Chao C Y H, Wan M P
Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong, China.
Indoor Air. 2006 Aug;16(4):296-312. doi: 10.1111/j.1600-0668.2006.00426.x.
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.
在一个具有两种不同理想化气流模式（顶部回风模式和单向向下模式）的封闭空间中，对呼气气溶胶的扩散特性进行了研究。采用了一个能够捕捉气溶胶多分散性和蒸发特性的多相数值模型。在具有向下气流模式的实验室内，采用光学技术进行实验，以测量气溶胶的扩散情况。将部分数值结果与室内测量结果进行了比较，发现二者吻合较好。小粒径气溶胶（初始粒径≤45微米）在向下气流中的沉降时间低于20秒，但在顶部回风气流中增加到32 - 80秒。在向下气流中，横向扩散限制在约0.3米左右，其中只有湍流扩散较为显著。在顶部回风气流中，横向扩散增加到超过2米，该气流模式同时具有湍流扩散和整体气流传输机制。整体气流传输气溶胶的作用比湍流扩散强约一个数量级。然而，结果也表明，如果气溶胶悬浮时间更长，仅靠湍流就可以使其扩散相当远的距离。大粒径气溶胶由于重力作用在很短时间内就沉降了。这些结果为医院病房环境中合适床位间距的设计提供了新的见解。本研究表明，在通风良好的室内环境中，整体气流传输是呼气气溶胶的主要扩散机制。湍流扩散比整体气流传输作用小约一个数量级，但通过足够长的沉降时间，湍流扩散也能使气溶胶扩散相当远的距离。结果表明，通过控制室内环境中的通风气流模式，可以控制呼气气溶胶的扩散。
