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The anti-freezing (or freezing delay) characteristics of sessile water droplets placed on bare and superhydrophobic surfaces were experimentally investigated. The change of the droplet temperature during the cooling and freezing processes was monitored under a constant surface temperature. The freezing delay time was calculated for a statistically large ensemble of droplets, which showed a random distribution. The minimum and maximum freezing delay times of droplets on the bare and superhydrophobic surfaces were analyzed by using a cumulative distribution function. The maximum freezing delay time of the superhydrophobic surfaces was more than three times longer than that of the bare surfaces. Moreover, about 7% of the water droplets on the superhydrophobic surface could be frozen earlier than or similarly with those on the bare surface. [ABSTRACT FROM AUTHOR]

We investigated the local frost growth in a louvered fin heat exchanger using a scaled-up model and analyzed the characteristics of frost behaviors and variation in the heat transfer rate according to frost formation compared to a flat-fin heat exchanger. In contrast to a uniform frost layer of the flat-fin heat exchanger, highly non-uniform frost layer between the front and rear sides was observed on the louvered fin heat exchanger. The heat transfer rate of the louvered fin heat exchanger more rapidly decreased with the frost formation than that of the flat-fin heat exchanger. The blocking of the spaces between the louvers on the front side by frost mainly induced the significant decrease of the heat transfer rate of the louvered fin heat exchanger. On the louvered fin front side, the spaces between the louvers near the redirection region were first blocked by frost formation before those near the air inlet region due to the higher mass transfer driving force around the redirection region. Once the louvered fin front side was totally blocked by frost, the louvered fin rear side did not effectively work anymore. [ABSTRACT FROM AUTHOR]

• Thermo-responsive, wettability switchable surfaces are fabricated. • Thermo-responsive surfaces are superhydrophobic below 60 °C and hydrophilic over 60 °C. • Frosting and defrosting performance is experimentally investigated. • The fabricated surfaces exhibit excellent frost retardation. • Wettability transition can greatly benefit to enhance defrosting performance. Combining hydrophobicity with hydrophilicity on hybrid surfaces generates unique phenomena in anti-frosting. However, limited attention has been paid to investigate the anti-frosting effects of wettability switchable surfaces in response to temperature changes. Herein, frosting and defrosting characteristics of thermo-responsive, wettability switchable surfaces are investigated. Specifically, the wettability switchable surfaces are successfully fabricated by coating a poly(ε-caprolactone) (PCL) layer onto the microstructured aluminum substrates, resulting in the wettability transition from superhydrophobicity with an apparent contact angle of water droplet as 152° in the surface temperatures under 60 °C to hydrophilicity with an apparent contact angle of water droplet as 63° in the surface temperatures over 60 °C. The wettability transition capabilities of the surfaces not only enable excellent frost retardation comparable to superhydrophobic surfaces in frosting process, but also realize fast full drying capabilities slightly shorter than superhydrophilic surfaces in defrosting process. The unique feature of wettability switchable surfaces represents an effort to combine the distinct advantages of superhydrophobic and superhydrophilic surfaces to realize anti-frosting functionalities. [Display omitted] [ABSTRACT FROM AUTHOR]

Experimental research was conducted to study the frost formation on superhydrophobic surfaces at ultra-low temperatures, which might broaden the insight into frost growth behavior on the heat exchangers. The frost properties were examined over time, and the frost retardation according to the frosting factor was identified using the frost reduction ratios. The ablimation-dominant (desublimation) mechanism was observed at ultra-low temperatures in which almost no water droplets were generated. Consequently, the influence of the superhydrophobic surface on frost retardation was insignificant within −5% owing to the frosting mechanism. It is difficult to expect an increase in thermal performance of heat exchangers by applying superhydrophobic surfaces; using superhydrophobic surfaces for the purpose of frost retardation at ultra-low temperatures is not recommended. • Frosting behavior on superhydrophobic surfaces at ultra-low temperatures was investigated. • Frost thickness and density were analyzed over time. • Frost retardation was identified using the frost reduction ratios. • The influence of the superhydrophobic surface on frost retardation was insignificant. [ABSTRACT FROM AUTHOR]

