Frost Accumulation Research Articles

Inhibition of surface ice nucleation by combination of superhydrophobic coating and alcohol spraying

Delaying and suppressing frost formation is of significance due to its wide application in many fields. In the present study, the characteristics of superhydrophobic surface inhibiting the condensation icing/frost were obtained and the turning point of coverage area was observed, which could be used as the new criterion for the onset of icing on the superhydrophobic surface. It was also found that during the frost accumulation period, the frost shedding occurred at the junction of hydrophilic and superhydrophobic regions when the hybrid surface was inverted. In addition, the results highlighted that the combination of active ethanol spraying and passive superhydrophobic coating could completely avoid the occurrence of condensation icing/frost.

Understanding the anti-icing property of nanostructured superhydrophobic aluminum surface during glaze ice accretion

Ice accumulation on power transmission lines has been a vital problem for power systems in cold regions. In this paper, the anti-icing property of the as-prepared superhydrophobic (SHP) surface on the aluminum substrate in glaze ice was studied in an artificial climate chamber. Results show that the as-prepared SHP surface demonstrates excellent anti-icing property with only 24.65% of the entire surface froze after spraying for 60 min. The nano-scale structures and the HDTMS molecule both contributed to the superhydrophobicity. The reason for the excellent anti-icing property of the as-prepared SHP surface can be ascribed to two reasons: on the one hand, the nanoscale structures can effectively help maintain the superhydrophobicity of the as-prepared SHP surface at low temperatures, which inhibited the frosting process for over 210 min at −5°C and −10 °C. On the other hand, the as-prepared SHP surface can completely repel the impacting millimeter-level cooled water droplets or reducing the critical diameter of micro-scale droplets in glaze ice leading to a reduction of ice accumulation. The nanostructured ZnO textures significantly helped improve the ability to resist frost and ice accumulation on the aluminum surface, which has a good prospect on power transmission lines against ice formation.

Experimental study on defrosting start control strategy for ASHPs

In applications, the defrosting operation of an air source heat pump (ASHP) unit is always started with a pre-set time, corresponding to a fixed mass of frost accumulation, and thus resulting in series of mal-defrosting phenomenon. The exact defrosting initiation time is hard to be quantitatively given due to the dynamic and uneven frosting conditions. For an ASHP unit with a multi-circuit outdoor coil, when the melted frost was locally drained during defrosting, the optimization of start defrosting control strategy becomes more complicated. Here, this fundamental problem was experimentally investigated, with frost evenly distributed on the surface of the outdoor coil at the start of defrosting. Defrosting performance of the experimental ASHP unit at different frost accumulations were then comparatively analyzed, with the melted frost local drainage specially considered. These physical parameters include the temperature of tube surface and melted frost, compressor suction and discharge pressures and their difference, thermal energy taken from indoor air and electricity inputs on compressor and fans during defrosting, etc. Results suggest that, the defrosting efficiency reached its peak at 46.05% when frost accumulation was at 930 g. After the melted frost was locally taken away during defrosting, it reached its peak at 51.80% when frost accumulation was at 933 g. Thus, the time-based start defrosting control strategy was demonstrated to be fundamentally optimized with this method. Contributions of this study could be used for adjusting the control strategy of ASHP units, which are valuable to energy saving in industrial applications.

An experimental study on time-based start defrosting control strategy optimization for an air source heat pump unit with frost evenly distributed and melted frost locally drained

Defrosting operation of an air source heat pump unit is always started with a pre-set time, corresponding to a fixed mass of frost accumulation. The exact defrosting initiation time is hard to be quantitatively given due to dynamic and uneven frosting conditions, and thus resulting in series of mal-defrosting phenomena. For an air source heat pump unit with a multi-circuit outdoor coil, when the melted frost locally drained during defrosting was considered, the optimization of start defrosting control strategy becomes more complicated. Here, this fundamental problem was experimentally investigated, with frost nearly evenly distributed on the surface of the outdoor coil at the start of defrosting. Defrosting performance of the experimental air source heat pump unit at different frost accumulations with the melted frost local drainage were then comparatively analyzed. These measured and calculated physical parameters include the temperature of tube surface and melted frost, compressor suction and discharge pressures and their difference, thermal energy taken from indoor air and electricity inputs on compressor and fans during defrosting, etc. Results showed that, the defrosting efficiency reached its peak at 51.80% when frost accumulation was at 933 g. Thus, the time-based start defrosting control strategy was demonstrated to be optimized with this method. Contributions of this study could be used for adjusting the control strategy of air source heat pump units, which are also benefit for energy saving in their industry and residential applications.

