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Experimental study on instantaneously shedding frozen water droplets from cold vertical surface by ultrasonic vibration
Experimental Thermal and Fluid Science 53 (2014) 17–25
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Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Experimental study on instantaneously shedding frozen water droplets from cold vertical surface by ultrasonic vibration Dong Li, Zhenqian Chen ⇑ Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, IIUSE, School of Energy and Environment, Southeast University, Nanjing 210096, PR China
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Article history: Received 27 March 2013 Received in revised form 2 October 2013 Accepted 14 October 2013 Available online 26 November 2013 Keywords: Ultrasonic vibration Frozen water droplets Temperature jump Interface transverse shear force Acoustic pressure
a b s t r a c t The ultrasonic vibration is introduced to remove adherent frozen water droplets from cold surface, which provides the possibilities for effective defrosting. The shedding processes of various frozen water droplets adhered to 70 mm 70 mm cold vertical surface by 20 kHz and 60 W ultrasonic vibration were experimentally studied. It was found that the frozen water droplets instantaneously crack and shed off from the vertical surface due to the combined effects of interface transverse shear force generated by ultrasonic mechanical vibration and impact force induced by ultrasonic acoustic pressure. However, the heating effect triggered by ultrasonic vibration has limited effect on the frozen water droplets removal. Moreover, the frozen water droplets in different diameters within 2–30 mm can be successfully removed and all the frozen water droplets in different positions of 70 mm 70 mm cold surface can be completely shed off by 20 kHz high frequency ultrasonic vibration. The results showed that the ultrasonic vibration has a very strong ability to remove the frozen water droplets, which are the parasitic substrates for frosting, from cold flat surface and thus it is a highly potential defrosting method for practical application. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Frost deposition is a well-known and undesirable phenomenon in numerous fields such as air conditioning, cryogenics and aeronautics which work under low temperature conditions. The frost layer accumulated on cold surface of heat exchanger inevitably decreases the heat transfer rate of refrigerating unit due to the increase in both thermal and flow resistance. Besides, frost accreted on aircraft structures compromises aircraft performance and might cause safety problems. Therefore, great attention has been paid to searching for the effective method for preventing frost formation or removing accreted frost layer on these equipments. Up to now, various methods concerning frost suppression and defrosting have been proposed. Thereinto, many studies have been performed to investigate the influence of surface energy on frost nucleation and growth [1–9]. In such studies, surface contact angle was changed by means of surface treatment to get the hydrophobic or hydrophilic surface. It was found that the hydrophobic/hydrophilic surface can significantly retard the initial frost nucleation. However, if the cold surface was completely covered by a thin frost layer, the surface energy no longer influences the subsequent frost growth process. Besides, numerous studies concerning the effect of various external fields on frost formation were reported [10–14]. Wang et al. [10] and Joppolo et al. [11] studied the frost formation ⇑ Corresponding author. Tel./fax: +86 25 83790626. E-mail address: [email protected] (Z. Chen). 0894-1777/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2013.10.005
in the presence of external electric field. They found that the frost structure subjected to external electric field is relatively skinny and fragile which could easily break up and fall off compared with that without electric field. In addition, Gou et al. [12] experimentally studied the frost formation under magnetic field. It was found that the water droplets are smaller and more homogeneous and the frost structure is easy to be removed under magnetic field. Furthermore, Cheng and Shiu [13] investigated the effect of external oscillation on the frost formation and liquid droplet solidification. It showed that the transverse oscillation of the cold plate results in a significant change in the solidification pattern of the liquid droplets. However, the frost crystals are not able to be removed from the cold plate by oscillation within the range of low frequency from 100 Hz to 200 Hz. The similar investigation was conducted by Wu and Webb [14]. Their experiment also showed that the low frequency mechanical vibration cannot successfully release the frost formed on the hydrophobic surface. In recent years, growing attentions have been concentrated on the ultrasonic technology for frost suppression and defrosting due to the ultrasonic high frequency and energy concentration. Kazunari et al. [15] observed the frost accumulation on a rectangular aluminum alloy plate surface subjected to approximately 37 kHz ultrasonic vibration. It was found that the high frequency vibration could suppress the frost accumulation by approximately 60%. Li et al. [16] visually studied the effect of 20 kHz ultrasound on frost formation on cold flat surface in atmospheric air flow. It indicated that the coverage of initial freezing droplets is all less
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Fig. 1. Schematic representation of the entire frosting process on cold surface.
Fig. 2. Real process of frost crystal growth on the frozen water droplet surface.
Data acquisition instrument
PC Threaded connection Temperature and humidity sensor Ultrasonic transducer Frozen water droplet
Photron CCD Ultrasonic generator Fig. 3. Schematic diagram of experimental apparatus for frozen water droplets shedding from cold vertical surface.
than 52% with the effect of ultrasound compared with that all more than 65% without ultrasound and the rate of frost layer thickness reduction with the effect of ultrasound compared with that without ultrasound is about 75%. However, in all the tests, the ultrasonic transducer is not in contact with the flat surface which will cause significant energy decrement. Wang et al. [17] experimentally studied the possibility of frost release from a finned-tube evaporator by ultrasonic vibrations in natural convection. Their results showed that the frost crystals and frost branches on the ice layer can be fractured and removed effectively. However, the ultrasonic vibrations cannot remove the basic ice layer on the fins. In addition, Yan et al. [18] developed an ultrasonic method for defrosting and their results showed that using more ultrasonic sound sources is better than using single sound source for defrosting. Palacios and Smith [19,20] introduced an ultrasonic de-icing system for helicopter rotor blades. Their studies showed that the ultrasonic shear actuators could melt a 1.5 mm thick ice layer in a time period under 5 min at the first ultrasonic resonance frequency (130 kHz) of the system. In summary, although a great number of investigations have been concentrated on the frost suppression and defrosting, sufficiently effective methods do not exist. Especially few effective defrosting methods have been proposed which are more attractive for the practical application. Despite quite a number of studies
70
Unit: mm
Position A
Φ0.8× 6
Frozen water droplet
Position B Φ10
70
Position C 15
Fig. 4. Locations of screw bolt and thermocouples.
indicated that ultrasonic vibration is an effective means for frost suppression, little research has been performed on the ultrasonic defrosting owing to the mechanism of ultrasonic vibration for defrosting has not been totally revealed.
