Frost Formation Research Articles

Image-Analysis-Based Approach for Identification of Air Cooler Heat Transfer Degradation during Frosting Process

Fin-and-tube heat exchangers have been extensively used in many fields, especially in heat, ventilation, air-conditioning, and refrigeration systems. In the case of the operation of a fin-and-tube heat exchanger as an air cooler, frost formation is an important effect that should be taken into account. The frost accumulation process is undesirable since it deteriorates heat transfer due to the insulation of the frost layer as well as causing excessive pressure loss. The analysis of the effect of the frosting process on a fin-and-tube air cooler performance is presented in this paper. Based on long-term experimental investigations applied to the air cooler in a cold storage chamber, the general degradation of the heat exchanger performance is discussed. The influence of frost on the cooling capacity, by-pass factor, and thermal resistance is analysed. The temperature distribution of the air passing through the air cooler before and after the defrosting process is presented and discussed. A method for the assessment of the amount of frost formed at the air cooler surface, based on visualisation of the air cooler during operation and synchronised with the thermal measurements, is developed. The results show that the frosting process causes deterioration of the cooling capacity by up to 40% in the analysed case. Correlation is demonstrated between frost formation and heat transfer degradation in the air cooler.

Simulation of Forced Convection Frost Formation in Microtubule Bundles at Ultra-Low Temperature

Hypersonic vehicles are an important area of research in the aerospace field today. One of the important issues is the power of the engine. In order to achieve large-span flight speeds, a more efficient approach is to use combined power systems. However, the problem of pre-cooler icing can occur in combined engine applications. The flow in the pre-cooler is extremely complex. Outside the tube is the high-temperature wet air entering from the engine intake, and the tube cooling is the ultra-low temperature cooling medium. Icing not only increases the heat exchange resistance of the pre-cooler during operation and affects the heat exchange performance of the pre-cooler, but also causes a large total pressure loss, resulting in a degradation of the engine performance. There is a lack of research on the icing law of the pre-cooler under different parameters. Therefore, it is necessary to conduct a corresponding numerical calculation study on pre-cooler icing and explore the influence of various influencing factors on icing. In this paper, a mathematical model of icing (frost) is established for the frosting phenomenon that may occur during the operation of the pre-cooler. Additionally, the principle of heat and mass transfer in the icing process is described by the mathematical model, and the influence of different parameters on the frosting parameters is explored by using the computational fluid dynamics (CFD) method. The law of tube bundle icing under different parameters was calculated, and the variation laws of frost layer morphology and wet air pressure drop were obtained. The laws of tube bundle icing under different parameters were calculated, and the changes in frost layer pattern and wet air pressure drop when each parameter was changed, which can provide guidance for the design and application of pre-coolers in the future.

Frosting and defrosting characteristics of multi-layer coated aluminum surfaces

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.

Porous Graphene‐Based Photothermal Superhydrophobic Surface for Robust Anti‐Icing and Efficient De‐Icing

AbstractThe problem of ice accumulation causes huge inconvenience in daily life and industrial application. Although superhydrophobic surfaces are promising for preventing ice formation, they may become ineffective in extreme environments. Here, a durable photothermal superhydrophobic surface on aluminum substrate (SHPo‐LIG@Al) is prepared through laser‐induced graphene (LIG) technology of polyimide (PI). Owing to the photothermal effect of graphene, the surface exhibits high light absorption rate (≈98%) and efficient photothermal conversion (surface temperature rises to 65 °C within 15 min under 1 sun). Compared to the conventional superhydrophobic surface (SHPo‐Al), ultra‐long droplet freezing time or even droplet un‐frozen is achieved on SHPo‐LIG@Al surface by varying the sunlight intensity. Moreover, it is interesting to find that satellite condensate droplets are formed around the three‐phase contact line of deposited droplet under 1 sun or 2 sun; the satellite condensate droplets would gradually evaporate till disappear and then bubbles start to nucleate on solid surface underneath the deposited droplet under 2 sun. Furthermore, the SHPo‐LIG@Al surface exhibits retarded frost formation and remarkable de‐icing/de‐frosting durability under sunlight irradiation, as well as self‐cleaning, chemical stability and corrosion resistance. This study may provide important insights for the development of photothermal superhydrophobic surface for anti‐icing/de‐icing applications.

Frost formation from general-low to ultra-low temperatures: A review

• 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.

