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Effect of ultrasound on frost formation on a cold flat surface in atmospheric air flow
Experimental Thermal and Fluid Science 34 (2010) 1247–1252
Contents lists available at ScienceDirect
Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Effect of ultrasound on frost formation on a cold flat surface in atmospheric air flow Dong Li, Zhenqian Chen *, Mingheng Shi School of Energy and Environment, IIUSE, Southeast University, Nanjing 210096, PR China
a r t i c l e
i n f o
Article history: Received 28 February 2010 Received in revised form 3 May 2010 Accepted 12 May 2010
Keywords: Ultrasound Freezing water droplets Frost layer structure Frost layer thickness
a b s t r a c t An experimental study concerning frost formation on a cold flat surface in atmospheric air flow subjected to the effect of 20 kHz ultrasound was conducted. A close observation of the frost nucleation and frost growth processes with and without the effect of 20 kHz ultrasound was made with a microscopic image system. The size and distribution of freezing water droplets during the initial nucleation period were described and the frost layer structure and thickness variation with time were presented. It was found that the freezing water droplets formed on the surface with the effect of ultrasound are smaller and sparser compared with those without the effect of ultrasound. The coverage of freezing droplets is all less than 52% with the effect of ultrasound compared with that all more than 65% without ultrasound under some tested conditions. Furthermore, the frost layer structure with a special pattern like ‘‘frost line” is observed on the cold surface and very small growth of the frost layer can be seen with the effect of ultrasound. The rate of frost layer thickness reduction with the effect of ultrasound compared with that without ultrasound is about 75%. The experimental results also showed that the frost formation process on the flat surface is remarkably restrained due to the effect of ultrasound. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Frost deposition is a well-known and undesirable phenomenon in numerous fields of industry that work with low temperatures such as aeronautics, air conditioning and cryogenics. When moist air comes in contact with a cold surface whose temperature is below both the dew point of the air and the freezing point of water, frost will form on it. Frost deposited on the cold surface of the heat exchanger inevitably degrades the performance of the refrigerating unit due to the increase of the air pressure drop and the decrease of thermal efficiency. In addition, frost accumulation on aircraft wings may cause serious safety problems. Therefore, development of an effective method to prevent the frost nucleation and growth is required in recent years. In the past few years, numerous studies have been performed to investigate the effect of surface energy of the cold surface on frost nucleation and growth. Forest [1] evaluated the use of polymer coatings to reduce the surface energy and concluded that a zero surface energy coating would be the most effective surface for releasing frost particles from the cold surfaces. Wu and Webb [2] investigated the possibility of frost release for hydrophilic and hydrophobic surfaces. Their experiment showed that frost formed on a hydrophobic surface cannot be removed successfully by mechanical vibration. Na and Webb [3] presented a theoretical analysis of the fundamental factors affecting frost nucleation and * Corresponding author. Tel.: +86 25 83790626; fax: +86 25 57714489. E-mail address: [email protected] (Z. Chen). 0894-1777/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2010.05.005
pointed out that a low energy surface requires a much higher supersaturating degree for frost nucleation than a high energy surface. Lee et al. [4] developed frost maps for two different surfaces having two different hydrophilic characteristics and concluded that surface with low dynamic contact angle (DCA) had lower frost thickness and higher frost density than that with high DCA. Liu et al. [5] observed the frost nucleation and deposition processes on a vertical plate and proposed that the initial water drops condensed on a hydrophobic surface are smaller and remain in the liquid state for a longer time compared with ones formed on a plain copper surface. However, their results showed that the influence of surface hydrophobicity is limited to the very initial period of the frost deposition process and cannot repress the frost layer growth. Recently, super-hydrophobic surface has been developed to prevent frost formation. Liu and Gou et al. [6] studied the frost formation on the super-hydrophobic surface with a contact angle of 162° and found that the super-hydrophobic surface can delay the frost deposition for 55 min compared with the plain copper surface under the tested conditions. Wang et al. [7] fabricated a super-hydrophobic coating with water contact angle of 155° from modified CaCO3 and polyacrylate. Their experiments showed the frost formation on the super-hydrophobic surface was greatly retarded compared with that on bare copper surface and the surface can keep super hydrophobicity even after freezing–thawing treating for 10 times. Furthermore, many studied concerning the effect of electric field on the frost formation were reported. Schaefer [8] firstly reported the influence of electrohydrodynamics (EHD) on frost
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Nomenclature Ta Tw u /
ambient air temperature (°C) cold surface temperature (°C) relative humidity average freezing droplet diameter (lm)
formation and found a rapid growth of ice in the form of whiskerlike aggregates in the presence of high electrical filed. Maybank and Barthakur [9] experimentally studied the effect of electric field on the ice crystal growth. Their results showed a rapid growth of ice crystals when the applied electric field strength is above 200 V/cm. The observed crystals are thinner and more fragile compared with those without electric field. Wang et al. [10] found that with the presence of EHD, the ice column is pulled up towards the electrode and the structure is relatively skinny and fragile which can easily break up and fall off due to the influence of gravity. It was also found that the electric polarity plays a significant role on the frost growing. For a negative polarity, the frost structure is thinner than that for a positive polarity and the break-off frequency of the ice column is more frequently compared to a positive polarity. In addition to above works, Gou et al. [11] theoretically and experimentally studied the frost formation under magnetic field. It is found that the water droplet is smaller and more homogeneous and the frost structure is easy to be removed under magnetic field. Cheng and Shiu [12] investigated the oscillation effects on the frost formation and the liquid droplet solidification on a cold plate. It is found 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 ranges of the frequency from 100 Hz to 200 Hz and amplitude from 40 lm to 100 lm. Lately, Yan et al. [13] developed a new method of ultrasonic defrost and their results showed it is feasible to use the ultrasonic defrost technology. However, the effect of ultrasound on the fundamental features of frost formation needs to be studied. Accordingly, the key focus of this paper is to study the effect of ultrasound on the frost formation experimentally. Comparative observations of the initial frost nucleation and the frost growth process on a cold flat surface will be performed with a microscopic image system. Meanwhile, the transient variation of frost layer thickness will be recorded to evaluate the effect of ultrasound on the frost growth.
