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Experimental study of thermo-physical properties and application of paraffin-carbon nanotubes composite phase change materials
International Journal of Heat and Mass Transfer 140 (2019) 671–677
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study of thermo-physical properties and application of paraffin-carbon nanotubes composite phase change materials Wu XueHong a,b,⇑, Wang ChunXu a, Wang YanLing a,b,⇑, Zhu YouJian a,b a b
School of Energy and Power Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China Collaborative Innovation Center of Food Production and Safety of Henan Province, Henan 450002, China
a r t i c l e
i n f o
Article history: Received 3 April 2018 Received in revised form 7 April 2019 Accepted 3 June 2019 Available online 24 June 2019 Keywords: Phase change material Carbon nanotube Thermo-physical properties Composite shelf
a b s t r a c t In order to improve the thermal conductivity of pure paraffin (RT4) which is regarded as cool storage of phase change material (PCM), four kinds of paraffin-carbon nanotubes cool storage of composite PCMs are prepared by adding different amount of carbon nanotube (CNT) into paraffin, and their thermo-physical properties are tested by using differential scanning calorimeter (DSC) and laser thermal conductivity meter. The results show that, with the increasing of carbon nanotubes, the phase transition temperature of the cool storage of composite PCMs diminishes, phase change latent heat dwindles while thermal conductivity gradually increases. Compared with the pure paraffin, increase in mass fraction of carbon nanotube 3% can provide an increase in thermal conductivity of the solid and liquid 30.3% and 28.5%, and a reduce in the melting and solidification phase change latent heat of composite PCM 8.9% and 9.3%, respectively. Further investigation applies the composite PCMs into the shelf of vertical open -type refrigerated display cabinet to improve the performance. The results show that the shelf filled with the composite PCMs can decrease the internal temperature of the VORDC and temperature fluctuation during the defrosting period. Compared with the ordinary shelf, the lengthways and depthwise temperature different of composite shelf filled into only RT4 reduce by 80.0% and 7.6%, respectively; the lengthways and depthwise temperature different of composite shelf filled into 3%-CNT composite PCMs reduce by 92.0% and 12.2%, respectively. It indicates that the composite PCMs further increases the thermal conductivity of composite shelf and improve the performance of VORDC. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Vertical open-type refrigerated display cabinet can not only provide suitable storage temperature for refrigerated food, but also comprehensively display food, beautify the shopping environment and stimulate consumption. After years of development, the vertical open refrigerated cabinet has become indispensable refrigerated equipment in the supermarket and retail stores. However, its food display surface is open and separates the inside environment from the outside only by air curtain, thus leading to the following disadvantages: food contained in the cabinet is vulnerable to external environment [1,2], the inside temperature distribution is uneven [3], and the food temperature fluctuates greatly during the defrosting period [4]. Evans et al. [5] pointed out that the temperature distribution in the refrigerated display cabinet was uneven. 60% of the high temperature part distributed was in the ⇑ Corresponding authors at: School of Energy and Power Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China. E-mail addresses: [email protected] (X. Wu), [email protected] (Y. Wang). https://doi.org/10.1016/j.ijheatmasstransfer.2019.06.008 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
lower part of the front row, and the food temperature in the front row rose greatly during the defrosting period. After investigating 32 supermarkets, Lunden et al. [6] found that the temperature of over 50% of the food exceeded the requirements for refrigerating, among which, the temperature of 17.9% of them surpassed the limit of refrigerating temperature over 3 °C. In the process of refrigerating, if the food temperature fluctuated or exceeded the temperature requirements for refrigerating, the moisture, vitamins, protein and other nutrients in food would be reduced greatly and the storage life of food would be shortened. In recent years, scholars had put forward an idea of applying the phase change material (PCM) into the open-type refrigerated display cabinet to solve the problems it has. Alzuwaid et al. [7] studied the influence of radiator which was used for filling the water gel PCM on the performance of the open-type refrigerated display cabinet’s refrigerating system. It proved that the radiator could save 5% of energy for the refrigerating system and reduced the temperature fluctuation in the cabinet during the defrosting period. Lu et al. [8] filled phase change materials such as water and borax into the shelf to investigate the fluctuation of food temperature in the cabinet
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during the defrosting period. Their results showed that the composite shelf could reduce the food temperature rise by about 1.5 °C during the defrosting period. As a kind of PCM, water had advantages including high latent heat value, non-toxicity, low price and pollution-free to the environment, however, in the solidifying process of water, the supercooling degree would delay the time for solidifying phase change materials and reduce the efficiency of cold storage. Wu et al. [9] found the heat transmission and cold storage characteristics and influence of the composite shelf filled with the PCMs (RT3, RT4, RT5) on the food temperature distribution in the cabinet. Their results showed that the cold storage effect of RT4 was the best, which could reduce the food temperature by 20.0%83.3% and the temperature rise by 83.3%- 87.5%. Paraffin was taken as a commonly used organic PCM due to its stable chemical performance, non-supercooling, non-phase separation, high phase change latent heat, non-toxicity, non-corrosiveness, low cost and accessibility [10], however, The low thermal conductivity was the major drawback which decreased the efficiency of heat stored and released during melting and freezing processes, and also limited their application [11]. At present, scholars have proposed many different methods to improve the thermal performance of PCMs, such as adding graphene oxide sheet [12], CuO [13], Al2O3 [14], Fe3O4 [15] and other nanometer materials to PCM, packaging phase change materials into polymers and inorganic materials [16,17], or preparing the shape-stabilized PCM by embedding metal structure or impregnating a porous material [18]. The carbon nanotube provide a new idea to improve the thermal conductivity due to its high thermal conductivity (2000–6000 W/mK) [19], low density and good thermal performance. Cui et al. [20] studied and found out that the thermal conductivity of composite PCMs increased by 15.4% in its solid state after 5% of the carbon nanotube (CNT) was added into the soybean wax. Li et al. [21] added the carbon nanotube (MWCNT) to the stearic acid, and the result showed that the thermal conductivity of composite phase change materials increased by 11.4% after 3% of the MWCNT was added. Babaei et al. [22] applied molecular dynamics simulations method to investigate the adding carbon nanotubes and graphene into long-chain paraffins which could increase the thermal conductivity. Therefore, based on the above literatures, this paper proposes adding the carbon nanotube (CNT) into the paraffin (RT4) to improve the thermal conductivity of composite PCMs. The thermo-physical properties of four composite PCMs with different mass fraction of carbon nanotube are investigated by DSC and LFA in order to obtain the best composite PCMs. Finally, its application in the VORDC’s shelf is also discussed.
