Supercooled Water Droplets Research Articles

A climatology of Arctic fog along the coast of East Greenland

AbstractThis study presents a comprehensive climatology of coastal fog from four synoptic weather stations operated by the Danish Meteorological Institute along the entire East Greenland coast between 1958 and 2016. Elements investigated include fog frequency, daily timing, temperature, wind, visibility and radiosonde profiles during fog. The spatiotemporal patterns in fog from the low‐ to high‐Arctic locations were related to varying regional seasonal temperatures, surface and upper‐air wind and sea ice conditions, and to correlations with the North Atlantic Oscillation (NAO) and the Greenland Blocking Index (GBI). Results indicate that ˜70–80% of East Greenland fog occurs in summer (MJJA), and yearly fog onset is near‐coincident with the start of sea ice break‐up. This warm‐season fog has the typical characteristics of advection fog, as shown in the radiosonde profiles and the association with a gentle sea breeze. More than 95% of warm‐season fog is warmer than −10°C, and peaks close to 0°C and, therefore, consists of liquid or supercooled water droplets. In the cold season, mixed‐phase fog prevails in the high‐Arctic locations, accounting for ˜70% of observations. Ice fog (T < −30°C) occurs in only 2% of observations and is limited to Northeast Greenland during the cold season. The cold‐season composite radiosonde fog profiles in the high‐Arctic locations are characterized by deep (˜1000 m) and strong (˜6°C) surface‐based temperature inversions. Visibility during most fog conditions is lowest during the warm season (<500 m) and highest during the cold season (<800 m). In Northeast Greenland, visibility during warm‐season fog has decreased by ˜50 m·dec−1 between 1981 and 2016. In Southeast Greenland, fog visibility is high during low GBI and a positive phase of NAO, but no other correlations with climate indices were found.

Numerical investigation on rebound dynamics of supercooled water droplet on cold superhydrophobic surface

The rebound dynamics of a microscale supercooled water droplet on cold superhydrophobic surface is numerically studied. A three-dimensional numerical model based on level set method and Enthalpy-Porosity model is developed to study the impacting-freezing behaviors of supercooled water droplet. The retraction rate and the morphology evolution of the supercooled water droplet are focused on. The accuracy of the adopted numerical model is validated via the well agreement with the experimental results. Four different regimes are observed for the impacting-freezing process: fully rebound, satellite rebound, breakup and fully adhesion. The numerical results indicate that the retraction rate is decreased with the cold surface temperature. The heat transfer rate between water droplet and surface is increased with the impact velocity, droplet diameter and supercooled degree of cold surface. The evolution of droplet morphology reveals the competitive relationship between droplet impact dynamics and solidification. Regarding the fully rebound regime, the rebound length increases with the Reynolds number. For the situations that rebound with satellite droplet, with the increase of the impact velocity, the generation of satellite droplet is delayed, while the size of satellite droplet is increased.

The viscosity and surface tension of supercooled levitated droplets determined by excitation of shape oscillations

Abstract. We report a new method for determining the viscosity and surface tension of supercooled liquid droplets using electrodynamic levitation and phase analysis of shape oscillations. The method uses a high-frequency alternating electrical potential to excite shape oscillations in a levitated droplet, and the phase shift in the oscillations is used to simultaneously determine droplet viscosity and surface tension. The advantages over existing contactless methods include its applicability to atmospherically relevant temperatures and the possibility of continuously monitoring changes in real time. We demonstrate proof-of-concept measurement for supercooled water droplets and dilute sucrose solution droplets, and we anticipate that the technique could be used to measure viscosity values for droplets containing dilute organics. The technique is especially well-suited for investigation of the role of atmospheric processing in the viscosity and surface tension of solution droplets in equilibrium with a given or changing relative humidity.