2. Experimental apparatus and procedure Fig. 1 shows the schematic diagram of experimental apparatus used in this study. It mainly consists of four parts: a cooling unit, a data acquisition system, a microscopic image acquisition system and ultrasonic vibration devices. A thermoelectric cooler (TEC) that could provide the desired temperature in the range of 0 °C to 30 °C is used as a cooling source for the frosting formation. A bare copper plate of 60 mm 60 mm 6 mm is mounted on the cold side of the TEC as the frosting surface. The hot side of the TEC is mounted on a U-type watercourse. The constant temperature water bath provided 10 °C water to flow through the U-type watercourse for heat removal from the hot side of the thermoelectric cooler. High thermal conductivity greave is used to connect the cooler and cold surface to minimize the contact resistance. The surface temperature of the copper plate can be adjusted by changing the input voltage of the semiconductor thermoelectric cooler and the water flow rate of the U-type watercourse. Four K-type ther-
A N
coverage area of freezing droplets on the cold surface (mm2) total number of droplets in the picture
Fig. 1. Schematic experimental apparatus for frosting tests.
mocouples fitted inside small holes drilled into the copper flat are installed to acquire the average surface temperature of the copper plate. The temperature data is recorded by a HP data acquisition system. Locations of thermocouples in detail are shown in Fig. 2. The frost formation processes on the surface are observed and recorded using a microscopic image acquisition system connected to the computer. The frost layer thickness was measured by micro-measurement software which is capable of an accuracy of 0.005 mm. The ambient humidity is regulated at the given value by an ultrasonic humidifier. A thermo-hygrometer is used to measure the relative humidity and the ambient temperature. The ultrasonic vibration devices include a 20-kHz ultrasonic transducer and an ultrasonic generation with the capacity of 0–100 W. The 20 kHz ultrasonic transducer with 50 mm in diameter which can directly
Fig. 2. Location of thermocouples.
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D. Li et al. / Experimental Thermal and Fluid Science 34 (2010) 1247–1252
(1) Ta = 24
Tw = -13
= 48%
(2) Ta = 26
Tw = -15
= 43%
(3) Ta = 25
Tw = -17
= 50%
(4) Ta = 25
Tw = -20
= 47%
Fig. 3. Location relationship between ultrasonic transducer and flat.
provide high-frequency ultrasonic vibration to moist air around the cold surface is placed in parallel with the cold surface. The distance between ultrasonic transducer and the cold flat surface is 2 mm which is kept unchanged in all experiments. Fig. 3 schematically presents the location relationship between ultrasonic transducer and cold flat. In these tests, the power applied to the transducer is controlled to be about 60 W. An uncertainty analysis is necessary for the experiment. In this study, the uncertainty is basically divided into four categories: environment-control uncertainty, calibration uncertainty, measurement uncertainty, and data acquisition uncertainty. The relative uncertainty of the relative humidity measurement is mainly caused by the uncertainties of the environmental condition control and the accuracy of the measurement instrument. In addition, the uncertainty of frost layer thickness is contributed by the accuracy of the microscopic image system and micro-measurement software. As a matter of fact, the magnitudes of the uncertainties of these variables are basically reasonable and acceptable since in experiments some unpredictable uncertainties may always exist inevitably.
(a) without ultrasound
(b) with ultrasound
Fig. 4. Comparisons of freezing water droplets shapes (magnification 65x).
