# Study on the Impact of Ventilation Methods on Droplet Nuclei Transmission in Subway Carriages
## 3 Results
### Analysis of Energy Consumption and Comfort in Different Ventilation Systems
Given the confined space within the carriage, it is particularly important to examine discomfort caused by airflow effects. We employed the Air Diffusion Performance Index (ADPI) to comprehensively compare the airflow distribution performance of two ventilation systems, aiming to determine the more comfortable air supply method. ADPI is defined as the percentage of measurement points within the target area (e.g., the occupant activity zone) that meet the effective air supply temperature and velocity requirements, as shown in Equation (16): [42]  
\[ADPI=\frac{\sum_{j=1}^{M}\left(P_{EDT}\right)_{j}}{\sum_{i=1}^{N}P_{i}}\times 1 00\% \tag{16}\]  
where Pi represents the measurement points in the target area (i = 1, 2,...N, where N is the total number of Pi); PEDT represents the measurement points that meet the temperature and velocity requirements (j = 1, 2,..., M, where M is the number of PEDT); and EDT is the effective draft temperature (\({}^{\circ}\)C) employed to evaluate the degree of blowing sensation in work area, as follows:  
\[EDT=\left(t_{x}-t_{m}\right)-9.1(\
u_{x}-0.15) \tag{17}\]  
where t\({}_{\rm{x}}\) represents the air temperature of the measurement points (\({}^{\circ}\)C); tm represents the indoor mean temperature (\({}^{\circ}\)C); and v\({}_{\rm{x}}\) represents the air velocity of the measuring points (m/s). When the EDT value varies between \(-\)2.2 \({}^{\circ}\)C and 2 \({}^{\circ}\)C and the air speed is greater than 0.2 m/s, most people can feel comfortable (in warm circumstances). Figure 14 presents the ADPI values for three different air supply velocities under two ventilation methods in this study. As shown in Figure 14, during summer conditions at Z = 1.2 m, the ADPI value of the SFRC system is generally higher than that of the MV system. However, when the air supply velocity is too high (V = 0.4 m/s), the ADPI value of the SFRC system falls below 80%. Although an ADPI value above 70% is generally considered acceptable for airflow \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Case No.**} & **Energy Consumption of** & **Energy Required for** & **Total Energy** & **Comparative** \\  & **the Ventilation System** & **Air Transport (kW)** & **Consumption (kW)** & **Energy-Saving Rate (\%)** \\ \hline Case 1 & 0.21 & 2.95 & 3.16 & 50.31 \\ Case 2 & 0.71 & 3.43 & 4.14 & 34.91 \\ Case 3 & 1.68 & 4.07 & 5.75 & 9.59 \\ Case 4 & 0.21 & 2.95 & 3.16 & 50.31 \\ Case 5 & 0.71 & 3.87 & 4.58 & 27.99 \\ Case 6 & 1.68 & 4.68 & 6.36 & / \\ \hline \end{tabular}
\end{table} 
Table 5: HVAC system energy consumption for each operating condition. distribution performance [43], an optimal air velocity is closer to the ideal comfort level. At this height, the SFRC system with low air velocity provides greater comfort, improving airflow distribution performance by 47% compared to the MV system, while medium air velocity improves performance by 18.7%. Since the SFRC system adopts a downward air supply method, with supply outlets closer to the breathing zone, discomfort occurs at higher air velocities. For standing occupants, both ventilation systems perform best at medium air velocities (V = 0.3 m/s for SFRC and V = 1.2 m/s for MV). However, from an overall perspective, the SFRC system still outperforms the MV system, with an improvement of 5% in airflow distribution performance. In addition to considering the typical breathing zone and the head-level region for standing occupants, we also examined the overall airflow distribution. As shown in the figure, at the overall level, the disadvantages of low air velocity are pronounced, as ADPI values fail to meet the 70% threshold. Although airflow distribution performance is higher at high velocities for both ventilation systems, it remains inferior to that at medium air velocities. Considering additional factors such as pollutant removal efficiency and the PRP index, higher air velocities perform better overall.