On the effect of HVAC recirculation on thermal comfort and indoor air quality in train cabins

Maintaining hygrothermal comfort and Indoor Air Quality (IAQ) in train cabins is crucial for the well-being of passengers and crew, particularly in mitigating the risks associated with airborne transmission. The Heating, Ventilation, and Air Conditioning (HVAC) systems play a critical role in determining airflow patterns, thermal comfort indices, and the distribution of airborne particles. This study investigates the effects of varying return air intake ratios, a practical and non-structural modification, using Computational Fluid Dynamics (CFD). Four HVAC configurations with different return air percentages (0 %, 25 %, 50 %, and 75 %) are assessed in terms of thermal comfort indices and the particles evacuation efficiency generated by a sneeze. The results reveal an optimal trade-off between thermal comfort and particle evacuation efficiency: low return intake (0 %) provides the best thermal comfort conditions; however, the optimal IAQ is achieved with a moderate return intake (50 %), which results in the highest particle evacuation rate (82.34 %) and significantly reduced particle deposition in critical areas, such as the seating zones. In contrast, a high return air intake (75 %) leads to recirculation zones that trap particles and hinder their removal, while a low return air intake increases the risk of particle deposition for passengers. This study underscores the importance of a multi-criteria approach in HVAC design for train cabins, emphasizing that configurations optimized solely for comfort may not ensure protection against airborne pollutants.