Introduction
Frost formation often occurs in conventional refrigeration systems, cryogenic equipment, and the aerospace industry. Significant frost accumulation results in additional thermal resistance and pressure drops. As reported by Emery and Siegel [
1], 50%-75% decreases in heat transfer, and a substantial increase in pressure drop are caused by frost formation on compact heat exchangers. Frost deposited on aircraft wings may cause aerodynamic penalties of lift and drag during take-off [
2]. Therefore, frost growth control and defrosting are necessary for the normal operation of these systems. Conventional defrosting methods, such as electric and hot gas heating, not only increase energy consumption but also decrease effective production. In practice, no clear and reliable methods exist to prevent frost formation on the cold surfaces of refrigeration systems. Thus, refrigeration system designers usually take into consideration the negative effects of frost deposition to insure that their designs operate normally under frost deposition conditions.
Numerous researchers in the past have studied the effects of ambient conditions and wall temperature on the frost structure and process. Hayashi et al. [
3] divided the frost formation process into several periods, marked by crystal growth, frost layer growth, and frost layer full growth. A large number of physical and mathematical models have also been proposed to predict frost layer growth [
4-
7]. Experimental correlations and mathematical models have been successfully applied in predicting frost growth. However, only a few works have investigated the effects of surface energy on frost formation. It is believed that changing the surface energy is a very attractive method of modifying frost properties and changing the frost deposition process. Okoroafor and Newborough [
8] applied a crosslinked hydrophilic polymeric coating to a cold surface to minimize frost growth. Results showed that frost growth can be reduced significantly over two hours of testing using a polymeric coating as thick as 700 μm. The mechanism for these coatings to minimize frost growth is not very clear, although it is believed that it may be related to the fact that these materials possess the ability to absorb large quantities of water from humid air and suppress the freezing of contained water down to temperatures of about -20°C [
9]. Liu et al. [
10] investigated the influences of surface hydrophilicity on frost formation on a vertical cold plate under natural convection conditions. Results demonstrated that surface hydrophobicity has strong influence on frost nucleation. They also showed that water drops that condense on a hydrophobic surface during the initial period of frost deposition are smaller and more spherical in shape compared to those that condense on hydrophilic surfaces. However, the experimental measurements also showed that surface hydrophobicity has no recognizable influence on frost layer thickness growth for long test runs.
Recently, a kind of hydrophilic polymer paint was formulated to counteract frost formation on cold surfaces. As reported by Liu et al. [
9,
11], this material can retard frost formation for up to three hours under high cold plate temperatures and moderate humidity conditions. However, this formula did not prove to be very effective in slowing down frost formation. In fact, it showed poor antifrosting performance when exposed to high humidity air streams and the coating used was thicker than 300 μm. Addressing these drawbacks, a new hydrophilic paint, developed by adding new additives, changing the paint constituent ratio, and improving the painting technology, is introduced in this paper. This new paint is better than the previous formulation because it requires thinner films and features stronger surface hardness and better antifrosting performance under low plate temperatures and high air humidity. The antifrosting performance of this paint is first tested on a vertical copper plate under different experimental conditions in the laboratory. It is then applied to a fin-and-tube heat exchanger at a coating thickness of 30 μm under more practical conditions. A series of frosting experiments are carried using both coated and uncoated heat exchangers for comparison in terms of the pressure drop, frost deposition, and outlet temperature. The antifrosting performance of this paint is also tested in a domestic refrigerator.