3. Results and discussion 3.1. Effect of ultrasound on the initial frost nucleation Visual observation of the initial frost nucleation process on the cold surface with and without the effect of ultrasound is carried out under difference test conditions. As the plate is cooling down, the water droplet eventually appears. A very tiny condensate firstly formed on the surface without the effect of ultrasound can be seen and then the small droplets grow up by direct condensation of water vapor and the coalescence of the adjacent droplets. In contrast, with the effect of ultrasound, the first observable water droplets appear much later. The water droplets grow up very slowly. During the experimental period, no coalescence of the neighboring droplets can be seen on the cold surface. However, all the droplets freeze abruptly at the same time both with and without the effect of ultrasound. Fig. 4 shows a group of typical pictures of freezing water droplets with different cold surface temperatures. As shown in Fig. 4, with the effect of ultrasound, regular shape of droplets in a form of hemisphere is seen compared with the extremely irregular shape without the effect of ultrasound. It is also observed the diameter of droplets is smaller and trends to uniform and the distribution of droplets is relatively homogeneous with the effect of ultrasound compared those without the effect of ultrasound. In order to obtain the average freezing droplets diameter, a new method has been developed. Taking Fig. 4-1 for an example to illustrate how to obtain the average freezing droplet diameter without measuring each droplet diameter. As shown in Fig. 5, the outline of each freezing water droplet was drawn in the original
picture (Fig. 5b) and black was filled in the outline (Fig. 5c). And then, the digital image processing method was used to convert the Fig. 5c into binary image which only contained two numbers –0 and 1 (0 is the gap and 1 is the coverage area of freezing droplets) and the coverage area of freezing droplets on the cold surface was easily obtained. Finally, the average freezing droplet diameter can be obtained using the following formula:
rffiffiffiffiffiffiffi A /¼2 pN
ð1Þ
where / is the average freezing droplet diameter, A is the coverage area of freezing droplets on the cold surface, N is the total number of droplets in the picture. Fig. 6 shows the average freezing droplets diameter with different cold surface temperatures with and without the effect of ultrasound. From the measurements, it is found that the ultrasound evidently influences the formation of the droplets and significantly decreases the diameter of the freezing droplets. As can be seen from Fig. 6, the diameter of the freezing droplets is much smaller with the effect of ultrasound than that without ultrasound under the same cold surface temperature. It is also found that the freezing droplets diameter decreases as the surface temperature decreases which is in agreement with those reported by Wu et al [14]. Although with the decrease of the cold surface temperature, the difference of the average freezing droplets diameter with and without the effect of ultrasound for the some cold surface temperature decrease, the diameter of the freezing droplets with the effect
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(a)
(b)
(c)
(a)
(b)
(c)
Fig. 5. Method to obtain the average freezing droplet diameter.
400
1.0 0.9
without ultrasound with ultrasound
coverage of freezing droplets %
average freezing droplet diameter/
350 300 250 200 150 100 50 -21
-20
-19
-18
-17
-16
-15
-14
-13
-12
cold surface temperature/ Fig. 6. Variations of average freezing droplet diameter with/without ultrasound.
of ultrasound are also much smaller than those without ultrasound. Fig. 7 presents the coverage of freezing droplets with different cold surface temperature with/without the effect of ultrasound. As shown in Fig. 7, the coverage of freezing droplets with the effect of ultrasound is much less than that without ultrasound with the same cold surface temperature. In the tests, the coverage of freezing droplets on the cold surface is all more than 65% without ultrasound. In contrast, the effect of ultrasound results in the coverage of freezing droplets all less than 52%. The above results indicate that the effect of ultrasound can cause the distribution of droplets much sparser than that without ultrasound. 3.2. Effect of ultrasound on the frost structure In order to study the effect of ultrasound on the frost growing process, some observations of frost growing process are made with/without the effect of ultrasound. Because what we focus on is the frost layer growth, all the tests here do not account for the nucleation process. Fig. 8 shows the top-view of the variation of the frost structure formed on the cold surface with/without effect
without ultrasound with ultrasound
0.8 0.7 0.6 0.5 0.4 0.3 -21
-20
-19
-18
-17
-16
-15
-14
-13
-12
cold surface temperature Fig. 7. Variations of coverage of freezing droplets with/without ultrasound.
