# Study on the Impact of Ventilation Methods on Droplet Nuclei Transmission in Subway Carriages
## 4 Discussion  
### Analysis of Particle Behavior in the MV and SFRC Systems  
While existing studies have examined the dispersion of aerosol droplets exhaled by occupants in various indoor and outdoor environments, research on the potential infection risk in subway carriages is still limited. The results indicated that ventilation methods significantly affect droplet dispersion within the carriage. Compared to other indoor environments, the MV method may increase the exposure risk to aerosol droplets within the carriage. Therefore, more suitable ventilation strategies are needed to control the spread of respiratory infectious diseases. The SFRC method is recommended for use in subway carriages. In this study, CFD methods were used to simulate the distribution of droplet nuclei in subway carriages under two ventilation methods (MV and SFRC). By comparing three different air supply velocities, the good ventilation performance of SFRC was validated. Figures 6 and 7 show that the diffusion range of the MV method varies at different air supply velocities due to the horizontal temperature differences in the breathing zone affecting the evaporation rate. Figures 6 and 7 illustrate that the dispersion and accumulation of particles differ for MV and SFRC at the same air supply velocity. For the SFRC system, due to the combined effects of exhaled airflow and thermal plumes, particles tend to disperse more towards the ceiling and then spread outwards, effectively removing pollutants. As shown in Figure 9, both the MV method and the SFRC method at low wind speeds exhibit a larger maximum reach for cough particles. When the air supply velocity of the SFRC method is increased, the maximum reach of the particles significantly decreases. This trend can be attributed to the similar patterns of particle emission during a single cough and the superiority of the ventilation method in effectively suppressing pollutant spread. For the conventional MV system, particles were observed to reach greater flow distances. This may be due to the smaller influence of inlet velocity in the duct and the  
Figure 15: The infection probabilities of different air supply methods (MV, SFRC) in the subway carriage under different air supply parameters. distance of the return air outlet from the infected individual, leading to less suppression of particles, resulting in farther spread that is difficult to remove. Additionally, at low wind speeds, both ventilation methods exhibit a larger maximum reach for particles, as lower wind speeds allow buoyancy to dominate, causing lighter particles to rise almost vertically, resulting in wider dispersion of smaller particles. As shown in Figure 10, the conditions of the two ventilation systems differ slightly, but this trend is consistent across the six examples. However, minor variations in deposition lengths were observed under both ventilation methods. It was observed that the droplet deposition distance in the SFRC system is slightly closer than that in the MV system. This can be attributed to the air conditioning flow field having a greater effect on heavier particles, as it is closer to the position of the infected individual. Consequently, heavier particles in this range fall to the ground over shorter flow distances. This conclusion is also supported by the PRP diagrams for intermediate diameters, which show a significant distance in the flow direction under high air supply velocities in the SFRC system, where PRP reaches zero. Figure 11 shows the quantities of suspended, deposited, and escaping droplet nuclei at different time intervals. The distribution of droplet nuclei varies over time based on the air supply velocity. The number of suspended, deposited, and escaping droplet nuclei varies significantly at 5 s, 20 s, 60 s, and 120 s for different ventilation methods, due to differences in the position and area of the return air outlets. The SFRC return air outlet is more centralized and larger, while droplet nuclei mainly diffuse upwards, resulting in higher removal efficiency.