form of agglomerate or winding form and very difficult to dissolve in the paraffin, so it is difficult to dispersible well in the paraffin wax. In order to obtain the composite PCMs with good stability and uniform dispersion, the different dispersion methods are investigated and compared before the preparation of the composite PCMs. The composite PCM with mass fraction of 1% carbon nanotubes is elaborated using different dispersion methods, which is kept static state for 48 h before the measurement as shown in Fig. 1. In Fig. 1, sample A does not use the dispersant and ultrasonic oscillation, it is obvious that the precipitation has formed owing to the strong Van der Waals forces among the CNTs. Sample B is only dealt with the ultrasonic oscillation (the working frequency of 40 KHZ and ultrasonic time of 75 mins) and carbon nanotubes are scattered into small coacervates, however, small coacervates are reunited quickly into a large one and precipitation is formed again due to the large Van der Waals forces among carbon nanotubes. The dispersant and ultrasonic oscillation are all used in sample C, composite PCM has a good stability and uniform dispersion. The reasons are the big coacervates of carbon nanotubes are broken into small ones, simultaneously, the dispersant is a surface-active substance added to a suspension of CNTs, which can be easy to adsorb on the carbon nanotube walls and prevent clumping. Based on the above results, CNTs/paraffin composites with different loadings of CNTs are prepared, as shown in Table 1. 2.3. Measurement and analysis of phase change latent heat A small sample with pure paraffin RT4 1%-CNT, 3%-CNT and 5%CNT/RT4 is placed in the DSC furnace. Nitrogen is used as protection gas and purge gas, in which the protection gas flow rate is 100 ml/min, and the purge gas flow rate is 50 ml/min. The samples are heated and cooled at constant temperature (5 °C/min). The thermal analyses are performed in the temperature range 20 °C to 25 °C under a constant stream of nitrogen at atmospheric pressure. Different samples of DSC curve of melting and solidification are showed in Fig. 2.
2. Experimental investigation of composite PCMs 2.1. Materials and equipment The PCM (RT4) was obtained from the German RUBITHERM company.Carbon nanotubes (CNT) is obtained from ZhongKe DeTong Technologies Co Ltd, its purity is >98% and its diameter and length are 10–20 nm and 10–30 lm, respectively; Solvent based dispersant (WinSperse3050) is obtained from WeiBoSi New Material Co Ltd. The instruments used in the experiment include ultrasonic cleaning machine C15, Electronic balance ME204, Differential scanning calorimeter DSC214 and Laser thermal conductivity meter LFA467. 2.2. Preparation of composite PCMs Owing to the strong Van der Waals force, great specific surface area and high diameter ratio of carbon nanotubes, it exists as a
Fig. 1. Dispersion of carbon nanotube after samples keeps static state for 48 h.
Table 1 Experimental samples. Samples
CNT addition/%
Dispersant content/%
Ultrasonic temperature /°C
RT4 1%-CNT 3%-CNT 5%-CNT
0 1 3 5
0.1 0.1 0.1 0.1
25 25 25 25
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Fig. 2. Heat flux curve of different composite PCMs.
Fig. 2 shows the measured heat flux when heating (positive values) or cooling (negative values) the different samples. As can be seen in Fig. 2, the positive flow represents the heat absorbed by the pure paraffin during melting, whereas the negative heat flow represents the heat released during solidification. Transition peak is clearly observed, which suggests that solid-liquid phase change of samples happens in the peak. Theoretically, the melting and solidifying curves of phase change materials are usually recognized as the exact mutually inverse process, however, according to results of measurement, the shapes and positions of melting and solidifying curves of the four samples are not completely symmetrical because the DSC curve is affected by the DSC heating and cooling rate, which means that different heating and cooling rate will make the phase change initial point, peak point and termination point measuring phase change materials have certain deviation. It can be seen from Fig. 2 that the solid-liquid phase transition temperature and the melting temperature are 32.87 and 51.81 °C, respectively. Besides, the initial temperature, peak temperature and final temperature of three composite PCMs coincide with that of the pure paraffin in the process of melting and solidifying, which shows that the phase change temperature of samples is less influenced by the amount of CNT in the process of melting and solidifying, these illustrate that both paraffin and the paraffin/CNT composite exhibit similar thermal characteristics. This is because there is no chemical reaction between the paraffin and the CNT. Table 2 shows the experimental values of phase change latent heat of the four samples in the process of melting and solidifying. In Table 2, as the mass fraction of CNT increases, the melting phase change latent heat and solidifying phase change latent heat decrease, because the CNT itself is not phase transition in the temperature of melting and solidifying process. As the mass fraction of CNT increases, the mass fraction of RT4 reduces; therefore, the total phase change latent heat will decrease. Compared with RT4,
Table 2 Different samples of phase change latent heat. Samples
Melting phase change latent heat (J/g)
Solidifying phase change latent heat (J/g)
RT4 1%-CNT 3%-CNT 5%-CNT
151.8 146.3 137 128.2
155.9 151.5 141.4 134.4
the melting phase change latent heat of 1%-CNT, 3%-CNT and 5%CNT reduces by 3.6%, 8.9% and 15.5%, respectively. 2.4. Measurement and analysis of thermal conductivity The thermal conductivity of samples is measured by NETZSCH LFA LFA 467. During the LFA measurement procedure of a sample, Helium is used as protective gas and purge gas, in which the protective gas flow rate and the purge gas flow rate are 20 ml/min and 60 ml/min, respectively. Temperature program of test sample is edited, the temperature range 20 °C to 25 °C, temperature interval of 5 °C and three times each temperature point shine are set. Thermal conductivity of four kinds of PCMs can be obtained by indirect calculation of kðTÞ ¼ qðTÞC P ðTÞaðTÞ, the specific heat used for calculating of the thermal conductivity is measured by the DSC test. The thermal conductivity of different samples along with the change of temperature and carbon nanotube content shows in Fig. 3. Presented in Fig. 3 are the measurement results of four samples between 20 and 25 °C. It can be seen that the thermal conductivity of the four groups of samples is almost no change in the range of 20 to 0 °C in the solid state, however, It is noteworthy that there increases dramatically within the range of 0 °C–7 °C due to occur
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Air curtain
Cold air channel
Evaporator Fig. 3. The thermal conductivity of four samples.