Heat transfer in the regenerative heat exchanger

This paper tackles the issue of frost formation in non-hygroscopic rotary heat exchangers used for energy recovery from exhaust air under high-speed rotor conditions by means of numerical simulations and experimental approaches. On the basis of some idealized assumptions, a frost growth submodel is presented to predict the behavior of the rotary heat exchanger under frosting conditions. Frost formation is modeled by considering the mass diffusion of water vapor through the frost layer, taking into account supersaturation phenomena. Calculations were carried out using a three-zone model based on the modified ε-NTU method. The local heat transfer coefficient and the NTU calculation method resulting from the influence of the heat exchanger entrance region were also applied. The obtained correlations for the temperature effectiveness agree with the simulation data within uncertainty bounds. The results of the numerical simulations allowed us to determine the outdoor air conditions that initiated the frost accumulation phenomenon inside the thermal wheel for two values of return air relative humidity: RH2i=20% and RH2i=40%. In both cases, the threshold temperature for unsafe operating conditions increases with increasing relative humidity of the outdoor air. Under ‘frost accumulation’ operating conditions, the frost growth rate is approximately five times higher at RH2i=40% than at RH2i=20%. In this regard, the need to implement frost protection techniques increases significantly with an increase in relative humidity of return airflow. Further analysis conducted for the operation of a thermal wheel’s operation under frosting conditions revealed that a latent heat flux contributed to the local frost density should not be neglected in a compact heat exchanger’s model. Interestingly, the operation time of the rotary heat exchanger, and hence the growth of the thickness of the frost layer, has a significant influence on a local heat transfer coefficient α, however, it does not affect the Number of Transfer Units (NTU) visibly.

Experimental data and modelling of a dual source reversible heat pump equipped with a minichannels evaporator

• Experimental investigation on a novel dual source (air/ground) reversible heat pump. • A minichannel heat exchanger working as evaporator and condenser is presented. • A physical model of the heat pump has been developed and validated. • R454B and R452B are assessed as possible low-GWP substitutes for R410A. • The model has been used to develop a switching strategy between the thermal sources. This paper presents an experimental investigation and a numerical analysis of an innovative dual source heat pump. The combined use of air or ground as heat source/sink can be a strategy to improve the efficiency of the heat pump at lower costs because the length of the borehole heat exchangers can be reduced. The present heat pump combines several solutions to go beyond the state of the art: it can work with air or ground as heat source/sink and it can operate with a traditional finned coil heat exchanger or with a minichannels heat exchanger. Experimental tests have been conducted in a climatic chamber to study the performance of the heat pump at full and partial loads when operating in summer and winter modes. The minichannels heat exchanger works both as the condenser and as the evaporator. In the winter configuration, the problem of frost formation in the minichannel heat exchanger is avoided because the heat pump operates in ground mode at low air temperature. In the summer mode, in the minichannels condenser the temperature approach between the air and the refrigerant is lower compared to the finned coil heat exchanger (on average 1.1 K lower) and the minichannels technology allows to reduce the refrigerant charge in the system. A mathematical model of the heat pump has been developed and validated against experimental data. The model provides accurate predictions of the heat pump performance with deviations below 10 %. The heat exchanger modelling is based on a physical approach that allows to calculate the heat transfer coefficients and pressure drop in the heat exchangers. This approach requires more computational effort, but it allows to predict the performance of the heat pump when considering different refrigerants as the low-GWP alternatives R454B and R452B. The model shows that when the heat pump works with R454B and R452B, the cooling and heating capacity are about 10 % lower compared to those obtained with R32, while the differences in terms of performance coefficients are negligible. The model is then used to implement a control strategy for switching between the two heat sources to achieve always the highest performance depending on the operative conditions. In the present case, the results of the model show when the air source should be preferred: in the winter season when the air temperature is by 5 K higher than the water temperature returning from the borehole heat exchanger, in the summer season when the air temperature is by 10 K lower than the water temperature from the ground.

