Sharif Digital Repository

In this research, two important mechanisms were discussed including droplet generation and jumping mechanisms using electro wetting-on-dielectric. For these purposes, the finite element method (FEM) was used to solve the unsteady Naiver-Stokes equation. In addition, the level set method was applied to capture the interface between two phases. In the current study, first, droplet generation in a T-junction fluidic channel device was studied through using electro wetting actuation. The efficacy of electro wetting on the droplet generation frequency as well as droplet diameter is visible in a T-junction fluidic channel since after applying voltages, specified with non-dimensional electro... 

The electric field induced response of a charged droplet in an uncharged gel is calculated theoretically. For modeling, the gel is considered as a soft porous solid that is saturated with a Newtonian electrolyte and modeled as a continuum that contains 3 phases: a porous, soft and compressible solid, an incompressible Newtonian fluid and the ions inside fluid. Droplet is modeled as a charged incompressible Newtonian fluid immiscible with the gel. The droplet-gel interface is considered as a surface with electrostatic potential ζ . After obtaining governing equations, they solved by using perturbation methodology and linear superposition. Boundary conditions, especially at the droplet-gel... 

Encompassing an interaction between the phase change material (PCM) and the droplets of a heat transfer fluid, the direct contact (DC) method provides a state-of-the-art solution for the meager melting and solidification rates of PCMs. In the DC procedure, when impinging on the molten PCM pool, droplets evaporate, solidifying the portion of the PCM. For the first time, the impact of single and simultaneous double ethanol droplets, having an average diameter of 2.68 mm, on the molten paraffin wax has been scrutinized exhaustively. Experiments have been carried out through high-speed imaging for various Weber numbers ranging from 179 to 464, pool temperatures from 70 to 95°C, and horizontal... 

The study of evaporation of fuel droplet and determination of the rate of vaporization are important in designing combustion chambers. For achieving high performance of a combustor, the evaporation of fuel droplets takes place within a high pressure environment. At these conditions, the use of low-pressure models is not appropriate and many effects that are assumed negligible at low ambient pressures become very important. For example, the solubility of the ambient gas into the liquid phase is increased by increasing the ambient pressure. In addition, the ideal gas assumption is not valid for these conditions and one should use an appropriate equation of state (EOS) that can predict the... 

The formation of droplets at a T-junction in a micro channel network is primarily influenced by the pressure difference across the interface in the squeezing regime. Accurate measurements of droplet velocity and pressure profiles are difficult to obtain experimentally, yet these are the basic parameters required. Droplet micro fluidics has shown great potential for biological assays, chemical reactions and polymer emulsions. High stability allows the droplets to work as stable and isolated reactors that open up for parallel and serial reactions where each droplet can be screened individually. The purpose was to fabricate such systems, establish stable droplet generation where droplet volumes... 

Injecting several droplets with a known size distribution function in the hot gas environment makes droplets smaller because of evaporation or reaction. Droplets lifetime specification is important in vapor-liquid interactive processes such as chemical or process industries. In previous works, different approximate semi-empirical methods are developed to specify droplets lifetime without considering their size distribution. In this work, governing equations is studiedconsidering a size distribution function for droplets. There is not any analytical method for solving this set of equations, so numerical methods using CFD packages such as Fluent, are applicable in these cases. In this study,... 

Condensation is one of the main processes in environment and engineering systems, including thermal power plants, desalination systems and air conditioners. Thus, improvement of its performance can lead to decrease of energy consumption and the resulting air pollutions. Recently, using nanotechnology and new coating methods, there have been great researches on stable superhydrophobic surfaces and using theme as condensing surfaces which because of jumping droplet phenomena can increase condensation performance. Because of the great influence of wetting and structure properties of surfaces on condensation, in this study nucleation and growth of condensing droplets on smooth and structured... 

Liquid-Liquid coalescers are devices used for increasing the droplet size of the dispersed phase in continuous phase flow such as water droplets in oil flow. The scope of this work is a literature study on the coalescence phenomenon and CFD modeling in general. A mathematical model for simulating coalescence of water droplets in continuous oil flow by the use of different commercial coalescers. The basis for the model are plate-type coalescer and swirl-based coalescer. So, the simulations are done by Eulerian-Lagrangian method. Different aspects of the performance of the coalescers are studied on the rate of coalescence such as impact of changing droplet’s inlet diameters, impact of the... 

Improving the discharge process of phase-change materials (PCMs) is of great importance. In this study, the process of acetone drop impact on molten paraffin, as a direct-contact solidification method, is experimentally investigated. Four Weber numbers (corresponding to heights of 10, 20, 30, and 40 cm) for the acetone drop and six surface temperatures (66, 68, 70, 75, 80, and 90 °C) for the molten paraffin are considered. As the acetone drop impacts the molten paraffin, the drop absorbs heat from the melting paraffin and boils, solidifying a portion of the molten paraffin. Two important parameters that govern the acetone drop dynamics and the solidification of the molten paraffin are the... 

Polymerase chain reaction, abbreviated as PCR, is a method of amplifying the number of copies of a desired DNA sequence through special protocols. The rapid advance of microfluidic devices and the emerging concept of digital microfluidics has resulted into the development of digital droplet PCR (ddPCR) systems with their significant uses in the detection of rare mutations, cancer diagnosis, and surveillance. A large number of micron-sized droplets are required to perform ddPCR. in this study, To investigate the gradient of confinements induced droplet self-breakup mechanism, we carried out the computational fluid dynamics (CFD) simulations for the two-phase flow using a commercial software... 

Surface Active Agents/surfactant have a strong tendency to adsorb at the interface between two immiscible fluids, due to the existence of hydrophobic portions in their molecules’ structure. Accumulation of these molecules on the liquid interface changes the intermolecular forces and causes the interfacial tension to decrease. Production of drops of the same size at a specified rate and controlling their movement’s speed are among the most important factors effective in the efficiency of processes associated with liquid drops, which could profoundly be influenced by the presence of a small amount of a surface active agent. In this study, the effects of surface active agents on the growth,... 

Emulsions are the intermittent dispersion of liquid droplets that have many applications such as polymerization, dyeing, cosmetics, food industry, etc. Droplets can be created in a variety of ways, but the most important point in emulsification and the greatest efficiency of its applications is when the formed droplets are uniform. In the emulsification process, there is a key parameter called interfacial tension, which affects the behavior of the formed droplets, such as their stability and morphology, and its precise measurement is very effective in the quality control of the mentioned applications. Be. In this research, droplet formation in microfluidic systems due to uniform droplet...