Compressor
Table 3 The thermal conductivity of different samples.
Shelf
Fan Condenser
Fig. 4. Side view of open-type refrigerated display cabinet.
Samples
20 °C
10 °C
10 °C
20 °C
PW PW-1%CNT PW-3%CNT PW-5%CNT
0.348 0.401 0.451 0.489
0.351 0.414 0.460 0.486
0.451 0.501 0.567 0.612
0.284 0.322 0.378 0.419
the solid-liquid phase change. In the process of phase change, the rise of temperature enhances the molecular vibration of phase change materials so as to improve the performance of thermal conductivity. The thermal conductivity of composite phase change materials increases as the mass fraction of CNT increases, because the thermal conductivity of CNT is greater than that of RT4. The experimental values of the thermal conductivity of the four samples under conditions of 10 °C and 20 °C in solid state and 10 °C and 20 °C in liquid state are shown in Table 3, respectively . In Table 3, compared with that of pure paraffin RT4, the thermal conductivity of samples 1%-CNT, 3%-CNT and 5%-CNT increases by an average of 16.6%, 30.3% and 30.3% in solid state, respectively. Compared with the results of Cui et al. [20] and Li et al. [21], The present results show that the present composite PCM can better improve the thermal conductivity in solid state. Their thermal conductivity increases by an average of 11.9%, 28.5% and 28.5% in liquid state respectively. The thermal conductivity of composite phase change materials increases with the increasing of CNT mass fraction. To sum up, the phase change latent heat of the four samples decreases as the mass fraction of CNT increases, and their ranges of phase change temperature are almost no change. In addition, the thermal conductivity of composite phase change materials increases as the mass fraction of CNT rises, but the growth rate is reduced. Therefore, there is a best value for the amount of adding CNT into RT4, which ensures not only the phase change latent heat has less changed, but also it is high thermal conductivity. Thus, composite phase change materials 3%-CNT/RT4 are used in this following experiment, which is filled into the composite shelves using the refrigerated display cabinet (RDC) in order to improve the performance of VORDC and uniformity of temperature. 3. Application of composite PCMs in the open-type refrigerated display cabinet
The nonuniformity and fluctuation of temperature have exited widely in the open-type refrigerated display cabinet, especially in the defrosting period. In recent, composite shelf is proposed, which is composed of hollow shelf, heat pipe and PCM filled between the heat pipe, structural diagram of composite shelf is shown in Fig. 5. The overall dimensions (length, depth, height) of shelf are 930 mm 300 mm 20 mm, six heat pipes are arranged uniformly in the shelf and the interval is 150 mm, which the external diameter is 16 mm and the length of evaporating section and condensing section are 290 mm and 60 mm, respectively. The detailed design of heat pipe of the present paper was stimulated by [8]. The composite shelf is placed in the third shelf of open-type refrigerated display cabinet to improve the uniformity of temperature distribution and reduces the food temperature fluctuation during defrosting, so which can improve the performance of refrigerated display cabinet. 3.2. Experimental devices and measurement methods Tests are carried out in the specified climate room in constant temperature and humidity air conditioning system, which is based on the Chinese Standard GB/T21001.2-2007 and European Standard EN441-1 for the test of open refrigerated display cabinet. The test conditions consider dry bulb temperature 25 °C, relative humidity 60%, temperature fluctuation range ±1 °C and relative humidity fluctuation range ±3％, respectively. Before the tests are started, the refrigerated display cabinet runs about 48 h at the specified climate class and then measures for continuous defrosting cycles. The T-type thermocouples are placed at the geometrical center of the food packages (M-packages) in accordance with Chinese Standard GB/T 21001.2–2007. The M-packages are made of tylose with their dimensions of 200 mm 100 mm 50 mm (length width height). Fig. 6 shows the arrangement of the M-packages and measuring points. Three layers and rows of M-packages are placed along the high and depth direction of all shelves and twelve measurement points are set to record the temperature of M-packages on the composite shelf, the six measurement points are set at the first and third layer M-packages in the front and rear, respectively, as shown in Fig. 6. 4. Experimental results and analysis
3.1. Encapsulation of composite PCMs into the composite shelf 4.1. Temperature uniformity on the lengthways shelf The vertical open-type refrigerated display cabinet which is single-layer air curtain is applied in present test, the side view of the vertical open-type refrigerated display cabinet is shown in Fig. 4.
