Comparison of Variation Between Strands

Another method of analysing the performance of a multi-strand tundish is to compare the C-curves for the individual strands. This is of practical importance to industrial practice since the melt arriving at the different outlets can have significantly different temperatures and compositions, most important being the size and quantity of inclusions, if the melt reaching these outlets have spent different times in the tundish. This will affect the way that the casting process is controlled, since either the casting in each strand will have to be operated under different conditions or a compromise has to be found between two strands, with both operating at non optimal conditions. One method of doing such a comparison is to calculate the parameters of the modified combined model for each outlet and compare the values. Such a comparison is summarised in Figure 6.5, by plotting the difference between each of the important properties for the outer and inner strands for the three configurations studied with the physical model. For all three cases it is apparent that the outer strand performs better than the inner strand, with the minimum, peak and mean residence times and the plug for volumes being higher and the dead volume being lower.

58 The outer strand is therefore likely to have fewer and smaller inclusions in the mold for all three configurations.

Figure 6.5 - Comparison of RTD parameters for the difference between in the outer and inner strands of the three configurations discussed

Comparing the different configurations, the variation between strands for the bare case is clearly much lower than for the two configurations using the turbulence inhibitor. However, it must be put into perspective by looking at the values summarised in Table 6.3. For all the properties the performance of inner strands of the cases using the turbulence inhibitor is still superior to the outer strand of the bare case. Nonetheless, it does show that the two cases using the turbulence inhibitor are not as superior to the bare case as would be concluded from the overall properties, since the performance of their inner strands are significantly inferior to their outer strands. There is clearly room to improve the design of the turbulence inhibitor and enhance the performance of the inner strand. Lastly, it can be noted that the TI case shows slightly better strand similarity than the TID case. This negates one of the arguments for the addition of holes and dams to the case using only the turbulence inhibitor (Kumar et al., 2008).

Table 6.3 - Comparison of RTD parameters for the outer and inner strands of the three configurations compared in the physical experiments

Configuration Outlet θmin θpeak θmean Vp Vd Vm

Bare Inner 0.010 0.0842 0.682 0.047 0.380 0.573 Outer 0.024 0.13 0.692 0.077 0.361 0.562 TID Inner 0.040 0.232 0.697 0.136 0.342 0.522 Outer 0.131 0.313 0.797 0.222 0.257 0.521 TI Inner 0.086 0.179 0.709 0.132 0.346 0.521 Outer 0.118 0.236 0.804 0.177 0.266 0.557

59 An alternative method that might be useful to quantify the variation between strands, is to calculate the ratio of the area between the C-curves of the two individual strands to the area under the overall C-curve. This value will give a measure of the difference in the RTD response at two different outlets. Calculating the ratio by integrating from the beginning of the experiment to , the strand variation for the bare tundish, the TI case and the TID case are 0.165, 0.222 and 0.254, respectively. This method therefore ranks the strand variability of the three configurations in the same order as was found when comparing the different RTD parameters. For this case, this ratio appears to be a good method of estimating the variation between strands while only using one parameter. Nonetheless, its validity over a range of applications should be tested further.

These comparisons, however, only give an estimate of the overall difference between the RTD responses at the outlets. For a more specific indication of where the variation occurs, the individual C-curves should be plotted on the same axis for each case. For the bare tundish, it can be seen in Figure 6.6 that the peak for the outer strand occurs at approximately double the time as for the inner outlet. However, it only takes a very short time for the concentrations of the tracer to become very similar at the two outlets.

Figure 6.6 - RTD responses for the individual strands for the bare tundish, as obtained from the physical experiments

Such a comparison for the TID, shown in Figure 6.7, shows that although the peaks are much closer to each other, the curves take much longer to establish a similar concentration.

60 Figure 6.7 - RTD responses for the individual strands for the TID tundish, as obtained from the physical experiments

For the TI case, the comparison in Figure 6.8 shows that the behaviour is very similar to that of the TID case, the main difference being slightly closer concentration values after the curves have crossed.

Figure 6.8 - RTD responses for the individual strands for the TI tundish, as obtained from the physical experiments

It can therefore be concluded that the strand similarity is much more for the bare tundish than for the two cases with turbulence inhibitors, with the TI case being the superior of the two by a small margin. Strand similarity should be improved by increased mixing in the tundish. The strand variations compare reasonably well with the calculated mixing volume fractions, shown in

61 Table 6.2, where the well-mixed fraction of the bare tundish is significantly higher than that of the two turbulence inhibitor cases. Unfortunately, this trend does not hold for the comparison of the TI and TID cases, where the variation between strands is less for the TI case, but the mixed volume fraction calculated from the overall C-curve is lower. However, when looking at the well-mixed volume fractions for the individual curves, the fractions are higher for the TI case. Therefore, generally, designing for a larger well-mixed volume fraction should decrease the variation between strands. However, the design should be done in such a way that the mixed volume is increased at the expense of the dead volume, instead of the plug flow volume, in order to still promote flow that is beneficial for inclusion removal.

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