Frost Crystal Growth Research Articles

Frost crystal growth behavior on a hydrophilic surface over a wide range of cold surface temperature

The initial frosting phenomenon is a discontinuous phase nucleation process, the cold surface temperature and properties have a decisive influence on this phenomenon, especially in the initial frosting stage. With the development of aerospace and energy transportation technology, frost formation at low temperatures (-100 °C∼-30 °C) and ultra-low temperatures (-273 °C∼-100 °C) has gradually attracted the attention of researchers. In this paper, the initial frosting phenomena on hydrophilic surfaces with a contact angle of 10° (CA= 10°) and ordinary (CA= 95°) surfaces are studied experimentally in a wide range of cold surface temperatures (-190 °C∼-30 °C). Four modes are confirmed: cold surface condensation frosting, cold surface sublimation frosting, air boundary layer condensation frosting and air boundary layer sublimation frosting. It is also found that the four frosting modes do not appear in turn with the decrease of the cold surface temperature, but two or more frosting modes appear at the same time. And the surface contact angle has an important influence on the frosting mode. The initial frost crystal morphology mainly depends on the cold surface temperature and the corresponding frosting mode. Four different forms of frost crystals are observed: hexagonal prism (feather), branch (pine needle), cluster (shrub) and floc (grape), in which the cluster frost crystal is more sensitive to the surface contact angle and can appear in different temperature ranges due to different contact angles. Based on the statistics of the size, quantity, and distribution of the initial frost crystals, it is found that -70 °C is a major turning point for frost formation from the cold surface sublimation frosting to the air boundary layer sublimation frosting, and an important change has taken place near this point. Furthermore, it affects the shape and size distribution of frost crystals. These findings are of great significance for the study and understanding of frost crystal growth mechanism in the initial stage of frost formation at low and ultra-low temperatures.

Experimental study on anti-frost property and edge effect of superhydrophobic surface with millimeter-scale geometries

Frost suppression ability and edge effect are crucial for the frost formation on engineering surfaces. However, certain fundamental aspects remain ambiguous, particularly in the context of superhydrophobic surfaces featuring millimeter-scale structures.. In this study, condensation, frosting, and defrosting experiments were conducted on superhydrophobic surfaces with multiple millimeter-scale geometries. The results indicate that condensate droplets tend to grow on the top of right-angled, semi-circular protrusions, and conical structures. In the course of frost formation, superhydrophobic surfaces with millimeter-scale structures exhibit certain anti-frost properties when compared to the non-structured superhydrophobic surface. Condensation state of droplets can be maintained for a long time on the bottom area of the grooves, creating a frost-free zone. By observing the growth of frost crystals in various regions of the millimeter structure, it has been revealed that the manipulation of water vapor contact spaces can lead to the design of anti-frost surfaces with adjustable rates of frost crystal growth. Frost formation process on superhydrophobic surfaces can generate the so-called "edge effect" and is influenced by the edge structure. Frost crystals tend to form first on conical edges due to their sharp structural features. The average height of frost crystals on 90° edges, obtuse edges, and semicircular edges is significantly lower than that on conical edges, with the reductions of 31.8 %, 28.4 %, and 14.3 %, respectively. During the defrosting process, two spontaneous methods were observed to remove the frost layer: a "hand-in-hand" manner lifting off the groove and an "airfoil retraction" contraction on the protruding structures. Regardless of the structure shape, defrosting water floats on top and falls off, allowing for rapid drying of the surface and groove. This work has broadened the understanding of frost formation and defrosting on millimeter-structured superhydrophobic surfaces, and holds potential for applications in anti-frost engineering.

Experimental study of frost crystal dendrite growth on a cryogenic cold surface under natural convection conditions

Quantitative measurements were carried out of the dendrite frost crystal growth under natural convection conditions on a frozen water drop sitting on a cold surface whose temperature is as low as -190 °C to study the effects of cold surface temperature, wet air state and original water drop size. In order to classify the influences of different cold surface temperature regions, the frost crystal dendrite growth of both ordinary-low and cryogenic cold surface temperatures were investigated. It was found that the cold plate surface temperature has a direct influence on frost formation mechanism. Decreasing the cold surface temperature for ordinary-low cold surface temperatures will enhance and for cryogenic cold surface temperatures will suppress the frost crystal dendritic growth of the frozen water drop. The measurements disclosed that the length of the longest frost crystal dendrite grows almost linearly with time, esp. in the early stage. The effects of the water drop sizes on the longest frost crystal dendrite growth become more and more obvious as the cold surface temperature decreases. The influence of air relative humidity on the frost crystal dendrite growth is something similar to that of air temperature under given conditions, since their change both changes the absolute air humidity. A very peculiar phenomenon, i.e. the influence of wet air state on frost crystal dendrite growth is not monotonous, was observed. These findings are important for understanding and studying the frost formation and growth, especially the initial frost formation period, on low temperature surfaces.

