The external and internal airflow and air renewal inside urban buses have taken especial relevance since the COVID-2 pandemic. Computational fluid dynamics (CFD) simulations, which focus on the estimation of indoor airflow are not conclusive about the impact of using Heat, Ventilation and Air Conditioning (HVAC) systems on diseases’ transmission risk while travelling with open windows has shown to be a good strategy to renew the indoor air. In order to estimate the COVID-2 airborne transmission by aerosols, a real urban bus was simulated by CFD. Twenty passengers (containing the driver) were included in the model with a typical inhalation–exhalation breathing cycle. The concentrations of air exhaled by ten of them were tracked during 30 min using Eulerian scalar tracer, and the concentrations inhaled by the twenty passengers were monitored. Then, the well-known Wells & Riley risk model was applied in order to estimate the cumulative inhaled viruses and the subsequent transmission risk.

Four scenarios were considered: HVAC off with closed windows (Case 1), HVAC on with closed windows and 100% of air recirculation (Case 2), HVAC on with closed windows and 75% of air recirculation (Case 3), and HVAC off and the bus moving at 20 km/h with some windows opened (Case 4). Results clearly showed that the motionless condition (Case 1) caused the highest transmission risk around the emitters with negligible risk far from them. On the contrary, the HVAC on reduced the maximum risk to only 6% (Case 2) and 3% (Case 3) of the risk estimated for Case 1. Finally, travelling with some open windows promotes a large air renewal, reducing almost completely the transmission risk.

自 COVID-2 大流行以来，城市公交车内的外部和内部气流和空气更新尤其重要。专注于室内气流估计的计算流体动力学 (CFD) 模拟对于在打开窗户旅行时使用供暖、通风和空调 (HVAC) 系统对疾病传播风险的影响尚无定论。更新室内空气的策略。为了估计气溶胶的 COVID-2 空气传播，使用 CFD 模拟了一辆真实的城市公交车。模型中包含 20 名乘客（包括驾驶员），具有典型的吸气-呼气呼吸循环。使用欧拉标量示踪剂在 30 分钟内跟踪其中 10 名乘客呼出的空气浓度，并监测 20 名乘客吸入的空气浓度。然后，

考虑了四种情况：关闭窗户的 HVAC 关闭（案例 1）、关闭窗户和 100% 空气再循环的 HVAC 开启（案例 2）、关闭窗户和 75% 空气再循环的 HVAC 开启（案例 3）和关闭 HVAC并且公共汽车以 20 公里/小时的速度行驶，并打开了一些窗户（案例 4）。结果清楚地表明，静止状态（案例 1）导致发射器周围的传播风险最高，而远离发射器的风险可以忽略不计。相反，HVAC 将最大风险降低到仅为案例 1 估计风险的 6%（案例 2）和 3%（案例 3）。最后，开着一些窗户旅行可以促进大量空气更新，几乎完全降低传播风险。