of ultrasound for the cold surface temperature of 15 °C with 18 °C ambient temperature and 54% relative humidity air. As shown, the characteristics of frost structure with the effect of ultrasound are totally different from those without ultrasound. At 50 s after the test starts, the cold surface without the effect of ultrasound has been already completely covered by a layer of frost crystal. However, during the same period of time, almost no frost crystal can be seen from the picture with the effect of ultrasound. It is found from Fig. 8b that frost does not uniformly form on the cold surface without the effect of ultrasound. Some frost branches shown in Fig. 8-1a firstly appear on the surface due to the required supersaturation degree for the frost formation locally differs which was reported by Na [3]. At the same time, with the effect of ultrasound, some special pattern like‘‘frost line”structure shown in Fig. 8-1b forms on the cold surface which is relatively uniform. With the time elapses, the frost branches without the effect of ultrasound significantly grow as shown in Fig. 8-1a to 8-4a. In contrast, with the effect of ultrasound, the surface is only covered with a layer of ‘‘frost line” at 600 s. Furthermore, converse to normal vertical direction which the frost grows along without the effect of ultrasound, the frost with the effect of ultrasound grows along the horizontal direction. This is because the mechanical effect of
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(1)
t = 100s
(2)
t = 350s
(1)
t = 270s
(2)
t = 420s
(3) t = 600s (a) without ultrasound (b) with ultrasound
(3)
t = 450s
Fig. 9. Side-view of frost formation comparison (magnification 49).
1.5
(4) t = 600s (a) without ultrasound (b) with ultrasound Fig. 8. Top view of frost structure comparison (magnification 23).
ultrasound which has the properties of high-frequency and strong directional transmission leads to the particles of water vapor along the propagation direction of the ultrasound cohesion and then deposition on the cold surface. 3.3. Effect of ultrasound on the frost thickness Fig. 9 presents the side-view of variation of the frost thickness formed on the cold surface with/without effect of ultrasound for the cold surface temperature of 23 °C with 10 °C ambient temperature and 40% relative humidity air. As shown, the frost crystal in a form of feather appears on the cold surface at about 270 s without the effect of ultrasound. The feather frost grows rapidly with time due to the deposition of the water vapor around the surface. As show in the Fig. 9-3, the feature shape of frost is more apparent. However, with the effect of ultrasound, almost no growth can be seen during the whole experimental process. As mentioned above, the ultrasound has a mechanical effect which can cause the periodic intensive vibration of water vapor around the flat surface. The acceleration of the water vapor due to intensive vibration is 104 magnitude of the acceleration of gravity [15]. Such a large acceleration can cause the vapor violent disturbances along propagation direction which destroy the ability of directional migration of the vapor to the cold surface due to the
frost layer thickness/mm
without the effect of ultrasound-A with the effect of ultrasound-B
1.0
0.5
0.0 120
180
240
300
360
420
480
540
600
660
720
time/s Fig. 10. Variations of frost thickness with/without effect of ultrasound.
concentration gradient. The water vapor deposition cannot appear to increase the frost thickness. This may be the dominant reason for almost no growth of frost with the effect of ultrasound. Fig. 10 compares the frost thickness with/without the effect of ultrasound. As shown in Fig. 10, the frost growth rate sharply decreases by the ultrasound. The experimental measurement shows that the frost thickness reduction with the effect of ultrasound compared with that without ultrasound is about 75% at the end of the test. 4. Conclusions A series of experiments concerning the initial frost nucleation and frost growth in atmospheric flow with the effect of 20 kHz ultrasound was conducted. The visual results showed that the size of deposited freezing droplets on the cold surface during the frost nucleation process with the effect of ultrasound are much smaller
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than that without ultrasound and the shape is relatively inerratic. The freezing water droplets distribution is apparently sparse compared with that without ultrasound. The frost with the effect of ultrasound grows along the horizontal direction compared with the normal vertical direction without the effect of ultrasound. The frost thickness reduction with the effect of ultrasound compared with that without ultrasound is about 75% in the test. The experimental results presented that the ultrasound has a strong ability to restrain the initial frost nucleation and frost growth process.
