Study On Collision Dynamics Of Irregular Aggregate In The Transition Regime

Nanoparticle synthesis via gas combustion relates to complex particle dynamic mechanisms including oxidation,nucleation,condensation,collision and sintering.Among these,particle collision events occur frequently due to a quite high particle number concentration.The particles that collide will irreversibly stick together to form inhomogeneous nanoaggregates rather than sphere particle,once their surface potential energy is overcome.The combustion-induced nanoparticles are mainly located in the transition regime,which is between the continnum regime and free molecular regime.The mobility of aggregate and particle collision dynamic have a great significance on the particle transport property(diffusion coefficient)and dynamic parameters(collision kernel,etc.)and it is still lacks mature theory to describe the collision dynamic of aggregate in the transition regime.In this paper,a fundermental study on the aggregate collision dynamics is carried out,mainly foucing on the study of gas-aggregate and aggregate-aggregate collision dynamics.The orientionally averaged patcile mobility is calculated with different aggregate morphology parameter and Kn number,and differentially weighted Monte Carlo method for particle collision,that is applicable for spherical and aggregate particle,is proposed.The main works in this paper include:(1)Comparison of four commonly used statistical models for gas molecular collision and parallel computing of the the direct simulation Monte Carlo method(DSMC)is achived.The two-dimensional Rayeligh flow and Poisellue flow are simulated,and the NTC(no-time-counter)method,TC(time-counter)method,RSF(randomly sampled frequency)method and modified Nanbu method is compared in terms of computational accuracy and cost,showing that the NTC method is more effective.Besides,GPU(graphic processing unit)-based parallel computing of direct simulation Monte Carlo(DSMC)is developed to improve the computation efficiency.An improved data transimission in multi-GPU parallel computing is devoted to promote parallel efficiency.A two-dimensional Couette flow and a lid-driven cavity flow are simulated by CPU,single GPU and double GPU parallel computing,respectively,in which the computational recision and cost are compared quantitatively.The results shows that the speedup ratio by double GPU acceleration is twice of that by single GPU acceleration and the speedup efficiency by multi-GPU is approximated to 100%.The DSMC method with high efficiency and precision provides a basic method for flow simulation on the study of the gas-aggregate particle collision dynamics.(2)Numerical simulation of irregular aggregate with different structural parameters is realized.Simulation of particle dynamic growth including ballistic aggregation model,diffusion limited aggregation model and reaction limited aggregation model by Monte Carlo(MC)method are realized to generate inhomogeneous aggregates,and the dynamic change of particle growth with diffusion limited cluster-cluster aggregation(DLCA)model is analyzed.The fractal dimension of aggregates is calculated by the gyration-radius method and then compared with that obtained through combustion-induced Ti O2 nanoparticles experiment qualitatively to study the dominant growth mechanism at different typical height above the burner.In order to generate aggregate particle with specific fractal dimension and primary particle number,the sequential algorithm(SA)is used to simulate three-dimensional aggregate with arbitrary specified fractal dimension and prefactors,which provides aggregate particle structure conforming to the actual experiment for the study of gas-aggregate particle collision dynamic.(3)Numerical calculation of aggregate particle mobility radius in the transition regime is realized.The mobility radius of irregular aggregate is a basic parameter of the particle collision kernel model and cannot be obtained by theoretical analysis.The three-dimensional aggregate mobility radius is obtained through gas-aggregate particle collision study,where the DSMC method is used to simulate the gas flow and the SA algorithm is used to generate three-dimensional irregular aggregate particle.Firstly,a numerical simulation of gas flow around over a three-dimensional sphere is carried out to study the influence of far-field inflow boundary conditions on the statistical results and accuracy.And then,the aggregate orientionally averaged mobility radius is calculated with Kn=1~10,primary particle number N=10~30,prefactor kf=1.1~1.5 and fractal dimension df=1.5~2.2.The results are fitted by the Levenberg-Marquardt iteration algorithm to gain the relationship among the mobility radius,gyration radius and primary particle number.At the same time,related aggregate hydrodynamic parameter such as adjusted sphere radius Radj,hydrodynamic radius RH and dynamic shape factor ? are studied.(4)The study of particle-particle collision dynamic by differentially weighted Monte Carlo method.The DSMC method is used to simulate particle collision and a probabilistic collision rule for particle collision with different number weights is developed.At the same time,two differentially weighted DSMC(DWDSMC)method are proposed to ensure computational efficiency and accuracy.One method is the split-restoration scheme(SRS),which could conserve the total mass,momemtum or energy.A coarse particle flow is simulated to validate the SRS method and compared with results by Time Calculation(TC)and equally weighted DSMC(EWDSMC)method.The results show that the SRS method can be used for particle collision with different number weights,however with statistical uncertainty.Therefore,a conservative particle weighting(CPW)scheme that could linearly conserve the total mass,momentum and energy is proposed and also validated with the coarse particle flow.Furthermore,the resultant LB-CA-CPW model,where the lattice Boltzmann-cellular automata probabilistic(LB-CA)model is used to describe gas-particle flows and the CPW scheme is used to deal with differentially weighted particle collision,is proposed for more complicated gas-solid flow simulation with the consideration of four-way coupling and used to simulate the classical back-ward facing step flow.The results show that the CPW scheme can not only be used for paticle collision in gas-solid flow,but also improve the statistical particle information in size spectrums with smaller particle number density.