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Aluminum surface Ultrasonic transducer
Close
Open
Pipettor
Swinging strut -16oC Commercial freezer
(a)
(b)
(c)
Fig. 5. Production procedure of frozen water droplets.
10mm
Shatter
Cracking
(a) 0ms
(b) 21ms
(c) 32ms
(d) 37ms
(e) 40ms
Falling off
Bounce off
(f) 47ms
(g) 68ms
(h) 76ms
(i) 93ms
(j) 108ms
Fig. 6. Detailed behavior of frozen water droplets shedding from cold vertical surface due to effects of ultrasonic vibration.
Therefore, in the present study, we introduce ultrasonic vibration to remove the adherent frozen water droplets from cold surface, which provides the possibilities for effective defrosting. Detailed observations of transient process of frozen water droplets shedding from the cold vertical surface in the presence of 20 kHz ultrasonic vibration were made by a high-speed digital camera. In particular, the mechanisms of ultrasonic vibration for the frozen water droplets removal were analyzed. Furthermore, the detailed removal of frozen water droplets in different sizes and positions on the cold surface due to the effect of ultrasonic vibration were presented to validate the feasibility of ultrasonic vibration for effective defrosting.
2. Basic ideas for ultrasonic defrosting It is necessary to fully understand the frosting mechanisms prior to introduce ultrasonic vibration into the effective defrosting. Just at the study of Hayashi et al. [21], it is well acknowledged that the entire frosting process can be divided into three stages: frost nucleation period, frost layer growth period and frost layer fully growth period (Fig. 1). As illustrated in the study of Piucco et al. [22], a heterogeneous nucleation firstly takes place and the embryo starts to grow (see inset 1 in Fig. 1), as soon as the necessary nonequilibrium conditions are satisfied. Then the adjacent embryos coalesce (see inset 2 in Fig. 1). The embryo surface temperature
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Depression
Crack
Fig. 7. Internal temperature variations of cold surface due to the effect of ultrasonic vibration.
becomes higher than that of the plate as its surface increases, demanding a higher energy removal rate to maintain its growth (see inset 3 in Fig. 1). When this amount becomes higher than the energy removal required to start nucleation on a new site, the embryo stops growing and a secondary nucleation takes place (see inset 4 in Fig. 1). During the frost layer growth period, as the new embryo grows (see inset 5 in Fig. 1), so does the energy removal needed to sustain its growth and, as a result, new nucleation spots appear on the surface of the original embryo (see inset 6,7 in Fig. 1). Successive nucleation and embryo growth processes go on until the supercooling and supersaturation degrees approach zero (see inset 8 in Fig. 1). From this point on, the frost layer fully growth period starts. The frost layer behaves as a porous medium into which diffusion of water vapor leads to an increase in both its thickness and density (see inset 9 in Fig. 1). In order to further verify the above frosting mechanism, a visual experiment on the detailed frosting process was performed in Fig. 2. It is clearly observed that the frost crystal grows on the surface of the initial frozen water droplet. Hence, the defrosting problem can be reduced to investigate the initial frozen water droplets removal from the cold surface. Therefore, the effective defrosting can be successfully achieved by removing its parasitic basis, i.e. the initial frozen water droplets adhered to the cold surface. 3. Experimental apparatus and procedure 3.1. Experimental setup The schematic diagram of experimental apparatus used in the tests is shown in Fig. 3. It mainly consists of four parts: a high
(a) 0ms
Fig. 9. SEM image of surface topography of aluminum plate in the tests.
frequency ultrasonic vibration system, a water droplet freezing system, a high-speed image acquisition system and a data acquisition system. The high frequency ultrasonic vibration system includes a 20 kHz ultrasonic transducer and an ultrasonic generator with a capacity of 0–100 W. A 70 mm 70 mm 3 mm 6061 aluminum plate is provided as the base surface for water droplets freezing. An ultrasonic longitudinal wave probe with 10 mm in diameter is directly attached to the center of the aluminum plate by a screw bolt, as shown in Fig. 4, which is different from the non-contact relationship between the ultrasonic transducer and the flat surface [16]. In addition, a pipette that could provide the desired water droplets in different diameters on the cold surface is utilized in these tests and a commercial freezer is used to provide a cold environment to freeze the water droplets. Also, three K-type thermocouples (OMEGA) fitted the small holes which is drilled into the flat are installed to acquire the internal temperatures of the aluminum plate. The wire core diameter of the thermocouples here is 2 0.127 mm. The detailed locations of thermocouples are shown in Fig. 4. A thermo-hygrometer is used to measure the ambient temperature. All the temperature data are recorded by a HP34970A data acquisition system. The shedding processes of frozen water droplets from the cold vertical surface are observed by a high speed video camera (Photron SA4) and recorded by an image acquisition system connected to the computer. In all the tests, the power applied to the transducer is controlled to be 60 W.
(b) 2ms
(c) 9ms
(e) 26ms
(f) 47ms
Upspring
(d) 17ms
Fig. 8. Upspring phenomenon of frozen water droplet on horizontal surface due to the effect of ultrasonic vibration.
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σv
Ultrasound
P0
Crack
σs Frozen water droplet-air interface
Air
Air
Air
Fragment Force direction
Air
P1
Depression
(a)
Shed and Bounce off
σv
(b)
(c)
(d)
Fig. 10. Schematic diagram of ultrasonic vibration for shedding frozen water droplet from cold surface.
