Freezing mechanism of supercooled water droplet impinging on metal surfaces

Ice accretion on power lines is a random natural phenomenon and may seriously harm to the safety of power network. However, the mechanism of the freezing process of supercooled water droplet impacting on wires is still not fully understood. In this study, an experimental investigation on the freezing mechanism of the supercooled water droplet impinging on cold metal surfaces was performed. The morphological characters and the dynamics of a single supercooled droplet collide on the cylindrical metal surfaces had been revealed with high-speed photographing. The experimental data for the surfaces of stainless steel, copper and aluminum, on which the supercooled droplets impinging with speeds of 2.3 m s⁻¹ and 4.3 m s⁻¹ had been plotted. The phenomena of instantaneous and non-instantaneous freezing of the supercooled impinging droplet were identified and the conditional boundaries for these two kinds of freezing were found statistically.     

The effect of supercooling on ice structure in tuna meat observed by using X-ray computed tomography

Several studies have reported that freezing a homogeneous food such as soy bean curd with deep supercooling (supercooled freezing) results in the formation of many particle ice and homogenous ice structure. However, ice crystal morphology may be affected by the cellular structure of the food. In this study, the ice crystal structure in tuna meat, a cellular food, frozen by the supercooled freezing method was investigated by X-ray computed tomography and compared with ice structures in tuna meat frozen by conventional freezing methods. The results showed that rod-like ice crystals grew parallel with the myofibers, and inhomogeneous ice structures formed in tuna meat frozen by the supercooled freezing method regardless of the degree of supercooling, in contrast to the ice structure in frozen soy bean curds. These ice crystals linked with each other to form rod-like ice structures due to mobility limitations imposed by the cellular structure.     

Ice formation on a smooth or rough cold surface due to the impact of a supercooled water droplet

Ice accretion is considered in the impact of a supercooled water droplet on a smooth or rough solid surface, the roughness accounting for earlier icing. In this theoretical investigation, the emphasis and novelty lie in the full nonlinear interplay of the droplet motion and the growth of the ice surface being addressed for relatively small times, over a realistic range of Reynolds numbers, Froude numbers, Weber numbers, Stefan numbers and capillary under-heating parameters. The Prandtl number and the kinetic under-heating parameter are taken to be order unity. The ice accretion brings inner layers into play forcibly, affecting the outer flow. (The work includes viscous effects in an isothermal impact without phase change, as a special case, and the differences between impacts with and without freezing.) There are four main findings. First, the icing dynamically can accelerate or decelerate the spreading of the droplet whereas roughness on its own tends to decelerate spreading. The interaction between the two and the implications for successive freezings are found to be subtle. Second, a focus on the dominant physical effects reveals a multi-structure within which restricted regions of turbulence are implied. The third main finding is an essentially parabolic shape for a single droplet freezing under certain conditions. Fourth is a connection with a body of experimental and engineering works and with practical findings to the extent that the explicit predictions here for ice-accretion rates are found to agree with the experimental range.     

Effects of poly-vinyl alcohol on supercooling phenomena of water

The effects of a polymer additive on the supercooling of water were investigated experimentally. Poly-vinyl alcohols (PVAs) were used as the additives, and samples were prepared by dissolving the PVA in water. Since the characteristics of PVA are decided by its degrees of polymerization and saponification, these were varied along with the concentration as the experimental parameters. Moreover, the effect of purity of the water was also considered. Each sample was cooled and the temperature at the instant when ice appeared was measured. Since the freezing of supercooled water is a statistical phenomenon, many experiments were carried out and the average degree of supercooling was obtained. It was found that PVA affects the nucleation of ice in supercooled water and the degree of supercooling increases with the addition of PVA even for water with low purity. The average degree of supercooling increases with an increase in the degree of saponification of PVA.     

