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    Numerical investigation of solidification of single droplets with and without evaporation mechanism

    , Article International Journal of Refrigeration ; Volume 73 , 2017 , Pages 219-225 ; 01407007 (ISSN) Mirabedin, S. M ; Farhadi, F ; Sharif University of Technology
    Elsevier Ltd  2017
    According to some experimental observations, water droplet with high initial temperature freezes faster than a cold one. There are some explanations to this problem such as sub-cooling, evaporation and radiation. In this work, solidification process of single droplets with and without the effect of evaporation is numerically investigated for three different drop diameters and initial temperatures. It seems that evaporation itself is able to explain why hot water freezes faster than cold water. © 2016 Elsevier Ltd and IIR  

    Electrowetting induced droplet jumping over a bump

    , Article Extreme Mechanics Letters ; Volume 32 , 2019 ; 23524316 (ISSN) ; https://www.sciencedirect.com/science/article/pii/S2352431619300410 Merdasi, A ; Daeian, M. A ; Moosavi, A ; Shafii, M. B ; Sharif University of Technology
    Elsevier Ltd  2019
    We study electrowetting induced droplet jumping over a system consisting of a flat surface and a topographical bump mounted on the surface. Different bump shapes including triangular and elliptical configurations are considered and the results are compared with the results of the flat surface. The results indicate that droplet jumping is enhanced over the bumps and the droplet jumps to larger heights compared with the flat surface because of the lower viscous dissipation. The shape of the bump can considerably affect the droplet dynamics. Between the considered shapes the triangular bump provides a larger dynamic and the droplet on the surface with this bump can jump with larger velocity.... 

    Numerical Modeling of Fuel Droplet Vaporization in Gas Phase at Supercritical Conditions

    , M.Sc. Thesis Sharif University of Technology Rajabi Matin, Zahra (Author) ; Hejranfar, Kazem (Supervisor)
    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... 

    Phase Extraction in Segmented Flow

    , M.Sc. Thesis Sharif University of Technology Amini, Mohammad (Author) ; Moosavi, Ali (Supervisor)
    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... 

    The effect of droplet size, channel length and the amount of electromagnetic actuation force on reciprocating movement of mercury droplets in the magneto mercury reciprocating (MMR) micropumps

    , Article Sensors and Actuators, A: Physical ; Volume 283 , 2018 , Pages 204-210 ; 09244247 (ISSN) Karmozdi, M ; Shafii, M. B ; Afshin, H ; Sharif University of Technology
    Elsevier B.V  2018
    Micropumps are regarded as one of the devices used in microsystems, which are responsible for pumping working fluid. The magnetic reciprocating micropump is an example of the existing micropumps in which the pumping agent includes three liquid metal droplets placed inside lateral channels and reciprocated by the electromagnetic force inside their channels. The working fluid located inside the main channel is pumped through due to the movement of these three droplets. The time duration in which the droplet traverses the sub-channel length is crucial in the operation of the suggested micropump. The present study aims to evaluate the effect of the length of sub-channels, moving droplet volume... 

    Droplet condensation and jumping on structured superhydrophobic surfaces

    , Article International Journal of Heat and Mass Transfer ; Volume 134 , 2019 , Pages 680-693 ; 00179310 (ISSN) Ashrafi Habibabadi, A ; Moosavi, A ; Sharif University of Technology
    Elsevier Ltd  2019
    A complete cycle of droplet nucleation, growth, coalescence and jumping on different textured hydrophobic and superhydrophobic surfaces is studied for the first time, using a 2-D double distribution function thermal lattice Boltzmann method. First, droplet nucleation mechanism on smooth and rough surfaces is studied in detail. The results reveal the presence of cooled vapor layer instability in the condensation on completely smooth surfaces. However, on the rough surfaces and near the roughness a completely different mechanism is observed and the nucleation occurs on the roughness wedges. Also, the condensation on different textured surfaces with nominal contact angles θa=90°,120°,155° is... 

    CFD-Modeling and Simulation of Droplets Evaporation With a Size Distribution Function

    , M.Sc. Thesis Sharif University of Technology Pirhadi, Zahra (Author) ; Farhadi, Fatollah (Supervisor)
    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,... 

    Numerical Investigation of Formation and Growth of Steam Condensing Droplets on Nanostructured Superhydrophobic Surface Using Lattice Boltzmann Method

    , M.Sc. Thesis Sharif University of Technology Ashrafi Habibabadi, Amir (Author) ; Moosavi, Ali (Supervisor)
    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... 

    CFD Simulation of Dispersed Drops in Contact with Different Coalescers for Liquid-Liquid Separation

    , M.Sc. Thesis Sharif University of Technology Mehdizadeh Chelebari, Yasin (Author) ; Farhadi, Fathollah (Supervisor)
    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... 

    An Experimental Investigation of Liquid Droplet Impingement on a Molten Phase Change Material as a Direct-Contact Solidification Method

    , M.Sc. Thesis Sharif University of Technology Faghiri, Shahin (Author) ; Shafii, Mohammad Behshad (Supervisor)
    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... 

    Design of a Microfluidic Digital Droplet PCR

    , M.Sc. Thesis Sharif University of Technology Abedini, Ali (Author) ; Saeedi, Mohammad Saeed (Supervisor)
    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... 

    Numerical and Experimental Study of the Effects of Surfactant on Droplet Motion and Deformation

    , Ph.D. Dissertation Sharif University of Technology Salehi, Moloud Sadat (Author) ; Firoozabadi, Bahar (Supervisor) ; Afshin, Hossein (Supervisor)
    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,... 

    Simulating of Droplet Formation and Calculation of Interfacial Tension by Microfluidic Method

    , M.Sc. Thesis Sharif University of Technology Ghanavati, Ashkan (Author) ; Mohammadi, Ali Asghar (Supervisor)
    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... 