[6]
[7]
[8] [9] [10]
References [11] [1] T.W. Forest, The adhesion of ice to low energy solids, in: ASME Winter Annual Meeting on Heat Transfer, Chicago, November 1980, paper 80-WA/HT-19. [2] X.M. Wu, R.L. Webb, Investigation of possibility of frost release from a cold surface, Experimental Thermal and Fluid Science 24 (2001) 151–156. [3] B. Na, R.L. Webb, A fundamental understanding of factors affecting frost formation, International Journal of Heat Mass Transfer 46 (2003) 3797–3808. [4] H. Lee, J. Shin, S. Ha, B. Choi, J. Lee, Frost formation on a plate with different surface hydrophilicity, International Journal of Heat Mass Transfer 47 (2004) 4881–4893. [5] Z. Liu, X. Zhang, H. Wang, S. Meng, S. Cheng, Influences of surface hydrophilicity on frost formation on a vertical cold plate under natural
[12]
[13] [14] [15]
convection conditions, Experimental Thermal and Fluid Science 31 (2007) 789–794. Z. Liu, Y. Gou, J. Wang, S. Cheng, Frost formation on a super-hydrophobic surface under natural convection conditions, International Journal of Heat Mass Transfer 51 (2008) 5975–5982. 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, Applied Surface Science 253 (2007) 8818–8824. V.J. Schaefer, P. Cirrus, Final Report-General Electric Research Laboratory, Schenectady, New York, 1950. p. 50. J. Maybank, N.N. Barthakur, Growth and destruction of ice filaments in an electric field, Nature 216 (1967) 50–52. C.C. Wang, R.T. Huang, W.J. Sheu, Y. Chang, Some observations of the frost formation in free convection: with and without the presence of electric field, International Journal of Heat Mass Transfer 47 (2004) 3491–3505. Y. Gou, Z. Liu, Y. Liu, L. Huang, M. Zhang, The study of frost formation under magnetic field, Journal of Engineering Thermophysics 30 (2009) 465–467. C.H. Cheng, C.C. Shiu, Oscillation effects on frost formation and liquid droplet solidification on a cold plate in atmospheric air flow, International Journal of Refrigeration 26 (2003) 69–78. Q. Yan, L. Zhu, N. Yan, Study on ultrasonic defrost technology of refrigeration fan, Journal of Agriculture and Machine 34 (2003) 74–75 (in Chinese). X. Wu, W. Dai, W. Xu, L. Tang, Mesoscale investigation of frost formation on a cold surface, Experimental Thermal and Fluid Science 31 (2007) 1043–1048. Y. Yuan, S. Chen, Principle and Application of Modern Ultrasound, Nanjing University Press, Nanjing, 1996 (in Chinese).
Contents lists available at ScienceDirect
Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Effect of ultrasound on frost formation on a cold flat surface in atmospheric air flow Dong Li, Zhenqian Chen *, Mingheng Shi School of Energy and Environment, IIUSE, Southeast University, Nanjing 210096, PR China
a r t i c l e
i n f o
Article history: Received 28 February 2010 Received in revised form 3 May 2010 Accepted 12 May 2010
Keywords: Ultrasound Freezing water droplets Frost layer structure Frost layer thickness
a b s t r a c t An experimental study concerning frost formation on a cold flat surface in atmospheric air flow subjected to the effect of 20 kHz ultrasound was conducted. A close observation of the frost nucleation and frost growth processes with and without the effect of 20 kHz ultrasound was made with a microscopic image system. The size and distribution of freezing water droplets during the initial nucleation period were described and the frost layer structure and thickness variation with time were presented. It was found that the freezing water droplets formed on the surface with the effect of ultrasound are smaller and sparser compared with those without the effect of ultrasound. The coverage of freezing droplets is all less than 52% with the effect of ultrasound compared with that all more than 65% without ultrasound under some tested conditions. Furthermore, the frost layer structure with a special pattern like ‘‘frost line” is observed on the cold surface and very small growth of the frost layer can be seen with the effect of ultrasound. The rate of frost layer thickness reduction with the effect of ultrasound compared with that without ultrasound is about 75%. The experimental results also showed that the frost formation process on the flat surface is remarkably restrained due to the effect of ultrasound. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Frost deposition is a well-known and undesirable phenomenon in numerous fields of industry that work with low temperatures such as aeronautics, air conditioning and cryogenics. When moist air comes in contact with a cold surface whose temperature is below both the dew point of the air and the freezing point of water, frost will form on it. Frost deposited on the cold surface of the heat exchanger inevitably degrades the performance of the refrigerating unit due to the increase of the air pressure drop and the decrease of thermal efficiency. In addition, frost accumulation on aircraft wings may cause serious safety problems. Therefore, development of an effective method to prevent the frost nucleation and growth is required in recent years. In the past few years, numerous studies have been performed to investigate the effect of surface energy of the cold surface on frost nucleation and growth. Forest [1] evaluated the use of polymer coatings to reduce the surface energy and concluded that a zero surface energy coating would be the most effective surface for releasing frost particles from the cold surfaces. Wu and Webb [2] investigated the possibility of frost release for hydrophilic and hydrophobic surfaces. Their experiment showed that frost formed on a hydrophobic surface cannot be removed successfully by mechanical vibration. Na and Webb [3] presented a theoretical analysis of the fundamental factors affecting frost nucleation and * Corresponding author. Tel.: +86 25 83790626; fax: +86 25 57714489. E-mail address: [email protected] (Z. Chen). 0894-1777/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2010.05.005