Laboratory experiment
Experimental apparatus
A thermoelectric cooler that can provide temperatures as low as -26°C was used as a cooling source for the frosting surface. A copper plate measuring 150 mm × 52 mm×6 mm and polished by a 2000 mesh polishing cloth was mounted on the cooling unit. During testing, the plate and data acquisition system were placed in a large enclosure to maintain natural convection conditions. The temperature and humidity inside the enclosure were regulated at the given values by an air-conditioning system and a humidity controller. The surface temperature of the plate was measured by four T-type thermocouples buried 0.5 mm beneath the test surface through four holes 1 mm in diameter and 13 mm in depth drilled into the plate. The temperature data were recorded by an HP data acquisition system before finally transferring to a personal computer for further analysis. The cold surface temperature is the average of the temperature readings of four thermocouples. The thermocouples were all precalibrated with a resolution of 0.1°C. The maximum uncertainty in the plate surface temperature measurements was estimated to be less than 0.5°C, including the error resulting from position errors and the fact that these thermocouples were not placed exactly on the surface. A microscopic imaging system, consisting of a CCD camera, a microscope, and a capture card, was used for micro- and transient observations of the frost deposition process and for measuring the frost layer thickness. The CCD camera and microscope, which had a maximum magnification of 110, were mounted over the cooled surface for taking photographs and observing the frost growth with the help of optical fiber luminescence.
The frost deposition process was recorded by the microscopic imaging system at a speed of 30 f/s, and the frost layer thickness was measured by a micro measurement system integrated into the microscopic image system every 5 min with an accuracy of±0.05 mm. A thermo-hygrometer was used to monitor the environmental conditions, including temperature and relative humidity. Humidity was measured by a humidity sensor with a 2% uncertainty. To facilitate testing under the exact same conditions, only the left half of the cold plate was coated with the new material. Figure 1 illustrates the experimental system.
Experimental procedure
To compare the experimental results of the coated and uncoated surfaces under the exact same conditions, only the left half of the plate was coated with the antifrosting paint. Before coating, the right half of the plate was covered with a plastic film. The mother material was ground into fine powder and then mixed with a kind of varnish at a specific ratio. The solution was then sprayed onto the plate several times according to the prescribed thickness of the coating. When the coating was finished and had dried at room temperature (about 30-60 min), the hydrophilic agent was sprayed onto the coating. After the agent was completely absorbed and the impregnation of the coating was finished, the coating thickness was measured by the microscope. The experimental data for the coated surface presented in the following sections are the results of a paint coating thickness of 80 μm. Figure 2 presents a picture of the cold plate (without frost deposition) coated with the antifrosting paint on its left half.
The test surface was cleaned according to a given procedure before each experiment. All frost deposition tests were conducted using a 150-mm high plate vertically oriented under natural convection conditions. The microscope was placed atop the plate and positioned in such a way that the frost layer on both the coated and uncoated surfaces could be observed and measured simultaneously. To measure the thickness of the frost layer, a magnification of 25 was used to create a visual field large enough to cover the entire testing period. Before starting the experiment, the microscope was adjusted to the right focus, the cooling water was turned on, and the temperature was set to the prescribed value. During the cooling process of the cold plate, the test surface was covered with a thin plastic film to isolate the surface from the air. When the plate temperature reached its prescribed value and the whole system became stable, the plastic film was removed and the timing and data acquisition began.
Experimental results and discussion
Comparison of frost layer thickness between the uncoated and coated surfaces
To prove the effectiveness of antifrosting paint in reducing frost deposition on coated surfaces, a series of comparative experiments were conducted for both coated and uncoated copper surfaces under various conditions. Figures 3 and 4 show typical experimental results obtained under different humidity conditions. Figure 3 compares the frost growth on the coated and uncoated surfaces under low air humidity (36%) and different cold plate temperatures (-15.1°C and -20.9°C) conditions. Figure 3(a) demonstrates that after three hours of testing, no frost deposition occurs on the coated surface, while the measured frost layer on the uncoated copper surface was as thick as 1.8 mm. Figure 3(b) reveals that the coated surface prolongs the onset of frost formation for about 120 min when the cold plate temperature is about -20.9°C. Three hours later, the frost layer on the uncoated copper plate is as thick as 2.6 mm, while the frost layer on the coated surface is only 1.3 mm, corresponding to a reduction of 50% in frost layer thickness. However, it should be pointed out that the average frost layer on the vertical plate was thinner than that on the top edge, that is, the measured frost layer is thicker than the average thickness of the plate. This suggests that heat and mass transfer rates on the plate between the frost surface and air decrease from top to bottom according to the heat and mass transfer theory [
12], and therefore, the frost deposition rate at the top edge of the plate is expected to be the largest. Furthermore, the experimental observation shows that the edge effect also causes more frost to deposit on the vicinity of the edge. These two factors make the frost layer at the top edge much thicker than the average.