Fig. 7 shows the temperature distribution on the first layer in the front row on the lengthways ordinary shelf, RT4 composite
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shelf and 3%-CNT composite shelf. In Fig. 7, It can be seen that the temperature and lengthways temperature difference of Mpackages on the ordinary shelf are bigger than these on the RT4 composite shelf and 3%-CNT composite shelf under the same test conditions. The reason is RT4 composite shelf and 3%-CNT composite shelf has cool storage, which can enhance heat conduction and thermal radiative. The temperature difference of M-packages on the left and right sides on the ordinary shelf, RT4 composite shelf and 3%-CNT composite shelf is 1.25 °C, 0.25 °C and 1.25 °C, respectively. Compared with the ordinary shelf, the average temperature difference of M-packages on the left and right sides on RT4 composite shelf and 3%-CNT composite shelf decreases by 80% and 92%, respectively. It means that composite PCMs adding carbon nanotubes can increase the performance of heat conduction of composite shelf. According to Fig. 7, during defrosting, the temperature rise of M-packages on RT4 composite shelf and 3%-CNT composite shelf obviously decreases. The reasons are the PCMs produce the liquid-solid phase change and have the cool storage in the refrigeration operation, which melt and absorb heat in the defrosting period and prevent the temperature of M-packages rise.
M-packes
Right Third layer
Left
Back Front
First layer
Composite shelf
Main view
Side view
Fig. 6. M-packages arrangement and measuring points.
4.2. Temperature uniformity on the depthwise shelf
6 Oridinary Left RT4 Left 3%-CNT Left
5
Oridinary Right RT4 Right 3%-CNT Right
o
Temperature/ C
Fig. 8 shows the temperature distribution of M-packages on the left and right sides in the front and rear rows on ordinary shelf, RT4 composite shelf and 3%-CNT composite shelf. During the thermal entrainment and ambient thermal radiation, the temperature of M-packages on the front is higher than that on the rear. Meanwhile, the M-packages on the rear are adjacent to the back panel which has low temperature, the big temperature difference between the front and rear M-packages has produced. The temperature difference of M-packages in the front and rear rows on ordinary shelf, RT4 composite shelf and 3%-CNT composite shelf is 3.68 °C, 3.4 °C and 3.68 °C, respectively. Compared with that on ordinary shelf, the temperature difference of M-packages in the front and rear rows on the RT4 composite shelf and 3%-CNT composite shelf decreases by 7.6% and 12.2%, respectively. From the Fig. 7 and Fig. 8, it can be seen that the temperature distribution of M-packages adding 3% CNT is more uniformity during the overall test period, the temperature and temperature rise of M-packages on the 3%-CNT composite shelf are lower than these on RT4 composite shelf. The reasons are the CNTs increase the thermal conductivity of PCMs and enhance the performance of shelf, so the 3%-CNT composite shelf can further improve the performance of VORDC.
4 3 2 1
0
200
400
600
800
Time/min Fig. 7. Average temperature of M-packages on the left and right side.
Fig. 9 shows the temperature distribution of composite shelf filled with 3%-CNT measured by the infrared thermal imager. The temperature at the point Sp1 and Sp2 is 0.6 °C and 0.5 °C respec-
Fig. 5. Structural diagram of composite shelf.
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7
Oridinary Front RT4 Front 3%-CNT Front
o
Temperature/ C
6
Oridinary Back RT4 Back 3%-CNT Back
5 4 3 2 1 0 -1
0
200
400
600
800
Time/min Fig. 8. Average temperature of M-packages on front and rear row. Fig. 10. Temperature distribution around 3%-CNT composite shelf by infrared thermal imager.
the melting and solidification phase change latent heat gradually decreases with the increase of the mass fraction of CNT, which the melting and solidification phase change latent heat of PCMs adding 3%-CNT are 8.9% and 9.3% lower than those of Pure paraffin (RT4). Secondly, the thermal conductivity of PCMs increases with the increase of the mass fraction of CNT, and the thermal conductivity of solid and liquid state of PCMs adding 3%-CNT are 30.3% and 28.5% higher than those of Pure paraffin (RT4), respectively. Thirdly, the application the composite PCMs into the shelf of VORDC, the composite shelf can improve effectively the temperature uniformity and decreases the temperature into the VORDC. Especially, it can reduce the temperature fluctuation during the defrosting period, so the proposed composite PCMs and composite shelf can improve the performance of VORDC. Declaration of Competing Interest Fig. 9. Temperature distribution on composite shelf of 3%-CNT by infrared thermal imager.
The authors declare no conflict of interest. Acknowledgements
tively, namely, the temperature distribution along the lengthways shelf is uniformity. The temperature at the point Sp3 and Sp4 is 1.4 °C and 0.7 °C respectively, namely, along the depth direction of the cabinet, the temperature increases from the inside to the outside, which is consistent with the temperature of M-packages measured by the thermocouple. Fig. 10 shows the temperature distribution of open-type refrigerated display cabinet. It can be seen that the temperature of M-packages on the composite shelf is lower than that without composite shelf in the same layer. Due to the cool storage and thermal radiation of composite shelf, the temperature of M-packages on the upper and lower layer shelf near the composite shelf is lower than that of M-packages in the same layer.
