.
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