Three-Dimensional Simulation of Droplet Motion in Air Channel

نوع: Type: Thesis

مقطع: Segment: masters

عنوان: Title: Three-Dimensional Simulation of Droplet Motion in Air Channel

ارائه دهنده: Provider: Vahid Asgari

اساتید راهنما: Supervisors: Dr. Amireh Nourbahksh

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Dr. Mohsen Goudarzi, Dr. Habibolah Sayevand

زمان و تاریخ ارائه: Time and date of presentation: 2025

مکان ارائه: Place of presentation: 50

چکیده: Abstract: Abstract Many natural and industrial phenomena involve complex interactions between different material phases with distinct inertial responses within surrounding fluid fields. These multiphase interactions have broad applications in engineering and scientific fields, including oil and gas transport, distillation operations in columns, chemical reactors, refrigeration and air conditioning systems, and water and wastewater treatment processes. In this study, a three-dimensional numerical simulation of droplet motion in an air channel was conducted within a limited Reynolds number range using the Volume of Fluid (VOF) method. This method is based on solving a single set of conservation equations over the entire computational domain and employs a volume fraction approach to accurately track the interface between phases (e.g., gas and liquid). The continuous phase was modeled using a structured and stationary mesh, while the droplet was discretized along with it. The primary objective of this research is to evaluate the effects of key dimensionless parameters—namely, the Capillary, Reynolds, and Weber numbers—on droplet dynamics. Furthermore, the critical Capillary number was investigated, and the effects of physical (viscosity and density), geometrical (droplet radius and initial position within the channel), and gravitational parameters on the droplet’s hydrodynamic behavior were analyzed. The results reveal that droplet behavior and motion are significantly dominated by these dimensionless parameters, while physical and geometrical factors play a crucial role in determining droplet stability, deformation, and trajectory. For Capillary numbers below the critical threshold, viscous forces are weaker than surface tension, maintaining the droplet’s overall shape. At the critical point, a balance between surface tension and viscous forces places the droplet at the instability limit, where the Reynolds number becomes a determining factor in deformation and breakup. For supercritical Capillary numbers, dominant viscous forces lead to strong elongation, fragmentation, or partial separation of the droplet. At low Capillary numbers, the droplet exhibits greater lateral displacement toward the channel wall. At intermediate values, a balance between forces maintains the droplet in a stable position between the centerline and the wall, with increased deformation and instability. At high Capillary numbers, the droplet undergoes severe deformation and breakup, accompanied by reduced lateral motion and stronger alignment with the main flow. Given the moderate Reynolds number range (viscous-dominated flow), the analysis primarily focuses on the Capillary number, since viscous and surface tension forces govern droplet behavior. At low Reynolds numbers, viscous effects dominate inertia, resulting in stable motion with minimal deformation. At moderate values, increased inertia enhances lateral displacement and deformation. At high Reynolds numbers, dominant inertial effects amplify hydrodynamic instabilities, leading to pronounced deformation or breakup. The density analysis indicates that at low droplet densities, buoyancy dominates, stabilizing the droplet near the channel center. At moderate densities, stronger gravitational effects cause partial breakup into smaller droplets. At high densities, gravity dominates entirely, pushing the droplet downward and ultimately leading to collapse and fragmentation. In terms of viscosity effects, droplets with lower viscosity exhibit greater lateral motion and deformation but lower axial velocity, while higher viscosity produces the opposite behavior and enhances stability. When gravity acts in the same direction as the flow, the droplet moves toward the channel center, where it remains more cohesive. Increasing the Capillary number in this case enhances deformation but reduces velocity. The influence of droplet radius shows that smaller droplets, due to stronger surface tension, experience lower deformation and axial velocity, while larger droplets with weaker surface tension exhibit greater deformation and higher velocity. The initial position of the droplet is also significant: droplets released at the channel center experience minimal lateral displacement, limited deformation, and higher axial velocity, while those near the wall show opposite effects. Eventually, droplets tend to reach an equilibrium position between the channel center and the wall, or, under specific conditions, collide with the wall. These findings provide valuable insights into the design of two-phase flow systems and can be applied to optimize related industrial processes.