A novel defrosting method in gasoline vapor recovery application

Condensation method is comprehensively applied for gasoline vapor recovery (GVR), of which frosts in the heat exchanger is the greatest challenge, especially for the continuous long running cases. A novel dual channel GVR cascade refrigeration system with shell-tube heat exchanger was presented and tested in this paper. With one-work-one-standby evaporator settings, combined with refrigerant evacuation and delay switching strategies, the defrosting of low temperature shell-tube heat exchanger was analyzed and solved. Also multi-stage cycle was introduced to supply three cooling stage, which cooled the gasoline vapor from ordinary temperature to about −70 °C. By the means of industrial application validation and process calculation, the ability of the non-stop cooling during defrosting was verified. The refrigerant evacuation was proposed to prevent high pressure drop caused by frost accumulation, which also improved the cooling capacity by 28.2% and approached the defrost efficiency of 55.4%. In addition, it was found that delay switching can effectively reduce the capacity fluctuation. Based on sensitivity studies, 20 min delay was identified as the best switching timing for this device. The capacity of this system performed lower reduction, higher duty ratio and defrost efficiency.

Freezing delay, frost accumulation and droplets condensation properties of micro- or hierarchically-structured silicon surfaces

The processes of single droplet freezing, frost accumulation and droplets flooding during condensation were analyzed and compared for surfaces of different morphologies. Four samples were first fabricated – a micro-cubes silicon, a micro-cylinders silicon, a hierarchically-structured silicon and a smooth silicon. After measuring the static contact angles and roll angles, the sessile water droplet freezing and frost accumulation under a series of surface temperature were studied. The droplets condensation processes were for the first time compared through environmental scanning electron microscopy (ESEM). The results showed that the droplet freezing time on hierarchically-structured silicon was 4 times longer than that of uncoated silicon. Micro-pillared and hierarchically-structured silicons could minimize the frost thickness only in the initial stage of frost formation and had quite different frost growth patterns. The ESEM images can facilitate explain the different patterns of freezing and frosting on micro-pillared and hierarchically-structured samples.

Effect of enthalpy exchanger on reducing frost accumulation on evaporator in frost-free refrigerator

Performance of a frost-free refrigerator may be deteriorated by frost accumulation on its evaporator surface. The objective of this study is to investigate the feasibility of using an enthalpy exchanger (EE) to reduce the frost accumulation on evaporator surface in frost-free refrigerator. The influence of EE on air circulation and the change process of air humidity is analyzed, and it is indicated that the application of EE is capable to decrease the amount of water vapor entering the evaporator and consequently reduce the frost accumulation on evaporator surface. The design process of EE in frost-free refrigerator is illustrated, and an optimal type of EE is presented. An experiment on a frost-free refrigerator shows that the application of EE decreases the frost accumulation by 18.8%. In addition, effects of EE on other main performances of frost-free refrigerator besides the frost accumulation are evaluated.

The role of surface wettability on natural convection frosting: Frost growth data and a new correlation for hydrophilic and hydrophobic surfaces

In refrigeration systems, frost accumulation on the evaporator demands periodic defrosting. Chemical coatings (either hydrophilic or hydrophobic) are sometimes used to mitigate frost build-up and/or improve drainage during the defrost cycle. Although several studies have been performed on frost growth behavior, relatively few studies have specifically examined the effect of surface wettability on the frost growth rate. This paper focuses on the influence of environmental operating conditions on the growth of a frost layer on surfaces which span a wide range of contact angles under natural convection conditions. A new semi-empirical correlation which includes the surface contact angle θ and the modified Jakob number Λ as parameters is reported with an average predictive error of 11.7% (N = 930) for all non-zero experimental data. The correlation was developed using multivariable regression analysis and data which span different surface temperatures (−13 °C to −5°C), relative humidities (40–80%), and static contact angles ranging from 45° (hydrophilic) to 160° (hydrophobic).