pointed out that a low energy surface requires a much higher supersaturating degree for frost nucleation than a high energy surface. Lee et al. [4] developed frost maps for two different surfaces having two different hydrophilic characteristics and concluded that surface with low dynamic contact angle (DCA) had lower frost thickness and higher frost density than that with high DCA. Liu et al. [5] observed the frost nucleation and deposition processes on a vertical plate and proposed that the initial water drops condensed on a hydrophobic surface are smaller and remain in the liquid state for a longer time compared with ones formed on a plain copper surface. However, their results showed that the influence of surface hydrophobicity is limited to the very initial period of the frost deposition process and cannot repress the frost layer growth. Recently, super-hydrophobic surface has been developed to prevent frost formation. Liu and Gou et al. [6] studied the frost formation on the super-hydrophobic surface with a contact angle of 162° and found that the super-hydrophobic surface can delay the frost deposition for 55 min compared with the plain copper surface under the tested conditions. Wang et al. [7] fabricated a super-hydrophobic coating with water contact angle of 155° from modified CaCO3 and polyacrylate. Their experiments showed the frost formation on the super-hydrophobic surface was greatly retarded compared with that on bare copper surface and the surface can keep super hydrophobicity even after freezing–thawing treating for 10 times. Furthermore, many studied concerning the effect of electric field on the frost formation were reported. Schaefer [8] firstly reported the influence of electrohydrodynamics (EHD) on frost
1248
D. Li et al. / Experimental Thermal and Fluid Science 34 (2010) 1247–1252
Nomenclature Ta Tw u /
ambient air temperature (°C) cold surface temperature (°C) relative humidity average freezing droplet diameter (lm)
formation and found a rapid growth of ice in the form of whiskerlike aggregates in the presence of high electrical filed. Maybank and Barthakur [9] experimentally studied the effect of electric field on the ice crystal growth. Their results showed a rapid growth of ice crystals when the applied electric field strength is above 200 V/cm. The observed crystals are thinner and more fragile compared with those without electric field. Wang et al. [10] found that with the presence of EHD, the ice column is pulled up towards the electrode and the structure is relatively skinny and fragile which can easily break up and fall off due to the influence of gravity. It was also found that the electric polarity plays a significant role on the frost growing. For a negative polarity, the frost structure is thinner than that for a positive polarity and the break-off frequency of the ice column is more frequently compared to a positive polarity. In addition to above works, Gou et al. [11] theoretically and experimentally studied the frost formation under magnetic field. It is found that the water droplet is smaller and more homogeneous and the frost structure is easy to be removed under magnetic field. Cheng and Shiu [12] investigated the oscillation effects on the frost formation and the liquid droplet solidification on a cold plate. It is found 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 ranges of the frequency from 100 Hz to 200 Hz and amplitude from 40 lm to 100 lm. Lately, Yan et al. [13] developed a new method of ultrasonic defrost and their results showed it is feasible to use the ultrasonic defrost technology. However, the effect of ultrasound on the fundamental features of frost formation needs to be studied. Accordingly, the key focus of this paper is to study the effect of ultrasound on the frost formation experimentally. Comparative observations of the initial frost nucleation and the frost growth process on a cold flat surface will be performed with a microscopic image system. Meanwhile, the transient variation of frost layer thickness will be recorded to evaluate the effect of ultrasound on the frost growth.
2. Experimental apparatus and procedure Fig. 1 shows the schematic diagram of experimental apparatus used in this study. It mainly consists of four parts: a cooling unit, a data acquisition system, a microscopic image acquisition system and ultrasonic vibration devices. A thermoelectric cooler (TEC) that could provide the desired temperature in the range of 0 °C to 30 °C is used as a cooling source for the frosting formation. A bare copper plate of 60 mm 60 mm 6 mm is mounted on the cold side of the TEC as the frosting surface. The hot side of the TEC is mounted on a U-type watercourse. The constant temperature water bath provided 10 °C water to flow through the U-type watercourse for heat removal from the hot side of the thermoelectric cooler. High thermal conductivity greave is used to connect the cooler and cold surface to minimize the contact resistance. The surface temperature of the copper plate can be adjusted by changing the input voltage of the semiconductor thermoelectric cooler and the water flow rate of the U-type watercourse. Four K-type ther-
A N
coverage area of freezing droplets on the cold surface (mm2) total number of droplets in the picture
Fig. 1. Schematic experimental apparatus for frosting tests.
mocouples fitted inside small holes drilled into the copper flat are installed to acquire the average surface temperature of the copper plate. The temperature data is recorded by a HP data acquisition system. Locations of thermocouples in detail are shown in Fig. 2. The frost formation processes on the surface are observed and recorded using a microscopic image acquisition system connected to the computer. The frost layer thickness was measured by micro-measurement software which is capable of an accuracy of 0.005 mm. The ambient humidity is regulated at the given value by an ultrasonic humidifier. A thermo-hygrometer is used to measure the relative humidity and the ambient temperature. The ultrasonic vibration devices include a 20-kHz ultrasonic transducer and an ultrasonic generation with the capacity of 0–100 W. The 20 kHz ultrasonic transducer with 50 mm in diameter which can directly
Fig. 2. Location of thermocouples.