5. Conclusion In this paper, in order to improve the thermal conductivity of paraffin as the phase change materials (PCMs), Carbon nanotubes added into the paraffin are as PCMs and apply in the vertical open refrigerated display cabinet (VORDC). Firstly, the thermo-physical properties and disperse method of composite PCMs are investigated by the DSC and LFA. Experimental results show that CNTs has little effect on the phase change temperature of PCMs, but
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Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study of thermo-physical properties and application of paraffin-carbon nanotubes composite phase change materials Wu XueHong a,b,⇑, Wang ChunXu a, Wang YanLing a,b,⇑, Zhu YouJian a,b a b
School of Energy and Power Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China Collaborative Innovation Center of Food Production and Safety of Henan Province, Henan 450002, China
a r t i c l e
i n f o
Article history: Received 3 April 2018 Received in revised form 7 April 2019 Accepted 3 June 2019 Available online 24 June 2019 Keywords: Phase change material Carbon nanotube Thermo-physical properties Composite shelf
a b s t r a c t In order to improve the thermal conductivity of pure paraffin (RT4) which is regarded as cool storage of phase change material (PCM), four kinds of paraffin-carbon nanotubes cool storage of composite PCMs are prepared by adding different amount of carbon nanotube (CNT) into paraffin, and their thermo-physical properties are tested by using differential scanning calorimeter (DSC) and laser thermal conductivity meter. The results show that, with the increasing of carbon nanotubes, the phase transition temperature of the cool storage of composite PCMs diminishes, phase change latent heat dwindles while thermal conductivity gradually increases. Compared with the pure paraffin, increase in mass fraction of carbon nanotube 3% can provide an increase in thermal conductivity of the solid and liquid 30.3% and 28.5%, and a reduce in the melting and solidification phase change latent heat of composite PCM 8.9% and 9.3%, respectively. Further investigation applies the composite PCMs into the shelf of vertical open -type refrigerated display cabinet to improve the performance. The results show that the shelf filled with the composite PCMs can decrease the internal temperature of the VORDC and temperature fluctuation during the defrosting period. Compared with the ordinary shelf, the lengthways and depthwise temperature different of composite shelf filled into only RT4 reduce by 80.0% and 7.6%, respectively; the lengthways and depthwise temperature different of composite shelf filled into 3%-CNT composite PCMs reduce by 92.0% and 12.2%, respectively. It indicates that the composite PCMs further increases the thermal conductivity of composite shelf and improve the performance of VORDC. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Vertical open-type refrigerated display cabinet can not only provide suitable storage temperature for refrigerated food, but also comprehensively display food, beautify the shopping environment and stimulate consumption. After years of development, the vertical open refrigerated cabinet has become indispensable refrigerated equipment in the supermarket and retail stores. However, its food display surface is open and separates the inside environment from the outside only by air curtain, thus leading to the following disadvantages: food contained in the cabinet is vulnerable to external environment [1,2], the inside temperature distribution is uneven [3], and the food temperature fluctuates greatly during the defrosting period [4]. Evans et al. [5] pointed out that the temperature distribution in the refrigerated display cabinet was uneven. 60% of the high temperature part distributed was in the ⇑ Corresponding authors at: School of Energy and Power Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China. E-mail addresses: [email protected] (X. Wu), [email protected] (Y. Wang). https://doi.org/10.1016/j.ijheatmasstransfer.2019.06.008 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
lower part of the front row, and the food temperature in the front row rose greatly during the defrosting period. After investigating 32 supermarkets, Lunden et al. [6] found that the temperature of over 50% of the food exceeded the requirements for refrigerating, among which, the temperature of 17.9% of them surpassed the limit of refrigerating temperature over 3 °C. In the process of refrigerating, if the food temperature fluctuated or exceeded the temperature requirements for refrigerating, the moisture, vitamins, protein and other nutrients in food would be reduced greatly and the storage life of food would be shortened. In recent years, scholars had put forward an idea of applying the phase change material (PCM) into the open-type refrigerated display cabinet to solve the problems it has. Alzuwaid et al. [7] studied the influence of radiator which was used for filling the water gel PCM on the performance of the open-type refrigerated display cabinet’s refrigerating system. It proved that the radiator could save 5% of energy for the refrigerating system and reduced the temperature fluctuation in the cabinet during the defrosting period. Lu et al. [8] filled phase change materials such as water and borax into the shelf to investigate the fluctuation of food temperature in the cabinet
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during the defrosting period. Their results showed that the composite shelf could reduce the food temperature rise by about 1.5 °C during the defrosting period. As a kind of PCM, water had advantages including high latent heat value, non-toxicity, low price and pollution-free to the environment, however, in the solidifying process of water, the supercooling degree would delay the time for solidifying phase change materials and reduce the efficiency of cold storage. Wu et al. [9] found the heat transmission and cold storage characteristics and influence of the composite shelf filled with the PCMs (RT3, RT4, RT5) on the food temperature distribution in the cabinet. Their results showed that the cold storage effect of RT4 was the best, which could reduce the food temperature by 20.0%83.3% and the temperature rise by 83.3%- 87.5%. Paraffin was taken as a commonly used organic PCM due to its stable chemical performance, non-supercooling, non-phase separation, high phase change latent heat, non-toxicity, non-corrosiveness, low cost and accessibility [10], however, The low thermal conductivity was the major drawback which decreased the efficiency of heat stored and released during melting and freezing processes, and also limited their application [11]. At present, scholars have proposed many different methods to improve the thermal performance of PCMs, such as adding graphene oxide sheet [12], CuO [13], Al2O3 [14], Fe3O4 [15] and other nanometer materials to PCM, packaging phase change materials into polymers and inorganic materials [16,17], or preparing the shape-stabilized PCM by embedding metal structure or impregnating a porous material [18]. The carbon nanotube provide a new idea to improve the thermal conductivity due to its high thermal conductivity (2000–6000 W/mK) [19], low density and good thermal performance. Cui et al. [20] studied and found out that the thermal conductivity of composite PCMs increased by 15.4% in its solid state after 5% of the carbon nanotube (CNT) was added into the soybean wax. Li et al. [21] added the carbon nanotube (MWCNT) to the stearic acid, and the result showed that the thermal conductivity of composite phase change materials increased by 11.4% after 3% of the MWCNT was added. Babaei et al. [22] applied molecular dynamics simulations method to investigate the adding carbon nanotubes and graphene into long-chain paraffins which could increase the thermal conductivity. Therefore, based on the above literatures, this paper proposes adding the carbon nanotube (CNT) into the paraffin (RT4) to improve the thermal conductivity of composite PCMs. The thermo-physical properties of four composite PCMs with different mass fraction of carbon nanotube are investigated by DSC and LFA in order to obtain the best composite PCMs. Finally, its application in the VORDC’s shelf is also discussed.
