A study of the dispersion of expiratory aerosols in unidirectional downward and ceiling-return type airflows using a multiphase approach.

Dispersion characteristics of expiratory aerosols were investigated in an enclosure with two different idealized airflow patterns: the ceiling-return and the unidirectional downward. A multiphase numerical model, which was able to capture the polydispersity and evaporation features of the aerosols, was adopted. Experiments employing optical techniques were conducted in a chamber with downward airflow pattern to measure the dispersion of aerosols. Some of the numerical results were compared with the chamber measurement results. Reasonable agreement was found. Small aerosols (initial size <or=45 microm) had settling times of below 20 s in downward flow but increased to 32-80 s in ceiling-return flow. Lateral dispersion was limited to around 0.3 m in downward flow, in which only turbulent dispersion was significant. It increased to over 2 m in ceiling-return flow, which had a combination of both turbulant dispersion and bulk flow transport mechanisms. The significance of aerosol transport by bulk flow was about an order of magnitude stronger than that by turbulent dispersion. However, results also show that aerosols could be dispersed for considerable distances solely by turbulence if they were suspended longer. Large aerosols settled within very short time due to heavy gravitational effects. The results provided new insights in designing proper bed spacing in hospital ward environments. This study shows that transport by bulk airflow stream was the major dispersion mechanism of expiratory aerosols in ventilated indoor environments. Dispersion by turbulence was about an order of magnitude less than that by bulk flow transport but considerable distance could be achieved by turbulent dispersion with long enough settling time. It was demonstrated that the dispersion of expiratory aerosol could be controlled by manipulating the ventilation airflow patterns in indoor environments.