Numerical study of supercooled water droplet impacting on cold superhydrophobic surface under electric field

The process of ice formation on outdoor electrical equipment is determined by the impact of supercooled droplets on the cold surface under an electric field. Clarifying the impact-freezing of a supercooled droplet under an electric field is important for designing anti-icing surfaces in the power industry. Here, we developed a coupled electro-solidification model by combining the volume of fluid (VOF) method, a dynamic contact angle model, the two-phase electrohydrodynamic (EHD) model, the modified enthalpy solidification method and the nucleation effect. Then, the impact dynamics of a water droplet on a cold superhydrophobic surface under an electric field were studied numerically. The effects of the electric field intensity, Weber number, initial droplet temperature and conductivity on the spreading factor βm and the freezing fraction αad are analyzed. By specifically considering the effect of the electric field enhancing the water to freeze, the bottom filament is successfully recreated in the simulation. This phenomenon was first reproduced in our simulation, and it would significantly increase the probability of icing on the cold surface under the electric field, even on the superhydrophobic surface. Finally, based on the numerical results and published experimental data, we fit the freezing fraction relation of the supercooled water droplet impinging on cold superhydrophobic surface under the electric field, which can be used to predict the icing growth and prevention of icing on transmission lines. The numerical model developed here can also be directly used to simulate ice growth and guide the design of anti-icing surfaces in power transmission systems.

A 1D Model for Nucleation of Ice From Aerosol Particles: An Application to a Mixed‐Phase Arctic Stratus Cloud Layer

AbstractMixed‐phase clouds (MPCs) have been identified as significant contributors to uncertainties in climate projections, attributable to model representation of processes controlling the formation and loss of supercooled water droplets and ice particles from the atmosphere. Arctic MPCs are commonly widespread and long‐lived, with sustained ice crystal formation processes that challenge current understanding. This study examines the ice‐nucleating particle (INP) reservoir dynamics governing immersion‐mode heterogeneous freezing in an observed case of Arctic MPCs using a simplified 1D aerosol‐cloud model. The model setup includes prescribed dynamical forcings and thermodynamic profiles, and represents INPs as multicomponent and polydisperse particle size distributions. Diagnostic and prognostic approaches to immersion freezing parameterization are compared, including time‐independent (singular) number‐ and surface area‐based descriptions and a time‐dependent description following classical nucleation theory (CNT). The choice of freezing parameterization defines the size of the INP reservoir. The CNT‐based description yields an orders of magnitude larger INP reservoir than the singular parameterizations, which is the dominant factor for sustained ice crystal formation. The efficiency of the freezing process and cloud cooling are of secondary importance. A diagnostic treatment neglecting INP loss is only accurate when the INP reservoir size is large and INP depletion weak. Since a larger INP reservoir sustains ice crystal formation substantially longer, and ice water path scales with ice crystal concentrations for the conditions considered, resolving the source of differences in INP reservoir dynamics due to model implementation is a high priority for advancing climate model physics.

Impact and freezing coupling processes of supercooled water droplets on cold superhydrophobic spheres at low Weber numbers

While the previous studies discussed the impact and freezing coupling processes of supercooled water droplets on cold superhydrophobic convex spheres at medium or high Weber numbers (We > 10), the cases on convex and concave spheres at low Weber numbers (We ≤ 10) are numerically studied. A numerical model using the VOF (Volume of Fluid) method and solidification-melting model is established and validated by comparing the calculated results with the experimental data in reference. The influences of the Weber number, supercooling degree, and sphere diameter (characterized by the sphere-to-droplet diameter ratio) on the dynamic behaviors, including the droplet profile, the spreading area and arc angle, and the final outcomes, are explored. As the Weber number increases and the supercooling degree decreases, the impact droplet yields a higher retraction speed and tends to rebound from the cold spheres more easily. The influence of the diameter ratio at low Weber numbers is distinct from that at medium Weber numbers. With the increase of the diameter ratio, the maximum spreading area factor on the convex sphere increases while that on the concave sphere decreases. The final droplet outcome transits from adhesion to rebound when the surface shape continuously changes from concave to convex. The rebound-adhesion morphology maps in terms of the Weber number and diameter ratio are obtained at various supercooling degrees. The rebound region of the convex sphere in the morphology map is wider than that of the concave sphere, and the adhesion region of the morphology map expands with a greater supercooling degree. A smaller diameter ratio yields a smaller critical Weber number between rebound and adhesion regions on the convex sphere but a greater one on the concave sphere. The innovation and optimization of the anti-icing surface may obtain theoretical reference and guidance from the above findings.

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.

Microstructure and crystal order during freezing of supercooled water drops.

