Figure 3 illustrated the details of boundary conditions in this case. The attached breathing zone in front of the cavity was set as zero-gauge pressure inlet to imitate the ambient environment. The outlet of the airway was set as velocity outlet by dividing the physiological volumetric flow rate with the area of the outlet. International Commission on Radiological Protection (ICRP) publication 66 suggests a tidal volume of 0.244 L with a frequency of 39 per minute for children subjects under light exercise condition. Therefore, the equivalent volumetric flow rate of 9.5 litre per minute (LPM) was employed for light exercise condition. Respiratory conditions are determined by human exercise and physical activity. To consider different physiological conditions, numerical simulations under three levels of inhalation flow rates, representing, respectively, resting (3.1 LPM), light exercise (9.5 LPM), heavy exercise (18.9 LPM) circumstances were performed in this study. For the nasal surface as well as surrounding face, it was assumed to be no-slip, stationary and perfect absorbed when predict particle transport.
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