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Heat Transfer Research
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Heat Transfer Research

DOI: 10.1615/HeatTransRes.2018025255
pages 217-231

WATER VAPOR CONDENSATION ON THE INNER SURFACE OF AN N95 FILTERING FACEPIECE RESPIRATOR

Yu Rao
School of Power and Mechanical Engineering, Wuhan University, Wuhan, China 430072
Hui Li
School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China; The Institute of Technological Sciences, Wuhan University, Wuhan 430072, China
Shengnan Shen
School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China; The Institute of Technological Sciences, Wuhan University, Wuhan 430072, China
Quan Yang
School of Power and Mechanical Engineering, Wuhan University, Wuhan, China 430072
Guoqing Zhang
School of Power and Mechanical Engineering, Wuhan University, Wuhan, China 430072
Xiaotie Zhang
Key Laboratory of Healthy and Intelligent Kitchen System Integration of Zhejiang Province, China 315336; Ningbo Fotile Kitchen Ware Company, Zhejiang Ningbo, China 315336
Mengfei Li
School of Power and Mechanical Engineering, Wuhan University, Wuhan, China 430072
Shuaichen Duan
School of Power and Mechanical Engineering, Wuhan University, Wuhan, China 430072

ABSTRACT

N95 filtering facepiece respirators (FFRs) are commonly used in China to reduce the inhalation of hazardous air pollutants, especially in industrial cities with elevated PM2.5 concentrations. N95 FFRs block at least 95% of particles, but the breath increases the relative humidity in the dead space, which can make the wearer uncomfortable. Furthermore, water vapor condensation, combined with an appropriate temperature and humidity inside the respirator, creates a favorable environment for bacterial growth and reproduction, which presents another health hazard. However, direct and real-time observation of water inside the dead space of an FFR is difficult, since water volatilizes quickly once the FFR is removed from the wearer's head. The objective of this research is to study the distribution characteristics of water vapor condensation on the inner surface of an FFR under different breathing conditions, including different environmental temperatures and breathing patterns. We used the computational fluid dynamics (CFD) method to simulate the flow-field in the upper respiratory system of a user wearing an N95 FFR. We analyzed the distribution of temperature, water vapor, and liquid water volume fraction on the inner surface of the FFR at various environmental temperatures. We also noted the variations of these factors when a sudden drop in environmental temperature occurred. Different breathing rates and frequencies were also investigated. When the environmental temperatures were 310.15 K, 296.6 K, and 273.15 K, the temperatures inside the FFR were 309.13 K, 302.55 K, and 291.03 K, respectively. Sudden drops in the environmental temperature from 296.60 K to 273.15 K and from 310.15 K to 273.15 K can increase the volume fractions of liquid water by 20.03% and 21.07%, respectively. Compared to the typical breathing rate ν0(t), when the breathing rates increased to 2ν0(t) and 3ν0(t), the maximum temperatures increased from 301.72 K to 303.02 K and 303.54 K, and the maximum fractions of liquid water decreased by 12.98% and 22.33%, respectively. When the breathing frequency was changed, the variation of the maximum temperatures was less than 1 K, and the maximum fractions of liquid water decreased by 40.08% at half the normal frequency and increased by 83.97% at double the normal frequency. The presence of liquid water can aid bacteria reproduction, which may be a health hazard to the wearer.


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