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Dispersion of expiratory airborne droplets in a model single patient hospital recovery room with stratified ventilation Aliabadi, Amir Abbas


Concerns about the environment, energy costs, and airborne infection risk have revived interest in ventilation systems for health care facilities. Low energy ventilation systems (e.g. stratified air ventilation) have received attention as a means of providing a better air quality at a lower energy cost. The sensitivity of such ventilation systems to boundary conditions in removing airborne contaminants produced by expiratory injections is of concern and studied experimentally and numerically in this work. A three step methodology is adopted. First, an air-assist internally mixing atomizer is developed to generate a poly-disperse distribution of droplets for ventilation testing. A series of near-field experiments reveal droplet size, velocity, and diffusivity in radial and axial directions for steady and transient atomization. Second, the atomizer is used to inject droplets into a mock-up of a patient recovery room with an underfloor air distribution ventilation system. A series of far-field size-resolved concentration measurements are conducted at locations representative of an occupant (receptor). Third, Computational Fluid Dynamics (CFD) simulations are used to predict airborne droplet exposure among various cases in the far-field experiments. Both tracer gas and discrete phase approaches are implemented. Based on the findings we recommend guidelines for ventilation design and room usage in real single patient hospital recovery rooms with stratified ventilation systems. It is desired to have expiratory injections at low momentum, preferably directed towards the walls or upwards. It is also advisable that occupant suspects spend most of their time away from the injection source, possibly at the corner of the room or behind the source. The variations in occupant thermal plume is not likely to affect exposure to airborne droplets in statistically significant ways. It is advisable to used air change rates greater than four since expiratory injections are likely to break down the vertical contaminant stratification. It is likely that dispersion rates be higher for sub micrometer droplets but lower for larger droplets. This has implications for ventilation design strategy as a function of pathogen or pathogen carrying droplet size.

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