| There are many mixed flow systems in nature and industrial production activities,which mainly include discrete phase and continuous phase according to the corresponding volume proportion.Compared with continuous phase,discrete phase charging plays a more important role in the aerosol measurement and purification.In this thesis,an instrument prototype is first proposed to measure aerosol particle morphology,mass and surface area concentrations in real-time on the basis of diffusion charging of discrete phase.This developed prototype is characterized by lost-cost,the portability and 1s time resolution measurement,which could fill in the deficiencies of current measuring instruments.Secondly,the effects of discrete phase charging conditions,industrial charging parameters,aerosol flow models and properties on aerosol purification are investigated based on the corona charging of fine particles and induction charging of large droplets.This part of study is aimed at expatiating the importance of discrete phase charging to the lost-cost modification and aerosol purification of conventional wet scrubbers.First of all,different experimental setups were adopted to study the unipolar diffusion charging without electric field and corona charging with electric field of of fine particles,induction charging of micron droplets,then experimental results were compared with corresponding classical charging theories.It is found that in the absence of external electric field,the influence of particle morphology on unipolar diffusion charging is important.The looser the particle morphology is,the smaller the fractal dimension is,corresponding to the higher particle charges acquired in corona charger.At the same mobility diameter,both surface area and diffusion charging of point-to-point agglomerates are affected by primary particle size to some extent,and this effect could be enhanced with increasing mobility diameter.When mobility diameter is larger than 100 nm,the agglomerate composed of large primary particles could acquire more diffusion charges at the same mobility diameter.Under the influence of external electric field,black carbon could acquire more charges than fly-ash particles based on the combined effect of diffusion charging and field charging.At the condition of the induction charging,the key factors of micron-droplet charging are operational parameters,such as induction voltage and spray flow-rate.Corresponding experimental results are in good agreement with the semi-empirical theoretical model.Based on the unipolar diffusion charging principle without external electric field,an instrument prototype that is composed of a jet-charger,two groups of ion trappers and electrometers was proposed to achieve the aerosol real-timemeasurement.Using this instrument prototype,two groups of instrument sensitivites at different trap voltages could be first acquired,then the combined instrument sensitivity could be fitted by the linear weighted-sum method.After that,the combined sensitivity was correlated with the measurement objectives in this study to establish the proposed method.After the calibration of monodisperse particles,the proposed instrument was validated by polydisperse particles and simultaneously compared with SMPS.The results show that the average data difference between developed instrument and SMPS is lower than 8%,which proves the precision of developed instrument for the measurement of spherical particles.On the basis of classifying particle morphology,this proposed instrument could also measure the surface area concentration,mass/volume concentration of non-spherical particles in different fractal dimensions.After the revision of raw SMPS data,the average difference between two instruments is lower than 10% for volume/mass concentration measurement,and 30% for surface area concentration measurement.Compared with SMPS,the best advantages of the developed instrument prototype are cost-effective,portable and 1s time resolution.Based on the induction charging of large droplets,effects of different parameters on aerosol purification efficiency were first analyzed using the dimensionless model,and the basic characteristics of wet electrostatic scrubbing in the creeping flow were studied experimentally.Results show that the droplet charging has positive influences on the collection efficiencies of both oil particles and salt particles,but the effect on oil particles is more significant.At the experimental conditions,the minimum collection efficiency of oil particles could be improved from 38% to 82%,meanwhile the operational stability of wet scrubbing could be enhanced.In contrast to spherical particles,the deposition rate of agglomerated particles on droplet surface is higher,but the addition of electrostatic field could weaken the gap of collection efficiency between agglomerates and spherical particles.Compared with large particles,the effect of electrostatic field on the collection efficiency of PM2.5 is more obvious,but it is could be weakened at very low particle concentration.Therefore,the biggest assistance of electrostatic field to improve particle collection efficiency occurs the appropriate particle concentration range.The above-mentioned experimental results were found to be in good agreement with the theoretical model in both nano-scale and micron-scale.Based on the corona charging of solid particles and the induction charging of large droplets,the effects of charging conditions,industrial parameters and gas temperature on collection efficiency in turbulent flow were studied experimentally.After that,the wet electrostatic scrubbing was compared with the wet electrostatic precipitator which is dominated by the corona field.Results show that the collection efficiency of black carbon in conventional wet scrubber is lower than that of fly ash,which suggests the particle wettability is more important than particle morphology.However,the efficiency increment of black carbon is significantly higher than that of fly ash under the effect of discrete phase charging.At the condition of droplet induction charging,the biggest efficiency increment of black carbon is 60% while that of fly ash is 40%.In contrast to droplet induction charging,the role of particle corona charging in improving collection efficiency is relatively weak.In addition,the gas temperature has a little influence on the particle deposition on the surface of droplets,but it is a minor factor compared with discrete phase charging.Except that,the comparison with other purification methods indicates that the dust abatement method mainly dominated by large charged droplets could not only improve the particle collection efficiency,but also significantly reduce the water consumption.At the electrostatic field produced by both particle charging and droplet charging,the particle trajectory,collision efficiency and deposition characteritics near a single charged droplet were studied a using numerical technique coupled with udf program.Results show that the turbulence diffusion makes the migration trajectory of small particles diverge,but it could not change the particle motion direction against the mainstream direction.Under the action of Coulomb force,particles could deposit on the backside of the charged droplet,but it was weakened with the increase of the initial position of particles.In the steady flow of boundary layer,Coulomb force has the significant effect on the deposition efficiency of small particles.However,the inertial impaction has the crucial effect on the collision event of particle-droplet in the separated boundary layer flow,which weakens the influence of electrostatic field on particle migration,collision and deposition to some extent.Combining the above-mentioned simulation with dimension-less model,a composite model was proposed to predict the collection efficiency under swarm droplets in turbulent flow.Finally,this composite model was validated by experimental results,which is helpful to the design and application of wet scrubber enhanced with electrostatic field. |
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