Droplet transmission is a critical pathway for the spread of respiratory infectious viruses. A thorough understanding of the mechanisms of droplet dispersion within subway carriages is crucial to curb the widespread transmission of the virus. This study utilizes computational fluid dynamics (CFD) to establish a full-scale numerical model of a subway carriage. The numerical model and droplet evaporation behavior are validated using experimental data and literature. The impact of primary parameters such as the initial droplet size, release velocity, release position, relative humidity, and passenger density on the droplet diffusion and probability of infection for passengers is investigated. The results indicate that large droplets (100 μm) are deposited on the carriage floor before complete evaporation, while tiny droplets (10 μm) evaporate rapidly, leading to a longer suspension time in the air within the carriage. The infected passenger’s position influences the ventilation system’s efficiency in removing the droplets; removal takes significantly longer when the infected passenger is closer to the carriage end. Additionally, a low relative humidity (35%) and high passenger density (4 p/m²) result in more droplets being trapped by passengers’ bodies. The infection probability for passengers depends on the initial size and quantity of droplets trapped by their bodies. Maintaining higher relative humidity levels and limiting the passenger numbers within the subway carriage can reduce the number of droplets captured by passengers’ bodies, thus helping to reduce the infection probability of fellow passengers.

Due to the rapidly increasing ridership and the relatively enclosed nature of subway spaces, passenger thermal comfort has become a primary design objective. In this paper, a series of human subject studies are conducted to investigate the exchange mechanism between passenger heat and the indoor environment in a type-B subway cabin. The impact of crowding levels (ideal, reasonable, and acceptable) and ventilation modes (internal ventilation mode, IVM, and external ventilation mode, EVM) are considered. The results indicate that under the IVM condition, passenger heat loss at various crowding levels has an approximately linear impact on indoor temperature, primarily driven by radiative heat loss. In contrast, the EVM leads to high radiative and convective heat loss, and the rise in indoor temperature remains modest. Furthermore, as an important parameter in the evaluation of thermal comfort models, passenger clothing surface temperature can be obtained using an iterative calculation method, with a relative error less than 2.2% compared to experimental measurements. It is also found that while the EVM mode quickly stabilizes the indoor thermal environment, excessive heat loss at low crowding degrades passenger comfort. The outcome can serve as an important reference for proceeding investigations on optimizing cabin design in public transport.

Cough droplets pose significant risks to human respiratory health, potentially leading to severe infections in indoor environments. In the confined and densely populated high-speed train compartment, passenger movement is unavoidable and follows a fixed path. This movement impacts the designed airflow and, consequently, influences the dispersion of cough droplets. In this study, a validated computational fluid dynamics overset mesh method was adopted to implement passenger movement along the aisle, and the impact of passenger movement on droplet dispersion inside a high-speed train compartment was investigated. The results show that the wake flow generated by moving passengers can carry cough droplets along the direction of movement. The timing and speed of passenger movement play a pivotal role in the extent of droplet dispersion. Premature and delayed interactions with the droplet cloud diminish engagement due to inadequate and excessive dispersion, respectively. When a passenger begins walking at the 10th second, droplet transfer in the direction of movement peaks, reaching up to 4.9 times that of the stationary case in the area of seat 13A, with droplet transmissions extending up to 6 m. The walking speed affects the intensity of the wake flow. A walking speed of 1.0 m/s or higher results in the noticeable transmission of droplets in the direction of the walking passenger. These findings underscore the necessity for incorporating human movement dynamic in the development of ventilation strategies and public health guidelines to mitigate airborne transmission risks in enclosed public spaces.