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In this study, the atomization of an effervescent atomizer with an elliptical and circular orifice was investigated experimentally in the gaseous crossflow. The shadowgraph technique was used to visualize the near field of the spray atomization. To measure the spray penetration height in crossflow, image processing was performed on the shadowgraph images based on two different approaches: (i) a threshold analysis of average spray images and (ii) standard deviation images of spray. By comparing these two approaches, it is found that the penetration heights obtained from the standard deviation images were much higher since these images were able to capture small droplets more accurately. Moreover, based on the standard deviation images of spray, an empirical correlation for the spray penetration height was developed as a function of gas-liquid mass flowrate ratio, liquid-air momentum flux ratio, orifice aspect ratio, and downstream location. The laser diffraction technique was used to analyze the particle size of the aerated elliptical and aerated circular jets in a gaseous crossflow. For each orifice shape, the effect of gas-liquid ratio and downstream location on the Sauter mean diameter (SMD) was studied. The results show that the aerated circular jet penetrates higher into the gaseous crossflow than the aerated elliptical jet. Besides, the aerated circular jet in crossflow mainly generates smaller drop sizes compared to the aerated elliptical jet.

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 of the entrapped bubble shows an optimum bubble size that facilitates the hollow droplet flattening. It is also shown that the ripples on splats produced by the hollow droplets with a larger bubble size are higher than those of small bubbles. In the end, a theoretical model is developed to analyse the maximum spreading diameter of the hollow droplet impact analytically. Its prediction is in good agreement with the experimental and numerical results.

The dynamics of round and elliptical liquid jets in subsonic crossflow was studied using high-speed imaging technique. The experiments were performed at constant gaseous Weber number and liquid–gas momentum flux ratios of 6.45 and 17.87, respectively, with orifices of different aspect ratios (AR) having an equivalent diameter of 0.43 mm. All of the cases were carried out inside an open-loop subsonic wind tunnel with a test section of 100 mm × 100 mm × 750 mm. For each case, dynamic modes were generated directly from the snapshots using a variant of the Arnoldi method known as the dynamic mode decomposition (DMD). The DMD results indicate that elliptical liquid jets have more small-scaled patterns with higher frequencies compared with the round liquid jets. As the first attempt to investigate the dynamics of elliptical liquid jets in crossflow, this study captures the dominant spatiotemporal structures. We also found that the orifice AR can significantly alter the jet wavelengths. The data from this study contribute to understanding the behavior of elliptical liquid jets exposed to gas crossflow in an enhanced capillary breakup regime.

Nowadays, in the aerospace industry, additive manufacturing and repairing damaged metallic components like Ti6Al4V samples have grabbed attention. Among repairing techniques, solid-state additive manufacturing processes like cold spray are promising because of their unique benefits such as high deposition rate with almost no oxidation in the deposited materials. However, its main drawback is the level of porosity of as-sprayed samples. To increase density and inter-particle bonding, deposited particles must go through more degrees of deformation by increasing particle velocity and particle temperature. In order to increase these two parameters simultaneously, high-velocity air fuel (HVAF) can be utilized. For understanding the effect of using HVAF on particle deformation, a proper elastic-plastic finite-element-based simulation is required. The obtained outcomes show that enhancing particle velocity and providing more kinetic energy will increase particle deformation and sample density. Importantly, increasing particle temperature will seize particle deformation by thermal softening effect, i.e., enhancing as-sprayed sample density, while rising substrate temperature by preheating will soften the substrate resulting in a decrease in particle deformation.

In developing wettable TiB2-based cathodes for Al production, the main problem encountered for decades is the difficulty to sinter or melt this refractory material. Here, suspension plasma spray (SPS) is used as this process makes it possible to use micrometric TiB2 particles to favor a high heat transfer per particle. In addition, an argon gas shield is used to protect the inflight TiB2 particles from oxidation with air during spraying. Our results confirm the efficiency of the argon shroud to limit TiB2 oxidation when compared with a TiB2 coating obtained without shroud. The SPS TiB2 coatings show a good adhesion to the graphite substrate. However, their high porosity indicates an insufficient TiB2 melting during the spraying, suggesting that the transit time spent by the TiB2 particles in the plasma is too short. The SPS TiB2 coatings present a good wettability for molten Al. However, Al infiltration through the porous coating results in the direct contact of molten Al with the graphite substrate, which may be detrimental for its mechanical integrity.