• Edge effect on droplet condensation and frosting characteristics are revealed. • Reverse melting frequency inside hole is from 1.78 × 10−2 hz to 2.61 × 10−2 hz. • Existed flat-plate fitted frost thickness is 14.59 % smaller than the measured value. • Frost grows faster in the edge region than inside the hole region. To reduce the negative impact of frost formation on cold surfaces in low-temperature environments, metal wire mesh can be set in the air flow direction from the cold surface to reduce air humidity. To quantitatively determine the effectiveness of dehumidification after passing through a metal wire, the frosting process needs to be studied. To analyze the mechanism of frost formation under the horizontal wire mesh surface, the wire mesh was simplified as a flat plate with holes, or aperture plate, with plate temperature varying at a range of -15.0 °C to -5.0 °C. The results show that, as the plate temperature decreases, the droplet condensation and growth and droplet solidification stages gradually decrease, the average frost layer thickness and frost layer growth rate rise, and overall frost layer surface roughness increases. For a frosting duration of 1,800 s, compared to -5.0 °C, the average frost layer thickness from -7.5 °C to -15.0 °C inside the hole increased by 56.68 % and 156.41 %, while the plate edge region away from the hole increased by 40.76 % and 169.92 %, respectively. The reverse melting frequency gap between the inside hole region and the plate edge region away from the hole increases with the decreasing aperture plate temperature. At -15.0 °C, the reverse melting frequency inside the hole is 5.00 × 10−3 Hz lower than in the region of the plate edge away from the hole. The results of this article are beneficial for understanding the frosting process of wire mesh or perforated-fin heat exchanger. [ABSTRACT FROM AUTHOR]

• Frost accumulation on an evaporator was measured using meltwater collectors. • Heat distribution of an evaporator was calculated by measuring the heat transfer from the heater. • Power optimization of the defrosting heaters was conducted. • The defrosting performance was improved at the optimum heater power. Power optimization was conducted for the defrosting heaters (conductive and radiant heaters) in a freezer to enhance their performance. The distribution of frost in the evaporator was measured experimentally, and the required defrosting energy corresponding to frost accumulation was calculated. The influence of defrosting heaters on frost was evaluated by measuring the heat quantity of the evaporator. The optimization was carried out such that the two heaters distribute the valid energy to the evaporator minimizing the excessive waste heat. At the optimum heater power, the maximum surface temperature of the evaporator was reduced, and the temperature distribution of the evaporator became uniform. The heater power and defrosting time were reduced through power optimization, thereby increasing the defrosting efficiency up to 6.7%. The effective heater power distribution could be further promoted to other heat exchangers with various heat transfer technologies. [ABSTRACT FROM AUTHOR]

• Frost growth behavior at ultra-low temperature were investigated. • Three unique frosting phenomena were observed. • Frost growth mechanism was classified by frost surface temperature. • The effect of frosting factors on the frost growth behavior was investigated. An experiment was conducted to investigate the frost growth mechanism and its behavior under forced convection and ultra-low temperature (−160°C < T w <−100°C) conditions. In ultra-low temperature frosting, peculiar phenomena such as formation of thread-shaped frost and a frost hill was observed. The point at which the frost growth mechanism changes was identified, and two stages of the frost growth mechanism were defined based on this point. The effect of cooling surface temperature, absolute humidity, air velocity, and air temperature on frost growth were investigated. As the cooling surface temperature decreased, the frost thickness increased, whereas the density was less affected. The results show that the frosting mechanism was dependent on the frost surface temperature, and it will be a useful guideline to solve the problems caused by frost generated under ultra-low temperature conditions. [ABSTRACT FROM AUTHOR]

• Frosting and fogging were investigated through experiments. • Frost and fog modeling at ultra-low temperatures was presented. • Frosting and fogging behaviors were analyzed experimentally and numerically. • The period of fog formation could be predicted using the model. When liquefied natural gas (LNG) is vaporized by an ambient air vaporizer (AAV), frost and fog appear, which reduce energy efficiency and cause environmental issues. A model for simulating frost growth behavior and fog formation was developed for evaluating an AAV. The mechanisms of frost and fog formation and their mass transfer rates were defined, and they were characterized numerically. Experimental tests were performed at ultra-low temperatures (below −100°C) to quantitatively verify the frost thickness and density and qualitatively verify the fog formation. Despite the irregular characteristics of the fog, it was found that fog generation was dominantly influenced by the frost surface temperature. Therefore, the period of fog formation could be predicted through frost and fog modeling, and the model would be useful in managing AAVs. [ABSTRACT FROM AUTHOR]