3.2. Production procedure of the frozen water droplet Since the interfacial adhesion between the frozen water droplet and cold surface, which is a key resistance for frozen droplet shedding off, is mainly dependent on the cooling and frozen process. Hence, in the experiments, we attempt to control the cooling and frozen process in the same experimental conditions so as to obtain the comparative frozen water droplets. In order to clearly describe the production procedure of the frozen water droplet, a simple schematic diagram is given in Fig. 5. As can be seen from the figure, the ultrasonic transducer connected with the aluminum plate is firstly put into the commercial freezer (see inset (a) in Fig. 5) and the swinging strut is justified to keep the aluminum surface horizontality. Next, a pipette is applied to drop the desired water droplet on the surface of the aluminum plate (see inset (b) in Fig. 5). And then the door of the freezer is closed and the water droplet’s cooling and freezing process gradually happens (see inset(c) in Fig. 5). It should be noted that, the freezer temperature in all the tests is set to 16 °C and the time of every water droplet’s cooling and frozen process is controlled to be about half an hour. 4. Results and discussion 4.1. Detailed behavior of frozen water droplets shedding from cold vertical surface by ultrasonic vibration Fig. 6 shows the detailed transient process of frozen water droplets shedding from the cold vertical surface. The simultaneous falling and bouncing off of frozen water droplets from the cold surface within 108 ms can be observed. As the ultrasonic generator is turned on, the frozen water droplet in the middle firstly cracks and a crazing can be seen clearly at just 21 ms (Fig. 6b) and subsequently the big part of the frozen water droplet breaks away from the surface (Fig. 6c). About 5 ms later, the remaining part of middle frozen water droplet abruptly shatters and bounces off from the cold surface (Fig. 6d–f). In contrast, the dynamic behaviors of the frozen water droplets on the top and lower surface are totally different from the middle one. As shown in Fig. 6g–j, the two frozen water droplets completely fall off from the vertical surface and no fractures can be seen on the frozen water droplets. Apparently, the ultrasonic vibration can effectively remove the frozen water droplets from the cold vertical surface with almost no time delay. 4.2. Mechanism analysis of ultrasonic vibration for frozen water droplets removal 4.2.1. Heating effect induced by ultrasonic vibration on removing frozen water droplets The internal temperature variations of cold plate during the frozen water droplets shedding process under the ultrasonic vibration are shown in Fig. 7 to explore the heating effect induced by ultrasonic vibration on the frozen water droplets removal. As shown, an
instantaneous internal temperature jump of cold flat occurs due to the heating effect caused by ultrasonic vibration and obviously the temperature distribution in the flat is not uniform. The temperature jump of Position B is especially remarkable compared with that of Position A and Position C. Almost 5.2° centigrade temperature rise can be seen in only one second. However, it is important to note that the maximum temperature tested in the cold flat after ultrasonic vibration is merely 7.2 °C, which is far from reaching the ice melting temperature, implying that it cannot be able to melt the frozen water droplets. Hence, the frozen water droplets removal cannot be attributable directly to the heating effect induced by ultrasonic vibration. 4.2.2. Mechanical effect generated by ultrasonic vibration on frozen water droplets removal It is well acknowledged that the mechanical effect generated by ultrasonic vibration could cause high frequency vibrations [23]. Due to the two material’s different ultrasonic frequency responses, the vibrations in the cold surface and frozen water droplets are not synchronous, which could cause the trend of high-speed relative motion between frozen water droplets and cold surface. Such a relative motion will cause a great transverse shear force rs, which may reach to about 106 Pa orders of magnitude [23], at the interface between frozen water droplets and cold surface. In contrast, the frozen water droplet shear adhesion strength on the 6061 aluminum is only in the range of 0.2–0.8 MPa, which was reported in [24–27]. The interface transverse shear force induced by ultrasonic mechanical effect far exceeds the shear adhesion strength, which could cause the delamination of frozen water droplets from cold surface. Therefore, the mechanical effect can be considered as a main reason for the shedding of frozen water droplets. 4.2.3. Acoustic pressure generated by ultrasonic vibration on frozen water droplets removal In addition to the above mentioned shear adhesion strength, the other vertical adhesion strength of frozen water droplet in the normal direction of cold surface is also an important resistance for the shedding of frozen water droplets from the cold vertical surface. Interestingly, it can be clearly observed in Fig. 8 that the ultrasonic vibration is able to provide an impact force in the normal direction to overcome such a vertical adhesion strength of the frozen water droplet. As shown in Fig. 8d, the whole frozen water droplet completely separates from the cold surface and up springs. At this moment, the maximum upspring height of frozen water droplet is about 1.8 mm. All the phenomenon confirms that ultrasonic vibration can generate intense impact force in the normal direction of the cold plate surface which can overcome the vertical adhesion strength of frozen water droplet at the droplet-surface interface. In order to explore the cause of the impact force in the normal direction of cold flat, the SEM image of surface topography of aluminum plate in the tests is presented. As shown in Fig. 9, tiny depression and crack can be seen on the rough aluminum surface. Hence, as the frozen water droplet adhered to the cold surface, the tiny depression will be covered and the air in the depression will
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Droplet-1
Droplet-2 20mm
Frozen water droplet in different diameters
125
Initial shedding time/ms
30mm
100 75 50 25 0
0
5
10
15
20
25
30
35
Diameters/mm Fig. 12. Initial shedding time of frozen water droplet in different diameters.
Droplet-3 10mm
impact stress could far exceed the frozen water droplet vertical adhesion strength of 0.4–0.8 MPa reported by [28] and the atmospheric pressure P0 and thus cause the separation of frozen water droplet from the cold surface. In summary, as a result of combined actions of the above mentioned interface transverse shear stress rs induced by the ultrasonic mechanical effect for overcoming the shear adhesion stress and the impact stress rv generated by ultrasonic acoustic pressure for conquering the vertical adhesion stress, the frozen water droplet can be completely shed and bounced off from the cold surface.
Droplet-4 5mm
Droplet-5 3mm
Droplet-6 2mm
Fig. 11. Ultrasonic vibration for shedding frozen water droplets in different diameters from cold vertical surface.