Ice formation on the aerodynamic surfaces of an aircraft is regarded as a major problem in the aerospace industry. Ice accumulation may damage parts, sensors and controllers and alter the aerodynamics of the airplane, leading to a range of undesired consequences, including flight delays, emergency landings, damaged parts and increased energy consumption. There are various approaches to reducing ice accretion, one of them being the application of icephobic coatings. In this work, commercially available polyurethane-based coatings were modified and deposited on NACA 0012 aircraft airfoils. A hybrid modification of polyurethane (PUR) topcoats was adopted by the addition of nanosilica and three-functional spherosilicates (a variety of silsesqioxane compound), which owe their unique properties to the presence of three different groups. The ice accretion on the manufactured nanocomposites was determined in an icing wind tunnel. The tests were performed under three different icing conditions: glaze ice, rime ice and mixed ice. Furthermore, the surface topography and wetting behavior (static contact angle and contact angle hysteresis) were investigated. It was found that the anti-icing properties of polyurethane nanocomposite coatings strongly depend on the icing conditions under which they are tested. Moreover, the addition of nanosilica and spherosilicates enabled the reduction of accreted ice by 65% in comparison to the neat topcoat.

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 the coatings. In this study, possible causes for the formation of metallic Ni during spraying are discussed.

The cold spray process can be used to form three-dimensional objects following the layer-by-layer approach which is called cold gas dynamic manufacturing or cold spray additive manufacturing (CSAM). In this study, a three-dimensional simulation of CSAM for a high-pressure nozzle with axial powder injection is investigated. During the manufacturing of a desirable object by CSAM, formation of the sharp edges, inclined surfaces, stagnation points, and corners will inevitably influence the trajectories of the particles. This leads to dispersion and lack of particle deposition in these areas which can eventually reduce the precision and efficiency of the deposition process. Three different objects of a cylinder and two frustums with different angles have been numerically simulated on a substrate to represent typical additively manufactured parts. Particle trajectories and impact conditions, i.e., size and velocity distributions, with and without these objects, have been compared. The results of numerical modeling provided useful information for understanding the limitations and challenges associated with CSAM, which can help us to improve the quality and precision of particle deposition.

Suspension plasma spraying process, a relatively new deposition technique in thermal spray coating, has been increasingly applied to deposit high-quality thermal barrier coatings using submicron particles. An accurate simulation of the process includes the development of a realistic model of the plasma both within and outside the torch. In this work, a three-dimensional time-dependent model has been developed to simulate the magnetohydrodynamic fields inside a DC plasma torch including arc fluctuations. The Reynolds stress model is used to simulate the time-dependent turbulent plasma flow. To investigate the effects of plasma arc fluctuation on the trajectory, temperature, and velocity of suspension droplets injected into the plasma jet, a two-way coupled Eulerian–Lagrangian method is employed. Submicron yttria-stabilized zirconia particles, suspended in ethanol, are modeled as multicomponent droplets. The Kelvin–Helmholtz Rayleigh–Taylor breakup model is used to simulate the droplet breakup. Particles are also tracked after the completion of suspension breakup and evaporation to obtain the in-flight particle conditions including the trajectory, size, velocity, and temperature. The arc attachment spots showed a good agreement with the experimental images. It was also shown that the properties of the particles are significantly affected by plasma arc fluctuations.

Aerosol Deposition (AD) method is an emerging coating process for deposition of ceramic particles for industrial applications such as MEMS, fuel cells, optical devices and radio frequency components. In this process, various parameters such as nozzle geometry, powder size and type, pressure inside the deposition chamber, and carrier gas flow rate have significant influence on the in-flight particle behavior before impact, and therefore, on the coating properties. In this study, a two-way coupled Eulerian-Lagrangian model is used to study the effects of gas flow rates and substrate location on the gas flow characteristics, bow shock near the substrate, and more importantly on the particle velocity and trajectory upon impact. The study is carried out with a rectangular sonic nozzle. To validate our simulations, locations of the Mach disks in highly under-expanded free-jet conditions are compared with the theoretical and experimental results in the literature. Two different computational geometries are utilized; a 2D planar and a quarter slice of 3D. It is found that, for the rectangular nozzle, the 3D geometry produces more accurate results and can capture the axis-switching phenomenon. Moreover, it is observed that the gas flow structure as well as the in-flight particle behavior obtained from 2D and 3D geometries are almost identical before the Mach disk. In addition, two different drag expressions are employed and it is shown that the influence of Mach and Knudsen numbers on particle behavior in vacuum condition is significant.

