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The interaction of a hollow droplet impacting a solid surface occurs in several applications, including controllable biomedicine and thermal spray coating. Understanding the physics of the hollow droplet spreading is the key to... more
The interaction of a hollow droplet impacting a solid surface occurs in several applications, including controllable biomedicine and thermal spray coating. Understanding the physics of the hollow droplet spreading is the key to maintaining the mass transfer process in all relevant applications. In this work, a comprehensive experimental, numerical and theoretical study is performed on water hollow droplets impacting a rigid surface to better understand the flattening process of a hollow droplet. In the numerical part, compressible Navier–Stokes equations are solved using the volume of fluid (VOF) method in a two-dimensional (2-D)-axisymmetric model. The comparison of simulation results with the experimental photographs shows that the numerical solution can correctly predict the hollow droplet shape evolution. The results show that the spreading diameter and height of the counter-jet formed after the hollow droplet impact grow with impact velocity. Investigating the size and location...
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An experimental study is performed to analyze the shear driven droplet shedding on cold substrates with different airflow speeds typical of those in the flight conditions. Understanding the mechanism of simultaneous droplet shedding,... more
An experimental study is performed to analyze the shear driven droplet shedding on cold substrates with different airflow speeds typical of those in the flight conditions. Understanding the mechanism of simultaneous droplet shedding, coalescence, and solidification is crucial to devise solutions for mitigating aircraft in-flight icing. To mimic this scenario, the experimental setup is designed to generate shear flow as high as 90 m/s. The droplet shedding at high-speed is investigated on a cold surface (0 and −5 °C) of different wettabilities ranging from hydrophilic to superhydrophobic. Result analyses indicate that on a hydrophilic substrate, the droplets form a rivulet, which then freezes on the cold plate. In contrast, on the superhydrophobic surface, there is no rivulet formation. Instead, droplets roll over the substrate and detach from it under the effect of high shear flow.
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The focus of this study is the formation of a solid solution and metallic nickel in the cobalt-nickel mixed oxide coatings during suspension plasma spray (SPS) deposition. The (Co,Ni)O solid solution is a potential material for inert... more
The focus of this study is the formation of a solid solution and metallic nickel in the cobalt-nickel mixed oxide coatings during suspension plasma spray (SPS) deposition. The (Co,Ni)O solid solution is a potential material for inert anode applications in aluminum production. SPS coatings and in-flight collected particles are studied to gain further insight into the melting and mixing phenomena of the NiO and CoO powders as well as phase formation in the deposited coatings. Moreover, the role of suspension feedstock particle sizes on the microstructure of coatings is discussed. SEM, EDS and X-ray diffraction studies helped better understanding the formation of different crystalline phases within the as-sprayed coatings. It was found that the formation of metallic nickel is possible in the coatings. The results support the importance of substrate temperature on the formation of metallic Ni, so that keeping the substrate at low temperature results in an increase of the Ni content in t...
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The focus of this study is the formation of a solid solution and metallic nickel in the cobalt-nickel mixed oxide coatings during suspension plasma spray (SPS) deposition. The (Co,Ni)O solid solution is a potential material for inert... more
The focus of this study is the formation of a solid solution and metallic nickel in the cobalt-nickel mixed oxide coatings during suspension plasma spray (SPS) deposition. The (Co,Ni)O solid solution is a potential material for inert anode applications in aluminum production. SPS coatings and in-flight collected particles are studied to gain further insight into the melting and mixing phenomena of the NiO and CoO powders as well as phase formation in the deposited coatings. Moreover, the role of suspension feedstock particle sizes on the microstructure of coatings is discussed. SEM, EDS and X-ray diffraction studies helped better understanding the formation of different crystalline phases within the as-sprayed coatings. It was found that the formation of metallic nickel is possible in the coatings. The results support the importance of substrate temperature on the formation of metallic Ni, so that keeping the substrate at low temperature results in an increase of the Ni content in t...
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This article describes the effect of electrode shape in electrowetting-based droplet actuation. A Volume of Fluid technique is applied to track the 3-D interface. The Laplace equation is solved to predict the time and space dependent... more
This article describes the effect of electrode shape in electrowetting-based droplet actuation. A Volume of Fluid technique is applied to track the 3-D interface. The Laplace equation is solved to predict the time and space dependent electric field in the domain. The onset of actuation, contact angle distribution at the three phase line, and the droplet morphology are studied for two electrode shapes; i.e. flat and crescent. The acceleration and the velocity of the droplet are also other important parameters to show the superior performance of crescent electrode in droplet actuation.