Figure 4 compares the frost growth on the coated and uncoated surfaces under high air humidity (74%) and different cold plate temperatures (-8.5°C and -20.5°C) conditions. Figure 4(a) illustrates that after three hours of testing, no frost deposition occurs on the coated surface at the plate temperature of -8.5°C. The coated surface delays the onset of frost formation for about 75 min under the lower cold plate temperature of -20.5°C. The frost growth on the coated surface of the paint is effectively retarded even under high air humidity.
Antifrosting performance of the coated surface
The strong antifrosting ability of the paint is described in Fig. 5, which presents a comparison of frost deposition in the central area of the coated (left) and uncoated (right) surfaces at different experimental times under high air humidity (72%) and low plate temperature (-15.3°C) conditions. The figure shows that, although the measured frost layer thickness of the coated surface was 1.7 mm during the three-hour period, no appreciable frost deposition could be observed in the central area of the surface. The difference between the thickness of the frost layer at the top edge and that at the central area of the uncoated surface is much smaller than that of the coated surface. Therefore, the antifrosting performance of the paint is actually much better than described by the measured frost layer thickness.
Application experiment
Numerous researchers have focused their study on the investigation of frost formation on finned heat exchangers. Kondepudi and O’Neal [
13] studied the effects of different fin configurations on the performance of finned tube heat exchangers under frosting conditions. They found that coil thermal performance is degraded by at least 15% due to frost formation. Min et al. [
14] reported that hydrophilic coatings on evaporators could reduce wet pressure drops. Yoshiyuki and Akiko [
15] studied the performance of heat exchangers with antifrosting-surface-treated fins in order to extend the defrost cycle time of air coolers. As a result, the compressor operating time between defrost processes increased to twice its original value compared to a conventional air cooler with the same cooling performance. Ryu and Lee [
16] studied frost formation in fin-and-tube heat exchangers with different surface energies. They found that the frost layer thickness of a hydrophilic heat exchanger became thinner than that of a conventional aluminum heat exchanger when smaller airside pressure drops were applied.
The principal purpose of this research is to investigate the possibility of using hydrophilic paints to delay the onset of frost formation and reduce pressure drops across heat exchangers. A series of frosting experiments are conducted for two heat exchangers: one is manufactured with aluminum fins coated with the hydrophilic paint, while the other has uncoated aluminum fins. Comparisons of frost deposition, pressure drop, and outlet temperature are made between the two heat exchangers.
Apparatus and instrumentation
The experimental setup and method are similar to those described in Ref. [
17]. The layout of the experimental system is presented in Fig. 6. The system consists of a cooling unit assembly, an air conditioning wind tunnel, a heat exchanger model, a coolant supply loop, a differential manometer, and a data acquisition system. The test section of the wind tunnel and a heat exchanger model was made up of a transparent acrylic material so that the frost formation can be observed clearly. It has an inlet and outlet for airflow passage across the heat exchanger. The air temperature, humidity, and velocity are regulated at the given values by the cooling unit assembly and the air conditioning system, while the relative humidity and the outlet and inlet air temperatures across the heat exchanger are measured by two thermo-hygrometers. The maximum relative uncertainty of the relative humidity was 3%; the uncertainty of the temperature measurement was 0.5°C. The coolant supply loop contained two stainless steel brine baths with different temperatures, providing a frosting-defrosting cycle for the test; one is connected to a heater and the other to a low-temperature refrigerating unit. A pump and a mass flow meter with an estimated uncertainty of 0.01 m/s were used to obtain the constant flow rate. The heat exchanger was connected to the coolant loop with one-touch fittings and the coolant temperature at the inlet and outlet of the heat exchanger was kept constant during the whole test. Two Pitot tubes connected to the differential manometer were mounted in the inlet and outlet of the test section to measure the pressure drop. The resolution and relative uncertainty of the differential manometer used in this study were 0.1 Pa and 2% at full range. In addition, a digital camera was used to take frosting pictures of the heat exchanger every 20 min for further comparison and analysis.