Supercooled water droplets are widely used to study supercooled water1,2, ice nucleation3-5 and droplet freezing6-11. Their freezing in the atmosphere affects the dynamics and climate feedback of clouds12,13 and can accelerate cloud freezing through secondary ice production14-17. Droplet freezing occurs at several timescales and length scales14,18 and is sufficiently stochastic to make it unlikely that two frozen drops are identical. Here we use optical microscopy and X-ray laser diffraction to investigate the freezing of tens of thousands of water microdrops in vacuum after homogeneous ice nucleation around 234-235 K. On the basis of drop images, we developed a seven-stage model of freezing and used it to time the diffraction data. Diffraction from ice crystals showed that long-range crystalline order formed in less than 1 ms after freezing, whereas diffraction from the remaining liquid became similar to that from quasi-liquid layers on premelted ice19,20. The ice had a strained hexagonal crystal structure just after freezing, which is an early metastable state that probably precedes the formation of ice with stacking defects8,9,18. The techniques reported here could help determine the dynamics of freezing in other conditions, such as drop freezing in clouds, or help understand rapid solidification in other materials.

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.

Skin‐Inspired Nanofluid‐Filled Surfaces with Tunable Icephobic, Photothermal, and Energy Absorption Properties

AbstractIce buildup can significantly and negatively impact system performance in various industrial sectors, and has remained a persistent challenge for decades. Many compliant materials exhibit excellent de‐icing performance but are easily eroded by impacts from supercooled water droplets, sand, dust, and debris. A composite panel inspired by animal skin, consisting of a facesheet protecting a nanofluid layer beneath, which exhibits durable anti‐icing and tunable photothermal properties is proposed. The viscous liquid layer beneath the facesheet increases flexural rigidity, preventing large deflections and increasing deformation resistance, which alters ice's adhesion to the surface. The non‐uniform fluid pressure exerted by the viscous nanofluid‐filled composite panels facilitates ice detachment, resulting in ice adhesion strengths as low as τice ≈ 10 kPa. Further, by altering the fluid properties, different additional functionalities can be endowed to the system. Incorporating fumed silica in a fluid‐filled composite panel results in rheopectic behavior, and this doubles their impact resistance when the shear thickening properties are properly tuned. Additionally, the combination of a transparent facesheet and a solar light absorbent nanofluid allows for tunable photothermal properties, further enhancing the anti‐icing performance of the system. This durable and tunable nanofluid‐filled composite panel shows great promise as a multifunctional de‐icing material.

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.

An experimental study of rime ice accretion on bundled conductors

The dynamic ice accretion process on bundled electrical power conductors and icing-induced effects on the aerodynamic forces acting on the conductors were examined experimentally. An Icing Research Tunnel was used to generate a typical atmospheric rime icing condition experienced by power transmission cables. Two pieces of Aluminum-Conductor-Steel-Reinforced (ACSR) cables were mounted in the icing wind tunnel with different spacing and angular displacements with respect to the incoming airflow. While ice accretion was found to take place primarily on the frontal surfaces of the bundled conductors, the accreted ice layers resembled well the twisted outer strands of the ACSR conductors. Averaged outer profiles of the iced conductors were elliptical with the semi-minor axis being the radius of the conductors and the semi-major axis passing through the foremost frontal points of the accreted ice layers. The icing process on the windward conductor was almost unaffected by the existence of the leeward conductor. The ice accretion and resultant aerodynamic force acting on the leeward conductor were found to vary significantly, depending on the spacing and angular displacements between the bundled conductors. The windward conductor was found to induce a “shadowing effect” by intercepting airborne supercooled water droplets to prevent them from impinging onto the leeward conductor, resulting in much less ice accretion and smaller aerodynamic drag acting on the leeward conductor. The aerodynamic force measurements were correlated with the acquired ice accretion images and wake flow field around the bundled conductors to elucidate the underlying physics.

