# Study on the Impact of Ventilation Methods on Droplet Nuclei Transmission in Subway Carriages
## 5 Conclusions  
This study employed the Euler-Lagrange method to analyze droplet dispersion in a subway carriage. It simulated droplet creation, propagation, and removal from coughing, and the effect of air supply on droplet dispersion was analyzed to determine the optimal ventilation system. The subsequent findings may be inferred:  
The coupling of turbulent airflow generated by coughing with thermal plumes is crucial for the initial spread of droplets. Smaller droplets (initial diameter less than 30 \(\upmu\)m) are airborne for a longer period, while larger droplets (initial diameter greater than 80 \(\upmu\)m) are mainly affected by gravity and settle quickly after release. Under different ventilation systems, droplet nuclei in the SFRC system are mainly distributed near the ceiling, while in the MV system, the distribution is more uniform. The number of droplet nuclei in the ceiling return air of the SFRC system is lower than that in the MV system, but the opposite is true for deposition and escape. Different ventilation systems have a significant impact on virus transmission in enclosed spaces. PRP curves show that increasing air supply speed can significantly reduce the concentration of aerosols and larger particles. In the SFRC system, an air speed of V = 0.8 m/s provides optimal performance, with lower droplet arrival probability, suspension rates, and pollutant concentrations within the safe social distance of 2-3 m.  
Air supply speed is critical to the distribution of droplet nuclei in subway cars. As the air speed increases, the observed data shows a decrease in suspended droplet nuclei, along with an increase in escaped droplet nuclei, and the range of pollutants decreases with increasing air speed, showing a lateral diffusion trend. (In the MV system, air speed increases from 0.8 m/s to 1.6 m/s due to the return air area being one-quarter the size of the SFRC system's air inlet). It is advisable to reduce the air supply temperature slightly, while maintaining thermal comfort. The SFRC air supply system provides passengers with a gentler airflow experience. Compared to the MV system, the droplet nuclei suspension rate drops by 18%, allowing for quicker and more efficient removal of droplets, reducing the likelihood of other passengers inhaling viral particles and thus lowering infection risks. The average pollutant spread is reduced by 60%. At higher air speeds, 99.9% of particles do not travel beyond 2 m, while in the MV system, at lower air speeds, over 53% of particles travel more than 3 m. The results show that midsized particles can travel farther and remain suspended for longer. Energy consumption, as an important indicator for evaluating the performance of transportation systems, was analyzed for three air supply velocities under two ventilation methods. The conclusion is that the SFRC air supply method results in lower energy consumption during the air conditioning process, making it more energy efficient. Additionally, the ADPI values for the SFRC system are higher than those for the MV system, making it a more comfortable air supply method. We conducted a quantitative analysis of infection risk, and the infection probability with the SFRC air supply method decreased by an average of 6.4%, with a maximum reduction of 8%. Considering energy consumption, comfort, and safety, the SFRC air supply method with high wind speed (V = 0.4 m/s) is an excellent choice for subway carriages. Conceptualization, G.F.; methodology, X.W. ([PERSON]) and G.F.; software, X.J., X.W. ([PERSON]) and G.F.; validation, R.L. and H.R. ; data curation, R.L. and H.R. ;  
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