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Biocompatibility is one of the essential characteristics of implants used in biomedical and dental applications. The biocompatibility may be affected by many factors, one of which and the most important one, is its corrosion resistance in... more
Biocompatibility is one of the essential characteristics of implants used in biomedical and dental applications. The biocompatibility may be affected by many factors, one of which and the most important one, is its corrosion resistance in highly aggressive body fluid. Corrosion of implants causes products of corrosion to accumulate in adjacent tissues and stimulate allergy in patients. Surface modification of
Research Interests: Computer Science, Materials Science, Dentistry, Density, Corrosion, and 15 moreThin Films, Porosity, Surface modification, Thin Film, Thermal spraying, Stainless Steel, Immune system, Surface Resistance, Powders, Wear resistance, Plasma Spraying, Biomedical Application, Corrosion Resistance, Nozzle, and High Velocity Oxygen Fuel
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The present study investigates the detailed visualization and behavior of a spray (e.g. multiple droplets) impinging on hydrophilic, and superhydrophobic aerodynamic shapes in isothermal room and icing conditions. These experiments can... more
The present study investigates the detailed visualization and behavior of a spray (e.g. multiple droplets) impinging on hydrophilic, and superhydrophobic aerodynamic shapes in isothermal room and icing conditions. These experiments can provide a fundamental understanding of in-flight icing. A superhydrophobic coating has a very low surface energy so it can be used to counteract ice accumulation. It also reduces the adhesion strength of ice to the surface which ensures easier removal of the ice during flight. The focus of the experiments primarily lies on the fundamental study of spray impact on an airfoil in room and icing conditions. Under such conditions, important icing features such as rivulets and runback flow are observed. This provides us with the basics of ice formation on an aerodynamic surface. The study also focuses on the comparison of ice accumulation on aluminum and superhydrophobic surfaces. All experiments are carried out in a small scaled icing wind tunnel using high speed photography with frame rates ranging from 4000 to 50000 frames per second. The experimental observations show that ice accumulation on a superhydrophobic surface is significantly less than that on a hydrophilic surface.
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The impact of a liquid droplet onto an elastic substrate is modeled in the current work. Volume of Fluid Method is used to model the interfacial flow in the fluid region which contains both liquid and gas phases. The droplet deformation... more
The impact of a liquid droplet onto an elastic substrate is modeled in the current work. Volume of Fluid Method is used to model the interfacial flow in the fluid region which contains both liquid and gas phases. The droplet deformation is precisely captured upon impact at impingement velocities as high as 200 m/s. In this regard, incompressible Volume of Fluid solver is used for low impact velocities and a compressible model is implemented at high impingement velocities. In addition, the solid substrate is modeled with Finite Element Method. A novel Fluid-Solid Interaction model is developed that couples the gas-liquid interfacial model with the structural solver utilizing the two-way coupling approach. The coupling between fluid and solid is achieved by using the stress continuity on the fluid-solid interface and no-slip velocity condition. The pressure field in the fluid domain and the stress field in the solid domain are obtained simultaneously by solving coupled fluid and solid equations. The effect of the fluid compressibility on the generated pressure build-up in the liquid and the resulting stress in the solid is investigated. The results obtained from the Fluid-Solid Interaction model are validated for an impingement velocity of 100 m/s.
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A theoretical study of liquid column deformation discharged perpendicularly into a subsonic transverse gas flow is carried out. Liquid jet trajectory prior to breakup is determined from an analytical approach for momentum flux ratios... more
A theoretical study of liquid column deformation discharged perpendicularly into a subsonic transverse gas flow is carried out. Liquid jet trajectory prior to breakup is determined from an analytical approach for momentum flux ratios lower than 100 and Weber numbers up to 100. Force balance on liquid element is analyzed in free stream direction assuming that surface tension and viscous forces are small compared to the aerodynamic force for the above range of momentum flux ratio and Weber number. Mass shedding from jet surface and liquid evaporation are neglected. A logarithmic correlation for penetration depth of liquid jet with circular cross-section is derived which takes into account the liquid to gas momentum flux ratio and drag coefficient. The changes in free stream properties e.g. the gas viscosity and velocity are incorporated in terms of the drag coefficient. The results are presented for various test conditions corresponding to their experimental counterpart in literature.
