| A Monte Carlo (MC) method was developed to study the multicomponent aerosol dynamics in the Sodium/Halide Flame and Encapsulation (SFE) process. Moving bins were incorporated in the MC method to eliminate numerical dispersion. The method was validated against analytical solutions, for both single-component and two-component aerosols undergoing coagulation and condensation. The improved MC method was employed to study the SFE process in a homogenous system with prescribed heat loss. Modeling results demonstrated that in the encapsulation process a few particles grow large very quickly through condensation and then the large coated particles scavenge the small uncoated particles. The frozen solution and fully coalesced solution for the core particles were studied and the results show that the SFE process can produce nanoparticles with much narrower size distribution than traditional flame synthesis processes. It is also found that more scavengers were obtained with higher heat loss, which results in smaller aggregates or coalesced core particles. To study the SFE process in the case of a non-premixed flame, a flame code with detailed chemistry and transport was developed to simulate a spherical CF4/Na diffusion flame. A two-component sectional method was developed and integrated into the flame code to simulate the particle condensation and encapsulation processes. The results show that near the flame, the temperature is high such that the salt produced is salt vapor. Downstream of the reaction zone, there is a narrow region where condensation of salt onto carbon particles occurs and a few large, heavily coated particles are formed. A large number of small particles still left uncoated in this region. Further downstream, the particles evolve through a scavenging process where small particles are scavenged by large, heavily coated particles, as anticipated by the Monte Carlo simulation. The investigation also shows that thermophoresis is important for this process, especially at low pressures. The effect of sticking coefficient on NaF(l) is minimal, which indicates that condensation of NaF occurs relatively fast and the amount of NaF(l) is determined by the diffusion rate of NaF vapor into the condensation region. The effect of pressure were also studied. |
