Movement and control of evaporating droplets released from an open surface tank in the push–pull ventilation system
Push–pull ventilation systems are effective local ventilation methods to control airborne contaminants generated in industrial buildings, among which droplets are typical. In this paper, the numerical simulations of water droplets released from an open surface tank into the push–pull flow field are carried out and the effects of ambient relative humidity and the pull-flow velocity on the ventilation system performance are discussed based on the droplet evaporation and movement. It was found that the movement and evaporation of droplets were closely related to the push–pull flow mechanism and the droplet initial diameter. When the control effect was good due to the presence of air closure in the flow field (pull-flow velocity ranging from 1.5 m/s to 3.0 m/s), droplets were unlikely to move away from the closure and the evaporation of droplets smaller than 40 μm was obvious. Whereas when the control effect was poor (pull-flow velocity equaling 1.0 m/s), large droplets still moved around the tank surface but small droplets were subject to dispersing, and in such a case droplets smaller than 60 μm evaporated obviously. Moreover, the effect of ambient relative humidity (ranging from 0 to 80%) on controlling droplets was rather limited and no more than ±6%. In addition, the system could save airflow rate and energy consumption by reducing the pull-flow velocity which was excessive originally in ventilation design. Finally, the paper put forward a new index to evaluate the control effect from another standpoint based on whether the droplets did harm to the environment.