Mechanical Engineering

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https://hdl.handle.net/10315/34419

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Now showing 1 - 9 of 9

Open Access
Asymmetric Behavior of a Drop Upon Impact onto a Surface
(2017-07-27) Almohammadi, Hamed; Amirfazli, Alidad
In this thesis, a systematic study was performed to understand drop impact onto hydrophilic and hydrophobic moving surfaces. Different systems (combination of liquids, surfaces, and drop impact conditions) were examined. Wide range of normal drop and surface velocities were studied; such normal and tangential velocity ranges are not available in systems where a drop impacts at an angle relative to a surface. The asymmetric nature of drop spreading on moving surfaces was elucidated. A model that for the first time is able to mathematically predict the time evolution of such asymmetric spreading was provided. Furthermore, a new model was developed to determine the splashing threshold of the drop impact onto a moving surface. The model is capable of describing the azimuthally different behavior of splashing. The effect of liquid viscosity on drop splashing was clarified. A comprehensive regime maps of drop impact outcome on a moving surface was provided.
Open Access
Behavior of Liquid Bridges between Nonparallel Surfaces
(2017-07-27) Ataei, Mohammadmehdi; Amirfazli, Alidad
Formation of liquid bridges between two solid surfaces is frequently observed in industry and nature, e.g. printing. When the two solid surfaces are not parallel (with dihedral angle between them), two significant phenomena emerge in the bridge behavior: First, if exceed a critical angle (_c), the bridge is no longer stable and propel itself horizontally towards the cusp of the surfaces. Second, if a stable bridge is squeezed and stretched, a horizontal bulk motion of the bridge along the surfaces can be observed. Through both experimental and numerical studies, we demonstrated that _c can be increased by increasing advancing contact angle (_a), and Contact Angle Hysteresis (CAH) of the surfaces. We also demonstrated that the magnitude of the bulk motion can be increased by increasing , the amount of compressing and stretching, and/or by decreasing _a and CAH of the surfaces.
Open Access
Characteristics of Air Flow Over a Sessile Droplet at the Verge of Shedding
(2020-08-11) Emami, Reza Yaghoubi; Amirfazli, Alidad
A Particle Image Velocimetry (PIV) study on the air flow over a sessile water droplet exposed to a boundary layer flow was done in a wind tunnel. Reynolds number based on free stream velocity and the height of the droplet was 600
Open Access
Drop Impact on an Inclined and a Moving Surface
(2018-11-21) Sookran, Salman Buksh; Amirfazli, Alidad
This thesis has made progress in two different areas related to drop impact onto a surface. Firstly, a systematic experimental study has been performed to understand asymmetric spreading of low and high surface tension liquids on a moving surface. A new time evolution model for droplet spreading on a moving surface was developed. This model regardless the value of surface tension of the liquid can predict the spreading of low viscous (1-4cSt) liquids on a moving surface. Secondly, liquids with viscosity (1-5cSt) and surface tension (17.4-72.8mNm) were used to study the drop impact on moving and inclined surface. Experiments performed with similar normal (0.9-2.9m/s) and tangential (0.8-2.9m/s) velocities on both surfaces to test our hypothesis that spreading/splashing for these two surface conditions should be same. Results indicates that our hypothesis is true, except for some special conditions when, normal and tangential velocities are greater than the range of our analysis.
Open Access
Drop Impact on Fabrics
(2020-08-11) Mozafari, Ali; Amirfazli, Alidad
Drop impact has been studied mostly for systems that are rough, porous, or deformable. Limited research on drop impact onto fabrics is available. Here, drop impact on fabrics is studied to understand whether fabrics features can change the dynamics of droplet. This study is done to develop a model to describe drop impact on fabrics. Five different woven fabrics are examined at different tensions. High speed images of the phenomena are analyzed to study spreading shape of droplet and measure its maximum diameter. Higher waviness (key factor) of fabric results in decrease in maximum spreading of droplet. Most part of the initial energy of droplet will be transformed to the surface energy of droplet at maximum spreading. Also, due to small fabric deflection and small void size, droplet behavior does not change drastically by changing fabric tension or void size and the energy loss of the droplet is not considerable.
Open Access
Droplet Impact on a Solid Surface: Analytical Modeling and Experimentation for Spreading Phase
(2025-04-10) Hu, Yating; Amirfazli, Alidad
