Water Droplet Impact Research Articles

Droplet impact dynamics on a flexible superhydrophobic cantilever wire mesh

Droplet impact on fixed permeable surfaces has many applications, including spray formation, coating deposition, and filtration processes. The present research studies the droplet and beam dynamics during the water droplet impact on the flexible superhydrophobic cantilever mesh, and the results are compared with the impact on a fixed-fixed superhydrophobic mesh. The effects of beam flexibility (droplet impact point), mesh opening size, and impact velocity on the rebound behavior of droplets, spreading diameter, contact time, beam deflection, and frequency were investigated. Although the contact time is reduced compared to the rigid surface, the well-known pancake rebound is not observed anymore on the cantilever beam. Therefore, the contact time reduction is not as pronounced as the fixed mesh for the cantilever beam. The spreading time and maximum spreading diameter decreased as the impact distance from the fixed point increased due to the reduced transmitted energy to the beam. However, the retraction time is affected by both beam dynamics and liquid penetration. Therefore, the shortest contact time depends not only on the oscillation and frequency of the beam and droplet but also on the liquid penetration. Moreover, the results prove that by defining an appropriate maximum spreading ratio, it becomes independent of the impact point. Finally, the beam deflection and frequency, in addition to the maximum spreading diameter of the droplet are modeled theoretically.

Hybrid Smoothed-Particle Hydrodynamics/Finite Element Method Simulation of Water Droplet Erosion on Ductile Metallic Targets

Erosion of metallic surfaces due to the permanent impact of high-speed water droplets is a significant concern in diverse industrial applications like turbine blades, among others. In the initial stage of water droplet erosion, there is an incubation regime with negligible mass loss whose duration is strongly dependent on water droplet sizes and velocities, the initial state of the surface, and the material properties of the target. The prediction of the incubation period duration is one of the main topics of research in the field. In this work, the interaction of the water droplets with a metallic surface is simulated using a hybrid Smoothed-Particle Hydrodynamics/Finite Element Method modeling scheme. The effect of multiple random impacts on representative target areas for certain ranges of impact angles and velocities was studied using a combination of simple material and damage models for the target surface of Ti-6Al-4V titanium alloy. The simulation is able to reproduce the main dependencies of the incubation regime and the first stages of water droplet erosion on the impact angle and velocity as reported in the literature. This framework can be considered a foundation for more advanced models with the goal of a better understanding of the physical mechanisms behind the incubation regime in order to devise strategies to extend it in real applications.

Developing RobEVELOPING DURABLE SUPERAMPHIPHOBIC COATING ON ROUGH ALUMINUM USING RESIDUAL FROM BURNED RECYCLED SILICONE RUBBER FOR ANTI-CORROSIONust Superamphiphobic Coating on Rough Aluminum Substrates Using Recycled Silicon Rubber and Simple Spray Pyrolysis Technique for Anticorrosion Applications

This study presents a new, cost-effective, and environmentally friendly approach for creating a superamphiphobic coating. The method involves using a spray coating technique to apply silicone rubber onto smooth and micro-rough aluminum (Al) substrates. To enhance the coating's surface energy reduction, a thin layer of silicone rubber – trifluorotoluene (SR-TFT) was added. The morphology and chemistry of the coatings were analyzed utilizing scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), and Fourier Transform Infrared Spectrometer (FTIR). The coatings displayed superamphiphobic properties with contact angles (CAs) of 170° for water and over 150° for both glycerol and ethylene glycol. Additionally, a remarkably low water droplet sliding angle (SA) of less than 2° was observed for surfaces coated with silicone rubber (SR)- trifluorotoluene (TFT), whether smooth or roughened. The coatings were tested for mechanical and chemical durability by impinging water droplets and immersing them in an acidic liquid respectively. The results showed that SR-TFT-coated silicone rubber on micro-rough Al substrates maintained its superamphiphobic property and exhibited excellent corrosion resistance compared to hydrophilic Al plates. Furthermore, the coatings displayed self-cleaning properties when water droplets were poured over a dusty surface, as the rolling water droplets effectively collected contaminant particles, leaving the surface of the sample clean. These findings indicate potential applications for the developed coatings as self-cleaning surfaces in challenging environmental conditions.