Biomimetic bamboo leaf double-sided microstructure composite surface and its delayed icing performance

The frost phenomenon occurred on the surface of engineering equipment can not only lead to a reduction in the efficiency of equipment but a higher energy consumption. How to effectively inhibit or delay the frost behavior on a material surface and improve the anti-frost property on the surface is of great importance. It is found that the nano needle structure on the surface of bamboo leaves can provide large Laplace pressure to drive droplets to bounce and hence delay the condensation process. In addition, in the frost crystal growth stage, the nano needle structure can reduce the contact area between droplets and the surface of bamboo leaves, and hence limit the phase-change heat transfer of water vapor and increase the thermodynamic barrier when nucleation of water vapor occurs, thereby delaying the frost crystal growth. In this paper, inspired by bamboo leaves, biomimetic structures are constructed on the surface of an aluminum alloy by laser processing and sol-gel method. Results reveal that delayed frost formation on the surface is realized, which is of immense practical value in engineering and many other aspects.

Effects of dissolved gas on the nucleation and growth of ice crystals in freezing droplets

The solidification process of water (liquid) can cause an intricate deformation of its structures, from the complex of ice crystals to dendritic frost crystal growth. The dissolved gas in water (liquid) is rejected and accumulated ahead of the ice-water interface during solidification and the resulting bubbles are incorporated into the growing ice crystal. In this study, the bubble formation process and the effect of the dissolved gas on the nucleation and growth of ice crystals in freezing droplets at various gas concentrations were experimentally investigated. The dissolved gas acted as a heterogeneous medium, and promoted the nucleation of the liquid droplets. Compared with the pure solid ice crystal, during the freezing process, the existence of bubbles altered the solid-liquid density ratio and the shape of the droplets. Coupled with experimental observations, a quantitative analysis of the tip angles was proposed to elucidate this influence, which indicated that the tip angle of the frozen ice droplet decreased with the gas concentration, and promoted the growth of the initial frost crystal.

Numerical study of a direct-expansion solar-assisted heat pump water heater under frosting conditions based on experiments

A direct-expansion solar-assisted heat pump (DX-SAHP) water heater is studied numerically. A simulation model, which includes the models of components and the frost formation of the collector/evaporator, is established to research the heating performance of the system and the growth of frost. The calculation results conform well to the experiment, which verifies the accurate predictions. On the basis of the model, the influences of the meteorological parameters on the system performance under frosting conditions are analyzed. It is found the formation of frost can be effectively inhibited by decreasing the ambient temperature below a specific value. What’s more, the collector efficiency can also be improved effectively during the incipient growth of frost crystals. Besides, higher solar radiation intensity can significantly improve the heating performance and reduce the frosting rate. By contrast, the enhancement in wind speed will have the same effect, but only slightly. The system coefficient of performance (COP) reaches above 2.75 with the reliable heating performance under typical frosting conditions, and the collector/evaporator presents the superiority in delaying frosting in comparison with the traditional evaporator for air source heat pump.

Suppression of frost layer growth on glass or copper surfaces with micro-scale lattice-patterned grooves

The growth of frost crystals and frost layers on solid surfaces causes serious problems, such as poor visibility of traffic lights or convex traffic mirrors and a deterioration of the performance of heat exchangers. Thus, controlling the growth of frost crystals and frost layers is very important for reducing these problems. In this study, the observation of frost growth was carried out for cooling glass or copper plates with micro-scale lattice-grooves. It was found from the captured images that frost crystals were formed from the surfaces of condensation droplets, which appeared on the protrusions of the grooved plates. The frost crystals growing from the surface of a droplet on one of the tops of the protrusions coalesced to those on other tops nearby. The coalescence caused local, gradual growth of a frost layer. It was also found that the droplets on some of the tops of the protrusions of the glass plates rapidly evaporated as a result of the growth of frost crystals near the tops. It was noted that new frost crystals did not appear on the surfaces of evaporating droplets and the dried tops of the protrusions. A further finding was that the porosity of the frost layer increased by coating a silane coupling agent onto the glass surfaces. It can be concluded from these findings that the glass or copper surfaces with micro-scale lattice-patterned grooves coated with the silane coupling agent are effective for attenuating the frost layer growth.

