# Comparison of Degassing Efficiency on a Foundry Degassing Unit Using Different Rotor Types
## 1 Introduction  
An ever-increasing use of aluminium alloys can be observed in many industries today. Its low weight combined with good mechanical properties and long service life make aluminium the second most important construction material in various industries. Thanks to its excellent properties, aluminium is widely used, for example, in the building industry, mechanical engineering, aviation, in food processing and automotive industries, etc. This material is popular especially due to its strength-to-weight ratio, corrosion resistance and high durability [1; 2; 3; 4]. The recyclability of aluminium alloys is a great advantage. In accordance with the principles of circular economy, great attention is focused on recycling of aluminium scrap and its reuse as a source material in the production of aluminium alloys [5; 6]. Aluminium and its alloys are also suitable as matrices of composite materials. The use of aluminium in composites makes it possible to further improve the properties of composites, such as hardness and the modulus of elasticity, while reducing the density of the material at the same time [7; 8]. Although substantial progress has been seen in metallurgy in recent decades, a lot of attention continues to be paid to optimization of metallurgical processes. The technology of aluminium melt refining using an inert gas is no exception, being a pivotal process inmaintaining aluminium casting quality. The requirements for high quality of aluminium products stress the importance of a better understanding of the processes that occur throughout the production process. Aluminium castings have complex shapes and limited post-casting processing options such as heat treatment and High Isostatic Pressing (H.I.P) [1]. It is essential to pay attention to both the melt preparation and casting process, as they determine the final properties of the casting, especially internal homogeneity. Various paths can be taken in the research of aluminium melt refining. The first consists in research of the technology under the operational conditions of a given production plant. Another approach is the modelling method, aimed at simulating the behaviour of a real system. Provided that certain requirements are adhered to, the model can then be used to predict the behaviour of a real system after a change in various conditions [9; 10]. This method enables us to study phenomena that are difficult to measure under operational conditions because the surrounding setting is more favourable than conditions during operation (e.g., temperature, melt splashes). There are two variants of the process modelling method. The first one is physical modelling. As described in [9; 10], physical modelling is usually done in scale models of operational equipment and under normal ambient conditions. In order to obtain representative results from physical modelling that can be transferred to real conditions, the physical models must be constructed according to exactly defined principles following the theory of similarity [11]. In terms of the construction of models of the studied aggregates, it is necessary to preserve geometric similarity of the model and the real object, and kinetic and dynamic similarity of the model and the real liquid. In accordance with the theory of dimensional analysis, these principles are complied with based on the principle of equality of relevant similarity criteria of the model and the studied object. The following numbers are adequate in terms of flow hydrodynamics and its properties for the construction of models of reactors used to refine aluminium melts using an inert gas: the Euler number (Eu), the Reynolds number (Re), the Froude number (Fr) and the Weber number (We). However, the Euler number is important for flows under pressure, and under conditions of intense turbulence, the Reynolds number reaches the area of self-modelling, and thus they can be neglected under certain conditions [9; 10].