be sealed (see inset (a) in Fig. 10). Therefore, as the ultrasonic generator is turned on, the sealed air in the depression will be instantly excited due to the alternated positive and negative acoustic pressure induced by the ultrasonic vibration and the air pressure inside will rapidly increase. Ultimately, the accumulated enormous pressure P1 of air in the depressions will instantaneously release, which could generate great impact stress rv in the vertical direction of the cold surface (see inset (b–d) in Fig. 10). Such a great
4.3. Feasibility of ultrasonic vibration for the frozen water droplets removal from the cold vertical surface In order to further validate and evaluate the feasibility of ultrasonic vibration for the frozen water droplets removal from the cold aluminum plates, the detailed behaviors of the frozen water droplets with different sizes, positions and thickness subjected to the ultrasonic vibration were explored. Fig. 11 presents the shedding processes of frozen water droplets with different diameters subjected to ultrasonic vibration on the same position of cold vertical surface. It can be visually observed that the frozen water droplets in different diameters can be all successfully removed by 20 kHz and 60 W ultrasonic vibration. Fig. 12 shows the initial shedding time of frozen water droplets in different diameters. As shown in the figure, the smallest frozen water droplet with 2 mm in diameter starts to shed after only 5 ms as the ultrasonic generator is turned on. In contrast, the initial shedding time of the biggest one with 30 mm in diameter could reach to 142 ms. It can be found that the bigger the frozen water droplet is, the longer the initial shedding time of the frozen water droplet by ultrasonic vibration will be. This is because the determination of the initial shedding time of frozen water droplets involves the ultrasonic power and the interfacial adhesion force. In addition, the interfacial adhesion force of frozen water droplets here is mainly dependent on the droplet size, i.e. the interfacial bonding area in the same cooling and frozen conditions. Based on the above analysis, we can conclude that, under the same ultrasonic power conditions, the size of interfacial bonding area is the dominant factor determining the initial shedding time. In this test, the bonding area of the frozen water droplet with 30 mm in diameter is about 225 times compared with the one with 2 mm in diameter. Hence, for the frozen water droplet in larger diameter, much more time should be consumed to overcome the whole adhesion on cold surface. Fig. 13 shows the ultrasonic vibration for the frozen water droplets removal in different positions on cold vertical surface. Eleven numbered frozen water droplets were adhered to the different
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3
2
1
4
5
6
7
8
9
10
11
(a) 0ms
(b) 27ms
(c) 35 ms
(d) 44ms
(e) 55ms
(f) 80ms
(g) 93ms
(h) 598ms
(i) 645ms
Fig. 13. Ultrasonic vibration for removing the frozen water droplets in different positions of the cold vertical surface.
700
600ms
Initial sheddiing time/ms
600 500 400 300 200
80ms
100 0
1
2
3
4
5
6
7
8
9
10
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Numbered frozen water droplets Fig. 14. Initial shedding time of the frozen water droplets in different positions of cold surface.
positions of the cold surface (see inset (a) in Fig. 13). As shown in Fig. 13, all the frozen water droplets can be shed and bounced off from the vertical surface. Fig. 14 presents the initial shedding time of the numbered frozen water droplets. It can be seen in Fig. 14 that at 80 ms, almost all the frozen water droplets have been removed from the cold surface except the frozen water droplet
(No. 3) which is the farthest from the ultrasonic source. About 500 ms later, the frozen water droplet (No. 3) successfully shed from the surface. All the frozen water droplets in different positions of cold surface can be totally shed off in less than 600 ms, as shown in Fig. 14, although the initial shedding time of each frozen water droplet in different positions from the cold surface is different. The results show that the ultrasonic vibration has a strong ability to affect almost the total area of this cold surface and remove all the frozen water droplets from cold surface. In order to explore the effect of the thickness on the frozen water droplet removal, a related experiment has been conduced. All the frozen water droplets used in the test are approximately 10 mm in diameter and driven by 60 W ultrasonic vibration. Moreover, the thickness of the five frozen water droplets provided here is 2.6 mm, 3.5 mm, 5.1 mm, 6.4 mm and 9.6 mm, respectively. A sequence of images is acquired at 3000 frames/s by high speed video camera. Fig. 15 presents the shedding process of frozen water droplets in different thickness by ultrasonic vibration. As can be seen from the Fig. 15b, the initial shedding time of these five frozen water droplets in different thickness is all in the range of 62–76 ms and almost no different can be seen in the initial shedding time. As mentioned above, the dominant factor for the frozen water droplet removal is the interfacial adhesion force and the ultrasonic power. Also, the interfacial adhesion force is mainly dependent on the
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Droplet Thickness
Droplet Thickness
Droplet Thickness
3.5 mm
2.6 mm
Droplet Thickness
Droplet Thickness
6.4 mm
9.6 mm
5.1 mm
10mm
(a)
(b)
0s
62ms
0s
0s
0s
0s
76ms
73ms
65ms
68ms
(c) Fig. 15. Shedding process of water droplets in different thickness due to ultrasonic vibration.
droplet size, i.e. the interfacial bonding area. In this experiment, all the five frozen water droplets with approximately 10 mm in diameter almost have the same interfacial bonding area. Therefore, under the same ultrasonic power conditions and with the same interfacial bonding area, there almost has no different in the initial shedding time. Based on the analysis, it can be concluded that the thickness of the frozen water droplet has negligible effect on the frozen water droplets removal by ultrasonic vibration. 5. Conclusions An idea for effective defrosting is proposed based on the ultrasonic vibration’s strong ability to remove adherent frozen water droplets from the cold surface which is the parasitic basis for frosting. The shedding processes of various frozen water droplets adhered to 70 mm 70 mm cold vertical surface by 20 kHz ultrasonic vibration were experimentally studied to validate the feasibility of ultrasonic defrosting. The conclusions can be summarized as (1) The frozen water droplets adhered to cold vertical surface can be instantaneously shed off in less than one second due to the combined effects of the interface transverse shear force induced by the ultrasonic mechanical effect and the impact force generated by the ultrasonic acoustic pressure. (2) The shedding process of frozen water droplets cannot be attributable to the heat effect generated by ultrasonic vibration, although the internal temperatures of the cold flat have an apparent rise due to the effect of ultrasonic vibration. (3) The frozen water droplets within 2–30 mm in diameter can be successfully removed and all the frozen water droplets in different positions of 70 mm 70 mm cold surface can be
completely shed off by the ultrasonic vibration, which further verify that the ultrasonic vibration has a very strong ability to remove the frozen water droplets, which is the parasitic basis for frosting, from the cold surface and thus it is a highly potential defrosting method for practical application.