Test sample preparation
The test samples are prepared basically in two physical forms: coated heat exchanger and uncoated heat exchanger. It is well established that good surface preparations are the key to obtaining good protection by surface coatings. Thus, the acid pickling method and chemical conversion treatment were used to remove the surface contaminants and improve the surface roughness before painting. The surface preparation procedure was the same as that described in Section 2.2. Figure 7(a) presents the picture of the fins coated with antifrosting paint at a coating thickness of 30 μm.
Experimental procedure
To prepare a good hydrophilic surface, the heat exchangers investigated in this chapter were made of 60 aluminum fins with dimensions of 110 mm × 90 mm × 0.1 mm, and copper tube was 8.5 mm in diameter. Figure 7(b) shows the heat exchanger assembled with the aluminum fins coated with the hydrophilic paint. Before testing, the static pressure at the inlet and outlet of the heat exchanger was checked to ensure that it was equal to the microdifferential pressure gauge. After the testing system was well assembled, the cooling water system, the air conditioning machine, the tunnel plant model system, and the two brine baths were switched on in succession, and the temperature of the brine (the cool brine was -9.0°C and the hot brine was 3.7°C) were set. When all the test conditions achieved their setting values and remained constant, the data acquisition system was started, and the stopwatch was clicked to measure the frost formation time, which started the first cycle of the experiment. All the tests were conducted under the conditions summarized in Table 1.
During testing, all measurements were logged and processed via a computer interfaced to a data acquisition system. When the ventilation resistance reached 294.0 Pa, the first cycle was completed. The cool brine was replaced by the hot brine automatically and the defrosting mode began. After 5 min, the defrosting mode was completed, and the second cycle started. The air relative humidity and the temperature were basically kept constant during the frost cycle, although they were affected by turning on the defrost mode. However, the effects of these slight variations on the experiment were negligible because the defrost time was very short and the air parameters can recover to their prescribed constant values during the frost cycle that followed.
To test the effectiveness of the antifrosting paint in releasing frost deposition on the heat exchanger, two groups of frosting experiments were conducted for the heat exchangers with different defrosting modes. In the first group, the defrosting operation lasted 5 min with no air flow; the hot brine temperature was set to 3.7°C during the defrosting process. In the second group, the defrosting operation lasted 5 min with an air flow rate of 23.8 m3/min; the hot brine temperature was set to 13.7°C. Extensive experimental measurements and observations were made for both the uncoated and coated heat exchangers. Some of the typical results are summarized as follows.