纳米氟碳涂层抑制过冷却器冰堵的机理

冰浆由于良好的热物特性,在许多领域得到广泛的应用,其制取方式也成为关注的焦点。过冷水动态制冰是目前最有发展前途的制取冰浆方式之一,但其主要缺陷是过冷却器易发生冰堵。在此,基于水溶液结晶的机理,从影响过冷却器冰堵的因素出发,指出纳米氟碳表面改性材料可改善表面状况,有效抑制壁面结冰,减少制冰过程中的冰堵问题,提高整个系统的制冰效率,降低能耗。纳米氟碳涂层抑制过冷却器冰堵的机理对进一步深化和开发新一代防结冰技术有一定意义。     

Freezing of a water droplet due to evaporation—heat transfer dominating the evaporation–freezing phenomena and the effect of boiling on freezing characteristics

One of the authors has proposed a novel transport/storage system for the waste cold from the gasification process of liquefied natural gas (LNG), which consists of an evaporator, a cold trap, and a pipeline. In order to estimate the performance of this system, one should know the pressure in the evaporator, in which evaporation–freezing of a PCM occurs, and in the cold trap, as well as the pressure drop of the pipeline due to the flow of low pressure vapor of the PCM. In this paper, the cooling/freezing phenomena of a water droplet due to evaporation in an evacuated chamber was experimentally examined, and the heat transfer dominating the evaporation-freezing phenomena was investigated in order to estimate the pressure in the evaporator. From the results, it was shown that the water droplet in the evacuated cell is effectively cooled by the evaporation of water itself, and is frozen within a few seconds through a remarkable supercooling state, and that the cooling rate of the water droplets were dominated by heat transfer within the droplet under the abrupt evacuation condition. The later result means that, in order to obtain an ice particle by evaporation–freezing, the surroundings of the water droplet should be evacuated at the pressure as low as the saturate pressure of water at the maximum supercooling temperature of the droplet.     

Laboratory experiments on frazil ice growth in supercooled water

Presented herein are the results of a laboratory investigation of the influence of turbulence and water temperature on the nature and rate of frazil ice growth in a turbulent body of supercooled water. The results indicate that the rate and the quantity of frazil ice growth increase with both increasing turbulence intensity and with decreasing water temperature at the instant of seeding, when a small fragment of ice is placed in the supercooled water. The turbulence characteristics of a flow affect the rate of frazil-ice growth by governing the temperature to which the flow can be supercooled; by influencing heat transfer from the frazil ice to surrounding water; and by promoting secondary nucleation, crystal, platelet and floc fracture, thereby increasing the number of nucleation sites available for further frazil ice growth.Larger frazil ice platelets, beginning as single crystals then becoming laminar fusions of crystals, were observed to form in water supercooled to lower temperatures. However, platelet size generally decreased with increasing turbulence intensity, as platelets with a major diameter larger than a certain value tend to break when buffeted by turbulence eddies.The investigation of frazil ice growth involved the use of a simplified analytical model, in which the rate of frazil ice growth is related to temperature rise of a turbulent volume of water due to the release of latent heat of fusion of liquid water to ice. Experiments conducted in a turbulence jar with a heated, vertically oscillating grid served both to guide and to calibrate the analytical model as well as to afford insights into frazil ice growth.     

Study on ice slurry production by water spray

A theoretical and experimental study was performed to examine the water spray method of ice slurry production. First, the conditions for the formation of ice particles were investigated theoretically by the diffusion-controlled evaporation model. The prediction of the model was proved to agree relatively well with experiments in which we examined the conditions for a droplet of initial temperature 20°C and size 50 μm to change into an ice particle in a chamber of height 1.33 m. Second, the production of cold storage heat will increase almost proportionally to the number of spray nozzles because no substantial difference was found in the Sauter Mean Diameter (SMD) of sprays from single and twin nozzle. Third, an ice slurry was experimentally obtained by spraying droplets of 7% ethylene glycol aqueous solution in a vacuum chamber where pressure is maintained below the freezing point of the solution. Finally, based on the theoretical and experimental results, we propose an optimizing chart for providing the operating conditions to make ice slurry using the relations of the staying time of the droplet in the chamber, the injection pressure, the spray droplet size and the chamber pressure.     

