# The Development of an Optimized Impact Pad for a Six-Strand Tundish Using CFD Simulations
## 4 Evaluation and Discussion
### RTD Curves  
From the first results of the simulations, the effect of the symmetry of the tundish was confirmed. Therefore, only three outlets on one side of the tundish were evaluated  
Figure 20: A comparison of velocity contours in the central part of the tundish near the slag line.  
The values of the minimum and maximum residence times, identified based on the C-curves at individual outlets for the analyzed configurations, and the percentage differences from the reference K configuration are listed in Table 3.  
One of the main goals of developing the new impact pad (SP-K4 configuration) was to ensure that the residence times between each casting strand are as consistent as possible. This was a challenging task as there are significant variations in the distances between the impact pad and the individual casting strands.  
In configuration SP-K4, we have made significant improvements in the liquid steel flow pattern in the tundish volume. We have successfully reduced the residence time of the farthest casting strand, CS1, by 61.2%, which will greatly decrease the dead zone volumes. Moreover, we have extended the residence time of the nearest casting strand, CS3, by 54.6%. This modification will significantly enhance the refining capacity of the tundish.  
When comparing the calculation of the dead volume based on the residence times on the individual casting strands in the K and SP-K4 configurations, the values were the same, and in the case of the SP-K4 configuration, there was a reduction in the plug flow and an increase in the mixing zone.  
Figure 23: Mathematical modeling-produced RTD curves for the CS3 outlet with the analyzed K, SP-K2, and SP-K4 configurations.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Resulting Min. and Max. Residence Times**} & \multicolumn{3}{c}{\(\frac{V_{p}}{V}\)} & \(\frac{V_{p}}{V}\) & \(\frac{V_{m}}{V}\) \\ \hline \multicolumn{1}{c}{} & \multicolumn{3}{c}{**Tmin [s]**} & \multicolumn{3}{c}{**Tmax [s]**} & \multicolumn{3}{c}{**[\%]**} \\ \hline \multirow{4}{*}{K configuration} & CS1 & CS2 & CS3 & CS1 & CS2 & CS3 & & & \\  & 168 & 56 & 28 & 387 & 103 & 37 & 19.9 & 25.8 & 54.3 \\  & 80 & 41 & 33 & 180 & 60 & 65 & 31.7 & 15.2 & 53.1 \\  & \(-\)52.4 & \(-\)26.8 & 17.9 & \(-\)53.5 & \(-\)41.7 & 75.7 & - & - & - \\  & 65.2 & 39.5 & 43.3 & 180 & 50 & 73.9 & 19.9 & 9.8 & 70.3 \\  & \(-\)61.2 & \(-\)29.5 & 54.6 & \(-\)53.5 & \(-\)51.5 & 99.7 & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 3: Calculation parameters for RTD and flow rate volume proportions for the K, SP-K2, and SP-K4 configurations.  
Furthermore, based on the comparison of the analyzed configurations, a ratio of mixed volume to the dead volume was calculated \(\frac{\mathrm{V_{m}}}{\mathrm{V_{d}}}\); it expresses the quality of thermal and chemical homogenisation in the tundish and is a ratio of plug flow to dead flow \(\frac{\mathrm{V_{p}}}{\mathrm{V_{d}}}\); it expresses the conditions for the favorable flotation of non-metallic inclusions [2], as shown in Table 4.  
It can be assumed that the initial experimental SP-K2 configuration will lower thermal and chemical homogeneity, leading to the favorable flotation of non-metallic inclusions, based on calculated values for the mixed zone and dead zone volumes. On the other hand, the second experimental SP-K4 configuration has better \(\mathrm{V_{m}}/\mathrm{V_{d}}\) and \(\mathrm{V_{p}}/\mathrm{V_{d}}\) ratios, leading to a significant improvement in steel cleanliness, as well as thermal and chemical homogeneity.