Acknowledgements The authors gratefully acknowledge the support provided by Scientific Research Foundation of Graduate School of Southeast University (No. YBJJ1203). References [1] Zhongliang Liu, Xinghua Zhang, Hongyan Wang, Sheng Meng, Shuiyuan Cheng, Influences of surface hydrophilicity on frost formation on a vertical cold plate under natural convection conditions, Exp. Therm. Fluid Sci. 31 (2007) 789–794. [2] Hyunuk Lee, Jongmin Shin, Samchul Ha, Bongjun Choi, Jaekeun Lee, Frost formation on a plate with different surface hydrophilicity, Int. J. Heat Mass Transfer 47 (2004) 4881–4893. [3] Sung Jhee, Kwan-Soo Lee, Woo-Seung Kim, Effect of surface treatments on the frosting/defrosting behavior of a fin-tube heat exchanger, Int. J. Refrig. 25 (2002) 1047–1053. [4] Jongmin Shin, Alexei V. Tikhonov, Cheolhwan Kim, Experimental study on frost structure on surfaces with different hydrophilicity: density and thermal conductivity, J. Heat Transfer 125 (2003) 84–94. [5] H. Wang, L. Tang, X. Wu, W. Dai, Y. Qiu, Fabrication and anti-frosting performance of super hydrophobic coating based on modified nano-sized calcium carbonate and ordinary polyacrylate, Appl. Surf. Sci. 253 (2007) 8818– 8824. [6] Zhongliang Liu, Yunjun Gou, Jieteng Wang, Shuiyuan Cheng, Frost formation on a super-hydrophobic surface under natural convection conditions, Int. J. Heat Mass Transfer 51 (2008) 5975–5982. [7] Kripa K. Varanasi, Tao Deng, J. David Smith, Ming Hsu, Nitin Bhate, Frost formation and ice adhesion on superhydrophobic surfaces, Appl. Phys. Lett. 97 (2010) 234102-1–234102-3.
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Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Experimental study on instantaneously shedding frozen water droplets from cold vertical surface by ultrasonic vibration Dong Li, Zhenqian Chen ⇑ Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, IIUSE, School of Energy and Environment, Southeast University, Nanjing 210096, PR China
a r t i c l e
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Article history: Received 27 March 2013 Received in revised form 2 October 2013 Accepted 14 October 2013 Available online 26 November 2013 Keywords: Ultrasonic vibration Frozen water droplets Temperature jump Interface transverse shear force Acoustic pressure
a b s t r a c t The ultrasonic vibration is introduced to remove adherent frozen water droplets from cold surface, which provides the possibilities for effective defrosting. The shedding processes of various frozen water droplets adhered to 70 mm 70 mm cold vertical surface by 20 kHz and 60 W ultrasonic vibration were experimentally studied. It was found that the frozen water droplets instantaneously crack and shed off from the vertical surface due to the combined effects of interface transverse shear force generated by ultrasonic mechanical vibration and impact force induced by ultrasonic acoustic pressure. However, the heating effect triggered by ultrasonic vibration has limited effect on the frozen water droplets removal. Moreover, the frozen water droplets in different diameters within 2–30 mm can be successfully removed and all the frozen water droplets in different positions of 70 mm 70 mm cold surface can be completely shed off by 20 kHz high frequency ultrasonic vibration. The results showed that the ultrasonic vibration has a very strong ability to remove the frozen water droplets, which are the parasitic substrates for frosting, from cold flat surface and thus it is a highly potential defrosting method for practical application. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Frost deposition is a well-known and undesirable phenomenon in numerous fields such as air conditioning, cryogenics and aeronautics which work under low temperature conditions. The frost layer accumulated on cold surface of heat exchanger inevitably decreases the heat transfer rate of refrigerating unit due to the increase in both thermal and flow resistance. Besides, frost accreted on aircraft structures compromises aircraft performance and might cause safety problems. Therefore, great attention has been paid to searching for the effective method for preventing frost formation or removing accreted frost layer on these equipments. Up to now, various methods concerning frost suppression and defrosting have been proposed. Thereinto, many studies have been performed to investigate the influence of surface energy on frost nucleation and growth [1–9]. In such studies, surface contact angle was changed by means of surface treatment to get the hydrophobic or hydrophilic surface. It was found that the hydrophobic/hydrophilic surface can significantly retard the initial frost nucleation. However, if the cold surface was completely covered by a thin frost layer, the surface energy no longer influences the subsequent frost growth process. Besides, numerous studies concerning the effect of various external fields on frost formation were reported [10–14]. Wang et al. [10] and Joppolo et al. [11] studied the frost formation ⇑ Corresponding author. Tel./fax: +86 25 83790626. E-mail address: [email protected] (Z. Chen). 0894-1777/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2013.10.005
in the presence of external electric field. They found that the frost structure subjected to external electric field is relatively skinny and fragile which could easily break up and fall off compared with that without electric field. In addition, Gou et al. [12] experimentally studied the frost formation under magnetic field. It was found that the water droplets are smaller and more homogeneous and the frost structure is easy to be removed under magnetic field. Furthermore, Cheng and Shiu [13] investigated the effect of external oscillation on the frost formation and liquid droplet solidification. It showed that the transverse oscillation of the cold plate results in a significant change in the solidification pattern of the liquid droplets. However, the frost crystals are not able to be removed from the cold plate by oscillation within the range of low frequency from 100 Hz to 200 Hz. The similar investigation was conducted by Wu and Webb [14]. Their experiment also showed that the low frequency mechanical vibration cannot successfully release the frost formed on the hydrophobic surface. In recent years, growing attentions have been concentrated on the ultrasonic technology for frost suppression and defrosting due to the ultrasonic high frequency and energy concentration. Kazunari et al. [15] observed the frost accumulation on a rectangular aluminum alloy plate surface subjected to approximately 37 kHz ultrasonic vibration. It was found that the high frequency vibration could suppress the frost accumulation by approximately 60%. Li et al. [16] visually studied the effect of 20 kHz ultrasound on frost formation on cold flat surface in atmospheric air flow. It indicated that the coverage of initial freezing droplets is all less
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Fig. 1. Schematic representation of the entire frosting process on cold surface.
Fig. 2. Real process of frost crystal growth on the frozen water droplet surface.
Data acquisition instrument
PC Threaded connection Temperature and humidity sensor Ultrasonic transducer Frozen water droplet
Photron CCD Ultrasonic generator Fig. 3. Schematic diagram of experimental apparatus for frozen water droplets shedding from cold vertical surface.
than 52% with the effect of ultrasound compared with that all more than 65% without ultrasound and the rate of frost layer thickness reduction with the effect of ultrasound compared with that without ultrasound is about 75%. However, in all the tests, the ultrasonic transducer is not in contact with the flat surface which will cause significant energy decrement. Wang et al. [17] experimentally studied the possibility of frost release from a finned-tube evaporator by ultrasonic vibrations in natural convection. Their results showed that the frost crystals and frost branches on the ice layer can be fractured and removed effectively. However, the ultrasonic vibrations cannot remove the basic ice layer on the fins. In addition, Yan et al. [18] developed an ultrasonic method for defrosting and their results showed that using more ultrasonic sound sources is better than using single sound source for defrosting. Palacios and Smith [19,20] introduced an ultrasonic de-icing system for helicopter rotor blades. Their studies showed that the ultrasonic shear actuators could melt a 1.5 mm thick ice layer in a time period under 5 min at the first ultrasonic resonance frequency (130 kHz) of the system. In summary, although a great number of investigations have been concentrated on the frost suppression and defrosting, sufficiently effective methods do not exist. Especially few effective defrosting methods have been proposed which are more attractive for the practical application. Despite quite a number of studies
70
Unit: mm
Position A
Φ0.8× 6
Frozen water droplet
Position B Φ10
70
Position C 15
Fig. 4. Locations of screw bolt and thermocouples.
indicated that ultrasonic vibration is an effective means for frost suppression, little research has been performed on the ultrasonic defrosting owing to the mechanism of ultrasonic vibration for defrosting has not been totally revealed.
