Effects of through-draught on the transmission and distribution of droplets within double-formation subway carriages

The situation of mass infections caused by respiratory diseases in public places continues to pose a significant threat to public health, particularly in the context of the subway, which is characterized by its high passenger density. While there exists a modicum of research on the transmission and spread of droplets within the subway compartment, these studies primarily focus on the environment of a stationary subway and a single carriage. There is a paucity of research on the specific flow field environment formed during subway operation, particularly in the case of through-draught. This study utilizes numerical simulation to analyze the transmission and distribution of droplets within the two-formation subway carriages under the influence of through-draught. The findings reveal that through-draught substantially augments the transmission distance of droplets in the direction of through-draught, reaching up to approximately 24 times the baseline value. Concurrently, through-draught suppresses the maximum transmission distances in the transverse and vertical directions, reducing them by approximately 29.7% and 22.0%, respectively. Furthermore, through-draught has been observed to reduce the suspension time of droplets and to augment the distribution range of their ultimate deposition positions within the passenger compartment. With the increase in the air volume of through-draught, droplets spread downstream in the passenger compartment, with more than 80% of the droplets depositing downstream of the release source. These findings elucidate the impact of through-draught on droplet transmission in the passenger compartment and further reveal the migration mechanism and distribution law of droplets in the carriages of an operating subway.