Frosting mechanism and behaviors on surfaces with simple geometries: A state-of-the-art literature review

• Correlations of frosting characteristic parameters are summarized and analysed. • Effects of frosting conditions on average frost layer characteristics are reviewed. • Frosting distribution characteristic for different cold surfaces are reviewed. • Eight research gaps relating to frosting mechanism and behaviors are identified. Frosting causes negative impacts in many technical and engineering fields, such as refrigeration, air source heat pump, aircraft, wind power generation, and power grids. To alleviate the negative impacts of frosting, studies on frosting characteristics, avoiding and delaying frosting, and defrosting mechanisms and methods have been carried out for decades. Among these studies, frosting characteristics, which are the fundamental basis for defrosting, avoiding, and delaying frosting are consistently the hot research spots. Considering that complex structures can be usually regarded as the combination of simple geometries, in this paper, a comprehensive review of the literature related to the frosting mechanism and behaviors on surfaces with simple geometries is presented. The former covers frost formation process and morphology, frost layer structure model, and frosting characteristic parameters, such as frost thickness and effective frost thermal conductivity. The latter is further divided into six parts based on the configurations of cold surfaces, including flat plates with a fixed surface temperature, flat plates with a fixed base temperature, cylinders, parallel plates, set of fins and semipermeable membranes. Based on the review, the research gaps in the related study fields are identified, and recommendations for further research are offered. The literature review presented in this paper can provide a comprehensive and systematic reference for the people who are working in the study fields where frosting is commonly encountered.

Transient cooling performance of R134a flash-evaporation spray enhanced by cold air jet: A novel heat transfer enhancement method in laser dermatology

Nevus of Ota is a dermal hyperplastic pigmented disease with a morbidity rate of 0.1–0.6% in the Asian population. Laser surgery with the assistance of cryogen spray cooling (CSC) is a potential clinical treatment for nevus of Ota. However, for Asians with high melanin content, it is necessary to enhance cooling capability without the increment of refrigerant amount. In this study, a R134a flash-evaporation cryogen spray cooling coupled with cold air jet (CSC-CAJ) was developed to improve the cooling capacity. Epoxy resin was employed as a skin phantom to investigate the heat transfer characteristics of CSC-CAJ with different air jet parameters. Air jet parameters of CSC-CAJ are recommended to be Ta = -7 ℃, Va = 3 m3/h Za = 10 mm and Φa = 30°. Compared to CSC, CSC-CAJ could reduce the minimum surface temperature from -39 to -42.1 ℃, and the average value of convective heat transfer coefficient (h) of CSC-CAJ could increase from 8 to 12.9 W/(m2·k) or 61.3% for the recommended parameters. The liquid film residence time (τ) decreases from 301 to 234 ms or 23.3%. In addition, based on the visualization observation and surface heat transfer measurement, the frost formation phenomenon on the surface and the heat transfer enhancement mechanisms are well understood. Four modes, including Leidonfrost mode, no runoff mode, runoff mode and evaporation mode, are proposed to describe the four stages of CSC-CAJ. A non-dimensional correlation of normalized Nusselt number (Nu*) and time (t*) is summarized. The primary heat transfer enhancement mechanism of CSC-CAJ is found to be the increase in cryogen evaporation rate and the ratio of heat removal from the phantom surface to phase change latent heat.

Innovative method to reduce frost formation in liquid-to-air membrane energy exchangers (LAMEE) based on 3D CFD simulation

Frost formation is an important and challenging problem in the proper operation of ventilation systems. In this study, a novel model is developed to predict the amount of frost formation in a Liquid-to-Air Membrane Energy Exchanger (LAMEE) based on the 3-Dimensional Computational Fluid Dynamics (CFD) technique. This model can be a robust way to find and analyze different proposed methods to combat frost. The results show that the presence of the membrane reduces the mass of frost compared to the impermeable surface by approximately 60% due to the mass transfer phenomenon. Moreover, the variations of air temperature along the LAMEE are sharper than the changes in humidity ratio, which is an important reason for more frost forming. Condensed Humidity Ratio (CHR) is presented as a valid criterion for analyzing the distribution of frost formation in heat and energy exchangers. Furthermore, an innovative method is proposed based on 3-fluid LAMEE to reduce frost formation. In the proposed method, the solution temperature is controlled with another fluid that flows in tubes located in the solution channel. Therefore, this model control heat transfer between air and solution flows in different parts and moderate air temperature variations for vapor reach the saturation line in a low humidity ratio. The developed CFD model predicts that the frost formation in the proposed LAMEE is around 25–28% saturation less than the typical LAMEE at an equal sensible cooling capacity.

CFD analysis of phase change behaviour and frost growth under various conditions using mass transfer theory

A frosting model based on mass transfer theory was developed using Computational Fluid Dynamics software to investigate frost growth on a horizontal surface. The mass transfer model utilizes dimensionless numbers to calculate the mass transfer coefficient to predict rate of frosting. The predicted frost thickness was generally within 20% of experimental data and was able to show the mass transfer behaviour for the frosting phenomenon. Compared to previous studies, water vapour condensation and frost growth in this study are governed by the 0 °C air temperature iso-surface which in turn is affected by the amount of frost developed as well as the operating conditions. The predictions by model show that mass transfer rates are higher at both the cold surface and the frost-air interface, which represents frost densification and thickness growth, respectively. A larger distinction between the two mass transfer rates is observed as the surface temperature becomes lower. Additionally, the effects of air humidity, velocity and temperature on frost formation at refrigeration conditions are investigated.