Freezing phenomena of supercooled water under impacts of ultrasonic waves

In order to clarify effects of ultrasonic waves on supercoold water, ultrasonic waves were applied to supercooled water. Frequency of the ultrasonic waves applied was 45 kHz, and the intensities of the waves were 0.13 and 0.28 W cm⁻², respectively. Four cases of experimental conditions were selected; a case of a free surface, a case of an oil-water surface, a case of a free surface with a dipped metal bar and a case of an oil–water surface with a dipped metal bar. For each experimental condition, water was cooled at a constant cooling rate and the ultrasonic wave was applied from underneath until the water in a test tube solidified. It was found that in the case of 0.28 W cm⁻², the ultrasonic wave had distinct effects on freezing of supercooled water for all experimental conditions. On the other hand, in the case of 0.13 W cm⁻², the ultrasonic wave had an effect on freezing of supercooled water only under existence of a free surface and a metal bar.     

Use of a droplet nucleation analyzer in the study of water freezing kinetics under the influence of ultrasound waves

In this report we present a new instrument (a droplet nucleation analyzer) to be used in the study of the influence of ultrasonic waves on the freezing of pure water. This influence can be of great interest in the cryopreservation of biological material. Two different types of experiments have been carried out. In the first set of experiments, ultrasound waves were used during the cooling process. In the second set, ultrasound was applied prior to the cooling process, trying to quantitatively reproduce some experiments in which the ice nucleation temperature of water was successfully decreased. A theoretical discussion of the results is also presented.     

Effect of bubble nuclei on freezing of supercooled water

In order to clarify effects of bubble nuclei on freezing of supercooled water, various kinds of experiments were carried out with invisible sizes of bubbles in supercooled water. Water samples were kept in a test tube and cooled down at a constant cooling rate until the water solidified. The degree of supercooling at freezing was then measured. Two kinds of water surfaces were applied. One was exposure to the atmosphere, and the other was covered with silicone oil. Three kinds of pressure conditions were applied. The first type was atmospheric pressure. The second type was compression up to 6.0 atm. The third type was evacuated down to 0.02 atm. Two holding time periods before starting the experiments were applied. One was 30 min and the other was 24 h. It was found that the degree of supercooling at freezing is high in the case of the free surface compared to the one with oil–water surface. The reason suggested was that the bubbles in the water can be released from the surface in the case of normal atmospheric exposure and trapped at the oil–water interface in the case of water covered with oil. Hence, it was clarified that the freezing of supercooled water is affected by the existence of bubble nuclei.     

The mechanism of repeated-segregation for the formation of thick layered ground ice

In this article, a mechanism of repeated-segregation for the formation of thick layered ground ice has been suggested. The mechanism consists of the following processes: (1) Moisture migration to the freezing front and ice lensing there as a result of upward freezing from permafrost. (2) Unequal law of migration of unfrozen water (the combined effect of the following processes: the upward migration of unfrozen water in a frozen active layer in the cold season; water migration and ice lensing in the frozen ground behind the freezing front during upward freezing; water migration and ice lensing in the still frozen ground beneath the thawing plane in the warm season). (3) Self-purification of ice. (4) Syngenetic growth of ground ice due to the addition of material onto the ground surface. (5) Annual repetition of the processes mentioned above. Thus, a new type of ground ice — repeated-segregation ice — is distinguished.     

Freezing a water droplet on an aligned Si nanorod array substrate

When a water droplet is dried on a vertically aligned Si nanorod array surface, the nanorods are bundled together. To understand how bundles are formed, a water droplet is frozen rapidly on a Si nanorod array surface observed under a cryo-SEM (scanning electron microscope). The nanorods in the precursor film form similar bundles as those dried in air. But the nanorods under the apparent frozen water droplet are only slightly deformed. We propose that the bundling of nanorods is caused by non-uniform water-nanorod interaction, which could happen either during the water spreading or drying process. Therefore, controlling the liquid-nanostructure interaction could minimize the bundling. In addition, the rapid freezing process does not preserve the water inside the nanochannels, and almost all the water forms ice on top of the nanorod surface, either as a planar interface or as particles, depending on the locations. The separated ice-nanorod interface will have potential applications in chemical separation and crystal growth.     