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Passive control can result in increasing fuel efficiency and reducing combustion instabilities of gas turbine spray combustors. Through the use of geometric modifications of the conventional circular nozzles, this method potentially... more
Passive control can result in increasing fuel efficiency and reducing combustion instabilities of gas turbine spray combustors. Through the use of geometric modifications of the conventional circular nozzles, this method potentially enhances mixing which is responsible for entraining the bulk air necessary for combustion. Several studies show that elliptic jets have higher mass entrainment and spreading rate compared to the equivalent circular jets [1]. The majority of these works have been limited to gaseous jets. The present study focuses on a liquid spray discharging into still ambient air from a single-hole injector with elliptic cross-section. The primary breakup is investigated using a theoretical approach. Characteristics of elliptic orifice jet are compared with circular orifice jet under different breakup regimes and various nozzle geometries.
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Numerical investigation is performed to study the droplet behavior under electrowetting actuation inside microchannels. Volume of Fluid (VOF) technique is employed to track the interface while the electric field is solved inside the whole... more
Numerical investigation is performed to study the droplet behavior under electrowetting actuation inside microchannels. Volume of Fluid (VOF) technique is employed to track the interface while the electric field is solved inside the whole domain in each time step simultaneously. The equations are solved in three dimensions for water as the liquid phase. Droplet morphology under the application of an electric field is investigated. Droplet velocity studied under different actuation voltages and compared to the experiments. Contact angle hysteresis and its effects on the threshold voltage are discussed.
Numerical study of elliptical liquid jets in cross flow is performed, using the Volume of Fluid (VOF) method coupled with Large Eddy Simulation (LES) turbulence model. Several elliptical geometries with different aspect ratios, i.e.... more
Numerical study of elliptical liquid jets in cross flow is performed, using the Volume of Fluid (VOF) method coupled with Large Eddy Simulation (LES) turbulence model. Several elliptical geometries with different aspect ratios, i.e. 0.25-4, including the cylindrical shape, were considered as the liquid jet nozzle exit. The simulations were performed with relative gas-liquid Weber numbers 18 and 30 which covers the bag and multimode primary breakup regime in crossflow. In addition the liquid to gas momentum ratio is kept at 50 and 100. The results show remarkable changes in liquid shapes before disintegration for different aspect ratios. Also the penetration depth, droplet size distribution and spray spreading angle has been affected by the change of disintegration model.
Surface tension is a dominant force for liquid handling and actuation in microscale. Application of an external electric field can change the surface tension between the liquidsolid interface, which reduces the meniscus contact angle and... more
Surface tension is a dominant force for liquid handling and actuation in microscale. Application of an external electric field can change the surface tension between the liquidsolid interface, which reduces the meniscus contact angle and induces motion of a droplet in a microchannel. Numerical simulation of droplet transport in microchannels under electrostatic actuation is investigated. Volume-of-Fluid (VOF) technique is employed, where electrowetting eects are implemented through Lippmann’s relation in the form of modified contact angles at the boundary. Experimental data for steady and transient velocities of droplets in microchannels are used to calibrate the code. Numerical simulation of a zero-leakage microvalve is investigated where a liquid droplet is used as a zero-leakage gate to regulate the flow in a T-junction. The droplet gate is activated by changing its surface tension via an applied electrostatic potential. Numerical simulation is used to predict the droplet behavior and to optimize valve design. It is found that the pressure break-down of the microvalve is significantly aected by the geometry of the T-junction corners. It is expected that such a microvalve design significantly improve the sensitivity and performance of many microfluidic devices.
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This paper studies the numerical simulation of the movement of a liquid droplet in a microchannel under electrostatic actuation. The surface tension between the liquid-solid interface is modified by an external electric field, which... more
This paper studies the numerical simulation of the movement of a liquid droplet in a microchannel under electrostatic actuation. The surface tension between the liquid-solid interface is modified by an external electric field, which reduces the meniscus contact angle and induces motion of a droplet electrowetting in microchannels and the droplet velocities of several centimetres per second has been observed.