Thermal Hydraulics Simulation of a Water Spray System for a Cooling Fluid Catalytic Cracking (FCC) Regenerator

Olefins are crucial building blocks for petrochemical industry, serving as raw materials for the production of various products such as plastics, synthetic fibers, detergents, solvents, and other chemicals. In FCC, heavy petroleum feedstocks are injected into a catalytic cracking unit, where they are mixed with a catalyst. The catalyst aids in breaking down the large hydrocarbon molecules into smaller fragments, including olefins like Propylene and Ethylene. These polymerization reactions occur at high temperatures. They demand that heat removal occurs as quickly as possible in order to control the reactor temperature and to avoid “hot spots” in the Regenerator or localized oxidation reactions (and to avoid creep rupture of the regenerator steel cladding). The cooling of the regenerator cladding surface can be achieved by impinging water droplets (spray), ejected from a spray nozzle. Spray cooling can provide uniform cooling and handle high heat fluxes in both a single phase and two phases. This research provides a thermal hydraulic design of regenerator spray cooling systems. In the framework of this research, Fire Dynamics Simulator (FDS) software was applied in order to simulate the temperature field and the water vapor mass fraction. A COMSOL Multiphysics finite element code was used in order to calculate the temperature field inside the regenerator cladding. The calculated surface temperatures and heat transfer convective coefficient, obtained using FDS software, were validated successfully against COMSOL numerical results and previous results in the literature. The numerical simulations were carried out for two cases. The first case was carried out at a distance of 0.5 m, and the second case was carried out at a distance of 0.2 m. A grid sensitivity study was carried out on the FDS model. Numerical integrations were carried out over time in order to calculate the average temperatures. The difference between these four average temperatures, calculated by applying different grids, is less than 7.4%. The calculated surface temperatures and heat transfer convective coefficient were validated successfully against COMSOL numerical results and previous research. It was shown that the calculated temperatures decrease in the second case. The water spray system managed to cool the steel wall more effectively as the water spray system approaches the steel cladding.

An analytical framework for predicting the total freezing delay time of water droplet impact on hydrophobic solid surfaces

An analytical framework is developed to investigate the impact and freezing delay of a water droplet on solid surfaces, particularly hydrophobic ones, at subfreezing temperatures. Submodules for droplet dynamics, supercooling, nucleation and recalescence, and equilibrium solidification are developed for different droplet impact and freezing stages. These submodules are then integrated to estimate the total freezing delay time per unit mass of the water droplet. It is found that the nucleation time depends on the impact height, surface temperature, droplet impact temperature, and the droplet-substrate contact angle, while the solidification time depends on the droplet size, the nucleation temperature, and the droplet final contact diameter, which is related to the initial droplet diameter, the Weber number, the Reynolds number, and contact angle. The analytical model is validated with results from droplet freezing experiments in a subfreezing environment. By simplifying the complex droplet dynamics, nucleation, and freezing processes, the new analytical framework provides a useful way to estimate the total freezing time of impacting water droplets on hydrophobic surfaces. Its accuracy needs to be improved for practical engineering applications.

Flow and Heat-Transfer Characteristics of Droplet Impingement on Hydrophilic Wires.

It is a common phenomenon that droplets collide with wires in industrial production, and their flow and heat-transfer behavior significantly impact the production efficiency. This article presents an experimental and numerical study on the impact of pure water droplets on hydrophilic stainless-steel wires. The dynamic behavior and solid-liquid heat-transfer law of droplet impacting the wire are emphatically analyzed. The impact position of the droplets has a significant effect on their morphology. Under the condition of low Weber number (We), eccentric impacts tend to cause droplets to separate from the wire. Additionally, both We and wire/droplet size ratio have noticeable effects on the droplet morphology. The smaller the We, the larger the wire/droplet size ratio, and the easier it is for droplets to be captured by wires. Conversely, as We increases and the wire-to-droplet size ratio decreases, some droplets become detached from the wire, primarily exhibiting a single-film falling mode. Furthermore, the impact morphology of droplets is influenced by the Ohnesorge number (Oh). The higher the Oh, the more inclined the droplet to develop a double-film falling mode. There is obvious field synergy in the process of droplet impacting on wire. The maximum heat flux is located at the three-phase contact line, while the minimum heat flux is observed at the bubble interface. The impact position of droplets influences the temperature distribution, although its impact on the magnitude of temperature variation is minimal.