    Evaluation of chemicals interaction with heavy crude oil through water/oil emulsion and interfacial tension study

    , Article Energy and Fuels ; Vol. 27, issue. 10 , September , 2013 , p. 5852-5860 ; ISSN: 08870624 Dehghan, A. A ; Masihi, M ; Ayatollahi, S ; Sharif University of Technology
    A newly-designed surfactant was formulated to tolerate the harsh conditions of oil reservoirs, including high salinity of the formation brine and temperature. The specific emulsion and interfacial tension (IFT) behavior of this new surface active agent were investigated by performing emulsion stability tests, emulsion size analysis, and IFT behavior in the presence of four different types of alkalis. Image processing was utilized to analyze the droplet size distribution using microscopic images of the samples. The results show that depending on the composition of the mixtures, the optimum phase region and interfacial tension behavior change considerably. Solutions containing a higher... 

    Numerical investigation of droplets breakup in a microfluidic T-junction

    , Article Applied Mechanics and Materials ; Volume 110-116 , 2012 , Pages 3269-3277 ; 16609336 (ISSN) ; 9783037852620 (ISBN) Bedram, A ; Moosavi, A ; Int. Assoc. Comput. Sci. Inf. Technol. (IACSIT) ; Sharif University of Technology
    A Volume of Fluid (VOF) method is used to stdy the breakup of droplets in T-junction geometries. Symmetric T-junctions, which are used to produce equal size droplets and have many applications in pharmacy and chemical industries, are considered. Two important factors namely "breakup time" and "breakup length" that can improve the performance of these systems have been introduced. In addition a novel system which consists of an asymmetric T-junction is proposed to produce unequal size droplets. The effects of the channel width ratio and the capillary number on the size and length of the generated droplets and also the time of the generation have been studied and discussed. For simulation the... 

    The effects of different jet velocities and axial misalignment on the liquid sheet of two colliding jets

    , Article Chemical Engineering Science ; Volume 206 , 2019 , Pages 235-248 ; 00092509 (ISSN) Kashanj, S ; Kebriaee, A ; Sharif University of Technology
    Elsevier Ltd  2019
    This experimental study investigated the patterns and characteristics of the liquid sheet formed by two axial misaligned colliding jets and two colliding jets with different velocities. The tests were limited to the low Reynolds number region, 100

    Electrowetting-induced droplet jumping over topographically structured surfaces

    , Article Materials Research Express ; Volume 6, Issue 8 , 2019 ; 20531591 (ISSN) Merdasi, A ; Moosavi, A ; Shafii, M. B ; Sharif University of Technology
    Institute of Physics Publishing  2019
    We analyze the process of electrowetting-induced jumping of droplets away from a substrate with a geometric heterogeneity in the form of a cone and compare the results with those of a flat substrate in different wettabilities and hydrophobicities. Our results reveal that the droplet dynamics can be enhanced through applying a topographic heterogeneity. However, increasing the height of the cones does not always provide a better condition for the jumping and there is an optimum value for the height of the cones. The enhancement is due to the fact that more liquid flowing affects the pressure gradient within the droplet leading to a higher jumping velocity. It is shown that for the flat... 

    Axis-switching and breakup of rectangular liquid jets

    , Article International Journal of Multiphase Flow ; Volume 126 , May , 2020 Morad, M. R ; Nasiri, M ; Amini, G ; Sharif University of Technology
    Elsevier Ltd  2020
    The behavior of low-speed liquid jets emerging from rectangular orifices into a quiescent air is studied numerically. After ejection, the rectangular cross-section transforms into an elliptical form along the jet and while axis-switching includes elliptical cross-sections only, the rectangular shape never establishes again. The optimum wavenumber, corresponding to the most dominant wave, is found to be greater in orifices with higher aspect ratios and, as a result, breakup length of the jet will be shorter. The breakup length decreases exponentially with the initial amplitude of disturbances. Moreover, it is observed that the form of final breakup leads to elimination of the satellite... 

    Investigating the effect of reagent parameters on the efficiency of cell lysis within droplets

    , Article Physics of Fluids ; Volume 32, Issue 6 , 2020 Shamloo, A ; Hassani Gangaraj, M ; Sharif University of Technology
    American Institute of Physics Inc  2020
    Cell lysis is an essential primary step in cell assays. In the process of cell lysis, the cell membrane is destroyed and the substances inside the cell are extracted. By utilizing a droplet-based microfluidic platform for cell lysis, the mixer unit that is required for mixing lysis reagents with the cells can be excluded, and thus, the complexity of the fabrication process is reduced. In addition, lysing the cells within the droplets will prevent the cells from exposure to the channel walls, and as a result, cleanliness of the samples and the device is maintained. In this study, cell lysis within the droplets and the parameters affecting the efficiency of this process are investigated using... 

    Ferrofluid droplet breakup process and neck evolution under steady and pulse-width modulated magnetic fields

    , Article Journal of Molecular Liquids ; Volume 343 , 2021 ; 01677322 (ISSN) Bijarchi, M.A ; Favakeh, A ; Mohammadi, K ; Akbari, A ; Shafii, M. B ; Sharif University of Technology
    Elsevier B.V  2021
    Numerous applications in engineering and biotechnology have attracted the attention of many researchers to the analysis of underlying physical phenomena during the droplet pinch-off. In this study, the neck evolution during the formation of a ferrofluid droplet from a capillary is investigated under two types of magnetic field for a drop-on-demand system. The two types are steady and Pulse-Width Modulated (PWM) magnetic fields. First, under steady magnetic field, the necking process is studied for different values of magnetic Bond number and various angles between magnetic coil centerline and gravity. Subsequently, self-similar behavior in the vicinity of the detachment moment is observed....