## 1 Introduction  
Stirred reactors with baffles are used in the chemical industry to obtain chemical products by mixing two or more components, [1]. The quality of the final chemical product obtained inside the industrial reactor is influenced by the performance of the stirring mechanism that equips the reactor. For the industrial reactors operating with a liquid-solid mixture it is of great importance that the stirring mechanism ensures a homogenous dispersion of the solid particles inside the entire volume of liquid and to prevent the sedimentation of the solid particles on the lower cover of the reactor. Using CFD for modelling the complex flow inside industrial reactors proves to be an efficient, time saving and tractable method to analyze the performances of the stirring mechanism that is present in the reactor. Many researchers have used different types of expert CFD codes (Fluent, CFX) [2; 3; 4; 5; 6; 7; 8; 9] for investigating the multiphase flow inside stirred reactors. A part of the previous research focused on identifying the appropriate turbulent model to be used [5; 6]. Another preoccupation of the researchers was to identify the best suitable model for modelling the solid suspension inside the liquid volume from the ones available in the expert CFD codes [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. Taking into consideration the results from [10; 11; 12; 13; 14], the best suited model for our research was the RNG k-\(\epsilon\) turbulence model, which allows sufficient accuracy of the turbulent quantities within a reasonable calculation time, and the Eulerian multiphase model for calculating the liquid-solid mixture. This research paper represents the third paper from a series of three that investigate the performance of a new stirring mechanism from a baffle industrial reactor [21; 22]. The main idea behind the three research papers is to develop a tractable and robust numerical approach for customized design and assessment of mixing impellers, beyond the rather classical approach of selecting on-the-shelf impellers based on empirical correlations. In the first paper [21], we investigate the performances of the original stirring mechanism fitted with an impeller with two blades, that was initially designed to operate with a liquid-liquid combination. An extensive presentation of the numerical model and numerical set-up is presented in this paper. This type of impeller proves to be not appropriate for mixing a liquid-solid combination because there is a consistent amount of solid particles sedimented on the bottom of the reactor and a large share of the volume of liquid is depleted of solid particles. We designed a new impeller with three blades and with a smaller diameter than the original one and, in our second research paper [22], we analysed the performance of the stirring mechanism fitted with the new impeller in the same position, regarding the bottom of the reactor, as the original impeller. The new impeller proved to have a better performance from the point of view of solid particle sedimentation, but it still cannot ensure a homogenous distribution of the solid particles in the entire volume of the liquid phase. Since the results presented in the previous two papers indicate that the two solutions investigated did not ensure a proper dispersion of the solid particles in the volume of the liquid phase, in the current paper we analyse a new solution consisting in fitting on the existing stirring mechanism two new designed impellers operating in tandem. A schematic representation of the investigated stirred reactor is presented in Figure 1, where the two new designed impellers are fitted on the stirring mechanism. The two impellers have three blades each and operate at the same rotational speed of 65 rpm. The lower impeller is positioned in the same place, regarding the lower cover of the reactor, as the original impeller. The upper impeller is positioned at a distance equal with the diameter of the impeller, regarding the position of the lower impeller, as suggested in [1].