# Comparison of Degassing Efficiency on a Foundry Degassing Unit Using Different Rotor Types
## 4 Results and Discussion  
The physical model was used to perform the measurements of the six rotor types illustrated in Figure 3. Due to the large number of results, Figure 5 visualizes only the flow and bubble dispersion in the vessel for the case with 350 rpm and gas flow rate of 17 Nl- min\({}^{-1}\) as used in operational conditions. The physical modelling provided concentration curves illustrating oxygen concentration loss from the model liquid. Figure 6 shows the concentration curves measured using the tested rotor types at 350 rpm and with various flow rates of the refining gas. The concentration curves were originally evaluated according to the so-called refining time. In this case, the refining time was understood as the time needed to achieve an oxygen concentration of 0.5 ppm in the water. For better clarity and simplification, the authors of this paper developed a method for evaluating the concentration curves based on a mathematical model [44]. Performance parameters of the rotors were determined for gas flow rates of 13, 15, 17, 19 and 21 Nl-min\({}^{-1}\). An overview of the calculated dimensionless performance parameters \(a\) according to Equation (2) is shown in Table 3.  
In Figure 5, it can be seen that the gas distribution in the liquid was similar for all rotors. The highest concentration of the gas was found around the impeller from where the gas bubbles spread radially and ascended to the surface of the liquid, while being concentrated predominantly around the shaft. Rotors A, D and F were relatively good at distributing the gas throughout the ladle volume. With these rotors, the gas was dispersed all the way to the walls of the vessel. At the same time, the gas was also distributed to the area below the rotor line. However, this applied predominantly to rotors D and F. With rotors B, C and E, the gas bubbles were not distributed all the way to the wall of the vessel and so-called \"dead zones\" (which are the volumes in which gas bubbles appear occasionally during the experiment) were formed. With rotor C, the gas bubbles spread more to the bottom part of the ladle, below the rotor line. As regards bubble dispersion, rotors D and E provided the best results; these rotors produced smaller bubbles that were distributed throughout the liquid volume. Therefore, rotors D and E provided the highest potential in terms of degassing efficiency. In contrast, larger bubbles can be observed in  
Figure 5: Demonstration of the character of the internal flow and distribution of the bubbles of inert gas; gas flow rate: 17 Nl-min\({}^{-1}\), rotor speed: 350 rpm; (**a**) rotor A; (**b**) rotor B, (**c**) rotor C, (**d**) rotor D, (**e**) rotor E, (**f**) rotor F.  
rotors B, C and E. Lower degassing efficiency was expected for these rotors given the larger bubbles and poorer distribution in the liquid volume. These findings are consistent with literature knowledge [15; 20; 21; 36; 41; 43; 46]. For clarity and to provide context, the parameters from Table 3 are shown in Figure 7a, which shows a comparison of rotor performance, with higher efficiency being exhibited by rotors with higher values of parameter \(a\). The evaluation of the measured data largely confirmed the assumptions determined from the visualization of the flow characteristics. The performance trend increases with increasing gas flow rates for all rotor types, which agrees with the findings of previous studies [15; 20; 23; 33; 41; 43; 46]. The highest efficiency, depending on the gas flow rate, was achieved by rotors D and F; these rotors also exhibited a high level of bubble dispersion throughout the liquid volume. Very similar results were obtained for rotors B and E, although rotor E provided better performance at lower gas flow rates (13 and 15 NI\(\cdot\)min\({}^{-1}\)) while rotor B achieved better results at higher gas flow rates (17, 19 and 21 NI\(\cdot\)min\({}^{-1}\)).