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Atomization of suspension solutions has a wide range of applications in many industrial processes. The application ranges from spray drug products, such as nasal spray and inhalation solution to spray drying, plasma spraying and coatings.... more
Atomization of suspension solutions has a wide range of applications in many industrial processes. The application ranges from spray drug products, such as nasal spray and inhalation solution to spray drying, plasma spraying and coatings. In any atomization process the main spray characteristics are defined as drop size and velocity distributions, the spray path and the distribution of the liquid throughout the spray. These distributions are affected by several parameters including liquid and gas physical properties, nozzle geometry and operating conditions [1]. Thus understanding the behavior of gas and liquid flow through the nozzle is crucial to predict the thickness and momentum of the liquid flow at the outlet of the atomizer. In this work the Multi-Fluid Marker and Cell (MFMAC) technique is used to numerically model the structure of internal two-phase flow inside an aerated-liquid jet. The behavior of liquid film in the discharge passage was investigated using different Gas to Liquid mass Ratio (GLR) and these numerical results were compared with the available experimental data. This work was done as a baseline to validate the simulations and the effect of suspension solid particles on the structure of the two-phase flow at the exit cross section of the nozzle was also studied. The effect of concentration of solid particles on the performance of the atomizer is considered through change in the liquid bulk density and viscosity. By increasing in the amount of aerating gas, the liquid film formed in the discharge orifice, becomes thinner and the gas/liquid velocity ratio and momentum flux at the nozzle exit increases. It was found that the variation of solid particle concentration can have an influence on the internal flow characteristics such as the liquid thickness and the flow pattern inside the nozzle discharge passage.Copyright © 2007 by ASME
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Widespread adoption of thermal spray technology requires the ability to apply a variety of coating materials, suited to each new application. Understanding the dependence of the microstructure of spray coatings on operating conditions of... more
Widespread adoption of thermal spray technology requires the ability to apply a variety of coating materials, suited to each new application. Understanding the dependence of the microstructure of spray coatings on operating conditions of the thermal spray system is of great practical ...
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ABSTRACT Coalescence of a falling droplet with a sessile droplet on solid surface with various wettabilities is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity,... more
ABSTRACT Coalescence of a falling droplet with a sessile droplet on solid surface with various wettabilities is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity, and the distance between the impacting droplets were controlled. The evolution of surface shape during the coalescence of two droplets on various surfaces is captured using high speed imaging and compared with numerical results. A two-phase volume of fluid method is used to determine the dynamics of droplet coalescence, shape evaluation, and contact line movement. The spreading length of two coalescing droplets along their original centers is also predicted by the model and compared well with the experimental results. The effect of different parameters such as impact velocity, center to center distance, droplet size, and surface wettability on maximum spreading length are studied and compared to the experimental results. Finally, correlations are developed for predicting the maximum spreading length using both experimental and numerical results.
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... Ghobad Amini, Ali Dolatabadi. Abstract. ... SE Bechtel, JA Cooper, MG Forest, NA Petersson, DL Reichard, A. Saleh, and V. Venkataramanan, A new model to determine dynamic surface tension and elongational viscosity using oscillating... more
... Ghobad Amini, Ali Dolatabadi. Abstract. ... SE Bechtel, JA Cooper, MG Forest, NA Petersson, DL Reichard, A. Saleh, and V. Venkataramanan, A new model to determine dynamic surface tension and elongational viscosity using oscillating jet measurements, J. Fluid Mech. ...
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Coalescence of a falling droplet with a stationary sessile droplet on a superhydrophobic surface is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity, and the... more
Coalescence of a falling droplet with a stationary sessile droplet on a superhydrophobic surface is investigated by a combined experimental and numerical study. In the experiments, the droplet diameter, the impact velocity, and the distance between the impacting droplets were controlled. The evolution of surface shape during the coalescence of two droplets on the superhydrophobic surface is captured using high speed imaging and compared with numerical results. A two-phase volume of fluid (VOF) method is used to determine the dynamics of droplet coalescence, shape evaluation, and contact line movement. The spread length of two coalesced droplets along their original center is also predicted by the model and compared well with the experimental results. The effect of different parameters such as impact velocity, center to center distance, and droplet size on contact time and restitution coefficient are studied and compared to the experimental results. Finally, the wetting and the self-cleaning properties of superhydrophobic surfaces have been investigated. It has been found that impinging water drops with very small amount of kinetic impact energy were able to thoroughly clean these surfaces.
