Study On The Basic Structure And Characteristics Of Charged Droplets

Particulate air pollution is associated with adverse human health and economy development impacts, especially in China. Basically, the features of particulate pollutant in the atmosphere are complicated composition and regional distribution difference. Controlling particulate emission is one of the most radical ways to improve the air quality. The wet electrostatic scrubber(WES) is a type of efficient hybrid system for particle abatement(particularly in the removal of PM10 and PM2.5 particles), which combines advantages of inertial wet scrubbers and electrostatic precipitators. The WES has great application potential in the areas related to gas cleaning. However, due to the complex removal mechanisms which involving multi-phase and multi-field effects, the fundamental theories of WES are still needed to explore. In this work, the issues in WES such as particle motion and interaction between particles and a charged droplet were investigated in both experimental and numerical ways, which will provide theoretical basis for developing WES technology. The main points are as following:1. Visualization investigation on charged droplet capturing particles. A high-speed digital camera with the micro-amplification technology was used to record the process that the pendant charged droplet capturing particles. The effect laws of particles motion influenced by the key parametes such as forces and charge of the particles were obtained, which also provided the necessary input parameters for simulation studies.The impaction of particles onto the charged droplet was studied. Particles could bounce off the interface after impacting the droplet. It was found that near 50% of the particles bounced off the droplet surface when the impact angle was smaller than 85°. Inclined impact leads to the tangential velocity of the particle along the interface. The existence of tangential velocity could substantially reduce the adhesion. Moreover, the normal component impact velocity of the particle was also reduced due to the inclined impact, which means the wetting area of the particle was shrunken, the adhesion provide by the interface was declined more significantly. Therefore, the particles are more likely to rebound. The rebound particles were charged with the same polarity with the droplet, they escaped due to the Coulomb repulsion.The mechanisms of particle clusters captured by the charged droplet was analyzed. It was found that charged droplet was ineffective to capture particle clusters. Compared to single particles, particle clusters have a larger volume but a looser structure. The larger volume of particle clusters enables them to capture more free charges during their motion, which even offset their initial polarity and repulsion by the charged droplet. Most particle clusters impacting the droplet surface were disintegrated due to their loser structure. Some child particles bouncing off the interface forms the reentrainment. In addition, the attraction between the like charged particle clusters were observed. In other words, two particle clusters with different sizes agglomerated into a large cluster and then moving towards the charged droplet. This phenomenon was theoretically analyzed and the criterion of agglomeration was derived.2. The process of charged droplet capturing particles was simulated numerically. Based on the experimental data, an Euler-Lagrange method based model for capturing particles by the charged droplet was established. This model includes the dynamic behaviors of the particles: adhesion, rebound and submergence. Particles captured by a pendant charged droplet was firstly simulated and compared with the experimental observations to validate the availability of the established model. The process that particles impacting the charged collector under the influence of air flow were studied numerically. The capture efficiency of the charged droplet affected by the Stokes number(St) and the Coulomb number(Kc) was discussed. The parameter range for capturing particles efficiently by the charged droplet was presented.3. Visualization investigation on characteristics of particles deposited on a charged droplet. A home made device was used to accomplish the process that particles capturing by a free fall charged droplet. The droplet eventually spreaded on a glass slide and the deposition morphology of the particles was observed. The deposition characteristics affected by the falling heights and charge conditions were analyzed. The results showed that the equivalent number of particles captured by the charged droplet were larger than that of uncharged ones by one order of magnitude at least. Due to the capillary attraction, particles on the charged droplet agglomerated remarkably. The front side of the droplet was the dominant area to cputer particles, but the actual deposition region was smaller than hemisphere. The experiments also showed that the capture efficiency of the droplet was linear-proportional to its charge in the limited range of the Re number.4. Based on the leaky dielectric hypothesis and the VOF method, the model that a charged droplet evaporating under the high temperature field was established. The evaporating charicteristics of the droplet affected by the electric field and temperature field were predicted by analyzing the inner flow field. The results shoed that the turbulent flow inside the evaporating droplet under the high temperature field can be restrained by the electric field effectively. That means the flow velocity and the transfer of heat inside the droplet were reduced. Consequently, the life of the evaporating charged droplet was extended in the high temperature condition.