Abstract
Cyclones are widely used equipment in the industry for dust collection from fluid flows to reduce industrial pollution on the environment. In cyclones, the effect of the centrifugal force of the fluid and the difference in particle density compared to the fluid are used for particle and dust separation. The main criteria for evaluating the performance of a cyclone in dust separation are the pressure drop and the particle collection efficiency. In this study, the effect of the diameter of the cyclone cylinders, where the ratio of the upper to the lower cylinder diameter is 0.5, 1, 1.5, and 2, on the particle collection efficiency and pressure drop was investigated. The simulation was performed with ANSYS Fluent 2023 software. Turbulent flow was considered. The inlet velocity and cyclone cylinder were 1 m/s and 20 cm respectively. The results showed that by increasing the diameter of the cyclone cylinders up to 4 times, the efficiency increases by more than twice, and the pressure drop is not changed significantly.

Introduction
The discharge of fluid flow in many industries, such as cement and steel, is accompanied by the dispersion of fine particles that need to be removed from the fluid flow to prevent environmental pollution. The volume of the exhaust flow in these industries is very high, and environmental regulations require these industries to not only remove pollutants but also separate the particles carried by the flow. The cyclone is a particle separation device that uses a swirling flow and centrifugal force to separate particles from the fluid flow based on the difference in density. The cyclone is widely used in industries to purify the fluid flow from polluting particles. Therefore, it is necessary to pay attention to the performance of cyclones that can separate more particles with the minimum pressure drop. Misiulia et al. (2017) investigated the effect of the inlet opening angle on the collection efficiency of a cyclone. They used five different angles of 7, 11, 15, 20, and 25 degrees. They observed that an increase in the inlet angle leads to a reduction in the cyclone's collection efficiency. Zhang et al. (2022) numerically and experimentally investigated the effect of the vortex finder dimensions on the cyclone performance. Their results showed that increasing the vortex finder diameter leads to an increase in the collection efficiency and a decrease in the pressure drop. Dziubak et al. (2023) numerically studied the effect of the axial inlet velocity and cyclone geometric parameters on the collection efficiency and pressure drop. They examined the impact of the cyclone length and inlet vane angle. Their results indicated the best performance was achieved for the cyclone with the maximum length, smaller diameter, and 90-degree inlet vane angle. Pandey and Brar (2023) investigated the effect of using a conical geometry on the cyclone collection efficiency and pressure drop. Their results showed that increasing the cone curvature diameter leads to a decrease in the cyclone collection efficiency and pressure drop. Shastri et al. (2022) studied the impact of changing the cylinder and cone lengths of the cyclone without changing the total cyclone length. The results showed that the particle collection efficiency and pressure drop are maximized when the cylinder length is half the cylinder diameter, and minimized when the cylinder length is 2.5 times the cylinder diameter. This study investigated the effect of the cyclone cylinder diameter on efficiency and pressure drop in four different cases of cyclone cylinder diameter size (D2=0.5D1, D2=D1, D2=1.5D1, D2=2D1).

Methodology
For the simulation, ANSYS Fluent 2023 software was used. In the software, the velocity condition was set for the inlet and the pressure condition was set for the outlet. The standard k-ε model was used for turbulence modeling, which focuses on the effects governing the turbulent kinetic energy. The cyclone collection efficiency is equal to the ratio of the collected particles to the total number of particles entering the cyclone.

Results
The results showed that increasing the ratio of the cyclone cylinder diameters led to a decrease in the centrifugal force and, consequently, a reduction in the suction region of the cyclone. Therefore, the cyclone with higher centrifugal force had the greatest pressure variations (figure 4.).
The results showed that the velocity at the beginning of the cyclone is changing between 0.2 to 0.5 m/s. It was observed that with an increase in the ratio of the cyclone cylinder diameters, the velocity in the middle section of the cyclone decreases due to the reduction of the centrifugal force and the decrease in the suction area (figure.5).
The results showed that with an increase in the ratio of the cyclone cylinder diameters, due to the reduction of the centrifugal force, the amount of flow rotation decreases, and the velocity vectors show less compaction. The largest vortex is for the case where D2=0.5D1, and the most stable flow condition is when D2=2D1 (figure 6.).

Conclusion
In this research, the simulation of a cyclone, which is used for the collection of dust particles in polluting industries, was performed for four different cases of the cyclone diameter ratios using the ANSYS Fluent software. The results showed that:
• With an increase in the ratio of the cyclone cylinder diameters, the efficiency increases due to the reduction of the centrifugal force and consequently the reduction of the suction in the central part of the cyclone.
• It was observed that when the ratio of the cyclone cylinder diameters increases 4 times, the particle collection efficiency increases by more than 2 times.
• When the ratio of the cyclone cylinder diameters is reduced by half compared to the case where the cyclone cylinder diameters are equal, due to the increase in the centrifugal force, the pressure drop is at its maximum and increases by up to 50%.
• With an increase in the ratio of the cyclone cylinder diameters, the pressure drop does not increase significantly, therefore it can be concluded that increasing the diameter of the cyclone cylinders leads to an improvement in the cyclone's performance.

Keywords
Pressure Drop; Particle Separation Efficiency, Industrial Pollution, Turbulence Flow