form of agglomerate or winding form and very difficult to dissolve in the paraffin, so it is difficult to dispersible well in the paraffin wax. In order to obtain the composite PCMs with good stability and uniform dispersion, the different dispersion methods are investigated and compared before the preparation of the composite PCMs. The composite PCM with mass fraction of 1% carbon nanotubes is elaborated using different dispersion methods, which is kept static state for 48 h before the measurement as shown in Fig. 1. In Fig. 1, sample A does not use the dispersant and ultrasonic oscillation, it is obvious that the precipitation has formed owing to the strong Van der Waals forces among the CNTs. Sample B is only dealt with the ultrasonic oscillation (the working frequency of 40 KHZ and ultrasonic time of 75 mins) and carbon nanotubes are scattered into small coacervates, however, small coacervates are reunited quickly into a large one and precipitation is formed again due to the large Van der Waals forces among carbon nanotubes. The dispersant and ultrasonic oscillation are all used in sample C, composite PCM has a good stability and uniform dispersion. The reasons are the big coacervates of carbon nanotubes are broken into small ones, simultaneously, the dispersant is a surface-active substance added to a suspension of CNTs, which can be easy to adsorb on the carbon nanotube walls and prevent clumping. Based on the above results, CNTs/paraffin composites with different loadings of CNTs are prepared, as shown in Table 1. 2.3. Measurement and analysis of phase change latent heat A small sample with pure paraffin RT4 1%-CNT, 3%-CNT and 5%CNT/RT4 is placed in the DSC furnace. Nitrogen is used as protection gas and purge gas, in which the protection gas flow rate is 100 ml/min, and the purge gas flow rate is 50 ml/min. The samples are heated and cooled at constant temperature (5 °C/min). The thermal analyses are performed in the temperature range 20 °C to 25 °C under a constant stream of nitrogen at atmospheric pressure. Different samples of DSC curve of melting and solidification are showed in Fig. 2.
2. Experimental investigation of composite PCMs 2.1. Materials and equipment The PCM (RT4) was obtained from the German RUBITHERM company.Carbon nanotubes (CNT) is obtained from ZhongKe DeTong Technologies Co Ltd, its purity is >98% and its diameter and length are 10–20 nm and 10–30 lm, respectively; Solvent based dispersant (WinSperse3050) is obtained from WeiBoSi New Material Co Ltd. The instruments used in the experiment include ultrasonic cleaning machine C15, Electronic balance ME204, Differential scanning calorimeter DSC214 and Laser thermal conductivity meter LFA467. 2.2. Preparation of composite PCMs Owing to the strong Van der Waals force, great specific surface area and high diameter ratio of carbon nanotubes, it exists as a
Fig. 1. Dispersion of carbon nanotube after samples keeps static state for 48 h.
Table 1 Experimental samples. Samples
CNT addition/%
Dispersant content/%
Ultrasonic temperature /°C
RT4 1%-CNT 3%-CNT 5%-CNT
0 1 3 5
0.1 0.1 0.1 0.1
25 25 25 25
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Fig. 2. Heat flux curve of different composite PCMs.
Fig. 2 shows the measured heat flux when heating (positive values) or cooling (negative values) the different samples. As can be seen in Fig. 2, the positive flow represents the heat absorbed by the pure paraffin during melting, whereas the negative heat flow represents the heat released during solidification. Transition peak is clearly observed, which suggests that solid-liquid phase change of samples happens in the peak. Theoretically, the melting and solidifying curves of phase change materials are usually recognized as the exact mutually inverse process, however, according to results of measurement, the shapes and positions of melting and solidifying curves of the four samples are not completely symmetrical because the DSC curve is affected by the DSC heating and cooling rate, which means that different heating and cooling rate will make the phase change initial point, peak point and termination point measuring phase change materials have certain deviation. It can be seen from Fig. 2 that the solid-liquid phase transition temperature and the melting temperature are 32.87 and 51.81 °C, respectively. Besides, the initial temperature, peak temperature and final temperature of three composite PCMs coincide with that of the pure paraffin in the process of melting and solidifying, which shows that the phase change temperature of samples is less influenced by the amount of CNT in the process of melting and solidifying, these illustrate that both paraffin and the paraffin/CNT composite exhibit similar thermal characteristics. This is because there is no chemical reaction between the paraffin and the CNT. Table 2 shows the experimental values of phase change latent heat of the four samples in the process of melting and solidifying. In Table 2, as the mass fraction of CNT increases, the melting phase change latent heat and solidifying phase change latent heat decrease, because the CNT itself is not phase transition in the temperature of melting and solidifying process. As the mass fraction of CNT increases, the mass fraction of RT4 reduces; therefore, the total phase change latent heat will decrease. Compared with RT4,
Table 2 Different samples of phase change latent heat. Samples
Melting phase change latent heat (J/g)
Solidifying phase change latent heat (J/g)
RT4 1%-CNT 3%-CNT 5%-CNT
151.8 146.3 137 128.2
155.9 151.5 141.4 134.4
the melting phase change latent heat of 1%-CNT, 3%-CNT and 5%CNT reduces by 3.6%, 8.9% and 15.5%, respectively. 2.4. Measurement and analysis of thermal conductivity The thermal conductivity of samples is measured by NETZSCH LFA LFA 467. During the LFA measurement procedure of a sample, Helium is used as protective gas and purge gas, in which the protective gas flow rate and the purge gas flow rate are 20 ml/min and 60 ml/min, respectively. Temperature program of test sample is edited, the temperature range 20 °C to 25 °C, temperature interval of 5 °C and three times each temperature point shine are set. Thermal conductivity of four kinds of PCMs can be obtained by indirect calculation of kðTÞ ¼ qðTÞC P ðTÞaðTÞ, the specific heat used for calculating of the thermal conductivity is measured by the DSC test. The thermal conductivity of different samples along with the change of temperature and carbon nanotube content shows in Fig. 3. Presented in Fig. 3 are the measurement results of four samples between 20 and 25 °C. It can be seen that the thermal conductivity of the four groups of samples is almost no change in the range of 20 to 0 °C in the solid state, however, It is noteworthy that there increases dramatically within the range of 0 °C–7 °C due to occur
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Air curtain
Cold air channel
Evaporator Fig. 3. The thermal conductivity of four samples.