A normal droplet impacting a flat, stationary, solid surface exhibits symmetric spreading. A key aspect of this symmetric spreading, particularly during deposition at low Weber numbers, is the formation of a rim along the lamella edge. The rim is crucial for predicting maximum spreading, and its dynamics can be captured using a set of ordinary differential equations in numerical calculations. However, establishing a reasonable initial condition remains challenging. In addition to symmetric spreading, various studies have explored how to disrupt symmetry for industry demands, for example, reducing contact time for anti-icing purposes. However, no attempts to date have been conducted to restore symmetry from asymmetric spreading, such as spreading over an inclined or moving surface, where tangential velocity naturally breaks the symmetry. To address these issues, this thesis investigates: 1) the rim genesis for a normal impact at low Weber numbers; 2) a model that elaborates how wettability can be manipulated to guide an asymmetric lamella towards a symmetric shape; and 3) experimental validation of this model. This work demonstrates that rim formation is driven by a developing motion at the lamella edge — the rim region — induced by deceleration due to capillary forces. The dynamic pressure from this motion creates a difference in curvature, and hence the dynamic pressure must be balanced by the Laplace pressure within the rim region. The rim forms when the rim region becomes thicker than the lamella. An analytical model was developed to predict the time when the rim starts to form and the configuration of the droplet at this moment, validated by OpenFOAM simulations and experimental recordings. Experimental results also support the proposed model for symmetry restoration based on the Taylor-Culick theory, demonstrating that manipulating wettability can accelerate the receding of the stretched contact line to counteract the stretch. A method for fabricating a patch with tunable wettability contrast has been developed for surface design to achieve symmetry restoration. Experimental results confirm the effectiveness of this patch design in correcting asymmetric spreading for water droplets impacting a surface under the following Weber number conditions: Wen ≤ 300, Wet ≤ 300, and 0.51 ≤Wen ⁄ Wet≤2.04.
Open Access
Droplet Impact onto a Spherical Particle in Mid-Air
(2019-11-22) Banitabaei, Sayed Abdolhossein; Amirfazli, Alidad
Collision between a droplet and a particle has a wide range of applications in chemical and petrochemical industries, polyethylene synthesis, and particle coating. Various studies in the literature indicate that the collision products are very different depending on the size and velocity of the particle and droplet, particle wettability and roughness, and physical properties of the liquid and the surrounding gas. The collision outcome is a liquid film (i.e. lamella) and the objective of this thesis is to identify various impact products in different conditions and to study how each category of the above mentioned parameters or a combination of them affect the lamella formation. Investigation of the droplet impact was divided into two parts: drop impact onto a still particle, and droplet impact onto a moving particle in mid-air. Contribution of this thesis to the field can be summarized as following. First, studying the impact phenomenon in a wider range of both Weber number (0.1
Open Access
Microfluidic Droplet Generation Device with Flexible Walls
(2022-08-08) Sajad Yazdanparast; Amirfazli, Alidad; Rezai, Pouya
Co-flow droplet microfluidics systems are used to generate discrete or dispersed microdroplets of one fluid (d-phase fluid) inside another immiscible fluid (continuous or c-phase fluid). Droplet sizes plays an important role for the physical and/or chemical attributes of a system. A planar device was developed with controllable c-phase channel width to tune droplet sizes. We found that the diameter of droplets was inversely proportional to the root square of channel width in constant c-phase velocity. This was due to the increased shear force due to the channel width decrease and controlled droplet size. Also, the effect of channel height on droplet size was examined. The results showed that at lower channel height, smaller droplets were generated because c-phase hardly bypassing the droplet enhanced the wall effect. The change of droplet size at constant c-phase flow rates was more than that at constant c-phase velocity. A decrease in channel width at constant flow rate not only increases the wall effect, but also increases the axial velocity which leads to a stronger drag force. The range of droplets generated in the developed device was exceptionally wider (by 111%) compared to the reported device in the literature.
Open Access
Shedding of Multiple Sessile Droplets
(2018-05-28) Razzaghi, Aysan; Amirfazli, Alidad
A droplet which is placed on a surface and is exposed to an airflow, can be shed, if the drag force overcomes the droplets adhesion force. Presence of other sessile droplets, in proximity, changes the drag force, so the minimum airflow velocity required to shed the droplets (Ucr) can vary. In this thesis, an experimental study on shedding of the multiple sessile droplets was performed on both hydrophilic and hydrophobic surfaces. The effects of the droplets arrangement type, and the spacing on Ucr were elucidate. For a pair of sessile droplets, a model was proposed to predict the Ucr based on droplets size, spacing, arrangement, and surface wettability. For three, or four sessile droplets arranged in triangle, square, reversed triangle, and diamond configurations, the effects of the droplets interaction on variation of the Ucr, was clarified. A critical value for spacing was determined beyond which multiple sessile droplets shed independently.

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Browsing Mechanical Engineering by Author "Amirfazli, Alidad"

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