Cold spray (CS) is a deposition technique to form a coating from the particles with temperature lower than their melting point. In this technique, particles are accelerated by a supersonic flow of a carrier gas such as air or nitrogen. Upon impact, particles undergo significant plastic deformation that bonds them to the substrate. Since the particles are not molten, this deposition method does not apply a lot of heat to the substrate and this makes CS the best candidate for temperature sensitive and oxygen sensitive materials. CS can be adapted to form 3D objects following layer-by-layer approach. This is called cold gas dynamic manufacturing (CGDM) or cold spray as additive manufacturing. Developing complex shapes by CGDM may result in formation of inclined surfaces, corners and sharp edges. Deposition in those regions is often accompanied with challenges that affect the accuracy and efficiency of the manufacturing. In this study, CGDM for two typical shapes such as cylinder and frustum on a flat substrate has been simulated to represent the additively manufactured parts. Particle trajectories and impact conditions i.e. velocity and size distributions have been compared. The results of numerical modelling provided useful information for understanding the limitations and challenges associated with CGDM that can help us to improve the quality and precision of particle deposition.

Understanding the mechanism of cloud-sized particles impact and freezing is crucial to find viable solutions to prevent ice accumulation on critical aerodynamic surfaces such as aircraft wing or nacelle. It has been reported that superhydrophobic surfaces (SHS) have promising anti-icing properties due to their excellent water-repellent characteristics. However, due to the complexity of the freezing phenomenon on superhydrophobic surfaces, the anti-icing performance of such surfaces has not been fully understood. A multi-region multiphase flow solver including phase change has been developed to model the icing of a micro-droplet as it impinges on a superhydrophobic substrate with a given thickness, texture, and solid material thermal properties. The Navier-Stokes equation expressing the flow distribution of the liquid and the gas, coupled with the volume of fluid (VOF) method for tracking the liquid-gas and liquid-solid interfaces, was solved numerically using the finite volume methodology. The superhydrophobic morphology is modeled through a series of micro-structured arrays with squared cross-sectional pillars. As such, the thermal contact resistance is inherently accounted by the inclusion of air pockets underneath the micro-droplet. Consequently, the direct effect of surface topology and thermal properties on droplet maximum spreading diameter, penetration to the surface asperities, contact time, and the freezing onset have been investigated. Finally, icephobicity of two similar SHSs with various thermal properties were compared.

Suspension plasma spray (SPS) is an emerging coating process for making surfaces with superior properties. In this process, in-flight spray particle characteristics such as size, velocity, and temperature have a direct influence on the properties of the deposited coatings. Accordingly, online diagnostic tools to characterize the in-flight particles in the SPS are sought by research laboratories and industrial centers for process optimization and control. However, small particle size, high temperature, and radiation of the plasma make it challenging to carry out these measurements. In this study, we used a light diffraction (LD) approach to measure online the size of in-flight particles sprayed from a well-predefined size distribution. Laser beam refraction by the hot plasma/gas jet is one of the main sources of noise for such size measurement. Successful measurements were achieved by shielding the measurement section and filtering the plasma radiation to reduce the influence of the laser refraction and plasma radiation. Results showed that the LD method has the potential to be used to monitor online the size distribution of in-flight particles in the SPS process.

Superhydrophobic coatings are a potential solution for mitigating the in-flight icing problem for aircraft. However, to develop a superhydrophobic coating which can be practically used for aircraft and that possesses sufficient durability is an ongoing challenge. In this work, superhydrophobic coatings are developed using suspension plasma spraying (SPS) as a flexible, versatile and scalable coating technique. The anti-icing and deicing performances of these SPS coatings are studied in icing wind tunnel experiments. Furthermore, the durability of these SPS coatings is tested in dry particle and cloud-sized water droplet erosion and icing/deicing cyclic tests. The capability of SPS superhydrophobic coatings to reduce ice accretion is comparable to that of commercial superhydrophobic coatings but perform better in deicing tests using heating. Additionally, compared to commercial superhydrophobic coatings, the SPS coatings demonstrate significantly better performance in dry particle and icing/deicing cyclic tests while showing comparable durability in cloud-sized water droplet erosion tests. It is also shown that in case of high intensity water erosion, when the superhydrophobicity of SPS coatings deteriorates, it can be restored using a simple and quick retreatment process due to the robustness of the hierarchical micro-textured TiO2 base coatings.