Comparative study on the water droplet erosion behavior between stretched and casted aeronautical PMMA

Rain droplet erosion is a potential threat in aviation, especially for forward-facing components such as windshields of aircraft. To comprehend and mitigate such hazards, a comparative study was conducted on two kinds of commonly used aeronautical PMMA materials: casted and biaxially stretched. The study aimed to investigate the water droplet erosion (WDE) behavior and the underlying damage mechanism. A modified pulsed water jet erosion facility was constructed to mimic the impact of successive water droplets. The influence of impact velocity on the WDE behavior was explored and quantitative evaluations of volume loss, surface entrance area, and erosion depth were conducted. Results revealed that the increase in impact velocity leads to faster damage initiation and a larger erosion rate. Compared with the stretched sample, the casted sample exhibits higher erosion resistance. The stretched sample exhibits a funnel-shaped cavity with a flat bottom, while the casted sample shows a tapered crater with shallow depth. The cavity shape can influence the subsequent flow fields and stress distribution, thereby affecting the development of erosion entrance and depth. Microscopic analysis of the erosion cavities revealed different damage mechanisms at each stage of erosion. The stretched sample exhibited exfoliation of fine flakes and denudation of large pieces due to stress waves and lateral jet-induced shear, whereas hydraulic penetration had a more pronounced effect on the casted sample. The cushioning effect of the water film was also studied, noting a gradual reduction in its effectiveness with increasing impact velocity.

Damage behavior and assessment of aeronautical PMMA subjected to high-velocity water-jet impact

Rain erosion of forward-facing aircraft components such as windshields can lead to a significant reduction in performance, hence a potential hazard to air safety, especially for supersonic fighters exposed to precipitation. To understand such threats, the damage behavior of two types of most commonly used aeronautical PMMA, i.e., oriented and non-oriented, under water droplet impact is investigated. A single-impact jet apparatus based on the one-stage gas gun was established to simulate the rain droplet impact. Two critical variables were evaluated, including water-jet velocities and incident angles. Typical damage characteristic features including annular depression and circumferential cracks were observed on non-oriented PMMA targets. In contrast, on oriented PMMA targets, massive subsurface craze delamination was observed without any sign of surface cracks. The distinct damage features are determined by the different damage behavior of the oriented and non-oriented PMMA targets during the initial stage of the impingement and the shearing effect of the lateral jet. Increasing impact velocity leads to surface peeling and bulk removal, escalating the damage severity. The damage mechanism of subsurface failure was revealed by observing stress wave interaction and the FEM simulation. An impulse method was developed to quantify each impact event and assess the damaged area. A linear relationship between the damaged area and the initial impact stage impulse was established, which can be applied for preliminary evaluation and prediction of the damage caused by high-velocity water droplet impact on aeronautical PMMA.

Effects of surface temperature and Weber number on the dynamic and freezing behavior of impacting water droplets on a superhydrophobic ultra-cold surface