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Aluminum surface Ultrasonic transducer
Close
Open
Pipettor
Swinging strut -16oC Commercial freezer
(a)
(b)
(c)
Fig. 5. Production procedure of frozen water droplets.
10mm
Shatter
Cracking
(a) 0ms
(b) 21ms
(c) 32ms
(d) 37ms
(e) 40ms
Falling off
Bounce off
(f) 47ms
(g) 68ms
(h) 76ms
(i) 93ms
(j) 108ms
Fig. 6. Detailed behavior of frozen water droplets shedding from cold vertical surface due to effects of ultrasonic vibration.
Therefore, in the present study, we introduce ultrasonic vibration to remove the adherent frozen water droplets from cold surface, which provides the possibilities for effective defrosting. Detailed observations of transient process of frozen water droplets shedding from the cold vertical surface in the presence of 20 kHz ultrasonic vibration were made by a high-speed digital camera. In particular, the mechanisms of ultrasonic vibration for the frozen water droplets removal were analyzed. Furthermore, the detailed removal of frozen water droplets in different sizes and positions on the cold surface due to the effect of ultrasonic vibration were presented to validate the feasibility of ultrasonic vibration for effective defrosting.
2. Basic ideas for ultrasonic defrosting It is necessary to fully understand the frosting mechanisms prior to introduce ultrasonic vibration into the effective defrosting. Just at the study of Hayashi et al. [21], it is well acknowledged that the entire frosting process can be divided into three stages: frost nucleation period, frost layer growth period and frost layer fully growth period (Fig. 1). As illustrated in the study of Piucco et al. [22], a heterogeneous nucleation firstly takes place and the embryo starts to grow (see inset 1 in Fig. 1), as soon as the necessary nonequilibrium conditions are satisfied. Then the adjacent embryos coalesce (see inset 2 in Fig. 1). The embryo surface temperature
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Depression
Crack
Fig. 7. Internal temperature variations of cold surface due to the effect of ultrasonic vibration.
becomes higher than that of the plate as its surface increases, demanding a higher energy removal rate to maintain its growth (see inset 3 in Fig. 1). When this amount becomes higher than the energy removal required to start nucleation on a new site, the embryo stops growing and a secondary nucleation takes place (see inset 4 in Fig. 1). During the frost layer growth period, as the new embryo grows (see inset 5 in Fig. 1), so does the energy removal needed to sustain its growth and, as a result, new nucleation spots appear on the surface of the original embryo (see inset 6,7 in Fig. 1). Successive nucleation and embryo growth processes go on until the supercooling and supersaturation degrees approach zero (see inset 8 in Fig. 1). From this point on, the frost layer fully growth period starts. The frost layer behaves as a porous medium into which diffusion of water vapor leads to an increase in both its thickness and density (see inset 9 in Fig. 1). In order to further verify the above frosting mechanism, a visual experiment on the detailed frosting process was performed in Fig. 2. It is clearly observed that the frost crystal grows on the surface of the initial frozen water droplet. Hence, the defrosting problem can be reduced to investigate the initial frozen water droplets removal from the cold surface. Therefore, the effective defrosting can be successfully achieved by removing its parasitic basis, i.e. the initial frozen water droplets adhered to the cold surface. 3. Experimental apparatus and procedure 3.1. Experimental setup The schematic diagram of experimental apparatus used in the tests is shown in Fig. 3. It mainly consists of four parts: a high
(a) 0ms
Fig. 9. SEM image of surface topography of aluminum plate in the tests.
frequency ultrasonic vibration system, a water droplet freezing system, a high-speed image acquisition system and a data acquisition system. The high frequency ultrasonic vibration system includes a 20 kHz ultrasonic transducer and an ultrasonic generator with a capacity of 0–100 W. A 70 mm 70 mm 3 mm 6061 aluminum plate is provided as the base surface for water droplets freezing. An ultrasonic longitudinal wave probe with 10 mm in diameter is directly attached to the center of the aluminum plate by a screw bolt, as shown in Fig. 4, which is different from the non-contact relationship between the ultrasonic transducer and the flat surface [16]. In addition, a pipette that could provide the desired water droplets in different diameters on the cold surface is utilized in these tests and a commercial freezer is used to provide a cold environment to freeze the water droplets. Also, three K-type thermocouples (OMEGA) fitted the small holes which is drilled into the flat are installed to acquire the internal temperatures of the aluminum plate. The wire core diameter of the thermocouples here is 2 0.127 mm. The detailed locations of thermocouples are shown in Fig. 4. A thermo-hygrometer is used to measure the ambient temperature. All the temperature data are recorded by a HP34970A data acquisition system. The shedding processes of frozen water droplets from the cold vertical surface are observed by a high speed video camera (Photron SA4) and recorded by an image acquisition system connected to the computer. In all the tests, the power applied to the transducer is controlled to be 60 W.
(b) 2ms
(c) 9ms
(e) 26ms
(f) 47ms
Upspring
(d) 17ms
Fig. 8. Upspring phenomenon of frozen water droplet on horizontal surface due to the effect of ultrasonic vibration.
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σv
Ultrasound
P0
Crack
σs Frozen water droplet-air interface
Air
Air
Air
Fragment Force direction
Air
P1
Depression
(a)
Shed and Bounce off
σv
(b)
(c)
(d)
Fig. 10. Schematic diagram of ultrasonic vibration for shedding frozen water droplet from cold surface.
