Dynamics of Droplet Shedding and Coalescence under the Effect of Shear Flow

Droplet shedding and coalescence has various industrial applications from ink-jet printing to ice accretion on wind turbine blades, power lines or aircrafts. It is known that the incipience of icing phenomenon in the mentioned applications arises from the shedding and coalescence of the rain droplets. The coalesced droplets then start to form a runback flow and when temperature is below the water freezing point, ice can be accumulated on these structures which alters the performance in the mentioned technologies. Accordingly this work is dedicated to a fundamental study on dynamics of droplet shedding and coalescence as a preliminary stage of ice formation. Sessile droplets are deposited on surfaces where various air flows are introduced to them for analyzing their shedding behavior. As it is believed that using superhydrophobic coatings decreases the amount of ice accumulation on the solid surfaces, the effect of different surface wettabilities ranging from hydrophilic to superhydrophobic is also studied on droplet dynamics. It is shown that on a hydrophilic substrate when the air speed is high enough, rivulets are formed from merging droplets. In contrast, on a superhydrophobic substrate there is no rivulet formation. Instead coalesced droplets roll on the surface and detach from it if the air speed is sufficiently high. In addition, the results indicate a contrast in the mechanism of the coalescence and subsequent detachment between a single and two droplets on a superhydrophobic surface. At low air speeds, the two droplets coalesce by attracting each other before detaching with successive rebounds on the substrate, while at higher speeds the detachment occurs almost instantly after coalescence, with a detachment time decreasing exponentially with the air speed. A wind tunnel experiment was designed to characterize the rivulet dynamics on various surface wettabilities. Smoothed Particle Hydrodynamics method was performed and its results were compared with the ones obtained from the experiments. The results indicate that increasing the air speed results in formation of waves with higher frequency in comparison with the lower air speeds. It was also demonstrated that as on superhydrophobic substrates instead of rivulets series of droplets are formed these substrates can be suitable candidates for anti-icing purposes

Enhancement of Asphalt Performance by Graphene-Based Bitumen Nanocomposites

As the State of California continues to grow, demand for enhanced infrastructure such as roadways and highways escalates. In view of the current average highway lifespan of 15–20 years, the improvement of asphalt binders leads to material sustainability by decreasing required maintenance and increasing the lifespan of roadways. In the present investigation, enhancement of asphalt binder properties was achieved by different methods of mixing varying compositions of graphene nanoparticles with an SBS polymer and asphalt binder. Additionally, experimental evaluation and comparison of the rheological and mechanical properties of each specimen is presented. Graphene nanoparticles have attracted great curiosity in the field of highway materials due to their incredible rigidity, even in small quantities. Addition of as little as 1.0%nanoparticles in combination with polymers in an asphalt binder is expected to increase the rigidity of the material while also maintaining the beneficial polymer characteristics. Evaluation of the effect of the mixing design established that the methods for application of graphene to the polymer-modified asphalt binder are critical in the improvement of a roadway, resulting in resistance to premature aging and strain from constant road operation

A Comprehensive Review on Fluid Dynamics and Transport of Suspension/Liquid Droplets and Particles in High-Velocity Oxygen-Fuel (HVOF) Thermal Spray

In thermal spraying processes, molten, semi-molten, or solid particles, which are sufficiently fast in a stream of gas, are deposited on a substrate. These particles can plastically deform while impacting on the substrate, which results in the formation of well-adhered and dense coatings. Clearly, particles in flight conditions, such as velocity, trajectory, temperature, and melting state, have enormous influence on the coating properties and should be well understood to control and improve the coating quality. The focus of this study is on the high velocity oxygen fuel (HVOF) spraying and high velocity suspension flame spraying (HVSFS) techniques, which are widely used in academia and industry to generate different types of coatings. Extensive numerical and experimental studies were carried out and are still in progress to estimate the particle in-flight behavior in thermal spray processes. In this review paper, the fundamental phenomena involved in the mentioned thermal spray techniques, such as shock diamonds, combustion, primary atomization, secondary atomization, etc., are discussed comprehensively. In addition, the basic aspects and emerging trends in simulation of thermal spray processes are reviewed. The numerical approaches such as Eulerian-Lagrangian and volume of fluid along with their advantages and disadvantages are explained in detail. Furthermore, this article provides a detailed review on simulation studies published to date

Numerical Study of the Effects of Twin-Fluid Atomization on the Suspension Plasma Spraying Process

Suspension plasma spraying (SPS) is an effective technique to enhance the quality of the thermal barrier, wear-resistant, corrosion-resistant, and superhydrophobic coatings. To create the suspension in the SPS technique, nano and sub-micron solid particles are added to a base liquid (typically water or ethanol). Subsequently, by using either a mechanical injection system with a plain orifice or a twin-fluid atomizer (e.g., air-blast or effervescent), the suspension is injected into the high-velocity high-temperature plasma flow. In the present work, we simulate the interactions between the air-blast suspension spray and the plasma crossflow by using a three-dimensional two-way coupled Eulerian–Lagrangian model. Here, the suspension consists of ethanol (85 wt.%) and nickel (15 wt.%). Furthermore, at the standoff distance of 40 mm, a flat substrate is placed. To model the turbulence and the droplet breakup, Reynolds Stress Model (RSM) and Kelvin-Helmholtz Rayleigh-Taylor breakup model are used, respectively. Tracking of the fine particles is continued after suspension’s fragmentation and evaporation, until their deposition on the substrate. In addition, the effects of several parameters such as suspension mass flow rate, spray angle, and injector location on the in-flight behavior of droplets/particles as well as the particle velocity and temperature upon impact are investigated. It is shown that the injector location and the spray angle have a significant influence on the droplet/particle in-flight behavior. If the injector is far from the plasma or the spray angle is too wide, the particle temperature and velocity upon impact decrease considerably