The dynamic and freezing behavior of a water droplet impacting a superhydrophobic cold surface is worthy of investigation. In this study, experiments are conducted to examine the impact and freezing of a water droplet on a cold superhydrophobic surface whose temperature changes in a wide range between −10 °C and −60 °C. A numerical model that introduces a continuous energy balance equation is developed to investigate the process numerically. The effects of surface temperature, Weber number, and initial droplet temperature on the droplet morphology, spread factor, contact time, and rebound height are investigated. The droplet exhibits three different behaviors as the surface temperature decreases: full rebound, partial rebound, and full adhesion. For the full rebound case, the results indicate that the growth rate of the contact time with decreasing surface temperature increases as the Weber number decreases. For the full adhesion case, the variation in the surface temperature minimally affects the dynamic behavior of the droplet impacting on an ultracold superhydrophobic surface (below −40 °C). The effect of surface temperature on the maximum spread factor is more pronounced at higher Weber numbers. Additionally, the ice nucleation rate is analyzed theoretically. The results show that the nucleation rate of droplets impacting an ultracold surface remains almost constant regardless of the surface temperature and surface roughness. Moreover, a regime map showing the different droplet morphologies correlating with the surface temperature and Weber number is established. The critical surface temperatures for different droplet morphologies of supercooled and room-temperature droplets are analyzed. This study allows us to better understand the dynamic and heat transfer mechanisms involved in the impact and freezing of a water droplet on a superhydrophobic surface.

Effect of attack angles on droplet impact on textured metal surfaces: Droplet morphisms, surface tilt angles, droplet deformation

The experimental results of the water droplet impact on the tilted metal substrates are given. To establish the effect of the texture on the spreading of droplets, parallel-directed scratches were applied using a grinding machine. To analyze the shape of water droplets after impingement, topological-based data analysis (TDA) was used. The TDA showed that the droplet could be represented as a group of trivial topological structures: spheres, discs, and hemispheres. The latter creates the prerequisites for the development of relatively simple mathematical models that describe the complex processes of liquid droplets impacting on tilted, textured substrates. Moreover, the time dependence of the droplet contact line (L) and the left and right contact angles (LCA and RCA) on a textured surface were established. It was shown that anisotropic modification of the substrate has a significant effect on the characteristics and conditions of the spreading of the coolant droplets.

Freezing behaviors of an impacting droplet on subcooled hydrophobic surfaces

The solidification of impacting water droplets on cold substrates wildly exists in nature and engineering devices, which would significantly affect their performances. The freezing mechanisms of impacting water droplets on hydrophobic surfaces are not well understood. In this work, we explore the freezing behaviors of impacting water droplets on a cold hydrophobic surface in the regime where the interaction between the droplet impact dynamics and solidification is strong. We observe six different freezing morphologies on hydrophobic surfaces via tuning the substrate temperature and impact velocity. The diverse freezing morphologies are resulted from the competition of timescales associated with the solidification and droplet impact dynamics. The final freezing area on surfaces shows a non-monotonic variation with impact height when Ts = -38° C. We thus developed a model to predict the final freezing area, which agrees well with the experimental results.

SEM and EDX analysis of erosion products formed on steam turbine blade

ABSTRACT Water droplet erosion (WDE), which is brought on by the high-energy impact of liquid water droplets, is a serious problem for steam turbine blades. Nevertheless, rather than addressing the WDE of actual steam turbine blades, the majority of the published research on this issue uses laboratory test rigs. In this study, scanning electron microscopy and energy-dispersive X-ray analysis are used to examine how the surface of low-pressure steam turbine blades that had been in service eroded over time. The results of this study provide valuable insights into the microstructural features and elemental composition of X20Cr13 steel in steam turbine blades, as well as the factors that can contribute to damage and failure.

Numerical Investigation on the influence of meteorological conditions on wind turbine icing

Keeping the blade structure and shape in the design condition is the premise and guarantee for efficient operation of wing turbine, but wet and cold climate will lead to icing blade, then the blade shapes were changed and the output power of the wind turbine is reduced. A numerical frame for wind turbine icing was developed in present work, which consists of a numerical method for impingement of water droplets based on the multiple frames of reference and Euler method and a 3D heat and mass transferring method for wind turbine icing, then the effects of several meteorological parameters, including the temperature, Liquid Water Content (LWC), Mean Volume Diameter (MVD) and the inflow velocity, on ice shape of the blade were investigated in the numerical frame. The present method can provide supports for prediction of icing process and aerodynamic characteristic after icing of wind turbine blades.

