Applications of CFD Simulations on Studying the Multiphase Flow in Microfluidic Devices

Abstract

Microfluidics has been extensively investigated as a unique platform to synthesize nanoparticles with desired properties, e.g., size and morphology. Compared to the conventional batch reactors, wet-chemical synthesis using continuous flow microfluidics provides better control over addition of reagents, heat and mass transfer, and reproducibility. Recently, millifluidics has emerged as an alternative since it offers similar control as microfluidics. With its dimensions scaled up to millimeter size, millifluidics saves fabrication efforts and potentially paves the way for industrial applications. Good designs and manipulations of microfluidic and millifluidic devices rely on solid understanding of fluid dynamics. Fluid flow plays an important role in heat and mass transfer; thereby, it determines the quality of the synthesized nanoparticles. Computational fluid dynamics (CFD) simulations provide an effective approach to understand various effects on fluid flows without carrying out complicated experiments. The goal of this dissertation is to utilize CFD simulations to study flow behaviors inside microfluidic and millifluidic systems. Residence time distribution (RTD) analysis coupled with TEM characterization was applied to understand the effect of reagent flow rates on particle sizes distribution. Droplet-based microfluidics, as a solution to the intrinsic drawbacks associated with single-phase microfluidics, depends on proper manipulation of the flow to generate steady droplet flow with desired droplet / slug sizes. The droplet/slug formation process inside a millifluidic reactor was investigated by both experiments and numerical simulations to understand the hydrodynamics of slug breaking. Geometric optimization was carried out to analyze the dependency of slug sizes on geometric dimensions. Numerical simulations were also performed to quantify the mixing efficiency inside slugs with the aid of mixing efficiency index. In some circumstances, the droplet sizes are difficult to control via manipulating the flow rates. By applying external electric field to the conventional droplet-based microfluidic systems, the electric force induced on the fluid interface can reduce the droplet sizes effectively. This work provides insight to understand fluid flow inside microfluidic and millifluidic systems. It may benefit the design and operations of novel microfluidic and millifluidic systems.