Movement And Control Of Evaporating Droplets Released From An Open Surface Tank In The Push-pull Ventilation System

Many industrial production processes, such as pickling, plating and metal smelting, employ open surface tanks that may contain harmful substances, of which the evaporation product, i.e. droplets, can enter into environment because of chemical mechanism and non-sealing. Pollutants have to be removed to achieve a sufficient level of comfort and safety in workplaces, otherwise the prolonged residence time of numerous gathering droplets indoor is a potential threat to the worker’s health, normal processes, manufacturing equipment and buildings envelope structures.Push-pull ventilation systems have been widely applied in industrial processes that require the control of harmful aerosol particles from a large open surface. The system has the noteworthy advantages of small demand for airflow rates, satisfying control effect, strong anti-jamming capability and ease of operation. However, it is interesting to find that in the push-pull ventilation researches contaminants are almost confined to gaseous and solid particles, rarely involving droplets. Droplet evaporation makes the mass and the forces acting on the droplet varying constantly, so the movement and control of droplets are certainly different from neither gas nor solid. In this paper, the numerical simulations of water droplets released from an open surface tank into the push-pull ventilation were carried out and the evaporation and movement and the control effect of droplets in push-pull flow filed were discussed.Based on the push-pull system parameters obtained following the flow rate ratio method, the effects of both pull-flow velocity and the distance between the liquid surface and tank upper surface on the push-pull flow physics were evaluated. It was showed that the control effect of flow field could be divided into either good(with pull-flow velocity ranging from 1.5m/s to 3.0m/s) or bad(with pull-flow velocity equaling to 1.0m/s) lying in the fact that whether pull-flow velocity was able to form closed control region. In practice, appropriately increasing the distance between the liquid surface and tank upper surface would help to improve control effect of the push-pull ventilation.Discrete phase model was employed to track droplets injected from the open surface tank into the calculated push-pull flow field, and the evaporation and movement of droplets were discussed. It was showed that when the control effect of flow field was good, droplets were unlikely to move away from the tank and droplets with initial diameter less than 40μm evaporated obviously. While when the control effect was bad, small droplets were subject to dispersing into the environment, and in such a case the evaporation of droplets with initial diameter less than 60μm could not be neglected.The influences of volatile component fraction, ambient relative humidity, pull-flow velocity and initial diameter on the control effect of droplets were analyzed by numerical simulation. It was showed for small droplets(1μm~60μm), evaporation could improve the control effect, while for large droplets(80μm~100μm), evaporation or not made little difference on capture efficiency. Moreover, the change of ambient relative humidity(0, 30%, 50%, 80%) made little difference on control effect of droplets, less than ±6%. Besides, the calculated pull-flow velocity was sufficient and appropriately reducing exhaust airflow rate could ensure a relatively high capture efficiency and meanwhile the system energy cost cut down significantly. Furthermore, when pull-flow velocity was 1.0m/s~1.5m/s, with initial diameter increasing, capture efficiency raised firstly and then decreased, whereas when pull-flow velocity was 2.0 m/s~3.0m/s, with initial diameter increasing, capture efficiency kept unchanged firstly and then decreased.Based on droplet capture efficiency, a new evaluation index, total capture efficiency, was proposed, which considered whether droplets dispersed into ambient environment. Increasing pull-flow velocity and initial diameter could both make the dispersing number raising. Besides differences between total capture efficiency and traditional capture efficiency became larger with either droplet initial diameter or pull-flow velocity increasing. The choice of the two evaluation indexes in practice depends on the research focus of ventilation systems.