Experimental results and discussion
Comparison of the pressure drop between uncoated and coated heat exchangers
A comparison of pressure drop between the uncoated and coated heat exchangers under the same conditions is shown in Fig. 8. In this case, the inlet and outlet air temperature are held constant at 2.2°C and -0.5°C, respectively, and the relative air humidity is about 90%. The test is performed for three frosting-defrosting cycles in this group. The pressure drop between the inlet and outlet air of the coated heat exchanger is 24.5 Pa before testing, which is much higher than that of the uncoated heat exchanger (14.2 Pa). The coated heat exchanger has a larger pressure drop because the coating reduces the gaps of the heat exchanger fins from 1.70 to 1.64 mm. The pressure drop of the uncoated and coated heat exchangers begins to increase at the early stage of the test because the appreciable frost deposition appears on the uncoated heat exchanger surface and the coating on the coated heat exchanger expands after absorbing a large amount of water. However, the increased speed of the uncoated heat exchanger is much greater than that of the coated heat exchanger 20 min after the tests started. The first cycle of the coated heat exchanger lasts for 137 min, while that of the uncoated lasts only for 80 min when the pressure drop across the heat exchanger reaches the largest permitted value of 294.0 Pa. The second and third cycles start after 5 min of defrosting, but the uncoated heat exchanger lasts only about 46 and 42 min, respectively, when the pressure drop across the heat exchanger reaches its largest permitted value, so the frosting time of the second and third cycles is shorter than that of the first cycle. This is attributed to the water droplets that remained on the fins during the defrosting cycle resulting in the rapid freezing of the retained condensate once the second cycle starts. The pressure drop on the coated heat exchanger also keeps increasing during the second cycle. However, pressure drop of the coated heat exchanger is larger than that of the uncoated at the beginning of the second cycle, and the slope of the pressure drop during the second cycle becomes steeper than the former cycle. This phenomenon occurs because the coating on the fins is not completely dried out by the arranged defrosting operation after absorbing large amounts of water, and the hydrophilic ability of the coating is reduced during the second cycle.
As described above, the increasing rate of the pressure drop during the second cycle becomes faster than that of the first cycle of the coated heat exchanger. This may be explained by the fact that the coating on the fins is not dried out completely by the arranged defrosting operation, which is accomplished by 3.7°C hot brine and without air flow. Thus, the defrost mode is changed by increasing the hot brine temperature from 3.7°C to 13.7°C and introducing an extra air flow from the inlet at a flow rate of 23.8 m3/min. The defrosting operation also lasts 5 min. The coated heat exchanger is the same as that of the first group. In order to dry the coated heat exchanger after it absorbs large amount of water during the first group experiment, it was put into the drying box for 6 hours. The relative humidity and temperature in the drying box are 50% and 40°C, respectively. The other experimental conditions are similar to those of the first group during the frost cycles.
Figure 9 present variations of the pressure drop between the uncoated and coated heat exchangers under the same experimental conditions. This group of tests also includes two frosting-defrosting cycles. At the early stage of the test, the air pressure drop between the inlet and outlet of the coated heat exchanger is slightly higher than that of the uncoated heat exchanger. Similar to the first group, the pressure drop of both the uncoated and coated heat exchangers begins to increase as the test continues. As such, the first frost cycles of the two types of heat exchangers lasts 176 min and 61 min, respectively. The second cycle starts after 5 min of defrosting operation with air flow and hot brine. The uncoated heat exchanger lasts 53 min, while the coated lasts 40 min when the pressure drop across the heat exchangers reaches its largest permitted value. Thus, the antifrosting performance has no appreciable improvement between two different defrosting modes for both the uncoated and coated heat exchangers. It may therefore be concluded from these experimental results that it is highly desirable to change the normal defrosting method and dry the coating surface before the next frosting cycle.
Antifrosting performance of the coated heat exchanger
The durability and cycling anti-frosting performance of the paint are shown in Fig. 10, where the frost deposition on the uncoated and coated heat exchangers at different experimental times under the same test conditions is compared. There is no appreciable frost deposition on the coated fin surface after 50 min of the first cycle, although a relative dense frost layer forms on the uncoated fins surface. Obvious frozen water drops also appear on the uncoated fins after 40 min of the second cycle because some water droplets remain on the fins after the 5 min reverse operation of the system, which quickly freezes once the second cycle begins.