Experimental validation and tightly coupled numerical simulation of hot air anti-icing system based on an extended mass and heat transfer model

When an aircraft flies through a cloud containing supercooled liquid water droplets, ice accretion may deposit, posing a threat to flight safety. Hot air anti-icing is one of the most widely used anti-icing techniques. An extended mass and heat transfer model for runback water on a hot air anti-icing surface is developed. The model is based on the mass conservation and energy conservation of continuous water film and rivulet water film, considering the conditions under which continuous water film may break up into rivulet water film. Additionally, the effect of water vapor resulting from liquid water evaporation is also taken into account. Then, a three-dimensional tightly coupled numerical method for hot air anti-icing systems is constructed based on the developed model. The method includes modules for the calculation of external cold air flow, solid skin heat conduction and internal hot air flow, the calculation of water droplet impingement, and the calculation of anti-icing thermodynamics. The accuracy of the method is validated against experimental studies on an enhanced heat transfer piccolo tube anti-icing system in the icing wind tunnel of China Aerodynamics Research and Development Center (CARDC). The temperature results obtained from the three-dimensional numerical simulations showed good agreement with the experimental results, indicating that this method can be utilized in preliminary and detailed designs to evaluate the thermal performance of hot air anti-icing systems.

Fast spreading of liquid on Leidenfrost vapor layer surface

Previous studies found that a surface with a Leidenfrost vapor layer can effectively decrease the drag of the flow field and even increase the slip length on the boundary flow. This study investigated this phenomenon by the experiment of impact of water droplets on surfaces both with and without Leidenfrost vapor layers in the film boiling regime. The rapid spreading behaviors are dominated by the inertia of the droplet, which mainly depends on the surface with or without Leidenfrost vapor film. In the cases with higher inertia force of critical Weber numbers, this study observed that a surface with a Leidenfrost vapor film can effectively increase the wetting velocity and induce larger maximum spreading factor, especially in the case of rough surfaces.

Numerical investigation of the freezing property of impinging water droplets on inclined cold superhydrophobic surfaces

We numerically study the freezing process of impinging water droplets on inclined cold superhydrophobic surfaces, concerning the dynamic and heat transfer properties of impinging water droplets. Increasing the surface inclination angle and decreasing the contact angle hysteresis (CAH) is found to promote the rebound of impinging water droplets at a higher cold superhydrophobic temperature (such as −10 °C). The influence of the cold surface temperature on the receding side of impinging water droplets is stronger than that on the front side, and so is the CAH. The frozen region inside impinging water droplets first appears on the receding side, and this becomes more obvious as the surface inclination angle increases. The relatively intensive heat transfer process appears early in the impinging process of water droplets on an inclined cold superhydrophobic surface. Increasing the surface inclination angle could weaken the heat transfer process of impinging water droplets at a higher cold surface temperature (such as −10 °C), but it is not remarkable at a lower cold surface temperature (such as −30 °C). By increasing the supercooling degree, impinging Weber number, and CAH, the freezing process of impinging water droplets is enhanced.

Bouncing Regimes of Supercooled Water Droplets Impacting Superhydrophobic Surfaces with Controlled Temperature and Humidity.

Superhydrophobic surfaces have shown significant potential for the passive anti-icing application due to their unique water repellency. Reducing the contact time between the impacting droplets and the underlying surfaces with certain textures, especially applying the pancake bouncing mechanism, is expected to eliminate droplet icing upon impingement. However, the anti-icing performance of such superhydrophobic surfaces against the impact of supercooled water droplets has not yet been examined. Therefore, we fabricated a typical post-array superhydrophobic surface (PSHS) and a flat superhydrophobic surface (FSHS), to study the droplet impact dynamics on them with controlled temperature and humidity. The contact time and the bouncing behavior on these surfaces and their dependence on the surface temperature, Weber number, and surface frost were systematically investigated. The conventional rebound and full adhesion were observed on the FSHS, and the adhesion is mainly induced by the penetration of the droplet into the surface micro/nanostructures and the consequent Cassie-to-Wenzel transition. On the PSHS, four distinct regimes including pancake rebound, conventional rebound, partial rebound, and full adhesion were observed, where the contact time increases correspondingly. Over a certain Weber number range, the pancake rebound regime where the droplet bounces off the surface with a dramatically shortened contact time benefits the anti-icing performance. By further decreasing the surface temperature, the pancake rebound turns into the conventional rebound, where the droplet is not levitated after the capillary emptying process. Our scale analysis indicates that the frost between the posts reduces the capillary energy stored during the downward penetration, resulting in the failure of the pancake bouncing. A droplet adheres onto the frosted surface at sufficiently low temperature, especially at larger Weber numbers, on account of the coupling influence of droplet nucleation and wetting transition.