• Frosting experiment was conducted under cryogenic forced convection condition. • Frosting model for cryogenic conditions was developed and validated. • Frost growth behavior was numerically analyzed under cryogenic conditions. • Frost properties were discussed qualitatively and quantitively. A frost model for predicting frosting behavior under cryogenic conditions was developed. The frosting behavior was visualized through numerical analysis, and the results were compared to experimental data to confirm the validity of the model. It was shown that under cryogenic temperatures, a considerably low-density frost layer was formed owing to the frosting mechanism, which was sublimation-dominant frosting, rather than conventional condensation-freezing. It was qualitatively confirmed through experiments that low-density frost formed near the frost surface, and high-density frost formed near the cryogenic cold plate. Furthermore, the frost density distribution of the interior frost was quantitatively analyzed through a numerical analysis. The surface temperature of frost was seen to rise rapidly during the initial stage of frosting. Thus, it is possible to estimate the time at which the frosting mechanism changes from sublimation to condensation-freezing. [ABSTRACT FROM AUTHOR]

• Heat and mass transfer analogy was proposed under cryogenic conditions. • A dimensionless correlation was developed for the frost density. • The correlation for thermal conductivity was derived via regression analysis. An experimental study on the phase change occurring on a cryogenic cooling surface was conducted under forced convection conditions. The mass and surface temperature of frost and the heat flux at the cooling surface were measured in the experiment. The heat and mass transfer analogy was investigated under cryogenic conditions. The mass estimated under various experimental conditions using the modified analogy differed by up to 8% from the experimentally measured mass. The density and thermal conductivity of frost at cryogenic temperatures were lower than those of general-low temperature frost; the dimensionless correlations of density and thermal conductivity of frost were derived via regression analysis. The calculated frost density and thermal conductivities were differed from the measured value by up to 9% and 11%, respectively. Therefore, a model was built to predict the heat and mass transfer and improve the thermal performance on the cooling surfaces of various cryogenic devices using the presented frost density and thermal conductivity correlations. [ABSTRACT FROM AUTHOR]

Fabrication methods of the hydrophobic property on metal surfaces and frosting characteristics on hydrophobic surfaces were investigated. A hydrophobic surface with a static contact angle of less than 150° was implemented by surface coating or etching, and a superhydrophobic surface with a static contact angle of greater than 150° was realized by a hybrid method using both coating and etching. The changes in surface properties affected the behaviors of the early stage frosting from the dry surface to the formation of ice crystals. On the hydrophobic surfaces, ice crystals were formed by freezing after condensation. Isolated-droplet freezing and inter-droplet freezing are mechanisms by which the condensate undergoes a phase change into ice crystals. Through isolated-droplet freezing, a supercooled condensate changes phase into ice crystals by forming ice nuclei based on the classical nucleation theory. In addition, through inter-droplet freezing, ice crystals are propagated due to the difference in saturation vapor pressure between supercooled condensates and ice crystals. The formation and propagation of ice crystals are delayed as the static contact angle increases. Additionally, based on a review, future researches that is needed to improve hydrophobic technologies are discussed. [ABSTRACT FROM AUTHOR]

• Ultra-low temperature frost growth was monitored on vertical plate under forced convection. • Different frost growth behaviors were observed according to airflow direction. • Correlations to predict the frost layer growth behavior were presented. Frost formed on equipment operating at ultra-low temperatures (below -100 °C) affects thermal performance and leads to maintenance problems. Despite this need to study frosting behaviors at ultra-low temperatures, studies on vertical plates under forced convection are lacking. In this study, frost growth behavior at ultra-low temperatures on vertical plates was experimentally investigated with buoyancy-assisting and buoyancy-opposing flows under forced convection. Different frost growth behaviors were observed owing to the combined effects of the gravity force of the frost and the drag force exerted on the frost. Specifically, the frost layer formed in the opposing flow had a higher density and lower thickness than that formed in the buoyancy-assisting flow due to the formation of unique frost walls in the opposing flow. The frost wall not only delayed frost growth on the downstream plates but also promoted condensation phenomena in the frost wall. In addition, frosting behavior at ultra-low temperatures on the vertical plate was observed in terms of air velocity (1–3 m/s), air temperature (15–25 ℃), cooling surface temperature (-160 to -100 ℃), and absolute humidity (0.0054–0.0094 kg/kg a). Additionally, correlation equations were suggested to predict ultra-low temperature frosting behaviors including average frost thickness and frost density within an error of 10%. [ABSTRACT FROM AUTHOR]