Inverse scattering problem for mixed-phase and ice clouds. I. Numerical simulation of particle sizing from phase-function measurements

We propose a new method of particle size retrieval for mixed-phase and ice crystal clouds. The method enables us to identify each component of a bicomponent cloud composed of water droplets and ice crystals and to retrieve a size distribution separately for each cloud component. We explore the method's capability by using sythetic multiangular data of scattered-light intensity. We model cloud microphysical characteristics by assuming two noninteracting cloud components, such as liquid or supercooled droplets and cubic or hexagonal ice crystals, with regular simple geometrical shapes as first approximation. The sensitivity of the method is tested for different relative concentrations of the cloud components that are varied over a wide range. First, we investigate the applicability limit of the single-component cloud approximation in retrieving particle size distributions of a bicomponent cloud. Second, we test the method to retrieve size distributions simultaneously for both components in mixed-phase clouds, and we discuss the conditions of its applicability.     

The modelling of the freezing process in fine-grained porous media: Application to the frost heave estimation

The solution of the moving-boundary problem, related to heat- and mass-transfer processes in freezing, fine-grained, porous media under phase-transition conditions is presented. It is assumed that a freezing zone, characterized by a wide temperature range of phase transitions, is formed. Therefore a three-zone model is developed. The preservation of the term ∂L/∂t(L is the ice content) in the system of equations has made it possible to determine the ice distribution within the frozen and the freezing zones. For loamy soils the dependence of the freezing process on the characteristic parameters, the Stefan and Lewis numbers, was analyzed. It was found that increasing the enthalpy of phase transition, i.e., decreasing the Stefan number Ste, resulted in diminution of the frozen zone but, at the same time, the ice content within this zone increased. Intensification of the migration process, i.e., increasing the Lewis number Le, also led to diminution of the frozen zone, in which the ice content and, consequently, the total moisture (including ice) were increased. For large Lewis numbers the freezing zone was observed to decrease. When the water migration process is absent (Le = 0), the calculations, which were based on the described model show that in the course of freezing the redistribution takes place only between moisture and ice contents. The total moisture remains constant and equal to the initial water content. The theoretical conceptions and results derived from the analytical solution are in agreement with experimental findings. The presented model predicts the freezing process in porous media and satisfactorily reflects observed phenomena. The utilization of the considered problem solution to the prediction of the frost heave phenomenon in soils freezing processes shown that the calculated frost heave curve matches the experimental results very closely indicating that the model can well reproduce the frost heaving process associated with the freezing. Propagation of the freezing front in the test is predicted the experimental results with reasonable accuracy.     

Rapid Solidification Process of a Water Droplet Due to Depressurization

This paper reports an experimental and numerical study of rapid solidification of a water droplet due to depressurization. During the experiment, a distilled water droplet was suspended on a thermocouple, which was also used to measure the droplet temperature, and the droplet surface temperature was captured by an infrared thermograph. The experimentally measured data indicates that freezing occurs from the droplet surface when the droplet temperature reaches a certain subcooling. A mathematical model was constructed to simulate the temperature transition and the temperature distribution within the sphere. The model considers the pressure reduction in the test vessel, the kinetic condition for undercooled solidification, and the heat transfers due to convection and sublimation at the ice surface. A coordinate transformation method was used to capture the two moving boundaries within the droplet, which are internal solidification interface and surface sublimation interface. The model-predictions agree well with the measured temperature data, demonstrating the soundness of the present model. The results show that the rapid solidification of a water droplet due to depressurization is a typical non-equilibrium phase transition, with a lower ambient pressure, the solidification speed will be faster, and the duration time for droplet center temperature keeps constant will be shorter.     