Alcohol-induced elevation in the dynamic Leidenfrost point temperature for water droplet impact

In the present study, an experimental investigation is conducted to explore the alcohol additive effect on water droplet impact on a smooth red copper surface and to construct impact regime maps. The observations show that the alcohol additives can effectively facilitate droplet atomization and breakup, and increase the dynamic Leidenfrost point temperature. An interesting phenomenon is noted that the dynamic Leidenfrost point temperatures of alcohol-added water droplets at lower impact velocities are obviously elevated compared with pure water at higher impact velocities. In other words, the addition of a small amount of alcohol additive can effectively compensate for the decrease in the dynamic Leidenfrost point temperature caused by smaller impact momentum. A new triggering mechanism of the dynamic Leidenfrost effect is proposed with the surface roughness effect taken into consideration. An approximation model based on the pressure balance criterion is then developed to discuss the underlying physics of the alcohol-induced elevation in the dynamic Leidenfrost point temperature. Combining the scaling analysis of vapor layer thickness, a scaling relation of the dynamic Leidenfrost point temperature is derived, and the predictions agree well with present and previous data. It is concluded that the present findings provide an effective method to expedite the transformation from film boiling to transition boiling for heat transfer improvement with an increased dynamic Leidenfrost point temperature, simply by adopting alcohol additives to regulate the droplet thermophysical properties.

Mechanically robust liquid-embedded coating with anti-icing/deicing durability

The slippery liquid-infused porous surface has been concerned for their excellent icephobicity. However, unavoidable lubricant consumption can result in the failure of anti-icing/deicing properties. Traditional coatings cannot escape the choice between anti-icing/deicing property and mechanical performance. This paper reports mechanically robust liquid-embedded coating (MRLC) with anti-icing/deicing durability. The MRLC is prepared by a facile one-step method via adding silica and silicone oil in the process of co-curing PDMS and epoxy resin. The prepared MRLC-5–5–80 have excellent anti-fouling, self-cleaning, anti-icing/deicing and anti-frost properties in high humidity and low temperature environment. This surface remained free of liquid residue and ice within 30 min in supercooled water droplet impact test. In addition, this coating exhibited a low τice (5.48 kPa) and excellent durability. The PDMS polymer and fumed silica serve as oil storage reservoir for the coating to ensure durability, and the anti-icing/deicing durability can be restored by self-replenishing and external-replenishing. More importantly, the coating guarantees anti-icing/deicing durability while possessing excellent mechanical robustness. The liquid repellency and anti-icing/deicing performance still prevailed after various mechanical damages. This method opens the door for production of durable and mechanically robust for various anti-icing/deicing practical applications.

Study on the water ingestion performance of compressor with inlet particle separator

Abstract For an aero turbo-shaft engine, rain water enters the intake protection device and the morphology of water droplets after hitting the wall will affect the internal flow of the compressor. In this paper, Sommerfeld number was used to determine the shape of water droplets after they hit the wall. The flow field and characteristics of the compressor with inlet particle separator under different water ingestion conditions were studied with considering the impact of water film and droplets splashing near the wall. The results show that the entry of water droplets deteriorates the internal flow field of the compression component, and the water film affects the flow state of the airflow near the wall. Compared to the effect of the splashing factor, the viscous resistance effect of water film near the wall on the airflow and boundary layer can cause a significant reduction in the characteristics and stability of the compressor.