Research On Evaporation And Movement Of Droplet Populations And The Control Methods

In industrial buildings, if droplets which are generated in the production process are not controlled effectively, they will disperse into the working environment, and do great harm to the workers, the environment and construction.Local ventilation system is the primary method for controlling droplets in the production process. It is necessary to pay more attention to both the droplets characteristics and new evaluation indexes for further improving the control effect and reducing energy consumption of the ventilation system on the basis of the presnent theories and methods. Therefore, researches on the control effect of the droplets, which are produced in industrial buildings in local ventilation system from the standpoint of droplets movement, are significant guidelines to make the designs of ventilation system reasonable, efficient and refined.Evaporation of droplets changes the acted forces dynamically and it plays an important role in droplet movement. The droplets evaporation is complex and influenced by many factors, and finding out the main factors is significant to effectively analyze the droplets movement. In this paper, a sensitivity analysis was carried out related to the evaporation rate of droplets in industrial environment. It was found that the main factors that affected the evaporation rate of a single droplet were the droplet temperature, water vapor partial pressure in the surrounding airflow and the droplet diameter. The finding provided a guidance for the choices of the factors that affected the evaporation and movement of droplets in industrial buildings for later study.In this paper, researches were carried out to study the evaporation and movement characteristics of droplets, analysed the change of the number, diameter and velocity of the droplet populations which are emitted from an open surface tank with uniform initial diameter at the same time, and finally obtained the distribution law of the diameter and velocity of droplet populations with time under different initial diameters. The results showed that the diameter and velocity of droplet populations presents obviously nonuniform distributions although they were emitted with identical initial diameter and velocity. Moreover, comparisons of the influences of both initial diameter and the ambient relative humidity on the evaporation and movement of droplet populations were made as well.The control effect of the upper suction hood on the droplets was performed. The influences of the initial diameter and the exhaust velocity on the number change of droplets emitted singly and continuously were researched, respectively. Based on evaluation of control effect on the dynamic particle number, the ratio of particle number was proposed to evaluate the control effect of ventilation system on droplets.In order to limit the control region and enhance the control effect of the exhaust hood, the flow field physics of the air curtain exhaust hood were experimentally studied, and the conditions for the jet to form air curtain were acquired. The study also obtained the axial velocity characteristics and distributions of the exhaust flow when the jet can form air curtain and the control effect of droplet populations. The results showed that the control effect of droplets could be significantly improved by placing a guard plate between the supply air inlet and the exhaust hood.The physics of flow field formed by the circular exhaust hood in non constrained condition was studied when the angle between the jet and the exhaust flow, θ, changed. The study attained the ranges of θ when stable air curtain could be formed, and then quantitatively analyzed the decay law of the axial velocity and the control range induced by the exhaust flow, finally obtained the corresponding fitting formula. Meanwhile, the control law of droplet populations in the flow field combined the jet with the exhaust flow was attained.