Effect of coating on heat transfer
To determine the influences of hydrophilic paint on heat transfer, comparisons of the outlet air temperature are also made between the uncoated and the coated heat exchangers. Figure 11 compares the outlet air temperature between the two heat exchangers under the exact same conditions. The average outlet air temperature is -0.81°C for the uncoated heat exchanger and -0.80°C for the coated one. The lack of significant difference between the outlet air temperatures of the heat exchangers during the frost cycles means that the coating produces no appreciable additional thermal resistance to the heat transfer process.
Influences of coating thickness on antifrosting performance
The coating thickness influences the antifrosting performance of the paint. The ability of the hydrophilic paint to absorb water during the test suggests that the antifrosting performance increases with increasing coating thickness. However, this may not be always true because the coating thickness has many influences. Figure 12 shows the pressure drop variation during the testing of two different coated heat exchangers, one with a coating thickness of 30 μm and the other with 50 μm. At the beginning of the test, the pressure drop between the inlet and outlet air of the 30 μm heat exchanger is 24.5 Pa, while that of the 50 μm heat exchanger is 73.7 Pa, which is three times higher than that of the 30 μm heat exchanger. As the test continues, the pressure drop of both samples begins to increase as the coating expands. However, the increases in the speed of the two samples differ. The 30 μm heat exchanger lasts for 137 min, while the 50 μm heat exchanger lasts for only 124 min in the first cycle. When the second cycle starts the normal defrosting operation for 5 min, the pressure drop of the 30 μm heat exchanger becomes 50.8 Pa, while that of the 50 μm heat exchanger becomes 213.4 Pa. It can be concluded that the thicker coating also means a larger occupation of the effective space for airflow and a faster increase of the pressure drop. Furthermore, increasing the coating thickness also induces a larger additional thermal resistance and thus may degrade the total performance of the system. Therefore, there should be a prescribed optimum coating thickness for meeting specific performance requirements.
Antifrosting performance in the domestic refrigerator
To test the antifrosting performance of the paint on a more practical heat transfer surface, the paint was also applied to a domestic refrigerator surface at a coating thickness of 200 μm. The freezing chamber of the refrigerator had an air temperature of -25.0°C and a wall temperature of -34.0°C, while the relative air humidity was 70%. Figure 13(a) depicts the frosting-defrosting processes between the coated (left) and uncoated (right) surfaces. Figure 13(b) shows that there are only few frost crystals on the coated surface, while the frost layer on the uncoated surface is as thick as 4.1 mm after six months of running. Figure 13(c) describes the different defrosting processes of both the coated and uncoated surfaces. A thick frost layer remains on the uncoated surface when the frost crystals on the coated surface have melted completely. Figure 13(d) reveals that even four months after defrosting, the coating still has effective repeated cycling antifrosting performance.
Theoretical analysis
The above experimental results demonstrate that the new hydrophilic paint can both minimize frost formation on cold plate in the laboratory and retard frost growth on practical refrigeration systems, such as heat exchangers and domestic refrigerators. However, the mechanism of its antifrosting ability remains unclear. The results observed may be related to the fact that the hydrophilic polymer paint contains a kind of super absorbent polymer (SAP) and polar substances. SAP can absorb a large amount of condensed water and polar substances can keep absorbed water in the liquid state even when the temperature is below its freezing point. Highgate [
18] studied the freezing behavior of water in a similar coating and found that the water contained in the polymer coating does not freeze until its temperature reaches as low as -20.8°C. The water inside the polymer goes through three typical states: free water, bonded water in weak interaction with the polymer, and unfrozen water in strong interaction with the polymer. The unfrozen water will not be frozen even when the temperature is below the crystallization temperature of water. During the frost formation process, the coating absorbs a large amount of condensed water first, after which frost crystals are deposited gradually onto the surface of the swollen coating. Therefore, both frozen and unfrozen water molecules are present at the interface of the coating and the frost layer. The adhesive force of the frost crystals to the coating surface is thus greatly reduced, which may explain the fragile structure of the frost layer on the coated surface.