3.2. Production procedure of the frozen water droplet Since the interfacial adhesion between the frozen water droplet and cold surface, which is a key resistance for frozen droplet shedding off, is mainly dependent on the cooling and frozen process. Hence, in the experiments, we attempt to control the cooling and frozen process in the same experimental conditions so as to obtain the comparative frozen water droplets. In order to clearly describe the production procedure of the frozen water droplet, a simple schematic diagram is given in Fig. 5. As can be seen from the figure, the ultrasonic transducer connected with the aluminum plate is firstly put into the commercial freezer (see inset (a) in Fig. 5) and the swinging strut is justified to keep the aluminum surface horizontality. Next, a pipette is applied to drop the desired water droplet on the surface of the aluminum plate (see inset (b) in Fig. 5). And then the door of the freezer is closed and the water droplet’s cooling and freezing process gradually happens (see inset(c) in Fig. 5). It should be noted that, the freezer temperature in all the tests is set to 16 °C and the time of every water droplet’s cooling and frozen process is controlled to be about half an hour. 4. Results and discussion 4.1. Detailed behavior of frozen water droplets shedding from cold vertical surface by ultrasonic vibration Fig. 6 shows the detailed transient process of frozen water droplets shedding from the cold vertical surface. The simultaneous falling and bouncing off of frozen water droplets from the cold surface within 108 ms can be observed. As the ultrasonic generator is turned on, the frozen water droplet in the middle firstly cracks and a crazing can be seen clearly at just 21 ms (Fig. 6b) and subsequently the big part of the frozen water droplet breaks away from the surface (Fig. 6c). About 5 ms later, the remaining part of middle frozen water droplet abruptly shatters and bounces off from the cold surface (Fig. 6d–f). In contrast, the dynamic behaviors of the frozen water droplets on the top and lower surface are totally different from the middle one. As shown in Fig. 6g–j, the two frozen water droplets completely fall off from the vertical surface and no fractures can be seen on the frozen water droplets. Apparently, the ultrasonic vibration can effectively remove the frozen water droplets from the cold vertical surface with almost no time delay. 4.2. Mechanism analysis of ultrasonic vibration for frozen water droplets removal 4.2.1. Heating effect induced by ultrasonic vibration on removing frozen water droplets The internal temperature variations of cold plate during the frozen water droplets shedding process under the ultrasonic vibration are shown in Fig. 7 to explore the heating effect induced by ultrasonic vibration on the frozen water droplets removal. As shown, an
instantaneous internal temperature jump of cold flat occurs due to the heating effect caused by ultrasonic vibration and obviously the temperature distribution in the flat is not uniform. The temperature jump of Position B is especially remarkable compared with that of Position A and Position C. Almost 5.2° centigrade temperature rise can be seen in only one second. However, it is important to note that the maximum temperature tested in the cold flat after ultrasonic vibration is merely 7.2 °C, which is far from reaching the ice melting temperature, implying that it cannot be able to melt the frozen water droplets. Hence, the frozen water droplets removal cannot be attributable directly to the heating effect induced by ultrasonic vibration. 4.2.2. Mechanical effect generated by ultrasonic vibration on frozen water droplets removal It is well acknowledged that the mechanical effect generated by ultrasonic vibration could cause high frequency vibrations [23]. Due to the two material’s different ultrasonic frequency responses, the vibrations in the cold surface and frozen water droplets are not synchronous, which could cause the trend of high-speed relative motion between frozen water droplets and cold surface. Such a relative motion will cause a great transverse shear force rs, which may reach to about 106 Pa orders of magnitude [23], at the interface between frozen water droplets and cold surface. In contrast, the frozen water droplet shear adhesion strength on the 6061 aluminum is only in the range of 0.2–0.8 MPa, which was reported in [24–27]. The interface transverse shear force induced by ultrasonic mechanical effect far exceeds the shear adhesion strength, which could cause the delamination of frozen water droplets from cold surface. Therefore, the mechanical effect can be considered as a main reason for the shedding of frozen water droplets. 4.2.3. Acoustic pressure generated by ultrasonic vibration on frozen water droplets removal In addition to the above mentioned shear adhesion strength, the other vertical adhesion strength of frozen water droplet in the normal direction of cold surface is also an important resistance for the shedding of frozen water droplets from the cold vertical surface. Interestingly, it can be clearly observed in Fig. 8 that the ultrasonic vibration is able to provide an impact force in the normal direction to overcome such a vertical adhesion strength of the frozen water droplet. As shown in Fig. 8d, the whole frozen water droplet completely separates from the cold surface and up springs. At this moment, the maximum upspring height of frozen water droplet is about 1.8 mm. All the phenomenon confirms that ultrasonic vibration can generate intense impact force in the normal direction of the cold plate surface which can overcome the vertical adhesion strength of frozen water droplet at the droplet-surface interface. In order to explore the cause of the impact force in the normal direction of cold flat, the SEM image of surface topography of aluminum plate in the tests is presented. As shown in Fig. 9, tiny depression and crack can be seen on the rough aluminum surface. Hence, as the frozen water droplet adhered to the cold surface, the tiny depression will be covered and the air in the depression will
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Droplet-1
Droplet-2 20mm
Frozen water droplet in different diameters
125
Initial shedding time/ms
30mm
100 75 50 25 0
0
5
10
15
20
25
30
35
Diameters/mm Fig. 12. Initial shedding time of frozen water droplet in different diameters.
Droplet-3 10mm
impact stress could far exceed the frozen water droplet vertical adhesion strength of 0.4–0.8 MPa reported by [28] and the atmospheric pressure P0 and thus cause the separation of frozen water droplet from the cold surface. In summary, as a result of combined actions of the above mentioned interface transverse shear stress rs induced by the ultrasonic mechanical effect for overcoming the shear adhesion stress and the impact stress rv generated by ultrasonic acoustic pressure for conquering the vertical adhesion stress, the frozen water droplet can be completely shed and bounced off from the cold surface.