Energy impacts of defrosting in household refrigerators: Lessons from field and laboratory measurements

Many modern household refrigerating appliances have automatic defrost systems that keep the evaporator clear of frost and ice and obviate any user interaction to remove frost accumulation that occurs during normal use. The energy impact of defrosting has received little attention to date and there is little understanding on how these systems operate in homes. This paper uses laboratory measurements for 110 appliances over a period of weeks and field measurements for 195 appliances over periods of up to one year to observe the energy characteristics of defrosting systems. Field data covering some 55,000 individual defrost events has been analysed to obtain unprecedented insights into defrosting intervals during normal use. This data also provides useful information about the pattern of defrosting and a comparison between laboratory measurements and field measurements. A method to estimate incremental defrost and recovery energy in normal use is proposed.

Modeling and Analysis of Frost Growth in Pilot-Scale Ambient Air Vaporizer

During the operation of an ambient air vaporizer (AAV) designed for vaporizing liquefied natural gas (LNG), the condensation of water vapor results in frost accumulation on the surface of the vaporizer tubes. The frost deposited on the surface lowers the heat transfer rate because the thermal conductivity of frost is less than 1/40 times the thermal conductivity of aluminum, which was used as the material for the tube and fin. It is essential to understand the behavior of frost accumulation because the actual challenge in this field is to predict frost growth in cryogenic conditions and the dynamic heat transfer performance of the AAV owing to frost. In this study, the formation of a frost layer on the surface of an LNG vaporizer and the reduction in heat performance are evaluated using a dynamic numerical model that combines heat transfer and mass transfer. The trend of frost growth and the temperature profile of LNG with position and time were studied by modeling and simulation. The numerical results pr...

Thermal performance of peripheral-finned tube evaporators under frosting

This study presents an experimental and theoretical evaluation of frost formation on the moist air side of a peripheral finned-tube (PFT) heat exchanger. Previous studies of this compact geometry correlated the friction and heat transfer parameters above the dew-point temperature of the air (no frost or condensate formation). Here, for the first time, the thermal-hydraulic performance of a PFT heat exchanger is analyzed under frosting. A PFT heat exchanger prototype was evaluated experimentally in a closed-loop wind tunnel calorimeter to determine the influence of the tube wall temperature, air velocity and psychrometric properties (temperature and relative humidity) on the heat transfer rate, air-side pressure drop and frost buildup on the surface. The thermal and hydrodynamic behavior of the enhanced air-side surface was analyzed using a distributed heat exchanger model in which mass, momentum and energy balances are applied to one-dimensional control volumes in the air flow direction. The model treats the air flow path as a porous medium in which the porosity, equivalent particle diameter and thermal properties vary as a function of time due to the frost accumulation. Good agreements (within 20% average error) between the model predictions and the experiments for the air-side pressure drop and heat transfer rate have been found. The enthalpy effectiveness was found to drop from 0.68 for dry to 0.50 under severe frosting, which suggests that the PFT heat exchanger continues to be effective under frost.

Performance evaluation of a novel frost-free air-source heat pump integrated with phase change materials (PCMs) and dehumidification

Abstract: Air-source heat pump (ASHP) has been widely used in domestic and commercial buildings because of its energy savings, high efficiency and environmental friendliness. However, the heating capacity of an ASHP system is greatly influenced by the frost accumulation on the surface of the outdoor exchanger. To solve this problem, a novel frost-free ASHP system, integrating with Phase Change Materials (PCMs) and dehumidification, has been developed. In this paper, the schematic design of this system is first presented. The effect of the match relationship between the desiccant materials and PCM thermal energy storage on the performance of the novel system is then studied using a dynamic mathematical model. The simulation results showed that the dehumidification efficiency was increased from 31.8 % to 34.7 % with the increase of the solid desiccant mass from 1.9 kg to 3.5 kg when the volume of the PCM was 1000 ml. The system COP was 2.87 when the desiccant was 2.2 kg and the volume of PCM was 1100 ml at a relative humidity of 80 % and ambient temperature of 0 °C. In addition, the water temperature was heated to 55 °C in one cycle which decreased the irreversible loss. Lastly, a correlation of the system COP with the amount of the solid desiccant and the PCM was obtained through a multivariate linear regression. The results obtained can facilitate optimal design and dynamic behavior investigation of this system.

