Numerical investigation of airborne transmission in low-ceiling rooms under displacement ventilation

Abstract

This study employs computational fluid dynamics (CFD) simulations to evaluate the risk of airborne transmission of COVID-19 in low-ceiling rooms, such as elevator cabins, under mechanical displacement ventilation. The simulations take into account the effects of the human body’s thermal environment and respiratory jet dynamics on the transmission of pathogens. The results of the study are used to propose a potential mitigation strategy based on ventilation thermal control to reduce the risk of airborne transmission in these types of enclosed indoor spaces. Our findings demonstrate that as the ventilation rate (Qv) increases, the efficiency of removing airborne particles (εp) initially increases rapidly, reaches a plateau (εp,c) at a critical ventilation rate (Qc), and subsequently increases at a slower rate beyond Qc. The Qc for low-ceiling rooms is lower compared to high-ceiling rooms due to the increased interaction between the thermal plume generated by the occupants or infectors and the ventilation. Further analysis of the flow and temperature fields reveals that εp is closely linked to the thermal stratification fields, as characterized by the thermal interface height and temperature gradient. When Qv < Qc, hT,20.7 < him (him is the height of infector’s mouth) and aerosol particles are injected into the upper warm layer. As Qv increases, the hti also increases following the 3/5 law, which helps displace the particles out of the room, resulting in a rapid increase of εp. However, when Qv > Qc, hT,20.7 > him and aerosol particles are injected into the lower cool layer. The hti deviates from 3/5 law and increases at a much slower rate, causing an aerosol particle lockup effect and the εp to plateau. In addition, as the Qc increases, the local flow recirculation above the infector head is also enhanced, which leads to the trapping of more particles in that area, contributing to the slower increase in εp. The simulations also indicate that the location of infector relative to ventilation inlet/outlet affects Qc and εp,c with higher Qc and lower εp,c observed when infector is in a corner due to potential formation of a local hot spot of high infection risk when infector is near the ventilation inlet. In conclusion, based on the simulations, we propose a potential ventilation thermal control strategy, by adjusting the ventilation temperature, to reduce the risk of airborne transmission in low-ceiling rooms. Our findings indicate that the thermal environment plays a critical role in the transmission of airborne diseases in confined spaces.