Multi-physics simulation of 3D in-flight ice-shedding

In-flight ice accretion may possibly jeopardize the safety of fixed- and rotary-wing aircraft. Icing can possibly occur if supercooled water droplets in clouds impinge on the aircraft surfaces and freeze upon impact. It may result in instrument failures and degradation of the aerodynamic performances. A major problem related to ice accretion is the possibility of ice shedding from the main body and impacting other parts of the aircraft or being ingested by the engines. In fixed-wing aircraft, shedding is caused by the action of the aerodynamic forces or the activation of an Ice Protection System. In the present work, a multi-physics framework is presented to simulate ice accretion and shedding from wings and engine nacelles. The aerodynamics is computed using the open-source toolkit SU2 (Economon et al., 2015)[1] . Cloud droplet trajectories are computed using the arbitrary-precision Lagrangian in-house solver PoliDrop. Then, the in-house ice accretion toolkit PoliMIce (Gori et al., 2015) is used to determine the ice layer. An algebraic level-set approach with iterative volume mesh refinement has been developed to extract the ice shape from the iced wing geometry. A FEM structural analysis is performed on the accreted ice shape by means of the open-source code MoFEM (Kaczmarczyk et al., 2014, 2020) [2]. Internal stresses within the ice geometry due to aerodynamic forces are computed. Three-dimensional ice accretion simulations are performed to check the validity of the present approach.

Numerical Simulation of Icing Characteristics on the Surface of an Aero-engine Guide Blade

This paper takes the guiding blade of a type of aero-engine needle as the research object for the icing problem of the guiding blade and uses the numerical simulation method to simulate the airflow field, supercooled water droplet impact and icing situation on the surface of the guiding blade under different flight conditions, to analyze the influence of flight speed on the icing situation of the guiding blade. It is found that the incoming flow velocity has a significant effect on the icing of the guide vanes, with the flight velocity increasing from 34m/s to 102m/s, the icing of the leading edge of the vanes increases by 98.1%; meanwhile, the icing range of the guide vanes is significantly related to the incoming flow velocity, with the flight velocity increasing, the icing range expands significantly; in addition, the icing range gradually expands from the root to the outermost end of the vanes, with the flow velocity The maximum icing range at 102m/s was extended from 17% to 20% of the chord length.

Flexible Electrothermal Hydrophobic Self-Lubricating Tape for Controllable Anti-icing and De-icing

In winter, the freezing phenomenon threatens the safety of people both in the work and home environments. At present, electrothermal de-icing technology is widely used, but how to improve the portability and anti-icing/de-icing efficiency of electrothermal de-icing technology has attracted extensive attention. Here, we proposed a simple yet universal flexible electrothermal hydrophobic self-lubricating tape (EHT) with a multilayer structure including an electrothermal layer and hydrophobic self-lubricating layer on a polyimide base. The resulting tape surface exhibits a hydrophobic/electrothermal synergistic effect on anti-icing and de-icing performance. Compared with the electrothermal tape (ET) without a hydrophobic self-lubricating layer, the friction coefficient and ice adhesion strength on the EHT surface were reduced by 29% and 57%, respectively. In addition, the hydrophobic self-lubrication effect of EHT can reduce the adhesion of supercooled water droplets on its surface in an extremely harsh environment to allow more de-icing easily under joule heat, and the different electrothermal power demand in diverse surroundings can be controlled by adjustable voltage. More importantly, the uniform EHT material can be prepared in a large area using a traditional blade-coating process that gives a convenient route to anti-icing in the engineering protection field.

Nucleation enhancement by energy dissipation with the collision of a supercooled water droplet

Lack of knowledge on nucleation with mechanical disturbances hinders researchers in understanding the anti-icing properties of superhydrophobic surfaces. In this research, experiments are designed to observe the freezing process of a supercooled water droplet at −14 °C with different levels of external disturbances by changing the impact velocity, and the nucleation rates are statistically analyzed. It is found that compared with the static state, the nucleation rates of droplets during the collision increase by three and five orders of magnitude on the hydrophilic epoxy resin and superhydrophobic polytetrafluoroethylene (PTFE) surfaces, respectively, which leads to a much higher instantaneous nucleation rate on the superhydrophobic surface than on the hydrophilic surface. Then, the mechanical energy dissipation during the impact is analyzed. The logarithm of the nucleation rate is approximately linear in relation to the logarithm of the average energy dissipation rate, while the nucleation rate of the rough PTFE surface increases more rapidly than that of the other smooth surface. Finally, the effect of the energy dissipation on the nucleation energy barrier is discussed theoretically and an impact-nucleation model is proposed, which drives the reconsideration of the design of a superhydrophobic anti-icing coating.