Most of the ice accretion on airframe is due to supercooled droplets in clouds which are located at altitudes below 2400 m which aircrafts frequently have to pass during takeoff and landing. In the present work, the impact and freezing of a supercooled droplet on a superhydrophobic surface with hysteresis is modeled based on (i) the volume of fluid (VOF) method coupled with a dynamic contact angle model, (ii) the modified momentum and the enthalpy formulation of the energy equations for the phase change during freezing, (iii) the nucleation theory, making use of Gibbs function as an energy barrier to be overcome before the supercooled liquid instantly freezes upon contact with the substrate. The simulation retrieves the characteristic concave ice-shape during droplet freezing, which is also found to promote the contact angle pinning. The solidification time which controls the type of ice is found to evolve exponentially with the droplet maximum spreading diameter. The simulations results agree well with the experimental data of supercooled droplet from the literature. The approach developed in this paper, which accounts for droplet nucleation and freezing mechanism, can be used to model and better understand the impact of supercooled water droplets of various sizes involved in ice accretion on aircraft wings leading edge.

In an effort to capture the complex evolving interface of internal and external flow in an effervescent atomizer, a compressible Eulerian method, along with the volume-of-fluid method coupled with the large eddy simulation model, are employed in a two-phase flow system. Water is injected into the atomizer with a constant mass flow rate of 0.0133 kg/s (i.e., 800 mL/min). The mass flow rate of air is adjusted to vary the gas-to-liquid ratio (GLR) from 0.55% to 2.6%. It is observed that the increase in the GLR is accompanied by an evolution of the internal flow from a complex bubbly flow to an annular flow, which consequently reduces the liquid film thickness at the discharge orifice. Further studies on the internal pressure illustrate the critical condition, which leads to choked flow and pressure oscillations at the discharge orifice. Increasing the GLR was found to affect the internal flow, resulting in changes to primary atomization parameters such as a shortening of the breakup length and a widening of the spray cone angle. The numerical predictions are in good agreement with the experimental results under the same operating conditions.

The focus of this study is to investigate and compare the behavior of a droplet on superhydrophobic (SHS) and slippery lubricant impregnated (SLIPS) surfaces under the effect of air shear flow. In this regard, both experimental and numerical analyses have been conducted to compare their performance on droplet mobility under different air speeds. Two different lubricants have been utilized to scrutinize their effect on droplet movement. The numerical simulations have been performed based on the volume of fluid method coupled with the large eddy simulation turbulent model in conjunction with the dynamic contact angle method in addition to a model that can represent the effect of lubricants on slippery surfaces. The numerical simulations are compared with the experimental study in order to shed light on the underlying mechanisms. The results showed that under the same conditions, the critical velocity for droplet movement on the superhydrophobic surfaces is lower than that on the slippery lubricant impregnated surfaces due to the smaller droplet base diameter and the larger contact angle. The hydrodynamics of droplet mobility on superhydrophobic surfaces exhibits a rolling behavior while for the slippery lubricant impregnated surfaces a combination of rolling and sliding is observed. Beyond the critical airflow speed, a complete droplet shedding on all surfaces occurs. The wetting length and position of the droplet on superhydrophobic and slippery surfaces have been measured. On slippery surfaces, the speed of droplets is greatly affected by the lubricant properties while similar behavior in the wetting lengths is observed.

This paper reports a novel method for fabrication of an electro-thermal heating element, as de-icer or anti-icer, for the polymer-based composites. The plasma spray technique was utilized for the deposition of a Nickel-Chrome-Aluminum-Yttrium (NiCrAlY) coating layer as a heating element on top of the glass/epoxy composite. To improve the adhesion strength and deposition efficiency of the coatings and to protect the composite fibers during grit blasting and spraying, a woven wire stainless steel mesh was added to the composite substrates during the composite fabrication process. Metal mesh will act as an anchor to keep the coating on the surface. Two types of woven wire and two types of NiCrAlY powder with the fine and coarse particle size distributions were used. Good processing parameters for grit blasting and plasma spraying are identified. It is found that the surface modification method applied to the composite substrates prior to the coating process makes a significant improvement in the coating thickness uniformity and deposition efficiency. Several tests were conducted on the coated samples for determination of their mechanical and electrical properties. Using flatwise tensile tests, it is shown that application of proper surface modification method and set of spray parameters could result in improving the coating bonding strength significantly. The electrical and thermal analyses of the coated samples are also performed. It is shown that the coated samples have a high capability in the generation of heat and can be used as a heating element.

