# Comparison of Degassing Efficiency on a Foundry Degassing Unit Using Different Rotor Types
## 1 Introduction
As determined by the authors of those studies, the mechanism of bubble fragmentation involves several steps: bubble transport to the zone behind the rotor blade due to a pressure gradient; compression and elongation of the bubble due to vortices; and bubble fragmentation through the action of high shear and tension due to the effects of the vortices on the bubble. The shape of the bubbles before disintegration indicates that vortex structures in the vessel are largely involved in this process. The authors also confirmed that the gas flow has an impact on bubble size, which increases with increasing gas flow and decreasing rotor rotational speed [31; 34; 35]. Bubble dispersion as a function of rotor speed was explored in [36]. As found in that study, the bubble size increases with increasing rotational speed of the rotor. This occurs due to bubble coalescence in the course of their journey through the vessel towards the surface of the liquid [37; 38]. As shown by the authors of [39], the bubbles collide at a certain distance from the rotor, and their size increases due to coalescence. On the other hand, the growth rate of the bubble size slows down with rising revolutions of the rotor because the turbulent field prevents bubble coalescence [9; 38]. As previously shown in many studies [15; 33], the overall efficiency of refining can be influenced by the rotor shape. The kinetics of the aluminium melt refining process in a mixing vessel was studied by the authors of [16; 21]. The authors carried out physical and numerical modelling in order to describe the kinetics of degassing and to compare various rotor types. As shown by numerical modelling [16], all tested rotors exhibited certain flow similarities. The fluid is pushed out of the rotor in the radial direction thanks to the momentum transmitted by the rotor and the flow of the gas. Two circulation loops thus form in the ladle--below and above the line of the rotor, while the upper circulation loop creates a weak flow, meaning that poor circulation is achieved in the upper part of the vessel [20; 36]. \"Dead volumes\" thus form near the surface of the metal melt, which do not react with the injected gas [23]. Furthermore, as reported in [21], the efficiency of degassing is derived from the so-called pumping effect, which is created in the form of a pressure drop below the rotor that balances the centrifugal forces. The pumping effect causes the development of a vortex in the lower part of the ladle under the rotor and affects its size. Bubble dispersion depends on the capability of the rotor to push out the liquid in the radial direction and is related precisely to the pumping effect and the capability of the rotor to transmit the momentum to the liquid. The pumping effect and the transmitted momentum also play a role in the development of turbulence in the vessel; this turbulence then supports bubble fragmentation and the transfer of mass [21; 31]. As reported in [23], the highest intensity turbulence was situated near the bottom of the ladle,near the vortex below the rotor, i.e., in an area directly affected by the presence of the rotor. This indicates that degassing largely occurs in the area of the rotor where the highest level of turbulence and the highest concentration of the refining gas are found. These variables can be influenced precisely by the shape of the rotor [21]. Furthermore, as shown by the results of the turbulent field calculation, higher refining gas flows are vigorously attenuated by speed fluctuations and by the turbulent flow formed in the vessel [23]. The intensity of refining is also affected by rotor speed. With a constant flow of the gas and higher speed, the bubbles exhibit smaller dimensions compared to a situation with lower speed [36; 40]. On the other hand, coalescence becomes easier with higher turbulence and the bubble dimensions increase again [36].