Numerical Study On The Dynamical Behaviors And Evolution Of Micron-nano-sized Particles

Environmental pollution is always one of the most concerned and to be solved issues during the development of the society,especially the rapidly deteriorating air quality in our main cities has been drawn more and more attentions on its impact on the environment and health of the people.The leading component in the air pollutant is the particle matters?PM?,especially PM2.5?particles with aerodynamic diameter is less than 2.5?m?,which plays the most important role in the air pollution.These particles have very small particle size,and contain many toxic and harmful components,which could suspend in the air for a long time and transmit for a long distance.Therefore,these particles have great impact on the air visibility,haze formation and the health of the people,and are the result of the interaction between certain climatic conditions and human activities.There exist several types of complicated dynamical behaviors of the fine and ultrafine particles in the atmospheric environment,including coagulation,breakage,condensation,evaporation,nucleation,deposition and surface chemical reaction.These behaviors dominate the particle size,concentration,spatial distribution and even chemical components,and as a result,decide the evolution law of the particles in time and space.Also,these dynamical characteristics are inhomogeneous in space and unsteady in time.This makes it difficult to simulate numerically and describe quantitatively,and has to solve the governing equation of the dynamical behaviors.This paper is exactly focused on how to describe the dynamical behaivors of particles more accurately,as well as to couple with flow field to simulate the particles evolution in the air flow.The main contents and achieved results are as follows:?1?The dynamical model of the deposition of the micron-nano sized particles are deduced and established by the Taylor-series method of moment considering both the effects of dispersion and gravitation.The linear partial differential equations are transformed to the form that could be solved by numerical method.The acquired moment equations are solved by the fourth Runge-Kuta method.The evolution of the related physical variables are acquired including particle number concentration,mean particle size,and degree of dispersion.The particle size range covers from free molecular regime to continumm regime?0.01?m⁴?m?.The numerical results are compared with the experimental data to validate the accuracy of the dynamical model.In addition,compared with the traditional method of moment,for there is no need to assume the form of the initial particle size distribution,the accuracy of the results is improved,as well as the expansibility of the method;?2?The effect of actual shape of the particle in the free molecular regime on the coagulation process is considered by introducing the fractal dimension D_f,and the mathematical equations solving the coagulation behavior are optimized.The moment equations are acquired under the non-spherical coagulation mode.Two particle sizes of 50 nm and 473 nm are selected in the calculation.The comparison results of the experimental data and the simulation results under spherical mode indicate that the optimized model could give a better result on simulating the change of number concentration with time,and the accuracy is improved greatly.Meanwhile,the relationship between the particle size,geometric size,and the fractal dimension is also discussed.The effect of D_f on the coagulation behavior is more obvious when the initial particle number concentration is higher or the initial particle size is smaller.In addition,the evolution of particles under the effects of both coagulation and deposition are also simulated and analyzed.Under the high particle concentration condition(initial concentration is more than 5×10¹⁰/m³),the evolution of particles is mainly affected by the coagulation,and the deposition is relative weak.This regularity appears more obvious when the initial number concentration is much higher;?3?The mathematical model of deposition coupled with three dimensional flow field is established by applying the Taylor-series method of moment.The RNG k^?turbulent model is selected to simulate the airflow field.Two sizes of101.8 nm and 10?m representing fine and coarse particles are selected in the calculation.The evolution processes of particle number concentration,mean particle size,and particle size distribution in an indoor ventilated environment are acquired.The simulated results are compared with that under experimental data and traditional method of moment,and it shows that the agreement presents much better.The simulation accuracy is improved with the same calculation cost.Besides,the evolution difference of particles under different inlet velocities is analyzed.When the inlet velocity is larger,the physical variables are more quickly to achieve the stable condition.The concentration distribution shows more obvious inhomogeneity due to the turbulent accumulation effect;?4?In terms of tailpipe emission process of fueled vehicles,the Taylor-series method of moment is expanded to solve the dynamics equations describing deposition,coagulation,condensation,and nucleation coupled with air flow field,and to establish the related mathematical model.The process of particles with initial size of 50 nm emitted from the tailpipe of the fueled vehicle is numerically simulated.By comparing with the experimental data,the applicability and the reliability of the model are validated.The results show that the accuracy of the mathematical model could be improved greatly by considering the real structure of the particles.In addition,the effects of different initial conditions including initial particle number concentation,initial particle mean diameter,environment temperature,relative humidity,fuel sulfur content and exhaust velocity on the particle number concentration and geometric mean diameter are analyzed and discussed.The conditions with higher initial concentration,larger initial diameter,higher environment temperature,lower relative humidity,lower sulfur content and larger exhaust velocity could make the particle number decrease more rapidly.Moreover,higher initial concentration and environment temperature condition could make the particle diameter get larger as well as increase more rapidly.