Mesoscopic investigation of frost crystal nucleation on cold surface based on the lattice-Boltzmann method

A lattice Boltzmann method (LBM) two-dimensional (2D) mesoscopic model is presented for studying the frost crystal nucleation process involved in liquid-solid phase change on a cold surface. In order to make the simulation more realistic, the actual frost crystal nucleus deformation process is considered in this model. The simulation results agree well with our experimental results. Detailed analysis was made on the dynamic behavior of liquid-solid phase change and heat transfer happening in the frost crystal nucleation process. Results are presented for the evolution law of frost crystal nucleus deformation, dynamic solid-liquid phase change process as well as the dynamic distribution of frost nucleus interior temperature during frost crystal nucleation. In addition, the occurrence of frost crystal nucleus deformation is explained by the theory of crystal growth. Finally, where and why the initial position of frost crystal growth happens is also revealed. Furthermore, the reason why the hydrophobic surface can delay the droplet freezing and why the frost growth is faster on the hydrophobic surface after the droplet is frozen is analyzed based on this model.

Frost formation and frost meltwater drainage characteristics on aluminum surfaces with grooved structures

Frost frequently forms on refrigeration and cryogenic engineering systems. Frost on finned surfaces increases the air-side thermal resistance and reduces the airflow area, which consequently reduce the heat transfer rate. Structured fin surfaces may be an effective solution to delay frost formation and promote meltwater drainage. This study experimentally studied condensation, frost crystal growth, frost melting and meltwater drainage characteristics on aluminum surfaces with parallel and crossed grooves with comparisons to these effects on a flat aluminum surface. The results show that condensed droplets freeze later with less frost accumulation on the grooved surfaces than on the flat surface and that the frost prefers to grow at the top edges and corners of the structures instead of inside the grooves on the grooved surfaces. During melting, the frost melts and the meltwater breaks up into small meltwater streams and droplets at the top of the structures or between the structures on the grooved surfaces. The parallel grooved surface has better drainage than the flat surface with a smaller meltwater retention ratio, while the surface with crossed grooves has worse drainage. In addition, the meltwater retention ratio is related to the frost thickness. Lower surface temperatures and longer frosting time results in more frost accumulation and larger meltwater films, which are more easily shed from the surface by gravity so that the meltwater retention ratio is smaller.

An experimental study of the effect of air quality on frosting on cold flat surface

The effects of air quality in the freezing process of droplets and the frost crystal growth on cold flat plate are studied experimentally. It is found that the air quality has important influence in the frosting process. The supercooling degree and freezing time change with solution concentration on different solutes. When supercooled droplets are in the instability state, the composition and concentration of solution can affect the nucleation and heat transfer, and most of the solute can increase the degree of supercooling and accelerate the freezing process. Solutions with cation of strong electrolyte have higher supercooling degree because the weak cation will hydrolyze. The hydrated ion can accelerate the formation of ice crystal. In addition, the frosting process on cold flat plate under different fine particle (PM2.5) concentrations is also studied in form of frost height and mass variations. The result shows that the greater the PM2.5 concentration is, the thicker the frost layer is and the greater the amount of the frost crystal is. This is because suspended particles in the air that can act as nucleating substrate to promote heterogeneous nucleation and the frost crystals are more easily deposited.

Modeling of frost crystal growth over a flat plate using artificial neural networks and fractal geometries

In the present study artificial neural network and fractal geometry are used to predict frost thickness and density on a cold flat plate having constant surface temperature under forced convection for different ambient conditions. These methods are very applicable in this area because phase changes such as melting and solidification are simulated by conventional methods but frost formation is a most complicated phase change phenomenon consists of coupled heat and mass transfer. Therefore conventional mathematical techniques cannot capture the effects of all parameters on its growth and development because this process influenced by many factors and it is a time dependent process. Therefore, in this work soft computing method such as artificial neural network and fractal geometry are used to do this manner. The databases for modeling are generated from the experimental measurements. First, multilayer perceptron network is used and it is found that the back-propagation algorithm with Levenberg–Marquardt learning rule is the best choice to estimate frost growth properties due to accurate and faster training procedure. Second, fractal geometry based on the Von-Koch curve is used to model frost growth procedure especially in frost thickness and density. Comparison is performed between experimental measurements and soft computing methods. Results show that soft computing methods can be used more efficiently to determine frost properties over a flat plate. Based on the developed models, wide range of frost formation over flat plates can be determined for various conditions.