Compressor
Table 3 The thermal conductivity of different samples.
Shelf
Fan Condenser
Fig. 4. Side view of open-type refrigerated display cabinet.
Samples
20 °C
10 °C
10 °C
20 °C
PW PW-1%CNT PW-3%CNT PW-5%CNT
0.348 0.401 0.451 0.489
0.351 0.414 0.460 0.486
0.451 0.501 0.567 0.612
0.284 0.322 0.378 0.419
the solid-liquid phase change. In the process of phase change, the rise of temperature enhances the molecular vibration of phase change materials so as to improve the performance of thermal conductivity. The thermal conductivity of composite phase change materials increases as the mass fraction of CNT increases, because the thermal conductivity of CNT is greater than that of RT4. The experimental values of the thermal conductivity of the four samples under conditions of 10 °C and 20 °C in solid state and 10 °C and 20 °C in liquid state are shown in Table 3, respectively . In Table 3, compared with that of pure paraffin RT4, the thermal conductivity of samples 1%-CNT, 3%-CNT and 5%-CNT increases by an average of 16.6%, 30.3% and 30.3% in solid state, respectively. Compared with the results of Cui et al. [20] and Li et al. [21], The present results show that the present composite PCM can better improve the thermal conductivity in solid state. Their thermal conductivity increases by an average of 11.9%, 28.5% and 28.5% in liquid state respectively. The thermal conductivity of composite phase change materials increases with the increasing of CNT mass fraction. To sum up, the phase change latent heat of the four samples decreases as the mass fraction of CNT increases, and their ranges of phase change temperature are almost no change. In addition, the thermal conductivity of composite phase change materials increases as the mass fraction of CNT rises, but the growth rate is reduced. Therefore, there is a best value for the amount of adding CNT into RT4, which ensures not only the phase change latent heat has less changed, but also it is high thermal conductivity. Thus, composite phase change materials 3%-CNT/RT4 are used in this following experiment, which is filled into the composite shelves using the refrigerated display cabinet (RDC) in order to improve the performance of VORDC and uniformity of temperature. 3. Application of composite PCMs in the open-type refrigerated display cabinet
The nonuniformity and fluctuation of temperature have exited widely in the open-type refrigerated display cabinet, especially in the defrosting period. In recent, composite shelf is proposed, which is composed of hollow shelf, heat pipe and PCM filled between the heat pipe, structural diagram of composite shelf is shown in Fig. 5. The overall dimensions (length, depth, height) of shelf are 930 mm 300 mm 20 mm, six heat pipes are arranged uniformly in the shelf and the interval is 150 mm, which the external diameter is 16 mm and the length of evaporating section and condensing section are 290 mm and 60 mm, respectively. The detailed design of heat pipe of the present paper was stimulated by [8]. The composite shelf is placed in the third shelf of open-type refrigerated display cabinet to improve the uniformity of temperature distribution and reduces the food temperature fluctuation during defrosting, so which can improve the performance of refrigerated display cabinet. 3.2. Experimental devices and measurement methods Tests are carried out in the specified climate room in constant temperature and humidity air conditioning system, which is based on the Chinese Standard GB/T21001.2-2007 and European Standard EN441-1 for the test of open refrigerated display cabinet. The test conditions consider dry bulb temperature 25 °C, relative humidity 60%, temperature fluctuation range ±1 °C and relative humidity fluctuation range ±3％, respectively. Before the tests are started, the refrigerated display cabinet runs about 48 h at the specified climate class and then measures for continuous defrosting cycles. The T-type thermocouples are placed at the geometrical center of the food packages (M-packages) in accordance with Chinese Standard GB/T 21001.2–2007. The M-packages are made of tylose with their dimensions of 200 mm 100 mm 50 mm (length width height). Fig. 6 shows the arrangement of the M-packages and measuring points. Three layers and rows of M-packages are placed along the high and depth direction of all shelves and twelve measurement points are set to record the temperature of M-packages on the composite shelf, the six measurement points are set at the first and third layer M-packages in the front and rear, respectively, as shown in Fig. 6. 4. Experimental results and analysis
3.1. Encapsulation of composite PCMs into the composite shelf 4.1. Temperature uniformity on the lengthways shelf The vertical open-type refrigerated display cabinet which is single-layer air curtain is applied in present test, the side view of the vertical open-type refrigerated display cabinet is shown in Fig. 4.