Microscopic seed behavior in the early stage of frost formation was experimentally observed with different fin surface contact angles from bare to superhydrophobicity under air-source heat pump operating conditions. The seed average height, radius, number, and frost density at the early stage of frost were analyzed. As the surface contact angle increased, the seed average height and number increased, while the seed radius and frost density decreased. A correlation with a Fourier number was proposed from the measured data. With the correlation, the large and small frost retardation effect regions were identified. [ABSTRACT FROM AUTHOR]

Delaying frost formation and shortening defrosting time were both of importance to enhance anti-frosting characteristics. To achieve those, functional multi-layers with a total thickness of <20 μm were successfully integrated over aluminum substrates by sequentially depositing a bottom electrical insulating layer, a middle heating layer, and a top superhydrophilic-superhydrophobic patterned layer. Frosting and defrosting characteristics of functional multi-layers were experimentally investigated. Coating functional multi-layers over aluminum substrates considerably enhanced anti-frosting characteristics. Specifically, the functional multi-layers effectively retarded frost formation on the surfaces, compared to superhydrophilic surfaces, while showing slightly higher frost formation than superhydrophobic surfaces. During defrosting, melting was completed in a fast manner due to the combined effects of the ultrafast temperature increase by the functional multi-layers and the excellent water drainage characteristics by the top superhydrophilic-superhydrophobic patterned layer. • Functional multi-layer surfaces are fabricated by layer-by-layer assembly. • Frosting and defrosting performance is experimentally characterized. • Surface wettability determines frosting formation on the surfaces. • Rapid and efficient defrosting is enabled by employing functional multi-layers. [ABSTRACT FROM AUTHOR]

We experimentally investigated the phenomena of frost retardation in terms of the heat exchanger fins with various water contact angles under normal air-source heat-pump operating conditions. We first introduced a new experimental method using a luminance meter, which measured the light intensity from the surface, and determined the phase-change time, for quantitatively defining the frost stages. The phenomena of frost retardation was analyzed for 10 samples with different water contact angles (75°–160°) at different refrigerant temperatures. When the refrigerant temperature was −10 °C or −12 °C, the effect of frost retardation increased remarkably with superhydrophobic surfaces. However, when the refrigerant temperature was −8 °C, the effect of superhydrophobicity diminished at water contact angles greater than 150°. [ABSTRACT FROM AUTHOR]

• Frosting mechanisms from general-low to ultra-low temperatures were investigated. • Frost morphology and properties were identified in terms of frosting factors. • Unique phenomena occurring at ultra-low temperatures were discussed. The frosting at ultra-low temperatures can cause significant degradation of the thermal performance of heat exchangers, which also leads to a decrease in the energy efficiency of air-source heat pumps. In this paper, the temperature terminologies for frosting were classified, and the frosting mechanism and frost properties were investigated from general-low to ultra-low temperatures. The dominant phase change mechanisms at general-low and ultra-low temperatures were condensation-freezing and desublimation, respectively. The frost layer growth mechanism at ultra-low temperatures can be classified into the formation of ice nuclei, growth of ice particles, and deposition of a frost layer in the initial stage of frosting. The frost morphology at ultra-low temperatures showed wisteria or shrub-shaped frost, which was not observed at general-low temperatures. Frost properties are determined depending on frosting factors such as cooling surface temperature, air temperature, absolute humidity, and air velocity, while the thermal conductivity of frost was determined by frost porosity. At ultra-low temperatures, some unusual phenomena were observed that were not observed at general-low temperatures. When the cooling surface temperature was at ultra-low temperatures, the fog was formed near the cooling surface along with the frost, and this fog was dominantly influenced by the frost surface temperature. In addition, unique phenomena such as thread-shaped frost and leading-edge frost concentrations were observed at ultra-low temperatures, and there was a clear difference in the frosting mechanism based on the temperature range. [ABSTRACT FROM AUTHOR]