Preparation of polycrystalline ice specimens for laboratory experiments

A perennial problem in preparing fine grained ice specimens is the convenient production of low porosity material which is homogeneous, of random c-axis orientation and of controlled grain size.A review of the nature of bubble formation in ice leads directly to the two factors which must be controlled in order to minimize gas bubble formation, and thus porosity. The factors are (1) reduction of the number of bubble nucleation sites and (2) reduction of the amount of dissolved gas available at the freezing front.An ice specimen preparation technique developed at CRREL addresses both these factors. Two techniques have proven effective in reducing the number of nucleation sites; (1) a purge technique using carbon dioxide gas and (2) a vacuum technique. A newly developed flushing technique reduces the amount of dissolved gas in the pore water during freezing.The resulting material is optically clear with some very small bubbles dispersed throughout. The radial freezing technique employed occasionally results in a narrow column of fine bubbles along the central axis of the specimen. The density is 0.917 ± 0.002 Mg/m³.An overview of techniques for forming, flooding and freezing ice specimens is presented and several major preparation methods are reviewed in detail.     

Numerical simulation of the process of airfoil icing in the presence of large supercooled water drops

We have developed a software package and related methodology that can be used to simulate the process of airfoil icing during flight in the presence of large supercooled liquid water drops in the oncoming airflow. The motion of a carrier medium is described using the Navier-Stokes equations for a compressible gas. The motion of water drops is described using an inertial model. The process of water deposition and its subsequent freezing on an airfoil surface are described by the method of control volumes based on the equations of conservation of mass, momentum, and energy for each element of the surface. The main results of simulations are presented for the icing of an NACA 0012 airfoil profile with “barrier” ice formation in the absence and presence of heating of the leading edge. The influence of the ice-growth thickness and position on the airfoil chord on the pattern of airflow and aerodynamic characteristics of airfoil is analyzed.     

Delaying Ice and Frost Formation Using Phase‐Switching Liquids

Preventing water droplets from transitioning to ice is advantageous for numerous applications. It is demonstrated that the use of certain phase‐change materials, which are in liquid state under ambient conditions and have melting point higher than the freezing point of water, referred herein as phase‐switching liquids (PSLs), can impede condensation–frosting lasting up to 300 and 15 times longer in bulk and surface infused state, respectively, compared to conventional surfaces under identical environmental conditions. The freezing delay is primarily a consequence of the release of trapped latent heat due to condensation, but is also affected by the solidified PSL surface morphology and its miscibility in water. Regardless of surface chemistry, PSL‐infused textured surfaces exhibit low droplet adhesion when operated below the corresponding melting point of the solidified PSLs, engendering ice and frost repellency even on hydrophilic substrates. Additionally, solidified PSL surfaces display varying degrees of optical transparency, can repel a variety of liquids, and self‐heal upon physical damage.     

The experimental study on freezing of supercooled water using metallic surface

Freezing of supercooled water on a metallic plate was studied, experimentally. First of all, a gold plated surface was selected as a metallic surface because the surface has very little change in the characteristics of the surface against time. The experiments on freezing under various cooling rates were carried out and the probability of freezing per unit surface area per unit time interval was calculated. It was found that the probability was independent of the cooling rate. Secondly, in order to clarify the effect of oxidation on freezing of supercooled water, an electrolytically polished copper surface was selected and a time variation of the probability of freezing was investigated. A large number of experimental data is required to obtain an accurate value for the probability, but it is impossible to perform it before the characteristics of the surface changes. Hence, in order to cover a wide range of the degree of supercooling at freezing in a short period of time, experiments were carried out under various cooling rates. It was found that the oxidation of the surface restrained the supercooled water on the surface from freezing. By comparing the results with the one for a gold plated surface, a parameter was obtained to express the characteristics of the surface.