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D. Li et al. / Experimental Thermal and Fluid Science 34 (2010) 1247–1252
(1) Ta = 24
Tw = -13
= 48%
(2) Ta = 26
Tw = -15
= 43%
(3) Ta = 25
Tw = -17
= 50%
(4) Ta = 25
Tw = -20
= 47%
Fig. 3. Location relationship between ultrasonic transducer and flat.
provide high-frequency ultrasonic vibration to moist air around the cold surface is placed in parallel with the cold surface. The distance between ultrasonic transducer and the cold flat surface is 2 mm which is kept unchanged in all experiments. Fig. 3 schematically presents the location relationship between ultrasonic transducer and cold flat. In these tests, the power applied to the transducer is controlled to be about 60 W. An uncertainty analysis is necessary for the experiment. In this study, the uncertainty is basically divided into four categories: environment-control uncertainty, calibration uncertainty, measurement uncertainty, and data acquisition uncertainty. The relative uncertainty of the relative humidity measurement is mainly caused by the uncertainties of the environmental condition control and the accuracy of the measurement instrument. In addition, the uncertainty of frost layer thickness is contributed by the accuracy of the microscopic image system and micro-measurement software. As a matter of fact, the magnitudes of the uncertainties of these variables are basically reasonable and acceptable since in experiments some unpredictable uncertainties may always exist inevitably.
(a) without ultrasound
(b) with ultrasound
Fig. 4. Comparisons of freezing water droplets shapes (magnification 65x).
3. Results and discussion 3.1. Effect of ultrasound on the initial frost nucleation Visual observation of the initial frost nucleation process on the cold surface with and without the effect of ultrasound is carried out under difference test conditions. As the plate is cooling down, the water droplet eventually appears. A very tiny condensate firstly formed on the surface without the effect of ultrasound can be seen and then the small droplets grow up by direct condensation of water vapor and the coalescence of the adjacent droplets. In contrast, with the effect of ultrasound, the first observable water droplets appear much later. The water droplets grow up very slowly. During the experimental period, no coalescence of the neighboring droplets can be seen on the cold surface. However, all the droplets freeze abruptly at the same time both with and without the effect of ultrasound. Fig. 4 shows a group of typical pictures of freezing water droplets with different cold surface temperatures. As shown in Fig. 4, with the effect of ultrasound, regular shape of droplets in a form of hemisphere is seen compared with the extremely irregular shape without the effect of ultrasound. It is also observed the diameter of droplets is smaller and trends to uniform and the distribution of droplets is relatively homogeneous with the effect of ultrasound compared those without the effect of ultrasound. In order to obtain the average freezing droplets diameter, a new method has been developed. Taking Fig. 4-1 for an example to illustrate how to obtain the average freezing droplet diameter without measuring each droplet diameter. As shown in Fig. 5, the outline of each freezing water droplet was drawn in the original
picture (Fig. 5b) and black was filled in the outline (Fig. 5c). And then, the digital image processing method was used to convert the Fig. 5c into binary image which only contained two numbers –0 and 1 (0 is the gap and 1 is the coverage area of freezing droplets) and the coverage area of freezing droplets on the cold surface was easily obtained. Finally, the average freezing droplet diameter can be obtained using the following formula:
rffiffiffiffiffiffiffi A /¼2 pN
ð1Þ
where / is the average freezing droplet diameter, A is the coverage area of freezing droplets on the cold surface, N is the total number of droplets in the picture. Fig. 6 shows the average freezing droplets diameter with different cold surface temperatures with and without the effect of ultrasound. From the measurements, it is found that the ultrasound evidently influences the formation of the droplets and significantly decreases the diameter of the freezing droplets. As can be seen from Fig. 6, the diameter of the freezing droplets is much smaller with the effect of ultrasound than that without ultrasound under the same cold surface temperature. It is also found that the freezing droplets diameter decreases as the surface temperature decreases which is in agreement with those reported by Wu et al [14]. Although with the decrease of the cold surface temperature, the difference of the average freezing droplets diameter with and without the effect of ultrasound for the some cold surface temperature decrease, the diameter of the freezing droplets with the effect
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(a)
(b)
(c)
(a)
(b)
(c)
Fig. 5. Method to obtain the average freezing droplet diameter.
400
1.0 0.9
without ultrasound with ultrasound
coverage of freezing droplets %
average freezing droplet diameter/
350 300 250 200 150 100 50 -21
-20
-19
-18
-17
-16
-15
-14
-13
-12
cold surface temperature/ Fig. 6. Variations of average freezing droplet diameter with/without ultrasound.
