# Study on the Impact of Ventilation Methods on Droplet Nuclei Transmission in Subway Carriages
## 3 Results
### Particle Reach Probability
This is related to the distribution of droplet nuclei under the two ventilation methods and air supply speeds. When the SFRC method is used, most droplet nuclei accumulate near the ceiling and are effectively removed by the return air vent in the ceiling. When using the MV method, the droplet nuclei disperse over a wider area, concentrating near the passengers' heads, resulting in less effective removal of the droplet nuclei. The temporal distribution of droplet nuclei varies depending on the air supply rate. In the case of the SFRC air supply system, at the initial moment, the largest discrepancies in the proportions of suspended, escaped, and deposited droplet nuclei were 29%, 43%, and 32%, respectively. At t = 120 s, the maximum differences were 6%, 14%, and 44%. In the case of the MV air supply system, the maximum discrepancies in the fractions of suspended, escaped, and deposited droplet nuclei were 16%, 0%, and 16%, respectively. Over time, many droplet nuclei remained suspended inside the subway carriage. At t = 60 s, the maximum differences in suspended, escaped, and deposited droplet nuclei were 28%, 49%, and 21%. Even at t = 120 s, droplet nuclei had not been fully removed in the MV air supply system with lower air supply speed. These data indicate that different air supply methods and speeds have a significant impact on the distribution of droplet nuclei inside the subway carriage. Droplet evaporation is directly influenced by temperature and relative humidity, which in turn are influenced by various other factors; but, the evaporation time into droplet nuclei is brief and the diffusion time scale is much longer than the evaporation time scale. This study uses typical summer air supply temperatures and relative humidity conditions. Therefore, the effect of relative humidity on droplets is not considered. Figure 12 shows the proportion of droplet nuclei across different vertical heights. The subway carriage, with a height of \(Z\) = 2.1 m, was divided into seven regions, each with a vertical height of 30 cm. It can be observed that under the SFRC air supply system, at a height of 0-1.5 m (where \(Z\) = 1.5 m represents the breathing zone for standing passengers), the droplet distribution is uniform and the quantity is very small, with a proportion below 4.45%. At lower wind speeds, the proportion of droplet nuclei reached about 20%, with most droplet nuclei concentrated near the ceiling, above head level. The higher the air supply speed, the more nuclei accumulate near the ceiling. the pollutant concentration in the SFRC air supply system is significantly lower than in the MV system. The tracer gas concentration in the low-speed Case 1 is lower than in the high-speed Case 4. It can also be observed that the pollutant concentration in each case increases gradually. Throughout the process, the higher the air supply speed, the lower the pollutant concentration, and the fewer suspended droplet nuclei. Higher air supply speeds lead to greater efficiency in pollutant removal. At t = 65 s, the concentration in the SFRC system stabilizes and approaches zero. In contrast, under the MV system, pollutants still remain in the subway car at t = 120 s, with Case 4 at a low air speed of V = 0.8 m/s being the worst-performing scenario. Increasing ventilation rates helps remove droplet nuclei from the subway car through the exhaust, reducing both deposition and suspension of the droplets. To track the dispersion of the contaminated fluid, a tracer gas with a density of 1.977 kg/m\({}^{3}\) and a mass diffusion coefficient of 2.5 \(\times\) 10\({}^{-5}\) kg/m\(\cdot\)s was introduced into the simulation. The tracer gas was incorporated into the model domain using a user-defined function (UDF) and was assigned a velocity inlet boundary condition. By implementing this UDF, a tracer gas jet was generated at the mouth of the coughing passenger, oriented at a 90\({}^{\circ}\) angle to the mouth plane, with a jet velocity of 13 m/s, a temperature of 310.15 K, and a duration of 0.2 s [36].