Modeling and simulation of titanium dioxide nanoparticle formation and growth in turbulent reacting flows.

摘要

The formation and growth of nanoparticles is of great scientific, industrial, and environmental interest. Nanoparticles have unique properties. These properties are primarily due to their size and surface-area-to-volume ratios. As a result nanoparticles are often the building-blocks to nano-structured materials. One such material is titanium dioxide, or titania. Titania appears in many industrial applications such as pigments, photo-catalysts and opacifiers. In this work, titania formation and growth in turbulent reacting flows are investigated via direct numerical simulations (DNS). DNS is unique in that it has the ability to provide solutions to the governing equations in a model-free manner. That is without the use of "turbulence" models and other assumptions which may reduce the fidelity of the obtained solution(s) with real world physics. In this work, all simulations are obtained by solving the fluid, thermal, and chemical fields as a function of space and time. This is accomplished by solving the Navier-Stokes equations along with the species mass fraction evolution equations. A nodal approach, which divides the particle field into three classes - monomers, clusters and particles - is used to represent the formation and growth of the nanoparticles, namely, nucleation, condensation, coagulation, and coalescence. The nodal method is advantageous in that there are no a priori assumptions regarding the particle size distribution. Titania is formed using the titanium tetrachloride (TiCl4) precursor. TiCl4 can be either oxidized or hydrolyzed to prepare titania. The simulation results provide insight into the particle-particle interactions as well as fluid-particle interactions, under the influence of different physico-chemical factors. The effect of the initial reactant concentration level, Reynolds number, gas mixing and fractal dimension on particle formation and growth are also studied to better understand and control the nano-particle synthesis processes. Results reveal the utility of DNS in elucidating the underlying formation and growth processes as well as the influence on fluid turbulence on nanoparticle dynamics. For brevity, specific conclusions are provided in each chapter.