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A three-dimensional model of a Cold Gas Dynamic Spray system with a peripheral nonaxisymmetric powder feeder is studied in this work. It is found that the stagnation pressure alternates for different substrate standoff distances due to... more
A three-dimensional model of a Cold Gas Dynamic Spray system with a peripheral nonaxisymmetric powder feeder is studied in this work. It is found that the stagnation pressure alternates for different substrate standoff distances due to the nature of the supersonic flow interaction with the substrate. One can find the optimum substrate location for any given operating condition, which results in minimum pressure buildup on the substrate. The three-dimensional analysis sheds more light on the complex gas and particle flow fields generated due to the three-dimensional particle injection process. In addition, the three-dimensional model allows us to further investigate the effect of practical substrate shapes (such as convex and concave) on the flow field and consequently to determine the optimum conditions to deposit coating particles.
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This study investigates the effect of water injection in the high pressure chamber of a cold spray nozzle. A De Laval nozzle geometry with constant back pressure and temperature is modeled numerically using Reynolds Stress Model coupled... more
This study investigates the effect of water injection in the high pressure chamber of a cold spray nozzle. A De Laval nozzle geometry with constant back pressure and temperature is modeled numerically using Reynolds Stress Model coupled equations. Water spray with a droplet size of 10-100 μm is modeled using both uniform and Rosin-Rammler size distributions. The two-phase flow of gas-liquid is modeled using an unsteady discrete phase mass source with two-way coupling with the main gas flow. Upon injection, the droplets in the water spray evaporate while travelling through the nozzle due to momentum and energy exchange with the gas flow. The evaporation behavior in the presence of water content is modeled and a correlation between the initial diameter and the diameter just before the throat is obtained. As a result, the proper droplet size distribution with a fully evaporative spray can be used as a carrier of nano-particles in cold spray nozzles. Having the results, guides us to substitute the un-evaporated part of the droplet with an equal diameter agglomerate of nano-particles and find a minimum fraction of nano-particles suspended in the liquid which guarantees fully evaporative liquid spray injection.
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... 5.2 Ceramic Coatings Finally, using the attachment enabled the authors to apply low-porosity alumina coatings with a deposition rate appropriate for industrial applications. As mentioned earlier, the particle tra-jectories are ...
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High velocity oxygen fuel process (HVOF) involves supersonic two-phase flow of gas-solid particles. Two kinds of shocks are formed in a typical high velocity oxygen fuel process. Adjustment of the overexpanded flow to the atmospheric... more
High velocity oxygen fuel process (HVOF) involves supersonic two-phase flow of gas-solid particles. Two kinds of shocks are formed in a typical high velocity oxygen fuel process. Adjustment of the overexpanded flow to the atmospheric pressure at the exit of the nozzle results in formation of shock diamonds while high speed flow impingement on a substrate creates bow shock. The latter is found to be responsible for deviation of the injected particles from their trajectories near the substrate, which significantly reduces the chance of some particles landing on the substrate. An attempt is made to study the behavior of particle trajectory as it interacts with the bow shock formed near the substrate. The strength and location of bow shock was found to vary for different substrate geometries and standoff distances. In this work, various particle sizes impinging on substrates with various configurations (flat, concave, and convex) are simulated and the effect of shock diamonds and bow shock on particle trajectory is studied.
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... During the past decade, computational fluid dynamics (CFD) made increasingly important contributions to the design of thermal spray nozzles (Ref 4, 5) (published in German). ... B. Jodoin,Cold Spray Nozzle Mach Number Limitations, J.... more
... During the past decade, computational fluid dynamics (CFD) made increasingly important contributions to the design of thermal spray nozzles (Ref 4, 5) (published in German). ... B. Jodoin,Cold Spray Nozzle Mach Number Limitations, J. Therm. Spray T
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ABSTRACT Ni-based electrode coatings with enhanced surface areas, for hydrogen production, were developed using atmospheric plasma spray (APS) and suspension plasma spray (SPS) processes. The results revealed a larger electrochemical... more
ABSTRACT Ni-based electrode coatings with enhanced surface areas, for hydrogen production, were developed using atmospheric plasma spray (APS) and suspension plasma spray (SPS) processes. The results revealed a larger electrochemical active surface area for the coatings produced by SPS compared to those produced by APS process. SEM micrographs showed that the surface microstructure of the sample with the largest surface area was composed of a large number of small cauliflower-like aggregates with an average diameter of 10 μm.