On the other hand, SAP is a kind of typical functional macromolecule material that has a strong ability for absorbing and storing water. It is also a macromolecule electrolyte that contains hydrophilic units and features a crosslinked structure. The macromolecule chains are close and are intertwined together to form a firm network structure before absorbing water. Water molecules can penetrate into the resin by capillary force and diffusion once they establish contact with the resin. The ability of absorbing water is limited by the crosslinked network structure at the same time. The hydrogel formed in the resin has intense viscosity, making it difficult to separate the water moisture from the resin even under pressure. Figure14 describes the strong water-absorbing ability of SAP. Figure 14(a) shows 0.1 g dry SAP in a vessel before absorbing water. It expands quickly and becomes a hydrogel upon absorbing 18 g distilled water; the water absorbed is 180 times heavier than the SAP.
Water molecules are associated by hydrogen bonds. However, a molecular force is not as firm as a chemical bond. The water molecule itself is neutral, but it has polarity due to the dissymmetry of the hydrogen bond. Figure 15 shows its charge distribution. Two hydrogen atoms point to the vertices of the tetrahedron and appear as positive electricity, while the two electron pairs of the oxygen atom point to the other vertices of the tetrahedron and appear as negative electricity. The four vertices with differently charged electricity can be combined with other dissimilar ions and molecules. Water molecules change into hexagonal crystal ice (Ih) by hydrogen bond combinations when the water temperature is below 0°C under constant pressure. The ice (Ih) structure is shown in Fig. 16. The frost crystal growth can be restrained by destroying the framework structure of the ice, specifically, the hydrogen bond that forms the ice structure. The polar material in the hydrophilic paint plays that a destructive function. During the frost formation process, SAP in the coating first absorbs a large amount of condensed water, after which the polar material dissolves in the water and ionizes to form negative and positive ions. These ionization ions interact with water molecules and break hydrogen bonds, thus suppressing ice formation. As such, the water inside the polymer remains in the liquid state and does not freeze even when the temperature is below the crystallization temperature of water. This is why this kind of polymer can retard the formation of incipient frost crystals.
Conclusions
The performance of a new anti-frosting paint was investigated experimentally in both laboratory and practical refrigeration systems. Based on the experimental results and discussion, the following conclusions can be made:
1) The coating thickness of the novel paint can be as thin as 30 μm and still retard frost crystal nucleation and decrease the frost deposition rate effectively. After three hours of testing, no frost deposition occurred on the coated surfaces when the plate temperature was above -8.5°C, and the air humidity was about 75%. It can shift frost appearance for two hours under low plate temperatures (-20.9°C) and air humidity (36%), and it can reduce the frost layer thickness by at least 50% in laboratory experiments. Under conditions of low cold plate temperature (-15.3°C) and high air humidity (72%), the coated surface was free of frost for at least three hours, while the uncoated surface exposed to the same conditions was completely covered by a dense and thick frost layer.
2) When used in heat exchangers, the paint prolonged the defrosting interval from 80 to 137 min under conditions of high air temperature (Tin = 2.2°C, Tout = -0.5°C) and relatively high air humidity (ϕ = 90%). Experimental observations demonstrated that the hydrophilic paint has effective antifrosting performance for both the heat exchanger fins and the domestic refrigerator wall even under harsh environmental conditions. Moreover, the coating brought no significant additional thermal resistance to the heat transfer process.
3) The hydrophilic paint was effective in controlling frost deposition and has vast development prospects in the field of refrigeration and cryogenic systems. However, the coated heat exchanger had poor antifrosting performance with defrosting cycles. This phenomenon occurred because the coating on the fins was not dried out completely by the arranged defrosting operation. Therefore, a new defrosting method should be designed if the hydrophilic ability of the coating is to be recovered during the cycling test. In addition, the expansion of coating led to weak adhesive forces and high pressure drops after absorbing water. Further research is required to strengthen the durability and cycling performance of the paint when used under practical conditions.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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