Fig. 7 shows the temperature distribution on the first layer in the front row on the lengthways ordinary shelf, RT4 composite
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shelf and 3%-CNT composite shelf. In Fig. 7, It can be seen that the temperature and lengthways temperature difference of Mpackages on the ordinary shelf are bigger than these on the RT4 composite shelf and 3%-CNT composite shelf under the same test conditions. The reason is RT4 composite shelf and 3%-CNT composite shelf has cool storage, which can enhance heat conduction and thermal radiative. The temperature difference of M-packages on the left and right sides on the ordinary shelf, RT4 composite shelf and 3%-CNT composite shelf is 1.25 °C, 0.25 °C and 1.25 °C, respectively. Compared with the ordinary shelf, the average temperature difference of M-packages on the left and right sides on RT4 composite shelf and 3%-CNT composite shelf decreases by 80% and 92%, respectively. It means that composite PCMs adding carbon nanotubes can increase the performance of heat conduction of composite shelf. According to Fig. 7, during defrosting, the temperature rise of M-packages on RT4 composite shelf and 3%-CNT composite shelf obviously decreases. The reasons are the PCMs produce the liquid-solid phase change and have the cool storage in the refrigeration operation, which melt and absorb heat in the defrosting period and prevent the temperature of M-packages rise.
M-packes
Right Third layer
Left
Back Front
First layer
Composite shelf
Main view
Side view
Fig. 6. M-packages arrangement and measuring points.
4.2. Temperature uniformity on the depthwise shelf
6 Oridinary Left RT4 Left 3%-CNT Left
5
Oridinary Right RT4 Right 3%-CNT Right
o
Temperature/ C
Fig. 8 shows the temperature distribution of M-packages on the left and right sides in the front and rear rows on ordinary shelf, RT4 composite shelf and 3%-CNT composite shelf. During the thermal entrainment and ambient thermal radiation, the temperature of M-packages on the front is higher than that on the rear. Meanwhile, the M-packages on the rear are adjacent to the back panel which has low temperature, the big temperature difference between the front and rear M-packages has produced. The temperature difference of M-packages in the front and rear rows on ordinary shelf, RT4 composite shelf and 3%-CNT composite shelf is 3.68 °C, 3.4 °C and 3.68 °C, respectively. Compared with that on ordinary shelf, the temperature difference of M-packages in the front and rear rows on the RT4 composite shelf and 3%-CNT composite shelf decreases by 7.6% and 12.2%, respectively. From the Fig. 7 and Fig. 8, it can be seen that the temperature distribution of M-packages adding 3% CNT is more uniformity during the overall test period, the temperature and temperature rise of M-packages on the 3%-CNT composite shelf are lower than these on RT4 composite shelf. The reasons are the CNTs increase the thermal conductivity of PCMs and enhance the performance of shelf, so the 3%-CNT composite shelf can further improve the performance of VORDC.
4 3 2 1
0
200
400
600
800
Time/min Fig. 7. Average temperature of M-packages on the left and right side.
Fig. 9 shows the temperature distribution of composite shelf filled with 3%-CNT measured by the infrared thermal imager. The temperature at the point Sp1 and Sp2 is 0.6 °C and 0.5 °C respec-
Fig. 5. Structural diagram of composite shelf.
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7
Oridinary Front RT4 Front 3%-CNT Front
o
Temperature/ C
6
Oridinary Back RT4 Back 3%-CNT Back
5 4 3 2 1 0 -1
0
200
400
600
800
Time/min Fig. 8. Average temperature of M-packages on front and rear row. Fig. 10. Temperature distribution around 3%-CNT composite shelf by infrared thermal imager.
the melting and solidification phase change latent heat gradually decreases with the increase of the mass fraction of CNT, which the melting and solidification phase change latent heat of PCMs adding 3%-CNT are 8.9% and 9.3% lower than those of Pure paraffin (RT4). Secondly, the thermal conductivity of PCMs increases with the increase of the mass fraction of CNT, and the thermal conductivity of solid and liquid state of PCMs adding 3%-CNT are 30.3% and 28.5% higher than those of Pure paraffin (RT4), respectively. Thirdly, the application the composite PCMs into the shelf of VORDC, the composite shelf can improve effectively the temperature uniformity and decreases the temperature into the VORDC. Especially, it can reduce the temperature fluctuation during the defrosting period, so the proposed composite PCMs and composite shelf can improve the performance of VORDC. Declaration of Competing Interest Fig. 9. Temperature distribution on composite shelf of 3%-CNT by infrared thermal imager.
The authors declare no conflict of interest. Acknowledgements
tively, namely, the temperature distribution along the lengthways shelf is uniformity. The temperature at the point Sp3 and Sp4 is 1.4 °C and 0.7 °C respectively, namely, along the depth direction of the cabinet, the temperature increases from the inside to the outside, which is consistent with the temperature of M-packages measured by the thermocouple. Fig. 10 shows the temperature distribution of open-type refrigerated display cabinet. It can be seen that the temperature of M-packages on the composite shelf is lower than that without composite shelf in the same layer. Due to the cool storage and thermal radiation of composite shelf, the temperature of M-packages on the upper and lower layer shelf near the composite shelf is lower than that of M-packages in the same layer.
5. Conclusion In this paper, in order to improve the thermal conductivity of paraffin as the phase change materials (PCMs), Carbon nanotubes added into the paraffin are as PCMs and apply in the vertical open refrigerated display cabinet (VORDC). Firstly, the thermo-physical properties and disperse method of composite PCMs are investigated by the DSC and LFA. Experimental results show that CNTs has little effect on the phase change temperature of PCMs, but
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