of ultrasound are also much smaller than those without ultrasound. Fig. 7 presents the coverage of freezing droplets with different cold surface temperature with/without the effect of ultrasound. As shown in Fig. 7, the coverage of freezing droplets with the effect of ultrasound is much less than that without ultrasound with the same cold surface temperature. In the tests, the coverage of freezing droplets on the cold surface is all more than 65% without ultrasound. In contrast, the effect of ultrasound results in the coverage of freezing droplets all less than 52%. The above results indicate that the effect of ultrasound can cause the distribution of droplets much sparser than that without ultrasound. 3.2. Effect of ultrasound on the frost structure In order to study the effect of ultrasound on the frost growing process, some observations of frost growing process are made with/without the effect of ultrasound. Because what we focus on is the frost layer growth, all the tests here do not account for the nucleation process. Fig. 8 shows the top-view of the variation of the frost structure formed on the cold surface with/without effect
without ultrasound with ultrasound
0.8 0.7 0.6 0.5 0.4 0.3 -21
-20
-19
-18
-17
-16
-15
-14
-13
-12
cold surface temperature Fig. 7. Variations of coverage of freezing droplets with/without ultrasound.
of ultrasound for the cold surface temperature of 15 °C with 18 °C ambient temperature and 54% relative humidity air. As shown, the characteristics of frost structure with the effect of ultrasound are totally different from those without ultrasound. At 50 s after the test starts, the cold surface without the effect of ultrasound has been already completely covered by a layer of frost crystal. However, during the same period of time, almost no frost crystal can be seen from the picture with the effect of ultrasound. It is found from Fig. 8b that frost does not uniformly form on the cold surface without the effect of ultrasound. Some frost branches shown in Fig. 8-1a firstly appear on the surface due to the required supersaturation degree for the frost formation locally differs which was reported by Na [3]. At the same time, with the effect of ultrasound, some special pattern like‘‘frost line”structure shown in Fig. 8-1b forms on the cold surface which is relatively uniform. With the time elapses, the frost branches without the effect of ultrasound significantly grow as shown in Fig. 8-1a to 8-4a. In contrast, with the effect of ultrasound, the surface is only covered with a layer of ‘‘frost line” at 600 s. Furthermore, converse to normal vertical direction which the frost grows along without the effect of ultrasound, the frost with the effect of ultrasound grows along the horizontal direction. This is because the mechanical effect of
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(1)
t = 100s
(2)
t = 350s
(1)
t = 270s
(2)
t = 420s
(3) t = 600s (a) without ultrasound (b) with ultrasound
(3)
t = 450s
Fig. 9. Side-view of frost formation comparison (magnification 49).
1.5
(4) t = 600s (a) without ultrasound (b) with ultrasound Fig. 8. Top view of frost structure comparison (magnification 23).
ultrasound which has the properties of high-frequency and strong directional transmission leads to the particles of water vapor along the propagation direction of the ultrasound cohesion and then deposition on the cold surface. 3.3. Effect of ultrasound on the frost thickness Fig. 9 presents the side-view of variation of the frost thickness formed on the cold surface with/without effect of ultrasound for the cold surface temperature of 23 °C with 10 °C ambient temperature and 40% relative humidity air. As shown, the frost crystal in a form of feather appears on the cold surface at about 270 s without the effect of ultrasound. The feather frost grows rapidly with time due to the deposition of the water vapor around the surface. As show in the Fig. 9-3, the feature shape of frost is more apparent. However, with the effect of ultrasound, almost no growth can be seen during the whole experimental process. As mentioned above, the ultrasound has a mechanical effect which can cause the periodic intensive vibration of water vapor around the flat surface. The acceleration of the water vapor due to intensive vibration is 104 magnitude of the acceleration of gravity [15]. Such a large acceleration can cause the vapor violent disturbances along propagation direction which destroy the ability of directional migration of the vapor to the cold surface due to the
frost layer thickness/mm
without the effect of ultrasound-A with the effect of ultrasound-B
1.0
0.5
0.0 120
180
240
300
360
420
480
540
600
660
720
time/s Fig. 10. Variations of frost thickness with/without effect of ultrasound.
concentration gradient. The water vapor deposition cannot appear to increase the frost thickness. This may be the dominant reason for almost no growth of frost with the effect of ultrasound. Fig. 10 compares the frost thickness with/without the effect of ultrasound. As shown in Fig. 10, the frost growth rate sharply decreases by the ultrasound. The experimental measurement shows that the frost thickness reduction with the effect of ultrasound compared with that without ultrasound is about 75% at the end of the test. 4. Conclusions A series of experiments concerning the initial frost nucleation and frost growth in atmospheric flow with the effect of 20 kHz ultrasound was conducted. The visual results showed that the size of deposited freezing droplets on the cold surface during the frost nucleation process with the effect of ultrasound are much smaller
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than that without ultrasound and the shape is relatively inerratic. The freezing water droplets distribution is apparently sparse compared with that without ultrasound. The frost with the effect of ultrasound grows along the horizontal direction compared with the normal vertical direction without the effect of ultrasound. The frost thickness reduction with the effect of ultrasound compared with that without ultrasound is about 75% in the test. The experimental results presented that the ultrasound has a strong ability to restrain the initial frost nucleation and frost growth process.
[6]
[7]
[8] [9] [10]
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