Droplet-4 5mm
Droplet-5 3mm
Droplet-6 2mm
Fig. 11. Ultrasonic vibration for shedding frozen water droplets in different diameters from cold vertical surface.
be sealed (see inset (a) in Fig. 10). Therefore, as the ultrasonic generator is turned on, the sealed air in the depression will be instantly excited due to the alternated positive and negative acoustic pressure induced by the ultrasonic vibration and the air pressure inside will rapidly increase. Ultimately, the accumulated enormous pressure P1 of air in the depressions will instantaneously release, which could generate great impact stress rv in the vertical direction of the cold surface (see inset (b–d) in Fig. 10). Such a great
4.3. Feasibility of ultrasonic vibration for the frozen water droplets removal from the cold vertical surface In order to further validate and evaluate the feasibility of ultrasonic vibration for the frozen water droplets removal from the cold aluminum plates, the detailed behaviors of the frozen water droplets with different sizes, positions and thickness subjected to the ultrasonic vibration were explored. Fig. 11 presents the shedding processes of frozen water droplets with different diameters subjected to ultrasonic vibration on the same position of cold vertical surface. It can be visually observed that the frozen water droplets in different diameters can be all successfully removed by 20 kHz and 60 W ultrasonic vibration. Fig. 12 shows the initial shedding time of frozen water droplets in different diameters. As shown in the figure, the smallest frozen water droplet with 2 mm in diameter starts to shed after only 5 ms as the ultrasonic generator is turned on. In contrast, the initial shedding time of the biggest one with 30 mm in diameter could reach to 142 ms. It can be found that the bigger the frozen water droplet is, the longer the initial shedding time of the frozen water droplet by ultrasonic vibration will be. This is because the determination of the initial shedding time of frozen water droplets involves the ultrasonic power and the interfacial adhesion force. In addition, the interfacial adhesion force of frozen water droplets here is mainly dependent on the droplet size, i.e. the interfacial bonding area in the same cooling and frozen conditions. Based on the above analysis, we can conclude that, under the same ultrasonic power conditions, the size of interfacial bonding area is the dominant factor determining the initial shedding time. In this test, the bonding area of the frozen water droplet with 30 mm in diameter is about 225 times compared with the one with 2 mm in diameter. Hence, for the frozen water droplet in larger diameter, much more time should be consumed to overcome the whole adhesion on cold surface. Fig. 13 shows the ultrasonic vibration for the frozen water droplets removal in different positions on cold vertical surface. Eleven numbered frozen water droplets were adhered to the different
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3
2
1
4
5
6
7
8
9
10
11
(a) 0ms
(b) 27ms
(c) 35 ms
(d) 44ms
(e) 55ms
(f) 80ms
(g) 93ms
(h) 598ms
(i) 645ms
Fig. 13. Ultrasonic vibration for removing the frozen water droplets in different positions of the cold vertical surface.
700
600ms
Initial sheddiing time/ms
600 500 400 300 200
80ms
100 0
1
2
3
4
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6
7
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Numbered frozen water droplets Fig. 14. Initial shedding time of the frozen water droplets in different positions of cold surface.
positions of the cold surface (see inset (a) in Fig. 13). As shown in Fig. 13, all the frozen water droplets can be shed and bounced off from the vertical surface. Fig. 14 presents the initial shedding time of the numbered frozen water droplets. It can be seen in Fig. 14 that at 80 ms, almost all the frozen water droplets have been removed from the cold surface except the frozen water droplet
(No. 3) which is the farthest from the ultrasonic source. About 500 ms later, the frozen water droplet (No. 3) successfully shed from the surface. All the frozen water droplets in different positions of cold surface can be totally shed off in less than 600 ms, as shown in Fig. 14, although the initial shedding time of each frozen water droplet in different positions from the cold surface is different. The results show that the ultrasonic vibration has a strong ability to affect almost the total area of this cold surface and remove all the frozen water droplets from cold surface. In order to explore the effect of the thickness on the frozen water droplet removal, a related experiment has been conduced. All the frozen water droplets used in the test are approximately 10 mm in diameter and driven by 60 W ultrasonic vibration. Moreover, the thickness of the five frozen water droplets provided here is 2.6 mm, 3.5 mm, 5.1 mm, 6.4 mm and 9.6 mm, respectively. A sequence of images is acquired at 3000 frames/s by high speed video camera. Fig. 15 presents the shedding process of frozen water droplets in different thickness by ultrasonic vibration. As can be seen from the Fig. 15b, the initial shedding time of these five frozen water droplets in different thickness is all in the range of 62–76 ms and almost no different can be seen in the initial shedding time. As mentioned above, the dominant factor for the frozen water droplet removal is the interfacial adhesion force and the ultrasonic power. Also, the interfacial adhesion force is mainly dependent on the
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Droplet Thickness
Droplet Thickness
Droplet Thickness
3.5 mm
2.6 mm
Droplet Thickness
Droplet Thickness
6.4 mm
9.6 mm
5.1 mm
10mm
(a)
(b)
0s
62ms
0s
0s
0s
0s
76ms
73ms
65ms
68ms
(c) Fig. 15. Shedding process of water droplets in different thickness due to ultrasonic vibration.
droplet size, i.e. the interfacial bonding area. In this experiment, all the five frozen water droplets with approximately 10 mm in diameter almost have the same interfacial bonding area. Therefore, under the same ultrasonic power conditions and with the same interfacial bonding area, there almost has no different in the initial shedding time. Based on the analysis, it can be concluded that the thickness of the frozen water droplet has negligible effect on the frozen water droplets removal by ultrasonic vibration. 5. Conclusions An idea for effective defrosting is proposed based on the ultrasonic vibration’s strong ability to remove adherent frozen water droplets from the cold surface which is the parasitic basis for frosting. The shedding processes of various frozen water droplets adhered to 70 mm 70 mm cold vertical surface by 20 kHz ultrasonic vibration were experimentally studied to validate the feasibility of ultrasonic defrosting. The conclusions can be summarized as (1) The frozen water droplets adhered to cold vertical surface can be instantaneously shed off in less than one second due to the combined effects of the interface transverse shear force induced by the ultrasonic mechanical effect and the impact force generated by the ultrasonic acoustic pressure. (2) The shedding process of frozen water droplets cannot be attributable to the heat effect generated by ultrasonic vibration, although the internal temperatures of the cold flat have an apparent rise due to the effect of ultrasonic vibration. (3) The frozen water droplets within 2–30 mm in diameter can be successfully removed and all the frozen water droplets in different positions of 70 mm 70 mm cold surface can be
completely shed off by the ultrasonic vibration, which further verify that the ultrasonic vibration has a very strong ability to remove the frozen water droplets, which is the parasitic basis for frosting, from the cold surface and thus it is a highly potential defrosting method for practical application.
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