Visualization of Effects of Ultrasound on Liquid Droplet Solidification and Frost Formation on Cold Flat Surface

An experimental study was conducted to study the effect of ultrasonic vibrations on frost suppression on a cold flat surface in atmospheric air flow. A microscopic visualization on the liquid droplet solidification and frost formation process with and without the effect of 20-kHz ultrasound was made. The water droplets solidification and frost crystal growth on the solidified water droplet surface were comparatively observed. Meanwhile, the frost thickness variations were comparatively analyzed. It was found that almost no difference can be seen between the water droplet solidification onset time with the effect of ultrasound and that without the effect of ultrasound. However, after the water droplet solidifies, almost no growth of the frost crystal can be observed on the solidified water droplets surface with the effect of ultrasound. In addition, during the whole frost formation progress, very small growth of the frost layer can be seen on the flat surface with the effect of ultrasound. The experimental results showed that the frost growth process on the cold flat surface can be significantly restrained due to the effect of ultrasound.

Experiment Study on Unsteady Frosting of Cryogenic Vertical Plate Surface under Natural Convection Conditions

A frosting test bench was designed, and unsteady frosting on vertical plate surface was dynamically described with liquid nitrogen as working medium under natural convection conditions. The variation of frost thickness was analyzed, and the flow state of moist air and temperature distribution near the vertical plate were described. The experimental results show the unsteady frosting of vertical plate surface under natural convection and cryogenic condition actually includes several steps: formation of frost crystals, growth of frost crystals and growth of frost layer. The initial frosting rate is large, but the rate decreases as time goes on until the frost layer thickness is unchanged. Moist air conducts a top-down flow near the vertical plate, and the rest of moist air flows into the main flow. The temperature of moist air decreased systematically along the tangential direction of vertical plate from top to bottom and along the normal direction of vertical plate from far to vertical plate surface.

Mesoscale investigation of frost formation on a cold surface

Frost formation on a horizontal flat copper surface was experimentally investigated using microscopic observations. The experiments were carried out on −20 to 0 °C copper surfaces with 22 °C air and 15–85% relative humidities. The experiments showed that the frost formation on a cold surface generally begins with the formation and growth of condensate droplets, freezing of the super-cooled condensate droplets, formation and growth of initial frost crystals on the frozen droplets, growth of frost crystals accompanied by the collapse of some of the crystals, and finally frost layer growth. The freezing onset time and diameter of the super-cooled condensate droplets were characterized. The initial frost crystals can be classified into four groups according to their appearance and shape, with the variations of the frost crystal shape as a function of the cold surface temperature and air humidity.

Frost formation and frost crystal growth on a cold plate in atmospheric air flow

The present study is concerned with the spatial variation of the frost thickness as well as the pattern of the frost crystals formed on a cold plate in atmospheric air. Experimental results regarding the two-dimensional variation of the frost thickness at the leading edge and observation on the growth of the frost crystals on the frost formation are provided. A microscopic image system is used to observe the structure of the frost layer. The environmental variables considered in this study include the velocity, temperature, and relative humidity of the air ( V, T a, and φ), as well as the surface temperature of the cold plate ( T w), which is varied by adjusting the cooling refrigerant temperature ( T ref). The ranges of the physical variables considered in this study are 2⩽ V⩽8 m/s, 18⩽ T a⩽30 °C, 40%⩽ φ⩽70%, − 18⩽T ref ( and T w )⩽0 °C.

Organic sequestration of copper

A swamp in southeastern New Brunswick contains up to 10% Cu (dry weight) in forest peat. This mineralization is unusual in that the metal is not present in megascopic lumps of native Cu but, rather, is invisible and appears to be in inorganic combination. Factors responsible for the movement of the metal in the mobile state are clearly diffusion, capillarity, evaporation, and the growth of frost crystals. The phenomena liable for the fixation of the Cu are less apparent. Vegetal concentration, sorption, and coprecipitation have no doubt produced minor modifications of the dispersion pattern. The dominant factor effecting the immobilization of the metal, however, appears to be sequestration of the Cu by the organic sediments, thereby forming a chelate compound. On the basis of this study, the only value that can be assigned to the activity of microbes is, possibly, a catalytic effect, speeding the formation of the Cu compound already stable under the chemical conditions imposed by the environment.