| Microbial contamination is one of the main pollution sources which affect the indoor air quality, while the human respiratory aerosols is an important source of microbial contamination. The hospital general ward is a relatively enclosed special area, where there is a big risk of cross infection among the indoor occupants. Therefore it is of great practical significance to study efficient ventilation and air conditioning system to control the spread of droplet contamination in the ward. Based on the the project “Rasic besearch on the influence of thermal and humidity environment on human health and comfort in medical buildings”(No.51278506), supported by National Natural Science Foundation of China(NSFC), this paper researches the evaporation and diffusion law of human droplet under the circumstance of different air distributions. And attention should be focused on the concentration distribution characteristics of droplet contamination in the activity area of indoor occupants, which provides the basis for the risk assessment of cross infection among the indoor occupants.Firstly, we conduct a spot investigation into the general ward indoor environment of three grade A tertiary hospitals in Chongqing, meanwhile refer to the relevant provisions of the domestic and international standards on the hospital ward, providing the realistic basis for the physical model construction of numerical simulation. Rased on the literature review on the characteristics of human droplet, a series of characteristic parameters of numerical simulation are setted theoretically.Then this paper establishes a mathematical model of droplet evaporation and diffusion in the general ward, which is verified by the experimental results of the existing literature. Different droplet pollution source state of patients and medical staff in double general ward are improved as follows: the two patients lying simulate the process of breathing accompanied by cough, without considering the impact of breathing, they cough 4 times in the former 60 s and each time lasts for 1s. The other two medical staff standing only consider the process of breathing. The air velocity generated by breath is described by a sine function. The droplet contamination generated by breath is defined as a steady pollution source and the droplet velocity is 0.2m/s, diameter is simplified as a uniform distribution of 5.0×10-7m. The air velocity generated by cough is described by a pulse function. The droplet contamination generated by cough is defined as an unsteady pollution source and the droplet velocity is also described by a pulse function, diameter is assumed as an inhomogeneous distribution, which is defined by the distribution function of Rosin-Rammler.Finally, the primary research is emphasized on the evaporation and diffusion law of human droplet under the condition of three typical air distributions, namely: ceiling-based-mixing type ventilation system, wall-attached-jet type ventilation system and recommended downward type ventilation system. On the basis of the concentration distribution characteristics of human droplet, we make a preliminary evaluation of the risk probability of cross infection among the indoor occupants. Through contrasting the evaluation index of droplet pollution control, we find out that under the circumstance of recommended downward type ventilation system, because of the closer distance between air inlet and pollution source, the pathline of droplet source is the shorter and the droplet range of motion diffusion is mainly concentrated in the vicinity of hospital beds. The total average unsteady ventilation efficiency is 1.4, which indicates that the recommended downward type ventilation system can remove more droplet contamination per unit time and has the strongest ability to remove indoor droplet contamination. Thus the risk of cross infection among the indoor occupants is the lowest. |
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