## 1 Introduction
This problem is exacerbated when two-way interactions between the gas and solid phases are considered in the simulation of complex two-phase flows within industrial classifiers. Therefore, it is common to ignore the influences of material particles on the fluid flow and their interactions [8; 17; 37; 38; 39; 40; 41; 42]. However, these interactions are highly important in many cases; for example, the cyclone's swirl intensity decreases and the gas flow's turbulence is greatly reduced by the presence of a solid phase [43]. An air classifier's inner flow field is composed of a highly turbulent and complex 3D swirling flow. The flow parameters become dynamically unstable as a result of this [44]. Therefore, the physical nature of the flow is not adequately represented if the characterization of the particles' impact on the gas phase is insufficient. To date, the number of studies considering the two-way interactions between the gas and solid phases is limited. Most studies have been performed on uncomplicated models, such as cyclones [33; 43; 45; 46; 47], or examined the fish-hook effect [34]. With the rapid development of materials science and computing science, widespread attention has been given to air classifiers with small cut sizes, high classification efficiency and accuracy, a narrow particle size distribution, and energy-saving features [10; 12; 13; 16; 48]. In the production of quartz-based engineered stone, to achieve the products' required quality, it is crucial to consider the particle size distribution curve (PSDC) of the material. It is not sufficient to evaluate the classification performance only through common indices such as \(d_{50}\) (cut size), \(K\) (classification sharpness index), and the pressure drop (\(\Delta P\)). When evaluating the separation performance, the particle size distribution curve of the powder material should be taken into account as a constraint condition. The required particle size distribution curve, referred to as the know-how curve (KHC) in this study, describes the proportions of particles in different particle size intervals according to the standard of quartz materials used in the production of quartz-based artificial stone, which was created by the Phenikaa Group [49]. The effect of various outlet mass airflow rates (from 4.934 to 6.806 kg/s) and a comparison of three coupled CFD-DPM models (one-way, two-way, and four-way coupled CFD-DPM models) were presented [2] for the same classifier considered in this study, highlighting the significant effects of material particles on the classification performance and flow field inside a real-size industrial-scale micron air classifier. It was noted that the four-way coupled CFD-DPM could provide the most accurate results when simulating a complex two-phase flow field, while the simulation results obtained under two-way coupling were comparable to those obtained under four-way coupling and could be considered acceptable. Moreover, there was no fish-hook effect in the investigated cases. In this study, the effects of the structural parameters (SPs) of the guide cone, such as the inclination and the material recirculation gap size, on the two-phase flow field and classification performance of a real-sized industrial-scale micron air classifier were investigated. It should be noted that the studied guide cone (Figure 1c) uses its inner surface to direct the flow inside the classifier, returning large-sized material particles downward. The objective of this study was to determine the optimal SPs of the guide cone of the classifier in order to achieve the best classification efficiency and satisfy the KHC for the particle size range (\(0\div 400\) um) used in the production of quartz-based artificial stone. Therefore, the characteristics of the flow field in the classifier, which are characterized by the sizes, shapes, positions, and rotational directions of the vortices in the secondary classification regions and the major classification chamber, were explored using a 3D transient simulation.