Fin and Temperature Effect of Frost in Ambient Air Vaporizer

Since liquefied natural gas (LNG) is imported in a liquid state of about -162℃ to increase transportation efficiency in Korea, it must be vaporized in a gaseous state to supply it to consumers. Among them, ambient air vaporizer (AAV) has caught attention due to eco-friendly and low costs characteristics. However, there is a disadvantage that the performance of the heat exchanger is deteriorated due to frost due to mist and icing when used for a long time. In this paper, frost generation model in AAV vaporizer was investigated with numerically to examine utilizing the vaporizer performance with the frost generation behavior. The frost generation behavior of AAV vaporizers was examined with humidity, fin characteristic, and temperature effects. As for the LNG discharge temperature, the 12 fin vaporizer showed the highest discharge temperature when the atmospheric temperature was 25℃, and the 8 fin vaporizer had the lowest LNG discharge temperature when the atmospheric temperature was 0℃. In the case of frost formation, in the case of the 12 fin vaporizer, it was formed the most at the atmospheric temperature of 25℃, and the least was formed in the vaporizer at the 0℃ condition of the atmospheric temperature of 8 fins.

Experimental and numerical investigation on frosting of finned-tube heat exchanger considering droplet impingement

Air source heat pumps are prone to frosting under low-temperature, and high-humidity conditions. Frosting in finned-tube heat exchangers increases their heat transfer and airflow resistances. Moreover, it leads to reductions in the evaporation and condensation temperatures of the unit, increased energy consumption, working performance deterioration, and even shutdown. Therefore, it is necessary to simulate the frost formation process, in order to provide a reference for frost-prevention design of finned-tube heat exchangers in cold, high- humidity areas. However, fin microchannel frosting is a complex process that involves heat and mass transfer. It is difficult to describe the frost layer formation process completely. This is, especially true for droplet formation during nucleation. This paper describes the mass transfer process that occurs during nucleation via a droplet impingement model and uses the enthalpy porosity method to solve the solidification problem. Air-source heat pump frosting tests performed in the enthalpy difference laboratory verify the numerical methods. It is determined that the model in this paper can simulate the frost growth process more comprehensively and accurately than a mathematical model that does not consider droplet formation. Then, the effects of the mass transfer rate, air temperature, humidity, inlet velocity, and fin surface temperature on the finned-tube heat exchanger frosting process are described. Frost growth is related positively to the rate at which water vapor impacts the fin surface. The frost is thickest when the moist air temperature range is between 1 °C and 3 °C. Moreover, the frost density varies little when the wet air temperature is above 0 °C. The variation of frost density and frost thickness shows an opposite trend under the wind speed factor.

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.

Frost prevention research of the enthalpy wheel in air conditioning systems and industrial heat recovery systems

Enthalpy wheels are often used for air dehumidification and waste heat recovery. Application scenarios include air conditioning systems and industrial systems.Frost formation inside the enthalpy wheel is common in cold climates and has serious impacts on operational reliability and heat recovery performance. It is found that frost type, frosting area and frosting limit vary between different scenarios. Experimental and numerical investigations are conducted in the present research. For a certain wheel, inlet parameters of supply air and outlet parameters of exhaust air are the key factors influencing frosting formation. The enthalpy wheel will operate regularly without frost formation if exhaust temperature is higher than the frosting limit. Reducing the enthalpy wheel rotation speed below 6 rpm is an effective method to prevent the enthalpy wheel from frosting, but heat recovery efficiency is correspondingly reduced. Industrial systems are less affected by the cold climate, especially at higher exhaust air temperatures. Even if the climate temperature drops to −30 °C, the heat recovery efficiency can still exceed 85%. The research is anticipated to be useful for the application of enthalpy wheels in cold climates.

