Effect of water microdroplet size on the removal of indoor particulate matter

Abstract

The effective removal of indoor particulate matter (PM) has received considerable attention due to the harmful effects of airborne PM on human health. Although various techniques have been introduced to remove indoor PM, no impeccable method has been established. In this study, we proposed the use of microscale water droplets produced by the ultrasonic module of a commercial household ultrasonic humidifier (UH) for effective PM removal. Water droplet size was regulated using micropinholes. The effect of water microdroplet size on the removal of indoor PM was experimentally investigated. Normalized PM concentration, PM removal efficiency and PM deposition constant were assessed by analyzing the measured temporal variations in PM concentration inside a test chamber. The PM deposition constants measured in this study were compared with those of other previous studies. Deposition events were quantitatively visualized by using a high-speed camera attached with a long-distance microscope. The increase in the deposition rate of surface area and the PM collection ratio were analyzed using digital image processing to examine the interactions between PM particles and water microdroplets with different droplet sizes. As a result, the removal efficiency of PM2.5 particles reaches up to 56.2% when water droplets of d = 2 μm are used. The PM10 removal efficiency is also increased by 73.9% within the initial 30 min. The deposition constant for the smallest water droplets of d = 2 μm is drastically increased. In addition, the increasing rate of the deposition surface area is exponentially increased, as the size of water droplets decreases. The small microscale water droplets generated by a commercial UH attached with a micropinhole are found to be effective for the removal of indoor PM.