Evaluating thermal comfort and air quality in buses with different ventilation strategies in Hong Kong

This study employed computational fluid dynamics (CFD) simulations to investigate the influence of air-conditioning exhaust position, supply velocity and direction on air quality and thermal comfort in a fully occupied bus. Air quality was evaluated employing CO₂ concentration as an indicator, while thermal comfort was assessed via the Predicted Mean Vote (PMV) index. The findings reveal that air supply velocity and direction have significant impacts on passengers’ thermal comfort. In winter, a closed air supply can maintain thermal neutrality in the cabin (|PMV|<0.5). Increasing the supply velocity from 0.5 m/s to 3 m/s shifts head-level sensation from slightly cool (PMV<-0.5) to cold (PMV<-1.5). Adjusting the air supply direction to the left or right can effectively eliminate cool areas (PMV<-1) at the head level otherwise caused by the 1.5m/s downward airflow. Additionally, both the supply velocity and the exhaust position predominantly affect the air quality in the cabin. At a low velocity of 0.5 m/s, the average CO₂ concentration at the breathing level reaches 6670.79 PPM, far exceeding the health threshold of 3500 PPM set by the Hong Kong bus air quality standard. An increase in velocity to 1.5 m/s alleviates the average CO₂ levels to 2863.85 PPM within the healthy range. Furthermore, relocating the exhaust forward can further improve air quality by 4.10 %, reducing CO₂ concentration to 2746.50 PPM. Optimal conditions for air quality and thermal comfort are achieved with an air supply velocity of 1.5 m/s, a leftward direction, and the exhaust located at the middle of the ceiling.