Fabrication of Anti-Icing Surface Structures on Aluminum Alloy for Aerospace Applications

Icing, the phenomenon of the formation and accumulation of ice or frost on a surface due to the solidification of water droplets at low temperature can be undesirable in many applications. Surface icing can lead to increased energy consumption in aerospace and automotive applications due to increased aerodynamic drag. Ice formation can also present a mechanical and electrical safety hazard, and as such significant work has been done to produce surfaces with anti-icing properties through surface modification to decrease ice formation and adhesion to surfaces. One route toward the generation of anti-icing surfaces is through laser surface processing. Laser micro/nanostructuring of surfaces has advanced greatly in recent years due to advancements in laser source technology and reduction in capital costs for ultrafast femtosecond pulsed machining lasers. Laser material processing offers a rapid, scalable, and non-contact method for fabricating large area anti-icing surfaces. In this work, the production of anti-icing surfaces using femtosecond laser micro-and nanostructuring on aluminum alloy 7075 surfaces was examined. With an aim to optimize the anti-icing properties of the substrates, laser parameters such as pulse energy, repetition rate and beam scanning speed were varied to produce highly defined microstructures on the aluminum surface.Various functional properties such as hydrophobicity and surface roughness are examined.

Molecular dynamic study of local interfacial thermal resistance of solid-liquid and solid-solid interfaces: Water and nanotextured surface

Degradation in the performance of air conditioners and refrigerators is caused by frost formation and adhesion on the surface. In the present study, by means of the classical molecular dynamics simulation, we investigated how and how much the nanotextured surface geometry influenced the interfacial thermal resistance (ITR) between the hydrophilic solid wall and the water/ice. The ITR of the interfacial region was comparable in both the water and the ice states. As the nanostructure gaps became narrower, the ITR of the interfacial region decreased. In addition, local ITRs of the nanotextured surfaces were characterized. Especially when the H2O molecules were in the crystal form, the local ITR significantly increased. A greater amount of the thermal energy was transferred through the material interface by means of the intermolecular interaction when more H2O molecules were located in the proximity area, which was closer to the Pt solid wall than the first adsorption layer density peak. When the H2O molecules were in the crystal form on the solid wall, the proximity molecules decreased, and then the local ITR significantly increased.

Polycyclic aromatic hydrocarbons (PAHs) in gas, PM2.5, and frost samples in a severely polluted rural site of the North China Plain: Distribution, source, and risk assessment

The process of frost formation has been studied as an important deposition pathway for removing atmospheric pollutants. However, the removal of organic species during frost formation remains unknown. Gas, PM2.5 and frost samples were collected in Wangdu, a severely polluted rural site in the North China Plain, during the winter of 2018. The concentrations, distributions, sources, and ecological and health risks of the 16 PAHs in the samples were determined. The 24 h-averaged concentrations of PAHs in the air (gas + PM2.5) reached 719 ng m−3, indicating that the local atmosphere was heavily contaminated by PAHs in winter. The average concentrations of PAHs in the frost insoluble reached 27.5 μg g−1, indicating the strong ambient PAHs deposition during the frost process. The distribution patterns of individual PAH species indicated that the PAHs in the gas influenced the frost samples, and the sources of PAHs in frost samples were highly similar to those in PM2.5 samples. The calculated lifetime lung cancer risk (LLCR) via inhalation of ambient PAHs was classified as high risks. The average total Benzo(a)pyrene toxic equivalent quotient (TEQ) of PAHs in the frost samples was estimated as 2.50 μg TEQ g−1, posing a high threat to the ecological environment and health. After the melting of frost, the PAHs are released into the soil. Although the concentrations are diluted, the PAH concentrations could adversely affect the growing vegetation or agricultural activities. The extremely high PAH concentrations and their ecological and health risks require special attention and strict regulation.

Sustainable anti-frosting surface for efficient thermal transport

Inhibiting frost formation is of fundamental importance for many industrial applications, ranging from various infrastructures to thermal systems. Sustaining a frost-free surface without reducing heat transfer efficiency is a great challenge. We hereby propose a counterintuitive approach to achieve sustainable anti-frosting under high heat flux subcooled condensation by accelerating the nucleation-to-departure cycle of condensed liquid, demonstrated by manipulating initial nucleation and reducing thermal resistance using a three-dimensional nanowire network. High heat flux subcooled condensation is maintained without frost formation on the frost-free surface at a low surface temperature of −15°C for more than 5 h in humid air with a relative humidity of 75%. More than 10 times enhancement in the steady-state heat flux is obtained on the frost-free surface compared with the state-of-the-art low solid fraction non-wetting nanostructured surfaces. This passive frost-free surface may hold promise for increasing the efficiency of thermal systems while inspiring the design of frost-free 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.