Frosting Behavior of Superhydrophobic Nanoarrays under Ultralow Temperature.

Retarding and preventing frost formation at ultralow temperature has an increasing importance due to a wide range of applications of ultralow fluids in aerospace and industrial facilities. Recent efforts for developing antifrosting surfaces have been mostly devoted to utilizing lotus-leaf-inspired superhydrophobic surfaces. Whether the antifrosting performance of the superhydrophobic surface is still effective under ultralow temperature has not been elucidated clearly. Here, we investigated the frosting behavior of fabricated superhydrophobic ZnO nanoarrays under different temperature and different environment. The surface showed excellent performance in anticondensation and antifrosting when the surface temperature was approximately -20 °C. Although the frosting event inevitably occurs on all surfaces when the temperature is decreased from -50 to -150 °C, the frost accumulation on the superhydrophobic surfaces is always less than that on the untreated surfaces. Interestingly, the frost layer detaches from the surface within a short time and keeps the surface dry in the very beginning of the defrosting process. Further, there is no frost formation on the surface at -20 °C during 10 min testing when blowing compressed air and spraying methanol together or spraying methanol individually. It can reduce the height of the frost layer and increases the density when spraying methanol at -150 °C. Furthermore, the frost crystals on the top surface can been blown away due to the low adhesion of ice or frost. It provides a basic idea for solving the frosting problem under ultralow temperature while combined with other defrosting methods.

Experimental investigation on a novel cold storage defrosting device based on electric heater and reverse cycle

It is universally acknowledged that the accumulation of frost on the evaporator surface will reduce the cooling capacity and performance of the cold storage. Therefore defrosting must be periodically and effectively carried out to avoid the evaporator blockage. The electric heater defrosting (EHD) and reversed cycle defrosting (RCD) methods are usually used in cold storages. However, no matter what method is adopted, there will be extra thermal load distributed to the compartments which lead to an unfavorable storage temperature fluctuation and then cause quality loss of the stored food. In order to solve the above-mentioned problems, a novel cold storage defrosting device based on EHD and RCD was proposed in this paper. Experiment was performed on the device to investigate its effect on several aspects including energy saving, reduction of cold storage temperature fluctuation and improvement of compressor operational stability etc. The results showed that this novel device could remarkably increase energy utilization and reduce temperature fluctuation of the cold storage, which was beneficial for long-term storage of products in the cold storage. Moreover, it was able to increase the operational stability of the compressor.

Effects of frost growth on louvered folded fins of microchannel heat exchangers on the time-dependent air side convective heat transfer coefficient

This paper investigated the frost formation on louvered folded fins in microchannel type heat exchangers and the effects of frost accumulation on the time dependent air side convective heat transfer coefficients during transient heat and mass transfer processes. Seven fin geometries, which were commonly used in outdoor microchannel type heat exchangers of air-source heat pump systems, were investigated for a wide range of temperatures and humidity promoting frost growth. The air-side heat transfer coefficient was depended on the rate of frost growth and it sometimes increased during early frost formation and then it decreased sharply. Frost thickness on the fin leading edge, air face velocity, and corresponding heat transfer coefficients and pressure drops were measured and semi-empirical correlations were developed. The frost thickness and air face velocity were key variables for estimating a local time-dependent air-side Reynolds number and for describing the transient behaviour of the local air-side heat transfer coefficient of the fins. The newly developed correlations identified the main parameters that describe frost growth and heat transfer on louvered folded fins under frosting operating conditions and they agree with the experimental data of the present work and from the literature.

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.

Tagging of Test Tubes with Electronic p-Chips for Use in Biorepositories.