Influence of positive DC electric field on rime icing on the insulator and its experimental verification in the natural environment

AbstractThe electric field imposes a significant effect on ice accretion on the insulator. The impact of an AC electric field on insulator icing in an artificial climate chamber has been addressed. Compared with AC, DC electric field exerts an attraction on supercooled water droplets, which is more detrimental to the power system. Moreover, the icing on energised insulators is rarely studied in the natural environment. In the present research, an electrical, fluid coupled‐field simulation was carried out on the insulator. The force characteristics and motion deviation features of water droplets at different positive DC electric field strengths were studied. On this basis, rime icing on the insulator energised with different positive DC voltages was investigated in the field. Results show that the charged droplet shifts along the electric field line under a positive DC voltage. The deviation degree of the droplet increases with the increment of the voltage and droplet size and decreases with an increase in wind speed. As the voltage rises, the surface roughness, icing amount and icing length of rime ice‐covered insulator increase. Consequently, compared with that without energisation, the local collision coefficient, icing amount and icing length increase by 17.6%, 46.4% and 63.9%, respectively, under a 40 kV DC voltage.

Three-dimensional numerical simulation of rime ice accumulation on silicone rubber insulator and its experimental verification in the natural environment

Icing on transmission lines is a challenging problem in the world. Rime icing on silicone rubber insulator was taken as research object in this paper. The position coordinates of supercooled water droplets collided with the surface of the insulator were extracted, the local collision coefficient (E) of the droplet was calculated by a triangular area ratio method. The “point-line-face-solid” ice shape reconstruction algorithm was proposed to realize three-dimensional numerical simulation of rime ice accretion on the insulator. The results shown that the effect of median volume diameter (MVD) of the droplet on the E is greater than that of wind speed (v). The most serious icing occurs on the rod and the edge of the shed. Due to the shielding effect of the shed, the icing on the rod presents the shape of the nose. The length, thickness and amount of rime icing on the insulator calculated by the model increases with time, whose average errors with that obtained by the tests are 7.68%, 7.96%, 10.34%, and 9.45%, respectively. Moreover, the simulated icing distribution is well consistent with the experimental results. Therefore, the effectiveness of the numerical simulation method was proved in the present research.

Quantitative Measurement Method for Ice Roughness on an Aircraft Surface

When an aircraft passes through clouds containing supercooled water droplets, the leading edge’s surface will gradually accumulate ice. Ice surface roughness is an important parameter affecting the local convective heat transfer coefficient and the water collection coefficient, which in turn affect the ice’s shape. However, because the surface roughness of aircraft icing is a transient value varying in time and space, it is extremely difficult to measure with existing methods in real time. In this study, a noncontact ultrasonic pulse-echo (UPE) technique is applied to characterize the ice roughness of an airfoil model’s surface. A multilayer model with equivalent bead-like roughness profiles is established to study the effects of changes in ice roughness on ultrasonic echo signals. A series of simulations indicated that ice roughness can be measured quantitatively and effectively in the range of [11.6, 120] μm. Based on these simulations, an experimental UPE device was developed to measure echo signals on top of the ice corresponding to surface roughness. The results show that for both the regular and irregular surface roughness samples, the maximum relative error in the roughness is less than 15%. Meanwhile, we designed and supplemented the experiment with the NACA-0012 airfoil model to realize the online measurement of ice roughness in an icing research tunnel.

Collision Characteristics of Water Droplets in Icing process of Insulators

Icing on the insulators of transmission lines threats the power system security. The flashover properties of ice-covered insulators have been widely studied while there is a lack of research on the icing properties of insulators. In this paper, the collision process of supercooled water droplets on insulators is studied. Based on the boundary element method (BEM), a mathematical model of airflow and water droplet motion outside insulators is established. Taking advantage of vector calculation and array tracking, the 3D distribution of collision droplets on insulators is obtained. The results show that the droplets mainly collide with windward shed edge and steel cap of insulators. With the increase of wind velocity and median volume diameter of droplets, the number of captured droplets increases and the collision area expands from the shed edge and steel cap centre to the inner surface and two sides, leading to the increase of icing rate and icing area. Moreover, the related icing tests of insulator LXY-120 were conducted on Xuefeng Mountain, the recorded icing mass and shape are compared with the simulation results, and it is found that the model in this paper has a relative error of 13.5%.