Abstract
This thesis presents a comprehensive numerical investigation into interface dynamics and phase change phenomena in multiphase flows, emphasizing methods to accurately capture complex interfacial behaviors and thermodynamic effects. It begins with a detailed overview of the fundamental physics governing phase transitions, such as evaporation of pure and multicomponent liquids, along with essential mathematical and thermodynamic formulations. The thesis categorizes interface-capturing methodologies into sharp interface and diffuse interface approaches, elaborating on theoretical foundations, numerical implementations, and specific challenges, including handling discontinuous properties and boundary conditions at evolving interfaces.
The first paper introduces a Volume-of-Fluid (VoF) method designed specifically for multicomponent droplet evaporation, incorporating Robin boundary conditions. This innovative numerical approach addresses critical complexities involved in modeling mass transfer across interfaces. It implements robust numerical techniques that accurately impose interfacial boundary conditions, thus ensuring reliable predictions of evaporation dynamics. Validation of this method through various benchmark test cases demonstrates its effectiveness in handling multicomponent droplet evaporation under diverse thermodynamic and physical conditions.
The second paper presents the development of a low Mach number diffuse-interface model for simulating multicomponent two-phase flows undergoing phase changes. This advanced modeling technique integrates the diffuse interface method while taking into account compressibility at low Mach numbers, effectively capturing intricate interactions between fluid dynamics, thermodynamics, and interfacial mass transfer processes.
Detailed validations conducted against theoretical predictions and experimental data confirm the model’s accuracy and robustness in resolving interface dynamics. The method is particularly suited for industrial and environmental scenarios that involve subtle thermal effects and complex phase transition dynamics in multicomponent fluid systems.
The third paper investigates finite-size droplet evaporation in weakly compressible homogeneous isotropic turbulent flows using direct numerical simulations (DNS). The study focuses on the comparative evaporation dynamics of ammonia and nheptane droplets, analyzing the influence of turbulence intensity, droplet interactions, coalescence, and breakup phenomena. When initialized with equal liquid volume fractions, ammonia droplets exhibit a higher initial evaporation rate due to their higher volatility and vapor diffusivity, but both the reduced available surface area caused by increased coalescence and the progressive saturation of the domain with vaporized ammonia lead to a decline in evaporation rate over time. Results reveal that turbulence significantly enhances mass and heat transfer, although the effect varies depending on the liquid’s thermophysical properties and vapor pressure. In cases where the total energy content (lower heating value – LHV) is matched, ammonia initially releases energy more rapidly, but the trend reverses as coalescence dominates, and n-heptane delivers a more consistent and sustained energy flux. Increasing turbulence intensity further promotes evaporation in both fuels; however, while n-heptane benefits throughout the entire simulation, ammonia shows early enhancement followed by saturation due to droplet interactions and vapor accumulation. Overall, n-heptane droplets demonstrate more stable evaporation behavior and sustained efficiency, making them generally more effective for prolonged combustion processes and energy transfer applications. This detailed numerical analysis contributes to a deeper understanding of the fundamental evaporation mechanisms, aiding the optimization of fuel injection and combustion processes in practical engineering applications.