A system has been developed to electronically tag and track test tubes used in biorepositories. The system is based on a light-activated microtransponder, also known as a "p-Chip." One of the pressing problems with storing and retrieving biological samples at low temperatures is the difficulty of reliably reading the identification (ID) number that links each storage tube with the database containing sample details. Commonly used barcodes are not always reliable at low temperatures because of poor adhesion of the label to the test tube and problems with reading under conditions of frost and ice accumulation. Traditional radio frequency identification (RFID) tags are not cost effective and are too large for this application. The system described herein consists of the p-Chip, p-Chip-tagged test tubes, two ID readers (for single tubes or for racks of tubes), and software. We also describe a robot that is configured for retrofitting legacy test tubes in biorepositories with p-Chips while maintaining the temperature of the sample below -50°C at all times. The main benefits of the p-Chip over other RFID devices are its small size (600 × 600 × 100 μm) that allows even very small tubes or vials to be tagged, low cost due to the chip's unitary construction, durability, and the ability to read the ID through frost and ice.

Suppression of Frost Nucleation Achieved Using the Nanoengineered Integral Humidity Sink Effect.

Inhibition of frost formation is important for increasing efficiency of refrigeration systems and heat exchangers, as well as for preventing the rapid icing over of water-repellant coatings that are designed to prevent accumulation of rime and glaze. From a thermodynamic point of view, this task can be achieved by either increasing hydrophobicity of the surface or decreasing the concentration of water vapor above it. The first approach has been studied in depth, but so far has not yielded a robust solution to the problem of frost formation. In this work, we systematically explore how frost growth can be inhibited by controlling water vapor concentration using bilayer coatings with a porous exterior covering a hygroscopic liquid-infused layer. We lay the theoretical foundation and provide experimental validation of the mass transport mechanism that governs the integral humidity sink effect based on this coating platform as well as reveal intriguing sizing effects about this system. We show that the concentration profile above periodically spaced pores is governed by the sink and source concentrations and two geometrical parameters: the nondimensional pore size and the ratio of the pore spacing to the boundary layer thickness. We demonstrate that when the ratio of the pore spacing to the boundary layer thickness vanishes, as for the nanoporous bilayer coatings, the entire surface concentration becomes uniform and equal to the concentration set by the hygroscopic liquid. In other words, the surface concentration becomes completely independent of the nanopore size. We identified the threshold geometrical parameters for this condition and show that it can lead to a 65 K decrease in the nucleation onset surface temperature below the dew point. With this fundamental insight, we use bilayer coatings to nanoengineer the integral humidity sink effect to provide extreme antifrosting performance with up to a 2 h delay in nucleation onset at 263 K. The nanoporous bilayer coatings can be designed to combine optimal antifrosting functionality with a superhydrophobic water repelling exterior to provide coatings that can robustly prevent frost, rime, and glaze accumulation. By minimizing the required amount of antifreeze, this anti-icing method can have minimal operational cost and environmental impact.

Effects of black silicon surface structures on wetting behaviors, single water droplet icing and frosting under natural convection conditions

Abstract In this research, the effects of black silicon and two-stage black silicon surface structures on the characteristics of wetting, icing process of a single water droplet, and frost accumulation under natural convection conditions were investigated. Black silicon and two-stage black silicon surfaces were first designed and fabricated, and then examined by means of scanning electron microscopy (SEM). Wetting behavior was measured by contact angle measurement system. The icing and frosting processes were acquired through high-speed photography. The test surfaces were cooled down from room temperature to well below 0 °C by circulating ethyl alcohol via a constant temperature system. The results show that compared with uncoated silicon, black silicon and two-stage black silicon surfaces can largely increase the surface hydrophobicity, reduce the freezing thickness deformation and show anti-frosting performance. This study provides a new type of hydrophobic surface structure and plays an important role in designing anti-icing/anti-frosting surfaces for practical applications.

Modeling and performance analyses of fin-and-tube heat exchanger operating under frost conditions

As far as the frost accumulation in fin-and-tube heat exchangers is concerned (regarding this process as a quasi-dynamic), a time-dependent distributed parameter model of fin-and-tube heat exchangers in the presence of frosting is presented. The model consists of two different sub-models: frosting and heat exchanger. The model results are compared with the experimental data, and an average error between them of ∼20% was found. In addition, the performances of the fin-and-tube heat exchangers under frost conditions, obtained from the experimental results, have been analyzed. These experimental results reveal that the inlet air velocity, temperature and relative humidity strongly affect frost thickness, frost mass accumulation, evaporating pressure and pipe wall temperature.