A three-dimensional unsteady numerical model is employed to study the in-flight droplets/particles' atomization in the suspension plasma spraying (SPS) process. A user-defined function (UDF) written in C programming adds the electromagnetic fields to the fluid flow field. A two-way coupled Eulerian-Lagrangian technique simulates the sprayed droplets' interaction with the plasma flow. The developed model is applied to investigate the droplets' atomization behavior in the SPS process. Droplets' atomization is simulated by applying the Kelvin-Helmholtz Rayleigh-Taylor (KHRT) breakup model. This model considers the effects of both aerodynamic forces (Wave model) and Rayleigh-Taylor instabilities on the atomization process combing a liquid core region (Levich model). Previous works to estimate the KHRT breakup model's coefficients were performed on a liquid jet (e.g., fuel jet) into the air. Because the density ratio of suspension in the plasma is higher than that of the liquid jet into the air (suspension density is higher than a liquid droplet and plasma density is lower than air), the model's coefficients should be modified for the suspension atomization in the SPS process. For this reason, nine case studies are investigated to see the effect of KH and RT instabilities and the liquid core length on the droplets/particles' atomization. The results obtained from the suggested values show a good agreement compared to the existing data.

Breakup of circular and elliptical liquid jets in subsonic gaseous crossflows is experimentally studied using shadowgraph technique. The experiments are performed at gaseous Weber numbers less than 15, the liquid-to-gas momentum flux ratio between 50 and 320, and the orifice aspect ratio between 0.22 and 4.47. It is found that in addition to the momentum flux ratio, the orifice aspect ratio can change the liquid penetration height significantly. For a fixed momentum flux ratio, the penetration of elliptical jets is less than that of circular jet. Moreover, for a given momentum flux ratio and Weber number, the column breakup location of elliptical jets is earlier than that of circular jet. Empirical correlations for the penetration height as well as the column breakup location of circular and elliptical jets are developed in this study.

Water droplet freezing affects many aspects of our daily lives, although there is no comprehensive model which retrieves all of the experimentally observed features when a liquid water droplet deposited on a cold substrate turns to ice. In this paper, we present general governing equations to describe water droplet freezing on a solid substrate by accounting for the physical properties of each phase present, namely the liquid and ice, in addition to the solid substrate. The approach, which takes advantage of the full mean curvature expression of both the droplet–air and liquid–ice interfaces, disjoining pressure, the Gibbs–Thomson effect, natural convection and the substrate thermal and physico-chemical properties, enables us to model a more realistic frozen droplet shape, without a prior assumption for the freezing growth angle. In addition to correctly predicting the freezing time, we capture both qualitatively and quantitatively the key experimentally observed features during water droplet freezing such as volume expansion, concave ice front and the cusp singularity. Furthermore, the proposed equation for the tip angle seems to explain its experimentally observed variability.

In this study, a three-dimensional DC plasma torch is modeled using Joule effect method to simulate the plasma jet and its voltage fluctuations. The plasma gas is a mixture of argon/hydrogen, and the arc voltage fluctuation is used as an input data in the model. Reynolds stress model is used for time-dependent simulation of the oscillating flow of the plasma gas interacting with the ambient air. The results are used to investigate the plasma oscillation effects on the trajectory, temperature, and velocity of suspension droplets. Suspensions are formed of ethanol and yttria-stabilized zirconia submicron particles and modeled as multicomponent droplets. To track the droplets/particles, a two-way coupled Eulerian–Lagrangian method is employed. In addition, in order to simulate the droplet breakup, Kelvin–Helmholtz/Rayleigh–Taylor (KH–RT) breakup model is used. After the completion of suspension breakup and evaporation, the sprayed particles are tracked to obtain the in-flight particle conditions including trajectory, size, velocity, and temperature. The arc voltage fluctuations were found to cause more than two times wider particle trajectories resulting in wider particle temperature, velocity, and size distributions compared with the case of constant voltage.

Droplet impact on a solid surface occurs in many natural phenomena and industrial applications such as raindrop impact on the ground, drip irrigation, spray cooling, spray coating, inkjet printing, etc. Importantly, in several cases such as aerosol transfer from the sea, oxygen dissolution, and thermally sprayed coatings the impinging droplets contain air bubble dealing with liquid-bubble-solid interaction. These hollow droplets upon impacting on the target surface, due to their controllable weight and embedded medium, control the splat properties by triggering the cavitation and releasing the inner substance. To better understand the flattening process of a hollow droplet, in this work a comprehensive experimental and numerical study is performed on water and glycerol droplets impacting on a rigid surface. The experiments are repeated on different types of surfaces including aluminum, sand-blasted steel, and superhydrophobic. The results show that the mechanisms of the post-impact process of hollow droplets are different from those of dense droplets in several aspects. We study the role of surface wettability, liquid properties, and impact velocity on the droplet flattening process. In the numerical work, we compare pressure contours and velocity vectors for dense and hollow droplets that provide an in-depth insight through the formation of splat and counter jets.