- 學位論文
微粒在纖維性濾材上再揚起之特性研究
Characteristics of Aerosol Resuspension from Fibrous Filters
摘要
2019年底爆發嚴重呼吸道傳染性肺炎(COVID-19)，空氣中可能存在有害的生物性氣膠，並藉由呼吸附著在口罩表面。而口罩表面的微粒是否會受到佩戴者咳嗽或打噴嚏的影響再揚起？因此本研究目的為探討影響微粒在纖維性濾材上再揚起的各項因素。 利用超音波霧化器產生多粒徑分布的氯化鈉及單一粒徑的壓克力球型微粒，經過電中和後導入到混合腔中，並控制混合腔的相對濕度在10%、50%及75%，以表面風速5、10、20 cm/s進行負載，以負載時間、濾材收集效率及微粒濃度計算負載量。當微粒負載完成後，套入一直管連接濾材及量測儀器，通入與負載方向相反之氣流，表面風速設定1-6 m/s，以氣動粒徑分析儀(APS)量測微粒負載及逸散之粒徑分布和數目，計算各粒徑微粒再揚起的比例。 以固定表面風速進行反吹時，大部分的微粒會在前10秒被吹出，當反吹的風速越快，再揚起的數目越多，並隨著粒徑越大，與再揚起的比例非線性增加，原因是微粒再揚起受到空氣氣流的拖曳阻力影響，與粒徑的平方相關。將5 µm的壓克力微粒分散於氯化鈉及油滴，發現氯化鈉乾燥後會在壓克力表面形成不規則的結晶，改變微粒的形狀、增加受力面積，使微粒更容易被帶走。0.9%的油滴可以使5 µm的壓克力微粒在6 m/s的反吹速度，再揚起比例從6%降到0.01%。增加負載的相對濕度也會抑制微粒再揚起，是由於油滴及水分子皆會增加微粒與纖維的附著力。靜電吸引可以顯著地抑制在N95口罩內沉積微粒的再揚起現象。 本研究中發現不論微粒特性，再揚起的比例都會受到相對濕度增加而受到抑制，且粒徑越小需要更大的啟動風速。而人體呼出的微粒幾乎不可能乾燥，因此認為沉積在濾材上的微粒不容易在一般使用情況下再揚起。
並列摘要
This study was motivated by curiosity about if deposited particles in a fibrous filter would be aerosolized again by sneezing or coughing. In the present study, we characterized the factors affecting aerosol resuspension from the N95 and fabric masks including face velocity, environmental conditions, particle type, and filter type. A variety of particles, including sodium chloride, di-ethyl-hexyl-sebacate (DEHS), and acrylic microspheres, were generated in the present study using an ultrasonic atomizing nozzle. The aerosol output was then introduced into the conditioning section before deposition in the filters. The relative humidity of the conditioning section varied from 10 to 75%. The filtration velocity was 5, 10, 20 cm/sec. The dried and filtered air was used to blow off the deposited particles. The blowing-off velocity ranged from 1 to 6 m/sec. An aerodynamic particle sizer (APS) was used to monitor the number concentrations and size distributions of the generated aerosols in the test chamber and the re-entrained aerosols in the vent duct. The load mass on the FFRs was estimated using the filtration efficiency data, the aerosol concentration in the test chamber, the sampling flow, and the sampling time. The N95 FFR and fibrous masks used for the resuspension experiment showed that most of the particles were resuspended within 10 seconds if there was enough blow-off face velocity. When the particles were dried, the resuspension increased with increasing load mass, particle size, and rough surface property of particles, apparently due to the change of adhering force on the particle-fiber. The resuspension also increased with decreasing relative humidity. The RH effect was particularly significant if the test particle had a unique hysteresis property. The RH also affected the resuspension of acrylic microspheres, likely due to a certain amount of water vapor condensed on the particles. It took only a tiny amount of DEHS in the acrylic microsphere's suspension to eliminate all resuspension. The filtration layer of N95 could decrease particles resuspension by attracting particles on the first layer, which was a most of particles resuspended from. The resuspension of exhaled droplets can only occur for the particles be totally dried. For particles that are not totally dried or with mucus residue coating, the particle reentrainment is nearly impossible under human breathing condition.
參考文獻
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